Cancer is one of the most significant causes of death in dogs. Antibody drugs targeting the PD-1/PD-L1 axis represent a promising
immunotherapy for both human and canine
cancers. However, the regulation mechanisms of PD-L1 expression in canine
cancers require further investigation to better understand the resistance mechanisms to anti-PD-L1
therapy. Recent reports have shown that CMTM6 and CMTM4 are critical regulators of
PD-L1 protein expression in human
cancer cells. By preventing PD-L1 from lysosome-mediated degradation, CMTM6 maintains PD-L1 expression on the cell surface. However, the literature has not reported on CMTM6 and CMTM4 in dogs, and their functions are completely unknown. To reveal a regulation mechanism of PD-L1 in canine
cancers, this study firstly identified the gene sequences of CMTM6 and CMTM4. Then, the expression analysis of these
proteins was performed by immunohistochemistry. Furthermore, the functions of CMTM6 and CMTM4 in regulating PD-L1 expression were examined by gene knockdown of CMTM6 and CMTM4. Canine CMTM6 and CMTM4 displayed high amino acid sequence identities compared with those of humans and mice. An immunohistochemical analysis using cross-reactive
antibodies revealed that canine
malignant melanoma and
osteosarcoma express CMTM6, CMTM4, and PD-L1 simultaneously. Gene knockdown of CMTM6 and CMTM4 with RNA interference significantly reduced the cell surface expression of PD-L1 in a canine cell line. These results suggest that CMTM6 and CMTM4 are regulators of PD-L1 expression in canine
cancers and could serve as potential therapeutic targets to enhance antitumor immunity.