The use of specific antibodies to mediate fusion between Sendai virus envelopes and living cells

Incubation of Sendai virus particles with non-ionic detergents such as Triton X-100 completely solubilizes the viral envelopes. Removal of the detergent from the supernatant (which contains the two main viral glycoproteins) leads to the formation of fusogenic, reconstituted viral envelopes. Soluble macromolecules such as DNA or proteins can be enclosed within the reconstituted vesicles, while membrane components can be inserted into the viral envelopes. Fusion of such loaded or 'hybrid' reconstituted envelopes with living cells in culture results in either microinjection or transfer of the viral components to the recipient cells. Thus such reconstituted envelopes can serve as efficient carriers for the introduction of macromolecules of biological interest into living cells in culture. A more specific vehicle has been constructed by chemically coupling anti-cell membrane antibodies (anti-human erythrocyte antibody) to the viral envelope. Such antibody-bearing intact virus particles or reconstituted envelopes bound to and fused with virus receptor-depleted cells. In addition, anti-Sendai virus antibodies were coupled to neuraminidase-treated human erythrocytes. Such antibodies mediated the binding and fusion of intact Sendai virus particles and their reconstituted envelopes to virus receptor-depleted cells.

Preclinical evaluation of two human anti-hepatitis B virus (HBV) monoclonal antibodies in the HBV-trimera mouse model and in HBV chronic carrier chimpanzees

Two human monoclonal antibodies (mAbs) against hepatitis B surface antigen (HBsAg) generated in the Trimera mouse system are described. Both mAbs 17.1.41 and 19.79.5 are of the IgG1 isotype and have high affinity constants for HBsAg binding in the range of 10(-10) mol/L. Monoclonal antibody 17.1.41 recognizes a conformational epitope on the a determinant of HBsAg whereas mAb 19.79.5 recognizes a linear one. The 2 mAbs bind to a panel of hepatitis B virus (HBV) subtypes with distinct patterns. The neutralizing activity of these antibodies was tested in 2 different animal model systems. Administration of each mAb to HBV-Trimera mice, a system that provides a mouse model for human hepatitis B infection, reduced the viral load and the percentage of HBV-DNA-positive mice in a dose-dependent manner. These 2 mAbs were more effective than a polyclonal antibody preparation (Hepatect; Biotest Pharma, Dreieich, Germany) in both inhibition of HBV liver infection and reduction of viral load. A single administration of a mixture of these mAbs into HBV chronic carrier chimpanzees resulted in immediate reduction in HBsAg levels followed by recurrence to initial levels within few days. Thus, these mAbs may be potential candidates for preventive therapy or in combination with other antiviral agents against HBV. Further studies in humans are needed to assess these mAbs in various clinical indications.

Human interleukin-6 facilitates hepatitis B virus infection in vitro and in vivo

BACKGROUND AND AIM

Research on hepatitis B virus (HBV) infection in vivo has been limited due to the absence of a suitable animal model. We have developed a human-mouse radiation chimera in which normal mice, preconditioned by lethal total body irradiation and radioprotected with SCID mouse bone marrow cells, are permissive for engraftment of human hematopoietic cells and solid tissues. This resulting human-mouse model, which comprises three genetically disparate sources of tissue, is therefore termed Trimera. This study was aimed at assessing the effect of human IL-6 on HBV infection in vivo in Trimera mice.

METHODS

Trimera mice were transplanted with human liver tissue fragments or with HepG2-derived cell lines, which had been previously infected ex vivo with HBV in the presence or absence of human interleukin-6 (hIL-6) and in the presence of anti-IL-6-neutralizing antibodies.

RESULTS

HBV sequences appeared in the sera of animals in which the liver tissue was incubated with both HBV and hIL-6 prior to transplantation. A similar result was obtained when a human hepatoblastoma cell line (HepG2), expressing the hIL-6 receptor, was infected ex vivo with HBV in the presence of hIL-6 prior to their injection into spleens of Trimera mice. However, when liver fragments were infected ex vivo and simultaneously treated with neutralizing antibodies against hIL-6 or were incubated with HBV prior to transplantation without hIL-6, the rate of mice positive for HBV DNA in their sera was lower. Human mononuclear cells are also permissive for HBV infection in vitro: in the presence of hIL-6 the infection of these cells is enhanced; and this infection is suppressed by the chimeric protein named Hyper-IL-6, generated by the fusion of hIL-6 to the soluble hIL-6 receptor (sIL-6Ralpha, gp80).

CONCLUSION

hIL-6 facilitates HBV infection in vitro and in vivo.

The hepatitis B virus-trimera mouse: a model for human HBV infection and evaluation of anti-HBV therapeutic agents

Previous studies have demonstrated the feasibility of implantation of human blood cells or tissues in lethally irradiated mice or rats, radioprotected with SCID mouse bone marrow cells: The Trimera system. In the present study, we describe the development of a mouse Trimera model for human hepatitis B virus (HBV) infection. In this model, viremia is induced by transplantation of ex vivo HBV-infected human liver fragments. Engraftment of the human liver fragments, evaluated by hematoxylin-eosin staining and human serum albumin mRNA expression, was observed in 85% of the transplanted animals 1 month postimplantation. Viremia levels were determined in these mice by measuring serum HBV DNA using polymerase chain reaction (PCR), followed by dot-blot hybridization. HBV DNA is first detected 8 days after liver transplantation. Viremia attains a peak between days 18 and 25 when HBV infection is observed in 85% of the transplanted animals. The HBV-Trimera model was used to evaluate the therapeutic effects of human polyclonal anti-HBs antibodies (Hepatect) and of two reverse-transcriptase inhibitors, lamivudine (3TC) and beta-L-5-fluoro-2',3'-dideoxycytidine (beta-L-5FddC). Treatment of HBV-Trimera mice with these drugs effectively reduced both the percentage of infected animals and the viral load in their sera. Treatment cessation resulted in rebound of viral load, indicating HBV replication upon drug withdrawal. These results show that the HBV-Trimera model represents a novel experimental tool for simulating human HBV infection and evaluating potential anti-HBV therapeutic agents.

Interaction of reconstituted Sendai viral envelopes with sperm cells: reconstituted Sendai virus envelope-induced fusion-mediated introduction of foreign material into bull sperm cells

Reconstituted Sendai virus envelopes (RSVE), i.e. membrane vesicles bearing the viral envelope glycoproteins and phospholipids, are able to fuse with bull sperm cells. This was inferred from the increase in the degree of fluorescence dequenching (DQ) obtained following incubation of fluorescently labeled (R18 labeled) RSVE with bull sperm cells and from electron microscopy studies of RSVE-sperm interaction. Only a low degree of DQ was observed, under the same conditions, with non-fusogenic fluorescently labeled RSVE. This, and electron microscopy results, show that binding and membrane fusion events occur between RSVE and sperm cells. In addition, DQ was observed following incubation of RSVE that had been pre-loaded with the self-quenched fluorochrome Calcein, with bull sperm cells, indicating fusion-mediated injection of the dye from the RSVE space into the sperm cells.

Functional and structural interactions between measles virus hemagglutinin and CD46

We analyzed the roles of the individual measles virus (MV) surface glycoproteins in mediating functional and structural interactions with human CD46, the primary MV receptor. On one cell population, recombinant vaccinia virus vectors were used to produce the MV hemagglutinin (H) and fusion (F) glycoproteins. As fusion partner cells, various cell types were examined, without or with human CD46 (endogenous or recombinant vaccinia virus encoded). Fusion between the two cell populations was monitored by a quantitative reporter gene activation assay and by syncytium formation. MV glycoproteins promoted fusion with primate cells but not with nonprimate cells; recombinant CD46 rendered nonprimate cells competent for MV glycoprotein-mediated fusion. Markedly different fusion specificity was observed for another morbillivirus, canine distemper virus (CDV): recombinant CDV glycoproteins promoted fusion with primate and nonprimate cells independently of CD46. Fusion by the recombinant MV and CDV glycoproteins required coexpression of H plus F in either homologous or heterologous combinations. To assess the role of H versus F in determining the CD46 dependence of MV fusion, we examined the fusion specificities of cells producing heterologous glycoprotein combinations. The specificity of HMV plus FCDV paralleled that observed for the homologous MV glycoproteins: fusion occurred with primate cells but not with nonprimate cells unless they produced recombinant CD46. By contrast, the specificity of HCDV plus FMV paralleled that for the homologous CDV glycoproteins: fusion occurred with either primate or nonprimate cells with no dependence on CD46. Thus, for both MV and CDV, fusion specificity was determined by H. In particular, the results demonstrate a functional interaction between HMV and CD46. Flow cytometry and antibody coprecipitation studies provided a structural correlate to this functional interaction: CD46 formed a molecular complex with HMV but not with FMV or with either CDV glycoprotein. These results highlight the critical role of the H glycoprotein in determining MV specificity for CD46-positive cells.

Fusogenic mechanisms of enveloped-virus glycoproteins analyzed by a novel recombinant vaccinia virus-based assay quantitating cell fusion-dependent reporter gene activation

The fusogenic activities of enveloped-virus glycoproteins were analyzed by using a quantitative, sensitive, rapid, and highly versatile recombinant vaccinia virus-based assay measuring activation of a reporter gene upon fusion of two distinct cell populations. One population uniformly expressed vaccinia virus-encoded viral glycoproteins mediating specific binding and fusion activities; the other expressed the corresponding cellular receptor(s). The cytoplasm of one population also contained vaccinia virus-encoded bacteriophage T7 RNA polymerase; the cytoplasm of the other contained a transfected plasmid with the Escherichia coli lacZ gene linked to the T7 promoter. When the two populations were mixed, cell fusion resulted in activation of the LacZ gene in the cytoplasm of the fused cells; beta-galactosidase activity was assessed by colorimetric assay of detergent cell lysates or by in situ staining. We applied this approach to study the human immunodeficiency virus type 1 envelope glycoprotein (Env)-CD4 interaction. Beta-Galactosidase was detected within 1 h after cell mixing and accumulated over the next several hours. Cell fusion dependence was demonstrated by the strict requirement for both CD4 and functional Env expression and by the inhibitory effects of known fusion-blocking monoclonal antibodies and pharmacological agents. Quantitative measurements indicated much higher sensitivity compared with analysis of syncytium formation. The assay was used to probe mechanisms of the cell type specificity for Env-CD4-mediated fusion. In agreement with known restrictions, cell fusion occurred only when CD4 was expressed on a human cell type. Membrane vesicle transfer experiments indicated that CD4 initially produced in either human or nonhuman cells was functional when delivered to human cells, suggesting that the fusion deficiency with nonhuman cells was not associated with irreversible defects in CD4. We also demonstrated that the infectivity specificities of different human immunodeficiency virus type 1 isolates for peripheral blood lymphocytes versus continuous CD4+ cell lines were associated with corresponding fusion selectivities of the respective recombinant Env proteins. The assay enabled analysis of the fusogenic activity of the fusion glycoprotein/hemagglutinin-neuraminidase of the paramyxovirus simian virus 5. This system provides a powerful tool to study fusion mechanisms mediated by enveloped-virus glycoproteins, as well as to screen fusion-blocking antibodies and pharmacological agents.

Sequences determining the pH dependence of viral entry are distinct from the host range-determining region of the murine ecotropic and amphotropic retrovirus envelope proteins

The entry of ecotropic and amphotropic murine leukemia retroviruses (MuLV) into cells was investigated by using viral vector particles carrying chimeric amphotropic-ecotropic envelope glycoproteins on their surface. Chimeras were made by joining, at or near the polyproline hinge, the N-terminal portion of the amphotropic (4070A) gp70 onto the C-terminal portion of the ecotropic (Moloney) gp70 and p15E (constructs AE2 and AE4) or vice versa (AE12). Transduction efficiency of the constructs was tested on target cells that either have only ecotropic receptors (CHO-2 and CHO-11 cells), only amphotropic receptors (mink lung fibroblasts and Cos 1 cells), or both types of receptors (NIH 3T3 cells). The assay made use of the fact that the mechanism for viral entry of ecotropic viruses is pH dependent while that of amphotropic viruses is pH independent. Treatment of target cells with NH4Cl, which prevents the reduction of pH within endosomes, reduced the titers of viral particles bearing the C-terminal moiety from the ecotropic envelope but did not reduce the titers of particles which had a C-terminal moiety from the amphotropic envelope. In addition, in contrast to other low-pH-dependent enveloped viruses, brief acid treatment did not allow surface-bound viruses to bypass the NH4Cl block. The results indicate that the pH dependence of viral entry is a property of the sequences C terminal to the polyproline hinge.

Analysis of the functional and host range-determining regions of the murine ectropic and amphotropic retrovirus envelope proteins

A series of Moloney murine leukemia virus (Mo-MuLV) envelope gene constructs were analyzed for biological activity. Three classes of recombinant envelopes were examined: insertions, deletions, and chimeras. Insertion (4 to 5 amino acids) and deletion (31 to 62 amino acids) mutants spanned most of the SU (gp70)-coding region and were all biologically inactive. Radioimmunoprecipitation demonstrated that the mutant envelope proteins were incorrectly processed. The Pr80env envelope precursor proteins failed to obtain the proper posttranslational modifications and were not cleaved into SU (gp70) and TM (p15E), suggesting that disruption of Pr80env structure prevents intracellular transport and processing. To analyze the functional domains of the SU portion of the Env protein, we assembled several chimeric constructs. In these constructs, portions of the ecotropic Mo-MuLV envelope gene were replaced with corresponding sequences from the 4070A amphotropic MuLV envelope. Using a retroviral vector pseudotyping assay, 5 of 12 chimeric envelope proteins were shown to be biologically active. Host range was determined by retroviral vector transduction of the appropriate cell, by viral interference studies, and by the productive infection of Chinese hamster ovary cells expressing the murine ecotropic receptor. These results permit assignment of the amino acids responsible for host range determination. Ecotropic host range is determined by the first 88 amino acids of the Mo-MuLV SU, while the amphotropic host range-determining region spans the first 157 amino acids of the 4070A SU.

Fusion of enveloped viruses with sperm cells: interaction of Sendai, influenza, and Semliki Forest viruses with bull spermatozoa

Incubation of fluorescently labeled Sendai, influenza, as well as Semliki Forest viruses with bull sperm cells resulted in fluorescence dequenching. Fluorescence dequenching was observed with Sendai virus at pH 7.4 while with influenza and Semliki Forest viruses at pH 5.0, a pH value which is required for triggering their fusogenic activity. Control experiments performed with nonfusogenic Sendai and influenza viruses, or with bull sperm cells from which the viral receptors have been removed by treatment with neuraminidase, showed little fluorescence dequenching. These results clearly indicate that animal enveloped viruses are able to interact and to fuse with bull sperm cells. The possibility that following virus-sperm fusion spermatozoa can serve as a carrier of the virus genome and introduce it into recipient eggs during fertilization is discussed.

Fusion of influenza virus particles with liposomes: requirement for cholesterol and virus receptors to allow fusion with and lysis of neutral but not of negatively charged liposomes

Influenza virus particles are able to fuse with liposomes composed of negatively charged or neutral phospholipids, as shown by using fluorochrome-labelled virions and fluorescence dequenching methods. Fusion with liposomes composed of only phosphatidylcholine (PC) was dependent on the presence of cholesterol (Chol), whereas fusion with liposomes containing negatively charged phospholipids, such as phosphatidylserine (PS), or of PC and phosphatidylethanolamine (PE) occurred in the absence of Chol. Fusion of influenza virions with PC:Chol liposomes was observed at pH 5.0, but not at pH 7.4, whereas a low degree of fusion with negatively charged liposomes or those containing PE was observed at pH 7.4. In addition, non-fusogenic influenza virions or HA0 influenza virions fused with PS- or PE-containing liposomes, especially at pH 5.0. Influenza virus particles were also able to induce the release of the fluorochrome calcein from negatively charged calcein-loaded liposomes at pH 5.0, as well as at pH 7.4, but failed to do so with PC:Chol liposomes. Lysis of PC:Chol by influenza virions was dependent on the presence of virus receptors, namely gangliosides (sialoglycolipids), and was observed only at pH 5.0. The results show that fusion of influenza virions with negatively charged or PE-containing liposomes does not reflect the biological activity of the virus needed for penetration and infection of living cells. On the other hand, fusion with PC:Chol liposomes is probably due to the activity of the viral fusion protein, the haemagglutinin glycoprotein.

An inducer of NKCF (NK cytotoxic factor) release: localization on target-cell membrane and initial characterization

Natural killer (NK) cells are probably involved in the elimination of virus-infected cells and of certain tumor cells. NK cell-mediated cytotoxicity (NK-CMC) was extensively studied and was found to consist of several steps. Following recognition and conjugation between the effector and the target cell, the latter one induces release of NK cytotoxic factor (NKCF) from the effector cells. The NKCF binds to the target cell which is subsequently killed. None of the molecules involved in these steps was completely characterized. In the present study it is demonstrated that isolated membranes of target cells can effectively induce the release of NKCF. Furthermore, the activity of such isolated membranes was found to be modulated by interferon (IFN) treatment of the cells prior to membrane isolation. It was therefore concluded that an NKCF-inducing structure (NKIS) is present on plasma membranes and is distinct from the NK-recognition structure. Similarly, the sensitivity to NK-CMC could be transferred from sensitive cells to IFN-gamma-treated (NK-resistant) cells by membrane fusion with the aid of Sendai virus envelope glycoproteins. It is proposed that transfer of NKIS is responsible for the acquired sensitivity to NK-CMC. In addition, it is shown that NKIS activity was recovered following membrane solubilization and reconstitution. Its level on cell surface was modulated by treatment of cells with tunicamycin, thus indicating that NKIS was probably a cell surface glycoprotein.

Resistance to NK cell-mediated cytotoxicity (in K-562 cells) does not correlate with class I MHC antigen levels

Natural Killer (NK) cells probably function as an early line of defense against virus-infected cells and tumor cells. In all cases, the killing by NK cell-mediated cytotoxicity (NK-CMC) is not MHC-restricted and the factors which determine the sensitivity to NK-CMC have not yet been identified. A positive correlation between resistance to NK-CMC and the level of class I MHC antigen (MHC I) expression on target cells has been reported in many studies, and in some cases a functional linkage between the two has been claimed. Several other studies have shown that there is no such correlation. By employing several experimental systems, we demonstrate here a lack of correlation between the level of MHC I and the sensitivity of K-562 cells to NK-CMC. Transfer of MHC I to MHC I-negative cells via vesicles had no effect on their resistance to NK-CMC. In addition, a decrease in resistance to NK-CMC and increase of MHC I levels was observed following target-cell membrane modulation by both application of cholesterol and hydrostatic pressure. Finally, no correlation between sensitivity to NK-CMC and MHC I expression was found in three sublines of K-562 cells. Since NK-CMC is a multistage process, it is concluded that components other than class I MHC antigens have a more prominent role in modulating the sensitivity of target cells to NK-CMC.

Enhancement of the fibrinolytic activity of sheep endothelial cells by retroviral vector-mediated gene transfer

In an attempt to enhance the fibrinolytic activity of endothelial cells (EC), a retroviral vector containing the human tissue-type plasminogen activator (t-PA) cDNA was constructed. Sheep EC were stably transduced with the vector, resulting in a 30-fold increase in t-PA activity over that detected in EC transduced with a control vector. Southern and Northern analyses confirmed the presence of both the vector sequence and the appropriate mRNA transcripts. Secretion of high levels of recombinant human t-PA continued in vitro for the duration of the experiments, up to 11 weeks after transduction, although the rate of t-PA secretion decreased in some of the EC lines. Zymographic analysis of conditioned medium from t-PA-transduced EC showed the presence of two new molecular species with plasminogen activator activity that could be specifically immunoprecipitated with a monoclonal antihuman t-PA antibody. The relative molecular masses of these species (60 to 80 and 110 Kd) suggest that they represent recombinant human t-PA both free and bound to sheep plasminogen activator inhibitor 1 (PAI-1). Consistent with this interpretation, the 110-Kd species could be specifically immunoprecipitated with antiserum to PAI-1. These studies demonstrate that retroviral vector-mediated gene transfer may be used to increase total EC fibrinolytic activity.

Reconstitution of fusogenic Sendai virus envelopes by the use of the detergent chaps

Sendai virus envelopes have been a useful tool in studying the mechanism of membrane-membrane fusion and have served as a vehicle for introducing foreign molecules (e.g., membrane proteins) into recipient cells. Reconstituted Sendai virus envelopes are routinely obtained following solubilization of virus particles with Triton X-100. This detergent has a low critical micellar concentration which precludes it from being the best detergent of choice in reconstitution studies. Nevertheless, it has remained in use since other detergents such as sodium deoxycholate and sodium cholate rendered the resultant vesicles inactive. Triton X-100 may be suboptimal for studies of some proteins that need be coreconstituted with the viral envelopes. Thus, alternative advantageous detergents, which retain the envelope fusogenic activity, have been sought. In this study we show that the synthetic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (Chaps) effectively solubilizes the Sendai virions, and that the vesicles formed by simple reconstitution protocols appear structurally and biochemically similar to those obtained with Triton X-100. The resultant vesicles retain functional integrity as assessed in both fusion and hemolysis assays. This protocol seems to be useful in sendai envelope-mediated reimplantation of Fc epsilon receptors into the plasma membranes of rat basophilic leukemia cells.

Animal viruses are able to fuse with prokaryotic cells. Fusion between Sendai or influenza virions and Mycoplasma

Sendai and influenza virions are able to fuse with mycoplasmata. Virus-Mycoplasma fusion was demonstrated by the use of fluorescently labeled intact virions and fluorescence dequenching, as well as by electron microscopy. A high degree of fusion was observed upon incubation of both virions with Mycoplasma gallisepticum or Mycoplasma capricolum. Significantly less virus-cell fusion was observed with Acholeplasma laidlawii, whose membrane contains relatively low amounts of cholesterol. The requirement of cholesterol for allowing virus-Mycoplasma fusion was also demonstrated by showing that a low degree of fusion was obtained with M. capricolum, whose cholesterol content was decreased by modifying its growth medium. Fluorescence dequenching was not observed by incubating unfusogenic virions with mycoplasmata. Sendai virions were rendered nonfusogenic by treatment with trypsin, phenylmethylsulfonyl fluoride, or dithiothreitol, whereas influenza virions were made nonfusogenic by treatment with glutaraldehyde, ammonium hydroxide, high temperatures, or incubation at low pH. Practically no fusion was observed using influenza virions bearing uncleaved hemagglutinin. Trypsinization of influenza virions bearing uncleaved hemagglutinin greatly stimulated their ability to fuse with Mycoplasma cells. Similarly to intact virus particles, also reconstituted virus envelopes, bearing the two viral glycoproteins, fused with M. capricolum. However, membrane vesicles, bearing only the viral binding (HN) or fusion (F) glycoproteins, failed to fuse with mycoplasmata. Fusion between animal enveloped virions and prokaryotic cells was thus demonstrated.

Fusion of membrane vesicles bearing only the influenza hemagglutinin with erythrocytes, living cultured cells, and liposomes

Membrane vesicles, bearing only the influenza viral hemagglutinin glycoprotein, were reconstituted following solubilization of intact virions with Triton X-100. The viral hemagglutinin glycoprotein was separated from the neuraminidase glycoprotein by agarose sulfanilic acid column. The hemagglutinin glycoprotein obtained was homogenous in gel electrophoresis and devoid of any neuraminidase activity. A quantitative determination revealed that the hemolytic activity of the hemagglutinin vesicles was comparable to that of intact virions. Incubation of fluorescently labeled hemagglutinin vesicles with human erythrocyte ghosts (HEG) or with liposomes composed of phosphatidylcholine/cholesterol or phosphatidylcholine/cholesterol/gangliosides, at pH 5.0 but not at pH 7.4, resulted in fluorescence dequenching. Very little, if any, fluorescence dequenching was observed upon incubation of fluorescently labeled HA vesicles with neuraminidase or glutaraldehyde-treated HEG or with liposomes composed only of phosphatidylcholine. Hemagglutinin vesicles were rendered non-hemolytic by treatment with NH2OH or glutaraldehyde or by incubation at 85 degrees C or low pH. No fluorescence dequenching was observed following incubation of non-hemolytic hemagglutinin vesicles with HEG or liposomes. These results clearly suggest that the fluorescence dequenching observed is due to fusion between the hemagglutinin vesicles and the recipient membranes. Incubation of hemagglutinin vesicles with living cultured cells, i.e. mouse lymphoma S-49 cells, at pH 5.0 as well as at pH 7.4, also resulted in fluorescence dequenching. The fluorescence dequenching observed at pH 7.4 was inhibited by lysosomotropic agents (methylamine and ammonium chloride) as well as by EDTA and NaN3, indicating that it is due to fusion of hemagglutinin vesicles taken into the cells by endocytosis.

Quantitative determination of virus-membrane fusion events. Fusion of influenza virions with plasma membranes and membranes of endocytic vesicles in living cultured cells

Incubation of fluorescently labeled influenza virus particles with living cultured cells such as lymphoma S-49 cells or hepatoma tissue culture cells resulted in a relatively high degree of fluorescence dequenching. Increase in the degree of fluorescence (35-40% fluorescence dequenching) was observed following incubation at pH 5.0 as well as at pH 7.4. On the other hand, incubation of fluorescently labeled influenza virions with erythrocyte ghosts resulted in fluorescence dequenching only upon incubation at pH 5.0. Only a low degree of fluorescence dequenching was observed upon incubation with inactivated unfusogenic influenza or with hemagglutinino-influenza virions. The results of the present work clearly suggest that the fluorescence dequenching observed at pH 5.0 resulted from fusion with the cells' plasma membranes, while that at pH 7.4 was with the membranes of endocytic vacuoles following endocytosis of the virus particles. Our results show that only the fluorescence dequenching observed at pH 7.4--but not that obtained at pH 5.0--was inhibited by lysosomotropic agents such as methylamine and ammonium chloride, or inhibitors of endocytosis such as EDTA and NaN3.

Reconstitution of functional influenza virus envelopes and fusion with membranes and liposomes lacking virus receptors

Reconstituted influenza virus envelopes were obtained following solubilization of intact virions with Triton X-100. Quantitative determination revealed that the hemolytic and fusogenic activities of the envelopes prepared by the present method were close or identical to those expressed by intact virions. Hemolysis as well as virus-membrane fusion occurred only at low pH values, while both activities were negligible at neutral pH values. Fusion of intact virions as well as reconstituted envelopes with erythrocyte membranes--and also with liposomes--was determined by the use of fluorescently labeled viral envelopes and fluorescence dequenching measurements. Fusion with liposomes did not require the presence of specific virus receptors, namely sialoglycolipids. Under hypotonic conditions, influenza virions or their reconstituted envelopes were able to fuse with erythrocyte membranes from which virus receptors had been removed by treatment with neuraminidase and pronase. Inactivated intact virions or reconstituted envelopes, namely, envelopes treated with hydroxylamine or glutaraldehyde or incubated at low pH or 85 degrees C, neither caused hemolysis nor possessed fusogenic activity. Fluorescence dequenching measurements showed that only fusion with liposomes composed of neutral phospholipids and containing cholesterol reflected the viral fusogenic activity needed for infection.

Membrane-bound antiviral antibodies as receptors for Sendai virions in receptor-depleted erythrocytes

Anti-Sendai virus antibodies were covalently coupled to neuraminidase-treated human erythrocytes by the use of the bifunctional crosslinking reagents, N-succinimidyl-3-(2-pyridyldithio)propionate or succinimidyl-4-(p-maleimidophenyl)butyrate. Neuraminidase-treated erythrocytes bearing antibodies were able to bind Sendai virus particles, while treated erythrocytes lacking the antibodies failed to do so. Virus particles attached to erythrocyte membranes via the antibodies were able to cause hemolysis (virus-cell fusion) and promoted cell-cell fusion. Similar results were obtained when the antibodies were coupled to cat erythrocytes which lack receptors for Sendai virus particles. Reconstituted Sendai virus envelopes, similar to intact virus particles, were able to hemolyze and to induce fusion of neuraminidase-treated antibody-bearing erythrocytes. However, reconstituted envelopes containing inactive HN (hemagglutinin-neuraminidase) but active F (fusion) glycoproteins, despite attachment to antibody-bearing erythrocytes, failed to hemolyze or to induce cell-to-cell fusion. Fusion could be restored by insertion of an active HN glycoprotein into the membranes of the reconstituted envelopes. These results suggest that the HN glycoprotein, besides being the viral attachment protein, also participates in the membrane fusion process.