PD-1 through Asparaginyl Endopeptidase Regulates Foxp3 Stability in Induced Regulatory T Cells

CD4⁺ T cell differentiation into multiple T helper lineages is critical for optimal adaptive immune responses. Lineage instability has been attributed to Tregs and Th17 cells but Th1 cells are acclaimed for their functional stability. In this report, we investigated the intrinsic mechanism by which PD-1 signaling imparted regulatory phenotype to Foxp3⁺ Th1 cells (denoted as Tbet⁺ iTreg_PDL1 cells). Tbet⁺iTreg_PDL1 cells were capable of preventing autoimmunity and alloimmunity in murine models of experimental colitis and experimental graft versus host disease (GvHD). PDL-1 binding to PD-1 imparted regulatory function to Tbet⁺iTreg_PDL1 cells by specifically down regulating the endo-lysosomal protease asparaginase endopeptidase (AEP). AEP cleaved Foxp3 in control Tbet⁺ iTregs and iTreg cells resulting in Foxp3 degradation. Blocking AEP, by knockdown and selective inhibitor, enhanced Foxp3 stability and regulatory function in Tbet⁺ iTreg cells. In addition, Aep^-/- iTregs significantly inhibited GvHD and maintained Foxp3. Of note, PD-1 mediated Foxp3 maintenance in Tbet⁺ Th1 cells was significantly operational in tumor infiltrating lymphocytes. Collectively, this report has identified a novel intrinsic function for PD-1 in converting Th1 cells into Tregs through an intrinsic proteolytic pathway.