CD77-DEPENDENT RETROGRADE TRANSPORT OF CD19 TO THE NUCLEAR-MEMBRANE -FUNCTIONAL-RELATIONSHIP BETWEEN CD77 AND CD19 DURING GERMINAL CENTER B-CELL APOPTOSIS

A region of the N-terminal extracellular domain of the B-cell restrict ed cell differentiation antigen, CD19, has high amino acid sequence si milarity to the receptor binding subunit B of verotoxin 1 (VT),,an Esc herichia coli elaborated cytotoxin, which specifically binds to the ce ll surface glycolipid, globotriaosylceramide, also known as the germin al center (GC) B-cell differentiation antigen, CD77. We have previousl y provided evidence of the association of CD19 and CD77 on the cell su rface and in CD19-mediated homotypic adhesion of the Daudi Burkitt Lym phoma cell line, one normal counterpart of which is a subset of GC B c ells. Evidence for the role of CD77 in CD19-induced apoptosis is now p resented. Initial cell surface distribution, antibody-induced redistri bution, internalization, and intracellular routing of CD19 were studie d by confocal microscopy, IF, and postembedding IEM in CD77(+ve) and C D77(-ve) cells to investigate the possible role of CD77 in CD19 intern alization and signaling. Daudi Burkitt's lymphoma cells were used as C D77(+ve) cells and as CD77(-ve) cells, Daudi mutant VT500 cells, and D audi cells treated with PPMP, an inhibitor of CD77 synthesis, were use d. Antibody ligated CD19 surface redistribution, internalization, and subcellular distribution of internalized CD19 was found to be differen t in CD77(+ve) and CD77(-ve) cells. A delay in internalization of anti body-CD19 complex was observed in CD77(-ve) cells. Internalized CD19 w as targeted to the nuclear envelope in CD77(+ve) cells in a manner sim ilar to that reported for VT, but not in CD77(-ve) cells. Internalizat ion of CD77 by ligation with verotoxin prevented the internalization o f ligated CD19. Induction of apoptosis following crosslinking of cell surface CD19 was greater in CD77(+ve) cells than in CD77(-ve) cells. T he nuclear targeting of internalized CD19 and induction of apoptosis f ollowing CD19 crosslinking only in CD77(+ve) cells indicates a role fo r CD77-dependent CD19 retrograde transport from the B cell surface via the FR to the nuclear envelope in CD19-mediated signal transduction f or apoptosis. (C) 1998 Wiley-Liss, Inc.