Changes in mast cell density, fibrosis, and immunohistochemical expression of tryptase, chymase, SCF, and PAR2 during regression of canine transmissible venereal tumors

This study aimed to evaluate the density, subtypes, and potential functional roles of mast cells in canine transmissible venereal tumors (TVTs) during tumor progression (pretreatment) and regression following vincristine therapy. Biopsy samples were collected from dogs at 3 different time points: day 0 (n = 20), day 7 (n = 20), and day 14 (n = 10). Total mast cell counts were assessed using toluidine blue (TB) staining, while immunohistochemistry for chymase and tryptase was performed to classify mast cell subtypes. Masson's trichrome staining was used to evaluate the severity of fibrosis during regression. In addition, immunohistochemical expression of stem cell factor (SCF) and protease-activated receptor 2 (PAR2) were examined to explore the possible roles of mast cells in the tumor microenvironment. In the late regression group (day 14), the number of TB-, tryptase-, and/or chymase-positive mast cells increased by 3.46, 2.49, and 3.4 times, respectively, compared with the progression group (day 0). In addition, the number of SCF-positive cells and the expression of PAR2 were 6- and 2.38-fold higher in the day 14 group compared with day 0, respectively. While mast cell accumulation is a known feature of various tumors, its precise role in tumor biology remains controversial. The findings of this study suggest a potential role of mast cells and their associated mediators in tumor regression and tissue remodeling processes of TVTs.