Properties of cold agglutinin and group carbohydrate-specific antibodies isolated from group C streptococcal antisera

The relationship between erythrocyte autoreactive cold agglutinin (CA) antibodies and group carbohydrate-specific antibodies in hyperimmune rabbit Group C streptococcal antisera was investigated. The different antibody preparations examined were isolated from autologous erythrocyte, Group C carbohydrate, and alpha-GalNAc immunoabsorbents. Each population was subsequently tested for SRBC reactivity, RRBC autoreactivity, and carbohydrate and ligand reactivity, in hemagglutination assay, direct biphasic hemolytic and hemolytic inhibition assays, and radioimmunoassay. In absorption experiments, CA antibodies present in unfractionated serum and represented as purified IgG and IgM preparations, were reactive with Group C carbohydrate, but poorly reactive with alpha-GalNAc immunoabsorbent. In addition, CA antibody activity present in carbohydrate-eluted material, was absent in alpha-GalNAc-eluted material as determined by hemagglutination and direct hemolytic assay. By radioimmunoassay, carbohydrate-eluted and alpha-GalNAc-eluted streptococcal antibodies, and alpha-GalNAc-eluted BSA-alpha-GalNAc antibodies, exhibited similar reactivity with 125I-BSA-alpha-GalNAc antigen, and with GalNAc in radioimmunoassay inhibition experiments. In contrast to these results, both IgG and IgM CA antibodies exhibited a relatively low avidity toward 125I-BSA-alpha-GalNAc. Yet relative to the other antibody populations tested in radioimmunoassay inhibition experiments, CA antibodies did not exhibit a particularly significant difference in reactivity with GalNAc. However, recognition of other Gal-containing ligands, e.g. Mel and Lac, was restricted to the CA antibody preparations. These data suggest that CA antibodies present in Group C streptococcal antisera do not represent a higher affinity cross-reactive anticarbohydrate population, but instead perhaps represent cross-reactive carbohydrate-specific antibodies produced in response to nonimmunodominant Group C carbohydrate determinants.