Human lactoferrin is a component of the non-specific immune system with distinct antiviral properties. We used alphaviruses, adapted to interaction with heparan sulfate (HS), as a tool to investigate the mechanism of lactoferrin's antiviral activity. Lactoferrin inhibited infection of BHK-21 cells by HS-adapted, but not by non-adapted, Sindbis virus (SIN) or Semliki Forest virus (SFV). Lactoferrin also inhibited binding of radiolabeled HS-adapted viruses to BHK-21 cells or liposomes containing lipid-conjugated heparin as a receptor analog. On the other hand, low-pH-induced fusion of the viruses with liposomes, which occurs independently of virus-receptor interaction, was unaffected. Studies involving preincubation of virus or cells with lactoferrin suggested that the protein does not bind to the virus, but rather blocks HS-moieties on the cell surface. Charge-modified human serum albumin, with a net positive charge, had a similar antiviral effect against HS-adapted SIN and SFV, suggesting that the antiviral activity of lactoferrin is related to its positive charge. It is concluded that human lactoferrin inhibits viral infection by interfering with virus-receptor interaction rather than by affecting subsequent steps in the viral cell entry or replication processes.

Previously, it has been shown that the exposure of Semliki Forest virus (SFV) to a mildly acidic environment induces a rapid and complete loss of the ability of the virus to bind and fuse to target membranes added subsequently. In the present study, incubation of SFV at low pH followed by a specific reneutralization step resulted in a partial reversion of this loss of viral fusion capacity, as assessed in a liposomal model system. Also, the ability of the viral E1 fusion protein to undergo liposome-stimulated trimerization was restored. Furthermore, acid-treated and neutralized SFV largely retained infectivity. Exposure of SFV to low pH induced dissociation of the E1/E2 heterodimer, which was not reversed upon neutralization. It is concluded that the SFV E1 fusion protein, after acid-induced dissociation from E2, rapidly adopts an intermediate, nontrimeric conformation in which it is no longer able to interact with target membrane lipids. Neutralization restores the ability of E1 to interact with membranes. This interaction, however, remains strictly dependent on low pH.Semliki Forest virus (SFV) is an enveloped positive-strand RNA virus belonging to the genus Alphavirus of the family Togaviridae. It is well established that SFV enters its host cell through receptor-mediated endocytosis via the clathrin-coated pit pathway, fusing subsequently from within acidic endosomes (14,20). Through this fusion reaction, the viral genome gains access to the host cell cytosol and initiates the infection process. The low-pH-induced fusion process of SFV has been studied extensively in cell-free model systems involving liposomes as receptor-free target membranes (3,16,25,37,39,40). These studies have demonstrated that low pH is the sole trigger for membrane fusion of SFV and that receptor interaction is not required for the induction of the process. In addition, these studies have revealed a striking dependence of SFV fusion on the presence of both cholesterol (Chol) and sphingolipids in the target membrane (3,5,15,16,24,25,36,37,39,40). Studies conducted with another prototype alphavirus, Sindbis virus, have led to similar conclusions (30,31,32,34).Membrane fusion of alphaviruses is mediated by the E1 component of the heterodimeric E1/E2 envelope glycoprotein (6, 37). Recent X-ray crystallographic analyses of the structure of the alphavirus membrane fusion protein (19) have revealed that it has striking similarities with the structure of membrane fusion protein E of flaviviruses (22, 27) and major differences with the structure of the fusion protein hemagglutinin (HA) of influenza virus (29, 41) and other HA-related viral membrane fusion proteins. This has led to the definition of class I (HA and related proteins) and class II (alphavirus and flavivirus proteins) viral fusion proteins. Interestingly, very recent structural analyses of the postfusion structures of two class II proteins (2, 10, 11, 23) suggest that there may well be mechanistic similarities between the fusion reactions mediated by the structurally distinct clas...