PD-L1 – PD-1 interactions limit effector regulatory T cell populations at homeostasis and during infection

Phenotypic and transcriptional profiling of Treg cells at homeostasis reveals that TCR activation promotes Treg cells with an effector phenotype (eTreg) characterized by the production of IL-10 and expression of the inhibitory receptor PD-1. At homeostasis, blockade of the PD-1 pathway results in enhanced eTreg cell activity while during infection with T. gondii early IFN-γ upregulates myeloid cell expression of PD-L1 associated with reduced Treg cell populations. In infected mice, the blockade of PD-L1, complete deletion of PD-1, or lineage-specific deletion of PD-1 in Treg cells prevents loss of eTreg cells. These interventions resulted in a reduced ratio of pathogen-specific effector T cells : eTregs and increased levels of IL-10 that mitigated the development of immunopathology, but which could compromise parasite control. Thus, eTreg cell expression of PD-1 acts as a sensor to rapidly tune the pool of eTreg cells at homeostasis and during inflammatory processes.