Serum programmed death ligand 2 is elevated in cats with mammary carcinoma

Since the PD-1/PD-L1/PD-L2 axis plays a vital role in immune tolerance and T-cell exhaustion, having emerged as a target for breast cancer immunotherapy, we investigated the relevance of serum PD-L2 (sPD-L2) levels in feline mammary carcinoma (FMC), to validate its potential use as a diagnostic and/or prognostic biomarker, and as a future target for immunotherapy. To accomplish that, sPD-L2 levels were quantified by enzyme-linked immunosorbent assay and compared between healthy control cats and cats with mammary carcinoma, also stratified by tumor molecular subtype. Statistical associations between sPD-L2 levels, clinicopathological features and other serum immune checkpoint molecules (sPD-1, sPD-L1, sCTLA-4, sTNF-α, sVEGF-A, sVEGFR-1, sVEGFR-2 and sLAG-3) were also analyzed. Results revealed that sPD-L2 levels were significantly higher in the FMC group (p < 0.0001), with a concentration of 1934 pg/mL established as the best cut-off value to distinguish sick from healthy cats (specificity: 96.6%; sensitivity: 93.6%; AUC = 0.980). Interestingly, cats with HER2-positive or Triple-Negative (TN) mammary carcinoma subtypes showed higher sPD-L2 levels (p < 0.0001). According to receiver-operating characteristic (ROC) curve analysis, a serum PD-L2 concentration of 5499 pg/mL represented the optimal cut-off for distinguishing cats with these two subtypes from cats with Luminal A (LA) and Luminal B (LB) carcinomas (specificity: 95.2%; sensitivity: 82.6%; AUC = 0.919). Furthermore, in the FMC group, positive correlations were found between sPD-L2 levels and sCTLA-4 (r = 0.496, p = 0.001), sTNF-α (r = 0.482, p = 0.0009), sVEGF-A (r = 0.54, p = 0.0002), sVEGFR-1 (r = 0.339, p = 0.025), sVEGFR-2 (r = 0.322, p = 0.033) and sLAG-3 levels (r = 0.324, p = 0.032). Finally, significant associations were found between sPD-L2 levels and progesterone receptor (PR) status (p = 0.002), HER2 status (p = 0.009) and Ki-67 index (p < 0.0001). A serum sPD-L2 concentration of 3732 pg/mL was identified as the optimal cut-off value for distinguishing FMCs with a high Ki-67 index (≥ 14%) from those with a low index (specificity: 80.0%; sensitivity: 94.1%; AUC = 0.906). In conclusion, our findings suggest that sPD-L2 is a potential biomarker for FMC, particularly in HER2-positive and TN subtypes.