Tumor-derived Small Extracellular Vesicles Inhibit the Efficacy of CAR T Cells against Solid Tumors.

Chimeric antigen receptor (CAR) T cell therapy has shown remarkable success in the treatment of hematologic malignancies. Unfortunately, it has limited efficacy against solid tumors, even when the targeted antigens are well expressed. A better understanding of the underlying mechanisms of CAR T cell therapy resistance in solid tumors is necessary to develop strategies to improve efficacy. Here we report that solid tumors release small extracellular vesicles (sEVs) that carry both targeted tumor antigens and the immune checkpoint protein PD-L1. These sEVs acted as cell-free functional units to preferentially interact with cognate CAR T cells and efficiently inhibited their proliferation, migration, and function. In syngeneic mouse tumor models, blocking tumor sEV secretion not only boosted the infiltration and anti-tumor activity of CAR T cells but also improved endogenous anti-tumor immunity. These results suggest that solid tumors use sEVs as an active defense mechanism to resist CAR T cells and implicate tumor sEVs as a potential therapeutic target to optimize CAR T cell therapy against solid tumors.