Serum opacity factor is a major fibronectin-binding protein and a virulence determinant of M type 2 Streptococcus pyogenes

Serum opacity factor (SOF) is a fibronectin-binding protein of group A stre ptococci that opacifies mammalian sera and is expressed by some strains tha t cause impetigo, pharyngitis and acute glomerulonephritis, Although SOF is expressed by approximate to 35% of known serotypes, its role in the pathog enesis of group A streptococcal infections has not been previously investig ated. The sof genes from M types 2, 28 and 49 Streptococcus pyogenes were c loned, sequenced, and their deduced amino acid sequences were compared. The gene for FnBA, a fibronectin-binding protein from Streptococcus dysgalacti ae, was also cloned and found to express an opacity factor. The leader sequ ences, the fibronectin-binding domains, and the membrane anchor regions of these proteins were highly conserved. Short spans of conserved sequences we re interspersed throughout the remaining parts of the proteins. The son gen e was insertionally Inactivated in an M type 2 S. pyogenes strain, T2MR. Th e resultant SOF-negative mutant (YL3) did not express SOF or opacify serum, and exhibited a 71% reduction in binding fibronectin. Complementation of t he SOF-negative defect with sof28 in the recombinant strain YL3(pNZ28) full y restored fibronectin-binding activity and the ability to opacify serum. T o determine whether sof plays a role in virulence, mice were challenged int raperitoneally with these strains. None of the 10 mice infected with YL3(pN Z28) survived and only 1 out of 15 mice challenged with T2MR survived, wher eas 12 out of 15 mice infected with YL3 survived. These data clearly indica te that SOF Is a virulence factor, and they provide the first direct eviden ce that a fibronectin-binding protein contributes to the pathogenesis of gr oup A streptococcal infections in vivo.