Exhaustion, rather than lack of infiltration and persistence, of CAR-T cells hampers the efficacy of CAR-T therapy in an orthotopic PDAC xenograft model

Background: The impressive success of chimeric antigen receptor (CAR)-T therapy in the treatment of hematologic tumors is leading researchers/clinicians to broaden the possible application of this approach to patients with solid tumors, especially patients with pancreatic ductal adenocarcinoma (PDAC). However, CAR-T cell therapy faces disappointing clinical results in solid tumors due to hurdles unique to solid, but not hematologic, tumors, which include restricted trafficking and limited infiltration into tumors, non-durable persistence, and T cell exhaustion. Since the tumor microenvironment composition varies between tumor type meaning CAR-T cells again face a unique set of challenges in each solid tumor, the major hurdle imposed by PDAC on CAR-T cell therapy remains elusive. Methods: In this study, we used immunodeficient NOD/Shi-scid, IL-2RγKO Jic mice orthotopically transplanted with human pancreatic tumor cell line, BxPC-3, and transferred with CAR-T cells specific to carcinoembryonic antigen (CEA), a model that allows analysis of quantitative, spatial and functional dynamics of CAR-T cells in tumor tissues unique to PDAC. Results: CAR-T cells suppressed tumor growth of orthotopic human PDAC tumors, but only within a limited time window. Contrary to previous studies that demonstrated a limited persistency and infiltration of CAR-T cells in many solid tumors, they persist and accumulated in PDAC tumor tissues, but CAR-T cells become progressively lost ability to control tumor growth. Ex vivo analysis revealed that CAR-T cells that had been recovered at different time points from mice bearing an orthotopic PDAC tumor exhibited a gradual loss of tumor reactivity. This loss of tumor reactivity of CAR-T cells was associated with the increased expression of AMP-activated protein kinase and Mitofusin1. Conclusions: The present results demonstrate the potential effectiveness of CAR-T cells in targeting CEA for the treatment of PDAC. However, optimization of CAR-T cells, especially in the areas of resistance to exhaustion is required.