PURIFICATION, AND BIOCHEMICAL AND BIOLOGICAL CHARACTERIZATION OF AN IMMUNOSUPPRESSIVE AND LYMPHOCYTE MITOGENIC PROTEIN SECRETED BY STREPTOCOCCUS-SOBRINUS

An immunosuppressive/mitogenic (ISM) protein was purified from the sup ernatants of cultures of Streptococcus sobrinus with an isoelectric po int of 4.75 and a relative molecular mass of 38 kDa (p38), Treatment o f C57BL/6 mice with p38 induced an increase in the numbers of non-spec ific splenic Ig-secreting plaque-forming cells (PFC) with peak respons es on day 3 for IgM-secreting PFC and on day 5 for IgG-secreting PFC, with an isotype pattern consisting predominantly of IgG2a and IgG2b, T his increase was accompanied by a lymphocyte blastogenic response of b oth T and B lymphocytes, The in vitro effects of p38 on pure B, T and total splenic lymphocytes indicated that this ISM protein was primaril y a B cell mitogen, being T cells activated subsequently by the genera tion of B blasts, Suppression of the murine primary immune response ag ainst sheep red blood cells was observed in C57BL/6 mice treated 4 day s before with p38, The amino acid sequence of the N-terminus of p38 ha s a significant similarity with several enolases: particularly with ra bbit enolase, However, the biological effects ascribed to p38 have not been detected after in vivo treatment with that enolase. The immunosu ppressive effect of p38 was abrogated by depletion of IL-10 but not of IL-4, In agreement with this observation IL-10 was the only cytokine detected in serum of C57BL/6 mice after p38 treatment and the peak of serum levels was observed as soon as 2 h after treatment.