Tumor-infiltrating lymphocytes vary in different canine soft tissue sarcoma histological types.

Soft tissue sarcomas (STSs) are conventionally viewed as poorly immunogenic tumors; however, some human STSs have recently been reported to elicit an immune response, thus representing potential candidates for immunotherapy. Data regarding immune cell infiltrates in canine STSs are limited and reported without tumor-type stratification. The aim of this study was to retrospectively assess tumor-infiltrating lymphocytes (TILs) in canine STSs of 5 different histotypes. Eighty-seven canine STSs were collected: 22 perivascular wall tumors (PWTs), 19 liposarcomas, 17 fibrosarcomas, 16 myxosarcomas, and 13 leiomyosarcomas. The tumors were graded and immunolabeled for CD3, CD20, and FoxP3, and slides were scanned. T-cell, B-cell, Treg, and total TIL densities were quantified with QuPath software and expressed as cells/mm. The B/T-cells ratio and Treg/T-cell proportions were calculated. Total TIL densities were higher in PWTs and myxosarcomas (median = 225 and 303, respectively). PWTs had higher T-cell density but lower Treg proportion (median = 152 and 7.6% respectively). Myxosarcomas had higher Treg densities and B/T-cell ratios (median = 24.4 and 1.57, respectively). No association with grade was found among STSs as a group. In myxosarcomas, higher grade was significantly associated with higher total TILs, and CD20+ and FoxP3+ cell densities ( < .05). The results suggest that PWTs and myxosarcomas may represent the most immunogenic STS types. Myxosarcomas elicit a B-cell and Treg-rich immune response; PWTs stimulate a T-cell-rich and Treg-poor reaction. The immune system response may contribute to the more aggressive behavior of myxosarcomas and the more indolent course of PWTs.