CD19 HAS A POTENTIAL CD77 (GLOBOTRIAOSYL CERAMIDE)-BINDING SITE WITH SEQUENCE SIMILARITY TO VEROTOXIN B-SUBUNITS - IMPLICATIONS OF MOLECULAR MIMICRY FOR B-CELL ADHESION AND ENTEROHEMORRHAGIC ESCHERICHIA-COLI PATHOGENESIS

The glycosphingolipid globotriaosyl ceramide (CD77) and other globe-series glycolipids containing terminal galactose (Gal)alpha 1-4Gal residues function as receptors for the verotoxin (Shiga-like toxin) family of Escherichia coli-elaborated toxins. CD77 is also a marker for germinal center B lymphocytes and Burkitt's lymphoma cells. The pan B cell marker CD19 is a 95-kD membrane protein that appears early in B cell differentiation and is only lost upon terminal differentiation to plasma cells. CD19 is involved in signal transduction and has a regulatory role in B cell proliferation and differentiation in response to activation in vitro. However, an endogenous ligand for CD19 has not yet been identified. We report herein that the extracellular domain of CD19 has a potential CD77-binding site with extensive sequence similarity to the verotoxin B-subunits. These B-subunit-like sequences on CD19 are in close proximity following the organization of intervening amino acids into disulfide-linked domains. Cocapping of CD19 and CD77 on Burkitt's lymphoma-derived Daudi cells with anti-CD19 antibodies indicates that CD19 and CD77 are associated on the B cell surface. Cell surface binding of anti-CD19 antibodies is decreased on CD77-deficient mutant Daudi cells, suggesting that CD77 expression influences the surface expression of CD19. Wild-type Daudi cells, but not the CD19/CD77-deficient mutants, bind to matrices expressing the carbohydrate moiety of CD77 or other Gal alpha 1-4Gal containing glycolipids. This binding can be inhibited by anti-CD77 antibodies, the CD77-binding verotoxin B-subunit or anti-CD19 antibodies. Daudi cells exhibit a degree of spontaneous homotypic adhesion in culture while the CD77/CD19-deficient Daudi mutants grow as single cells. The stronger homotypic adhesion that occurs in B cells after antibody ligation of CD19 and that involves, to some extent, the integrin system, is also dramatically lower in the mutant cells relative to the parent cell line. However, reconstitution of mutant cells with CD77 restores the anti-CD19 mAb-induced adhesion to wild-type Daudi cell levels. These studies represent the first time that CD19-mediated signaling has been reconstituted in a low-responder B cell line. These convergent observations provide compelling evidence that CD19/CD77 interactions funct in adhesion and signal transduction at a specific stage in B cell development and suggest that such interactions have a role in B lymphocyte homing and germinal center formation in vivo. By targeting CD77(+) B cells, verotoxins may suppress the humoral arm of the immune response during infection. This may explain the infrequent and sporadic nature of antitoxin antibody responses in individuals exposed to verotoxin-producing E. coli.