AbstractIntroductionChronic infections lead to the functional exhaustion of T cells. Exhausted T cells are phenotypically differentiated by the surface expression of the immunoinhibitory receptor, such as programmed death‐1 (PD‐1). The inhibitory signal is produced by the interaction between PD‐1 and its PD‐ligand 1 (PD‐L1) and impairs the effector functions of T cells. However, the expression dynamics of PD‐L1 and the immunological functions of the PD‐1/PD‐L1 pathway in chronic diseases of pigs are still poorly understood. In this study, we first analyzed the expression of PD‐L1 in various chronic infections in pigs, and then evaluated the immune activation by the blocking assay targeting the swine PD‐1/PD‐L1 pathway.MethodsIn the initial experiments, anti‐bovine PD‐L1 monoclonal antibodies (mAbs) were tested for cross‐reactivity with swine PD‐L1. Subsequently, immunohistochemical analysis was conducted using the anti‐PD‐L1 mAb. Finally, we assessed the immune activation of swine peripheral blood mononuclear cells (PBMCs) by the blockade with anti‐PD‐L1 mAb.ResultsSeveral anti‐PD‐L1 mAbs tested recognized swine PD‐L1‐expressing cells. The binding of swine PD‐L1 protein to swine PD‐1 was inhibited by some of these cross‐reactive mAbs. In addition, immunohistochemical analysis revealed that PD‐L1 was expressed at the site of infection in chronic infections of pigs. The PD‐L1 blockade increased the production of interleukin‐2 from swine PBMCs.ConclusionsThese findings suggest that the PD‐1/PD‐L1 pathway could be also involved in immunosuppression in chronic infections in pigs. This study provides a new perspective on therapeutic strategies for chronic diseases in pigs by targeting immunosuppressive pathways.