Differences in Histopathology and Local Immune Response in Steady and Progressive Natural Transmissible Venereal Tumors in Mexican Dogs.

The canine transmissible venereal tumor (TVT) is a neoplasm of the external genitalia of dogs, considered one of four reported contagious tumors in animals. These tumors have different presentations, with steady, progressive, or regressive stages. In some areas of Mexico, where the prevalence of TVT is high (5.15%), two morphological types are usually observed: one steady, pedunculated, strawberry-like (Type A) and one progressive, multilobulated, cauliflower-like (Type B). This study aimed to characterize the histopathological and inflammatory infiltrate patterns in eight stray dogs showing both morphological types of natural TVT (n = 4 each), to identify potential differences between tumor morphologies. Histopathological and immunohistochemical techniques were applied to tumor samples to evaluate the interaction between pathological morphology and the following cell markers: ionized calcium-binding adaptor molecule 1 (IBA1) for activated macrophages (including resident macrophages), inducible nitric oxide synthase (iNOS) for M1 macrophages, CD163 for M2 macrophages, CD3 for T lymphocytes, CD20 for B lymphocytes, and lambda light chain for plasma cells. The results showed a greater inflammatory infiltrate in Type A tumors than in Type B ones, with a parallel increase in activated macrophages and B lymphocytes. The presence of M1 and M2 macrophages was scarce in both types of tumors, and T lymphocytes were almost absent. This study reveals a stronger and more balanced local immune response in dogs with Type A TVTs compared with Type B tumors, which may underlie differences in tumor characteristics, although individual tumor heterogeneity should be considered.