Targeting CD19-negative relapsed B-acute lymphoblastic leukemia using trivalent CAR T cells.

121Background: Chimeric antigen receptor (CAR) T cells targeting CD19 have shown remarkable efficacy in treating relapsed B cell acute lymphoblastic leukemia (B-ALL). However, recent reports show that up to 40% of patients who relapse after CD19 CAR T cell therapy have CD19-negative disease, justifying a need to expand CAR T cell therapy for B-ALL to include additional tumor-associated antigens. We hypothesize that targeting CD19, CD20, and CD22 will improve B-ALL therapy outcomes and control disease progression during CD19-negative relapse. Methods: We designed two trivalent CAR T cell products with exodomains derived from single chain variable fragments (ScFv) targeting CD19, CD20, and CD22. Each CAR contains the 4-1BB and T-cell receptor zeta chains. Donor T cells were engineered to express the CARs using a retroviral system. We used primary CD19-negative relapsed bone marrow samples and CRISPR CD19 knockouts of primary ALL to model CD19 escape and standard cytotoxicity and immune assays to evaluate an...