Expression of the T-cell activation antigens CD27 and CD28 in normal and psoriatic skin.

Activated T lymphocytes are thought to be involved in the pathogenesis of psoriasis. From studies with peripheral blood T lymphocytes it is known that T cells show a decrease in membrane expression of CD27 molecules during continuous antigenic stimulation. The T-cell activation molecule CD28 is thought to be involved in the transduction of an antigen-non-specific costimulatory signal. Therefore, in order to elucidate further the pathogenesis of psoriasis we studied the expression of CD27 and CD28, together with CD4, CD8 and CD45RA in this benign inflammatory dermatological disease. We used immunohistochemical techniques to determine absolute numbers of T lymphocytes and expression of these T-cell activation and T-subset-specific molecules in normal (n = 7), uninvolved perilesional (n = 7) and lesional psoriatic (n = 7) skin. We found that not only lesional but also clinically uninvolved perilesional skin showed an increased number of T cells. Further, immunohistochemical studies showed that CD27 is expressed by a minority of normal skin T cells, while in lesional psoriatic skin, expression was even lower, and almost absent in perilesional skin sections. In contrast to normal skin, both perilesional and lesional psoriatic skin contained no CD28 positive T cells. In lesional psoriatic skin, however, T cells showed predominantly the CD4 phenotype, while in perilesional skin CD8 positive T cells were dominant. Two conclusions were reached: first, the absolute number of T cells, their CD27, CD28 and CD45RA expression, and the influx of CD8 positive T cells, indicate that perilesional psoriatic skin is different from normal and lesional psoriatic skin; and secondly, the data on CD27 and CD28 suggest that not only lesional but also perilesional psoriatic skin is subject to continuous antigenic stimulation, thus leading to decreased CD27 and CD28 expression on skin T cells.