Utility of tumour-infiltrating CD25+FOXP3+ regulatory T cell evaluation in predicting local recurrence in vertical growth phase cutaneous melanoma

Abstract

Tumour-infiltrating lymphocytes (TILs) represent the local immune response to cancer, however, their correlation with tumour behaviour is not unanimously considered in the literature. Most studies have not characterized TILs, that are known to comprise distinct subsets, bearing different roles in the complex tumour microenvironment. Characterization of patient lymphocytes has been mainly performed in peripheral blood, that is not always representative of the local immune status. Only few investigations have been performed at the tissue level in cancer, including melanoma. TILs encompass different populations of effector and regulatory T cells (Tregs), and the relevance of the latter in tumour progression is widely accepted. The transcription factor gene product FOXP3 is considered the most reliable marker of Tregs. However, it has not been extensively evaluated in primary cutaneous melanoma. We analyzed 66 vertical growth phase primary cutaneous melanomas, aiming at finding differences in TIL subsets between two groups of cases, that behaved differently in terms of local recurrence. In our study, the percentage of Tregs, as characterized by CD25 and FOXP3 expression, both among tumour cells, inside tumour parenchyma and at its periphery, and among TILs, at the tumour-stroma boundary, was significantly higher in cases that recurred than in those that did not (p=0.00065; p=0.00014; p<0.00001, respectively). TIL characterization by immunohistochemistry in melanoma diagnostic reports, could add further information. The analysis of a larger series of patients and correlation with other clinical parameters, such as distant metastases and/or patient survival, are mandatory for validating its use as a prognostic indicator.