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Streptococcus dysgalactiae-derived mitogen (SDM), a novel bacterial superantigen: characterization of its biological
Tohru Miyoshi-Akiyama1, Jizi Zhao, Hidehito Kato
1Department of Microbiology and Immunology, Tokyo Women's Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan. akiyamat@research.twmu.ac.jp
Abstract:
A mitogenic substance, designated Streptococcus dysgalactiae-derived mitogen (SDM), was purified from S. dysgalactiae culture supernatant, and the gene encoding the mitogen was cloned. Both native and recombinant SDM expressed in Escherichia coli significantly activated human V beta 1+ and V beta 23+ T cells in association with major histocompatibility complex (MHC) class II molecules on accessory cells, indicating that SDM possesses superantigenic properties. The sdm gene consists of two segments encoding a signal peptide and a mature 25 kDa protein composed of 212 amino acids. Three of 34 S. dysgalactiae strains but none of 28 Streptococcus pyogenes strains examined carried sdm. Phylogenetic analysis indicated that SDM belongs to a family distinct from established bacterial superantigens. SDM showed around 30% homology with other superantigens at the amino acid sequence level. The tertiary structure of SDM was predicted by modelling onto streptococcal pyrogenic exotoxin C and streptococcal mitogenic exotoxin Z-2, both of which share highly homologous structure-determining regions. SDM showed overall structural similarity to both these superantigens. This is the first study to characterize fully a bacterial superantigen from S. dysgalactiae.
Insights
Streptococcus dysgalactiae-derived mitogen (SDM) is a novel bacterial superantigen. This study fully characterizes SDM, revealing its distinct properties and structural similarity to other superantigens.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Bacterial superantigens are potent immune modulators.
- Streptococcus dysgalactiae is a known pathogen, but its superantigens remain uncharacterized.
Purpose of the Study:
- To identify and characterize a novel superantigen from Streptococcus dysgalactiae.
- To investigate the molecular properties and T cell activation capabilities of this new superantigen.
Main Methods:
- Purification of Streptococcus dysgalactiae-derived mitogen (SDM) from culture supernatant.
- Cloning and recombinant expression of the sdm gene in Escherichia coli.
- Analysis of T cell activation using human V beta 1+ and V beta 23+ T cells and MHC class II molecules.
- Phylogenetic analysis and tertiary structure prediction of SDM.
Main Results:
- SDM was purified and its gene cloned, encoding a 25 kDa protein.
- Both native and recombinant SDM activated human T cells, confirming superantigenic properties.
- Phylogenetic analysis revealed SDM belongs to a distinct superantigen family with ~30% homology to known superantigens.
- Tertiary structure prediction showed overall similarity to streptococcal pyrogenic exotoxin C and streptococcal mitogenic exotoxin Z-2.
Conclusions:
- This study provides the first full characterization of a bacterial superantigen from Streptococcus dysgalactiae.
- SDM represents a novel class of bacterial superantigens with unique evolutionary and structural features.
- SDM's ability to activate T cells highlights its potential role in streptococcal pathogenesis and immune evasion.