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

Abstract

121 Background: 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 anti-tumor efficacy. Results: Due to the use of viral 2A sequences we detected near equal expression of each CAR by flow cytometry. The first T cell product expresses three CARs individually (TriCAR), and the second expresses a single CAR targeting CD19 and a second bi-specific CAR targeting CD20 and CD22 via a tandem arrangement (SideCAR). Using primary B-ALL cells, we observed that TriCAR and SideCAR T cells killed ALL cells more robustly than CD19 CAR T cells at low E:T ratios. Further, in ImageStream analysis of single cell interactions between CAR T cells and primary B-ALL cells, TriCAR T cells exhibited increased actin polymerization compared to CD19 CAR T cells, suggesting remodeling and increased cell activation. Finally, in multiple models of CD19 escape in primary ALL, we showed that trivalent CAR T cells mitigated CD19 negative relapse, producing IFN-γ/TNF-α and killing CD19-negative primary ALL, while CD19 CAR T cells remained ineffective. Conclusions: Trivalent CAR T cells effectively target primary ALL cells with varying antigen profiles and mitigate CD19-negative relapse. This strategy has the potential for use as an initial CAR therapy in relapsed ALL or a salvage therapy for patients with CD19-negative disease.