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Saad S. Kenderian, Reona Sakemura, Nan Yang, Michelle Cox, Sutapa Sinha, Mehrdad Hefazi, Michael Hansen, Kendall Schick, Cynthia Forsman, Justin Boysen, Wei Ding, Sameer Parikh and Neil Kay

Despite the remarkable outcomes of CD19-directed chimeric antigen receptor T (CART19) cell therapy in B-cell malignancies, the durable responses in diffuse large B-cell lymphoma are less than 40%, and strategies to enhance this response are desperately needed. Inhibition of AXL RTK with TP0903, a high-affinity AXL inhibitor has been found to induce robust apoptosis of malignant B cells. Here, we aimed to examine the role of AXL RTK inhibition with TP0903 on T-cell function in B-cell malignancies. First, we investigated the influence of TP0903 on CART19 cell phenotype and functions. Here, we used 41BB costimulated, lentiviral-transduced CART cells. AXL inhibition led to polarization of CART cells into a Th1 phenotype when T cells were stimulated with the CD19+ mantle cell lymphoma (MCL) cell line Jeko or with leukemic B cells isolated from patients with chronic lymphocytic leukemia (CLL), in the presence of TP0903 (). Exposure of activated CART cells to TP0903 also resulted in significant downregulation of inhibitory receptors on activated CART cells (), a reduction of conical cytokines known to be associated with the development of cytokine release syndrome (CRS) (). To investigate the effect of AXL RTK inhibition of CART cells with TP0903 in vivo, we established MCL xenografts through the injection of 1.0x106 of Jeko into NSG mice. A week later, mice were treated with either vehicle alone, TP0903 (20 mg/kg/day) alone, 0.5x106 of CART19 alone, or TP0903 (20mg/kg/day)+0.5x106 of CART19. Three weeks after the treatment, mice were rechallenged with 1.0x106 of Jeko. Mice treated with CART19 and TP0903 rejected the tumor challenge while mice previously treated with CART19 alone redeveloped MCL, suggesting that AXL inhibition enhanced CART cell persistence (). Finally, we validated our preclinical findings in correlative analyses of phase 1 clinical trial of TP0903 in patients with solid tumors (NCT02729298). Similar to our findings, there was a significant reduction in Tregs, reduction of inhibitory receptors and polarization to a Th1 phenotype. These findings will be further investigated in a planned phase 1 clinical trial of TP0903 in relapsed/refractory CLL (NCT03572634) In summary, we demonstrated for the first time that AXL inhibition polarizes T cells into a Th1 phenotype, downregulates inhibitory receptors, and synergizes with CART cells in B-cell malignancies. These findings encourage further study of TP0903 as an enhancer of T-cell immunotherapies.

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Saad S. Kenderian, Michelle Cox, Reona Sakemura, Fabrice Lucien-Matteoni, Nan Yang, Sutapa Sinha, Cynthia Forsman, Mehrdad Hefazi, Kendall Schick, Justin Boysen, Sameer A. Parikh and Neil E. Kay

Background: Unprecedented clinical outcomes were reported after CD19 chimeric antigen receptor T-cell (CART19) therapy. However, the complete response rate in chronic lymphocytic leukemia (CLL) is much lower, at approximately 20%–30%. Several immune defects have been identified in CLL that result from the complex interaction between CLL cells and the microenvironment. This leukemic microenvironment is rich with extracellular vesicles (EVs) secreted by B-CLL cells. Here, we aimed to investigate the role of EVs play in the diminished CART response seen in some CLL patients. Methods: EVs were isolated from peripheral blood of 16 patients with untreated CLL at different Rai stages and risk profile by FISH. Cytometry was used to determine size, number of particles per μl, and CD19 expression. To investigate the impact of EVs on CAR T-cell functions, CART19 cells were stimulated with either CLL B cells or the CD19-positive cell line JEKO and different effector functions were analyzed. Results: Two patterns of EVs in CLL patients were identified; a single versus 2 distinct EV populations, characterized by size (small [EVssmall] and large [EVslarge], ). In 25% of patients, EVs were CD19 positive (EVCD19+). CD19 positivity was detected only in patients with the EVslarge (). The EVs concentration, CD19 expression (EVsCD19+ vs EVsCD19-), or the size (EVssmall vs EVslarge) did not correlate with disease stage (early vs advanced Rai stage) or risk profile of CLL (low vs high risk). To investigate our hypothesis that EVs could modulate CART19 function, CART19 cell effector functions were examined in the presence of EVsCD19+, EVsCD19-, EVssmall, or EVslarge. EVs alone were insufficient to stimulate CART19 cells. However when CART19 cells were stimulated with the CD19-positive cell line JEKO, their effector functions were reduced only in the presence of EVsCD19+ but not EVsCD19-. This included a significant reduction in CART-specific killing () and a reduction in cytokine production. The impairment of CART cell functions was independent of the size of EVs, ie, there was no impairment of CART functions with large or small size EVCD19- in co-culture. Conclusion: We identify CD19-positive large size EVs from patients with CLL and demonstrate that these EVs play a role in the leukemic microenvironment by reducing CAR T-cell activity. Studies are ongoing to define the mechanism(s).