The immunology and inflammatory responses of human melanocytes in infectious diseases.

Melanin is a canonical and major defense molecule in invertebrates but its role in mammalian immunity remains unexplored. In contrast, several recent studies have highlighted the emerging innate immune activities of human melanin-producing cells which can sense and respond to bacterial and viral infections. Indeed, the skin is a major portal of entry for pathogens such as arboviruses (Chikungunya, Dengue) and bacteria (mycobacterium leprae, Leptospira spirochetes). Melanocytes of the epidermis could contribute to the phagocytosis of these invading pathogens and to present antigens to competent immune cells. Melanocytes are known to produce key cytokines such as IL-1ß, IL6 and TNF-α as well as chemokines. These molecules will subsequently alert macrophages, neutrophils, fibroblasts and keratinocytes through unique crosstalk mechanisms. The infection and the inflammatory responses will control melanocyte's immune and metabolic functions and could contribute to skin manifestations (rash, hyper or de-pigmentation, epidermolysis and psoriasis-like lesions). This review will address the potential role of melanocytes in immunity, inflammation and infection of the skin in health and diseases.

The role of toll-like receptors in host defenses and their relevance to dermatologic diseases.

The family of toll-like receptors (TLRs) plays a central role in the cutaneous immune defense system. To date, different TLRs have been found on several major cell populations of the skin, such as keratinocytes, fibroblasts, antigen-presenting cells, and melanocytes. Activation of TLRs leads, via different intracellular signaling pathways, to the production of pro-inflammatory stimuli, and is considered a danger signal that should transform the skin in to the functional state of defense. However, TLRs have also been implicated in tissue homeostasis and renewal. Within the group of TLRs, two types have been identified: surface-expressed TLRs, which are predominantly active against bacterial cell wall compounds; and intracellular receptors, which preferentially recognize virus-associated pattern molecules. In addition, surface-expressed receptors trigger phagocytotic and maturation signals, while the intracellular TLRs lead to the induction of antiviral genes. Our review aims to outline the importance of TLRs in the pathogenesis of numerous skin diseases and the potential of TLR agonists as a treatment option for various skin diseases.

A symbiotic concept of autoimmunity and tumour immunity: lessons from vitiligo.

Vitiligo is a skin disease in which melanocytes (MCs) are eradicated from lesional epidermis, resulting in disfiguring loss of pigment. MCs are destroyed by MC-reactive T cells, as well as other non-immune and immune components. Similarities exist between the autoimmunity observed in vitiligo and the tumour immunity observed in melanoma immuno-surveillance. An analysis of these mechanisms might lead to the development of new therapies for both vitiligo and melanoma.

A novel, antigen-presenting function of melanocytes and its possible relationship to hypopigmentary disorders.

It is now well established that cultured human melanocytes are capable of expressing immunologically important cell surface molecules and that they can produce cytokines. Not all cells with the ability to express MHC class II molecules are capable of effective Ag presentation. However, the dendritic nature of melanocytes, their strategic position within the skin, and their phagocytic capacity seem to suggest a role for these cells in processing and presenting Ag. This study demonstrates that cultured normal human skin melanocytes can present peptide Ag, and process and present the mycobacterial HSP65 kDa protein and whole Mycobacterium leprae sonicate to CD4+ cytotoxic proliferative T cell clones in an Ag-specific and HLA-class II-restricted manner. T cell stimulation was dependent on costimulatory signals, i.e., LFA-3/CD2 and LFA-1/ICAM-1. Besides eliciting a T cell proliferative response, our studies further demonstrate that melanocytes can function as target cells for T cell-mediated cytotoxicity. The described Ag-processing and -presenting functions of melanocytes, taken together with in vivo behavior of melanocytes in hypopigmentation, provide new clues for the etiopathogenesis of melanin pigmentary disorders.