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The superantigen streptococcal pyrogenic exotoxin C (SPE-C) exhibits a novel mode of action
P L Li1, R E Tiedemann, S L Moffat
1Department of Molecular Medicine, University of Auckland, 92019 Auckland, New Zealand.
Abstract:
Recombinant streptococcal pyrogenic exotoxin C (SPE-C) is a potent superantigen that stimulates Vbeta2-bearing human T cells, but is inactive in mice. SPE-C binds with high affinity to both human HLA-DR and murine I-E molecules, but not to murine I-A molecules in a zinc-dependent fashion. Competition binding studies with other recombinant toxins revealed that SPE-C lacks the generic low affinity major histocompatibility complex (MHC) class II alpha-chain binding site common to all other bacterial superantigens. Despite this, SPE-C cross-links MHC class II to induce homotypic aggregation of class II-bearing B cells. Nondenaturing sodium dodecyl sulfate electrophoresis and size exclusion chromatography revealed that both wild-type and recombinant SPE-C exist in a stable dimer at neutral or alkaline pH. These data support a recent crystal structure of SPE-C and reveal yet another mechanism by which bacterial superantigens ligate and cross-link MHC class II.
Insights
Streptococcal pyrogenic exotoxin C (SPE-C) uniquely binds human MHC class II and cross-links B cells, despite lacking a common binding site. This superantigen functions as a stable dimer, revealing novel mechanisms of T cell activation.
Area of Science:
- Immunology
- Microbiology
- Structural Biology
Background:
- Streptococcal pyrogenic exotoxin C (SPE-C) is a bacterial superantigen known to stimulate T cells.
- Superantigens typically bind to major histocompatibility complex (MHC) class II molecules.
- Understanding SPE-C's unique binding and activation mechanisms is crucial for immunological research.
Purpose of the Study:
- To investigate the binding characteristics of recombinant SPE-C to MHC class II molecules.
- To elucidate the mechanism by which SPE-C induces T cell stimulation and B cell aggregation.
- To characterize the structural properties of SPE-C, such as its oligomeric state.
Main Methods:
- Recombinant SPE-C production and characterization.
- Zinc-dependent binding assays with human and murine MHC class II molecules.
- Competition binding studies with other bacterial superantigens.
- Analysis of SPE-C's effect on B cell aggregation.
- Nondenaturing electrophoresis and size exclusion chromatography to determine SPE-C's oligomeric state.
Main Results:
- SPE-C binds human HLA-DR and murine I-E molecules with high affinity in a zinc-dependent manner, but not murine I-A.
- SPE-C lacks the conserved MHC class II alpha-chain binding site found in other superantigens.
- SPE-C induces homotypic aggregation of MHC class II-bearing B cells.
- Wild-type and recombinant SPE-C exist as stable dimers at neutral or alkaline pH.
Conclusions:
- SPE-C employs a distinct mechanism for ligating and cross-linking MHC class II molecules compared to other bacterial superantigens.
- The dimeric structure of SPE-C likely contributes to its stability and function.
- These findings provide new insights into the molecular interactions of bacterial superantigens with the immune system.