Lymphocyte activation. I. Expression of theta, H-2 and immunoglobulin determinants on lymphocytes stimulated by phytohaemagglutinin, pokeweed mitogen, concanavalin A or histocompatibility antigen.

Mouse lymphocytes were cultured with phytohaemagglutinin, pokeweed mitogen, Concanavalin A or allogeneic cells. The expression of theta, H-2 and Ig determinants on the blasts and surviving lymphocytes was assessed by indirect autoradiography using unlabelled mouse antisera and iodinated rabbit anti-mouse light chain. The great majority of the responding cells carried the θ-antigen (82–100%) and few carried Ig determinants (0–12%). There was no difference in the concentration of antiserum required to demonstrate H-2 or θ on fresh lymphocytes, cultured lymphocytes or PHA-induced blasts. In mixed leucocyte cultures between parental and F1 cells most of the blasts were derived from the parent (90%) whereas in cultures of two parental strains both strains contributed equally to the response. These data support the view that thymus-derived cells predominate in the response to non-specific mitogens and histocompatibility antigen, and that parental/F1 cultures exhibit a unidirectional response.