Estimation of group B streptococcus type III polysaccharide-specific antibody concentrations in human sera is antigen dependent

The presence of immunoglobulin G (IgG) antibodies against group B streptoco ccus (GBS) type III polysaccharide (PS) has been correlated with protection against GBS disease. The GBS type III PS is structurally similar to the pn eumococcal type 14 PS, differing only in the presence of sialic acid residu es. Four different preparations of GBS type III PS were evaluated for their specificity in enzyme-linked immunosorbent assay (ELISA): free PS, free PS mixed with methylated human serum albumin (mHSA), PS conjugated to biotin and PS conjugated to human serum albumin. Three groups of human sera were u sed to evaluate these PS preparations: sera from recipients of a GBS PS vac cine, sera from women receiving a GBS type III PS-tetanus toroid conjugate vaccine, and sera from nonimmunized healthy women of childbearing age. Esti mated antibody concentrations were different depending on the PS preparatio n used. Using any of the four preparations, we were able to measure less th an or equal to 0.05 mu g of IgG antibody to the GBS type III PS per ml. The specificity of the assay was determined by competitive inhibition with hom ologous and heterologous PS. The pneumococcal type 14 PS did not inhibit bi nding of antibody to the native GBS type III PS in sera from adults receivi ng the GBS PS vaccine or in sera from nonimmunized adults (except serum G9) . The pneumococcal type 14 PS inhibited 50% in sera from recipients of GBS type III conjugate vaccine and in serum G9 when GBS type III PS conjugated to biotin or to HSA was used as antigen in ELISA. These data show that free GBS type III PS or PS mixed with mHSA is a sensitive and specific antigen for ELISA and that conjugation can alter the antigenic specificity of a PS.