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A conserved 3D pattern in a Streptococcus pyogenes M protein immunogen elicits M-type crossreactivity
Kuei-Chen Wang1, Eziz Kuliyev1, Victor Nizet2
1Department of Chemistry & Biochemistry, University of California, San Diego, California, USA.
Abstract:
Coiled coil-forming M proteins of the widespread and potentially deadly bacterial pathogen Streptococcus pyogenes (strep A) are immunodominant targets of opsonizing antibodies. However, antigenic sequence variability of M proteins into >220 M types, as defined by their hypervariable regions (HVRs), is considered to limit M proteins as vaccine immunogens because of type specificity in the antibody response. Surprisingly, a multi-HVR immunogen in clinical vaccine trials was shown to elicit M-type crossreactivity. The basis for this crossreactivity is unknown but may be due in part to antibody recognition of a 3D pattern conserved in many M protein HVRs that confers binding to human complement C4b-binding protein (C4BP). To test this hypothesis, we investigated whether a single M protein immunogen carrying the 3D pattern would elicit crossreactivity against other M types carrying the 3D pattern. We found that a 34-amino acid sequence of S. pyogenes M2 protein bearing the 3D pattern retained full C4BP-binding capacity when fused to a coiled coil-stabilizing sequence from the protein GCN4. We show that this immunogen, called M2G, elicited cross-reactive antibodies against a number of M types that carry the 3D pattern but not against those that lack the 3D pattern. We further show that the M2G antiserum-recognized M proteins displayed natively on the strep A surface and promoted the opsonophagocytic killing of strep A strains expressing these M proteins. As C4BP binding is a conserved virulence trait of strep A, we propose that targeting the 3D pattern may prove advantageous in vaccine design.
Insights
Streptococcus pyogenes M proteins vary widely, limiting vaccine development. However, a conserved 3D pattern on M proteins, which binds complement C4b-binding protein (C4BP), can elicit cross-reactive antibodies, offering a promising vaccine strategy.
Area of Science:
- Microbiology
- Immunology
- Vaccine Development
Background:
- Streptococcus pyogenes (strep A) M proteins are key virulence factors and antibody targets.
- Antigenic variability of M proteins, with over 220 types, hinders vaccine efficacy due to type-specific antibody responses.
- Previous trials showed unexpected cross-reactivity with multi-hypervariable region (HVR) immunogens, suggesting a conserved recognition mechanism.
Purpose of the Study:
- To investigate if a conserved 3D structural pattern in M protein HVRs, which binds complement C4b-binding protein (C4BP), could be the basis for cross-reactive antibody responses.
- To determine if a single M protein immunogen displaying this 3D pattern could elicit cross-reactive antibodies against other M types possessing the same pattern.
Main Methods:
- A 34-amino acid sequence from Streptococcus pyogenes M2 protein, containing the conserved 3D pattern and retaining C4BP-binding capacity, was fused to a GCN4 coiled coil-stabilizing sequence to create the M2G immunogen.
- Antibody responses to M2G were analyzed for cross-reactivity against various M types.
- The ability of elicited antibodies to recognize native M proteins on strep A surface and promote opsonophagocytic killing was assessed.
Main Results:
- The M2G immunogen successfully elicited cross-reactive antibodies against Streptococcus pyogenes M types that share the conserved 3D pattern.
- Antibodies generated against M2G did not cross-react with M types lacking this specific 3D pattern.
- The antibodies recognized M proteins displayed on the strep A surface and enhanced the killing of strep A strains via opsonophagocytosis.
Conclusions:
- A conserved 3D structural pattern in M protein HVRs, associated with C4BP binding, can serve as a target for eliciting cross-reactive antibodies.
- This conserved pattern represents a potential conserved epitope for developing broadly protective Streptococcus pyogenes vaccines.
- Targeting this C4BP-binding 3D pattern may overcome the limitations of type-specific variability in M proteins for vaccine design.
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