Possible mechanisms of the crosstalk between Langerhans cells and regulatory T cells in extramammary Paget disease by receptor activator of nuclear factor kappa B (RANK) ligand/RANK pathways

Background Extramammary Paget disease (EMPD) is a skin adenocarcinoma of apocrine gland origin, in which Paget cells express receptor activator of nuclear factor kappa B (RANK) ligand (RANKL) and matrix metalloproteinase (MMP)-7, and release soluble (s) RANKL into the tumour microenvironment. We previously reported that about 60% of the RANK(+) cells among the stromal cells are M2 macrophages, but the identity of the remaining population of RANK+ cells is still unknown. Objectives To investigate the unknown subpopulation of RANK-expressing cells in EMPD. Methods The main population of RANK-expressing cells in the epidermis was composed of epidermal Langerhans cells (LCs). To explore the effects of RANKL on LCs, we stimulated LCs generated from human CD34(+) hematopoietic progenitor cells with graded concentrations of sRANKL. To further examine the correlation between LCs and regulatory T cells (Tregs) in EMPD, we employed immunohistochemical staining. Results sRANKL stimulation was shown to augment the production of C-C motif chemokine ligand 17 (CCL17) from LCs. We additionally demonstrated CCL17 expression by CD1a(+) LCs in EMPD in an immunofluorescence study. Spearman's rank correlation test confirmed a correlation between the number of LCs and the number of Foxp3(+) Tregs in the lesional skin of invasive EMPD. In addition, the numbers of Foxp3(+) Tregs in the sentinel lymph nodes of metastatic EMPD were significantly higher than those of metastatic melanoma, which did not express RANKL. Conclusions The findings suggest that the RANKL/RANK pathway in EMPD might contribute to the recruitment of Tregs and to maintenance of the tumour microenvironment.