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Infection and Immunity, September 2003, p. 5097-5103, Vol. 71, No. 9
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.9.5097-5103.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Veterans Affairs Medical Center,1 Department of Medicine,2 Department of Anatomy and Neurobiology, University of Tennessee, Memphis, Tennessee3
Received 29 January 2003/ Returned for modification 5 May 2003/ Accepted 24 June 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 µg 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.
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