Fine-tuning affinity and spacer design enhances T cell potency in DLL3 and BCMA CAR T cells

Chimeric antigen receptor (CAR)-modified T cells have garnered substantial attention due to their clinical success, culminating in six Food and Drug Administration-approved therapies for hematological malignancies. Notably, CD19-specific CAR T cell therapies have achieved remarkable clinical efficacy in treating B-cell malignancies, but these profound and durable responses are not observed in CAR T therapies targeting other indications, particularly solid tumors. Key design elements of CAR constructs - namely, antigen binding affinity and spacer length - play critical roles in determining T cell effector function and overall therapeutic effectiveness. Refining CAR designs may enhance T cell functionality, extend clinical application, and potentially apply CAR T cell therapies across a wider array of malignancies. In this study, affinity variant and spacer variant CARs targeting BCMA and DLL3 tumor antigens were evaluated using <i>in vitro</i> measurements of antigen-binding properties and effector function. Each panel of CARs spanned 2-3 logs of antigen binding affinity (BCMA: 181 pM KD to 74 nM KD, DLL3: 417 pM to 407 nM). Additionally, CAR T cells were challenged with tumor spheroids composed of BCMA⁺ H929 and DLL3⁺ SHP77 tumor cells. We show that for both tumor models, higher affinity CARs (KD stronger than approximately 100 nM) paired with an intermediate length spacer (IgG1 Fc, CH2-CH3, 230AA) elicited the strongest levels of tumor killing, CAR⁺ T cell expansion, and proinflammatory cytokine production. These CARs displayed the strongest cellular affinity when measured in a conjugation assay, suggesting a relationship between cellular affinity and T cell functional performance. This study highlights the critical role of CAR design in enhancing T cell functionality, demonstrating that high-affinity CARs combined with intermediate-length spacers yield superior performance in targeting BCMA and DLL3 antigens. This study provides a framework for rational CAR design, informing strategies to broaden the clinical utility of CAR T-cell therapies beyond hematologic cancers.