IMMUNOHISTOCHEMICAL DETECTION OF CD25+LYMPHOCYTES IN FELINE INJECTION SITE SARCOMA AND POST-INJECTION PANNICULITIS

Introduction: CD25 is the a-subunit of the membrane-bound interleukin-2 receptor, which is mainly expressed by regulatory T cells (Tregs). Normally these cells regulate immune system activity and prevent autoimmunity. Imbalanced function or number of Tregs, either enhanced or decreased, might lead to tumour development and autoimmunity, respectively. In the present study we hypothesized the presence of CD25+ cells among tumour infiltrating lymphocytes (TILs) that are normally associated with feline injection site sarcoma (FISS). To this end we analyzed CD25 protein expression by immunohistochemistry in 18 cases of FISS and in 15 cases of post-injection panniculitis (PIP). Materials and Methods: Sections (4 mm) were immunolabelled using the avidinebiotin technique for CD25 (MAb). Labelling was evaluated semiquantitatively for percentage of positivity (10 fields at 40) and the expression was considered as: percentage !20%, weak positivity; 21e50%, moderate positivity; O50%, strong positivity. Results: All FISS specimens and 14 of 15 PIP specimens had lymphocytes positive for CD25. The immunolabelling appeared as distinct membrane or diffuse cytoplasmic expression. Nine cases of FISS strongly expressed CD25; the median expression percentage for all 18 cases was 55%. Six cases of PIP had strong positivity, with a median expression percentage for all the cases of 41%. Conclusions: The present study identified large numbers of CD25+ cells among TILs and in the lesions of panniculitis. CD25+ lymphocytes inhibit Th1 and CD8+ immune responses and consequently they could represent a permissive and stimulatory factor for tumour development.