Serum opacity factor (SOF) of Streptococcus pyogenes evokes antibodies that opsonize homologous and heterologous SOF-positive serotypes of group A streptococci

Infect Immun. 2003 Sep;71(9):5097-103. doi: 10.1128/iai.71.9.5097-5103.2003.


Serum opacity factor (SOF) is a protein expressed by Streptococcus pyogenes that opacifies mammalian serum. SOF is also a virulence factor of S. pyogenes, but it has not been previously shown to elicit a protective immune response. Herein, we report that SOF evokes bactericidal antibodies against S. pyogenes in humans, rabbits, and mice. Rabbit antiserum against purified recombinant SOF2 opsonized SOF-positive M type 2, 4, and 28 S. pyogenes in human blood but had no effect on SOF-negative M type 5 S. pyogenes. Furthermore, affinity-purified human antibodies against SOF2 also opsonized SOF-positive streptococci. A combination of antisera against M2 and SOF2 proteins was dramatically more effective in killing streptococci than either antiserum alone, indicating that antibodies against SOF2 enhance the opsonic efficiency of M protein antibodies. Mice tolerated an intravenous injection of 100 microg of SOF without overt signs of toxicity, and immunization with SOF protected mice against challenge infections with M type 2 S. pyogenes. These data indicate that SOF evokes opsonic antibodies that may protect against infections by SOF-positive serotypes of group A streptococci and suggest that different serotypes of SOF have common epitopes that may be useful vaccine candidates to protect against group A streptococcal infections.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Antibodies, Bacterial / biosynthesis*
  • Cross Reactions
  • Genes, Bacterial
  • Humans
  • In Vitro Techniques
  • Mice
  • Molecular Sequence Data
  • Opsonin Proteins / biosynthesis
  • Peptide Hydrolases / genetics
  • Peptide Hydrolases / immunology*
  • Peptide Hydrolases / toxicity
  • Rabbits
  • Serotyping
  • Streptococcus pyogenes / classification
  • Streptococcus pyogenes / genetics
  • Streptococcus pyogenes / immunology*


  • Antibodies, Bacterial
  • Opsonin Proteins
  • opacity factor
  • Peptide Hydrolases

Associated data

  • GENBANK/AY162273