Superantigenic Activity of emm3 Streptococcus pyogenes Is Abrogated by a Conserved, Naturally Occurring smeZ Mutation

Abstract

Streptococcus pyogenes M/emm3 strains have been epidemiologically linked with enhanced infection severity and risk of streptococcal toxic shock syndrome (STSS), a syndrome triggered by superantigenic stimulation of T cells. Comparison of S. pyogenes strains causing STSS demonstrated that emm3 strains were surprisingly less mitogenic than other emm-types (emm1, emm12, emm18, emm28, emm87, emm89) both in vitro and in vivo, indicating poor superantigenic activity. We identified a 13 bp deletion in the superantigen smeZ gene of all emm3 strains tested. The deletion led to a premature stop codon in smeZ, and was not present in other major emm-types tested. Expression of a functional non-M3-smeZ gene successfully enhanced mitogenic activity in emm3 S. pyogenes and also restored mitogenic activity to emm1 and emm89 S. pyogenes strains where the smeZ gene had been disrupted. In contrast, the M3-smeZ gene with the 13 bp deletion could not enhance or restore mitogenicity in any of these S. pyogenes strains, confirming that M3-smeZ is non-functional regardless of strain background. The mutation in M3-smeZ reduced the potential for M3 S. pyogenes to induce cytokines in human tonsil, but not during invasive infection of superantigen-sensitive mice. Notwithstanding epidemiological associations with STSS and disease severity, emm3 strains have inherently poor superantigenicity that is explained by a conserved mutation in smeZ.

Community outbreaks of group A Streptococcus revealed by genome sequencing

“…The superantigens speA , speH and speI are highlighted in grey in emm 6 strains as these are atypical variants that may not be functional. Similarly, all emm 3 strains were found to carry a non-functional smeZ gene, as previously described 16 . The DNase spd1 in three emm 28 strains is highlighted grey to indicate that it is a different variant to that found in the other emm 28 strains.…”

The contribution of group A streptococcal virulence determinants to the pathogenesis of sepsis

“…This pattern of antigenic variation suggests that SMEZ is uniquely important for survival of GAS during invasive pathogenesis, and that the need to escape antibody neutralization has driven the progressive variation within the smez locus, without affecting the mitogenicity of the molecule. Despite these observations, a highly conserved naturally occurring smez mutation has been shown to abrogate production of SMEZ by M3 GAS isolates, including those recovered from active STSS cases 91 . Taken together these data suggest that a combination of GAS virulence factors that include, but are not limited to, the SAgs are required to stimulate the signs of STSS.…”

Bacterial superantigens and superantigen-like toxins

“…However, a possible correlation of certain smez alleles with invasive disease has not been analyzed. Notably, the STSS-associated emm3 strain carries a smez variant with a 13-bp deletion that causes a frame shift and consequently disrupts SAg activity [60].…”