FUNCTION OF HUMAN CYTOMEGALOVIRUS GLYCOPROTEIN-B - SYNCYTIUM FORMATION IN CELLS CONSTITUTIVELY EXPRESSING GB IS BLOCKED BY VIRUS-NEUTRALIZING ANTIBODIES

We report that U373 glioblastoma cells constitutively producing human cytomegalovirus (HCMV) glycoprotein B (gB), the product of open readin g frame UL55 of the HCMV genome, formed syncytia that contained 5 to 2 5 nuclei. Flow cytometry with a panel of monoclonal antibodies (mAbs) to the extracellular domain of HCMV gB showed that these cells express ed high densities of gB in the plasma membrane. We studied the propert ies of five clonal UB cell lines and the results are as follows. React ivity of a panel of mAbs to HCMV gB showed that Us cells forming syncy tia expressed gB with all of the conformational and sequential epitope s contained in the viral gB made in HCMV-infected cells. Syncytium for mation in LIB cells was independent of low pH and proteolytic cleavage of gB and was blocked by drugs that inhibit glycosylation and translo cation of gB to the cell surface. Infected UB cells formed more syncyt ia than infected U373 cells, virions entered UB cells more rapidly, an d higher virus titers were produced. Incubation of UB cells with compl ement-independent neutralizing antibodies to gB, which prevent virion entry into cells, cell-to-cell transmission of infection, and fusion o f HCMV-infected glioblastoma cells, significantly reduced syncytium fo rmation in UB cells. These results show that overlapping functional do mains on HCMV gB promote fusion of the virion envelope with the cell s urface, fusion of infected U373 cells, and syncytium formation in UB c ell lines expressing high densities of gB in the plasma membrane. Our findings indicate that the functional regions of gB can be subjected t o detailed analysis without constructing viral mutants by expressing m utated forms of gB with site-directed changes that preclude syncytium formation of U373 cells expressing these gene products. (C) 1994 Acade mic Press, Inc.