Compromised antigen binding and signaling interfere with bispecific CD19 and CD79a chimeric antigen receptor function.

Therapy with CD19-directed chimeric antigen receptor (CAR) T cells has transformed the treatment of advanced B-cell malignancies. However, loss of or low antigen expression can enable tumor escape and limit the duration of responses achieved with CAR T-cell therapy. Engineering bispecific CAR T cells that target 2 tumor antigens could overcome antigen-negative escape. We found that CD79a and b, which are heterodimeric components of the B-cell receptor, were expressed on 84.3% of lymphoma cases using immunohistochemistry, and 87.3% of CD79ab-positive tumors also coexpressed CD19. We generated 3 bispecific permutations: tandem, bicistronic, and pooled products of CD79a-CD19 or CD79b-CD19 CAR T cells and showed that bispecific CAR T cells prevented the outgrowth of antigen-negative cells in a CD19-loss lymphoma xenograft model. However, tandem and bicistronic CAR T cells were less effective than monospecific CD19 or CD79a CAR T cells for the treatment of tumors that only expressed CD19 or CD79, respectively. When compared with monospecific CAR T cells, T cells expressing a tandem CAR exhibited reduced binding of each target antigen, and T cells expressing a bicistronic CAR vector exhibited reduced phosphorylation of downstream CAR signaling molecules. Our study showed that despite added specificity, tandem and bicistronic CAR T cells exhibit different defects that impair recognition of tumor cells expressing a single antigen. Our data provide support for targeting multiple B-cell antigens to improve efficacy and identify areas for improvement in bispecific receptor designs.