In situ identification of T lymphocyte subsets and HLA-DR expressing cells in the human skin tuberculin reaction.

T lymphocyte subsets and HLA-DR expressing cells were studied with an immunohistochemical double staining technique in frozen sections of human skin 6, 48 and 96 hr after intradermal PPD injections. The number of lymphocytes reacting with monoclonal Leu 1 antibodies (all mature peripheral T cells) increased with time. The majority of the T lymphocytes at 48 and 96 hr reacted with Leu 3a ('helper/inducer' phenotype) antibodies and a few with Leu 2a ('suppressor/cytotoxic' phenotype) antibodies. Apposition of T lymphocytes of both subsets to HLA-DR expressing cells occurred in the dermis as well as in the epidermis. The study gives a morphological picture of the cell-mediated immune reactions in delayed type of hypersensitivity consistent with in vitro experiments on proliferative responses to soluble antigens.