Alpha C protein-specific immunity in humans with group B streptococcal colonization and invasive disease

Alpha C protein, found in 76% of non-type III strains of group B Streptococcus (GBS), elicits antibodies protective against α C-expressing strains in experimental animals, making it an appealing carrier for a GBS conjugate vaccine. We determined whether natural exposure to α C elicits antibodies in women. Geometric mean concentrations of α C-specific IgM and IgG were similar by ELISA in sera from 58 α C GBS strain colonized and 174 age-matched non-colonized women (IgG 245 and 313 ng/ml; IgM 257 and 229 ng/ml, respectively), 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.