Human keratinocytes express HLA-DR antigens in the tuberculin reaction

The cellular response in the human skin tuberculin reaction was studied with immunohistochemical double-staining techniques in frozen sections of skin biopsies taken 6 h to 8 days after intradermal PPD injections. Cell infiltrates were observed from day 2 onwards and increased in size up to 4 days. Most of the infiltrating cells reacted with anti-Leu 3a (T 'helper/inducer' phenotype) antibodies. In contrast to normal epidermis, not only Langerhans cells but also keratinocytes expressed HLA-DR antigens from day 4 onwards. The induction of HLA-DR antigens on keratinocytes may be secondary to T-cell activation. Since the HLA-DR expression on keratinocytes appeared late in the tuberculin reaction, the function may be to suppress rather than enhance the immune response.