Isolation of a new superantigen with potent mitogenic activity to murine T cells from Streptococcus pyogenes

A mitogenic substance on murine lymphocytes was detected in the culture supernate of Streptococcus pyogenes type 12 strain. This substance had a molecular weight of 28 000 and p I 9.2, and was designated as S. pyogenes mitogen (SPM). The proliferative response of C3H/HeN spleen cells began at 1 ng ml −1 and reached a maximal response at 100 ng ml −1 of SPM for 4 days culture. Anti‐Thy 1.2 mAb and complement‐treated spleen cells abrogated the proliferative response to any dose of SPM. Although the anti‐major histocompatibility complex class I mAbs had no blocking effect on proliferation by SPM, this proliferation was substantially inhibited by the addition of either anti‐I‐A or anti‐I‐E mAb, and complete inhibition was produced by the addition of both mAbs. Fixed antigen‐presenting cells still induced T cell proliferation by SPM. A significant expansion of T cells bearing Vβ13 T‐cell receptor was observed up to 73% among the Thy1.2 + cells in cultures stimulated with SPM, indicating expansion in a Vβ‐specific manner. Immunoblotting of IEF‐separated proteins showed that anti‐streptococcal pyrogenic exotoxin (SPE) C reacted with a protein of p I 6.9 and anti‐SPEB did not show any reactivity. SPEA was reported to expand Vβ8.1 and 8.2 bearing murine T cells, and SPM did not. SPM also exhibited potent mitogenic activity on human T cells and Vβ21 + T cells were selectively expanded. These results lead to the conclusion that SPM was neither SPEA, B nor C, but a new protein belonging to a group of streptococcal superantigens with activity on not only human but also murine lymphocytes.