Identification and immunoreactivity of proteins released from Streptococcus agalactiae.

The aim of the present study was to identify released proteins of Streptococcus agalactiae and to investigate their immunoreactivity with human sera to determine whether such proteins might be viable as carrier proteins in conjugate vaccines. Infections with S. agalactiae are the leading cause of sepsis and meningitis in neonates. Vaccination of women of childbearing age would be a desirable alternative to intrapartum antibiotic prophylaxis, but factors that mediate S. agalactiae invasive disease and virulence are poorly defined. Capsule-based vaccines have shown only low immunogenicity to date, and interest has shifted towards S. agalactiae proteins, either as candidate vaccine antigens or as carrier proteins for serotype-specific S. agalactiae polysaccharides. In this study, some major released proteins of S. agalactiae could be identified, including molecules known to be present on the surface of bacterial cells but not previously described as released proteins, such as CAMP factor, a phosphocarrier protein, aldolase, enolase, PcsB, and heat-shock protein 70. Serotype-specific differences in the protein patterns of extracellular products and immunoreactivity with human sera could be detected by SDS-PAGE and Western blot. The identification of unexpected released proteins may indicate secondary functions for these proteins. In addition, the widespread immunoreactivity of these proteins with human sera as shown by Western blot indicates that released proteins may be promising candidates as carrier proteins in conjugate vaccines.