T Lymphocytes

Antigen-stimulated, so-called armed T cells are generally of CD8 (mainly "killer" T cells) or CD4 (T helper [Th]) type. CD8 cells recognize peptides bound to major histocompatibility complex (MHC) class I molecules. The membrane-bound receptor on these cells is a ligand for Fas and is involved in apoptosis. CD4 cells recognize the antigen-MHC class II complex. They are further subdivided into two groups by the cytokine profile they produce. Type 1 T-helper (Th1) cells secrete IL-2, IL-12, and interferon (IFN)-y. They are involved in cell-mediated immune reactions and stimulate B cells to produce complement-fixing antibodies. Type 2 T-helper (Th2) cells secrete IL-4, IL-5, IL-6, and IL-10 and are involved in IgE-mediated allergic reactions and the production of non-complement-fixing antibodies (Janeway and Travers 1994). In addition, there are also so-called Th0 cells producing IL-4 and IFN-y. Also, IL-10, a cytokine that has down-regulating properties, is produced by both Th1 and Th2 cells (Stevens and Bergstresser 1998). In normal human skin, some T cells are found, mostly around dermal postcapillary venules and appendages. Most of them express the aß T-cell antigen receptor, and vßs 1,7,14, and 16 are enriched compared with circulating T cells (Sugerman and Bigby 2000). yô T cells have cytotoxic potential and are present in murine epithelia with a possible role in immunosurveillance of epithelia but are not found in normal human skin (Alaibac et al. 1992, Bos et al. 1990).

Based on CD45 isoforms, T cells can be divided into CD45RA+ (suppressor inducer) cells, with functional characteristics of naive T lymphocytes, and CD45RO+ (helper inducer) cells, with functional characteristics of memory T cells, responding to recall antigens (Kristensson et al. 1992, Morimoto et al. 1985a, b). Also, CD31 cell surface antigen (platelet endothelial adhesion molecule-1) has been shown to define naive T cells (Morimoto and Schlossman 1993,Torimoto et al. 1992). UVB irradiation has been shown to recruit nonactivated CD4+CD45RO+ T cells into both the epidermis and the dermis. Antigen presentation to these cells is thought to result in activation of the suppressor pathway (Di Nuzzo et al. 1998). Contrary to this, UVA irradiation has been shown to deplete skin-infiltrating T cells via apoptosis (Morita et al. 1997).

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