A deletion mutant lacking the non-essential vaccinia virus gene K2L, a member of the serine protease inhibitor superfamily, was constructed. This virus replicates in all cell types tested and its virulence and immunogenicity are comparable to those of the parent virus in intranasally inoculated mice. However, in a variety of cell lines the cytopathic effect of the deletion mutant (vKL4) is markedly different from that caused by the parent virus: the absence of K2L in infected cells results in extensive polykaryocytosis. Reinsertion of the K2L gene into vKL4 abolishes this fusion activity, thus confirming that the polykaryocytosis is the result of the deletion of K2L rather than of spontaneous mutations elsewhere in the genome, and that in cells infected with the WR strain of vaccinia virus the K2L gene product prevents fusion. The cell type-specific polykaryocytosis induced by vKL4 is apparent at late times post-infection, occurs from within and requires the synthesis of at least one late virus protein. Other vaccinia virus proteins known to be involved in fusion of infected cells are a 14K membrane protein which is required for fusion, and the haemagglutinin which prevents fusion. The haemadsorption properties of cells infected with the parent virus and the deletion mutant were indistinguishable: both haemadsorbed chicken erythrocytes. A monoclonal antibody against the 14K protein inhibited fusion of vKL4-infected cells, thus demonstrating that in addition to the absence of the K2L gene product, the 14K protein is required for fusion to occur.


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