Target and Effector Cell Fusion Accounts for B Lymphocyte-mediated Lysis of Mouse Hepatitis Virus-infected Cells

In confirmation of previous reports, we observed lysis of mouse hepatitis virus (MHV)-infected target cells in the presence of spleen and lymph node cells from nonimmunized mice possessing a B cell surface phenotype (IgM+, IgG+, J11d+, Ia+, Fc+, Thy1-, MAC-1- and asialo-GM1-). Lysis was inhibited by MHV-specific antisera. The presence of immunoglobulin at the surface of B cells is not required for cytolysis since MHV-infected target cells are lysed in the presence of the B cell hybridoma Sp2/0, which fails to synthesize immunoglobulin. Using 51Cr-labelled Sp2/0 cells, both target and effector cells were shown to undergo cytolysis. Direct observation of target and effector cells co-incubated after labelling with different fluorescent dyes demonstrated that lysis correlates with the fusion of B cells and MHV-infected cells. These findings are consistent with the idea that the E2 protein of MHV, which is expressed on the infected cell surface and has receptor and membrane-fusion activities at neutral pH, selectively mediates fusion with cells of B lymphocyte lineage. This may represent a general mechanism by which enveloped viruses with fusion proteins that function at neutral pH can interfere with the function of subsets of immune cells.