Inhibitory signals through the PD-1 pathway regulate T cell activation, tolerance, and exhaustion. To study the role of PD-1 in Treg cells, we generated mice that selectively lack PD-1 in T reg cells. PD-1–deficient Treg cells exhibit an activated phenotype and enhanced immunosuppressive function. The potent suppressive capacity of PD-1–deficient Treg is illustrated by protection from diabetes in NOD mice and ameliorated  EAE in mice lacking PD-1 selectively in Treg cells. Reduced PI3K–AKT pathway signaling is a mechanism underlying enhanced suppressive capacity of PD-1–deficient Treg. Our results show PD-1 is an “equal-opportunity” inhibitor of T cell activities. When PD-1 is lost on cells whose primary function is to suppress immune responses, the consequence is a stronger suppressor cell. This means that the outcome of PD-1 pathway blockade in vivo is a summation of increased Teffector and enhanced Treg suppressive activities.  The consequences for CTLA-4 combination therapy and for hyperprogression will be discussed. The frequency of PD-1+CD8+ T cells to PD-1+ Treg in the tumor microenvironment is a potential biomarker of PD-1 efficacy. Additional immune inhibitory pathways contribute to tumor immune evasion. HHLA2, a member of the B7 family of immunoregulatory ligands, has costimulatory effects through the CD28 family member TMIGD2, but HHLA2 also has inhibitory effects on T cells. We identified KIR3DL3 as an inhibitory receptor for HHLA2 in T and NK cells. We generated HHLA2 and KIR3DL3 antibodies that block the immune inhibitory activity of HHLA2 and enhance T cell and NK activities, preserving the costimulatory signal. HHLA2 is frequently expressed in several tumor types including clear cell renal cell carcinoma (ccRCC). HHLA2 expression was non-overlapping with PD-L1 expression, suggesting that HHLA2 mediates a mechanism of tumor immune evasion that is independent from PD-L1. Blockade of both PD-1 and KIR3DL3 pathways may be a more effective way to reverse tumor immune evasion.

Learning Objectives:

1. Understand how the outcome of PD-1 blockade is a summation of increased Teffector activity and increased Treg suppressive activity

2. Explain how PD-1 blockade leads to increased T reg activity

3. Understand a new checkpoint pathway, HHLA2-KIR3DL3