Immunophenotype

The immunophenotype of LCs has been studied extensively in normal and inflamed skin by us and others using immunohistochemistry and flow cytometry. It is important to distinguish between normal and inflamed human skin because the immunophenotype of LC is subject to highly complex regulatory mechanisms: (a) freshly isolated LC change their phenotype (and function) during short-term culture toward highly stimulatory mature DCs, (b) the inflammatory microenvironment as such alters the immunophenotype of the LC in situ, and (c) in some skin diseases, for example, atopic dermatitis, a subset of membrane receptors shows evidence for disease-specific regulation (Wollenberg et al. 1999).

A variety of immunoglobulin receptors, MHC class I and class II molecules, and multiple adhesion molecules are the immunophenotypic hallmark of normal LCs. The nonclassic MHC class Ib molecule CD1a is regarded as the most specific LC marker for normal human skin currently available (Fithian et al. 1981). This does not apply to inflammatory skin conditions because another CD1a-expressing population cell (IDEC) is present in the epidermis and may be mistaken for LCs (Wollenberg et al. 1996).

In the lesional epidermis of LE, a reduction in HLA-DR+ DCs compared with CD1a+ cells suggests reduced expression of HLA-DR antigen by CD1a+ DCs (Mori et al. 1994, Sontheimer and Bergstresser 1982). In vivo studies of LCs in LE show altered morphologic characteristics when examined by cell surface ATPase activity, HLA-DR antigens, and OKT-6 antigens (Sontheimer and Bergstresser 1982). The most impor tant of these changes were reduced surface density, loss of dendritic processes, and bizarre shapes (Sontheimer and Bergstresser 1982). The morphologic and density alterations are said not to be associated with a decreased alloantigen-presenting capacity of LCs. Yet, a newer study revealed a diminished T-cell stimulatory capacity in the allogeneic mixed lymphocyte reaction in DC-enriched APCs from patients with SLE compared with healthy individuals (Scheinecker et al. 2001).

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