Tregs integrate native and CAR-mediated costimulatory signals for control of allograft rejection

Regulatory T cells (Tregs) expressing Chimeric Antigen Receptors (CARs) are a promising tool to promote transplant tolerance. The relationship between CAR structure and Treg function was studied in xenogeneic, immunodeficient mice, revealing advantages of CD28-encoding CARs. However, these models could underrepresent interactions between CAR-Tregs, antigen-presenting cells (APCs) and donor-specific antibodies. We generated Tregs expressing HLA-A2-specific CARs with different costimulatory domains and compared their function in vitro and in vivo using an immunocompetent model of transplantation. In vitro, the CD28-encoding CAR had superior antigen-specific suppression, proliferation and cytokine production. In contrast, in vivo, Tregs expressing CARs encoding CD28, ICOS, PD1 and GITR, but not 41BB or OX40, all extended skin allograft survival. To reconcile in vitro and in vivo data, we analyzed effects of a CAR encoding CD3ζ but no co-stimulatory domain. These data revealed that exogenous co-stimulation from APCs can compensate for the lack of a CAR-encoded CD28 domain. Thus, Tregs expressing a CAR with or without CD28 are functionally equivalent in vivo, mediating similar extension of skin allograft survival and controlling the generation of anti-HLA-A2 alloantibodies. This study reveals a new dimension of CAR-Treg biology and has important implications for the design of CARs for clinical use in Tregs.