Group A streptococcus-liposome ELISA antibody titers to group A polysaccharide and opsonophagocytic capabilities of the antibodies.

Antibodies reactive with group A streptococci (GAS) carbohydrate were studied by ELISA and in an indirect bactericidal assay. The ELISA used GAS carbohydrate covalently bound to phosphatidylethanolamine incorporated into liposomes so that both precipitating and nonprecipitating antibodies were measured. Sera from children from different geographic areas exhibited marked differences in levels of anti-GAS carbohydrate antibody, which increased with age. The antibodies were predominantly of IgG. In bactericidal assays, most of these sera promoted phagocytosis of several type-specific M-positive strains. Opsonization was also related to serum levels of anti-GAS carbohydrate antibodies. These opsonizing antibodies were depleted from the serum by absorption of the sera on an N-acetyl-D-glucosamine affinity column. Antibody eluted from this column could partially restore opsonization of GAS. Anti-GAS carbohydrate antibodies play a major role in these opsonophagocytosis assays.