Phase- and stage-related proportions of T cells bearing the transcription factor FOXP3 infiltrate primary melanoma

Although tumor-infiltrating lymphocytes (TILs) of primary cutaneous melanoma WCM) include cytolytic T cells able to exert anti-PCM immunity, progression of PCM most frequently occurs, raising the hypothesis that the PCM microenvironment may also exert suppressive forces, for example, possibly developed by regulatory T (TREG) lymphocytes. The aim of this study was to investigate whether TILs of PCMs include lymphocytes bearing the transcription factor forkhead box protein P3 (FOXP3), which is the T-REG lineage specification molecule in mice, and is debated to have a similar role in humans. Fourteen patients with PCM were selected, of which four had radial growth phase (RGP) stage I melanoma, five had vertical growth phase (VGP) stage I melanoma, and five had VGP stage III-IV melanoma. Formalin-fixed, paraffin-embedded sections were utilized for immunohistochemical single and double stainings. TILs of PCMs included FOXP3-bearing lymphocytes, which predominantly were CD20- and CD8-negative, but CD3-, CD4-, and CD25-positive, thus consistent with the standard immunophenotypical characteristics of "natural" T-REG cells. Further, the proportions of FOXP3-bearing lymphocytes were higher in vertical than in RGP (P-0.001), as well as in late than in early melanoma stages (P<0.001). Should these FOXP3-bearing lymphocytes actually exert regulatory capabilities within the PCM microenvironment, they may suppress "in vivo" the local anti-PCM immune response, thus favoring melanoma progression.