TIM-3 Is a Potential Immune Checkpoint Target in Cats with Mammary Carcinoma

Simple Summary Feline mammary carcinomas are highly prevalent tumors, with aggressive behavior and scarce therapeutic options. Thus, there is a high need for novel biomarkers and therapeutic targets. In human medicine, immune checkpoint inhibitors targeting TIM-3 are emerging as promising treatment strategies; however, the clinical relevance of immune checkpoints in feline cancers is sparse. To unravel the clinical importance of TIM-3 in feline mammary carcinoma, we analyzed the intratumor expression and serum levels of the TIM-3 receptor in tumor-bearing cats. The results obtained show that TIM-3 is highly expressed in both immune and cancer cells, and the localization of TIM-3 expression can predict clinical outcomes in feline mammary carcinoma. TIM-3 levels in serum were reduced in diseased compared to healthy animals. These findings shed light on the biological role of TIM-3 in feline mammary carcinoma and support clinical testing of novel therapies targeting TIM-3. Recent findings in human breast cancer (HBC) indicate that T-cell immunoglobulin and mucin-domain-containing molecule-3 (TIM-3)-targeted therapies may effectively activate anticancer immune responses. Although feline mammary carcinoma (FMC) is a valuable cancer model, no studies on TIM-3 have been developed in this species. Thus, we evaluated the expression of TIM-3 by immunohistochemistry in total (t), stromal (s), and intra-tumoral (i) tumor-infiltrating lymphocytes (TILs) and in cancer cells, of 48 cats with mammary carcinoma. In parallel, serum TIM-3 levels were quantified using ELISA and the presence of somatic mutations in the TIM-3 gene was evaluated in 19 tumor samples. sTILs-TIM3+ were more frequent than iTILs-TIM-3+, with the TIM-3 ex-pression in sTILs and cancer cells being associated with more aggressive clinicopathological features. In contrast, the TIM-3 expression in iTILs and tTILs was associated with a more benign clinical course. Moreover, the serum TIM-3 levels were lower in animals with FMC when compared to healthy animals (p < 0.001). Only one somatic mutation was found in the TIM-3 gene, at intron 2, in one tumor sample. Altogether, our results suggest that the expression of TIM-3 among TILs subpopulations and cancer cells may influence the clinical outcome of cats with FMC, in line with the previous reports in HBC.