Age distribution and regression-associated immunophenotypic features in canine cutaneous histiocytoma.

Canine cutaneous histiocytoma (CCH) is a benign tumor characterized by spontaneous immune-mediated regression. Although common in young dogs, it also occurs in older animals. This retrospective study evaluated histologic and immunohistochemical features of CCH across age groups, focusing on mitotic count, lymphocytic infiltration grade, major histocompatibility complex II (MHC-II) expression score and pattern, and CD206 expression. Cases were divided using an a priori 4-year cutoff based on reported age distributions. Immunohistochemistry was performed for CD3, MHC-II, and CD206, with MHC-II expression categorized by score and pattern (membranous versus cytoplasmic). Thirty-eight cases were included, and the age range was 0.75-14 years (median, 7 years). MHC-II score differed significantly between age groups (P=0.0118), whereas mitotic count, lymphocytic infiltration grade, CD206 score, and MHC-II pattern did not. Age showed a weak negative correlation with MHC-II score (r=-0.3299, P=0.0431). However, multivariable logistic regression demonstrated that age was not independently associated with the membranous MHC-II pattern after adjustment for lymphocytic infiltration grade (OR=1.20, 95% CI 0.99-1.50). In contrast, higher lymphocytic infiltration was associated with membranous MHC-II expression (OR=9.87, 95% CI 1.96-77.54). These findings suggest that, in this cohort, regression-associated MHC-II localization was more closely related to lymphocytic infiltration grade than to age. In conclusion, CCH in older dogs showed largely comparable histologic and immunophenotypic features to those in younger dogs, although age-related differences in MHC-II expression score should be interpreted cautiously.