Functional and immunological characterization of the stimulation of human lymphocytes with a mitogen from group A streptococci (SM)

Streptococcal mitogen (SM), an extracellular product of group A streptococci, is non specifically mitogenic for both B and T lymphocytes. The mitogenic activity of SM is resistant to digestion with trypsin, to heating at 100 °C for 5 min, and to treatment with dithiothreitol. The proliferative response of lymphocytes from patients with a history of rheumatic fever is similar to that of lymphocytes from healthy donors when stimulated with optimal concentrations of SM, but is significantly reduced when low doses of SM are used. T lymphocytes stimulated with SM acquire Ia antigens and the ability to stimulate allogeneic and autologous lymphocytes in mixed lymphocyte reactions. An involvement of Ia antigens in these reactions is indicated by the specific block by monoclonal antibodies to human Ia antigens.