The most common CAR-T-related toxicities include cytokine release syndrome (CRS) resulting from immune activation; neurotoxicity, also referred to as immune effector cell-associated neurotoxicity syndrome (ICANS); cytopenias; and infections (Fig. 2). Less common toxicities include secondary T-cell malignancies and “on-target/off-tumor” (i.e., recognition of the target antigen on normal cells) and “off-target/off-tumor” (i.e., recognition of an unrelated antigen; cross-reactivity) toxicities [43, 45,46,47,48].

Overview of the main toxicities associated with CAR-T therapies. CAR-T; chimeric antigen receptor T cell; CRS, cytokine release syndrome; ICANS, immune effector cell-associated neurotoxicity syndrome

CAR-T infusion can lead to CRS, ICANS, cytopenias, and infections. CAR-T causes inflammation that can lead to CRS and ICANS. CRS may lead to ICANS, cytopenias, and infections. In addition, lymphodepletion may lead to cytopenias and infections as well.

The incidence and severity of toxicities associated with CAR-T therapies vary largely between studies, likely due to various CAR-T types, the infusion time and dose, and co-administration of other therapies (Fig. 3).

Incidence and severity of the most common toxicities associated with CAR-T therapies. The circles represent the incidence reported in individual studies. Data sources are listed in Electronic Supplementary Material 1; the individual studies were published between 2014 and 2023 for CRS and ICANS, and between 2017 and 2023 for cytopenias and infections. CAR-T; chimeric antigen receptor T cell; CRS, cytokine release syndrome; ICANS, immune effector cell-associated neurotoxicity syndrome

Figure 4 provides an overview of clinical laboratory parameters that are associated with the risk of developing each of these toxicities.

Clinical laboratory parameters correlated with the risk of developing CAR-T-associated toxicities. Ang, angiopoietin; CA, catecholamines; CAR-T, chimeric antigen receptor T cell; CRP, C-reactive protein; CRS, cytokine release syndrome; F, factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; ICANS, immune effector cell-associated neurotoxicity syndrome; IFN, interferon; IL, interleukin; MCP, monocyte chemoattractant protein; MIP, macrophage inflammatory protein; pTF, plasma tissue factor; sE-selectin, soluble E-selectin; sICAM, soluble intercellular adhesion molecule; TNF, tumor necrosis factor; VCAM, vascular-cell adhesion molecule; vWF, von Willebrand factor

CRS is one of the most common life-threatening toxicities associated with engineered CAR-T therapies that was not observed in preclinical models, but first noted in subsequent clinical studies [43]. CRS is a systemic inflammatory response that can be induced by the binding of CAR-T cells to a specific antigen on the surface of target cells, inducing the release of cytokines such as interferon (IFN)-γ or tumor necrosis factor (TNF)-α. Subsequent activation of bystander immune and non-immune cells, such as monocytes, macrophages, dendritic cells, and endothelial cells, results in the hypersecretion of proinflammatory cytokines, initiating a cascade of events leading to CRS (Fig. 5) [49, 50].

Sequence of events leading to cytokine release syndrome, adapted from Cosenza et al. Int J Mol Sci. 2021;22:7652 [49]. CAR-T cells bind to the tumor cells and induce the release of cytokines such as IFN-γ or TNF-α, leading to the activation of bystander immune and non-immune cells, which further release proinflammatory cytokines, triggering a cascade reaction in which high levels of released IL-6 activate T cells and other immune cells, leading to a cytokine storm. CAR-T, chimeric antigen receptor T cell; DC, dendritic cell; GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; IL, interleukin; MCP, monocyte chemoattractant protein; NK, natural killer; TNF, tumor necrosis factor

The incidence and severity of CRS vary depending on the CAR-T product, dose, and disease burden at the time of infusion [51, 52]. Symptoms of CRS may occur immediately after the administration of CAR-T cells or may be delayed until days or weeks after treatment, with a median time of onset of 2–3 days following CAR-T infusion [51, 53, 54]. The severity of CRS ranges from mild, flu-like symptoms, including fever and chills, to severe and life-threatening symptoms, including hypotension, tachycardia, pleural effusion, pulmonary edema, capillary leak syndrome, and hypoxia, which may ultimately lead to multisystem organ failure and death [51, 55].

When treated effectively, CRS is manageable. Nevertheless, following CAR-T infusion, patients should be closely monitored at the hospital for at least 7 days, including daily assessments of biochemistry and blood counts, review of organ systems and physical exam, and assessment of vital signs at least every 4 h [56,57,58]. The Society for Immunotherapy of Cancer recommends daily monitoring of clinical laboratory values associated with CRS for a postinfusion period related to a specific CAR-T product (typically several weeks); in addition, patients with a high disease burden require specific attention, including cardiac function monitoring [59].

The current gold standard of CRS treatment is anti-cytokine therapy that is initiated as soon as symptoms of CRS begin to occur to prevent progression to severe CRS [53]. Tocilizumab, an IL-6 antagonist, has been approved by the US FDA for the treatment of CRS occurring after CAR-T therapy [14, 53] and is commonly used in the management of CRS [60, 61]. Prophylactic tocilizumab prior to administration of T-cell therapies has also been evaluated. For instance, in a recent single-center study, the use of prophylactic tocilizumab prior to the administration of another T-cell redirecting therapy, based on the bispecific antibody teclistamab, decreased the incidence and severity of CRS; CRS occurred in 26.3% of patients who received prophylactic tocilizumab versus 73.3% of patients who did not receive prophylactic tocilizumab [62]. In another study, among 20 patients with non-Hodgkin lymphoma who received prophylactic tocilizumab 1 h prior to infusion of anti-CD19 CAR-T cells, only low-grade CRS was observed in 50% of patients, indicating that prophylactic tocilizumab is a viable option to mitigate high grade CRS [63]. Another anti-IL-6 monoclonal antibody, siltuximab, has also been used, although it has not been approved by the US FDA for treatment of CRS following CAR-T therapy [53, 64, 65]. Other agents for the treatment of CRS include corticosteroids, dasatinib, A3 adenosine receptor agonists, JAK/STAT pathway inhibitors, or lenzilumab [53, 66].

CRS should be managed according to the toxicity grade [59, 67, 68]. The American Society of Clinical Oncology (ASCO) guidelines recommend mostly supportive care for grade 1 CRS and tocilizumab for higher grades; steroids may be considered earlier in treatment depending on the CAR-T product [67].

Several clinical trials focusing on CRS treatment are ongoing (Electronic Supplementary Material 2).

The broad use of CAR-T therapy necessitates the identification of clinical laboratory parameter values associated with the risk of developing severe CRS. Several studies showed that severe CRS following CAR-T therapy is associated with an increase in certain cytokine levels [69,70,71,72,73]. Such cytokine activation profiles could be established following CAR-T infusion to help mitigate the severe effects, and close monitoring of those levels is suggested to be a valuable tool in assessing the risk of CRS [43]. The main cytokines implicated in the pathogenesis of CRS include IL-1, IL-2, IL-5, IL-6, IL-8, IL-10, IFN-γ, monocyte chemoattractant protein 1 (MCP-1), macrophage inflammatory protein-1α, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (Fig. 4) [51, 74,75,76,77,78]. Higher levels of these cytokines have been shown to be associated with severe CRS, indicating their potential as clinically relevant diagnostic predictors for high-grade CRS [70, 71, 79,80,81]. Among these, high serum IL-6 levels are detected in nearly all patients with CRS [49], and several studies found these levels to be associated with the severity of CRS after CAR-T therapy [69, 70, 72, 73, 79, 82,83,84,85]. Furthermore, an increase in C-reactive protein (CRP) levels correlating with increased IL-6 levels has also been detected in patients with CRS [68, 69, 82, 86,87,88,89]; elevated ferritin levels have also been shown to correlate with CRS [72, 90, 91] (Fig. 4). Surrogate markers of systemic inflammation could identify patients with CRS and potentially be used to guide intervention to either suppress cytokine release or eliminate CAR-T cells in the event of severe toxicity. While the results of a full cytokine panel may not be promptly available in many hospitals, CRP and ferritin may serve as surrogate markers of CRS. In addition, monitoring lactate dehydrogenase levels, complete blood count, coagulation, and uric acid levels is also recommended [51, 92].

Other, less common clinical laboratory parameters have also been shown to be predictive of the occurrence and severity of CRS, including coagulation parameters; levels of the plasma tissue factor, Factor X, Factor XII, and P-selectin [93]; angiopoietin (Ang)-2 and von Willebrand factor (vWF); soluble E‐selectin and soluble intercellular adhesion molecule-1 [71, 94]; or catecholamines [95]. Furthermore, a regression modeling study predicted a 3-cytokine signature (i.e., IFN-γ, IL-6, and soluble IL-2 receptor α) associated with progression to severe CRS [70].

Clinical factors that increase the risk of developing higher-grade CRS include disease burden and marrow involvement, common lymphodepletion regimens such as fludarabine and/or cyclophosphamide, and higher CAR-T infusion doses. Patient-specific factors include age, bulky disease, comorbidities, early-onset CRS (within 3 days of cell infusion), pre-existent inflammation, and baseline thrombocytopenia [53, 56, 69, 72, 86, 96]. Of note, high tumor burden (≥ 40% lymphoblasts) in the bone marrow has been identified as the major risk factor for severe CRS [72, 97]. Although elevated levels of the abovementioned cytokines in the blood of patients following CAR-T infusion are generally considered predictors of CRS, individual patient factors might affect these levels, making it difficult to predict the grade of CRS in these individuals. Furthermore, a certain level of IL-6, for instance, might be associated with grade 3 CRS in one patient but not in another, indicating that other individual patient factors need to be considered. Such factors, as well as the factors contributing to the extent of cytokine release, e.g., the CAR-T type, dose, and their abundance in the patient, may help with predicting the effects of cytokine release.

As patients with a higher baseline disease burden are likely to have more severe CRS, imaging techniques such as positron emission tomography–comp