Cytokine Release Syndrome 2.0: Managing Immune-Effector Toxicity as Cell Therapy Moves Earlier, Broader, and Beyond CAR T

Introduction

Cytokine release syndrome has become one of the defining toxicities of modern cellular therapy. In early clinical experience, CRS was largely viewed as an inpatient oncology, transplant, or cellular therapy unit complication. That view is now too narrow. Cellular therapy is expanding across hematologic malignancies, moving into earlier treatment lines, and extending beyond conventional CAR T-cell constructs into engineered TCR products and tumor-infiltrating lymphocyte platforms.[15-25]

This shift has practical implications for oncologists, hospitalists, emergency physicians, intensivists, cardiologists, pulmonologists, nephrologists, neurologists, infectious diseases specialists, gastroenterologists, pharmacists, and advanced practice clinicians. A patient with fever, hypotension, hypoxia, diarrhea, encephalopathy, renal injury, arrhythmia, or shock after cellular therapy may present first to an emergency department, community hospital, infusion clinic, subspecialty office, or intensive care unit rather than to the treating cellular therapy team.

CRS is not simply “inflammation.” It is a dynamic immune-effector toxicity that can progress from fever to vasodilatory shock, capillary leak, hypoxemic respiratory failure, coagulopathy, renal injury, and cardiac dysfunction. CRS may overlap with ICANS or other neurotoxicity, but neurotoxicity is not simply the most severe end of the CRS continuum.[1,5]

The same syndrome may also be mild, self-limited, and manageable with close observation and reassessment. The clinical challenge is identifying when post-infusion inflammation is developing a trajectory that requires anticytokine therapy, corticosteroids, ICU-level monitoring, or urgent reassessment for an alternative or concurrent diagnosis.

The “2.0” era of CRS management is defined less by a single new drug than by more precise toxicity recognition, stronger cross-specialty coordination, product-specific label awareness, and careful distinction between established practice and evolving evidence.

Why This Topic Matters Now

Several developments have changed the clinical context for CRS. Approved cellular therapies no longer fit one toxicity profile. CD19-directed CAR T-cell therapies, BCMA-directed CAR T-cell therapies, engineered TCR therapies, and tumor-infiltrating lymphocyte products differ in disease indications, lymphodepletion regimens, antigen targets, manufacturing logistics, toxicity timing, monitoring requirements, and label warnings.[7-11,15-25]

Regulatory expectations have also evolved. In June 2025, FDA eliminated the Risk Evaluation and Mitigation Strategies (REMS) for six autologous CAR T-cell products: ABECMA, BREYANZI, CARVYKTI, KYMRIAH, TECARTUS, and YESCARTA. FDA determined that a REMS was no longer necessary to ensure that the benefits of these products outweighed their risks.[12]

The removal of those REMS requirements should not be misread as evidence that CRS and neurotoxicity have become minor concerns. Current labeling continues to include boxed warnings and product-specific monitoring and management instructions. Institutional readiness, clinician education, rapid access to appropriate treatment, and long-term safety surveillance remain essential.[7-12]

FDA also required boxed-warning language in 2024 for T-cell malignancies after BCMA-directed and CD19-directed genetically modified autologous CAR T-cell immunotherapies. Reports arose from clinical trials and postmarketing data and included CAR-positive T-cell malignancies. Patients should be monitored lifelong for secondary malignancies. A new malignancy after treatment should prompt manufacturer notification and collection of samples for CAR-transgene testing according to manufacturer instructions.[13]

The toxicity landscape is also broadening. CRS remains central, but clinicians must recognize ICANS, HLH/MAS, prolonged or recurrent cytopenias, infections, delayed neurologic syndromes, and product-specific complications. One important example is immune effector cell-associated enterocolitis after ciltacabtagene autoleucel.[9,14]

For internal medicine subspecialists, the relevant question is no longer whether they “manage CAR T.” It is whether they can recognize immune-effector toxicity when it enters their clinical setting.

Defining CRS: A Clinical Syndrome, Not a Cytokine Panel

CRS is a systemic inflammatory syndrome that occurs after immune-effector activation. In cellular therapy, it usually reflects therapeutic T-cell activation, antigen engagement, downstream cytokine signaling, and activation of bystander immune cells, including monocytes and macrophages.[5]

Interleukin-6 is clinically important because IL-6 receptor blockade can reverse many systemic manifestations of CRS, but CRS is not an IL-6-only disorder. IL-1, interferon-gamma, tumor necrosis factor, granulocyte-macrophage colony-stimulating factor, endothelial activation, and coagulation pathway disruption may also contribute. The relative importance of these pathways may differ by product, patient, disease burden, and clinical phase.[5]

The ASTCT consensus grading system remains the most widely used clinical framework for immune-effector cell-associated CRS. CRS begins with fever of at least 38.0 degrees C that is not attributable to another cause. Once CRS has been established and antipyretics, anticytokine therapy, or corticosteroids have been administered, fever is no longer required for subsequent grading.[1]

Severity is then determined by hypotension and hypoxia. Organ dysfunction remains clinically important, but it does not independently determine the ASTCT CRS grade. When hypotension and hypoxia fall into different grades, the higher grade is assigned.[1]

An elevated C-reactive protein (CRP), ferritin, or cytokine concentration may support concern for systemic inflammation or help track a trajectory, but laboratory markers do not independently establish or grade CRS. A cytokine panel should not delay stabilization, infection evaluation, or grade-directed treatment.

Table 1. ASTCT-Based CRS Grading in Practical Clinical Terms

Grade	ASTCT clinical anchor	Practical caution
Grade 1	Fever at least 38.0 degrees C without hypotension or hypoxia	Continue close monitoring and do not dismiss infection, particularly during neutropenia
Grade 2	Hypotension not requiring a vasopressor and/or hypoxia requiring low-flow nasal cannula at no more than 6 L/min or blow-by oxygen	Inpatient evaluation is usually appropriate; escalate earlier in patients with limited cardiopulmonary or renal reserve
Grade 3	Hypotension requiring one vasopressor, with or without vasopressin, and/or hypoxia requiring high-flow nasal cannula, facemask, nonrebreather mask, or Venturi mask	Use an ICU or ICU-capable setting and reassess for sepsis, ICANS, HLH/MAS, and cardiopulmonary disease
Grade 4	Hypotension requiring multiple vasopressors, excluding vasopressin, and/or hypoxia requiring positive-pressure ventilation	Provide urgent critical care, organ support, multidisciplinary management, and product-specific protocol review

Incidence and Severity Vary by Product, Disease, and Trial Context

CRS frequency varies widely across cellular therapy products and studies. In many CD19-directed and BCMA-directed CAR T-cell trials, any-grade CRS was common, while grade 3 or higher CRS was less frequent but clinically consequential. Rates are influenced by antigen target, construct, costimulatory domain, tumor type, disease burden, lymphodepletion regimen, bridging therapy, patient selection, reporting period, and grading system.[15,16,19,20,22-25]

This variability is why product-specific CRS rates should be taken from current prescribing information rather than copied across therapies. Even within the same broad category, the timing and severity of CRS may differ.

For example, current AUCATZYL prescribing information reports CRS in 75% of patients in the pivotal safety population, including grade 3 CRS in 3%, with a median time to onset of 8 days. Current CARVYKTI prescribing information reports CRS in 84% of patients across CARTITUDE-1 and CARTITUDE-4, including grade 3 or higher CRS in 4%, with a median time to onset of 7 days.[8,9]

These figures should not be used for direct product comparison because trial populations, disease settings, follow-up periods, and management practices differ. The clinical lesson is not that one product is “safe” and another is “dangerous.” CRS risk is product-specific, population-specific, and time-dependent.

A febrile patient after cellular therapy should be evaluated with the exact product, infusion date, conditioning regimen, disease burden, baseline organ function, and current prescribing information in mind.

Risk Stratification Before CRS Occurs

Risk stratification begins before infusion. High tumor burden, rapid disease kinetics, elevated inflammatory markers, baseline cytopenias, occult infection, limited cardiopulmonary reserve, renal impairment, hepatic dysfunction, and frailty may reduce physiologic reserve or be associated with more complicated toxicity in selected settings.[2-5]

The strength and applicability of these predictors vary by disease and product. They should guide monitoring intensity, location of care, and contingency planning rather than create false certainty about whether CRS will occur or how severe it will become.

Cardiologists may be asked to assess ventricular function, ischemic disease, arrhythmia risk, pulmonary hypertension, and tolerance of fluid resuscitation or vasopressors. Pulmonologists may need to distinguish CRS-related hypoxemia from infection, pulmonary edema, pulmonary embolism, diffuse alveolar injury, or disease-related lung involvement.

Nephrologists may become involved when CRS overlaps with acute kidney injury, tumor lysis syndrome, fluid overload, or electrolyte instability. Infectious diseases specialists are essential because fever after lymphodepletion and cellular therapy cannot be assumed to represent sterile inflammation.

Neurologists are needed when systemic toxicity overlaps with ICANS, seizures, aphasia, encephalopathy, concern for cerebral edema, or delayed movement and cognitive syndromes. Gastroenterology involvement may be necessary for severe or prolonged diarrhea after products associated with delayed enterocolitis.[9,14]

The practical pre-infusion question is not only whether the patient is eligible for cellular therapy. It is whether the treating center has anticipated how that patient may declare toxicity, where the patient may present, and who will be contacted when the first abnormal vital sign or neurologic change appears.

Diagnostic Considerations: Treat CRS Without Missing Sepsis

A common early diagnostic error is framing fever after cellular therapy as either CRS or infection. In practice, it may be both. Neutropenia, mucosal barrier injury, indwelling catheters, lymphodepletion, prior therapies, corticosteroids, hypogammaglobulinemia, and prolonged cytopenias increase infectious risk.[2-4]

A clinically useful evaluation includes the vital-sign trajectory, oxygen requirement, volume status, mental status, infusion timeline, complete blood count with differential, comprehensive metabolic panel, coagulation studies, and fibrinogen when clinically indicated. CRP and ferritin may be followed as trend markers, but neither independently establishes the diagnosis or severity of CRS.

Blood cultures, urine and respiratory testing, lactate in unstable patients, and symptom-directed imaging should be obtained when appropriate. Electrocardiography, troponin, BNP or NT-proBNP, and echocardiography may be appropriate when shock, arrhythmia, chest pain, unexplained dyspnea, myocardial injury, or volume intolerance is present.

The differential diagnosis includes ICANS, HLH/MAS, tumor lysis syndrome, adrenal insufficiency, pulmonary embolism, transfusion reaction, anaphylaxis, disease progression, drug fever, and infusion-related reactions unrelated to cellular expansion. The diagnostic workup should not delay resuscitation, empiric antimicrobials when clinically indicated, or timely anticytokine therapy for clinically significant CRS.[1-4]

Therapeutic Principles

CRS management should be grade-based, product-specific, and responsive to patient physiology. Institutional pathways remain essential because product labels, clinical trial protocols, and toxicity-management practices differ. The following framework is intended for clinical orientation and does not replace current prescribing information or local cellular therapy protocols.[1-4,7-11]

Grade 1 CRS is generally managed with close monitoring, antipyretics, infection evaluation, and repeated reassessment. The product, day after infusion, comorbidity profile, disease burden, distance from the treating center, and ability to return rapidly for care determine whether monitoring can safely occur outside the hospital.

Grade 2 CRS usually warrants inpatient management or urgent transfer to the treating cellular therapy center. Supportive care includes supplemental oxygen, cautious intravenous fluids, cultures and empiric antimicrobials when clinically indicated, and consideration of tocilizumab according to the product label and institutional criteria.

In patients with heart failure, severe valvular disease, pulmonary hypertension, advanced kidney disease, or frailty, formally “fluid-responsive” hypotension may still carry substantial risk. Escalation should occur before repeated fluid boluses produce preventable pulmonary edema or respiratory failure.

Grade 3 or 4 CRS requires ICU-level care or an ICU-capable environment. Management commonly includes vasopressors, escalating respiratory support, organ support, tocilizumab, corticosteroids, and continuous reassessment for overlapping sepsis, ICANS, HLH/MAS, tumor lysis syndrome, cardiogenic shock, pulmonary edema, or adrenal insufficiency.[1-4]

Table 2. Practical CRS Management by Severity

Severity	Immediate priorities	Immune-directed treatment and escalation
Grade 1	Confirm the product and infusion date; evaluate for infection; monitor vital signs, oxygenation, and mental status	Supportive care is often sufficient; follow the product-specific protocol and escalate for persistent fever, hypotension, hypoxia, or neurologic change
Grade 2	Admit or urgently transfer; provide oxygen; use cautious fluids; obtain cultures and give antimicrobials when indicated	Tocilizumab is commonly considered; corticosteroid use depends on the product, protocol, trajectory, ICANS, and patient risk
Grade 3	Provide ICU-level care, vasopressor support, and high-flow oxygen or equivalent respiratory support	Tocilizumab and corticosteroids are generally used according to the product protocol; reassess urgently for worsening shock, organ injury, ICANS, and HLH/MAS
Grade 4	Provide full critical care, mechanical ventilation or other positive-pressure support, and multiple vasopressors as needed	Use urgent anticytokine therapy and corticosteroids with specialist oversight; investigate refractory shock, multiorgan failure, cerebral edema, and uncontrolled infection

Tocilizumab and Corticosteroids

Tocilizumab remains the principal FDA-approved anticytokine therapy for CAR T-cell-induced severe or life-threatening CRS. ACTEMRA prescribing information includes adults and pediatric patients aged 2 years and older. The labeled CRS regimen is intravenous and administered over 60 minutes.[6,7]

For patients weighing at least 30 kg, the dose is 8 mg/kg. For patients weighing less than 30 kg, the dose is 12 mg/kg. Tocilizumab may be administered alone or with corticosteroids. If clinical improvement does not occur after the first dose, up to 3 additional doses may be administered at intervals of at least 8 hours. Doses exceeding 800 mg per infusion are not recommended for CRS, and subcutaneous administration is not approved for CRS.[7]

The formal contraindication in the ACTEMRA label is known hypersensitivity. The label also warns against administration during an active infection and describes serious and sometimes fatal bacterial, mycobacterial, invasive fungal, viral, protozoal, and opportunistic infections in other treated populations.[7]

CRS creates a difficult clinical context because tocilizumab may be needed in patients who are already neutropenic, lymphodepleted, or receiving antimicrobials. Infection and CRS may also coexist. Tocilizumab suppresses acute-phase reactants, so fever and CRP responses may become less reliable after treatment. Bedside reassessment, cultures, imaging, organ-function trends, and appropriate antimicrobial management remain important.[2-4,7]

Corticosteroids are commonly used when CRS is persistent, recurrent, severe, or accompanied by concerning organ dysfunction, according to product-specific and institutional protocols. They are central to the management of clinically significant ICANS, particularly moderate, severe, progressive, or otherwise concerning presentations.[2-4]

Tocilizumab does not reliably treat ICANS itself and may not prevent neurotoxicity progression. Corticosteroid selection, dose, timing, and taper should follow the relevant product label and institutional protocol rather than a universal regimen.[2-4]

Earlier concerns that corticosteroids might uniformly compromise cellular therapy efficacy have given way to a more nuanced interpretation. Product-specific exploratory and observational studies suggest that earlier or prophylactic corticosteroid strategies do not necessarily eliminate antitumor response, but these findings cannot be generalized across every product, disease, dose, or timing strategy.[26]

For refractory CRS, clinicians should first revisit the diagnosis. Persistent shock or inflammation despite tocilizumab and corticosteroids may reflect uncontrolled infection, HLH/MAS, progressive malignancy, occult cardiopulmonary disease, adrenal insufficiency, or another inflammatory complication.

Anakinra, siltuximab, and other cytokine-directed approaches may be considered in selected refractory cases under specialist supervision. Their use is often off-label, product-specific, and supported largely by retrospective or observational data, institutional experience, or biological extrapolation rather than definitive randomized toxicity-management trials.[27]

Product-Specific and Platform-Specific Safety Issues

The current product label should be reviewed whenever a patient with recent cellular therapy presents for acute care. This is clinically necessary because indications, contraindications, monitoring instructions, warnings, and postmarketing safety findings can change.

For ABECMA, BREYANZI, CARVYKTI, KYMRIAH, TECARTUS, and YESCARTA, FDA eliminated the prior REMS in 2025. The change removed specific REMS certification and on-site tocilizumab requirements, but it did not remove boxed warnings, product-specific treatment requirements, safety monitoring, or the need for rapid escalation pathways.[12]

Secondary malignancy surveillance is part of the long-term safety framework for BCMA-directed and CD19-directed genetically modified autologous CAR T-cell therapies. Patients should be monitored lifelong. A suspected secondary malignancy should prompt manufacturer reporting and appropriate evaluation for the CAR transgene.[13]

CARVYKTI illustrates why current product-specific monitoring matters. Its July 2026 prescribing information includes boxed-warning risks involving CRS, neurologic toxicities, HLH/MAS, prolonged or recurrent cytopenias, immune effector cell-associated enterocolitis, and secondary hematologic malignancies.[9]

Immune effector cell-associated enterocolitis may develop weeks to months after CARVYKTI infusion. Reported manifestations include severe or prolonged diarrhea, abdominal pain, and weight loss that may require parenteral nutrition. Fatal outcomes from perforation or sepsis have occurred. Persistent gastrointestinal symptoms should prompt review of the cellular therapy history, infection evaluation, and gastroenterology and infectious diseases involvement according to institutional guidance.[9,14]

Engineered TCR therapy adds another layer of selection and toxicity complexity. Current TECELRA labeling indicates afamitresgene autoleucel for adults and pediatric patients aged 12 years and older with unresectable or metastatic synovial sarcoma after prior chemotherapy, specified HLA-A*02 positivity, and tumor MAGE-A4 expression confirmed by an FDA-approved or cleared companion diagnostic.[10]

TECELRA is contraindicated in patients who are heterozygous or homozygous for HLA-A*02:05P. The label also states that it should not be administered to patients with active infections or inflammatory disorders. CRS occurred in 72% of treated patients, including grade 3 or higher CRS in 2%, with a median onset of 2 days. Febrile neutropenia may occur concurrently with CRS, and immediate access to medications and resuscitative equipment is required.[10,18]

Tumor-infiltrating lymphocyte therapy requires attention to the entire treatment regimen. AMTAGVI is a tumor-derived autologous T-cell immunotherapy approved under accelerated approval for selected adults with unresectable or metastatic melanoma after PD-1 blockade and, for BRAF V600-positive disease, a BRAF inhibitor with or without a MEK inhibitor.[11,17,21]

CRS was reported in 3.2% of patients in the AMTAGVI label, but the boxed warning focuses on treatment-related mortality, prolonged severe cytopenia, severe infection, and cardiopulmonary and renal impairment. AMTAGVI must be administered in an inpatient hospital setting with an intensive care facility and appropriate specialists available.[11]

The relevant exposure is not only the infused cell product. The regimen also includes lymphodepleting chemotherapy followed by high-dose intravenous IL-2, or aldesleukin, beginning 3 to 24 hours after AMTAGVI infusion. Aldesleukin-related toxicity and the cumulative effects of the regimen are therefore central to clinical assessment.[11]

Table 3. Platform-Specific Toxicity Considerations

Platform or example	Verified safety nuance	Practical clinician action
CD19- or BCMA-directed CAR T-cell therapy	CRS and ICANS remain central acute toxicities; product timing, severity, and monitoring differ	Confirm the exact product and infusion date; review the current label and treating-center protocol
CARVYKTI	Delayed neurologic syndromes, HLH/MAS, cytopenias, secondary malignancies, and immune effector cell-associated enterocolitis require product-specific vigilance	Consider CARVYKTI history in prolonged diarrhea, weight loss, neurologic change, cytopenia, or late inflammatory illness
TECELRA	Boxed warning for CRS; HLA and MAGE-A4 selection are required; HLA-A*02:05P is a contraindication	Verify HLA eligibility and current indication; ensure immediate access to CRS treatment and resuscitative support
AMTAGVI	CRS is less frequent than in many CAR T-cell trials, but regimen toxicity includes lymphodepletion, cytopenias, infection, cardiopulmonary and renal impairment, and high-dose aldesleukin effects	Manage the complete cellular therapy regimen, not the cell infusion in isolation

A Practical Approach for Clinicians

When a patient presents with fever, hypotension, hypoxia, confusion, diarrhea, renal injury, arrhythmia, or shock after cellular therapy, first identify the exact product, infusion date, treating center, and current day after infusion.

Stabilization, infection evaluation, and empiric antimicrobial treatment when clinically indicated should proceed concurrently. Grade CRS using ASTCT criteria and document oxygen and vasopressor requirements clearly.[1-4]

Contact the cellular therapy team early, before deterioration. Review current prescribing information and institutional algorithms at the bedside. Treatment should be grade-based, but escalation thresholds must be individualized.

A patient with severe aortic stenosis, pulmonary hypertension, advanced chronic kidney disease, or frailty may require earlier ICU involvement than a younger patient with greater physiologic reserve and the same formal CRS grade.

Monitoring should include serial vital signs, oxygen requirements, fluid balance, mental status, laboratory trends, and reassessment after each intervention. A patient who does not improve after tocilizumab and corticosteroids should not simply receive repeated treatment without diagnostic reconsideration.

Persistent inflammation or organ dysfunction should prompt renewed evaluation for sepsis, HLH/MAS, ICANS, cardiopulmonary disease, occult bleeding, adrenal insufficiency, tumor lysis syndrome, and product-specific complications.[2-4,27]

Clinical Implications for Internal Medicine Subspecialists

CRS increasingly belongs to multiple disciplines. Cardiologists should anticipate vasodilatory shock, arrhythmias, myocardial injury biomarkers, stress cardiomyopathy-like presentations, and the hazards of excessive fluid resuscitation.

Pulmonologists and intensivists should treat CRS-related hypoxemia while evaluating for infection, pulmonary edema, embolic disease, diffuse alveolar injury, and disease-related lung involvement. Nephrologists should consider CRS in the context of acute kidney injury, tumor lysis syndrome, electrolyte abnormalities, and fluid-management constraints.

Infectious diseases clinicians should maintain diagnostic discipline because early CRS and sepsis may be clinically indistinguishable and may coexist. Neurologists should recognize that ICANS may occur before, during, or after CRS, and may require corticosteroids even when IL-6 blockade improves systemic findings.[1-4,9]

Gastroenterologists should be aware of delayed immune effector cell-associated enterocolitis, particularly after products with relevant label warnings. Severe or prolonged diarrhea should not automatically be attributed to routine infection, medication intolerance, or checkpoint inhibitor colitis without reviewing the cellular therapy history.[9,14]

Pharmacists play a central safety role. They help ensure timely access to tocilizumab, verify dosing and administration, reconcile corticosteroid pathways, review antimicrobial prophylaxis and treatment, identify interactions, assess renal and hepatic dosing, and improve transitions between cellular therapy centers and outside hospitals.

Limitations of the Evidence

CRS management is evidence-informed but is not uniformly supported by randomized toxicity-management trials. Many pivotal cellular therapy studies were single-arm trials in selected populations, and grading systems changed over time. Product labels may use trial-specific populations, definitions, treatment algorithms, and follow-up intervals, limiting direct comparison and generalizability.[1-5,15-25]

Management recommendations therefore combine consensus guidance, product labeling, protocol experience, retrospective studies, observational data, and biological plausibility. Evidence for prophylactic tocilizumab, early corticosteroids, anakinra, siltuximab, and other cytokine-directed strategies continues to evolve and cannot be generalized across all products.[2-4,26,27]

Observational evidence suggesting that earlier immunosuppression does not necessarily compromise antitumor response or cellular therapy efficacy should not be converted into an unrestricted claim of safety. Product, disease, cumulative dose, timing, infection risk, and the severity of toxicity remain important.[26]

Current institutional pathways, FDA labeling, and professional society guidance should be checked whenever protocols are updated. Cellular therapy indications and postmarketing warnings have changed rapidly, including material labeling changes during 2025 and 2026.[9-14]

Future Directions

Future CRS management will likely become more product-specific and risk-adapted. Areas of investigation include validated prediction models, biomarker-guided escalation, outpatient monitoring systems, earlier identification of patients who cannot tolerate aggressive fluid resuscitation, improved differentiation of CRS from infection, and preventive strategies that preserve antitumor activity.[5]

Engineering approaches may reduce toxicity through altered costimulatory domains, controllable activation systems, modified cytokine signaling, or more selective antigen recognition. These approaches remain product-specific and, in many cases, investigational.

The most important near-term improvement may be operational rather than pharmacologic. As cellular therapies spread across indications and care settings, outcomes will depend on whether emergency departments, ICUs, subspecialty services, and community hospitals can rapidly identify the product, grade the syndrome, contact the treating center, and initiate appropriate therapy.

References

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