Intracellular Signaling triggered by engagement of PD-1 by PD-L1

Abstract

Abstract Blockade of PD-1/PD-L1 is in effective anti-tumor immunotherapy. To further elucidate the mechanism by which PD-1/PD-L1 engagement causes T cell dysfunction, we investigated intracellular signaling triggered by PD-1/PD-L1 ligation in T cells. We describe a novel antibody that detects PD-1 signaling. This antibody detects phosphorylation of the immunotyrosine switch motif (ITSM) in the intracellular tail of both human and mouse PD-1 (phospho-PD-1). We first examined the expression of PD-1 and phosphorylated PD-1 in T cells from human PBMC. We found that without CD3 and CD28 mAb stimulation, PD-1 and phospho-PD-1 had minimal expression on CD3+CD4+ and CD3+CD8+ cell. Upon stimulation with CD3 and CD28 mAb, we observed increased expression of PD-1 and phospho PD-1 in both CD3+CD4+ and CD3+CD8+ cells. Furthermore, upon CD3 and CD28 mAb stimulation and PD-L1 recombinant protein treatment, the level of phospho PD-1 increased in CD3+CD4+ and CD3+CD8+ cells. Utilizing a co-culture system of Jurkat-hPD-1 and 300-hPD-L1, we mimicked the cell-cell interaction through molecular ligation of PD-1 and PD-L1. Upon stimulation with CD3 and CD28 mAb in this co-culture system, we detected increased phospho PD-1 signal and phospho SHP-2 signal by Western blot. When CD3 and CD28 mAb stimulated Jurkat-hPD-1 cells were treated with recombinant PD-L1 protein, we observed decreased IL-2 secretion which indicates PD-1/PD-L1 ligation suppresses T cell activation and corresponds with phospho PD-1 expression. This study examines the intrinsic intracellular signals upon PD-1/PD-L1 engagement, and the results may promote a better understanding of the mechanism of T cell dysfunction by PD-1/PD-L1 ligation.