Targeting CAR T Resistance Due to CD19 Loss with CD79b-Specific CAR T Cells in B-Cell Malignancies

Targeting CAR T resistance due to CD19 loss with CD79b-specific CAR T cells in B-cell malignancies Fuliang Chu, JingJing Cao, Jingwei Liu, Xiaoyun Cheng, Swathi Karri, Adithi Reddy, Shao Qing Kuang, Jinsheng Weng, Sattva S. Neelapu Department of Lymphoma and Myeloma, The University of Texas M. D. Anderson Cancer Center. Houston, TX 77030 USA Chimeric antigen receptor (CAR) T cells targeting CD19 are highly effective in B-cell malignancies and two CD19-targteing CAR T-cell therapy products were recently approved by the US FDA for relapsed or refractory B-cell acute lymphoblastic leukemia and/or large B cell lymphoma. In pivotal trials, durable remissions lasting more than 1 year have been observed in ~40-50% of these patients. However, relapse or progression occurs in ~50-60% and a major cause of resistance appears to be due to CD19 antigen loss. Thus, there is an urgent need to develop CAR T-cell therapies against novel targets to further improve outcomes in these patients. CD79b is a pan B-cell linage marker and an important component of the B-cell receptor complex. CD79b is broadly expressed in normal B cells and B-cell malignancies and its expression is usually retained in CD19 negative tumors progressing after CD19-specific CAR T-cell therapy. Here, we report our results with a novel CD79b-specific CAR T cell product in in vitro and in vivo models. We generated five murine monoclonal antibodies against human CD79b by hybridoma technology and demonstrated that they bind specifically to recombinant human CD79b, have high affinity (Kd range of 1.44-18.1 nM), and stain multiple B-cell lymphoma cell lines comparable to the commercial CD79b fluorochrome-labeled antibodies. Next, we cloned the variable regions of the heavy and light chains of the CD79b antibodies, and developed lentiviral constructs for CD79b-specific CARs with CD3z and CD28 costimulatory domains. We demonstrated that the CD79b-CAR lentiviral constructs can be transduced into primary CD4+ and CD8+ T cells from healthy donors to >85% transduction efficiency. We observed that the CD79b-specific CAR T cells but not untransduced T cells had significant cytotoxic activity that was comparable to control CD19-specific CAR T cells against Daudi Burkitt lymphoma and Mino mantle cell lymphoma cell lines. More importantly, CD79b- but not CD19-specific CAR T cells lysed CD19-CD79b+ lymphoma cells. Significant CD107a/b degranulation was also observed in both CD4+ and CD8+ CD79b-specific CAR T cells when t