Distribution and maturation of skin dendritic cell subsets in two forms of cutaneous T-cell lymphoma: mycosis fungoides and Sézary syndrome.

Dendritic cells (DCs) critically regulate immune responses and the "immune-surveillance" of tumours. This study retrospectively analysed the distribution and maturation status of DC-subsets in T-cell lymphoma of the skin. Mycosis fungoides and Sézary syndrome (n = 25) were investigated immunohistochemically for DC subsets, based on C-type lectin receptor expression: Langerhans' cells (langerin/CD207+, DEC-205/CD205+), dermal DCs (DC-SIGN/CD209+, CD205+) and plasmacytoid DC (BDCA-2/CD303+). Maturation status was assessed by double-labelling for CD83 and CD208/DC-LAMP. DCs were interspersed between the neoplastic infiltrate, and a marked increase in numbers of all three subsets was noted, DC-SIGN+ dermal DCs constituting the majority. Substantial numbers of plasmacytoid DCs were consistently observed. Most DCs in epidermis and dermis were phenotypically immature. Amongst the relatively few mature DCs in the dermis, langerin+ cells predominated. There was a positive correlation between the histological intensity of the tumour infiltrate and DC numbers. It is possible that mature DCs reflect ongoing anti-tumour immune responses, and immature DCs the induction of tumour tolerance.