Impact of stromal-targeting antitumor CAR T cells in solid tumors.

Abstract

2541 Background: While chimeric antigen receptor (CAR) T cells have shown tremendous success in hematological malignancies, but such efficacy has not been achieved in the setting of solid tumors. One of the hurdles to CAR T cells therapy in solid tumors is the presence of physical stroma and cancer associated fibroblasts (CAFs), which inhibit the entry of activated T cells to the tumor sites. Membrane bound protein Leucine-rich repeat containing 15 (LRRC15) has been shown to be highly expressed on CAFs in many solid tumors including pancreatic cancer as well as directly expressed on tumors of mesenchymal origin including sarcomas, glioblastomas and melanomas. LRRC15 has very limited expression in normal tissues. The goal of the current study was to explore the impact of stromal targeting antitumor (STAT) in solid tumors using LRRC15-directed CAR T cells. Herein we demonstrate that CAR T cells directed to tumor stroma can eradicate solid tumor. Methods: LRRC15-directed CAR T cells were validated in in vitro assays that included specific lysis, cytokine secretion, and proliferation. STAT CAR T cells were administered intravenously into NSG mice after engrafted with osteosarcoma SaOS2 and pancreatic PANC1 tumor cell lines as well as patient derived tissues (PDXs). The efficacy of STAT CAR T cells was also assessed in syngeneic mouse model engrafted with murine OS F420 and pancreatic KPC tumor cell lines. Tumor was harvested from mice at various timepoints and analyzed for LRRC15 expression as well as for the presence of T cells. Results: LRRC15-directed CAR T cells specifically lysed SaOS2 cells. Upon stimulation with SaOS2 cell line, LRRC15-directed CAR T cells resulted in robust expansion and secreted cytokines including IL2, IFN-ϒ, GM-CSF and TNFα. The LRRC15-directed CAR T cells were able to eliminate tumor in NSG mice xenografted with SaOS2 cell line as well as OS PDX. LRRC15-directed CAR T cells significantly increased the survival of mice. Next, we sought to test the efficacy of LRRC15-directed CAR T cells in NSG mice engrafted with LRRC15 - tumor/CAFs + cell line and PDX. We showed that the NSG mice injected with the LRRC15 - PANC1 cell line acquired stroma with CAFs positive for LRRC15 within 2-3 weeks. LRRC15-directed CAR T cells were able to significantly inhibit the progression of both PANC1 tumors as well as PDAC PDX in NSG mice. In syngeneic mouse model, LRRC15-directed CAR T cells resulted in tumor regression of both LRRC15 + sarcoma F420 and LRRC15 - pancreatic KPC tumors. Conclusions: To our knowledge this is the first study demonstrating targeting stroma in solid tumors using STAT CAR T cells. LRRC15-directed CAR T cells showed antitumor efficacy in mouse models engrafted with LRRC15 + as well as LRRC15 - tumors. Collectively, we show that targeting LRRC15 + CAFs in the tumor with CAR T cells has the potential to inhibit solid tumor progression as well to circumvent the challenge of limited penetration of T cells into the tumor site by disrupting the stroma.