Canine Mast Cell Tumour-Immune System Interaction: An Immunohistochemical Investigation of Tumour Infiltrating T Cells

Introduction: Mast cell tumour (MCT) is one of the most common skin neoplasms in dogs. Although various methods have been proposed for prognostic assessment, the prognosis of grade II/low-grade MCT still remains controversial. In the last few years, the interaction between tumour and immune system has been extensively studied, allowing important progresses in prognostic assessment. In particular, the role of tumour infiltrating lymphocytes (TILs) has been proven useful as a prognostic marker in various human neoplasms. The aim of this study was to characterize the T cell infiltration in MCT and evaluate its relationship with location (cutaneous, subcutaneous) and nodal metastatic spread. Materials and Methods: Thirty-nine MCTs [26 cutaneous (all grade II/low-grade), 13 subcutaneous (various growth patterns)] from 34 dogs with known sentinel lymph node (SLN) metastatic status (according to Weishaar et al, 2014) were selected. Samples were routinely processed for histopathological examination and immunohistochemistry for CD3. A semiquantitative grading of interstitial and perivascular CD3+ cells was performed, followed by digital imaging analysis of CD3+ % area in six hot spot fields at 40x (ImageJ software). Results: Intratumoural CD3+ cells were significantly increased in subcutaneous MCTs as compared with cutaneous MCTs (mainly perivascular infiltrates), and in non-metastatic (HN0) cutaneous MCTs as compared with metastatic (HN1-HN2-HN3) cutaneous MCTs (mainly interstitial infiltrates). Conclusions: The results suggest that cutaneous and subcutaneous MCTs have a different tumour immune microenvironment and that increased intratumoral T cell infiltration might contribute to the control of nodal metastatic spread of cutaneous MCT