03 Tumor Lysis Syndrome (TLS)

TLS is a syndrome caused by massive tumor cell lysis, releasing intracellular contents and metabolites that lead to electrolyte disturbances, including hyperuricemia, hyperphosphatemia, hypocalcemia, hyperkalemia, acute uric acid nephropathy, and even death [1]. Unlike CRS, TLS primarily occurs at the peak of T-cell activation, delayed relative to CRS. It typically occurs in patients with high tumor burden prior to treatment; therefore, tumor burden reduction through pretreatment is essential to prevent severe TLS. For patients with TLS, close monitoring of vital signs, fluid input/output, hepatic and renal function, and electrolytes is critical. Treatment follows physician orders including vigorous hydration, alkalinization, and diuresis. For hypocalcemia, manifestations include twitching and hand/foot numbness; when seizures occur, the patient should be placed flat with head turned to one side to maintain airway patency, and calcium gluconate should be administered by slow intravenous drip as ordered. For hyperkalemia, characterized by generalized weakness and arrhythmias [2], patients should be instructed to limit high-potassium foods and reduce consumption of potassium-rich fruits; calcium gluconate and insulin may be given as ordered [3]. Severe cases require early dialysis; proper dialysis catheter maintenance is essential to ensure patency. As shown in Figure 1, the "Expert Consensus on Clinical Management of Toxicities in CAR-T Cell Therapy for NHL," issued by the Biotherapeutic Committee of the Chinese Research Hospital Association, provides detailed recommendations for diagnosis, prevention, and treatment [1].

(Figure 1: Prevention and treatment of tumor lysis syndrome)

04 Macrophage Activation Syndrome (MAS) / Hemophagocytic Lymphohistiocytosis (HLH)

HLH/MAS is a severe immune dysregulation syndrome characterized by excessive activation of macrophages and lymphocytes, overproduction of pro-inflammatory cytokines, lymphohistiocytic infiltration, and immune-mediated multi-organ failure. Its clinical features and laboratory findings overlap with CRS, including high fever, multi-organ failure, elevated serum ferritin, lactate dehydrogenase, soluble CD25, cytokines (e.g., IFN-γ and IL-6), and decreased fibrinogen. Thus, HLH/MAS and CRS may represent similar systemic hyperinflammatory responses, making diagnosis of HLH/MAS in the context of CRS challenging.

IFN-γ promotes macrophage secretion of pro-inflammatory cytokines and generation of superoxide anions and oxygen free radicals, enhancing cytotoxic activity. While pro-inflammatory cytokines are essential for host defense, excessive activation and massive cytokine release can damage the host, sharing pathophysiological features with CRS. Therefore, MAS shares similar pathophysiological foundations with CRS and is easily overlooked. Treatment should include IL-6 receptor antagonists, corticosteroids, and symptomatic therapy. If symptoms do not improve within 48 hours, etoposide should be considered; for HLH-associated neurological toxicity, intrathecal cytarabine and/or hydrocortisone may be administered [4].

05 Peripheral Blood Cytopenia

Following CAR-T cell infusion, some patients may develop severe thrombocytopenia and neutropenia that do not recover within one month, potentially related to bone marrow hematopoietic suppression. Fried et al. [5] found that prolonged cytopenia was more common in patients with high-grade CRS (NCI 2017 criteria), and dysregulation of stromal cell-derived factor-1 (SDF-1) may contribute to neutropenia, possibly competing with B-cell recovery for hematopoietic factors. Nahas et al. [6] identified pre-lymphodepletion platelet count <75,000/μL and high-grade CRS on day 0 or day 1 post-infusion as independent predictors of persistent cytopenia.

The delayed hematopoietic recovery period is also the most common time for infections after CAR-T therapy. CRS severity is the only independent risk factor for infection; thus, controlling CRS is critical. However, tocilizumab use may exacerbate neutropenia and increase infection risk. Therefore, prophylactic antibiotics during the lymphodepletion phase may reduce life-threatening infections. Growth factor support and transfusions can be used to manage neutropenia, anemia, and thrombocytopenia. Patients with prolonged grade 4 neutropenia should consider antibacterial/antifungal prophylaxis. As shown in Figure 2, the "Expert Consensus on Clinical Management of Toxicities in CAR-T Cell Therapy for NHL" [1] provides management recommendations to ensure treatment continuity and reduce myelosuppression-related infections and complications.

(Figure 2: Grading and management of cytopenia)

06 B-Cell Aplasia / Hypogammaglobulinemia

B-cell aplasia/hypogammaglobulinemia is one of the characteristic adverse effects following CAR-T cell therapy. Nearly all patients receiving CAR-T cell therapy experience varying degrees of B-cell aplasia and associated infection risks due to humoral immune dysfunction [1]. Clinical manifestations include absolute B-cell count <61 cells/µL and IgG ≤400 mg/dL.

After anti-CD19 CAR-T therapy, particularly when CAR-T cells persist for >3 months, off-tumor adverse effects such as B-cell depletion and hypogammaglobulinemia may occur. B-cell recovery occurs within 24 months in 75% of patients with sustained responses, with some recovering as early as 9 months. To promptly detect B-cell aplasia, immunoglobulin levels should be monitored after CAR-T therapy. For high-risk populations (IgG ≤4 g/L, severe infection, persistent or recurrent infection), regular monitoring of serum IgG, IgM, IgA, and peripheral blood CD19⁺ or CD20⁺ B-cell counts is recommended.

 Prophylaxis

· Prophylactic antibiotics and immunoglobulin replacement therapy may be administered to prevent recurrent infections.

Immunoglobulin Replacement Therapy²

(Figure 3: Prevention and treatment of B-cell aplasia)

07 Opportunistic Infections

Risk factors for opportunistic infections include prior ASCT/allo-HSCT, bridging therapy, and corticosteroid/tocilizumab use for CRS/ICANS management. Most early infections (within 30 days) are bacterial or respiratory viral; invasive fungal infections are rare. After 30 days, viral infections predominate. The negative impact of early G-CSF administration after CAR-T therapy has not been definitively established; a recent report of G-CSF starting on day 5 post-infusion showed no increase in CRS or ICANS, suggesting early use may be safe and may shorten neutropenia duration. Further data are needed to support this observation. For patients with febrile neutropenia, empirical broad-spectrum antibiotic therapy is strongly recommended [1].

(Figure 4: Management strategies for opportunistic infections)

08 On-Target/Off-Tumor Effects

The ideal target antigen is one expressed exclusively on tumor cells (tumor-specific antigen, TSA), providing essential survival signals to malignant clones. Unfortunately, most CART cell targets are tumor-associated antigens (TAAs) shared with normal tissues to varying degrees. CAR-T cells, while killing tumor cells, may also attack normal tissues expressing the TAA, causing the so-called "on-target/off-tumor" adverse effects. These can range from mild single-lineage deficiencies such as B-cell aplasia to severe adverse events and death, and are predictable in many organs including the gastrointestinal tract, hematopoietic system, and respiratory system.

Currently, few TSAs are known, with limited CAR-T trials targeting PSMA and EGFRvIII. Exploring strategies to avoid off-tumor effects has become a research focus. Current approaches include: (1) Dual-target and multi-target antigen development – designing CARs requiring two distinct target signals for activation, thereby reducing damage to normal cells; (2) Incorporating suicide genes or inhibitory genes into CAR-T cells to enable drug-induced apoptosis when adverse events occur [7,8]. Off-tumor effects are managed symptomatically, e.g., immunoglobulin replacement for hypogammaglobulinemia. However, offtumor effects may reflect CAR-T cell activity and provide insight into their in vivo function.

09 Other Adverse Events

Allergic Reactions: Currently used transgenic T cells contain antigen-recognition domains, mostly derived from murine monoclonal antibodies. Therefore, CAR-T cell infusion may elicit cellular and humoral immune responses due to the immunogenicity of foreign proteins. Efforts are underway to humanize the expressed proteins to achieve durable and robust clinical antitumor effects. More acute reactions occur when the host recognizes infused foreign components, leading to anaphylactic reactions.

Insertional Mutagenesis: The U.S. FDA has required a black-box warning for secondary T-cell malignancies on approved CAR-T therapies, but the benefits of CAR-T therapy still outweigh the risks. Over a dozen experts, including CAR-T pioneers Bruce L. Levine and Carl H. June from the University of Pennsylvania, published an article in Nature Medicine titled "Unanswered questions following reports of secondary malignancies after CAR-T cell therapy," noting that among an estimated 34,400 patients treated with CAR-T, the observed incidence of T-cell malignancies is far lower than that of some other therapies [9]. In fact, any chemotherapy, radiotherapy, or targeted immunotherapy carries a risk of secondary malignancies, but this probability is very low, approximately 0.1%-1%. This risk is considered acceptable for both clinicians and terminally ill patients.

Viral Reactivation: B-cell-directed agents may cause hepatitis B virus (HBV) reactivation, leading to fulminant hepatitis, liver failure, and death. HBV infection (HBsAg positive) in DLBCL ranges from 25% to 61%, raising concerns about HBV reactivation after CAR-T therapy in these patients. Yang et al. [10] found that 20% (3/15) of patients with chronic HBV infection experienced HBV reactivation after CAR-T therapy, but this did not affect treatment efficacy. To avoid viral reactivation, screening for HBV, HCV, and HIV should be performed before mononuclear cell collection according to routine clinical practice, with regular follow-up and antiviral therapy as indicated.

References

[1] Biotherapeutic Committee of the Chinese Research Hospital Association. Expert consensus on clinical management of toxicities in CAR-T cell therapy for NHL. Journal of Translational Medicine. 2021;10(1):1-11. [in Chinese]

[2] Ding YG, Zhao W, Wu XY, et al. Nursing care of adverse reactions in children with neuroblastoma after chimeric antigen receptor-modified T-cell infusion. Chinese Journal of Nursing. 2017;52(3):307-310. [in Chinese]

[3] Yang CC, Zhong ZQ, Luo YM, et al. Nursing care of patients with refractory multiple myeloma undergoing two CAR-T cell therapies. Journal of Nursing Science. 2019;34(4):34-36. [in Chinese]

[4] Schram AM, Nancy B. How I treat hemophagocytic lymphohistiocytosis in the adult patient. Blood. 2015;125(19):2908-2914.

[5] Fried S, Avigdor A, Bielorai B, et al. Early and late hematologic toxicity following CD19 CAR-T cells. Bone Marrow Transplant. 2019;54(10):1643-1650.

[6] Nahas GR, Komanduri KV, Pereira D, et al. Incidence and risk factors associated with a syndrome of persistent cytopenias after CAR-T.

[7] Russo V, Bondanza A, Ciceri F, et al. A dual role for genetically modified lymphocytes in cancer immunotherapy. Trends Mol Med. 2012;18(4):193-200.

[8] Straathof KC, Pule MA, Yotnda P, et al. An inducible caspase 9 safety switch for T-cell therapy. Blood. 2005;105(11):4247-4254.

[9] Levine BL, Pasquini MC, Connolly JE, et al. Unanswered questions following reports of secondary malignancies after CAR-T cell therapy. Nat Med. 2024 Feb;30(2):338-341. doi: 10.1038/s41591-023-02767-w. PMID: 38195751.

[10] Yang C, Xie M, Zhang K, et al. Risk of HBV reactivation post CD19-CAR-T cell therapy in DLBCL patients with concomitant chronic HBV infection. Leukemia. 2020. doi: 10.1038/s41375-020-0913-y.