Review Article: Infectious and Non-infectious Precipitants of Sarcoidosis

Introduction

Although involvement can occur in any organ, sarcoidosis is a complex disorder characterized by the presence of non-caseating granulomas primarily the lungs and lymph nodes¹. Although the etiology(ies) is not known, independent investigations support infectious and non-infectious antigens in sarcoidosis pathogenesis. It has also been proposed that autoimmunity plays a crucial role in the pathogenesis of this disease. In this review, we will discuss the current knowledge regarding factors shown to initiate sarcoidosis (such as infectious and self-antigens), modify disease (checkpoint inhibitors, vaccines and antibody-mediated immunosuppression), as well as factors associated with disease (microbiome, cholesterol and environment). We will emphasize the cellular and molecular mechanisms that lead to granuloma formation and tissue damage.

IA. Infectious antigens associated with sarcoidosis pathogenesis

Mycobacterial antigens

One of the earliest descriptions of sarcoidosis (as a skin condition) by the French dermatologist Ernest Henri Besnier reveal the significant clinical overlap with tuberculosis. Besneir reported cutaneous lesion of “lupus pernio de la face-synovites fongueuses (scrofulo-tuberculeuses) symmetriques des extremities superieures.”² There remains striking clinical, histologic and pathogenic overlap between sarcoidosis and tuberculosis. Molecular analysis of sarcoidosis granulomas from American patients reveals the presence of novel mycobacterial sequences among mycobacteria that cause tuberculosis-like syndrome³; mass spectral analysis of sarcoidosis diagnostic specimens reveals mycobacterial virulence factors, such as early secreted antigenic target, 6 kDa (ESAT-6)⁴. MTB antigens have been detected in the granulomas of Iranian sarcoidosis patients⁵. Antibody immune responses to mycobacterial virulence factors have been reported⁶. In a complementary fashion, adaptive T cell immune responses in sarcoidosis local and systemic environments reveal Th1 and Th17 responses to mycobacterial virulence factors, such as ESAT-6, superoxide dismutase A⁷, antigen 85A and katG^8–10, as well as heat shock protein¹¹. In need, administration of superoxide dismutase A to wildtype (WT) mice recapitulates sarcoidosis histologically and immunologically^12–14. Interestingly, a Phase I investigation of antimycobacterial therapy of concomitant levaquin, ethambutol, azithromycin and rifampin reduced size and granuloma burden in cutaneous sarcoidosis lesions¹⁵; however, these results have not been confirmed in a Phase II investigation. Administration of this same regimen to pulmonary sarcoidosis did reduce immune responses to mycobacterial virulence factors but did not result in improvement in sarcoidosis forced vital capacity (FVC)¹⁶. This is lack of improvement in lung function is akin to findings seen after administrating antimycobacterial therapy to tuberculosis patients¹⁷ or those with nontuberculosis mycobacterial (NTM) infection¹⁸. Because antigens such as ESAT-6 do not persist but rather are actively secreted through the ESX-1 secretion system¹⁹, a reduction in immunity against mycobacterial virulence factors following antimycobacterial therapy correlates well with bacterial clearance^20–22.

Proprionibacteria

The agent formerly known as Propionibacterium acnes, has been renamed, Cutibacterium acnes. Similarly, Cutibacterium acnes (CA) DNA has been found in areas of granulomatous and nongranulomatous involvement of sarcoidosis specimens^4,23, and antibodies against CA have been detected in the sera of these patients²⁴. Using the Clone Library Method detected Streptococcus gordonii and Cutibacterium acnes in sarcoidosis lymph nodes²⁵. CA DNA or protein has been detected in ocular, cutaneous and pulmonary sarcoidosis specimens. Administration of heat-killed CA to wild-type mice resulted in the granulomatous inflammation and lung fibrosis that was ameliorated with antibody against IL-17A²⁶. Also, administration of antimicrobial therapy such as azithromycin, which targets mycobacteria and cutibacteria, has been shown to resolve sarcoidosis granulomas¹⁵.

Viruses

Several viruses have been investigated as potential triggers for sarcoidosis, including human herpesvirus 6 (HHV-6)²⁷, and lymphotrophic viruss such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV)²⁸. The role of these viruses in the development of sarcoidosis remains controversial. The appearance of SARS-CoV-2 led many clinicians to assess for increased morbidity and mortality among sarcoidosis patients. It was determined that age at diagnosis of SARS-Cov-2 infection and comorbidities, such as diabetes and severe pulmonary disease, were associated with increased risk for hospitalization²⁹, and that sarcoidosis patients could be safely vaccinated, even if immunosuppressed³⁰.

Fungus

There have been numerous case reports of pulmonary sarcoidosis complicated by known fungal pathogens such as cryptococcus³¹, mucormycosis³² and coccidiomycosis³³, but no convincing reports support that they contribute to sarcoidosis granuloma formation. A potential role for Aspergillus spp was reported by another group who found enrichment of a peptide from Aspergillus nidulans in the bronchoalveolar lavage (BAL) fluid of nine patients with Lofgren’s syndrome, an acute form of sarcoidosis. These patients also showed an increase in A. nidulans-specific IgG in their serum, compared to three controls³⁴. Also, fungal cell wall antigens in organic dust have been demonstrated to modulate immunity in sarcoidosis peripheral blood mononuclear cells (PBMC)³⁵. These findings support a role for fungal antigens as modulators of sarcoidosis pathogenesis.

IB. Noninfectious agents associated with sarcoidosis pathogenesis

A review of sarcoidosis innate and adaptive immunity is detailed in this journal. As cited above, many labs have reported local and systemic immune responses against microbial antigens, particularly pathogenic mycobacteria. In terms of innate immunity, potential non-mycobacterial-associated pathogen-associated molecular patterns (PAMP) include lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria, peptidoglycan, lipoteichoic acid, bacterial DNA, viral DNA/RNA, chitin, flagellin, leucine-rich repeats (LRR), mannans in the yeast cell wall, and microbial HSPs³⁶.

In terms of adaptive immunity, single cell and special transcriptomics of lungs from a sarcoidosis murine model and human cutaneous sarcoidosis lesions revealed that granuloma formation followed characteristic spatial patterns and involved genes linked to immunometabolism, cytokine and chemokine signaling, and extracellular matrix remodeling. Three cell types emerged as key players in granuloma formation: metabolically reprogrammed macrophages, cytokine-producing Th17.1 cells, and fibroblasts with inflammatory and tissue-remodeling phenotypes³⁷. Independent laboratories report Th17 and T regulatory immune responses against mycobacterial virulence factors^38,39. A recent report using a sarcoidosis murine model demonstrated that rescuing T regulatory cell function against mycobacterial SodA reduced granuloma burden¹⁴. Sarcoidosis patients also have an increased number of CD163+ macrophages which promote sarcoidosis granuloma formations, correlating with the M2 phenotype and sarcoidosis disease severity⁴⁰.

Many investigators and clinicians view mycobacterial infections and autoimmune disease separately; however, mycobacterial infections induce autoantibody production^41–44. For example, a recent investigation reveals among pulmonary tuberculosis patients the presence of rheumatoid factor, the components of complement system and antibodies against modified citrullinated vimentin⁴⁵. In addition, there were complex alterations in B cells and follicular Th cell subsets, suggesting autoimmunity in mycobacterial pathogenesis⁴⁵. For the purpose of this section, we expound upon the role of immunity against self-antigens, another important contributor to sarcoidosis pathogenesis.

Self Antigens

There is a clear association between sarcoidosis pathogenesis and autoimmunity. Sarcoidosis patients are more prone to being autoantibody-positive compared to controls (10.4 vs. 3%, p = 0.031). In particular, autoantibody profiling reveals anti-mitochondrial antibody-M2, anti-Ro52, anti-Ro60, anti-SSB, anti-P0, anti-CCP, anti-β2-GP, antinuclear antibodies, anti-Sm antibody, and rheumatoid factor (RF)⁴⁶. The frequent presence of associated immune-mediated disease, among which autoimmune thyroid disorders, Sjögren Syndrome and Ankylosing Spondylitis are also frequent⁴⁷.

Vimentin:

One of the self-antigens that have been proposed to play a role in sarcoidosis is vimentin, a cytoskeletal protein expressed in various cells, including macrophages and fibroblasts. Vimentin has been detected in granulomas and autoantibodies in the sea of sarcoidosis patients, and its expression correlates with disease severity^{48 49,50}. Epitopes of native vimentin are antigenic also in sarcoidosis patients with good prognosis, such as HLA-DRB1*0301+ sarcoidosis patients⁵¹. It is also noteworthy that vimentin autoantibodies are not specific for sarcoidosis as post-translationally modified and native forms of vimentin are present in many other autoimmune diseases: rheumatoid arthritis, systemic lupus erythematosus, antiphospholipid syndrome, Crohn’s disease, ankylosing spondyloarthritis and idiopathic pulmonary fibrosis⁵¹.

Heat Shock Protein 70 (HSP70):

HSP70, a stress-induced protein expressed in response to cellular damage has also been implicated as an auto-antigen. HSP70 has been detected in the granulomas of sarcoidosis patients and has been shown to activate T cells and induce the release of cytokines. Autoantibodies against HSP70 have been found in the sera of sarcoidosis patients, and their levels correlate with disease activity^52,53.

Serum amyloid A (SSA):

SSA is an acute phase protein produced primarily by the liver. Serum amyloid A has been identified to be abundant in sarcoidosis tissues, and this promiscuous host protein can serve as an innate ligand to regulate experimental granulomatous inflammation⁵⁴. Higher levels of SSA were found localized to macrophages and giant cells within sarcoidosis granulomas, amplifying local Th1 responses to mycobacterial antigens. It has been shown that SSA mediates NF-κB activation through Toll-like receptor-2 (TLR2) in sarcoidosis bronchoalveolar lavage (BAL) macrophages; it has been suggested that this is a likely mechanism for sarcoidosis chronic lung disease⁵⁵. However the SAA is not specific for sarcoidosis but present to the same degree in other chronic fibrotic lung diseases, such as hypersensitivity pneumonitis (HP), (eosinophilic) granulomatosis with polyangiitis ((E)GPA) patients and Idiopathic Pulmonary Fibrosis (IPF)⁵⁶.

II. Disease associated factors by drug-induced sarcoidosis-like reaction (DISR)

Clinical Presentation-

DISR is a systemic granulomatous reaction that is indistinguishable from sarcoidosis in terms of clinical, biological, radiological and pathological presentation. DISR induced by immunotherapy is typically associated with respiratory and skin involvement⁵⁷. Dermatological symptoms may precede the discovery of pulmonary abnormalities. Conversely, clinically silent intra-thoracic lymphadenopathy may be detected in 5% of patients treated with immune checkpoint inhibitors (ICIs). Other clinical features include fever, extra-thoracic lymphadenopathy, uveitis, hypercalcemia, nervous system, hepato-splenic, muscle, and osteo-articular involvement^57,58. Whatever the responsible drug, the radiological picture of DISR completely resembles sarcoidosis, with typical bilateral hilar lymphadenopathy and/or perilymphatic pulmonary micronodules, and ¹⁸F-fluorodeoxyglucose uptake in involved organs on positron emission tomography scan⁵⁸. Serum angiotensin-converting enzyme levels can also be increased⁵⁸. Although the list is not exhaustive, below we discuss the more common drugs associated with DISR.

Immune checkpoint inhibitors-

Increased checkpoint inhibitor expression, specifically PD-1, PD-L1 and CTLA-4, has been reported on sarcoidosis granulomas and immune cells^59–61. This therapy enhances the antitumor response by blocking down regulators of T-cell immunity, specifically Programmed Cell Death 1 (PD-1) and Programmed Death Ligand 1 (PD-L1). Common ICIs are antibodies targeting PD-1 (Nivolumab, Pembrolizumab), the PD-L1 (Atezolizumab, Avelumab, Durvalumab), or Cytotoxic T-lymphocyte Antigen 4 (CTLA-4) (Ipilimumab). The mean time from immunotherapy initiation to DISR onset was 7.1+ 9 months. In a series of 32 cancer patients with preexisting sarcoidosis who received immune checkpoint inhibition (ICI), only one case with a 20-year remote history had a symptomatic exacerbation requiring systemic corticosteroids⁶². Another retrospective study of ICI-treated patients without a prior diagnosis of sarcoidosis, DISR occurred in approximately 3.7% of patients treated with either anti-CTLA-4 or anti-PD-1 antibody and 6.3% of patients treated with a combination of both. Melanoma patients who developed de novo sarcoidosis with anti-PD-1 therapy had more Th17.1 cells in blood at baseline (pre-anti-PD-1 therapy) compared with melanoma patients who did not, suggesting that like classical sarcoidosis, Th17.1 cells might play an important role in de novo sarcoidosis-ir adverse events^63,64. The mean interval from the initiation of immunotherapy to development of DISR was 5.5 months (range 2.3-13.5 months). Mean interval from radiological detection of DISR to imaging evidence of resolution was 5.8 months (range 1.6-18.3 months). Three patients out of 81 (3.7%), 11/297 (3.7%), and 5/79 (6.3%) developed sarcoidosis-like reaction after treatment with anti-CTLA-4 antibody, anti-PD-1 antibody, and a combination of both, respectively. Most patients with DISR were asymptomatic and did not require systemic therapy^65,66. Further studies to assess if DISR T cells recognized mycobacteria, propionibacteria or self-antigens is warranted to further delineate if these Th17 cells are antigen-specific.

Antiretroviral therapy-

when HIV-positive patients are treated with combination antiretroviral therapy (cART) and CD4+Tcell counts rise to > 150–200 cells/microliter, a DISR indistinguishable from sarcoidosis may develop as part of immune reconstitution^58,67. Additionally, cART may also exacerbate prior sarcoidosis^67–69. DISRs are on average reported 9–20 months after cART initiation, although reports of Sarcoidosis occurring in HIV patients prior to cART initiation are present^58,68.

Interferons-

IFN-alpha (IFNα) is produced by leukocytes, specifically by macrophages and dendritic cells. IFN-beta (IFNβ) is produced by fibroblasts. The third form of IFN is IFN-gamma (IFNγ), which is produced by T-cell lymphocytes. Lung and mediastinal lymph nodes (70%) and skin (60%) were the most commonly reported organs involved with DISRs from IFNα therapy^6,45,77. IFNα-induced DISRs have been detected from 6 to 104 weeks after the beginning of therapy. While the exact mechanism associated with IFN-associated DISR, researchers suspect enhanced Th1 cell polarization, as well as induction of cytokines associated with granuloma formation⁷⁰. Symptoms typically resolve with cessation of therapy or with corticosteroid use.

It is also noteworthy that some immune-mediated drugs that are used to treat sarcoidosis has also been reported to induce it, such as Tumor-necrosis factor alpha antagonists and anti-IL-6 antibody (tociclixumab).

Tumor-necrosis factor alpha antagonists-

A systematic literature review on publications with DISRs related to anti-TNFα therapy was recently conducted, excluding cases with a previous sarcoidosis diagnosis⁷¹. Over 100 DISRs were identified: 49% due to treatment with Etanercept, 25% received adalimumab and 20% of patients were treated with infliximab. Most patients were on anti-TNFα therapy for more than two years with a mean treatment duration of 25.6 months (range 1–132 months)⁷¹. Because TNFα expression is important for granuloma formation, the rare observation of sarcoidosis granuloma formation with antibody against TNFα is paradoxical. While the mechanism is unclear, neutralization of soluble TNF-α that allows activation of specific autoreactive T cells has been proposed⁷².

Tociclizumab-

Toziclizumab has been reported to reduce pulmonary sarcoidosis symptoms⁷³. Paradoxically, it has also been demonstrated to induce pulmonary and/or cutaneous sarcoidosis. Both clinical phenotypes of sarcoidosis respond well to corticosteroids or cessation of the agent⁷⁴.

Vaccines:

The Bacille Calmette-Guérin (BCG) vaccine, a live attenuated strain of Mycobacterium bovis, is widely used for the prevention of TB. BCG vaccination has been associated with the development of sarcoidosis granulomas in some individuals, although the incidence of this association is rare⁷⁵. The pathogenesis of sarcoidosis granulomas in response to BCG vaccination is not clear, but it may be related to immune dysregulation, by shifting towards a Th1 immune response. It is important to note that sarcoidosis granulomas due to BCG vaccination is common; the vaccine remains an important tool for the prevention of tuberculosis. There is a recent investigation of the sarcoidosis incidence rate during periods of high versus low BCG vaccination among the Danish. The investigation noted that the sarcoidosis incidence rate was increased for individuals born during low BCG vaccination rates compared with individuals born during high BCG vaccination rates. The IRR of sarcoidosis for men born during low BCG vaccine uptake versus high BCG vaccine uptake was 1.22 (95% confidence interval [CI] 1.02-1.45)⁷⁶. This is in contrast to a report of 54,000 English children who underwent BCG vaccination in which no efficacy against sarcoidosis was noted^77,78. Other vaccines have been reported to induce sarcoidosis. For example, sarcoidosis has been reported as a possible adverse reaction to the Covid-19 Vaccine^79,80,81. However, in a study monitoring autoinflammatory and autoimmune adverse events in patients that received the Covid-19 vaccine, the incidence of sarcoidosis following vaccination was rare (2 [0.3%] out of 756 patients) ⁸².

Adjuvants:

The adjuvants associated with autoimmune/inflammatory syndrome (ASIAs) include silicone⁸³, hyaluronic acid⁸⁴, and mineral oil⁸⁵. Several sarcoidosis reactions related to ASIAs, from acute presentations such as Lofgren’s syndrome to more chronic presentations such as neurosarcoidosis, have been reported after silicone breast implantation^83,86–88 with resolution of the sarcoidosis after removal of the implant⁸⁹. Sarcoidosis has been reported in two cases after hip replacement^90,91. Both cases were associated with systemic dissemination of prosthetic particles that were found in distant organs. These findings were suggestive of a granulomatous response to the hip prosthetic particles from wear and tear of the prosthesis. Multiple organ granulomas were found in both cases, including in the lung, liver, spleen, eye, and lymph nodes.

DISR Outcomes-

The majority of DISRs are characterized by a relatively indolent course with resolution in most cases, either spontaneously or after withdrawal of pharmacological triggers⁵⁸. Specific treatment was required in 58.8% of cases, including topical corticosteroids (12.9%), systemic corticosteroids (43.5%), methotrexate (2.4%), infliximab (3.5%), and hydroxychloroquine (1.2%). After a mean follow-up of 8.3 9.9 months, DISR was stable in 12% of cases, improved in 38.7%, and resolved in 49.3%. Importantly, the decision made on immunotherapy seemed to have no impact on DISR outcomes. A table of common pharmacologic inducers of sarcoidosis, the induction period and therapeutic intervention are listed in Table 1.

Table 1.

Infectious and noninfectious drivers of sarcoidosis pathogenesis

I. Infectious antigens	Potential Mechanism(s)	References
A. Mycobacteria	Secreted antigens are targets of local and systemic adaptive immune responses	4–10
B. Propionibacteria	Antigens can induce granuloma formation; antigen present in nongranulomatous regions also	23
C. Fungal antigens	Antigens augment adaptive immune response	31–33
D. Self-antigens	Present in granulomas; expression associated with disease severity	51–56
II. Disease Associated Factors
A. Drugs
1) Checkpoint Inhibitors	Reverse PD-1, PD-L1-or CTLA-4-mediated immune dysfunction	59, 60
2) Antiretroviral Therapy	Reverse immune dysfunction	67–69
3) Interferons	Enhanced Th1 cell polarization; induction of cytokines associated with granuloma formation	70
4) Immunomodulators	May permit the activation of specific autoreactive T cells.	72
B. Adjuvants	Induces foreign body granuloma formation	83–85
III. Disease Modifying Factors
A. Occupations	Associated with “Sarcoidosis clusters”	92–95, 99
B. Organic Dust	Mechanism(s) unclear	35
C. Inorganic Dust	Mechanism(s) unclear	106
D. Metals	Drive foreign-body granuloma formation	108
E. Microbiome	Dysbiotic communities drive Th17 cell differentiation	113, 119
F. Cholesterol	Aberrant lipid metabolism in sarcoidosis macrophages drives lung granuloma formation	127

III. Disease modifying factors

Environment

The clinical observation of “sarcoidosis clusters” among firefighters and nurses, as well as Japanese and Swedish families suggests that environment is important^92–95. Environmental factors are increasingly recognized as potential contributors to sarcoidosis development and progression. The highest prevalence of sarcoidosis in the world is in the southeastern United States among African Americans, as well as among Swedes and Danes. The highest numbers of environmental opportunistic mycobacteria are located within the brown-water swamps of the southeastern coastal United States⁹⁶, as well as waters draining from boreal forest soils and peat lands in Finland⁹⁷. Elucidating the role of these environmental factors and the underlying mechanisms involved could improve our understanding of sarcoidosis pathogenesis, facilitate risk assessment, and guide preventive strategies and therapeutic interventions⁹⁸.

Occupational Exposures

Occupational exposure has been associated with an increased risk of developing sarcoidosis. These exposures can occur in various occupational settings, such as construction, agriculture, healthcare, and manufacturing. While the exact mechanisms by which these agents contribute to sarcoidosis development are not fully understood, various reports suggest a causal element related to occupational exposure such as inhalation of World Trade Center dust by fire fighters, or exacerbation of pre-existing disease due to repeated or prolonged exposures. Studies using the Bradford Hill criteria for causation to determine if a causal relationship can be established between occupational exposure and sarcoidosis noted association between World Trade Center (WTC) dust and sarcoidosis, which illustrates a causal relationship, compared to other occupational exposures⁹⁹. Various occupational settings, such as construction, mining, agriculture, and healthcare, involve exposure to inorganic dusts (e.g., silica, beryllium, aluminum), which have been implicated as potential triggers of sarcoidosis^100–102. Using univariable analysis, one group identified elevated risk of sarcoidosis for workers with industrial organic dust exposures, suppliers of building materials, hardware, and gardening materials, as well as educators. Work providing childcare was negatively associated with sarcoidosis risk¹⁰³. These studies provide valuable insights into occupational exposures that have been associated with sarcoidosis. However, it’s important to note that occupational exposure does not account for all cases of sarcoidosis, and the disease can also occur in individuals without any known occupational risk factors. Further research is needed to better understand the relationship between occupational exposures and sarcoidosis development.

Chronic Beryllium Disease (CBD)-

Similar to most immune-mediated diseases, disease susceptibility in CBD and sarcoidosis is driven by the expression of certain MHCII molecules, primarily HLA-DPB1 in CBD and several HLA-DRB1 alleles in sarcoidosis. One of the defining features of both CBD and sarcoidosis is an infiltration of activated CD4+ T cells in the lung. CD4+ T cells in the bronchoalveolar lavage (BAL) of CBD and sarcoidosis patients are highly Th1 polarized, and there is a significant increase in inflammatory Th1 cytokines present in the BAL fluid. In sarcoidosis, there is also a significant population of Th17 cells in the lungs that is not present in CBD. Due to persistent antigen exposure and chronic inflammation in the lung, these activated CD4+ T cells often display either an exhausted or anergic phenotype. Evidence suggests that these T cells are responding to common antigens in the lung. In CBD there is an expansion of beryllium-responsive TRBV5.1+ TCRs expressed on pathogenic CD4+ T cells derived from the BAL of CBD patients that react with endogenous human peptides derived from the plexin A protein¹⁰⁴. performed logistic regression modeling and generated ROC curves to determine which genes could most accurately differentiate: 1) CBD versus sarcoidosis 2) CBD versus BeS 3) sarcoidosis versus controls 4) non-progressive versus progressive sarcoidosis. Results CD55 and TNFα were significantly upregulated, while CXCL9 was significantly downregulated in CBD compared to sarcoidosis (p < 0.05). The ROC curve from the logistic regression model demonstrated high discriminatory ability of the combination of CD55, TNFα, and CXCL9 to distinguish between CBD and sarcoidosis with an AUC of 0.98. CD55 and TNFα were significantly downregulated in sarcoidosis compared to controls (p < 0.05)¹⁰⁵.

Inorganic dust.

Inorganic dusts have been implicated in the development of sarcoidosis. While the exact mechanisms are not fully understood, it is believed that the interaction between these inorganic dusts and the immune system drive granuloma formation. Investigators performed MELISA lymphocyte proliferation test on lymphocytes from the blood of 13 sarcoidosis patients using silica and metals, such as aluminum, titanium, nickel, chromium, mercury, and palladium. Of the patient group, nine tested positive to at least one of these metals; two tested positive to beryllium; and two, to silica. Twenty-six metal- or silica-exposed patients with sarcoidosis, seven unexposed patients with sarcoidosis, and 19 control subjects were tested. Only patients with sarcoidosis exhibited immunoreactivity to at least one of the antigens tested (n ¼ 7; P ¼ .039). Correlation between the LPT (ATS protocol) and the MELISA LPT was strong (1.00; P < .01) when testing reactivity to beryllium¹⁰⁶. These findings suggest an etiologic mechanism for sarcoidosis similar to that for chronic beryllium disease. Sarcoidosis is also linked to both more subtle metal exposures, such as the copper, iron, and silica in photocopier toner¹⁰⁷, as well as a number of vocations that directly handle metals⁹⁸. Indeed, one study found a significant correlation between exposure to man-made mineral fibers and the development of sarcoidosis. This association was further supported by electron microscopy quantitative analysis on previously collected lung specimens from the sarcoidosis group, which identified silica, aluminum, and/or titanium in 50% of samples¹⁰⁸. Despite these associations, one report noted that jobs with metal dust or metal fume exposures were negatively associated with sarcoidosis risk, especially in Caucasian workers¹⁰³.

Organic Dusts:

Composition of organic dust is very complex, involving particles of microbial, animal and plant origin. Agricultural work, particularly farming and animal husbandry, has been associated with an increased risk of sarcoidosis. Exposure to organic dusts serves as potential triggers of granulomatous inflammation in susceptible individuals³⁵. An investigation assessed the in vitro effects of the co-exposure of fungal cell wall agents (FCWAs) and bacterial lipopolysaccharide (LPS) on inflammatory immune responses of peripheral blood mononuclear cells (PBMCs) from patients with pulmonary sarcoidosis. They reported significantly higher secretion of inflammatory cytokines TNF-α, IL-6, IL-10 and IL-12 (1.7-fold, 2.0-fold, 2.2-fold, and 2.8-fold, respectively; all p < 0.05) after in vitro co-stimulation of PBMCs with FCWAs and LPS³⁵. Because organic dust has a complex composition, future studies to identify the relevant infectious and noninfectious component(s) is warranted.

Tattos-

Cosmetic tattoos despite small size and thereby low relative dose of pigment injected in the skin can trigger fully developed systemic sarcoidosis. It is hypothesized that iron oxide pigments popular in cosmetic tattoo inks of red or brown color may be prone to elicit sarcoid reactions and thus carry a special risk of granuloma. In decorative tattoos, carbon black is the commonest trigger. Both local and systemic sarcoidosis involvement can appear after application of cosmetic tattoos¹⁰⁹. Cutaneous, pulmonary and/or ocular involvement in the form of uveitis are among the more common manifestations following cosmetic tattoos. The sarcoidosis lesions induced by tattoos are characterized by increased CD11+ macrophages and CD3+ lymphocytes. 18F-fluorodeoxyglucose (18F-FDG) PET is a nuclear medicine imaging study supports that some sarcoidosis lesions associated with cosmetic tattoos are metabolically over-active areas¹¹⁰. This is one of the first investigations to detail enhanced metabolomics in sarcoidosis granulomas induced by tattoos.

Microbiome

Perturbations to the lung microbial community, such as the introduction of a respiratory virus or induction of variation by disease or antimicrobials, influence pathogenesis. For example, murine models of pulmonary influenza virus infection increase Enterobacteriaceae while reducing Lactobacilli and Lactococci in the intestinal microbial community¹¹¹.

Lung microbiome:

While a specific microbial agent has not been identified as an etiologic agent of pulmonary sarcoidosis, a growing body of literature supports that a lack of pulmonary microbial diversity is associated with more severe forms of disease. For example, a team of investigators performed a sampling of oral wash (OW), protected bronchoalveolar lavage fluid (PBAL) and left protected sterile brushes (LPSB) of 35 sarcoidosis patients and 35 healthy controls¹¹². The pulmonary microbiome was profiled by sequencing the fungal internal transcribed spacer 1 (ITS1) region and the V3V4 region of the bacterial 16S rRNA gene There was no significant difference in fungal alpha diversity between sarcoidosis and controls, but compared to controls, the bacterial alpha diversity in sarcoidosis was significantly lower in oral wash (p = 0.047) and BAL (p = 0.03). Aspergillus dominated the PBAL samples in sarcoidosis ¹¹².

An independent investigation of BAL from stable COPD patients, exacerbated COPD (ECOPD), interstitial lung disease (ILD) and sarcoidosis was conducted to analyze SSU rRNA gene sequences in order to define the alveolar microbiome. Shannon diversity was significantly higher in stable COPD when compared to ECOPD (p = 0.0061) and ILD (p = 0.037) patient samples. No significant difference in observed richness was noted between the diseases (p = 0.099, Kruskal–Wallis test). A predominance of Proteobacteria followed by Firmicutes, Bacteroidetes, Actinobacteria, and Fusobacteria was observed in all the disease subsets. However, using Shannon diversity index, statistically significant differences between diseases (p = 0.001, Kruskal–Wallis test) with post hoc tests revealed higher diversity in stable COPD compared to ILD (p = 0.037) and sarcoidosis (p = 0.004); Mann–Whitney test, Bonferroni adjustment). Bray–Curtis based PCA plots were analyzed at the genus level to understand the community ordination. This approach showed extensive overlap in membership between the bacterial communities of the ECOPD, stable COPD, ILD, and sarcoidosis disease groups. These findings suggest that while there is extensive microbial community overlap among chronic pulmonary diseases, quantifiable distinctions in the percentages of microbes vary by disease¹¹³. Atopobium and Fusobacterium spp were noted in significantly higher prevalence among sarcoidosis BAL, compared to IPF and healthy control specimens¹¹⁴.

Similarly, another group of investigators performed 16sRNA gene sequencing of BAL obtained from the middle lobe or the lingula of sarcoidosis and ILD patients recruited within the PULMOHOM study, a prospective cohort study to characterize inflammatory processes in pulmonary diseases. The sarcoidosis patients were sorted according to Scadding classification. There were no significant differences between patients with sarcoidosis or other ILDs with regard to microbiome composition and diversity. In addition, the abundance of the genera Atopobium, Fusobacterium, Mycobacterium or Propionibacterium were not different between the two groups ^115.

The way the investigations are conducted carry significant implications as bacterial 16S and fungal ITS1 ribosomal RNA gene metagenomic analysis of sarcoidosis specimens, such as formalin-fixed, paraffin-embedded lymph node biopsies, BAL, Kveim reagent, and fresh granulomatous spleen, revealed enrichment of microbes in single types of sarcoidosis samples but limited concordance across sample types. Statistical analysis accounting for environmental contamination was essential to avoiding false positives¹¹⁶. Although the microbiome investigations have not identified an etiologic cause of sarcoidosis, independent investigations support that low microbial diversity contributes to the pathogenesis of fibrotic lung diseases, such as in IPF patients, in which lung bacterial burden predicts fibrosis progression, and microbiota diversity and composition correlate with increased alveolar profibrotic cytokines¹¹⁷.

Gut Microbiome.

The gut microbiome has been shown to play a crucial role in immune system development and homeostasis. Dysbiosis of the gut microbiome, characterized by alterations in the composition and function of the microbiota, has been implicated in the pathogenesis of several autoimmune and inflammatory diseases, including rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis ¹¹⁸.

Another investigation reported the significance of gut microbial diversity to lung fibrosis pathogenesis, reporting the reduced gut microbial diversity leads to increase generation of pulmonary PD-1+Th17 cells, which augments collagen deposition thereby augmenting lung fibrosis severity¹¹⁹. This investigation also demonstrated that the addition of lactobacillus supernatant to human lung fibroblasts reduced the capacity of IL-17A and TGF-β1 to induce collagen 1A levels. These findings suggest that secreted microbial products, rather than the microorganism itself, can directly impact HLF collagen production¹¹⁹. Similar rigorous metagenomic investigations are warranted in sarcoidosis subjects because while independent investigations support a role of microbial dysbiosis in more severe forms of disease, there is a lack of congruence regarding key microbial populations. The dysbiosis of the gut microbiome in sarcoidosis may contribute to the development of granulomas through several mechanisms. First, dysbiosis can lead to alterations in the gut epithelial barrier and increased permeability, allowing for the translocation of bacterial products and antigens into the systemic circulation ¹²⁰. These antigens can activate the immune system and promote the formation of granulomas. Second, dysbiosis can lead to alterations in the production of short-chain fatty acids, which are important for the maintenance of intestinal homeostasis and immune regulation¹²¹. Third, dysbiosis can modulate leukocyte production of cytokines and chemokines, which can affect the recruitment and activation of immune cells¹²².

Cholesterol

Independent investigations demonstrate significantly increased risks of sarcoidosis among obese compared with nonobese patients; risk estimates ranged from 1.42 [95% confidence interval (CI), 1.07-1.89] to 3.59 (95% CI, 2.31-5.57) ^123–125. However, it was unclear if obesity was an inciter, a consequence or both of sarcoidosis emergence; a growing body of literature supports that it contributes to sarcoidosis pathogenesis and emergence. In a cohort of untreated, new diagnosed sarcoidosis subjects, metabolic syndrome was more common in sarcoidosis than controls (odds ratio, OR: 5.3; 95% confidence interval, CI 95%: 2.4-11.5;p<0.001). In addition, triglyceride and glucose levels, diastolic blood pressure measurements, and waist circumference of female sarcoidosis patients were significantly higher than in controls¹²⁶. Recent studies reveal several mechanisms by which obesity may impact sarcoidosis pathogenesis. Using a sarcoidosis murine model, investigators noted that aberrant lipid metabolism in sarcoidosis macrophages drives lung granuloma formation¹²⁷. Independent researchers noted increased IL-15 in the sarcoidosis lung of murine model and humans. The presence of obesity was an important driver of IL-15 expression¹²⁸. In tuberculosis subjects, pathogenic mycobacteria use cholesterol as a carbon source^129,130 and cholesterol also serves as a driver of inflammation.

Conclusion

Sarcoidosis is a complex, granulomatous disease with significant clinical and immunologic overlap with known infectious diseases, such as tuberculosis, as well as granulomatous diseases of noninfectious origin, such as CBD. While the etiology(ies) of sarcoidosis remains unknown, it is an exciting time to be a sarcoidosis researcher as significant progress has been achieved in understanding sarcoidosis pathogenesis. The identification of microbial targets of sarcoidosis local and systemic immune response had led to potential therapeutic targets, and the emergence of new sarcoidosis animal models, which further enhance understanding of sarcoidosis pathogenesis. It lends hope that enhanced understanding of how infectious antigens drive sarcoidosis pathogenesis, as well as noninfectious factors such as metals or organic dust, contribute to pathogenesis. Further research is also needed to understand the role of relevant mitigating factors, such as occupational hazards, immunomodulatory medications, the gut microbiome and cholesterol on sarcoidosis progression. Dysbiosis of the gut microbiome may play a role in the pathogenesis of sarcoidosis, with alterations in the composition and function of the microbiota contributing to profibrotic cytokine expression. Further studies are needed to elucidate the mechanisms underlying the gut microbiome-sarcoidosis connection and to determine whether targeting the gut microbiome may be a viable therapeutic strategy for this complex disease.

Figure 1.

Microbial and Non-infectious causes of sarcoidosis pathogenesis

1. This work provides a detailed list of potential infectious causes.

2. This works provides a detailed list of non-infectious contributors.

3. A synopsis figures of potential drives of sarcoidosis progression is also included.