Insight to Neglected Biomarkers in COVID-19: A Comprehensive Narrative Review”

Abstract

In the context of COVID-19, a range of neglected biomarkers provide critical insights into the mechanisms of the disease and potential therapeutic targets. This review aims to address this gap by systematically analyzing the diagnostic and prognostic potential of these neglected biomarkers, with particular emphasis on their mechanistic connections to COVID-19 pathophysiology. Reduced levels of adiponectin and prostacyclin (PGI2) and elevated level of endothelin are associated with endothelial dysfunction, whereas elevated levels of endocan and endoglin are indicative of elevated vascular inflammation. Increased concentrations of markers such as angiopoietin, E-selectin, P-selectin, ICAM-1, and VCAM-1 suggest endothelial activation, while higher levels of fractalkine, galectin, HMGB1, and osteopontin reflect an ongoing inflammatory state. Immunological markers, including HMGB1, neopterin, and serum amyloid A, are significantly elevated, underscoring prolonged immune activation associated with severe COVID-19. Elevated levels of matrix metalloproteinases (MMPs) and soluble urokinase plasminogen activator receptor (SuPAR) highlight tissue remodeling and fibrinolytic imbalance related to vascular injury. Additionally, increases in soluble fms-like tyrosine kinase-1 (sFlt-1) and pentraxin reflect inflammatory pathways that exacerbate endothelial dysfunction. Elevated levels of syndecan-1 reflect endothelial glycocalyx degradation and impaired endothelial barrier integrity. Increased von Willebrand factor (vWF) indicates endothelial activation and injury with a prothrombotic shift. Elevated surfactant protein D (SP-D) is a marker of pulmonary epithelial injury and disruption of the alveolar–capillary interface. Other biomarkers, such as the receptor for advanced glycation end products (RAGE) and MR-proADM, signal oxidative stress and endothelial damage. Collectively, these biomarkers emphasize the extensive vascular and endothelial impairment in COVID-19, suggesting their utility as diagnostic tools and potential targets for therapeutic intervention against the systemic effects of the disease. This review advocates for the integration of these biomarkers into standard monitoring and treatment protocols for COVID-19, thereby enhancing personalized care. Furthermore, our analysis underscores the necessity for additional research into the roles of these biomarkers in other endothelial disorders, ultimately contributing to a more nuanced approach to managing viral infections characterized by vascular complications.

Introduction

In the global effort against COVID-19, it is fundamental to identify the complex strategies by which the virus interacts with its host. A key aspect of COVID-19 disease progression is its capacity to disrupt the endothelium, leading to a sequence of biological processes. The endothelium plays a crucial role in maintaining vascular homeostasis by regulating vascular tone, inflammation, and coagulation. Upon exposure to SARS-CoV-2, the virus infiltrates host cells via the angiotensin-converting enzyme 2 (ACE2) receptor, which is predominantly located on endothelial cells. Following entry into endothelial cells, viral replication occurs, triggering a robust immune response and resulting in endothelial dysfunction. Endothelial dysfunction is characterized by increased permeability, vasoconstriction, the release of pro-inflammatory cytokines, and impaired antithrombotic properties. Such dysregulation compromises normal vascular function, leading to microvascular damage, thrombotic events, and organ dysfunction and increased mortality.^1,2

Endothelial dysfunction assumes a pivotal role in the onset of acute respiratory distress syndrome (ARDS), a severe complication associated with COVID-19 that is marked by extensive inflammation and impaired gas exchange within the pulmonary system. Additionally, endothelial injury and the formation of microvascular thrombi can initiate multi-organ dysfunction, which may include acute kidney injury, myocardial infarction, and stroke.^3,4 Furthermore, endothelial dysfunction may contribute to the disease of “long-COVID,” wherein patients experience persistent symptoms long after the acute phase of the illness. Ongoing endothelial injury and inflammation could play a significant role in the enduring vascular dysfunction, fatigue, cognitive impairment, and other long-term complications observed in certain COVID-19 survivors. ⁵

With the emergence of the COVID-19 pandemic, an array of biomarkers has been introduced for prognosis, diagnosis, and disease surveillance. These biomarkers are generally categorized into several classes, including endothelial, cardiovascular, respiratory, tissue matrix, cell adhesion, and inflammatory biomarkers (Figure 1). Among the most widely recognized biomarkers utilized during pandemics are those referenced in studies^6,7 (Figure 2). However, there are significant gaps in the understanding of often-neglected endothelial and vascular biomarkers in the context of COVID-19, which could potentially serve as early indicators of disease severity. While extensively studied markers such as C-reactive protein (CRP) and D-dimer have garnered considerable attention, lesser-known biomarkers may yield valuable insights into the disease process. A comprehensive understanding of these biomarkers and their relationship with clinical outcomes (such as severe respiratory distress or thrombotic events) could facilitate early risk assessment and inform treatment decisions. Investigating the underlying mechanisms of these biomarkers may reveal novel pathways for the development of tailored therapies, thereby enhancing our understanding and management of COVID-19. This study aims to investigate alternative biomarkers to evaluate their efficacy in predicting, diagnosing, and monitoring the status of COVID-19. The review will specifically focus on biomarkers that are frequently overlooked or underutilized.

Figure 1.

The spectrum of biomarkers in COVID 19, with an emphasis on their interrelationships and roles in disease pathogenesis.

Figure 2.

The conventional and most extensively researched biomarkers in COVID-19, which have garnered significant utilization and attention during the pandemic. Their recognized attributes and accessibility render them vanguard biomarkers in COVID-19 surveillance.

Neglected Biomarkers in COVID

Adiponectin

Adiponectin, a protein consisting of 224 amino acids, is primarily secreted by adipose tissue and functions as a significant adipokine that regulates various metabolic and immunological processes. It plays crucial roles in glucose homeostasis, lipid metabolism, and the modulation of inflammatory responses, thereby contributing to metabolic equilibrium, exhibiting anti-inflammatory properties, and possessing the capacity to influence the endocrine system. ⁸ Obesity is a metabolic disease characterized by a state of low-grade inflammation; circulating levels of most adipokines are elevated due to increased production in adipose tissue. However, adiponectin is an exception, as both its synthesis in adipocytes and circulating concentrations decline with the progression of obesity. In severe cases of COVID-19 hyperglycemia with insulin resistance and decreased levels of adiponectin was reported. ⁹ Moreover, inflammatory cytokines have been shown to inhibit adiponectin production. Individuals experiencing viral infections frequently present with diminished serum levels of this adipokine. ¹⁰

Research has indicated that serum adiponectin levels are inversely correlated with the severity of COVID-19. A study comparing circulating adiponectin concentrations among healthy controls and COVID-19 patients with varying disease severity demonstrated that severe cases exhibited significantly lower adiponectin levels compared to moderate cases and healthy individuals, who displayed comparable concentrations of this adipokine. ¹¹ In a study involving 123 hospitalized COVID-19 patients, reduced adiponectin levels were associated with an increased likelihood of mortality and respiratory distress, suggesting that adiponectin may serve as a predictive biomarker for the severity of COVID-19. ¹² Given that obesity is a recognized risk factor for adverse COVID-19 outcomes, likely due to dysregulated adipokine production and persistent low-grade inflammation, lower adiponectin levels in individuals with obesity have been linked to poorer clinical outcomes following SARS-CoV-2 infection. ¹³ Some studies propose that adiponectin levels increase during COVID-19 and correlate with disease severity. ¹⁴

Inconsistent associations between adiponectin levels and COVID-19 outcomes primarily arise from heterogeneity in patient populations and differences in disease stage at assessment. Reduced adiponectin is typically linked to obesity, insulin resistance, and chronic low-grade inflammation, conditions that increase susceptibility to severe COVID-19 and poorer outcomes. Conversely, elevated adiponectin reported in advanced or critical illness likely reflects a compensatory or maladaptive response to systemic inflammation, endothelial dysfunction, and catabolic stress, rather than a true protective effect. Variability in sampling time points, age distribution, comorbidity burden, and the lack of differentiation between biologically active adiponectin isoforms further contribute to divergent findings. Therefore, adiponectin should be interpreted within the broader clinical and pathophysiological context rather than be used as an isolated prognostic biomarker for COVID-19 severity or mortality. However, the preponderance of research indicates a reduction in this biomarker, underscoring the need for further investigation into the role of adiponectin in the context of COVID-19.

Angiopoietin

Angiopoietin is a member of a family of vascular growth factors that are essential for both embryonic and postnatal angiogenesis. These factors facilitate the assembly and disassembly of endothelial linings within blood vessels, thereby regulating microvascular permeability as well as vasodilation and vasoconstriction through signaling to the smooth muscle cells surrounding the vessels. ¹⁵ Angiopoietin-1 (Ang-1) promotes vascular maturation by constricting endothelial cell-to-cell junctions and enhancing the recruitment of pericytes and smooth muscle cells to the vessel wall. Overall, Ang-1 preserves vessel integrity and prevents leakage. ¹⁵ Conversely, Angiopoietin-2 (Ang-2) destabilizes blood vessels, disrupts endothelial cell-to-cell connections, and promotes angiogenic sprouting. Notably, Ang-2 can compete with Ang-1, inhibiting Ang-1-mediated vascular stability. ¹⁶

Importantly, Ang-2 has been implicated in vascular necroptosis associated with COVID-19-induced vascular injury, while Ang-1 demonstrates significant vascular protective properties, including the suppression of plasma leakage,the inhibition of vascular inflammation, and the prevention of endothelial dysfunction. ¹⁷ Elevated levels of angiopoietin have been correlated with COVID-19 outcomes. In comparison to healthy controls, COVID-19 patients exhibit reduced levels of Ang-1 and increased levels of Ang-2.^18,19 Elevated concentrations of Ang-2 have been associated with the severity of illness and are proposed as potential indicators for predicting intensive care unit (ICU) admission, the onset of acute kidney injury (AKI), and ARDS, as well as for forecasting 90-day mortality among individuals affected by COVID-19. ²⁰ One study found that circulating Ang-2 levels may accurately and early estimate the likelihood of in-hospital mortality and persistent pulmonary complications in COVID-19 patients. ²¹ Another investigation identified circulating Angiopoietin-like 4 (ANGPTL4) as a novel indicator of COVID-19 severity, potentially linked to increased hospital mortality. ²² Research utilizing a Vasculature-on-a-chip platform demonstrated that the peptide QHREDGS, derived from Ang-1, shows promise as a therapeutic agent capable of significantly reducing the inflammatory response of cells to levels comparable to those observed in uninfected controls; this effect enhances the barrier function and survival of endothelial cells against SARS-CoV-2 infection in the presence of peripheral blood mononuclear cells (PBMC). ²³ Based on the findings presented, angiopoietin holds potential both as a biomarker and as a therapeutic intervention for COVID-19.

Apolipoprotein M (ApoM)

ApoM is a key regulator of sphingosine-1-phosphate (S1P), a signaling molecule that plays a crucial role in maintaining endothelial integrity, vascular permeability, and inflammatory regulation. S1P signaling is vital for endothelial cell survival and vascular membrane function. ApoM is instrumental in the regulation of cholesterol transport and high-density lipoprotein (HDL) metabolism. ²⁴ Individuals diagnosed with COVID-19 frequently display significant abnormalities in lipid metabolism, including disruptions in HDL composition and function; these alterations are particularly pronounced in severe cases of the disease. The ApoM/S1P complex, which is associated with HDL particles, possesses anti-inflammatory properties that may be diminished in the context of COVID-19. Animal studies have demonstrated that the loss of ApoM results in compromised endothelial barrier function within the lungs. Additionally, reduced plasma levels of ApoM are correlated with impaired endothelial function. A research report indicates that ApoM levels were significantly lower in COVID-19 patients admitted to the intensive care unit (ICU) compared to those not requiring such intensive care. ²⁵ Moreover, ApoM levels within HDL were found to be inversely associated with the risk of mortality due to complications arising from COVID-19. Specifically, for every 1 standard deviation increase in ApoM, the odds ratio for death decreased to 0.27. ²⁶ Given that ApoM exhibits anti-inflammatory properties, a reduction in ApoM levels could result in decreased S1P levels, heightened inflammatory responses, and exacerbation of the cytokine storm associated with severe COVID-19. A decline in ApoM levels was observed in COVID-19 patients during a 6-month follow-up investigation. ²⁷

While the findings are promising, it is imperative to acknowledge that further research is required to comprehensively establish ApoM as a reliable biomarker for COVID-19. Given that decreased ApoM levels are consistently linked to poorer outcomes and persist in survivors, ApoM remains a candidate for additional investigation, both for its predictive potential and for a deeper understanding of the long-term effects of COVID-19.

Cell-Free DNA (cfDNA), DNA Myeloperoxidase (DNA-MPO), and Neutrophil Elastase (NE)

Neutrophil extracellular traps (NETs) play an essential role in the pathophysiology of COVID-19, especially in promoting hyperinflammation, endothelial dysfunction, and thrombotic complications. NETs are web-like structures formed from decondensed chromatin fibers released by activated neutrophils. These structures are accompanied by antimicrobial proteins, including myeloperoxidase (MPO), neutrophil elastase (NE), and citrullinated histone H3 (Cit-H3). In SARS-CoV-2 infection, the formation of NETs, known as NETosis, is initiated through various pathways. These include direct viral recognition by Toll-like receptors (TLRs) on neutrophils, complement activation, and cytokine storms that involve IL-8 and G-CSF. ²⁸

Cell-free DNA (cfDNA), DNA-MPO, and NE are novel biomarkers in COVID-19 clinical research, with increasing applications in diagnostics, risk stratification, therapeutic monitoring, and studies on long COVID. cfDNA, DNA-MPO, and NE levels exhibit a strong correlation with the severity of COVID-19, organ damage, ICU admission, and mortality, facilitating the early identification of at-risk patients. ²⁹ Moreover, increased NET markers are considered factors contributing to enduring symptoms and tissue damage linked to long COVID. Research is examining the serial assessment of NET biomarkers, including DNA-MPO complexes, to better understand chronic inflammation, develop anti-inflammatory therapies, and forecast long-term outcomes such as acute respiratory distress syndrome, thromboembolism, and higher mortality. ³⁰

Persistence of higher levels of NET markers, including NET-inducing enzymes (NE, MPO) and cell-free DNA, is observed for months following COVID-19 infection and is associated with the severity of long COVID. NETs are composed of DNA, MPO, histones, and various neutrophil proteins, functioning as neoantigens that may initiate autoimmune responses, thereby sustaining inflammation. Persistent formation of neutrophil extracellular traps (NETs) or impaired clearance of these structures could maintain the chronic inflammation observed in long COVID patients, contributing to pulmonary fibrosis, cardiovascular disorders, and neurological dysfunctions. ³¹

Multiple therapies targeting COVID-19 NETs are either authorized or currently being investigated and are being developed to prevent lung injury, kidney damage, endotheliosis, and immunothrombosis. These drugs inhibit molecules that contribute to NETs, such as PAD4 and Gasdermin D. PAD4 inhibitors reduce COVID-19 thrombosis by mediating NET production. Disulfiram, an inhibitor of Gasdermin D, has the potential to enhance COVID-19 treatment. Recombinant human thrombomodulin, activated protein C, high mobility group box-1 inhibitors (methotrexate, hydroxychloroquine, inflachromene, glycyrrhizin, and salicylic acid derivatives), C1 esterase inhibitor, heparin, and recombinant human DNase can protect against endothelial injury, improve lung oxygenation, reduce ARDS, and prevent coagulopathy, decreasing the severity of COVID-19. ³²

Endocan

Endocan, previously referred to as endothelial cell-specific molecule-1 (ESM-1), functions as a soluble dermatan sulfate proteoglycan released by activated endothelial cells and plays a significant role in various inflammatory processes and endothelial dysfunction. Endocan is a molecule with an approximate molecular weight of 50 kDa, consisting of a core protein linked to glycosaminoglycan chains that are crucial to its biological activity. It regulates endothelial cell activation, permeability, and proliferation. The production of endocan can be upregulated by pro-inflammatory cytokines, including TNF-α and IL-1β. This biomolecule has been associated with numerous medical conditions, such as cardiovascular disease, diabetes, and cancer. In the context of cardiovascular diseases, endocan serves as a biomarker of endothelial dysfunction and is associated with atherosclerosis due to its role in enhancing the expression of adhesion molecules like intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). ³³ Recent research indicates that individuals diagnosed with COVID-19 exhibit significantly elevated serum endocan levels compared to healthy controls. ³⁴ Moreover, studies have reported even higher levels in severe cases, suggesting that endocan may function as a valuable biomarker for assessing the severity of COVID-19. The increased endocan levels correlate with the severity of illness, underscoring its potential as a predictive marker for adverse outcomes, such as intensive care unit (ICU) admission and mortality. ³⁵ Notably, some studies indicate that unvaccinated COVID-19 patients have significantly higher endocan levels compared to vaccinated individuals, suggesting a complex interplay between viral infection, immune response, and endothelial function. ³⁶ Endocan is emerging as a critical biomarker for the detection and assessment of the severity of COVID-19, with implications for patient care and prognosis. However, further research is necessary to elucidate its precise relevance in this context and its practical application in clinical treatment.

Endothelin

Endothelin-1 (ET-1), a potent vasoconstrictor, plays a significant role in the pathophysiology of COVID-19. Research has demonstrated that dysregulation of endothelial function, characterized by elevated levels of ET-1, is associated with severe manifestations of COVID-19 and an increased mortality rate. ³⁷ It appears that Weibel–Palade bodies, which are intracellular organelles, and subsequent cellular apoptosis within endothelial cells contribute to the heightened levels of endothelin. ³⁸ Furthermore, the dysregulation of ET-1 has been implicated in various cardiovascular conditions, including hypertension, heart failure, and atherosclerosis. Endothelial dysfunction disrupts vascular homeostasis, leading to severe outcomes such as vascular injury, inflammation, oxidative stress, and coagulopathy in patients with COVID-19. This dysfunction, which is characterized by diminished nitric oxide bioavailability and an increase in vasoconstrictive agents such as endothelin, is considered a critical factor in the progression of COVID-19. Pulmonary hypertension emerges as one of the potential complications of COVID-19- and is characterized by elevated levels of endothelin, which may serve as a valuable biomarker for monitoring disease status. ³⁹ It is noteworthy that elevated endothelin levels are observed not only in patients with COVID-19 but also in individuals with diabetes, heart failure, and hypertension. Given that these populations are recognized as high-risk groups for severe COVID-19 outcomes, the potential therapeutic efficacy of endothelin receptor antagonists in reducing endothelial inflammation in these patients merits further investigation. ⁴⁰ Additionally, increased levels of ET-1 have also been reported in patients with ARDS, ⁴¹ suggesting that endothelin may serve as a predictor of COVID-19 status and outcomes.

Endoglin

Endoglin is a homodimeric transmembrane protein that serves as a principal glycoprotein of the vascular endothelium and functions as a coreceptor for the transforming growth factor β receptor, also known as CD 105. It plays a critical role in maintaining vascular homeostasis and mediating inflammatory responses. Recent studies suggest that endoglin may be significantly implicated in the pathogenesis of COVID-19, particularly concerning vascular dysfunction and inflammation, which are common hallmarks observed in severe cases of the disease. ⁴² Endoglin is involved in regulating vascular remodeling, angiogenesis, and the activity of endothelial nitric oxide synthase, and its involvement in endothelial dysfunction has led to its consideration as a potential biomarker in various pathological conditions. ⁴³ Research has indicated that endoglin levels are markedly elevated in COVID-19 patients who succumbed to the illness. Moreover, univariate Cox regression models from the same study demonstrated a correlation between endoglin levels and mortality. ⁴⁴ The cleavage of endoglin from the cell surface results in the generation of soluble endoglin (sEng) in the bloodstream, which may serve as a biomarker for vascular dysfunction in COVID-19 patients. It is noteworthy that this biomarker has been found to be more effective than COVID-19 itself in identifying septic shock; Statistically, there is a more significant correlation between elevated endoglin levels and septic shock than with COVID-19. ⁴⁵

E-Selectin, L-Selectin, and P-Selectin

E-selectin, a cell adhesion molecule and a recognized marker of endothelial activation, is instrumental in regulating the inflammatory process. Its primary function is to facilitate leukocyte rolling and subsequent adhesion to the endothelial lining, a critical mechanism for attracting immune cells to sites of inflammation or tissue injury. E-selectin is expressed on endothelial cells, and its expression is tightly regulated, with inflammatory stimuli leading to its upregulation. This molecule enables immune cells to migrate from the bloodstream into target tissues, where they fulfill essential roles in immune surveillance, defense mechanisms, and tissue repair processes. ⁴⁶

Comprehensive investigations have revealed elevated levels of E-selectin in individuals infected with COVID-19, correlating with the severity of the disease. Furthermore, a significant relationship has been established between E-selectin levels and neutrophil counts, as well as the duration of hospitalization.^{47 -49}

L-selectin, a cell adhesion molecule (CD62L) expressed on the surface of leukocytes such as lymphocytes, neutrophils, and monocytes, directs leukocytes to sites of tissue damage or infection, thereby stimulating inflammatory and immunological responses. ⁵⁰ L-selectin, similar to other members of the selectin family, plays a critical role in the pathophysiology of COVID-19. Patients with COVID-19 exhibit elevated levels of soluble L-selectin (sL-selectin) compared to healthy individuals, with significantly higher levels observed in those experiencing thrombosis. This increase correlates with severity markers such as C-reactive protein (CRP), lactate dehydrogenase (LDH), and interleukin-6 (IL-6). L-selectin appears to function as an active participant in the pathogenesis of COVID-19, potentially serving as both an indicator of disease severity and thrombotic risk, as well as a candidate for future therapeutic interventions. However, further research is necessary to fully elucidate its role and therapeutic potential in the management of COVID-19 patients. ⁴⁸

P-selectin plays a pivotal role in the pathophysiology of COVID-19, particularly concerning endothelial injury, coagulopathy, and inflammation. Studies have shown that the spike proteins of SARS-CoV-2 induce the expression of P-selectin in human lung microvascular endothelial cells and neutrophils, subsequently exacerbating inflammation, NETosis, and coagulopathy. ⁵¹ Reports indicate elevated levels of P-selectin in COVID-19 patients, which are associated with disease severity and deep vein thrombosis.^51,52 Additionally, increased P-selectin expression has been noted in cases of ARDS. Furthermore, one study reported that post-recovery COVID-19 patients exhibited higher levels of P-selectin, indicative of enhanced platelet activation, thus suggesting potential long-term effects on platelet function and the manifestation of long-COVID symptoms. ⁵³ An investigation utilizing a cutoff value of 4.125 ng/mL demonstrated that serum soluble P-selectin levels possessed a sensitivity of 97.5% and a specificity of 80% for diagnosing COVID-19. These findings indicate a potentially significant role for P-selectin in aiding the clinical diagnosis of affected patients. ⁵⁴

Fractalkine (CX3CL1)

Fractalkine, also known as CX3CL1, is a substantial cytokine protein consisting of 373 amino acids and is recognized as the sole member of the CX3C chemokine family implicated in the pathophysiology of COVID-19, particularly with thrombosis. The onset of hyperinflammatory syndrome and thrombosis has been associated with COVID-19. Evidence suggests that infection with SARS-CoV-2 may induce an increase in CX3CL1 expression in endothelial cells, thereby creating a pro-thrombotic environment. ⁵⁵ In a study examining cytokine alterations during COVID-19, CX3CL1 was identified as the only factor exhibiting variations in expression levels based on gender, with female COVID-19 patients presenting higher levels compared to their male counterparts. ⁵⁶ Additional research indicates that elevated levels of CX3CL1 are associated with severe COVID-19 and neurological manifestations. Furthermore, investigations conducted using K18-hACE2 transgenic mice have demonstrated the efficacy of the CX3CR1 antagonist AZD8797 against SARS-CoV-2, as confirmed through histopathological analyses. ⁵⁷ Another study in this field illustrates the potential effectiveness of inhibiting the CX3CL1 pathway to manage hyperinflammation in COVID-19, as well as to prevent the infiltration of cytotoxic T-cells into vital organs. ⁵⁸ Based on the research conducted in this area, it can be concluded that CX3CL1 holds potential utility both as a biomarker and as a target for therapeutic interventions.

Galectins

Galectins are carbohydrate-binding proteins that play critical roles in a variety of biological and pathological processes. Galectin-3 (Gal-3), a β-galactoside-binding lectin with a molecular weight of 29 to 35 kDa, primarily binds to β-galactosides and is involved in the regulation of cellular functions. Galectins influence cell cycle processes, including apoptosis and autophagy, through interactions with intracellular signaling pathways. They also contribute to both acute and chronic inflammation by recruiting immune cells, such as dendritic cells and macrophages, to sites of injury or infection, and they promote cell-cell and cell-matrix interactions, which are essential for tissue integrity and repair. In pathological conditions, galectins are implicated in cancer angiogenesis, and their dysregulation has been observed in metabolic and autoimmune diseases. ⁵⁹ By binding directly to pathogens, galectins can modulate immune responses to infections and may facilitate SARS-CoV-2 entry through interactions with the spike protein. Gal-3 is known to exacerbate viral infections by enhancing host inflammatory responses and stimulating the production of cytokines, including IL-6 and TNF-α, which are prevalent in severe cases of COVID-19. Elevated levels of Gal-3 are also thought to promote neutrophil infiltration, contributing to acute respiratory inflammation. ⁶⁰ The study revealed a strong positive correlation with IL-1β, moderate positive correlations with TNF-α and D-dimer, and a significant negative correlation with pO₂ and SaO₂. ⁶¹ Gal-3 levels were found to increase with disease severity and the likelihood of ARDS. ⁶² Furthermore, elevated Gal-3 levels are associated with higher mortality rates in intensive care units. ⁶³ Thus, galectin-3 emerges as a promising predictive biomarker for COVID-19 outcomes, underscoring its potential in guiding clinical decision-making.

It is important to note that inhibiting Gal-3 has shown potential as a therapeutic approach by reducing the host’s inflammatory response through multiple mechanisms. Gal-3 inhibitors can diminish the intensity of the inflammatory response in individuals with COVID-19 by significantly reducing the synthesis of inflammatory mediators such as IL-1 and IL-6. ⁶⁴ One potential therapeutic agent is GB0139, an inhaled Gal-3 inhibitor that has demonstrated promise in reducing the fibrosis and inflammation associated with COVID-19 pneumonitis. ⁶⁵ Galectin-3 serves as both a diagnostic tool for assessing COVID-19 severity and a therapeutic target for strategies aimed at decreasing inflammation and improving patient prognosis.

HMGB1

HMGB1, a highly conserved non-histone protein, plays a critical role in the condensation and folding of intranuclear DNA through non-specific DNA interactions. It is classified as a member of damage-associated molecular patterns (DAMPs) and has been shown to activate inflammatory pathways during viral respiratory infections by promoting the synthesis of cytokines and chemokines. ⁶⁶ . Elevated serum levels of HMGB1 have been associated with increased viral replication and the severity of lung injury. The protein’s capacity to bind DNA connects it to essential cellular processes, including transcription, replication, repair, and recombination. HMGB1 interacts with Toll-like receptor 4 (TLR4) and receptor for advanced glycation end products (RAGE), which modulate the production of pro-inflammatory cytokines, activate inflammasomes, induce pyroptotic cell death, and initiate coagulation; these mechanisms facilitate viral infection and tissue damage. Such feedback loops may ultimately culminate in a cytokine storm.^67,68 . Additionally, HMGB1 regulates autophagy and may serve as a biomarker for acute lung injury and ARDS.^66,69 In a rat model, increased levels of angiotensin-converting enzyme II (ACE II) were found to decrease HMGB1 levels, as well as apoptosis in the myocardium following an infarction. This observation suggests that a reduction in ACE II levels due to viral infection could lead to an increase in HMGB1, thereby contributing to the “cytokine storm” and the severe complications associated with COVID-19 infection. ⁷⁰ Elevated HMGB1 levels significantly correlate with disease severity in COVID-19. Patients with severe COVID-19 exhibit markedly higher HMGB1 levels compared to those with mild cases, with intensive care unit (ICU) patients demonstrating greater levels than non-ICU patients. Furthermore, patients with chronic medical conditions, such as hyperglycemia and cardiovascular disease, often present elevated HMGB1 levels, which may contribute to the adverse outcomes observed in these populations following COVID-19 infection. ⁷¹ A reduction in HMGB1 levels has been associated with clinical improvement. ⁶⁶ Blood HMGB1 levels exceeding a specific threshold (eg, 125.4 ng/mL) may indicate an increased risk of mortality in COVID-19 patients. High HMGB1 levels are also correlated with other biomarkers, including D-dimer and liver enzymes (aspartate aminotransferase and alanine aminotransferase), highlighting its association with multi-organ failure. ⁷² Given the role of HMGB1 in the progression of COVID-19, targeting this protein presents dual potential as both a predictive biomarker and a therapeutic target. Modulating HMGB1 activity may reduce cytokine release, thereby limiting excessive immune activation and tissue damage. This strategy could help mitigate severe inflammatory responses, such as cytokine storms, ultimately improving outcomes for patients with severe COVID-19 and positioning HMGB1 as a promising target for intervention.

ICAM-1 and VCAM-1

Intercellular Adhesion Molecule 1 (ICAM-1), a member of the immunoglobulin superfamily, is induced by inflammatory cytokines such as interleukin-1β (IL-1β) and tumor necrosis factor (TNF). Functionally, ICAM-1 interacts with integrins, including Lymphocyte function-associated antigen 1 (LFA-1) and Macrophage-1 antigen (Mac-1), thus facilitating the adhesion and transmigration of leukocytes across endothelial cells. Furthermore, ICAM-1 has been implicated in various conditions, including inflammation and viral infection. ICAMs contribute to the inflammatory response to COVID-19 by promoting the adhesion and migration of immune cells to the site of infection. Additionally, ICAM-mediated interactions between immune cells and endothelial cells may lead to vascular leakage and subsequent tissue damage. ⁷³ The most severe presentations of COVID-19-associated ARDS are characterized by predominant endothelial injury over alveolar damage. This is evidenced by elevated levels of Angiopoietin-2 (Ang-2) and ICAM-1 in non-survivors compared to survivors. While both COVID-19 ARDS and classical ARDS exhibit comparable impairments in gas exchange, they display differing biomarker expressions, suggesting distinct pathological mechanisms. In instances of COVID-19-associated ARDS, plasma concentrations of ICAM-1 upon admission were found to be significantly higher in non-survivors compared to survivors. Moreover, levels of ICAM-1 in COVID-19-related ARDS were observed to be elevated compared to classical ARDS, ⁷⁴ This indicates that ICAM-1 could potentially serve as a valuable biomarker to differentiate between classical and COVID-related ARDS.

The discovery of Vascular Cell Adhesion Molecule 1 (VCAM-1) as a component of the endothelial cell surface marked a significant advancement in understanding its pivotal role in regulating vascular adhesion associated with inflammation, as well as facilitating the trans-endothelial migration of leukocytes such as T-cells and macrophages. Its involvement in inflammatory processes suggests its potential as a therapeutic target for conditions related to immunology and cancer. ⁷⁵ Several studies have found that COVID-19 patients exhibit elevated levels of VCAM-1 compared to healthy controls, with the severe group demonstrating higher quantities than those in the mild or moderate group.^76,77 The dysregulation of VCAM-1 in individuals with COVID-19 has been linked to a dysfunctional host response and severe conditions, underscoring its importance in disease progression. Furthermore, men have been found to exhibit significantly higher circulating VCAM-1 levels than women, and analyses of COVID-19 cohorts have shown that plasma concentrations of VCAM-1 in COVID-19 patients demonstrated a significant positive correlation with D-dimer levels.^78,79

It is important to note that the migration of natural killer (NK) cells into the lungs appears to occur through the Very Late Antigen-4 (VLA-4)/Vascular Cell Adhesion Molecule 1 (VCAM-1) adhesion pathway, which plays a critical role in inflammation. The interruption of this pathway may be the primary reason for the reduction in natural killer cells in the peripheral blood of patients with severe COVID-19, ⁸⁰ ). The utilization of anti-trafficking agents such as natalizumab, which blocks VLA-4, could provide valuable insights into strategies for preventing lung inflammation in COVID-19 patients. ⁸¹

Osteopontin

Osteopontin, also known as bone sialoprotein, is a protein encoded by the SPP1 gene in humans and consistsof approximately 300 amino acids. It functions as a component of the extracellular matrix as well as a soluble cytokine. Osteopontin is involved in a wide array of biological processes, including development, immune responses, carcinogenesis, and bone resorption. It serves as a chemotactic factor that recruits neutrophils, mast cells, and macrophages. Osteopontin inhibits the synthesis of interleukin-10 (IL-10) in T-helper 2 (TH2) cells, thereby enhancing the inflammatory response of T-helper 1 (TH1) cells.^82,83 Furthermore, studies have indicated that osteopontin is a significant biomarker for various autoimmune and inflammatory conditions, including inflammatory bowel diseases, systemic lupus erythematosus, asthma, and rheumatoid arthritis. ⁸⁴

Emerging evidence suggests that osteopontin is a promising biomarker for assessing the severity of COVID-19 in pediatric populations. Preliminary investigations into plasma osteopontin levels in hospitalized children with COVID-19 have demonstrated significantly elevated levels in those exhibiting moderate to severe symptoms. This biomarker has shown substantial ability to differentiate between varying degrees of disease severity, surpassing conventional inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). ⁸⁵ Additional research has indicated that osteopontin levels correlate with disease severity and serve as an independent risk factor for more severe clinical outcomes. Elevated osteopontin levels have been associated with increased mortality risk and a higher likelihood of requiring mechanical ventilation in individuals diagnosed with COVID-19. ⁸⁶

One potential mechanism linking osteopontin to adverse COVID-19 outcomes involves an axis mediated by high glucose and angiotensin II, which activates osteopontin and subsequently triggers the p38 mitogen-activated protein kinase (MAPK) pathway. This cascade ultimately activates the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and furin. Notably, the susceptibility of the SARS-CoV-2 spike protein to proteolytic cleavage by furin may significantly enhance the virus’s ability to infect host cells and disseminate to secondary organs, thereby exacerbating disease severity and potentially leading to fatal outcomes. ⁸⁷ A study has established a direct relationship between serum osteopontin levels and various serum inflammatory markers, including D-dimer, CRP, white blood cell (WBC) count, and procalcitonin. These findings suggest that osteopontin may serve as a valuable routine laboratory parameter for the management and monitoring of patients with COVID-19. ⁸⁸

Matrix Metalloproteinases (MMP)

Matrix metalloproteinases (MMPs) may be considered potential endothelial biomarkers. MMPs are critical contributors to numerous physiological and pathological processes, particularly those related to endothelial function. They are also involved in the remodeling of the extracellular matrix (ECM). Endothelial cells and vascular smooth muscle cells (VSMCs) produce matrix metalloproteinases, including MMP-1, MMP-2, MMP-3, MMP-7, and MMP-9, which indicates their significant role in endothelial biology and functionality. ⁸⁹ Elevated MMP levels have been associated with pathological conditions such as atherosclerosis, where they contribute to plaque destabilization by degrading ECM components. This degradation may result in vascular inflammation and other complications. ⁹⁰

MMP-1 levels are markedly elevated in the peripheral blood of patients hospitalized with COVID-19, particularly among those classified as severe or critical. This metalloproteinase has been identified as a potential peripheral blood biomarker for idiopathic pulmonary fibrosis. The overexpression of MMP-1 leads to hyperactivation of MMP-1/PAR1 signaling, resulting in increased expression of VEGF receptor 2, compromised endothelial cell functionality, and inappropriate recruitment and activation of inflammatory cells. ⁹¹ Research has indicated that MMP-2 levels differ between COVID-19 patients and healthy controls; specifically, individuals with COVID-19 exhibit lower levels of MMP-2 compared to the control group. Notably, hypertension appears to affect MMP-2 levels in COVID-19 patients, as hypertensive individuals demonstrated elevated MMP-2 levels relative to their non-hypertensive counterparts, although these levels remained lower than those found in the control group. ⁹² Furthermore, both MMP-2 and MMP-8 have been positively correlated with immune responses associated with the release of immunosuppressive mediators in severe COVID-19 lung pathology. This correlation suggests that MMP-2, in conjunction with inflammatory responses and oxidative stress, may contribute to severe lung injury. Additionally, lower MMP-2 levels were associated with an increased risk of in-hospital mortality, underscoring its potential as a prognostic marker. ⁹³

Matrix metalloproteinases (MMPs) play a critical role in the pathogenesis of COVID-19, particularly concerning lung injury and inflammation. MMPs contribute to the “cytokine storm,” a hyper-inflammatory state that can lead to severe outcomes. Pro-inflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α), activate MMP-9, resulting in the degradation of the extracellular matrix. This activation has the potential to exacerbate lung injury and perpetuate inflammation. MMPs, particularly MMP-3 and MMP-9, have been identified as potential biomarkers for assessing the severity of COVID-19. Serum levels of MMP-3 are elevated in hospitalized COVID-19 patients, with a significant increase correlating with disease progression as classified by the World Health Organization (WHO). MMP-9 levels were also significantly elevated in patients; however, they did not correlate directly with disease severity at initial presentation. Nevertheless, these levels increased over time, suggesting a role in subsequent inflammatory and repair processes. ⁹⁴

Furthermore, a study identified MMP-7 as the most reliable biomarker for assessing the need for invasive mechanical ventilation (IMV) in individuals with COVID-19. The average concentration of soluble MMP-7 was found to be significantly higher only in patients requiring IMV, indicating a link between MMP-7 levels and disease severity. MMP-7 has been associated with inflammatory lung damage in ARDS and is elevated in the respiratory microenvironment, as well as in the serum of patients with various interstitial lung diseases that may progress to fibrosis, including idiopathic pulmonary fibrosis. Notably, MMP-7 levels remained elevated in IMV survivors for several months and were associated with fibrosis, similar to findings in a recent study where early fibrotic changes were correlated with higher levels of soluble MMP-7. ⁹⁵

In a meta-analysis, it was reported that patients with severe COVID-19 exhibited significantly elevated blood levels of MMP-9 compared to those with non-severe cases. Additionally, patients with ARDS demonstrated substantially higher MMP-9 levels compared to those without ARDS. No significant difference in MMP-9 levels was observed between non-survivors and survivors. Additionally, patients with neurological disorders and obese diabetics exhibited significantly higher levels of MMP-9 compared to individuals without comorbidities. ⁹⁶

MR-proADM

Mid-regional proadrenomedullin (MR-proADM) is a biomarker derived from the precursor of adrenomedullin (ADM), a peptide that plays a crucial role in various biological processes, including vascular function. MR-proADM is involved in the regulation of vascular permeability and the maintenance of endothelial integrity. As a vasodilator, it facilitates the modulation of blood flow and the organism’s response to stressors such as infection and inflammation. The significance of MR-proADM as a reliable biomarker for assessing endothelial dysfunction and systemic inflammation is considered more stable than that of full-length ADM, which is rapidly degraded in the bloodstream. ⁹⁷

Numerous studies suggest that MR-proADM may be instrumental in monitoring sepsis and ARDS in the context of COVID-19. These investigations have established that MR-proADM serves as a valuable prognostic biomarker in these clinical scenarios. A critical finding in this body of research is the predictive capability^{98 -100} of MR-proADM; elevated levels of this biomarker are associated with adverse outcomes in COVID-19 patients, including an increased likelihood of requiring mechanical ventilation and a higher mortality rate. A cutoff level of approximately 0.87 nmol/L has been determined to be significant for predicting these negative outcomes. ¹⁰¹ Moreover, research indicates that MR-proADM levels correlate with the severity of COVID-19 pneumonia, with elevated concentrations linked to severe disease and organ dysfunction. This suggests that MR-proADM may reflect the extent of endothelial damage induced by the virus. ¹⁰² In comparative analyses with other biomarkers, one study reported that the area under the curve (AUC) for MR-proADM was significantly higher when evaluating mortality risk. This finding suggests that MR-proADM is a more reliable predictor of mortality in COVID-19 patients compared to other biomarkers. ¹⁰³ In summary, MR-proADM emerges as a valuable biomarker for the early identification of patients at risk of severe COVID-19 outcomes, thereby facilitating improved resource allocation and treatment strategies in emergency settings. Its integration into clinical protocols may enhance patient triage and care during pandemics or outbreaks of severe respiratory illnesses.

MR-proADM has been identified as a significant prognostic biomarker in patients with COVID-19, particularly in those who are critically ill, and consistently shown that elevated levels of MR-proADM are associated with higher mortality rates in COVID-19 patients.^{104 -106} This is attributed to its role as a marker for endothelial dysfunction, which is a key factor in the severity of COVID-19. A meta-analysis of 14 studies demonstrated that patients who did not survive COVID-19 had significantly higher MR-proADM concentrations than survivors (1.692 ± 0.761 nmol/L vs 0.841 ± 0.295 nmol/L). ¹⁰⁴ Consistent results were reported in an ICU-based study, in which non-survivors exhibited markedly elevated MR-proADM levels compared with survivors (2.65 ± 2.33 nmol/L vs 1.18 ± 0.47 nmol/L). This study suggested a cutoff of 1.8 nmol/L was associated with a significantly increased risk of death in ICU patients, and a value of >1.5 nmol/L was identified as an independent risk factor for mortality at day 28 in a prospective study of critically ill patients, which had the best accuracy in predicting 28-day mortality in critically ill patients when compared to procalcitonin (PCT), lactate dehydrogenase, and D-dimer. ¹⁰⁵ Another study reported a cutoff of 1.73 nmol/L, which predicted mortality with 90% sensitivity and 95% specificity. ¹⁰⁷

The measurement of MR-proADM upon hospital or ICU admission can aid in the early risk stratification of COVID-19 patients^106,108 This can help clinicians in triaging patients, deciding on therapy escalation, and anticipating potential complications. Monitoring the trend of MR-proADM levels over time can also provide additional prognostic information during the course of the hospital stay. ¹⁰ However, it is important to note that larger, prospective studies are still needed to fully confirm these findings and to better understand the mechanisms behind the increase of MR-proADM in COVID-19 patients.^104,109

Neopterin (NPT)

Neopterin is synthesized by macrophages in response to induction by interferon-gamma (IFN-γ) and serves as an indicator of macrophage activation, cellular immune activity, and T helper type 1 (TH1) immune responses related to antiviral cellular immunity. Elevated neopterin synthesis is associated with increased formation of reactive oxygen species, thereby facilitating the assessment of oxidative stress induced by the immune system. ¹¹⁰ Moreover, neopterin (NPT) enhances endothelial function and reduces vascular damage by activating nitric oxide synthase. The increased release of NPT triggers negative feedback regulation of macrophage activity, leading to the polarization of macrophages from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype. ¹¹¹ Viral infections can stimulate the secretion of IFN-γ from infected cells, which drives macrophages to produce and release NPT. Consequently, NPT concentrations reflect underlying cell-mediated immunological activation. Elevated serum levels of NPT have been associated with various viral diseases, including rubella, cytomegalovirus, hepatitis B and C, and dengue fever. ¹¹²

In several investigations, a strong correlation has been observed between neopterin levels and the severity of COVID-19. For instance, elevated serum neopterin (NPT) levels signify increased T cell activation associated with acute lung injury (ALI) and ARDS in cases of SARS-CoV-2 pneumonia. Serum NPT levels exceeding 45 nmol/L upon admission are predictive of both disease severity and the necessity for mechanical ventilation in the intensive care unit (ICU). ¹¹³ Hospitalized individuals with severe COVID-19 exhibit significantly higher serum neopterin levels compared to those with moderate COVID-19. ¹¹⁴ A recent study found that critically ill patients had twice the average neopterin levels when compared to those with moderate symptoms. While neopterin levels eventually decreased in both groups, they remained elevated for a more extended period in severe cases. ¹¹⁵ Neopterin is a valuable biomarker in COVID-19, demonstrating immunological activity and correlating with disease severity. Elevated levels can assist in distinguishing between mild and severe cases, provide prognostic information, and guide treatment strategies. Further research may enhance its therapeutic value in evaluating disease progression and managing immune responses in COVID-19 and other inflammatory disorders.

Pentraxin 3 (PTX3)

Pentraxin 3 (PTX3) is an acute-phase protein belonging to the pentraxin family and plays a crucial role in the immune response and inflammation. PTX3 is rapidly synthesized by fibroblasts and endothelial cells in response to inflammatory stimuli, such as Toll-like receptor (TLR) activation and cytokines, including tumor necrosis factor-alpha (TNFα) and interleukin-1 beta (IL-1β). Under normal physiological conditions, PTX3 levels in the bloodstream are low; however, they can increase significantly to 200-800 ng/mL within 6 to 8 hours following inflammatory stimulation related to conditions such as sepsis, cardiovascular complications, and COVID-19. This rapid elevation positions PTX3 as a potential prognostic indicator for inflammation. ¹¹⁶ PTX3 functions as a pattern-recognition receptor that binds to apoptotic cells and pathogens, facilitating their clearance by immune cells. It enhances the recognition of pathogens by macrophages and dendritic cells through the activation of the classical complement pathway. PTX3 modulates the immune response with both pro-inflammatory and anti-inflammatory properties, prompting researchers to characterize its role in inflammation as a “yin and yang” effect due to this dual nature. ¹¹⁷

Significantly elevated levels of PTX3 have been observed in individuals with COVID-19 compared to healthy controls, particularly in those with severe disease. ¹¹⁸ One study indicates that PTX3 outperforms conventional inflammatory biomarkers in predicting patient outcomes. Patients with elevated PTX3 levels exhibited a markedly higher risk of intubation and mortality relative to those with lower levels. Elevated blood concentrations of PTX3 have been identified as strong, independent predictors of 28-day mortality following hospitalization. ¹¹⁹ In conclusion, PTX3 is significant in the immunoinflammatory aspect of COVID-19, contributing to inflammation, influencing vascular integrity, and serving as a biomarker for disease severity and prognosis. Its dual role in pathogen protection and harmful inflammation underscores the complexities of immune responses during SARS-CoV-2 infection. Further investigation into PTX3 may yield novel pharmaceutical targets and treatment strategies for COVID-19.

Prostacyclin (PGI2)

Prostacyclin, a member of the lipid compound family known as eicosanoids, is synthesized in endothelial cells by prostacyclin synthase from prostaglandin H2. This molecule serves 2 primary functions: it inhibits platelet activation and induces vasodilation. Additionally, prostacyclin plays a role in suppressing the proliferation of fibroblasts and immune cells, thereby exhibiting anti-inflammatory effects in various organs, including the kidneys, liver, lungs, and heart. ¹²⁰

The inflammatory response to SARS-CoV-2 infection involves prostacyclin. Elevated levels of PGI2 have been reported in COVID-19 patients, particularly among those with more severe conditions. ¹²¹ The vasodilatory actions of prostacyclin may mitigate some of the endothelial dysfunction and vascular constriction observed in severe cases of COVID-19. Furthermore, by inhibiting platelet aggregation, prostacyclin may reduce the risk of thrombosis. Due to its anti-inflammatory and immune-regulatory properties, PGI2 may also modulate the immunological response to COVID-19. ¹²² It can attenuate the activity of NF-κB, which is associated with the inflammatory response. ¹²³

Notably, as a potent vasodilator and platelet aggregation inhibitor, prostacyclin has emerged as a promising therapeutic option for COVID-19, particularly in individuals experiencing ARDS. Its ability to enhance pulmonary hemodynamics and regulate endothelial function is primarily responsible for its therapeutic efficacy. Research indicates that inhaled prostacyclin may improve oxygenation in patients with COVID-19-induced ARDS. For instance, a randomized study demonstrated that inhaled prostacyclin enhanced the oxygenation index by 34 mmHg (P = .04) by Day 5 compared to saline. A systematic review also indicates that inhaled prostacyclins may improve the PaO2/FiO2 ratio, suggesting enhanced gas exchange in the lungs.^121,124 For COVID-19 patients suffering from severe hypoxia due to ARDS, prostacyclin represents a promising adjunctive therapeutic strategy. However, further extensive research is required to fully establish its effectiveness and safety in this context.

Receptor for Advanced Glycation Endproducts (RAGE)

The receptor for advanced glycation endproducts (RAGE) is a transmembrane receptor that is a member of the immunoglobulin superfamily and is encoded by a gene located within the major histocompatibility complex (MHC) class III region on chromosome 6 . ¹²⁵ RAGE consists of 2 primary domains: the transmembrane domain, which anchors the receptor to the cell membrane, and the cytosolic domain, which regulates signal transduction processes. RAGE is capable of recognizing several ligands, including advanced glycation endproducts (AGEs), high-mobility group box 1 (HMGB1), S-100 calcium-binding proteins, amyloid-β protein, Mac-1, and phosphatidylserine. ¹²⁶ RAGE plays a critical role in various cellular processes, such as inflammation, development, apoptosis, and autophagy. ¹²⁷ Activation of the RAGE signaling pathway, which involves phosphatidylinositol-3 kinase (PI-3K), Ki-Ras, and the mitogen-activated protein kinases (MAPKs) Erk1 and Erk2, ultimately leads to the induction of nuclear factor-kB (NF-kB), thereby initiating a cascade of inflammatory reactions. ¹²⁸ RAGE binds to the N-protein of SARS-CoV-2, which triggers lung injury through the RAGE-ERK1/2 -NF-κB pathway. The upregulation of RAGE in bronchoalveolar lavage (BAL) fluid and lung tissue following exposure to the N-protein indicates its involvement in acute lung injury. Furthermore, RAGE deficiency or antagonist treatment provides partial protection to mice against N-protein-induced injury, suggesting that RAGE may serve as a potential therapeutic target for COVID-19. ¹²⁹

The interaction between the RAGE–AGE and inflammasome pathways appears to play a significant role in the development of severe pneumonitis and the extent of lung damage in various pathological conditions, including severe pneumonitis associated with COVID-19. ¹³⁰ Disruption of RAGE and its ligands is associated with the onset of various human diseases, including chronic inflammatory disorders. RAGE is predominantly expressed by alveolar epithelial cells and macrophages, which are central to the lung inflammation caused by COVID-19. ¹³¹ Elevated levels of RAGE have been linked to several conditions, such as diabetes, obesity, and cardiovascular disease, all of which are known to exacerbate the morbidity and mortality associated with COVID-19. ¹³²

The role of RAGE in mediating acute lung injury induced by the N-protein emphasizes its significance as a potential target for therapeutic interventions. Blocking RAGE may mitigate the cytokine storm and ARDS associated with COVID-19. ¹²⁹

Serum Amyloid A (SAA)

Serum amyloid A (SAA) proteins are apolipoproteins associated with high-density lipoprotein (HDL) in plasma. They are implicated in cholesterol transport to the liver, recruitment of immune cells to sites of inflammation, and activation of enzymes that degrade the extracellular matrix. SAA proteins can be produced continuously or in response to inflammatory stimuli. Primarily synthesized by the liver, SAA levels rise significantly during acute inflammatory episodes. SAA exhibits strong immunomodulatory effects, promoting cytokine production and directing the migration of neutrophils and mast cells through chemotaxis. It activates various cell-surface receptors, including Toll-like receptors (TLR 2 and 4) and formyl peptide receptors, thereby supporting the innate immune response. ¹³³ Elevated levels of SAA are correlated with infections and various chronic inflammatory disorders, such as rheumatoid arthritis and inflammatory bowel disease (IBD). In these conditions, SAA serves as a biomarker for disease progression and severity. In addition to promoting pro-inflammatory responses, SAA facilitates the clearance of harmful lipids and cellular debris during inflammation, thereby aiding tissue healing. In the context of COVID-19, SAA levels are consistently elevated, with higher concentrations associated with more severe cases. ¹³⁴

Research indicates that critically ill patients exhibit significantly higher SAA concentrations compared to those with mild illness. This elevation reflects the acute-phase response, as SAA levels may surge up to 1000-fold shortly after infection or inflammatory onset. Increased SAA levels may exacerbate clinical conditions by promoting coagulopathy, impairing pulmonary and tissue gas exchange, and contributing to atherogenesis. ¹³⁵ SAA also promotes platelet aggregation by interacting with integrin receptors on platelets, potentially leading to complications such as deep vein thrombosis and pulmonary embolism. ¹³⁶ Notably, non-survivors exhibit higher SAA levels than survivors, suggesting its potential utility in prognostic assessment. ¹³⁴ SAA functions as both a marker and an inflammatory mediator, a role that has gained increasing relevance during the COVID-19 pandemic. Its association with disease severity and heightened thrombotic risk underscores its clinical significance, highlighting the necessity for further investigation into its potential as a biomarker and therapeutic target.

Soluble fms-Like Tyrosine Kinase 1 (sFlt-1)

The primary function of sFlt-1, a soluble variant of vascular endothelial growth factor receptor 1 (VEGF-R1), is to inhibit angiogenesis. sFlt-1 exerts its inhibitory effect by binding to vascular endothelial growth factor (VEGF) and placental growth factor (PlGF), thereby obstructing their interaction with cell surface receptors. This mechanism is essential for the formation of blood vessels in organs such as the kidneys, cornea, and uterus. Lacking a transmembrane domain allows sFlt-1 to circulate freely in the bloodstream. By sequestering VEGF and PlGF, sFlt-1 diminishes their bioavailability, leading to reduced endothelial cell migration and proliferation. While this mechanism is crucial for maintaining vascular homeostasis, elevated levels of sFlt-1 can contribute to pathological conditions. ¹³⁷ sFlt-1 plays a particularly significant role during pregnancy, as it is produced by trophoblastic cells in the placenta. Increased levels of sFlt-1 have been associated with preeclampsia, a condition characterized by hypertension and proteinuria. The sFlt-1/PlGF ratio serves as a biomarker for preeclampsia, with higher ratios indicating an increased risk of the condition. ¹³⁸

Recent studies have identified a significant association between COVID-19 infection during pregnancy and the development of preeclampsia. The infection appears to exacerbate the elevation of sFlt-1 levels, potentially contributing to the increased risk of this condition. ¹³⁹ As angiotensin II (ANG-II) binds to its receptor (AT1) in response to hypoxia, the synthesis of sFlt-1 is induced. It is well established that sFlt-1 leads to endothelial dysfunction, rendering the endothelial cells throughout the placenta and systemic circulation more sensitive to the effects of ANG-II. ¹⁴⁰ Serum levels of sFlt-1 are elevated even in non-pregnant individuals with severe COVID-19. This increase can inhibit VEGF activity, potentially impairing lung tissue repair and regeneration, which may contribute to respiratory complications in severe COVID-19 cases. ¹⁴¹ Although investigations indicate that sFlt-1 levels are elevated during COVID-19, the concurrent rise in both COVID-19 cases and preeclampsia undermines its utility as a reliable predictor of disease severity.

Soluble Urokinase Plasminogen Activator Receptor (SuPAR)

In the context of kidney injury, the soluble urokinase plasminogen activator receptor (suPAR) functions as an immunological mediator. The urokinase receptor system is essential for the regulation of coagulation, inflammation, and immune responses. SuPAR is generated when membrane-bound uPAR is cleaved in response to cardiovascular risk factors, such as diabetes and smoking, as well as inflammatory stimuli, including viral infections. Elevated levels of suPAR are frequently associated with an increased risk of acute kidney injury (AKI) in various clinical settings. This correlation may be attributed to the impact of suPAR on mitochondrial respiration and its promotion of reactive oxygen species production in proximal tubular cells, thereby rendering them more susceptible to subsequent damage. ¹⁴² Currently, 3 forms of suPAR are present in circulation: suPAR DI-III, suPAR DI, and suPAR DII-III. Among these, suPAR DI-III is regarded as the functional form of suPAR, as it possesses the ability to bind to uPA, which plays a critical role in chemotaxis. ¹⁴³

Given that nearly 50% of hospitalized COVID-19 patients develop acute kidney injury (AKI), with 20% requiring dialysis, soluble urokinase plasminogen activator receptor (suPAR) appears to be a contributing factor to kidney damage in COVID-19. ¹⁴² A hypothesis suggests that endothelial activation occurs early in the disease, during which the urokinase plasminogen activator receptor (uPAR) bound to the endothelium is cleaved. This process results in elevated levels of its soluble form, suPAR, which is associated with severe respiratory failure. ¹⁴⁴ Enhanced release of suPAR into circulation may compete with urokinase-type plasminogen activator (uPA) on cell surfaces, thereby inhibiting plasmin production. This inhibition could lead to hypofibrinolysis, a significant characteristic of the hypercoagulable state observed in severe COVID-19 patients, consequently exacerbating thrombosis severity through a synergistic mechanism. ¹⁴⁵

Despite extensive research on the potential of this biomarker as a predictor of COVID-19 severity and progression, its role has not been thoroughly addressed. A meta-analysis indicates that suPAR levels are elevated in COVID-19 patients compared to healthy individuals, with significantly higher levels observed in critically ill patients. A meta-analysis further confirmed that suPAR levels are elevated in severe COVID-19 illness and are useful in predicting mortality, with higher levels observed in non-survivors. ¹⁴⁶ Additionally, in animal models, the administration of an anti-suPAR monoclonal antibody reverses the detrimental effects of suPAR on the renal system, suggesting that suPAR may serve as a therapeutic target for COVID-19-associated AKI. ¹⁴⁷ SuPAR has the potential to predict patients who are predisposed to adverse outcomes such as organ failure and mortality.

Recent evidence strengthens the prognostic value of suPAR. A multicenter prospective trial ranked suPAR as the most significant predictor of death or mechanical ventilation, showing greater discriminatory accuracy (AUC 0.712) than CRP, ferritin, IL-6, and D-dimer (Vasbinder, 2024). This study also defined a suPAR level below 4.0 ng/mL as a sensitive marker for identifying low-risk patients. These findings align with earlier research highlighting suPAR and IL-6 as superior to many routine biomarkers in forecasting poor outcomes. ¹⁴⁸ One study also reported suPAR to be a potential biomarker to assess the severity of COVID-19, with a better correlation to increasing severity than CRP.

By stratifying patients based on suPAR levels, healthcare providers may more effectively identify individuals who could benefit from intensive monitoring and early intervention, thereby positioning suPAR as a valuable tool for risk assessment in COVID-19 and other inflammatory diseases.

Surfactant-D

Surfactants, which are composed of approximately 90% lipids and 10% proteins, serve to effectively reduce surface tension, thereby facilitating the exchange of oxygen and carbon dioxide during respiration. Surfactant protein D (SP-D), a member of the collectin family, functions as a pattern-recognition molecule and plays a crucial role in innate immunity by acting as a pattern-recognition receptor. ¹⁴⁹ SP-D has been identified as an effective biomarker for acute lung injuries and ARDS, as well as a protective factor in various cases of acute lung injury. The association between serum SP-D levels and mortality in respiratory disorders, particularly COPD, underscores the importance of surfactant protein D in prognostic assessments. Increased levels of SP-D in the bloodstream have been correlated with lung dysfunction following acute lung injury, such as that caused by COVID-19 infection. Notably, the elevation of SP-D levels commenced early in the course of pneumonia. As the condition progressed from non-severe to severe, serum SP-D levels exhibited a significant increase. Furthermore, SP-D demonstrates a higher efficacy against coronaviruses; for instance, SP-D binds more avidly to the SARS-CoV-2 spike protein compared to other surfactants. Elevated SP-D levels were noted in cases of mild-to-moderate pneumonia, in contrast to lower levels observed in severe or critical COVID-19 infections. Additionally, both SP-D and interleukin-6 (IL-6) levels were significantly elevated in COVID-19 patients, although they exhibited a decline over time. ¹⁵⁰ Follow-up studies of COVID-19 patients showed a positive correlation between SP-D levels and markers such as IL-6, ferritin, CRP, troponin-I, and D-dimer, while exhibiting an inverse relationship with arterial oxygen partial pressure to fractional inspired oxygen (PaO₂/FiO₂) levels. ¹⁵¹ The observed increase in pulmonary surfactant levels may represent a compensatory response by the human body in the early stages of pneumonia to address surfactant deficiency. However, as pneumonia progresses and alveolar type II cells sustain damage, pulmonary surfactant levels decline, resulting in reduced lung function. ¹⁵⁰ Monitoring fluctuations in surfactant levels may serve as a valuable marker for the progression of COVID-19 pneumonia.

SP-D is a key component of the innate immune system and has emerged as a significant prognostic biomarker in COVID-19, Elevated levels of circulating SP-D are believed to reflect damage to the alveolar-capillary barrier, which is a hallmark of severe lung injury in COVID-19 patients. Multiple studies have confirmed a strong correlation between high SP-D levels and increased COVID-19 severity and mortality.^152,153 A 2024 meta-analysis encompassing over 5400 patients found that elevated SP-D levels were significantly associated with more severe disease outcomes. ¹⁵² Specifically, higher plasma concentrations of SP-D have been observed in patients who develop ARDS and pulmonary fibrosis compared to those with milder forms of the disease. ¹⁵³ According to one study, imatinib therapy was more beneficial for a subphenotype of patients with high SP-D levels, which are symptomatic of alveolar damage and systemic inflammation. This indicates that SP-D could be valuable for selecting patient groups for targeted treatments. ¹⁵⁴ Compared to other inflammatory markers, SP-D has shown excellent effectiveness for assessing the possibility of ARDS and lung fibrosis in COVID-19 patients. ¹⁵³ Its capacity to detect alveolar epithelial cell injury makes it a useful biomarker for determining the prognosis of COVID-19 patients. ¹⁵⁵

Syndecan-1 (SDC-1)

Class I transmembrane heparan sulfate proteoglycan Syndecan-1 (SDC-1) is indispensable for a variety of biological functions, including cell adherence, migration, endocytosis, and signaling. It is predominantly expressed in epithelial cells and facilitates connections between cells and the extracellular matrix (ECM). Structurally, SDC-1 comprises a cytoplasmic domain that interacts with intracellular signaling molecules, a transmembrane domain that anchors it to the cell membrane, and an extracellular region that binds cytokines and growth factors through heparan sulfate chains.^156,157 SDC-1 is primarily involved in 3 functions: facilitating cell adhesion and migration, regulating growth factors and angiogenesis by binding to fibroblast growth factors and vascular endothelial growth factors, and mediating inflammation and immune responses through the recruitment of leukocytes and modulation of cytokine activity. Furthermore, SDC-1 is critical for barrier function, maintaining the integrity of cellular barriers and protecting underlying tissues. ¹⁵⁸

Numerous research efforts have underscored the significance of SDC-1 in the context of COVID-19. The spike protein of the virus interacts with the heparan sulfate domains on SDC-1, thereby facilitating its attachment to host cells. This interaction suggests that SDC-1 may influence the early stages of infection by promoting viral uptake. ¹⁵⁹

A study revealed that SDC-1 levels were significantly elevated in COVID-19 patients compared to healthy individuals. Moreover, serum SDC-1 levels were notably higher in intensive care unit (ICU) patients compared to non-ICU patients. The levels of SDC-1 exhibited a dynamic pattern, increasing during the early phase of the disease and gradually decreasing over time. Specifically, from day 0 (the day of hospitalization) to day 5, SDC-1 levels rose; however, from day 5 to day 10, SDC-1 levels progressively declined in both ICU and non-ICU patients. ¹⁶⁰ A retrospective study of a Japanese cohort found a positive correlation between SDC-1 levels and the severity of COVID-19, indicating that patients who succumbed to the disease exhibited significantly higher SDC-1 levels compared to survivors. Area under the receiver operating characteristic curve (AUC) analysis suggested that SDC-1 could serve as a potential predictor of mortality, with an AUC value of 0.714. Additionally, Kaplan-Meier (K-M) analysis demonstrated a significant difference in survival based on an SDC-1 cutoff value of 10.65 ng/mL. ¹⁶¹ Although the precise mechanism by which SARS-CoV-2 induces the shedding of SDC-1 from the glycocalyx remains unclear, elevated levels of granzyme B and elastase 2 have been observed in severe inflammatory conditions and are known to contribute to endothelial damage. These serine proteases may also play a role in the degradation of the glycocalyx and the subsequent upregulation of SDC-1. ¹⁶² The elevation of SDC-1 correlates with endothelial cell dysfunction and could serve as a biomarker for disease severity and mortality. SDC-1 is a multifunctional proteoglycan that is essential for cell signaling and adhesion, and emerging data suggest that it plays a significant role in the pathogenesis of COVID-19 through mechanisms related to viral entry and inflammatory pathways.

Tissue Plasminogen Activator (tPA)

Tissue plasminogen activator (tPA) is an enzyme naturally produced primarily by endothelial cells. tPA plays a critical role in maintaining the balance between coagulation and fibrinolysis. Disruptions in tPA levels may result in various clotting and bleeding disorders. ¹⁶³ In patients with COVID-19, thrombotic and thromboembolic events are prevalent, with documented incidence rates ranging from 21% to 49% in cases categorized as moderate to severe; significantly higher rates have been observed in patients who do not recover from the disease.^164,165 Research indicates that individuals with severe COVID-19 exhibit markedly elevated plasma levels of tPA when compared to healthy controls. For example, a meta-analysis found an average tPA concentration of approximately 26.67 ng/mL in COVID-19 patients, in contrast to 4.68 ng/mL in non-infected individuals. ¹⁶⁶ This increase is associated with poorer clinical outcomes, including elevated mortality rates and a heightened need for admission to intensive care units (ICU). ¹⁶⁷ Elevated tPA levels in COVID-19 reflect a complex interaction between fibrinolysis and coagulation processes. While higher tPA levels may suggest enhanced fibrinolytic activity, some studies have revealed a paradoxical condition known as hypofibrinolysis, wherein the body inadequately breaks down clots despite increased tPA levels. This impaired fibrinolysis may contribute to the thromboembolic complications reported in COVID-19 patients, potentially resulting in severe outcomes such as pulmonary embolism or stroke. ¹⁶⁸

Given tPA’s dual role as an initiator of fibrinolysis and a potential contributor to bleeding complications, its therapeutic significance in the context of COVID-19 remains under investigation. Some studies suggest that administering tPA may benefit certain patient populations, particularly if given early in the disease course or tailored to individuals exhibiting distinct coagulation profiles. However, further research is necessary to delineate the optimal timing, dosage, and patient selection criteria for tPA treatment in this context. Achieving a balance between minimizing thrombosis and preventing severe bleeding is essential for the effective management of COVID-19 patients. ¹⁶⁹

Von Willebrand Factor (vWF)

Von Willebrand factor (vWF) is a large multimeric glycoprotein that plays a crucial role in normal hemostasis by facilitating platelet adhesion, serving as a carrier for Factor VIII, and triggering inflammation and endothelial cell activation. ¹⁷⁰ Patients with COVID-19 exhibit hypercoagulability attributed to elevated FVIII-vWF binding and increased platelet adherence to endothelial cells resulting from vWF secretion triggered by the SARS-CoV-2 spike protein. ¹⁷¹

In COVID-19, both von Willebrand factor (vWF) antigen and vWF activity are typically elevated, particularly in moderate to severe disease. ¹⁷² In individuals diagnosed with COVID-19, circulating vWF levels and activity are generally elevated, suggesting a correlation between higher vWF levels and disease severity. Moreover, vWF levels in intensive care unit (ICU) patients are significantly higher compared to non-ICU individuals. ¹⁷³ While vWF levels increase during COVID-19 infection, vWF activity is reduced in ICU-admitted patients. ¹⁷³

ADAMTS13 is a metalloprotease enzyme that plays a critical role in regulating blood clotting and preventing excessive platelet aggregation. The ratio of von Willebrand factor (vWF) to ADAMTS13 serves as an important marker of hemostatic balance, which becomes dysregulated in COVID-19. ¹⁷⁴ Reduced ADAMTS13 activity has been associated with an increased risk of thrombotic events. A higher vWF/ADAMTS13 ratio correlates strongly with adverse outcomes and acute kidney injury in the context of COVID-19.^175,176 The disparity between ADAMTS13 and vWF in COVID-19 is comparable to that observed in thrombotic thrombocytopenic purpura (TTP), albeit with key distinctions. Patients with COVID-19 frequently experience low to moderate declines in ADAMTS13 activity, in contrast to TTP, which presents with a significant reduction. This decrease may often be insufficient to result in the development of excessive ultra-large vWF multimers, which are characteristic of TTP. ¹⁷²

The vWF/ADAMTS13 axis shows potential as a prognostic marker in COVID-19, aiding in the prediction of disease severity and mortality risk, identifying individuals at high risk for thrombotic complications, and guiding decisions regarding anticoagulant therapy.^172,177 In summary, von Willebrand factor (vWF) plays a significant role in the coagulopathy associated with COVID-19. In severe cases, elevated vWF levels and an imbalance in the vWF/ADAMTS13 axis are primary contributors to the hypercoagulable state and increased risk of thrombosis. Understanding these fundamental processes could facilitate the development of more effective diagnostic and therapeutic approaches for COVID-19-related thrombotic complications. All biomarkers are summarized in Table 1. (A table of characteristics of the studies included in this review is provided in the Supplemental File).

Table 1.

Summary of Neglected biomarkers in COVID-19.

Biomarker	Alteration in COVID-19	The main function
Adiponectin	Decreased	Association with Mortality and Respiratory Failure, severity prediction 12
Angiopoietin	Increased	Regulates vascular development, angiogenesis, vascular permeability, and inflammation 15
Apolipoprotein M (ApoM)	Decreased	Mediate Lipid transport and metabolism and inflammation 24
Endocan	Increased	Regulates cell adhesion and inflammatory processes and Reflects endothelial dysfunction and inflammation 33
Endothelin	Increased	Vasoconstriction and vascular homeostasis. Promotes vasoconstriction and inflammation 37
Endoglin	Increased	Involved in Angiogenesis and vascular remodeling 42
E-selectin and P-selectin	Increased	Facilitate leukocyte adhesion and inflammation 46
Fractalkine (CX3CL1)	Increased	Facilitate Leukocyte adhesion, chemotaxis, and, inflammation 55
Galectin	Increased	Tissue integrity, and cancer angiogenesis, contribute to acute respiratory inflammation interaction with SARS-CoV-2 Spike protein. 60
HMGB1	Increased	Activate inflammatory response, activate autophagy, and contribute to cytokine storm67,68
ICAM-1 and VCAM-1	Increased	Facilitate Cell adhesion, leukocyte transmigration, and vascular inflammation 73
Osteopontin	Increased	Regulates immune responses and tissue repair 82
Matrix metalloproteinases (MMP)	Increased	Extracellular matrix degradation and tissue remodeling 89
MR-proADM	Increased	Reflects cardiovascular stress and endothelial dysfunction Vasodilation and cardiovascular homeostasis. 97
Neopterin (NPT)	Increased	Macrophage activation, Activating nitric oxide. 110
Pentraxin	Increased	Acute phase protein is elevated in sepsis, viral infection, and, cardiovascular diseases. 116
Prostacyclin (PGI2)	Decreased	Vasodilation and inhibition of platelet aggregation. 120
Receptor for Advanced Glycation Endproducts (RAGE)	Increased	Promote Inflammation and oxidative stress. 125
Serum Amyloid A	Increased	Transportation of lipids, response to inflammation, strong immunomodulatory effect, promoting cytokine production, and activation of TLRs. 133
Soluble fms-like tyrosine kinase 1 (sFlt-1)	Increased	Contributes to endothelial dysfunction and coagulation. Regulation of angiogenesis 137
Soluble urokinase plasminogen activator receptor (SuPAR)	Increased	Reflects immune activation and disease severity 142
Surfactant-D	Increased	Plays a role as PRR, immunomodulatory effects in the lungs. 149
Syndecan-1 (SDC-1)	Increased	Indicates endothelial glycocalyx damage. Cell adhesion and signaling 156
Tissue plasminogen activator (tPA)	Increased	Promotes fibrinolysis and may contribute to coagulopathy and vascular remodeling 163
Von Willebrand Factor (vWF)	Increased	Enhances platelet adhesion and coagulation 170

Discussion

Endothelial and vascular biomarkers hold significant importance for the study of COVID-19 pathophysiology, particularly regarding the virus’s impact on the vascular system and associated complications such as acute respiratory distress syndrome, thrombosis, and multiorgan failure. Despite this relevance, traditional biomarkers have predominantly shaped clinical evaluations, leading to the underutilization of other crucial endothelial and vascular indicators. The neglect of these biomarkers may hinder a comprehensive understanding of the vascular dimensions of disease progression and impede healthcare providers’ access to pertinent information. Incorporating these additional biomarkers could enhance our understanding of endothelial health and facilitate the identification of individuals at heightened risk for severe outcomes. These biomarkers provide essential insights into endothelial activation, glycocalyx damage, and immune-endothelial interactions, which are pivotal factors in severe COVID-19 cases. The clinical application of these biomarkers might inform treatment strategies, such as the tailored administration of anticoagulants or anti-inflammatory agents, to mitigate endothelial injury and manage excessive inflammation. Moreover, these markers illuminate the unique vascular ramifications of COVID-19 and may be applicable to other conditions characterized by endothelial dysfunction and coagulopathy. Their integration into routine clinical practice could standardize vascular health assessments across diverse patient populations, ultimately enhancing the precision of clinical interventions.

From a prognostic standpoint, several of these underutilized biomarkers have been independently validated as predictors of clinical deterioration and mortality in COVID-19. Longitudinal proteomic studies in severe cases reveal that signatures of endothelial and vascular injury reliably differentiate survivors from non-survivors, highlighting the prognostic importance of endothelial pathways.

Additionally, biomarkers such as MR-proADM and suPAR consistently stratify the risk of respiratory failure, the need for mechanical ventilation, and short-term mortality, often outperforming traditional inflammatory markers like CRP or PCT in multivariable models. Similarly, barrier-related markers (including vWF/ADAMTS13 imbalance, syndecan-1, and SP-D) correlate with ARDS, thromboembolic events, and persistent pulmonary dysfunction. This links biochemical evidence of endothelial and alveolar–capillary damage to clinically useful prognostic information.

In this review, we critically examine a selection of vascular and endothelial biomarkers frequently overlooked in COVID-19 research while emphasizing their diagnostic and prognostic potential. A notable finding from our analysis includes the roles of biomarkers such as adiponectin, angiopoietin, and ApoM. For example, adiponectin levels, which are inversely correlated with COVID-19 severity, function both as markers of disease prognosis and as potential therapeutic targets due to their anti-inflammatory properties. Angiopoietin-2, which exerts destabilizing effects on vascular integrity, sharply contrasts with the stabilizing effects of angiopoietin-1, highlighting the dynamic interplay between angiogenesis and disease severity. Further comparisons indicate differences in biomarker levels between ICU and non-ICU patients, reflecting a graded response correlated with disease intensity.

A substantial gap identified in the literature pertains to the limited understanding of the specific mechanistic pathways that connect these biomarkers to COVID-19 outcomes. While biomarkers such as endocan and endothelin have been associated with vascular dysregulation, their interactions with immune modulators in the context of COVID-19 necessitate further exploration. Moreover, the variability in data regarding the role of adiponectin underscores the need for standardization in the measurement and interpretation of biomarker levels, which could clarify the contradictory findings across different studies. The absence of longitudinal studies that investigate biomarker levels beyond the acute phases of COVID-19 presents a significant challenge in understanding the persistent vascular impacts observed in long-COVID cases. It should be noted that most of the studies were conducted in severe and hospitalized patients. The discrepancies in findings related to adiponectin and ApoM highlight conflicting perspectives regarding their roles in COVID-19. Integrating disparate research through meta-analyses and expanded cohort studies may yield greater clarity. Additionally, exploring combinatory therapies that target multiple biomarkers, such as ICAM-1 in conjunction with HMGB1 antagonists, may address both inflammatory and vascular components, potentially mitigating severe outcomes. The observed correlations between biomarkers, such as syndecan-1 and disease severity, suggest potential applications in patient stratification for critical care. Endothelin inhibitors, which are already employed in cardiovascular treatments, may also be beneficial for COVID-19 patients who are at an elevated risk of vascular complications. These clinical applications advocate for a more tailored approach to treating COVID-19 patients based on their biomarker profiles, which could optimize outcomes in both acute and long COVID cases.

This evidence points to risk-adapted integration of specific biomarkers into COVID-19 evaluation algorithms as preferable to universal screening. The use of prognostically informative markers, such as MR-proADM, suPAR, vWF/ADAMTS13 imbalance, and SP-D, in hospitalized or otherwise high-risk patients is supported by current findings. Utilization may enhance early risk assessment, initiate rapid care escalation, and direct the distribution of endothelial-targeted medicines and intensive monitoring.

The identified biomarkers are not only pertinent to virology but also have implications in related fields such as immunology, cardiovascular medicine, and endocrinology. For instance, the role of galectin-3 in modulating inflammation and endothelial adhesion underscores its broader relevance in inflammation-related conditions beyond COVID-19. Additionally, research on adiponectin and ApoM may contribute to studies in metabolic disorders, linking viral infections with chronic disease pathways.

Therapeutic strategies that target multiple biomarkers may offer a dual approach to addressing both the immediate and long-term effects of COVID-19. For instance, therapies aimed at modulating levels of HMGB1 and ICAM-1 could potentially alleviate acute symptoms while also diminishing the long-term endothelial damage characteristic of long-term COVID-19.

Future research should prioritize the adoption of innovative methodologies, such as high-throughput proteomics and endothelial cell-based assays, to enhance the understanding of the mechanistic roles of biomarkers like ICAM-1 and VCAM-1 in endothelial dysfunction. Additionally, broadening the scope to include biomarkers associated with lipid metabolism, such as ApoM, may uncover novel pathways for the management of COVID-19. Furthermore, the utilization of vascular-on-a-chip technology, as demonstrated by QHREDGS, could yield valuable insights into therapeutic interventions designed to restore endothelial function in the context of COVID-19.

To bridge available findings, it is important to combine a wide range of previously neglected biomarkers into an integrated model of COVID-19 etiology. This study innovatively classifies these indicators into distinct but correlated groups (Figure 3).

Figure 3.

The neglected biomarkers in COVID-19 can be classified into 5 main categories.

This combined assessment of existing evidence reveals that the neglected biomarkers are not independent signs but rather an interconnected network that affects the immediate inflammatory response, long-COVID syndrome, and the subsequent phases of recovery following SARS-CoV-2 infection. Recent clinical trials confirm that specific treatment strategies, such as sulodexide, can substantially decrease main biomarkers of endothelial dysfunction (eg, thrombomodulin, von Willebrand factor, IL-6, CRP, and D-dimer), illustrating the therapeutic potential for recovering vascular health. ¹⁷⁸ Combining these indicators provides a complete framework for patient categorization, risk prediction, and multi-targeted therapy methods.

Conclusion

Incorporating previously overlooked endothelial and vascular biomarkers into COVID-19 and even other endothelial and vascular abnormalities surveillance facilitates more precise and personalized treatment methodologies. However, integrating these biomarkers into standard clinical protocols should be based on prospective validation experiments that demonstrate incremental prognostic value over current scores and routine laboratory markers, as well as feasibility and cost-effectiveness analyses in real-world settings. By integrating these biomarkers, healthcare providers are better equipped to assess disease severity, accurately predict outcomes, and implement targeted interventions for individuals at high risk. This approach not only enhances the management of COVID-19 but also lays the groundwork for improved biomarker-based assessments in other vascular diseases, thereby expanding the scope of endothelial dysfunction management.