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Conservation of protective and nonprotective epitopes in M proteins of group A streptococci
L Miller1, V Burdett, T P Poirier
1Department of Microbiology, Medical School, University of Newcastle upon Tyne, England.
Abstract:
Carefully controlled hybridization experiments with probes from a cloned serotype 5 M protein (M5) gene (smp5) were performed with DNA isolated from heterologous M types of group A streptococci, and the homologies detected by hybridization were compared with the ability of anti-pepM5 serum to cross-opsonize heterologous M types. As previously reported (J.R. Scott, S.K. Hollingshead, and V.A. Fischetti, Infect. Immun. 52:609-612, 1986), extensive structural homologies exist among the 3' ends of heterologous M protein genes, but there appears to be an increase in sequence variation as one moves towards the 5' ends. However, a clear, predictive correlation between the hybridization patterns and cross-opsonization was not observed. Antibodies raised to a synthetic peptide corresponding to central, conserved sequences adjacent to the C-terminal sides of the pepsin cleavage sites in M5, serotype 6 M protein, and serotype 24 M protein cross-reacted with heterologous acid-extracted M antigens but were not protective and did not bind to intact streptococcal cells, indicating that these epitopes are inaccessible on the intact cell surface. Removal of the N-terminal half of M5, serotype 6 M protein, or serotype 24 M protein by pepsin exposed the conserved epitope on the cell surface. These results suggest that immunoaccessible protective epitopes are confined to the highly variable N-terminal halves of M proteins and that a single, broadly conserved protective M protein epitope does not exist.
Insights
Group A Streptococcus M protein genes show sequence variation, with conserved regions at the 3' ends but not the 5' ends. Protective epitopes are located in the variable N-terminal regions, not conserved central areas.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Group A Streptococcus (GAS) M proteins are key virulence factors.
- M proteins are highly variable, contributing to GAS serotype diversity.
- Understanding M protein structure-function is crucial for vaccine development.
Purpose of the Study:
- To investigate the correlation between M protein gene homology and cross-opsonization.
- To identify conserved and variable regions within M protein genes.
- To determine the location of protective epitopes on the M protein surface.
Main Methods:
- DNA hybridization experiments using M protein gene probes.
- Opsonization assays with anti-M protein antibodies.
- Epitope mapping using synthetic peptides and pepsin digestion.
Main Results:
- Significant homology was found in the 3' ends of M protein genes, with increasing variation towards the 5' ends.
- No direct correlation was observed between hybridization patterns and cross-opsonization.
- Conserved epitopes in central M protein regions were inaccessible on intact cells but exposed after pepsin treatment.
Conclusions:
- Immunoaccessible protective epitopes are located in the variable N-terminal regions of M proteins.
- A single, broadly conserved protective M protein epitope does not exist.
- M protein variability likely contributes to the evasion of host immunity by GAS.