Human CD8(+)CD28(−) T Suppressor Cells Expanded by IL-15 In Vitro Suppress in an Allospecific and Programmed Cell Death Protein 1-Dependent Manner

CD8(+)CD28(−) T suppressor cells (Ts) have been recently documented to play an important role in alloimmunity. Therefore, understanding and optimizing the conditions under which these cells are generated and/or expanded would greatly facilitate further research and potential clinical use. In this study, we describe rapid expansion of human allospecific CD8(+)CD28(−) Ts cells through coculture of CD8(+) T cells with human leukocyte antigen-mismatched donor antigen-presenting cells plus IL-15 in a relative short period of time in vitro. Interestingly, IL-15 promotes the expansion of CD8(+)CD28(−) Ts cells through several parallel mechanisms. The expanded CD8(+)CD28(−) Ts cells upregulate expression of CD132, CD25, and programmed cell death protein 1 (PD-1), but downregulate expression of CD122, GZM-B, and perforin, while exhibiting no cytotoxicity. Most importantly, the expanded CD8(+)CD28(−) Ts cells vigorously inhibit CD4(+) T cells proliferation in a contact-dependent and donor-specific manner both in vitro and in vivo. Interestingly, the co-inhibitory molecules PD-1 and programmed death-ligand 1 play an obligatory role in the mechanisms of CD8(+)CD28(−) Ts cells suppression. Taken together, our study report novel methodology for IL-15-induced expansion of human CD8(+)CD28(−) Ts cells and possible mechanisms. These findings may facilitate understanding of transplant rejection and promote clinical application of CD8(+)CD28(−) Ts cell-based strategies for inducing and monitoring transplant tolerance in the future.