The development of CRS in patients treated with CAR-T cells is very frequent, with an incidence ranging from 48% to 93% in different studies and from 3% to 48% for severe CRS. The syndrome is somewhat less frequent in patients treated with bispecific antibodies (7%–72% and 17%–29%, respectively).6 The significant variation observed among different studies is due, not only to differences in the treatments administered and the patient’s clinical context, but also to the various staging systems used before 2019.7 Patients treated with CAR-T cells will often require ICU admission due to the development of CRS. Two multicenter studies, one European8 and the other American,9 have described ICU admission rates of 27% and 35%, respectively. The most frequent manifestations were arterial hypotension (70%) and respiratory failure (10%).8 ICU mortality was less than 5% in both studies.

Several risk factors for the development of CRS have been identified. Some are intrinsic to the patient's disease, such as tumor burden, tumor type, presence of basal inflammation, pre-treatment thrombocytopenia, and early cytokine elevation. Others are directly related to the administered treatment. In CAR-T therapy, the type of construct involved, the cell dose infused, the degree of expansion, and the lymphodepletion protocol all influence the risk of developing CRS.10–13

CAR-T cells are T-cells from the patient to which a chimeric antigen receptor (CAR) has been added ex vivo. This receptor allows the cells to specifically recognize a tumor antigen and subsequently activate it, thanks to the presence of co-stimulatory molecules.13 Bispecific and trispecific antibodies, on the other hand, bind to an antigen of the tumor cell on one end and to CD3 on the T-cells the other end. This brings the patient's T-cells into close contact with the tumor cells, facilitating their identification and destruction.14

The recognition of neoplastic cells by CAR-T cells and bispecific and trispecific antibodies activates the T-cells. These activated T-cells produce effector cytokines such as interferon-γ (IFN- γ), tumor necrosis factor-α (TNF-α), and granulocyte-monocyte colony-stimulating factor (GM-CSF). This stimulates their proliferation and the recruitment of other immune system cell lines, including B-cells, NK cells, monocytes, macrophages, and dendritic cells. This results in the release of multiple cytokines.15–18 The destruction of the tumor cells contributes to the release of their own cytokines and the exposure of damage-associated molecular patterns (DAMPs), which are recognized by macrophages and amplify the response.17 Among the mediators involved in CRS, interleukins 1 and 6 (IL-1 and IL-6) play a central role in its pathophysiology,19 as demonstrated in several animal20 and clinical studies.21

This excessive immune-inflammatory response leads to endothelial and complement activation, activation of the coagulation cascade, and increased capillary permeability, which can result in multiorgan dysfunction. There is an increase in the production of endothelial growth factor, which internalizes endothelial cadherin — a structural protein that mediates cell adhesion — generating a state of capillary leakage. The activated endothelium also contributes to IL-6 production, amplifying the response15,22–26 (Fig. 1).

Figure 1.

Physiology of cytokine release syndrome (CRS). BiTe: bispecific T-cell engager (activator); CAR: chimeric antigen receptor; VEGF: vascular endothelial growth factor; TF: tissue factor; IFN-γ: interferon gamma; IL-1: interleukin-1; IL-6: interleukin-6; GM-CSF: granulocyte-monocyte colony-stimulating factor; TNF-α: tumor necrosis factor alpha.

Source: adapted from García Roche et al.26

CRS is characterized by the presence of fever and constitutional symptoms. It is often accompanied by arterial hypotension (67%) and/or respiratory failure (20%),27 although it can also be associated with organ dysfunction in multiple systems, including the gastrointestinal, hepatic, renal, cardiac, cutaneous, and hematological systems. Neurological manifestations are considered a distinct entity known as immune effector cell-associated neurotoxic syndrome (ICANS),5 and appear within the first 14 days after CAR-T cell infusion,28 though they typically begin within the first 5–7 days.17 In contrast, the onset is earlier among patients treated with specific antibodies due to differences in their pharmacokinetics.29 The severity of symptoms can range from flu-like manifestations, with fever and constitutional symptoms, to multi-organ failure leading to the death of the patient.5

The diagnosis of CRS is based on the presence of compatible clinical manifestations following the administration of certain immune therapies, as well as the exclusion of other possible causes, such as infections,30 tumor lysis syndrome,28 hematological disease progression,30 or HLH.31

For patient management, it is important to use an internationally validated staging system. In the early years of advanced therapy development, a variety of coexisting grading systems were used,19,32–34 which made it difficult to compare the severity of CRS across different studies. Since 2019, however, unified criteria have been proposed by the American Society for Transplantation and Cellular Therapy (ASTCT).5 According to these criteria, CRS is staged into four grades based on the presence of fever, arterial hypotension, and/or hypoxemia (Table 1).

When using the ASTCT grading system, it is important to note that fever, arterial hypotension, and/or hypoxemia cannot be attributed to any cause other than CRS. The grade of CRS is determined by the most severe event, regardless of specific organic toxicities. For patients with CRS who have received antipyretic and/or anticytokine treatment (e.g., tocilizumab or corticosteroids), the presence of fever is no longer required for staging. In these cases, the severity of CRS is determined by arterial hypotension and/or hypoxemia.5

The treatment of CRS requires a holistic and multidisciplinary approach. Different measures are adopted depending on the stage of the disease11,13,35–37 (Fig. 2).

Figure 2.

CRS management algorithm. i.v.: intravenous; s.c.: subcutaneous; IMV: invasive mechanical ventilation; NIMV: noninvasive mechanical ventilation.

Source: own elaboration.

Grade 1 CRS: Management in the Hematology Department is advised, along with conventional supportive care, such as antipyretic medication and fluid therapy. A differential diagnosis should be made with other syndromes, especially with infectious processes, initiating empirical antibiotic therapy (with an antipseudomonal spectrum) and continuing the recommended prophylactic treatment depending on the type of immunotherapy the patient is receiving. If the patient has persistent fever for more than 24 h, is at high risk of an unfavorable outcome (due to advanced age, significant comorbidities, and/or severe biochemical alterations), or does not improve within the first 72 h, the early administration of intravenous (i.v.) tocilizumab 8 mg/kg (one-hour infusion without premedication, up to a maximum of 800 mg per dose, with a maximum of 3 doses within 24 h and a maximum of 4 doses) should be considered. Tocilizumab binds to both soluble and membrane-bound IL-6 receptors, inhibiting signaling mediated by both.35–37 If there is a high risk of developing neurotoxicity, it is advisable to administer a single intravenous dose of 10 mg of dexamethasone when tocilizumab is employed.

Grade 2 CRS: The ICU should be notified to assess the patient initially in the Hematology ward. Then, a joint decision should be made as to whether a transfer is required. Supportive measures include administering low-flow oxygen to maintain an oxygen saturation level (SpO2) > 90%, as well as fluid resuscitation to maintain a systolic blood pressure (SBP) > 90 mmHg and diastolic blood pressure (DBP) > 60 mmHg. Due to the high risk of capillary leak syndrome and/or acute respiratory distress syndrome, caution should be exercised with fluid administration. Hemodynamic monitoring is advised to individualize resuscitation. Tocilizumab should be started, and if no clinical improvement is observed, dexamethasone i.v. 10 mg every 6−8 h can be initiated.11,35–37

Grade 3 CRS: If the patient has not been transferred to the ICU, it is advisable to do so. If the mean arterial pressure (MAP) is <65 mmHg, support with vasopressor drugs should be innitiated. Administer oxygen via high-flow therapy and/or start noninvasive mechanical ventilation to maintain a SpO2 > 90%. Initiate treatment with dexamethasone i.v. at a dose of 10 mg every 6 h if it has not been started previously. Corticosteroid treatment will be maintained for 1–3 days and/or until CRS decreases to grade 1.11,13,35–37 Maintaining tocilizumab i.v. every 8 h (up to a maximum of 4 doses) may be considered. If refractoriness occurs, increase the dose of intravenous dexamethasone to 20 mg/6 h (for 1–3 days).35 Consider starting subcutaneous (s.c.) anakinra 8 mg/kg/day divided into 4 doses, for a variable duration depending on the response.13,36 Anakinra competitively inhibits the binding of IL-1α and IL-1β to the interleukin-1 type I receptor. The addition of thiamine i.v. 500 mg every 8 h (first 72 h) followed by 500 mg every 24 h (until resolution of CRS),37 may also be considered.

CRS grade 4: Patient management should be performed in the ICU in all cases. It is advisable to intensify vasoactive and respiratory support, as well as increase the dexamethasone dose to 20 mg intravenously every 6 h for 3 days. If CRS improves, gradually withdraw the medication over 3–7 days. If refractoriness occurs, start a regimen of methylprednisolone i.v. at 1000 mg/24 h for 2 days, followed by 250 mg/12 h for 2 days, then 125 mg/12 h for 2 days, and finally 60 mg/12 h for 2 days (with subsequent suspension).13,36

Some patients may present with severe CRS (grades 3–4) that does not respond to the recommended treatment, even with the optimal use of tocilizumab and corticosteroids.38 In these cases, the available scientific evidence is limited.

In this regard, the literature provides positive data on various interventions, though publication bias cannot be ruled out. Immunomodulatory drugs, such as anakinra,39 siltuximab,40 ruxolitinib,41 emapalumab,42 or etanercept43 have been tested, with good results. Anakinra is the drug for which the most experience exists, and it is advisable to administer it via the intravenous route (instead of s.c.) to patients with edema. Another possibility is the use of treatments that block CAR-T in a reversible (dasatinib)44 or irreversible (cyclophosphamide)45 manner. Favorable responses to extracorporeal therapies such as hemofiltration,46 hemadsorption,47 or plasma exchange48 have also been described.

It is also essential to rule out other conditions that could be either an alternative diagnosis or a contributing factor, such as infection/sepsis, immunotherapy-associated HLH, or progression of the underlying oncological disease.

While waiting for the patient to respond to the initiated treatments, supportive management of multiorgan dysfunction should be established. In the absence of specific recommendations, it seems logical that the guidelines for monitoring and treating refractory septic shock could be applied to these patients,49 given the underlying pathophysiology.

IEC-HS is caused by a defect in the mechanisms that inhibit inflammation. This defect triggers the uncontrolled release of IFN-γ, which activates macrophages with hemophagocytosis and the release of inflammatory cytokines (ferritin, TNF-α, and IL-6, among others). This results in an exaggerated inflammatory response that contributes to tissue damage and multiorgan failure (Fig. 3).

Figure 3.

Pathophysiology of HLH.

Source: own elaboration.

Since our focus is on HLH secondary to immune effector cell therapies treatment, we will henceforth refer to the syndrome as IEC-HS.

In IEC-HS, the immune effector cells are activated upon the recognition of antigens presented by tumor cells. This leads to tumor cell lysis and expansion of the immune effector cell population, followed by proliferation and sustained cytokine release. Risk factors for developing HS-ICD include the hematological disease itself (tumor burden, proliferation dynamics, or immune resistance mechanisms), treatment (target antigen, co-stimulators, or cell dose), and patient-specific factors (baseline inflammation, immunosuppression, cytopenias, or genetic predisposition).4,31,50 IEC-HS and CRS thus share pathophysiological mechanisms. In fact, IEC-HS is usually preceded by CRS, though not always. It is difficult to determine when the transition occurs, since the two conditions can also share clinical manifestations such as fever, arterial hypotension, and hypoxemia. IEC-HS has characteristic alterations, however.50

The typical clinical presentation of IEC-HS includes fever, lymphadenopathy, and hepatosplenomegaly. With regard to the laboratory test findings, cytopenias and elevated transaminase levels are common. More specific alterations include elevated ferritin, triglycerides, and soluble IL-2 receptor, decreased fibrinogen, and the presence of hemophagocytosis in various organs, particularly the bone marrow and liver.50

Patients with IEC-HS usually require ICU admission due to acute respiratory failure and/or distributive shock. Other frequent causes of admission include acute renal failure, encephalopathy, acute liver failure, hemorrhagic complications, and multiorgan failure.4,31,50–53

The ICU mortality rate among patients presenting with HLH generally varies between 40%–70%.51–53 Factors associated with mortality include patient age, the presence of hemophagocytic cells in the bone marrow aspirate, multiorgan failure on admission, and an SOFA score increase during ICU stay.50

Patient management consists of immunomodulatory (etiological) treatment and multi-organ supportive therapy.

Regarding immunomodulatory treatment specifically, it should be noted that IEC-HS is typically accompanied by CRS or ICANS symptoms, meaning the patient has likely already received tocilizumab (8 mg/kg i.v.) and corticosteroids.

In cases of suspected progression of CRS to IEC-HS, anakinra is suggested as the first-line treatment, although the existing evidence is scarce. There is no consensus on the appropriate dosage. High doses of anakinra (>200 mg/day i.v.) in the form of 100−200 mg every 6–12 h i.v. could be used safely in severe cases, and may result in a superior response compared to low doses (100−200 mg/day s.c.).60 This treatment can be combined with corticosteroids (dexamethasone 10 mg every 6 h i.v.).31,59 In some case series, anakinra treatment lasted up to 7 days.61

Some authors prefer not to extend the anakinra treatment. If no improvement is observed within 48 h (clinically and biochemically, with triglyceride and IL-2 determinations), they suggest switching to another strategy, such as ruxolitinib. Ruxolitinib is a selective inhibitor of Janus kinases (JAKs) JAK1 and JAK2, which mediate the signaling of various cytokines and growth factors that are important for hematopoiesis and immune function. Ruxolitinib suppresses the macrophage overstimulation that characterizes IEC-HS in murine models.62,63 The suggested dose is 0.3 mg/kg/day orally, distributed in two doses (10 mg/kg/12 h, for an adult weighing 70 kg),31 although dosage adjustment is necessary when co-administered with CYP450 inhibitors.

Other therapeutic options that may be considered include:

• •

  Etoposide: A topoisomerase-II inhibitor that effectively induces DNA damage and subsequent apoptosis in hyperactivated immune cells. It has been widely used to treat HLH, usually at doses of 75−100 mg/m2/4–7 days.58,64

• •

  Intrathecal cytarabine (100 mg) with or without intrathecal hydrocortisone (50−100 mg) if there is associated ICANS.58

• •

  Emapalumab: This is a human monoclonal antibody directed against IFN-γ. It is the first FDA-approved therapy for pHLH (pending approval in Europe).65

• •

  A recent multicenter clinical trial (phase 2, non-randomized) has demonstrated the benefit of combining ruxolitinib (0.3 mg/kg/day for 5 days) with doxorubicin (25 mg/m2, on day 1), etoposide (100 mg/m2, on day 1) and methylprednisolone (2 mg/kg/day, on days 1 and 5 of treatment) as IEC-HS rescue therapy.66