648. Mitigating Tumor Escape: Tandem Anti-CD20- and CD19 SCFV-Based Chimeric Antigen Receptors (CARs) in Leukemia/Lymphoma

Abstract

In recent years, adoptive cell therapy using T cells engineered with chimeric antigen receptors (CAR T) specific for CD19 has shown marked clinical efficacy. Nevertheless, cases of tumor escape due to tumor escape in B cell malignancies could be mitigated by CAR T the loss of CD19 antigen have been reported. We hypothesized that approaches that target the CD19 and CD20 tumor antigens simultaneously. Second generation CARs containing scFv specific for CD19 and CD20 were generated by lentiviral transduction of human primary T cells. The configurations were: a) single targeting domain, b) tandem targeting domains, or c) two full-length CAR receptors co-expressed bicistronically in the same T cell. CAR T expression for all constructs confirmed by Western blotting and protein L flow cytometry. CAR T cytolytic activity and IFN gamma secretion were evaluated by killing assays and ELISA, respectively. Single, tandem, and bicistronic CAR T cells exhibited CAR surface expression of 50-70%. All anti- CD19, anti-CD20 CAR T cells demonstrated target-specific cells lysis and induction of IFN-gamma. Interestingly, bicistronic CAR19, CAR20 construct yielded high CAR T surface expression and IFN-gamma production, but inferior in vitro cytolytic activity. To confirm CAR T specificity, K562 CD19+ and K562 CD20+ cell lines were generated. In flow-based 1h killing assays, CAR19 and CAR20 lysed their respective target lines only, whereas tandem CAR constructs lysed both K562 CD19+ and K562 CD20+ cells, but not K562 control. In a subsequent series of co-culture experiments, Raji target cells were combined with very low ratios of CAR T cells, to allow for the evolution of escape variants. On day 5 of co-incubation in the presence of CAR19 T cells, the surviving Raji cells numbers were comparable to control (85,000 cells/well vs 77, 000 cells/well in sham transduction). In comparison, viable Raji numbers decreased drastically post co-incubation with CAR20-, CAR19_20- and CAR20_19-T cells (546, 1,355 and 1,543 cells/well, respectively). Among the surviving Raji population, CD19 surface expression was reduced to 1.6%, vs 93.29% for sham control, whereas the expression of CD20 and CD22 remained high (97% and 78%, respectively). By contrast, the CD20 surface marker was less prone to down-regulation by CAR-T cells expressing anti-CD20 scFv. Similar results were seen when experiments were of longer, 7 days, or shorter, 1 day, duration. Thus, targeting two tumor antigens simultaneously using tandem CAR T cells is a promising new strategy for mitigation of tumor escape via antigen down-regulation. Moreover, expression of two scFv in a single CAR format is superior to expression of two independent CARs from a bicitsronic vector. It also appears that the order of the targeting domains within the tandem CAR impacts anti-tumor activity. These findings are currently being explored further in animal model systems.