The
glycosphingolipid globotriaosyl
ceramide (CD77) and other globo-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 function 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.(ABSTRACT TRUNCATED AT 400 WORDS)