Basic streptococcal superantigens (SPEX/SMEZ or SPEC) are responsible for the mitogenic activity of the so-called...

Area of Science:

• Microbiology
• Immunology
• Enzymology

Background:

• Group A Streptococcus mitogenic factor (MF) was previously identified as streptococcal pyrogenic exotoxin F (SPEF), a superantigen.
• MF possesses both mitogenic and DNase activities.
• Superantigens are known to stimulate T cells via specific Vbeta chains.

Purpose of the Study:

• To purify and characterize the mitogenic factor (MF) from Streptococcus pyogenes.
• To determine the precise nature of MF's superantigenic and DNase activities.
• To clarify the classification of MF, specifically its relationship to superantigens and DNases.

Main Methods:

• Purification of MF from culture supernatants of various Streptococcus pyogenes strains.
• Assessment of Vbeta specificity of MF preparations.
• Neutralization assays using antibodies against known streptococcal superantigens (SPEC, SPEX/SMEZ3) and MF.
• Analysis of DNase activity of MF.
• Molecular characterization of streptodornase type B (SDB) and comparison with MF.

Main Results:

• MF preparations from different strains exhibited varying Vbeta specificities, linked to SPEC or SMEZ3 expression.
• Mitogenic activity decreased with purification and was neutralized by anti-SPEC/SPEX antibodies, not anti-MF antibodies.
• DNase activity of MF was completely neutralized by anti-MF antibodies.
• MF was identified as streptodornase type B isoform II (SDBII), lacking superantigenic properties.
• SDBII differs from SDBI by an additional N-terminal arginine.

Conclusions:

• Mitogenic factor (MF) from Group A Streptococcus is identical to streptodornase type B isoform II (SDBII).
• MF lacks superantigenic properties and should be reclassified, no longer termed a superantigen or pyrogenic exotoxin.
• The study clarifies the dual activity of MF, attributing mitogenicity to superantigen contamination or interaction and the primary function to DNase activity.