Cytokine Release Syndrome and Sepsis: Analogous Clinical Syndromes with Distinct Causes and Challenges in Management

Introduction

Cytokine release syndrome (CRS) and sepsis are clinical syndromes with considerable overlap and heterogeneous clinical presentations. The goal of this review is to define a spectrum of diseases that are associated with CRS and sepsis, including other closely related immunopathologic syndromes such as hemophagocytic lymphohistiocytosis (HLH), to describe several well-recognized CRS triggers (including chimeric antigen receptor (CAR) T-cell therapy, allogeneic transplants, novel drugs, and COVID-19), and to provide a framework for delivering therapy that covers both clinical syndromes when the etiology is not yet known.

CRS was first described to define the body’s hyperinflammatory immune response to a stimulus, including infection or sterile inflammatory processes¹. One of the first reported descriptions of CRS in the literature was in a patient who developed acute graft-versus-host disease (GVHD) during hematopoietic stem cell transplantation (HSCT) in the early 1990s². CRS has been used to describe the clinical syndrome of elevated cytokines with exaggerated inflammation and organ dysfunction¹. This definition bears similarity to the Society of Critical Care Medicine/European Society of Intensive Care Medicine guidelines most recent definition of sepsis as “life-threatening organ dysfunction caused by a dysregulated host response to infection.”³ Neither the definition of CRS nor sepsis have complete consensus, and there is significant overlap in the clinical presentations of both. Furthermore, the two entities may be linked, as many of the underlying disorders and treatments that predispose to CRS are states of profound immunosuppression, which carry an increased risk of infection and sepsis. However, as management strategies for both syndromes have become more targeted in recent years, accurate diagnosis is essential to facilitate early initiation of appropriate therapies.

CRS Triggers

Chimeric Antigen Receptor T (CAR-T)-cell Therapies:

Over the past decade, CRS has been used more specifically to describe the syndrome of elevated cytokines with associated fevers, hypotension, hypoxia and multiorgan dysfunction that can result after CAR-T⁴. CRS associated with CAR-T cells has been well studied with now agreed upon standardized classification⁵ (Table 1). In addition to fever (defined as temp ≥ 38°C), the extent of hypotension and hypoxia are used to classify the grade of CRS, with grade 5 signifying death due to CRS. Furthermore, central nervous system associated toxicities, often referred to as immune effector cell-associated neurotoxicity syndrome (ICANS) or less commonly cytokine release encephalopathy syndrome (CRES), are classified separately given their distinct treatments and outcomes^5,6. However, as discussed previously, CRS is not unique to CAR-T cell therapies and can be seen in several novel therapies, including the now widely used checkpoint inhibitors (CPIs).

Table 1:

Cytokine Release Syndrome Clinical Grading Criteria as agreed upon by the American Society for Transplantation and Cellular Therapy

CRS GRADE	Hypotension	Hypoxia
1	None	None
2	Not requiring a vasopressor	Requiring NC or blow-by oxygen
3	Requiring a single vasopressor (with or without vasopressin)	HFNC, facemask, NRB, or Venturi
4	Requiring multiple vasopressors	Requiring invasive or noninvasive positive pressure ventilation

All CRS grades must have a temperature ≥ 38°C

abbreviations NC: nasal cannula; HFNC: high-flow nasal cannula, NRB: non-rebreather

CPI Therapy Associated CRS, and the Overlap with Sepsis

There are three widely used categories of CPIs: cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) inhibitor (ipilimumab), programmed cell death protein 1 (PD-1) inhibitors (cemiplimab, nivolumab, and pembrolizumab), and programmed death ligand 1 (PD-L1) inhibitors (atezolizumab, avelumab, and durvalumab), with several new targets (LAG-3, TIM-2, B7-H3, and others) in ongoing clinical trials⁷. CPIs have revolutionized outcomes in oncology. The mechanism of CRS associated with CPIs is not well known, but presumably arises from priming of T cells/cancer cells with resultant cellular destruction and subsequent inflammation⁸. A recent World Health Organization (WHO) global database survey noted 58 cases of worldwide CRS in over 130 member countries surveyed, with the highest total cases in North American (United States and Canada with n = 37), and with the highest incidence in Australia (0.14%)⁹. The malignancies most associated with CPI-related CRS included melanoma (n = 17) and hematologic malignancies (n = 16). Six of the cases (~10%) had concurrent infections, thereby suggesting some overlap between sepsis and CRS⁹. Another study evaluated 25 patients with CRS after CPI therapy in two tertiary hospitals and classified these patients using the CRS grading scale detailed in Table 1¹⁰. In this study, three patients suffered from Grade 5 (fatal) CRS, and Grade 3–4 CRS was seen in another five patients¹⁰. This study raises concerns that the overall incidence of CRS after CPI is underreported, when compared to numbers in the WHO global database.

Underreporting could also be due to the overlap in immune related adverse events (IRAEs) associated with CPI use¹¹. IRAEs affect multiple organ systems and are used to describe organ specific toxicities that result after CPI use. Most commonly, dermatitis, pneumonitis and gastroenteritis can be seen¹². The IRAEs can result in disruption of the epithelial barrier and predispose to secondary infections, including sepsis. In addition, IRAEs are treated with immunosuppression which can result in additional infectious risks. When IRAEs are associated with hypotension due to secondary infection or hypophysitis with multi-organ involvement, there can be overlap with CRS, and even sepsis. Thus, the exact classification of the syndrome impacting these patients can be challenging. Would the patient benefit from additional immunosuppression vs. antimicrobial therapy vs. immunomodulation of cytokines? These questions remain challenging for the bedside clinician.

CPIs are now being considered for use in sepsis with ongoing clinical trials¹³. Septic patients have been noted to have marked immunosuppression and defects in adaptive immune cell responses to infection, thus placing them at an increased risk of infection^14,15. This phenomenon of immune exhaustion during sepsis is similar to the immune exhaustion seen in cancer patients¹⁶. Hence, it is not surprising that immunotherapy is now being considered for potential treatment of sepsis in non-cancer patients. CPIs have shown remarkable responses in certain infections, such as JC virus associated progressive multifocal leukoencephalopathy, but it is notable that even in that small study one patient (of eight studied) was identified as having an inflammatory syndrome classified as immune reconstitution inflammatory syndrome (IRIS)¹⁷. Thus, as the indications for CPIs broaden, in addition to CRS, overlap with infectious diseases/sepsis, and additional inflammatory syndromes (IRAEs or IRIS) will likely become more common¹⁸.

CRS Associated with Antibody Therapy

Apart from the CPIs, almost all monoclonal antibodies could be associated with CRS^19,20. The incidence of CRS after the use of muromonab-CD3 (OKT3) was among the first published cases of CRS in the literature²⁰. Antibody-mediated reactions may result in the production of excessive cytokines resulting in a hyperinflammatory syndrome that could be classified as CRS²¹. Specifically, the novel CD19/CD3 bispecific T-cell receptor-engaging (BiTE) antibody, blinatumomab, has been associated with CRS along with neurological toxicities. The CRS and neurological toxicities associated with blinatumomab are treated with steroids and CRS specific anti-cytokine antibody treatment²². Several other BiTEs are now in clinical trials with their unique side-effect CRS profiles. In addition to CRS, certain antibody therapies are associated with very specific side-effects. For example, the novel agent tagraxofusp-erzs is a CD123 targeting IL-3 antibody fused with diphtheria toxin that has revolutionized responses to blastic plasmacytoid dendritic-cell neoplasm (BPDCN) but is associated with capillary leak syndrome, which will be discussed later in this article²³.

CRS Associated with HSCT:

With the availability of haploidentical (or half-matched donors), the ability to provide allogeneic HSCT has greatly increased²⁴. The infusion of hematopoietic stem cells, particularly from a haploidentical donor, has been associated with an increased incidence of CRS akin to what is seen after CAR-T cell therapy²⁵. Several risk factors, perhaps most predominantly the use of peripheral blood stem cells, have been associated with an increased incidence of CRS with higher grades of CRS corresponding to increased mortality and worse overall outcomes^25–28. Haploidentical HSCT-associated CRS generally abates with the administration of cyclophosphamide on day three and four post-transplant, but in high CRS grades with multiorgan dysfunction, the elevated interleukin (IL)-6 levels in particular are highly responsive to steroids and tocilizumab, an IL-6 receptor antibody^29–31.

This patient population is generally neutropenic following conditioning chemotherapy treatment in anticipation of a new hematopoietic stem cell population, and thus is also at an increased risk of sepsis, and particularly neutropenic sepsis necessitating early intervention with supportive care and appropriate antimicrobial therapy. In addition to the overlap with sepsis, several other peri-transplant syndromes, including acute GVHD and engraftment syndrome may also be present in this patient population, further complicating the diagnosis³².

CRS after Engineered Virus Specific T-Cells:

In addition to CAR-T cells, other engineered cellular products may also contribute to CRS. With the advent of new virus specific T-cells (VST), case reports have noted associated CRS^33,34. With VST therapy, T cells are obtained either from a viral-experienced allogeneic donor or more recently virus-naïve umbilical cord blood or adult donors, stimulated and expanded ex-vivo against a single or multivalent viral target, and infused into the recipient to confer immediate antiviral immunity³⁵. While this method provides a potentially powerful tool against difficult to treat viruses, the direct infusion of a large number of primed T cells into a host with significant antigen burden poses a risk for acute release of inflammatory cytokines resulting in CRS and subsequent infiltrative tissue damage. Several groups have reported success with VST in Phase 1 and 2 studies and case series, demonstrating microbiologic and clinical cure with limited adverse events^34,36,37. In total, CRS is felt to be rare post-VST, complicating less than 2% of recipients in one study³⁸. However, it must be noted that these patients are largely clinically stable at the time of infusion and while they may have evidence of tissue-invasive disease (i.e. cytomegalovirus pneumonitis or retinitis, BK virus associated hemorrhagic cystitis), they are not generally critically ill with evidence of multisystem organ dysfunction. There are limited data to guide whether VST may be of potential clinical benefit or harm in patients who are systemically ill with concurrent organ dysfunction, and thus remains an important clinical question about optimal timing of treatment. Clinically significant and potentially fatal CRS has been reported following VST³³. The authors show an acute rise in serum ferritin and IL-6 levels and concurrent fall in C reactive protein (CRP) and interferon (IFN) ɣ, immediately following anti-BK VSTs in a patient with severe BK virus hemorrhagic cystitis. The patient received tocilizumab but developed progressive multiorgan failure secondary to sinusoidal obstructive syndrome (SOS)/veno-occlusive disease (VOD) and acute hypoxic respiratory failure and subsequent death on day 63 posttransplant. Postmortem sampling demonstrated hepatic SOS/VOD with hemorrhagic necrosis, acute renal tubular injury, and early pulmonary exudative phase diffuse alveolar damage without any evidence of polyomavirus on SV40 staining, supporting the hypothesis that the fatal organ injury was likely driven by overwhelming inflammatory cytokines and cellular damage rather than direct microbial-mediated damage. Thus, while VST is an important weapon in a limited arsenal of therapy against challenging viral infections, it is not without risk. Vigilance for signs of CRS is essential, and further data are needed to evaluate for safety in setting of systemic illness.

Inflammatory Syndromes Resembling CRS:

HLH:

HLH clinically presents similarly to sepsis and CRS but has well-established criteria for the diagnosis. A vast majority of the HLH in adult patients arises due to ongoing stimulation from either an infection, malignancy, or autoimmune disease. This type of HLH is referred to as a secondary HLH, rather than genetic or primary HLH which is more common in pediatric patients³⁹. The diagnosis of secondary HLH requires five of the following eight criteria: 1) fever, 2) cytopenias in at least two cell lines, 3) hypertriglyceridemia or hypofibrinogenemia, 4) splenomegaly, 5) elevated soluble IL-2 receptor α (CD25), 6) decreased or absent NK cell activity, 7) ferritin elevation, and 8) hemophagocytosis in tissue⁴⁰. Once this diagnosis is established, the primary objective is to treat the underlying trigger. In general, over a third of secondary HLH is due to malignancy, under a third due to infection, over ten percent due to autoimmune disease (referred to as macrophage activation syndrome)⁴¹, and about ten percent remains idiopathic⁴². Thus, as expected there is significant overlap between HLH, CRS, and sepsis. In one of the largest retrospective intensive care unit (ICU) studies of HLH (40 patients with HLH of 2643 total ICU patients), the authors found that a higher ferritin cut-off value (9,083 μg/L; AUC 0.963, 95% CI, 0.949–0.978) correlated with HLH when compared to patients with sepsis⁴³. Given that 1/3 of HLH is triggered by infection, distinguishing sepsis from HLH can be difficult.

In patients that develop refractory CRS after CAR-T cell therapy, there is a concern that their clinical disease may start to resemble HLH. Based on the HLH diagnostic criteria, several CRS patients would meet criteria for HLH at baseline (or merely Grade 1–2 CRS toxicity), and thus deciding whether therapy targeting the HLH is merited can be challenging. Several institutions have adapted treatment guidelines to further distinguish refractory CRS and HLH. For instance, most centers recommend first treating for CRS, and then monitoring for improvement at 48hrs, prior to initiating chemotherapy (generally etoposide)⁴⁴. Regardless, these patients are often covered for sepsis as well with broad-spectrum antibiotics given their immunosuppressed state.

IRIS and Human Immunodeficiency Virus (HIV) Malignancies:

IRIS is generally used to describe the phenomenon of immune recovery, and specifically T cell recovery, in HIV patients after initiation of antiretroviral therapy⁴⁵. Although IRIS can be applied to a variety of immune reconstitution phenomenon it is characterized by elevated cytokines and diffuse organ involvement with vasodilation, and thus can have overlap with CRS and sepsis syndromes. In addition, several HIV and human herpes virus-8 associated malignancies, including Kaposi sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease are characterized by elevated cytokines, and a “cytokine storm” like phenomenon akin to CRS, and even sepsis^46,47. Smaller studies have demonstrated the role of tocilizumab (commonly used in CAR-T associated CRS) in these patients^48,49.

Capillary Leak Syndrome:

Capillary leak syndrome is another clinical syndrome with significant overlap between CRS and sepsis. It is characterized by increasing capillary permeability with loss of oncotic pressure, and resultant leakage of fluid into the interstitium and hypotension⁵⁰. It can be idiopathic, secondary to sepsis, secondary to the treatment of sepsis, or it can be seen with acute respiratory distress syndrome (ARDS). It can also be associated with the use of certain targeted therapies employing toxins. For instance, Tagraxofusp-erzs is a novel CD123-directed cytotoxin of IL-3 fused to truncated diphtheria toxin that is used for the treatment of blastic plasmacytoid dendritic cell neoplasm and is sometimes complicated by capillary leak syndrome²³. In this disease with poor outcomes, Tagraxofusp-erzs demonstrated a 90% overall response rate. However, two of the 47 patients enrolled died from capillary leak syndrome. The package insert for the drug recommends aggressive management of capillary leak syndrome in these patients. As newer antibody and cytokine mediated therapies develop, we are likely to see more patients with CRS and associated syndromes^51–53.

Distinguishing Between CRS and Sepsis to Target Clinical Management

Both CRS and sepsis can be associated with fever, hypotension, vasodilatory shock, and multiorgan failure. Making the distinction between CRS and sepsis can be challenging but important such that targeted therapy can be initiated promptly. When one is unable to distinguish between CRS and sepsis it is imperative to treat both, focusing on maintaining end-organ perfusion, maintaining oxygenation, supporting hemodynamics, and monitoring inflammatory markers frequently. Supportive treatment of coagulopathies and symptom relief are also important parts of clinical management. Understanding the timing of CRS may also help distinguish this from sepsis and allow for targeted therapies. In general, the timing of CRS after infusion of CAR-T depends on the CAR-T cellular construct, but on an average is seen seven days after infusion⁵⁴. Although delayed complications can be seen in a subset of population⁵⁵.

Anti Interleukin-6 Therapy and Steroids:

CAR-T cell therapy revolutionized outcomes in refractory diseases, but initially there was concern for an unacceptably high mortality associated with CRS. This was mitigated using tocilizumab, an anti-IL-6 receptor antibody^56–58. Tocilizumab is being explored for sepsis in clinical trials, and the results of these studies are still pending⁵⁹. In higher grade CRS, or ICANS, rapid initiation of steroids is recommended for their cytolytic and antiinflammatory activity on CAR-T cells, along with their ability to cross the blood-brain barrier⁶⁰. However, this infusion of steroids and resultant cytolysis of the CAR-T cells could be deleterious. A recent single center study evaluated 60 CAR-T recipients (of a 100 total) who had received steroids⁶¹. The study found that higher doses of steroids and earlier use of steroids after infusion of CAR-T cells were both associated with overall shorter survival. The role of steroids in septic shock remains contentious though large studies have demonstrated a trend towards accelerated recovery and possible improved mortality^62,63. Most guidelines recommend steroids in patients with refractory septic shock⁶⁴.

Fluid Resuscitation Strategy:

Recent studies have highlighted challenges with guideline mandated volume resuscitation in sepsis, along with choices of fluid (balanced crystalloid versus normal saline)⁶⁵. Unlike sepsis, in CRS, cautious and judicious use of fluids is recommended given a tendency for patients to develop pulmonary edema^58,66. Most institutional guidelines recommend early use of vasopressors after a modest fluid challenge to minimize expected third-spacing of fluid⁶⁰.

Antimicrobial Therapy:

In septic shock, early antibiotic administration has been shown to improve mortality⁶⁷. Patients with CRS after CAR-T therapy have a high risk of concomitant infection, and most guidelines recommend early administration of antibiotics with first fever^51,68. In fact, a vast majority of patients with CRS have concomitant neutropenia after conditioning therapy requiring immediate initiation of broad-spectrum antibacterial agents. The European Society for Blood and Marrow Transplantation has also recommended the initiation of antiviral prophylaxis (along with CMV monitoring), with delayed initiation of pneumocystis pneumonia prophylaxis, along with conditional mold prophylaxis and immunoglobulin replacement⁶⁹. Centers in the United States tend to follow a similar paradigm, with more institution specific guidelines^70,71.

COVID-19 complicated with CRS and Sepsis:

The interplay between CRS and sepsis has further been highlighted with the recent COVID-19 pandemic. Early in the pandemic, clinicians noticed a hyperinflammatory subtype in some of the patients with COVID-19 which mimicked a secondary HLH⁷². This response represented the exaggerated host immune response to the viral infection, and the need to mitigate this response became the focus of several trials. RECOVERY remains among the first studies in COVID-19 to demonstrate a mortality benefit in hypoxic patients that were treated with steroids⁷³. The role of steroids in CRS and sepsis has been discussed above.

On the heels of the RECOVERY trial, several studies assessed the roles additional antiinflammatory agents may play in COVID-19. The recommended treatments for COVID-19 include, anti-IL-6 agents, specifically tocilizumab and sarilumab (anti-IL-6 receptor antibodies). These agents have been shown to prevent need for mechanical ventilation and death^74–76. In addition, inhibition of the janus tyrosine kinase (JAK) and signal transducers and activators of transcription (STAT) pathway with agents such as baricitinib and ruxolitinib has been shown to help in recovery from COVID-19^77,78. Interestingly, these agents have also been used in HLH with some success⁷⁹.

A particularly inflammatory endotype of patients with hyperinflammation and COVID-19 has been classified as multisystem inflammatory syndrome (MIS) in adults (A) and children (C). The clinical criteria for MIS-C are slightly variable among the WHO, Centers for Disease Control and Prevention (CDC), and the Royal College of Pediatrics and Child Health⁸⁰. For the purposes of this review, we have focused on the CDC definitions⁸¹. Table 2 details the clinical criteria for definition of MIS-A. Patients must present with fevers and elevated inflammatory markers in the presence of confirmation of infection with SARS-CoV-2, followed by the inclusion of at least three clinical criteria, with one being a primary clinical criterion. MIS-C, by CDC definition, is used to describe patients < 21 years with COVID-19 who are hospitalized with elevated inflammatory markers and evidence of two or more organ system dysfunction without other plausible explanations.

Table 2:

Classification of Multisystem Inflammatory Syndrome in Adults (MIS-A)

Age	> 21 years
Duration	Hospitalized for ≥ 24hrs, or with illness resulting in death
Fever	(≥38.0 C) for ≥ 24 hours prior to hospitalization, or fevers within three days of hospitalization
Primary (must include at least one of the following)	Severe Cardiac Dysfunction (cardiac arrest alone does not qualify) Myocarditis Pericarditis Coronary artery dilation or aneurysm New left ventricular or right ventricular dysfunction 2nd/3rd degree A-V block Ventricular Tachycardia Rash and non-purulent Conjunctivitis
Secondary (must have at least one primary, and two additional secondary to meet criteria)	Neurologic dysfunction: Encephalopathy Seizures Meningismus Peripheral Neuropathy Acute Inflammatory Demyelinating Polyneuropathy (Guillain-Barré syndrome) Shock or hypotension Gastrointestinal symptoms: abdominal pain, vomiting, or diarrhea Thrombocytopenia (platelet count <150,000/ microliter)
Laboratory Markers	Elevated levels of at least TWO of the following: C-reactive protein, ferritin, interleukin-6, erythrocyte sedimentation rate, procalcitonin A positive SARS-CoV-2 test for current or recent infection by RT-PCR, serology, or antigen detection

In the largest case series of MIS-C published in the United States, 186 patients with a median age of 8.5 years were included with almost 80% requiring ICU care and four deaths (though almost a third of the patients remained hospitalized at the time of study publication)⁸¹. This hyperinflammatory state has been treated with immunomodulators including intravenous immunoglobulins, steroids, IL-6 antagonists, and anakinra (IL-1 receptor inhibitor).

Beyond the inflammatory subtype of COVID-19, there has also been concern for secondary infections, particularly pneumonia, in damaged lungs of patients with COVID-19 resulting in increased mortality and septic shock⁸². Yet, larger group studies have recommended judicious use of antibiotics given the lower incidence of concomitant bacterial or fungal infection⁸³. There may be some utility in using infectious markers, such as C-reactive protein and procalcitonin, in making decisions regarding antimicrobial stewardship⁸⁴. Overall, infection with COVID-19 serves to highlight the overlap between sepsis and CRS, and the challenges in distinguishing between the two.

Conclusion:

This review provides a broad framework for understanding CRS and other closely related syndromes. We have discussed several triggers of CRS, along with clinical syndromes that share significant overlap with both CRS and sepsis. Figure 1 details CRS-associated conditions and demonstrates the overlap between CRS and sepsis, along with the interplay of HLH and the COVID-19 pandemic. Given that both CRS and sepsis are clinical syndromes rather that distinct diseases, identifying and classifying patients at the bedside as having one or the other, or both, will remain challenging. The backbone for treatment of both CRS and sepsis remains excellent supportive care to maintain oxygenation and end-organ perfusion, with a focus on making the distinction quickly such that targeted therapies can be employed.

FIGURE 1:

Overlap between Cytokine Release Syndrome (CRS) and Sepsis with the inclusion of other diseases and clinical syndromes.

Abbreviations: CLS: Capillary Leak Syndrome; CPI: Checkpoint Inhibitor; IRAE: Immune-Related Adverse Events; HSCT: hematopoietic stem cell transplant; IRIS: Immune Reconstitution Inflammatory Syndrome; MAS: Macrophage Activation Syndrome; HLH: Hemophagocytic Lymphohistiocytosis; CAR-T: Chimeric Antigen Receptor T cell

Key points:

1. differentiate between sepsis and cytokine release syndrome (CRS)

2. identify CRS triggers

3. discuss syndromes with clinical overlap between sepsis and CRS

Synopsis:

Both CRS and sepsis are clinical syndromes rather than distinct diseases and share considerable overlap. It can often be challenging to distinguish between the two, but it is important given availability of targeted treatment options. In addition, several other clinical syndromes overlap with CRS and sepsis, further making it difficult to differentiate them. This has particularly been highlighted in the recent COVID-19 pandemic. As we start to understand the differences in the inflammatory markers and presentations in these syndromes, hopefully we will be able to enhance treatment and improve outcomes.