Regulation of T cell activation and tolerance by PDL2.

T cell activation and tolerance are regulated by costimulatory molecules. Although PD-1 serves as a crucial negative regulator of T cells, the function of its ligands, PDL1 and PDL2, is still controversial. In this study, we created a PDL2-deficient mouse to characterize its function in T cell activation and tolerance. Antigen-presenting cells from PDL2-/- mice were found to be more potent in activation of T cells in vitro over the wild-type controls, which depended on PD-1. Upon immunization with chicken ovalbumin, PDL2-/- mice exhibited increased activation of CD4(+) and CD8(+) T cells in vivo when compared with WT animals. In addition, T cell tolerance to an oral antigen was abrogated by the lack of PDL2. Our results thus demonstrate that PDL2 negatively regulates T cells in immune responses and plays an essential role in immune tolerance.