Immunohistochemical evaluation of CD4 and CD8 tumour infiltrating lymphocytes and their prognostic significance in canine mammary tumours

Tumour-infiltrating lymphocytes (TILs) are critical components of the tumour microenvironment (TME), comprising T cells, B cells and natural killer cells that influence tumour progression and clinical outcome. Among these, CD4 and CD8 T cells play pivotal roles, with CD4 cells regulating immune responses and CD8 cells mediating cytotoxic effects. Their distribution between stromal and intratumoural regions holds prognostic significance. The objective of the present study was to investigate the localisation of TILs in relation to histological grade, as well as to evaluate the prognostic relevance of the CD4CD8 ratio. Immunohistochemistry was performed on 25 tumour samples, which revealed abundant CD4 and CD8 TILs in grade I tumours, which declined as grade advanced, whereas CD4 T cells showed a slight predominance in high-grade tumours. Lymphocytes were primarily confined to stromal regions, followed by peritumoural areas, while intratumoural localisation was consistently sparse. Importantly, the CD4CD8 ratio, increased with tumour grade, reflecting a relative predominance of CD4 T cells in high-grade tumours and suggesting progression towards an immunosuppressive microenvironment. KaplanMeier survival analysis revealed that dogs with a low CD4CD8ratio exhibited more favourable overall survival compared to those with a higher ratio (p