Human Epidermal T Cells Predominately Belong to the Lineage Expressing

Normal human epidermis, long since dismissed as merely a protective covering for the body, is now recognized as a complex immunological unit . This concept is based on the demonstration that Langerhans cells (LCs)t are potent APCs for a variety of T cell (TC) responses and that keratinocytes (KC), at least upon stimulation, can secrete a plethora of immunomodulating cytokines (reviewed in reference 1) . Finally, it has been known for almost 40 years that the normal human epidermis harbors lymphocytic-appearing cells (2), yet surprisingly this cell population has received little attention since then . The relatively recent discovery ofaTCRy/S-bearing lymphocyte population (DETC, dendritic epidermal Tcell) in the mouse epidermis (reviewed in reference 1) has rekindled interest in the possibility not only that the epidermis mayfunction as an extrathymic microenvironment for the differentiation and/or "education" of TCs but also that a resident TC population may exist in the human epidermis, particularly an equivalent to the murine DETC. While some investigators never, or only rarely, observed epidermal TCs in normal skin (3, 4), we and others have found them to be routinely detectable in nonlesional, clinically normal skin (5-9). In this study we have sought to (a) more fully describe the heterogeneous phenotype ofepidermal TCs, particularly with regard to TCR-a/o-and --y/b-bearing subpopulations, (b) examine ultrastructural features of immunolabeled epidermal TCs, (c) quantitate this population in various body regions, (d) examine their in