Delayed hypersensitivity in the guinea-pig to a protein-hapten conjugate and its relationship to in vitro transformation of lymph node, spleen, thymus and peripheral blood lymphocytes.

Guinea-pig delayed hypersensitivity to purified protein derivative (PPD) and guinea-pig albumin-orthanilic acid (AO) was produced in the absence of detectable antibody formation to the conjugate. Ten days after sensitization the guinea-pig peripheral leucocytes, lymph nodes, spleen and thymus cell suspensions were cultured from 1 to 5 days with various concentrations of immunizing antigens, unconjugated hapten, a hapten—ovalbumin conjugate or phytohaemagglutinin (PHA). All the cultures of `draining' lymph node cells, and about 40 per cent of the spleen and peripheral leucocyte cultures manifested increased lymphocyte transformation on radioautographs, and by total tritiated thymidine incorporation when stimulated by PPD or AO. In addition all the cultures responded well to PHA. However, lymphocytes from the mediastinal and cervical lymph nodes from the immunized, and most of the lymphoid organ cultures from unimmunized guinea-pigs were not stimulated by antigens but responded only to PHA. Cultured guinea-pig thymocytes did not respond to any stimulus. The lymphocyte proliferation was carrier specific. It did not occur in response to unconjugated hapten. However, the response to AO was partially inhibited in the presence of the hapten. The kinetics and morphological changes in the cultures also were investigated, and the immunological significance and specificity of lymphocyte transformation are discussed.