The bootstrap consensus tree was inferred from 1000 replicates with the neighbor-joining method using the MEGA 7.0 software. the expression of PD-L1 in tumor tissues of equine malignant melanoma (EMM). The amino acid sequences of EqPD-1 and EqPD-L1 share a considerable identity and similarity with homologs from non-primate species. Two clones of the anti-bovine PD-L1 mAbs recognized EqPD-L1 in flow cytometry, and one of these cross-reactive mAbs blocked the binding of equine PD-1/PD-L1. Of note, immunohistochemistry confirmed the PD-L1 expression in EMM tumor tissues. A cultivation assay revealed that PD-L1 blockade enhanced the production of Th1 cytokines in equine immune cells. These findings showed that our anti-PD-L1 mAbs would be useful for analyzing Azithromycin Dihydrate the equine PD-1/PD-L1 pathway. Further research is warranted to discover the immunological role of PD-1/PD-L1 in chronic equine diseases and elucidate a future application in immunotherapy for horses. == Introduction == Programmed death-1 (PD-1) is an immunoinhibitory receptor, which is expressed on activated and exhausted T cells [13]. Programmed death ligand 1 (PD-L1), also called CD274, is the ligand of PD-1 expressed on immune cells, including antigen-presenting cells, and tumor cells [1,4,5]. The interaction of PD-1 and PD-L1 suppresses the activation signal mediated by T-cell receptors and inhibits effector functions of T cells, Azithromycin Dihydrate including cytokine production and cell proliferation [14]. This pathway is invaluable for regulating excessive immune responses [68]; however, in cancers, tumor cells utilize the suppression of T cells mediated by PD-1/PD-L1 to circumvent anti-tumor immune responses [911]. In human medicine, the blocking antibodies targeting PD-1 or PD-L1 have been leveraged for treatment of various types of cancers and resulted in remarkable outcomes with 20%90% response rates in multiple clinical trials [1215]. Equine malignant melanoma (EMM) is a common neoplasm among aged gray horses, resulting in dermal tumors at multiple sites [16]. A previous study reported that approximately 80% of aged gray horses developed dermal melanoma and speculated that all gray horses would develop this tumor as they reach old age [17]. Although cellular immune response is critical for eradicating melanoma, but several mechanisms have been suggested to limit anti-tumor immunity in EMM based on the findings for human malignant melanoma [18]. However, no studies are available on immune evasion mechanisms in EMM, and immune exhaustion mediated by PD-1 and PD-L1 has not been investigated in horses. In our previous research, we established anti-bovine PD-L1 monoclonal antibodies (mAbs) [19]. We found that PD-1 and PD-L1 play crucial roles in immune exhaustion and disease progression in bovine chronic infections [2024] and in canine cancers including malignant melanoma [25,26]. Importantly, we noted that the PD-1/PD-L1 blockade enhances T-cell responses in cattle and dogs [2026] and exhibits therapeutic effects in bovine chronic infections and canine malignant melanoma [2731]. Until now, no information was available on cDNA sequences, expression, and function of PD-1/PD-L1 in horses. Azithromycin Dihydrate Furthermore, the role of the PD-1/PD-L1 pathway in EMM remained unclear. Based on the findings of our previous studies, we hypothesized that PD-1 and PD-L1 may provide potential targets for immunotherapy against EMM. Therefore, in this study, we identified cDNA sequences of equine PD-1 (EqPD-1) and PD-L1 (EqPD-L1), evaluated the blocking effects of our anti-bovine PD-L1 mAbs against EqPD-L1, and confirmed the expression of PD-L1 on EMM. == Materials and methods == == Horse blood samples and cell preparation == Heparinized blood samples were collected from Thoroughbred horses in farms and veterinary hospitals in Hokkaido, Japan. Peripheral blood mononuclear cells (PBMCs) were purified using density gradient centrifugation on Percoll (GE Healthcare, Little Chalfont, UK), washed three times with phosphate-buffered saline (PBS), and suspended in PBS. All experimental procedures were conducted following approval from the local committee for Ctsk animal studies according to the Hokkaido University (200093). Informed consent was obtained from all.