Melanocytes are potential immunocompetent cells: Evidence from recognition of immunological characteristics of cultured human melanocytes

The dendritic nature, the strategic location, and other accumulated evidence about the immunologic characteristics of melanocytes suggest that they are not only professional melanin producing cells but are also immunocompetent cells. In this study, we demonstrated that cultured melanocytes express low levels of some immunologically important surface markers such as intercellular adhesion molecule-1 and CD40. Moreover, we report for the first time CD40 expression by melanocytes can be up-regulated by interferon-gamma ( IFN-gamma) stimulation. Optimal enhancement of CD40 expression was observed at an IFN-gamma concentration of 300 U/ml after a co-culture period of 72 h. Maximal melanocyte-driven T lymphocyte proliferation and interleukin-12 secretion were also observed following the same treatment and proved to be CD40-dependent. Our data further suggest that upon CD40 ligation, melanocytes up-regulate their co-stimulating and adhesion molecules. In addition to previous descriptions about the melanocyte's antigen processing and presenting capacity, we therefore hypothesize a dynamic model in which melanocytes alternatively work as heterogeneous antigen presenting cells. As a result of CD40 expression on the cell surface, melanocytes might contact and subsequently stimulate CD8+ cytotoxic T lymphocytes directly via CD40 CD40L interaction in some cases.