Isolation and characterization of a novel streptococcal superantigen

The involvement of S. pyogenes (group A Streptococcus) in severe invasive disease, toxic-shock-like syndrome, and episodes of rheumatic fever led us to explore the possibility that these strains produce a novel superantigen. By using a superantigen-specific assay, we purified a 28-kDa protein from culture supernatants that stimulated T cells in an MHC class II-dependent, V beta-specific manner and designated it SSA, streptococcal superantigen. The amino terminus of SSA showed striking resemblance to SEB, SEC1, and SEC3. The structural homology exhibited by SSA to SEB was reflected functionally in that both of these superantigens bound the same class II isotypes. In contrast, SSA differed from SEB and other known bacterial superantigens with respect to its pattern of V beta-specific T-cell activation. SSA stimulated human T cells that expressed V beta 1, 3, 15, and perhaps V beta 5.2. Using SSA-specific antibodies in an immunoblot assay, we screened 26 strains of Lancefield group A Streptococcus and 16 strains of group B, C, and G Streptococcus. We found that SSA was expressed with high frequency in group A strains, but was absent from all other groups tested. These data establish SSA as a novel superantigen secreted by S. pyogenes. Further study of the structure and expression of SSA may reveal a role for this molecule in current episodes of severe streptococcal diseases.