Blastogenic responses of human lymphocytes to structurally defined polypeptide fragments of streptococcal M protein

Highly purified peptic extracts of types 5, 6, and 24 M proteins (pep M5, pep M6, and pep M24), whose partial covalent structures have been determined, were cultured with the lymphocytes obtained from human fetal cord blood, from unimmunized and pep M24 immunized human adults, and from the peripheral blood of mice, rats, guinea pigs, and rabbits. None of the lymphocytes from the unimmunized laboratory animals responded to any of the pep M proteins tested. In contrast, all human adult as well as neonatal cells tested responded vigorously to each of the pep M proteins as measured by [³H]-thymidine incorporation. The responses increased in a linear fashion over 5 days. Autoradiographic analysis of the responding lymphocytes indicated that only a small population of lymphocytes were sensitive to the stimulatory effects of the pep M proteins. Depletion of the mononuclear cell cultures of T cells or of adherent cells abolished the responsiveness to each of the M proteins tested. The kinetics of the responses of cord blood lymphocytes was similar to those obtained from adults. Treatment of pep M24 protein with rabbit antiserum prepared by immunization with structurally defined fragments (CB1 and CB7) of the pep M24 molecule reduced lymphocyte stimulation to less than 40% that of pep M24 similarly treated with preimmune serum. Furthermore, although nonimmune rabbit lymphocytes were unresponsive to the pep M proteins, lymphocytes of rabbits immunized with any of the three pep M proteins, or a purified peptide fragment (CB2) responded vigorously to all 3 serotypes of pep M protein, suggesting that a) the cell-mediated response is directed against the M protein molecule itself and not a common contaminant, and b) the sensitized lymphocytes recognize common structures shared among these M protein molecules. Possible mechanisms of the responses of nonimmune human lymphocytes to the streptococcal M proteins are discussed.