Evaluation of circulating PD-1 and PD-L1 as diagnostic biomarkers in dogs with tumors

BACKGROUNDProgrammed cell death protein-1 (PD-1) and programmed cell death ligand-1 (PD-L1) have important roles in tumor evasion of the immune system. OBJECTIVESThis study aimed to assess the diagnostic utility of circulating PD-1 and PD-L1 levels in healthy dogs and dogs with tumors. METHODSCirculating PD-1 and PD-L1 levels in the serum of 71 dogs with tumors were compared with those of 52 healthy dogs by performing enzyme-linked immunosorbent assay (ELISA). RESULTSThe ELISA results revealed higher circulating PD-1 and PD-L1 levels in dogs with tumors (2.9 [2.2-3.7] ng/mL; median [IQR] and 2.4 [1.4-4.4] ng/mL, respectively) than in healthy dogs (2.4 [1.9-3.0] ng/mL; p = 0.012 and 1.4 [0.9-2.1] ng/mL; p < 0.001, respectively). Especially, there was a significant difference in circulating PD-1 levels between healthy dogs and dogs with malignant epithelial tumors (2.4 [1.9-3.0] ng/mL and 3.1 [2.6-4.4] ng/mL, respectively; p < 0.01). In addition, there was a significant difference in circulating PD-L1 levels between healthy dogs and dogs with lymphomas (1.4 [0.9-2.1] ng/mL and 2.7 [1.6-5.8] ng/mL, respectively; p < 0.001). CONCLUSIONThis study indicates that circulating PD-1 and PD-L1 have potential as tumor diagnostic biomarkers in dogs with tumors.