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Spontaneous M6 protein size mutants of group A streptococci display variation in antigenic and opsonogenic epitopes
K F Jones1, S K Hollingshead, J R Scott
1Laboratory of Bacteriology and Immunology, Rockefeller University, New York, NY 10021.
Abstract:
Deletions of highly, but not entirely, homologous intragenic sequence repeats result in amino acid sequence and conformational changes in the M proteins of spontaneous M protein-size variants of group A streptococci. To determine if antigenic changes occurred as a result of these deletion mutations, monoclonal and polyclonal antibodies with defined epitopes were used in competition assays. Competing antigens were either purified pepsin-derived fragments (representing the amino-terminal half of the molecule) of parent and mutant M proteins or were intact bacterial cells. These assays showed that antigenic variation occurred at the site(s) of these deletions but not at adjacent or distant epitopes. Once cleaved from the bacterium by pepsin, the M molecules also underwent conformational changes, which were reflected in their ability to compete. A monoclonal antibody opsonic for M6 streptococci lost its ability to completely opsonize one of the size mutants in this study. Therefore, spontaneous intragenic events between repeats within emm-6, the structural gene for the M6 protein, do result in structural variations within the mutant M molecules. This variation alters the ability of certain antibodies, originally produced in response to sequences in the parental M molecule, to bind to the mutant M molecules or opsonize the mutant organisms. Group A streptococci have evolved a mechanism for generating antigenic diversity that differs from currently known mechanisms in other bacterial species.
Insights
Spontaneous mutations in group A streptococci M proteins cause structural changes, leading to altered antibody binding and immune evasion. This study reveals a novel mechanism for bacterial antigenic diversity.
Area of Science:
- Microbiology
- Immunology
- Molecular Biology
Background:
- Group A streptococci (GAS) possess M proteins that are crucial virulence factors.
- M proteins contribute to GAS survival by inhibiting phagocytosis and complement-mediated killing.
- Antigenic variation of M proteins is a key strategy for GAS immune evasion.
Purpose of the Study:
- To investigate whether intragenic deletions in M protein genes lead to antigenic changes.
- To determine if these structural variations affect antibody recognition and opsonization.
Main Methods:
- Utilized spontaneous M protein-size variants of GAS.
- Employed competition assays with monoclonal and polyclonal antibodies.
- Used purified pepsin-derived M protein fragments and intact bacterial cells as antigens.
Main Results:
- Deletions in homologous intragenic repeats resulted in amino acid and conformational changes in M proteins.
- Antigenic variation was observed specifically at the sites of deletions.
- A loss of opsonizing ability by a specific monoclonal antibody against a size mutant was noted.
- Pepsin cleavage of M molecules induced conformational changes affecting antibody competition.
Conclusions:
- Intragenic deletions within the emm-6 gene generate structural variations in M6 protein.
- These structural variations alter the binding of specific antibodies and reduce opsonization of GAS.
- GAS employs a unique mechanism for generating antigenic diversity through intragenic recombination.