Class II transplantation antigens in the late human skin tuberculin reaction.
The presence of class II transplantation antigens on keratinocytes and the phenotypes of the inflammatory cells in the late human skin tuberculin reaction were analysed with immunohistochemical double staining techniques in frozen sections of skin biopsies taken 10-45 days after intradermal purified protein derivative (PPD) injection. Dermal cell infiltrates decreased with time but were found throughout the observation period. Most of the cells in the perivascular infiltrates expressed HLA-DR antigens. Some of these cells are probably 'activated' macrophages, since they expressed OKM1 and OKT9 antigens in consecutive sections. Another less frequent cell population which reacted with RFD1 antibodies are presumably interdigitating cells. About half the perivascular cells were anti-Leu 3a-positive (T 'helper/inducer' phenotype). HLA-DR but not HLA-DQ antigens were detected on keratinocytes. This acquired expression of HLA-DR antigens on the epithelial cells disappeared between 17 and 30 days. If HLA-DR-expressing keratinocytes have immunoregulatory functions these might be different from those of other HLA-DR-expressing cell types that also express HLA-DQ molecules.