Antigenic Specificity of Opsonophagocytic Antibodies in Rabbit Anti-Sera to Group B Streptococci

Abstract

An opsonophagocytic assay was developed which requires human polymorphonuclear leukocytes, immune serum and complement for optimal klling of Group B streptococci. Only with all 3 of these components was killing of greater than 1.0 log10 of the initial inoculum achieved, using rabbit antisera directed to homologous strains of each of the 5 known serotypes of Group B streptococci. Titers of specific antisera, which opsonized the strains and resulted in > 1 log10 reduction of colony-forming units, ranged from 1:100 (serotype Ib) to 1:3200 (serotype Ia). Cross-reactions between serotype-specific sera and heterologous strains were seen in certain instances. Type Ic strain and serotype Ic antiserum demonstrated cross-reactions with types Ia and Ib which were explainable by known shared antigens among these types. The only other cross-reaction which resulted in > 1 log10 reduction in colony-forming units was when unabsorbed antiserum to strain Ia was used to opsonize a strain of serotype III. Opsonization of 10 serotype III strains was demonstrated with a single type III antiserum. Killing of 9 of these strains required polymorphonuclear leukocytes, complement and antiserum; but 1 strain, D136C, the reference strain, could be killed (> 1 log10 reduction in colony-forming units) without complement or specific antiserum. Inhibition studies were performed utilizing large MW polysaccharide antigens extracted from each serotype. These antigens inhibited opsonization of homologous strains by homologous antisera with 50% inhibition points ranging between 0.5 and 4 .mu.g.