T cells can be genetically modified to target tumors through the expression of a chimeric antigen receptor (CAR). Most notably, CAR T cells have demonstrated clinical efficacy in hematologic malignancies with more modest responses when targeting solid tumors. However, CAR T cells also have the capacity to elicit expected and unexpected toxicities including: cytokine release syndrome, neurologic toxicity, “on target/off tumor” recognition, and anaphylaxis. Theoretical toxicities including clonal expansion secondary to insertional oncogenesis, graft versus host disease, and off-target antigen recognition have not been clinically evident. Abrogating toxicity has become a critical step in the successful application of this emerging technology. To this end, we review the reported and theoretical toxicities of CAR T cells and their management.

The efficacy of chimeric antigen receptor (CAR) T cell therapy against poorly responding tumors can be enhanced by administering the cells in combination with immune checkpoint blockade inhibitors. Alternatively, the CAR construct has been engineered to coexpress factors that boost CAR-T cell function in the tumor microenvironment. We modified CAR-T cells to secrete PD-1-blocking single-chain variable fragments (scFv). These scFv-secreting CAR-T cells acted in both a paracrine and autocrine manner to improve the anti-tumor activity of CAR-T cells and bystander tumor-specific T cells in clinically relevant syngeneic and xenogeneic mouse models of PD-L1 hematologic and solid tumors. The efficacy was similar to or better than that achieved by combination therapy with CAR-T cells and a checkpoint inhibitor. This approach may improve safety, as the secreted scFvs remained localized to the tumor, protecting CAR-T cells from PD-1 inhibition, which could potentially avoid toxicities associated with systemic checkpoint inhibition.

The engineered expression of chimeric antigen receptors (CARs) on the surface of T cells enables the redirection of T-cell specificity. Early clinical trials using CAR T cells for the treatment of patients with cancer showed modest results, but the impressive outcomes of several trials of CD19-targeted CAR T cells in the treatment of patients with B-cell malignancies have generated an increased enthusiasm for this approach. Important lessons have been derived from clinical trials of CD19-specific CAR T cells, and ongoing clinical trials are testing CAR designs directed at novel targets involved in haematological and solid malignancies. In this Review, we discuss these trials and present strategies that can increase the antitumour efficacy and safety of CAR T-cell therapy. Given the fast-moving nature of this field, we only discuss studies with direct translational application currently or soon-to-be tested in the clinical setting.

Summary Sotillo and colleagues describe the molecular events associated with apparent loss of target antigen expression following CAR T-cell therapy. We propose that broader immune activation is required to prevent outgrowth of tumor antigen escape variants following targeted therapies.

survival (OS) and progression free survival (PFS) in MM patients. Furthermore, the preclinical activity of a new GPRC5DxCD3 bispecific antibody (JNJ-7564) in development for the treatment of MM was evaluated, as well as biomarkers for in vitro JNJ-7564 response. Methods: GPRC5D protein expression was assessed by flow cytometry on BM MNCs derived from healthy donors (HD) and MM patients. GPRC5D gene expression levels were analyzed in purified CD138 + MM cells derived from patients who participated in 5 large randomized clinical trials (HOVON65, MRC-IX, TT2, TT3 and APEX). MM cell lysis by JNJ-7564 (0.00064-4 µg/ml; 48 hour-incubation) was evaluated in MM cell lines and whole BM samples from newly diagnosed (ND) and relapsed/refractory (RR) MM patients. At baseline, the MNCs were characterized for the composition of T-cell subsets. T-cell activation and degranulation were measured by flow cytometry based on expression of CD25 and CD107a, respectively. Results: GPRC5D protein expression was significantly higher on MM cells compared to other BM cells, including HD plasma cells (fig 1A). GPRC5D protein expression was positively correlated with BCMA expression (r = 0.44; P = 0.02), but was independent of tumor load, and CD38 or PD-L1 expression on MM cells. Gene expression levels of GPRC5D were highly variable in MM, and independent of age and ISS stage, but were significantly higher in patients with t(4;14) or gain 1q. There was no association with OS or PFS in the 5 clinical trials (n = 1421). JNJ-7564 effectively killed GPRC5D + MM cell lines (MM1.S, UM9 (fig 1B) and RPMI8226) in a dose-dependent manner using HD peripheral blood MNCs as effector cells. Co-incubation with patient-derived BM stromal cells resulted in a modest impairment of killing capacity in MM1.S and RPMI8226 cells. In MM patient samples (n = 20), the mean lysis of MM cells with 4.0 µg/mL JNJ-7564 was 62% (range: −8-97%; fig 1C), while NK-cell and T-cell frequencies were not affected. JNJ-7564 was also effective in samples from extensively pretreated daratumumab (DARA)-refractory patients (DRMM; n = 6; median of 6 prior lines; mean lysis with 4.0 µg/mL: 70%; range: 44-97%). Maximal lysis of primary MM cells was not associated with the level of GPRC5D expression on MM cells, effector:target ratio, or frequency of regulatory T-cells. JNJ-7564-mediated MM cell lysis was associated with activation and degranulation of CD4+ and CD8+ T-cells (fig 1D).