Alpha C protein of group B Streptococcus as a potential vaccine candidate

Group B Streptococcus (GBS) is a leading cause of life-threatening infection in neonates and young infants, pregnant women, and non-pregnant adults with underlying medical conditions. Immunization has theoretical potential to prevent significant morbidity and mortality from GBS disease. Alpha C protein (α C), found in 70% of non-type III capsule polysaccharide group B Streptococcus, elicits antibodies protective against α C-expressing strains in experimental animals and is an appealing carrier for a GBS conjugate vaccine. We determined whether natural exposure to α C elicits antibodies in women and if high maternal α C-specific serum antibody at delivery is associated with protection against neonatal disease. An ELISA was designed to measure α C-specific IgM and IgG in human sera. A case-control design (1:3 ratio) was used to match α C-expressing GBS colonized and non-colonized women by age and compare quantified serum α C-specific IgM and IgG. Sera also were analyzed from bacteremic neonates and their mothers and from women with invasive GBS disease. Antibody concentrations were compared using t-tests on log-transformed data. Geometric mean concentrations of α C-specific IgM and IgG were similar in sera from 58 α C strain colonized and 174 age-matched non-colonized women (IgG 245 and 313 ng/ml; IgM 257 and 229 ng/ml, respectively). Delivery sera from mothers of 42 neonates with GBS α C sepsis had similar concentrations of α C-specific IgM (245 ng/ml) and IgG (371 ng/ml), but acute sera from 13 women with invasive α C-expressing GBS infection had significantly higher concentrations (IgM 383 and IgG 476 ng/ml [p=0.036 and 0.038, respectively]). Convalescent sera from 5 of these women 16-49 days later had high α C-specific IgM and IgG concentrations (1355 and 4173 ng/ml, respectively). In vitro killing of α C-expressing GBS correlated with total α C-specific antibody concentration. Invasive disease but not colonization elicits α C-specific IgM and IgG in adults. Whether α C-specific IgG induced by vaccine would protect against disease in neonates merits further investigation. ^