The bacterial superantigen streptococcal mitogenic exotoxin Z is the major immunoactive agent of Streptococcus pyogenes.

The gene encoding streptococcal mitogenic exotoxin Z (SMEZ) was disrupted in Streptococcus pyogenes. Despite the presence of other superantigen genes, mitogenic responses in human and murine HLA-DQ transgenic cells were abrogated when cells were stimulated with supernatant from the smez(-) mutant compared with the parent strain. Remarkably, disruption of smez led to a complete inability to elicit cytokine production (TNF-alpha, lymphotoxin-alpha, IFN-gamma, IL-1 and -8) from human cells, when cocultured with streptococcal supernatants. The potent effects of SMEZ were apparent even though transcription and expression of SMEZ were barely detectable. Human Vbeta8(+) T cell proliferation in response to S. pyogenes was SMEZ-dependent. Cells from HLA-DQ8 transgenic mice were 3 logs more sensitive to SMEZ-13 than cells from HLA-DR1 transgenic or wild-type mice. In the mouse, SMEZ targeted the human Vbeta8(+) TCR homologue, murine Vbeta11, at the expense of other TCR T cell subsets. Expression of SMEZ did not affect bacterial clearance or survival from peritoneal streptococcal infection in HLA-DQ8 mice, though effects of SMEZ on pharyngeal infection are unknown. Infection did lead to a rise in Vbeta11(+) T cells in the spleen which was partly reversed by disruption of the smez gene. Most strikingly, a clear rise in murine Vbeta4(+) cells was seen in mice infected with the smez(-) mutant S. pyogenes strain, indicating a potential role for SMEZ as a repressor of cognate anti-streptococcal responses.