ABSTRACT
Streptococcus pyogenes (group A Streptococcus (GAS)) causes â¼700 million human infections each year, resulting in over 500,000 deaths. The development of a commercial GAS vaccine is hampered by the occurrence of many unique GAS serotypes, antigenic variation within the same serotype, differences in serotype geographical distribution, and the production of antibodies cross-reactive with human tissue that may lead to autoimmune disease. Several independent studies have documented a number of GAS cell wall-associated or secreted metabolic enzymes that contain neither N-terminal leader sequences nor C-terminal cell wall anchors. Here, we applied a proteomic analysis of serotype M1T1 GAS cell wall extracts for the purpose of vaccine development. This approach catalogued several anchorless proteins and identified two protective vaccine candidates, arginine deiminase and trigger factor. These surface-exposed enzymes are expressed across multiple GAS serotypes exhibiting ≥99% amino acid sequence identity. Vaccine safety concerns are alleviated by the observation that these vaccine candidates lack human homologs, while sera from human populations suffering repeated GAS infections and high levels of autoimmune complications do not recognize these enzymes. Our study demonstrates anchorless cell surface antigens as promising vaccine candidates for the prevention of GAS disease.
Subject(s)
Bacterial Proteins/metabolism , Cell Wall/metabolism , Hydrolases/metabolism , Peptidylprolyl Isomerase/metabolism , Streptococcal Infections/prevention & control , Streptococcal Vaccines/metabolism , Streptococcus pyogenes/immunology , Adolescent , Animals , Bacterial Proteins/immunology , Cell Wall/immunology , Child , Female , Humans , Hydrolases/immunology , Immune Sera/immunology , Immunity, Active , Male , Mice , Mice, Inbred BALB C , Mice, Inbred C57BL , Microscopy, Immunoelectron , Peptidylprolyl Isomerase/immunology , Proteome/immunology , Proteome/metabolism , Rabbits , Recombinant Proteins/immunology , Recombinant Proteins/metabolism , Streptococcal Infections/immunology , Streptococcal Vaccines/administration & dosage , Streptococcal Vaccines/immunology , Streptococcus pyogenes/enzymology , Streptococcus pyogenes/ultrastructure , Vaccination , Young AdultABSTRACT
Group A streptococci (GAS) display receptors for the human zymogen plasminogen on the cell surface, one of which is the plasminogen-binding group A streptococcal M protein (PAM). Characterization of PAM genes from 12 GAS isolates showed significant variation within the plasminogen-binding repeat motifs (a1/a2) of this protein. To determine the impact of sequence variation on protein function, recombinant proteins representing five naturally occurring variants of PAM, together with a recombinant M1 protein, were expressed and purified. Equilibrium dissociation constants for the interaction of PAM variants with biotinylated Glu-plasminogen ranged from 1.58 to 4.99 nm. Effective concentrations of prototype PAM required for 50% inhibition of plasminogen binding to immobilized PAM variants ranged from 0.68 to 22.06 nm. These results suggest that although variation in the a1/a2 region of the PAM protein does affect the comparative affinity of PAM variants, the functional capacity to bind plasminogen is conserved. Additionally, a potential role for the a1 region of PAM in eliciting a protective immune response was investigated by using a mouse model for GAS infection. The a1 region of PAM was found to protect immunized mice challenged with a PAM-positive GAS strain. These data suggest a link between selective immune pressure against the plasminogen-binding repeats and the functional conservation of the binding domain in PAM variants.
