A BCL2L1 Armoured BCMA Targeting CAR T Cell to Overcome Exhaustion and Enhance Persistence in Multiple Myeloma

Chimeric antigen receptor (CAR) T cells targeting the B-cell maturation antigen (BCMA) have resulted in deep responses in patients with relapsed MM however most remissions are not sustained. While cellular and molecular mediators of relapse post CAR T therapy in MM are not fully delineated, current data suggest three possible mechanisms including the lack of persistence of the CAR T cell product, acquired exhaustion and less commonly loss of BCMA expression.

Using CITE-seq we measured the expansion of variable T cell subsets, T cell specific activation and inhibitor markers and their functional states in serial blood and marrow samples (n=10) collected from patients treated with BCMA targeting CAR T cells. CAR T cells were identified by the expression of the chimeric CAR T cell transcript. With the exception of one patient where biallelic loss of BCMA was identified at relapse, CAR T cells of resistant patients were enriched with terminally exhausted CD45RA+ cells with loss of CD28, low BCL2L1 (gene encoding BCL-XL) expression, high CD57 with co-expression of checkpoint inhibitors (LAG3, TIGIT and PD1). The lack of persistence of the CAR T cells product was notable in all relapsing patients consistent with an activation induced cells death (AICD) specially in the setting of chronic antigenic stimulation. Cognizant of the role BCL-XL plays in T cells survival in response to CD28 co-stimulatory signaling, we postulated that increasing BCL-XL expression is a feasible strategy to enhance CAR T cell resistant to AICD, improve their persistence and anti-BCMA reactivity. To this goal, we designed a 2nd generation lentiviral CAR construct where the anti-BCAM scFV-41BBz CAR and the BCL2L1 cDNA were linked with self-cleaving 2A sequence. The efficiency in eradicating MM cells of this BCL-XL armored CAR (BCMA_BCL2L1_CAR) was compared to that of non-unarmored CAR (BCMA_CAR) in vitro and i n vivo studies. While BCMA_BCL2L1_CAR and BCMA_CAR were equally cytotoxic to OPM2 MM cells, in MM cell lines expressing the FAS death receptor ligand FASLG (MM1S, OCMY5 and H929) BCMA_BCL2L1_CAR viability and cytolytic activity was significantly superior to that of unarmored BCMA_CAR. Of note, the expression of FASLG, a known interferon response gene, was upregulated in H929 cells when co-cultured with CAR T cells. Importantly, under chronic antigenic stimulation conditions (FIG 1A), where CAR T cells were stimulated every 6 days over a 28 days period with irradiated OPM2 cells, we found no phenotypic difference between BCMA_BCL2L1_CAR and BCMA_CAR with respect to the composition of effector memory T cells (Tem: CCR7− CD45RO+ CD45RA−) or central memory T cells (Tcm: CCR7+CD45RO+CD45RA−) or terminal effector / exhausted T cells. However, under these chronic antigenic stimulation conditions, the CAR T cells viability, proliferation (FIG 1B) and anti-MM cytotoxic activities (FIG 1C) of the BCMA_CAR were dramatically reduced compared to that of the BCL2L1 armored CAR. Furthermore, in initial animal studies where NOD-SCID mice were tail vein injected with 2e6 OPM2 MM cells transduced with a luciferin reporter gene, followed 10 days later by control T cells, BCMA_CAR or BCMA_BCL2L1_CAR T cells IV injection, and despite a skewing to a larger initial disease burden in the BCMA-BCL2L1-CAR group, BCL2L1 armored CAR T cells resulted in more prolonged disease control and animal survival compared to the BCMA_CAR treated mice (FIG 1D).

Our studies indicate that BCL2L1 blockade of AICD not only enhanced the viability and proliferation of BCMA targeting CAR T cells but surprisingly also reduced their functional exhaustion. Our findings provide an novel approach for CAR T optimization and overcoming disease relapse resulting from lack of persistence and/or T cells exhaustion.

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