OC 56.2 - Targeting Autoreactive B Cells Using ADAMTS13-Based Chimeric Auto-Antigen Receptors (CAARs) T Cells for the Treatment of Immune Thrombotic Thrombocytopenic Purpura (iTTP)

Background: iTTP is a life-threatening thrombotic microangiopathy characterized by thrombosis due to dysregulated platelet activation. iTTP results from an autoantibody response directed against a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS13), a plasma protease that regulates platelet activation via cleavage of platelet-activating von Willebrand Factor (vWF) multimers. Autoantibodies bind to and reduce ADAMTS13 activity resulting in ultralarge circulating vWF multimers that induce abnormal platelet activation and microvascular thrombi leading to the clinical features of TTP.

Aims: To develop CAAR T cells targeting ADAMTS13-specific autoimmune B cells as a targeted and potentially-curative therapeutic strategy for iTTP.

Methods: We generated CAARs employing different combinations of ADAMTS13 domains as extracellular targeting domains. We also created a panel of target B cell-like Nalm6 cell lines expressing iTTP patient-derived autoantibodies. ADAMTS13-CAAR T cells were tested for efficacy, specificity, and long-term fitness in in-vitro co-culture assay with Nalm6 target cells. In vivo experiments utilized Nalm6 targets in a murine xenograft model to test the efficacy of ADAMTS13-CAAR T cells compared to a non-specific CD19-targeted CAR T (CART-19). Lastly, the ability of ADAMTS13-CAARs to target polyclonal patient B cell was indirectly assessed using a serum adsorption assay (7 patients).

Results: ADAMTS13-CAAR T cells exhibited specific cytotoxicity against Nalm6 cells-expressing human iTTP autoantibodies. Long-term restimulation in vitro demonstrated ADAMTS13-CAAR T cell fitness comparable to CART-19. In-vivo, ADAMTS13-CAAR T cells effectively controlled Nalm6 expressing anti-CS domain iTTP antibodies (Figures 1 & 2) with no signs of toxicity. Adsorption of iTTP sera with ADAMTS13-CAAR T cells significantly depleted autoantibodies compared to adsorption with control non-CAAR T cells (p < 0.05, t-test).

Conclusion(s): We have successfully created ADAMTS13-based CAAR T cells that effectively and specifically target B cells expressing anti-ADAMTS13 autoantibodies. This approach has the potential to target autoimmune B cells in iTTP patients with the promise of a curative treatment.