Enveloped viruses are wrapped in a lipid membrane; the viral envelope fuses with a host (plasma or endosomal) cell membrane, allowing penetration of the viral core into the host cell. The extracellular form of Vaccinia virus (EEV, for extracellular enveloped virus) is wrapped in two lipid envelopes, however, posing a challenge to viral entry: Fusion of the outer envelope will result in the intracellular release of a form called the intracellular mature virus (IMV), which is still surrounded by a lipid membrane. Law et al. used immunoelectron microscopy to investigate EEV invasion of PtK2 potoroo kidney cells and saw that, rather than fusing with the plasma membrane, the outer membrane became disrupted at the site of cell contact. It remained outside the cell, allowing the inner viral membrane to fuse with the plasma membrane. Outer membrane disruption occurred only at the site of cell contact and was not stimulated by binding to glass. Exposure to polyanions (PA), heparin, or dextran sulfate, however, appeared to damage the outer membrane, allowing neutralizing antibodies directed against the IMV to permeate. Outer membrane rupture depended on glycosaminoglycans on the host cell membrane, and analysis of viral mutants implicated the viral A34 and B5 proteins in the response. Intriguingly, the combination of antibodies against the IMV with intranasal administration of PA appeared to synergistically protect mice from Vaccinia-induced pneumonia, suggesting that this research not only elucidates a mechanism whereby viruses with double envelopes gain entry to cells but also may lead to new therapeutic approaches.
M. Law, G. C. Carter, K. L. Roberts, M. Hollinshead, G. L. Smith, Ligand-induced and nonfusogenic dissolution of a viral membrane. Proc. Natl. Acad. Sci. U.S.A. 103, 5989-5994 (2006). [Abstract] [Full Text]