HB4 ANTIBODY RECOGNIZES A CARBOHYDRATE STRUCTURE ON LYMPHOCYTE SURFACE-PROTEINS RELATED TO HB6, CDW75, AND CD76 ANTIGENS

Cells regulate the specificity of the carbohydrate chains on their mem brane-bound glycoconjugates by differential expression of glycosyltran sferases. In lymphocytes, beta-galactoside alpha2,6-sialyltransferase is reportedly involved in the generation of epitopes recognized by HB6 , CDw75, and CD76 mAb. The HB4 mAb binds to an Ag present on subpopula tions of B and NK cells. We now show that this Ag represents another m ember of a set of neuraminidase-sensitive, alpha2,6-sialyltransferase- generated sugar Ag. Transient expression of a cDNA encoding this enzym e in COS cells generated a minor population of HB4+ cells that was com pletely contained within the HB6+ COS cell population. Using various p roteinases and an inhibitor of N-linked carbohydrate processing, we sh ow both epitopes to represent components of N-glycosylated membrane pr oteins. Remarkably, porcine thyroglobulin, an alpha2,6-NeuAc+ glycopro tein, is specifically recognized by both mAb. These data underline a c lose relationship between HB4 and HB6 epitopes and imply further that both mAb react with oligosaccharide chains irrespective of the carrier molecule nature. Thus, the terminal sugar residue sialic acid plays a pivotal role in at least four distinct epitopes that are expressed di fferentially in immune cells. This may point at an important role for these epitopes in biologic recognition.