Inactivation of the a C protein antigen gene, bca, by a novel shuttleysuicide vector results in attenuation of virulence and immunity in group B Streptococcus (Streptococcus agalactiaeysurface antigensybacterial pathogenesisybacterial vaccines)

The a C protein of group B Streptococcus (GBS) is a major surface-associated antigen. Although its role in the biology and virulence of GBS has not been defined, it is opsonic and capable of eliciting protective immunity. The a C protein is widely distributed among clinical isolates and is a potential protein carrier and antigen in conjugate vaccines to prevent GBS infections. The structural gene for the a C protein, bca, has been cloned and sequenced. The protein encoded by bca is related to a class of surface-associated proteins of Gram-positive cocci involved in virulence and immunity. To investigate the potential roles of the a C protein, bca null mutants were generated in which the bca gene was replaced with a kanamycin resistance cassette via homologous recombination using a novel shuttleysuicide vector. Studies of lethality in neonatal mice showed that the virulence of the bca null mutants was attenuated 5- to 7-fold when compared with the isogenic wild-type strain A909. Significant differences in mortality occurred in the first 24 h, suggesting that the role of the a antigen is important in the initial stages of the infection. In contrast to A909, bca mutants were no longer killed by polymorphonuclear leukocytes in the presence of a-specific antibodies in an in vitro opsonophagocytic assay. In contrast to previous studies, a antigen expression does not appear to play a role in resistance to opsonophagocytosis in the absence of a-specific antibodies. In addition, antibodies to the a C protein did not passively protect neonatal mice from lethal challenge with bca mutants, suggesting that these epitopes are uniquely present within the a antigen as ex- pressed from the bca gene. Therefore, the a C protein is important in the pathogenesis of GBS infection and is a target for protective immunity in the development of GBS vaccines.