Lymphocyte subsets and their proliferation in a model for a delayed-type hypersensitivity reaction in the skin.

A delayed-type hypersensitivity (DTH) reaction was induced in the skin of young pigs, by local injection of phytohaemagglutinin, and evaluation was carried out on the resulting accumulation of lymphocyte subsets and lymphocyte production by incorporation of bromodeoxyuridine in the skin and the draining lymph node. There was a rapid increase in mononuclear cells, which were found in clusters around venules. These included very few B lymphocytes, and CD8+ lymphocytes far outnumbered CD4+ cells. Underlining the importance of determining absolute numbers, the relative and absolute numbers of lymphocyte subsets showed quite different patterns during the development of the skin reaction. Lymphocytes in the normal skin incorporated the DNA precursor bromodeoxyuridine at higher rates than have been found for peripheral lymphoid organs. After intradermal phytohaemagglutinin injections, all subsets showed high proliferation rates in the skin, with kinetics which differed from the reaction in the draining lymph node. The labelling indexes of cells labelled with bromodeoxyuridine in vitro and in vivo were comparable. The phytohaemagglutinin injections also caused a marked and rapid increase in the proliferation of the cells in the basal layer of the epidermis. This model DTH-like reaction in skin with major CD8+ T-cell accumulation and proliferation locally and in the lymph nodes provides a reliable model for study of such reactions and for investigation of the regulatory role of cytokines.