Relationship between PD-L1 Expression and Tumor-Infiltrating Lymphocytes in Canine Mammary Tumor

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Background: Studies pointed out that the tumor-infiltrating lymphocytes (TILs) have considerable importance in canine mammary tumor (CMT). On the other hand, cancer cells sometimes find ways to use immune checkpoint proteins as a shield to avoid being identified and attacked by the immune system as programmed death 1 ligand 1 (PD-L1). In this study, it was investigated the relationship between PD-L1 expression, stromal tumor-infiltrating lymphocytes (TILs) in canine mammary tumor (CMT), and the association with clinical and pathological characteristics of the tumors.Materials, Methods & Results: PD-L1 expression and TILs were assessed in 23 female dogs with CMT. The tumors were grouped into simple carcinoma (CA, n = 8) and complex carcinoma (CC, n = 15). Stromal TILs were assessed using two thresholds as TILs-Low representing < 50% of infiltrate within stromal area and TILs-High representing ≥ 50% of stromal area. Clinicopathological data of CMT was characterized according to key parameters, as well as survival rates. TILs evaluation within tumor stroma revealed that 65.2% (n = 15) of tumors had TILs-Low. PD-L1 expression and stromal TILs were significantly associated (P = 0.009). PD-L1 expression was observed in 39% (n = 9) of all tumors of which 17.4% (n = 4) were from CA group and 21.7% (n = 5) were from CC group. PD-L1 expression within TILs was observed in 39% (n = 9) of the tumors. PD-L1 in malignant epithelium was present in all lymph node metastasis (n = 5). PD-L1 was associated with involvement of regional lymph nodes (P = 0.034). Survival curves demonstrated TILs-Low had higher (P = 0.010) overall survival (OS) compared with TILs-High, and PD-L1+ and PD-L1– (P = 0.06) did not differed. The clinicopathological variables significantly correlated with OS by univariate analysis were the histological grade (P = 0.009), lymph node involvement (P = 0.004), stromal TILs (P = 0.016), and PD-L1+/TILs-High vs. PD-L1–/TILs-Low (P = 0.010). Multivariate analysis revealed that group of tumors with grade II-III was independent and negative prognostic factors for OS.Discussion: In this study, PD-L1 was differently expressed according to the histologic subtypes of TMC. Currently, has been showed the presence of PD-L1 in several canine cancer. Nevertheless, only a few studies have described PD-L1 protein expression in dog tumors and showed PD-L1 was constitutively expressed on canine tumor cell lines, although the levels of basal expression were very variable. This expression can be modulated by IFN-γ exposure. In the present study, it was found a strong PD-L1 expression on TILs. The increase in PD-L1 cell surface expression by tumor cells can lead to decreased T-cell proliferation and increased apoptosis. In human breast cancer (BC) the PD-L1 expression was expressed in TILs and tumor epithelium. It has been reported the association of stromal TILs and PD-L1 expression with aggressive types and stages of BC. In this study, it was detected PD-L1 expression in malignant epithelium in all lymph node metastasis. PD-L1 overexpression was significantly associated with a series of clinicopathological parameters. It was demonstrated that PD-L1+/TILs-High had higher risk of overall survival (OS) than another group of interaction. High PD-L1 expression may be a prognostic indicator for reduced OS, while tumor PD-L1+ was associated with poorer disease-free survival. The presence of TILs has shown to be potentially predictive and a prognostic factor in BC subtypes. In CMT, it has been reported that a high proportion of TILs was correlated to several malignancy characteristics. In relation to PD-L1, further research is necessary to clarify this immune checkpoint as a potential therapeutic target and its application in clinical practice in CMT.