Serologic cross-reactions among streptococcal group A, A-variant, and C polysaccharides

Serologic cross-reactions among streptococcal groups A, A-variant (A-V), and C cell wall polysaccharides were found previously in studies employing capillary or quantitative precipitin techniques. Similar cross-reactions occur with radioimmune precipitation using extrinsically labeled ¹²⁵I-streptococcal antigens. This study was performed to determine the degree of cross-reactivity when intrinsically labeled ¹⁴C-polysaccharide antigens were used in the radioimmune precipitin assay. Unadsorbed antisera from rabbits immunized with group A streptococci bound 3-5% as much ¹⁴C-A-V antigen as homologous A carbohydrate but undetectable amounts of C polysaccharide. Similarly, A-V antisera bound 3-5% as much ¹⁴C-A or ¹⁴C-C carbohydrate as A-V antigen. Group C antiserum bound 1-2% as much A and A-V antigens as C carbohydrate. Thus, less than 3% of intrinsically labelled ¹⁴C-A or ¹⁴C-C carbohydrate represents exposed A-V antigenic sites, i.e., exposed polyrhamnose backbone. Otherwise, group A, C, or A-V carbohydrates failed to exhibit heterologous determinants. Anti-peptidoglycan antibodies in antisera did not result in serologic cross-reactivity. These data suggest that formamide-extracted streptococcal group-specific polysaccharides, intrinsically labeled with ¹⁴C, may possess greater group specificity than ¹²⁵I-carbohydrates and yield only negligible cross-reactivity with heterologous antisera. This degree of cross-reactivity does not appear to be sufficient to account for the persistently elevated serum levels of antibody to group A carbohydrate in patients with rheumatic heart disease.