The uncoating of paramyxoviruses may not require a low pH mediated step

Virosome and ISCOM vaccines against Newcastle disease: preparation, characterization and immunogenicity

The purpose of this study was to prepare and characterize virosomes and ISCOMs containing envelope proteins of Newcastle disease virus (NDV) and to evaluate their immunogenicity in target animals (chickens). Virosomes were prepared by solubilization of virus with either Triton X-100 or octyl glucoside (OG) followed by detergent removal. Biochemical analysis revealed that these virosomes contained both the haemagglutinin-neuraminidase protein (HN) and the fusion protein (F), with preserved biological activity. Acidic environment triggered the fusion between virosomes and chicken erythrocyte ghosts. Formation of ISCOMs was achieved by solubilizing phospholipids, cholesterol, envelope protein antigen and Quil A in Triton X-100. The ISCOM particles were formed by removal of the detergent. In each formulation the relative HN content correlated with the capability to agglutinate red blood cells. The immunogenicity of these lipid-based subunit vaccines was determined in chickens after subcutaneous immunization. The relative HN content of the subunit vaccines correlated with the haemagglutination-inhibition (HI) antibody titres. Virosomes prepared with Triton X-100 and ISCOMs offered high clinical protection (> 80%) upon challenge with virulent NDV. Virosomes prepared with OG yielded lower clinical protection despite high HI antibody titres. Virosomes with reduced antigen density showed poor immunogenicity and protection. In conclusion, ND virosomes and ISCOMs were found to be immunogenic and provided good protection.

Acidic pH enhancement of the fusion of Newcastle disease virus with cultured cells

Fusion of the lentogenic strain "Clone 30" of Newcastle disease virus (NDV) with the cell line COS-7 has been studied. Fusion was monitored using the octadecylrhodamine B chloride dequenching assay [Hoekstra, D., de Boer, T., Klappe, K. and Wilschut, J. (1984). Biochemistry 23, 5675-5681]. In the present work, fusion of NDV with COS-7 cells was found to occur in a time- and temperature-dependent fashion. Significant dequenching of the probe occurred at temperatures higher than 28 degrees C. A 20-fold excess of unlabeled virus inhibited fusion by about 53% compared with the control, whereas 62% inhibition of fusion was obtained after digestion of viral glycoproteins with trypsin. The data are discussed in terms of the nonfusion transfer of the probe. In addition, preincubation of cells with 50 mM ammonium chloride or 0.1% sodium azide prevented NDV from fusing with COS-7 cells by about 30% in comparison with the control. The cytopathic effect of NDV infection in cell culture in the presence of ammonium chloride was reduced compared with control. Moreover, viral preincubation at pH 5 yielded a mild inhibition of fusogenic activity. Our results suggest that NDV may use the endocytic pathway as a complementary way of entering cells by direct fusion with the plasma membrane.

Coronavirus IBV-induced membrane fusion occurs at near-neutral pH

The lysosomotropic agent NH4Cl caused a reduction of 80-95% in the number of chick kidney (CK) cells and Vero cells infected by infectious bronchitis virus (IBV) strain Beaudette, as determined by immunofluorescence at the end of the first replication cycle. Inhibition only occurred when NH4Cl was present during the first 2 h after infection. Syncytium formation was studied during replication of IBV-Beaudette in Vero cells. Some cell-cell fusion occurred at pH 7.0 and pH 6.5 but it was optimal at pH 6.7. IBV strain UK/123/82 did not replicate in Vero cells and was studied in CK cells in which it grew well but without forming syncytia. In contrast to IBV-Beaudette, NH4Cl had virtually no effect on the replication of UK/123/82. The results show that the IBV spike glycoprotein induces membrane fusion at near neutral pH although some IBV strains may require a mildly acidic environment for the efficient uncoating of the virion RNA.

Exposure to acidic pH causes irreversible conformational changes in the measles virus hemagglutinin

Temporary exposure to acidic pH caused the hemagglutinin (H) protein of measles virus to lose hemagglutinating activity and undergo antigenic changes. The antigenic changes, detected as alterations in reactivity of several anti-H protein monoclonal antibodies, were characterised by ELISA and radioimmune precipitation assay. The possible biological instances of such conformational changes within the paramyxovirus H protein are discussed.

Differentiation of acid-pH-dependent and -nondependent entry pathways for mouse hepatitis virus

Early events of infection of MHV were studied in comparison with those of VSV, which is known to enter cells by an endocytic pathway. Treatment of mouse L-2 fibroblasts with ammonium chloride, chloroquine, or dansylcadaverine inhibited infection of MHV to a much lesser degree than that of VSV, suggesting a relatively minor role for the endocytic pathway and functional endosomes in MHV infection. Endocytosis of MHV and VSV into L-2 cells was assayed by the recovery of infectious (i.e., not uncoated) viruses from homogenates of cells harvested within the first few minutes of infection (and treated with protease to remove surface-bound virus). The results thus suggest that while a small proportion of the MHV inoculum is internalized by endocytosis, productive infection does not depend on functional endocytosis as utilized by VSV. Studies on direct virion-mediated cell fusion showed that MHV can induce fusion at pH 7.4, whereas VSV causes fusion at pH 5.0. Taken together, the above results suggest that MHV enters L-2 cells predominantly by membrane fusion with a non-acidified compartment such as the plasma membrane, endocytic vesicles, or endosomes (prior to their acidification). Results obtained from cell lines which differed in permissiveness to MHV infection suggested that the ability to support MHV infection does not correlate with endocytosis. Rather, nonpermissive cells, such as rat astrocytoma (C-6) and Vero cells, showed higher levels of recoverable internalized MHV than did fully permissive L-2 cells. Cells which are normally nonpermissive to MHV, could be rendered MHV-susceptible by PEG-induced fusion of cell surface-bound virus. Such PEG-mediated susceptibility to MHV infection was insensitive to inhibition by ammonium chloride, supporting the idea that host cell restriction of MHV infection in C-6 and Vero cells may be due to a block in nonendosomal membrane fusion. Thus endocytic internalization of MHV, which clearly occurs in a variety of cells, does not guarantee productive infection.

Solubilization of matrix protein M1/M from virions occurs at different pH for orthomyxo- and paramyxoviruses

Enveloped viruses, of which the orthomyxo- and paramyxoviruses are members, are known to be uncoated by nonionic detergents in a salt concentration-dependent manner. In this study we have shown that detergent uncoating of myxoviruses depends not only on salt concentration but also on pH. Treatment of orthomyxoviruses with Nonidet-P40 or Triton N-101 at low salt concentrations results in solubilization of surface virion glycopolypeptides in alkaline and neutral pH (9.0-6.5), but in acidic pH (6.0-5.0) the viral matrix protein M1 is also removed, and the viral ribonucleoprotein complex is released. Conversely, the paramyxovirus matrix protein M is more completely solubilized in alkaline pH (pH 9.0) than in neutral and acidic pH 7.4-5.0. The described pH-dependent differences are discussed in terms of orthomyxo- and paramyxovirus uncoating in target cells.

Pathways of viral antigen processing and presentation to CTL: defined by the mode of virus entry?

Processing requirements for antigen presentation to cytotoxic T lymphocytes (CTL) vary among viruses and between major histocompatibility complex (MHC) class I- and class II-restricted responses to the same virus. In this article, Eric Long and Steven Jacobson argue that the mode of virus entry may define processing pathways and that the invariant chain associated with MHC class II molecules may account for the distinct processing requirements for MHC class I- and class II-restricted CTL.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Herpes simplex virus type 1 penetration initiates mobilization of cell surface proteins

Changes in membrane structure resulting from herpes simplex virus 1 (HSV-1) penetration were detected using fluorescence photobleaching recovery methods. The effect could be blocked by inhibitors of viral and cellular processes involved in virus penetration. A rapid mode of HSV-1 strain KOS penetration into VERO cells at 37 degrees C normally occurs after a 5 min lag period and is 90-95% complete within 20-30 min. Rates of cell surface protein diffusion increase 2-3-fold after 5 min and return to normal after 25-30 min, this return correlating temporally with the penetration of the virus. At pH 6.3 the lag period preceeding penetration of HSV is increased to 20 min and penetration proceeds much more slowly than at pH 7.4. Inhibition of virus penetration with cytochalasin B or with the antiherpes drug tromantadine also prevents the HSV-1-induced increase in cell surface protein mobility. Colchicine, which does not block HSV-1 penetration, prevents the recovery of the membrane following virus penetration. Therefore, the changes in membrane structure characterized by increased cell surface protein mobility seem to be caused by virus penetration. Cytoskeletal function and integrity are required for the initiation of, and cell recovery from, virus penetration. A pH-sensitive activity, likely to be a virion fusion glycoprotein, is also required.

Antigen presentation to HLA class II-restricted measles virus-specific T-cell clones can occur in the absence of the invariant chain

A human fibroblast line expressing HLA-DR1 antigen on its surface was generated by transfection with DR alpha and DR beta cDNAs. Expression of the invariant chain gene was not detectable in the transfected fibroblasts and was not induced by infection with measles virus. Lysis of measles virus-infected cells occurred with DR1- but not with DR4-restricted measles virus-specific cytotoxic T-lymphocyte (CTL) clones and was inhibited by a monoclonal antibody specific for DR antigen. Therefore, the invariant chain is not required for DR-restricted presentation of measles virus antigens by this fibroblast line. Transfected fibroblasts were lysed as efficiently as an autologous B-cell line even though they expressed much less surface DR antigen. Lysis of both the transfected fibroblasts and the B-cell line was insensitive to treatment with chloroquine. These results demonstrate that expression of a DR alpha beta heterodimer at the surface of this fibroblast line is necessary and sufficient for presentation of measles virus antigens to specific CTL clones.

Newcastle disease virus and two influenza viruses: differing effects of acid and temperature on the uptake of infectious virus into bovine and canine kidney cell lines

The entry of 2 influenza viruses was compared to the entry of the Ulster strain of Newcastle disease virus (NDV) by measuring the escape rate of preadsorbed virus from neutralization using a microwell assay. The 2-minute entry of Ulster into Madin-Darby bovine kidney (MDBK) cells increased exponentially over the temperature range for fusion of 30-37 degrees C and was prevented by pH 4.6 which corresponded to the pH within endosomes. The 2-hour entry of both influenza viruses into MDBK cells increased exponentially over the temperature range of 0-30 degrees C with Q10 values similar to Q10 for the endocytosis of horse radish peroxidase and their entry times were reduced to less than 10 minutes at pH 4.6. When Madin-Darby canine kidney (MDCK) cells were used the entry of one influenza virus, NIB-8, took 2 minutes whereas the other, A/Jap/Bel, took 2 hours. The reason for this was unknown. NIB-8 contains genes 1, 2, 3, 7, 8 from A/Puerto Rico/8/34 (H1N1) virus and genes 4, 5, 6 from A/Bangkok/1/79 (H3N2) virus. A/Jap/Bel is a triple reassortant which shares genes 3 and 7 with NIB-8 whereas 1, 2, 4, 5 are from A/Japan/305/57 (H2N2) and genes 6, 8 are from A/Bel/42 (H1N1).

Ecotropic murine leukemia virus-induced fusion of murine cells

Extensive fusion occurs upon cocultivation of murine fibroblasts producing ecotropic murine leukemia viruses (MuLVs) with a large variety of murine cell lines in the presence of the polyene antibiotic amphotericin B, the active component of the antifungal agent Fungizone. The resulting polykaryocytes contain nuclei from both infected and uninfected cells, as evidenced by autoradiographic labeling experiments in which one or the other parent cell type was separately labeled with [3H]thymidine and fused with an unlabeled parent. This cell fusion specifically requires the presence of an ecotropic MuLV-producing parent and is not observed for cells producing xenotropic, amphotropic, or dualtropic viruses. Mouse cells infected with nonecotropic viruses retain their sensitivity toward fusion, whereas infection with ecotropic viruses abrogates the fusion of these cells upon cocultivation with other ecotropic MuLV-producing cells. Nonmurine cells lacking the ecotropic gp70 receptor are not fused under similar conditions. Fusion is effectively inhibited by monospecific antisera to gp70, but not by antisera to p15(E), and studies with monoclonal antibodies identify distinct amino- and carboxy-terminal gp70 regions which play a role in the fusion reaction. The enhanced fusion which occurs in the presence of amphotericin B provides a rapid and sensitive assay for the expression of ecotropic MuLVs and should facilitate further mechanistic studies of MuLV-induced fusion of murine cells.

Cell-mediated lysis of heat-inactivated influenza virus-coated murine targets

The involvement of inoculated virus antigens in the induction of target susceptibility to cytotoxic T lymphocyte (CTL)-mediated lysis was investigated using heat-inactivated influenza virus, PR8 strain, and various inhibitors in comparison to the cases for live or ultraviolet (u.v.)-irradiated influenza and Sendai viruses. Induction of target susceptibility with heated PR8 was not inhibited by cycloheximide and actinomycin D as in the case of u.v.-irradiated Sendai virus, whereas live virus and u.v.-irradiated PR8 were inhibited under conditions which suppress protein synthesis. Induction of target susceptibility with the live and inactivated PR8 tested was suppressed in the presence of chloroquine, contrary to the case of Sendai virus, and was dependent on the cleavage type of influenza virus haemagglutinin. These findings suggest that the viral target antigens recognized by CTL in heated PR8-coated targets came from inoculated virus proteins, whereas those in PR8-infected or u.v.-irradiated PR8-coated targets involved newly synthesized viral proteins. The former viral target antigens seem to be transferred or processed from the endosome, depending on low pH fusion in the endosomes into which they were engulfed. In this point, the induction of viral target antigens with heated PR8 was different from that induced by u.v.-inactivated Sendai virus. Targets made with heated PR8 were recognized by cross-reactive CTL over the HA subtype.

Infectious entry of murine retroviruses into mouse cells: evidence of a postadsorption step inhibited by acidic pH

The entry into cells by many enveloped RNA viruses is accomplished by endocytosis and subsequent penetration of the endosomal membrane by an acidic pH-dependent fusion event. In the current study, we examined early events in the infectious entry of mouse retroviruses, using as a framework the observation that infection of a mouse tail skin cell line by the ecotropic virus Friend murine leukemia virus was inhibited at mildly acidic pH (pH 6). This inhibition operated on a postadsorption step, since binding of virus was unaffected at this pH. The rate of penetration of preadsorbed virus, which displayed first-order kinetics, was markedly affected by changes in the pH of the medium. The half-time for disappearance of infectious cell surface virus at 37 degrees C was approximately 10 min at pH 7.6. At pH 6.0, however, greater than 98% of the adsorbed infectivity remained at the cell surface after 45 min. This cell surface virus, though not infecting the cell at pH 6.0, retained its capacity to enter and infect the cell when the pH of the medium was raised. Acidic pH had little effect on the rate of fluid uptake by the cells, as measured by internalization of [3H]sucrose, indicating that global inhibition of endocytosis had not occurred. In contrast, cell fusion induced by Friend murine leukemia virus was optimal at pH 7.6 but markedly inhibited at a pH of less than 6.4. This inhibitory effect of acidic pH on membrane fusion is unique among the enveloped viruses which have been studied and would preclude entry of Friend murine leukemia virus from within acidified endocytic vesicles. Entry of other members of the ecotropic, mink cell focus-forming, and xenotropic host range groups displayed similar pH sensitivity. However, one xenotropic virus was relatively resistant to the effect of acidic pH, suggesting that differences might exist in the requirements for entry of different retroviruses.

Interferon induction by viruses. XI. Early events in the induction process

The mechanism by which NDV, Sindbis virus, and VSV enter "aged" primary chick embryo cells to initiate IFN induction was studied by using NH4Cl, a lysosomotropic weak base that compromises low pH-dependent membrane fusion. NH4Cl was used to perturb the early steps in virus entry into the cytosol thought to result ultimately in presentation of dsRNA to the cell's first-stage recognition system for IFN induction. Three parameters were monitored: (i) the nature of the dose (multiplicity)-response (IFN yield) curve, (ii) the activity of IFN-inducing particles (IFP), and (iii) the quantum yield of IFN. In all tests, the qualitative nature of the dose-response curve was not altered by NH4Cl treatment. NH4Cl had no effect on infectivity of IFN induction by NDV in keeping with a mode of entry involving acidic-independent fusion through the plasma membrane. Sindbis virus infectivity and IFN-inducing particle activity were inhibited similarly in an NH4Cl concentration-dependent manner. While the infectivity of VSV was very sensitive to the action of NH4Cl, virtually all IFN-inducing particles were functional; however, the quantum yield of IFN they induced was reduced in an NH4Cl concentration-dependent manner. Only 1 of 6 VSV [+/-]DI-011 particles registered in NH4Cl-treated cells; however, they induced more than one-half the normal quantum yield of IFN. The mechanism of entry of Sindbis virus (acidic-fusion) and VSV (acidic-endocytosis) was distinguished by the action of cytochalasin B. Infectivity and IFN induction by Sindbis virus and VSV share a common, limiting step in NH4Cl treated cells: transfer of viral RNA from basic vacuoles into the cell cytosol. The similar sensitivity of Sindbis IFP and PFP to NH4Cl suggests that both activities respond the same to increased intravacuolar pH; neither can be expressed. VSV-IFP, requiring only partial genomic expression (or none in the case of DI-011) can register in NH4Cl-treated cells to varying degrees under conditions that prevent expression of the full genome.