Lymphocyte trafficking in psoriasis: a new perspective emphasizing the dermal dendrocyte with active dermal recruitment mediated via endothelial cells followed by intra-epidermal T-cell activation.

Prominent within the inflammatory infiltrate of psoriasis are HLA-DR positive T lymphocytes and factor XIIIa positive dermal dendrocytes. Many investigators studying psoriasis have assumed that the HLA-DR positive T cells are activated, and thereby capable of producing lymphokines such as gamma interferon. However, by immunohistochemical analysis, greater than 95% of the dermal T cells in psoriatic lesions are Ki-67 negative, which suggests that they are in a resting or non-cycling (Go) state. In contrast to the dermal T-cell population, the epidermal T-cell population contains a greater population of Ki-67 positive lymphocytes. The entry of the T cells into the epidermis is, therefore, apparently associated with an important activation event, which in all likelihood involves interaction with the keratinocyte. The presence of activated intraepidermal T cells has been substantiated by the ability to detect gamma interferon mRNA by polymerase chain reaction in epidermal sheets of psoriatic lesions. The pathophysiologic implication in psoriasis for these distinctions and compartmentalization involving dermal and epidermal T cells are placed into the context of a cascade of cellular trafficking events, which are further dissected into a specific network of molecular mediators of inflammation. This report suggests that more attention should be placed on the microenvironment of the skin, with specific emphasis on the mechanism by which T cells accumulate in the dermis and epidermis, and elucidation of the selective inductive and recruitment capabilities of endothelial cells, perivascular dermal dendrocytes, and keratinocytes.