Two Human IgM Myeloma Proteins with Unusual Specificities for Streptococcal Carbohydrate‐Associated Epitopes

Five hundred and fifty human sera from patients with IgM myeloma or Waldenstrom's macroglobulinaemia were screened by a solid-phase enzyme-linked immunoassay for binding to the carbohydrate of group A streptococci (A-CHO). Two of them (AC8 and AC 179) contained immunoglobulin. which bound specifically to A-CHO even at serum dilutions of 1:107. Using synthetic oligosaccharides coupled to protein for inhibition studies, the fine specificities of AC8 and AC179 were determined. AC179 is directed to α-linked rhamnose oligosaccharides, AC8 appears to be specific for N-acetyl-D-glucosamine (GlcNAc) side chains β(12)-linked to rhamnose, whereas GlcNAc side chains in A-CHO are reported to be β(13)-linked to the rhamnose backbone. Naturally occurring anti-A-CHO antibodies consist mainly of low-affinity antibodies to such β(13)-linked GlcNAc. In contrast, both myeloma antibodies show more than 10 times higher relative affinities to A-CHO than antibodies prepared from normal human serum (anti-GlcNAc and anti-A-CHO, respectively) by selection for high affinity in the elution procedure. AC179 induced complement activation in the presence of A-CHO.