Myeloid-Derived Suppressor Cell Regulation of PD-1 Expression on, and Longevity of, Human CD8⁺ T Cells

Persistent viral infections and tumor burdens are associated with limited longevity of functional CD8⁺ T cells. Longevity of effector or memory CD8⁺ T cells may depend on a balance of costimulatory and coinhibitory signals. 4–1BB (CD137), induced as a costimulatory receptor, is known to promote longevity of anti-virus and tumor CD8⁺ T cells, whereas ‘exhausted’ CD8⁺ T cells conversely induce coinhibitory receptors such as programmed death-1(PD-1) in order to limit long term CD8⁺ T cell survival. We hypothesized that myeloid-derived suppressor cells (MDSC) often recruited to the tumor microenvironments might control expression of coinhibitory and costimulatory receptors, thereby regulating longevity of effector and memory CD8⁺ T cells. To this end, we developed various myeloid-derived dendritic cell (DC) types in vitro from human CD14⁺ monocytes using M-CSF and GM-CSF in the presence of IL-4 with/without other cytokines, and characterized the DC according to their capacity to regulate expression of CD137 and PD-1 on CD8⁺ T cells, and their impact on longevity of CD8⁺ T cells following coculture. In contrast to conventional DC generated with GM-CSF and IL-4 (GM-DC), DC produced with M-CSF and IL-4 (M-DC) were highly suppressive to CD8⁺ T cell cytotoxic activity, inhibiting induction of costimulatory CD137 on CD8⁺ T cells during allogeneic cell coculture. M-DC generated in the presence of IL-10 (IL-10/M-DC) uniquely induced inhibitory PD-1 in CD8⁺ T cells, while maintaining the ability to inhibit stimulatory CD137 expression. The presence of TNF-α or LPS during IL-10/M-DC development, on the other hand, deprived the DC of their PD-1 inducing ability. This suggests that pro-inflammatory factors may have converted IL-10/M-DC into a more mature form of DC that no longer possesses the ability to induce PD-1. Expression of the ligand for PD-1 (PD-L1) in M-DCs was increased greatly by IL-10. Importantly, IL-10/M-DC significantly reduced CD8⁺ T cell viability during coculture. TGF-β, known to demonstrate similar immune suppressive activities as IL-10, failed to promote M-DC to induce PD-1 on CD8⁺ T cells, suggesting that IL-10 plays a unique role in inducing PD-1 on CD8⁺ T cells through M-DC. Neither IL-10 nor M-DC alone affected PD-1 expression on CD8⁺ T cells. We then evaluated whether IL-10/M-DC affected expression of Sirt1, an NAD⁺-dependent deacetylase known to extend life span of many cell types, in CD8⁺ T cells. Sirt1 was detected at only marginal levels in freshly isolated naïve CD8⁺ T cells, but its expression was increased upon CD3 activation. Coculture with IL-10/M-DC reduced induction of Sirt1 in anti-CD3 activated CD8⁺ T cells. Various immune suppressive cells are found in tumor microenvironments and chronically virus infected patients. Our results suggest that IL-10/M-DC may play a role as immune suppressor cells in evasion of a host’s immune system by tumors and viruses, via modulation of CD137 and PD-1 signals, both of which affect longevity of CD8⁺ T cells. Longevity may relate to expression of Sirt1.