Xie et al. developed CAR T cells using variable regions of heavy-chain-only antibodies (VHHs) to target solid tumors. PD-L1-targeting CAR T cells generated from PD-L1-/- T cells slowed B16 or MC38 tumor growth and increased survival in PD-L1-/- immunocompetent mice. Generating PD-L1-targeted CAR T cells in the presence of PD-L1-blocking VHH prevented early activation and exhaustion, and improved the antitumor response. CAR T cells targeting EIIIB (a splice variant of fibronectin expressed specifically in the stroma and neovasculature of some tumors) delayed B16 tumor growth, increased T cell infiltration, and improved survival in WT mice.
Chimeric antigen receptor (CAR) T cell therapy has been successful in clinical trials against hematological cancers, but has experienced challenges in the treatment of solid tumors. One of the main difficulties lies in a paucity of tumor-specific targets that can serve as CAR recognition domains. We therefore focused on developing VHH-based, single-domain antibody (nanobody) CAR T cells that target aspects of the tumor microenvironment conserved across multiple cancer types. Many solid tumors evade immune recognition through expression of checkpoint molecules, such as PD-L1, that down-regulate the immune response. We therefore targeted CAR T cells to the tumor microenvironment via the checkpoint inhibitor PD-L1 and observed a reduction in tumor growth, resulting in improved survival. CAR T cells that target the tumor stroma and vasculature through the EIIIB(+) fibronectin splice variant, which is expressed by multiple tumor types and on neovasculature, are likewise effective in delaying tumor growth. VHH-based CAR T cells can thus function as antitumor agents for multiple targets in syngeneic, immunocompetent animal models. Our results demonstrate the flexibility of VHH-based CAR T cells and the potential of CAR T cells to target the tumor microenvironment and treat solid tumors.