The major aim of this project is to develop and standardize an ELISA assay that measures IgG antibody to the Group B streptococcus (GBS) capsular polysaccharide (PS). In order to select the optimal assay, four published methods were compared for sensitivity and specificity. We choose to measure only IgG, because IgG antibodies are able to cross the placenta and provide protection for the newborn. Four of the GBS types, Ia, II, III and V, will be examined in these studies. To determine the best method for measuring specific IgG antibody to GBS PS, we are comparing four different ELISA methods to measure IgG antibody to GBS type III PS. The relative assay sensitivity was examined, and all four methods measured 0.1 ug/ml or less of GBS-III PS antibody. Specificity of immune GBS-III antibody was evaluated for all the ELISA assays by competition inhibition assay using GBS and pneumococcal type PS's. Using 5 ug of type III PS, 100% inhibition was achieved for the mHSA method while the inhibition was approximately 85% for the biotin and HSA conjugated methods. We will also determine the specificity of all assays using non immune sera. The group specific PS antigen, common to all GBS types, purified from a nonencapsulated GBS strain did not inhibit type III binding. We are presently comparing the avidity of both vaccine induced and normal GBS type III antibodies using three ELISA methods. Since we are interested in measuring GBS IgG antibodies that are protective, we will determine which ELISA method best correlates with functional antibodies using opsonophagocytosis.
