Structure and development of viruses as observed in the electron microscope. IX. Entry of parainfluenza I (Sendai) virus

Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling

Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.

Use of Human Hybridoma Technology To Isolate Human Monoclonal Antibodies

The human hybridoma technique offers an important approach for isolation of human monoclonal antibodies. A diversity of approaches can be used with varying success. Recent technical advances in expanding the starting number of human antigen-specific B cells, improving fusion efficiency, and isolating new myeloma partners and new cell cloning methods have enabled the development of protocols that make the isolation of human monoclonal antibodies from blood samples feasible. Undoubtedly, additional innovations that could improve efficiency are possible.

Membrane uncoating of intact enveloped viruses

Experiments in the 1960s showed that Sendai virus, a paramyxovirus, fused its membrane with the host plasma membrane. After membrane fusion, the virus spontaneously "uncoated" with diffusion of the viral membrane proteins into the host plasma membrane and a merging of the host and viral membranes. This led to deposit of the viral ribonucleoprotein (RNP) and interior proteins in the cell cytoplasm. Later work showed that the common procedure then used to grow Sendai virus produced damaged, pleomorphic virions. Virions, which were grown under conditions that were not damaging, made a connecting structure between virus and cell at the region where the fusion occurred. The virus did not release its membrane proteins into the host membrane. The viral RNP was seen in the connecting structure in some cases. Uncoating of intact Sendai virus proceeds differently from uncoating described by the current standard model developed long ago with damaged virus. A model of intact paramyxovirus uncoating is presented and compared to what is known about the uncoating of other viruses.

Functions of surface glycoproteins of myxoviruses and paramyxoviruses and their inhibition

Two glycoproteins, HN and F, are present on the surface of paramyxoviruses. HN has receptor-binding amd neuraminidase activities. F is involved in viral penetration, cell fusion and haemolysis and is activated by proteolytic cleavage by a host enzyme into two disulphide-bonded subunits (f1 and F2). The ability of the virus to initiate infection and undergo multiple cycle replication depends on the presence of an activating protease in the host; thus cleavage of F is a major determinant of pathogenesis. The new N-terminus generated on F1 by cleavage is involved in biological activity, and the amino acid sequence of this region of F1 by cleavage is involved in biological activity, and the amino acid sequence of this region of F1 is hydrophobic and highly conserved among para-myxoviruses. In an attempt to design specific inhibitors, oligopeptides and analogous to this region were synthesized and found to be highly active, specific inhibitors of viral penetration, cell fusion and haemolysis. Inhibition is amino-acid-sequence-specific and affected by peptide length, steric configuration and addition of groups to the n-terminal and C-terminal amino acids. Replication of influenza virus was also specifically inhibited by oligopeptides resembling the N-terminus of the HA2 polypeptide. Like that of F1 protein the N-terminus of HA2 is generated by a proteolytic cleavage that activates infectivity. These results have provided information on the action of proteins in viral penetration and membrane fusion and they suggest a possible new approach to chemical inhibition of viral replication. Studies with specific antibodies to each of the paramyxovirus glycoproteins have shown that antibodies to the F protein are essential for effective prevention of the spread of infection. Antibodies to the HN protein, although capable of neutralizing released virus, do not prevent spread to adjacent cells through membrane fusion mediated by the F protein. These findings have implications for the design of effective vaccines against paramyxoviruses and also provided additional insight into the mechanisms involved in the atypical and severe infections observed in individuals who received inactivated paramyxovirus vaccines and were later infected.

Parainfluenza (Sendai) virus infects ciliated cells and secretory cells but not basal cells of rat tracheal epithelium

Sendai virus is a common respiratory pathogen in rodents. In the airways of rats infected with Sendai virus, viral antigen is present in epithelial cells, but whether all types of epithelial cells are infected is unknown. Because each type of epithelial cell has specific functions that could be affected by viral infection, we asked whether ciliated cells, secretory cells, and basal cells of the rat tracheal epithelium become infected by Sendai virus. We inoculated pathogen-free rats intranasally with Sendai virus, killed the rats 1 to 12 days after inoculation, and prepared the tracheas for double-labeling immunohistochemistry and for electron microscopy. In other studies, we maximized the infection by inoculating rats with a 100-fold higher titer of the virus, by inoculating weanling rats, or by inoculating tracheal explants with Sendai virus in vitro. We also determined whether Sendai virus can infect basal cells of tracheal explants after removal of the overlying columnar epithelial cells. Immunohistochemical studies showed that at the peak of the infection (5 days after inoculation), 30% of the surface epithelial cells stained for Sendai virus antigen, but no basal cells were stained. Electron microscopic examination confirmed the presence of viral particles in ciliated cells and secretory cells, but none were found in basal cells. No basal cells were infected under the conditions that maximized the infection. We conclude that ciliated cells and secretory cells of the rat tracheal epithelium become infected by Sendai virus, but basal cells do not become infected.

Fusion of Sendai virus with human HL-60 and CEM cells: different kinetics of fusion for two isolates

The kinetics of fusion of Sendai virus (Z strain) with the human promyelocytic leukemia cell line HL-60, and the human T lymphocytic leukemia cell line CEM was investigated. Fusion was monitored by fluorescence dequenching of octadecylrhodamine (R-18) incorporated in the viral membrane. For one virus isolate (Z/G), the overall rate of fusion (at 37 degrees C) increased as the pH was lowered, reaching a maximum at about pH 5, the lowest pH tested. For another isolate (Z/SF) the rate and extent of fusion were lower at pH 5 than at neutral pH. Lowering the pH from neutral to 5 after several minutes of incubation of either isolate with HL-60 cells resulted in an enhanced rate of fluorescence dequenching. Nevertheless, experiments utilizing NH4Cl indicated that fusion of the virus with cells was not enhanced by the mildly acidic pH of the endosome lumen. Analysis of the kinetics of fusion by means of a mass action model resulted in good simulation and predictions for the time-course of fusion. For the isolate which showed maximal fusogenic activity at pH 5, the rate constant of fusion (approx. 0.1 s-1) at neutral pH was in the range found previously for virus-liposome fusion, whereas the rate constant of adhesion was close to the upper limit for diffusion-controlled processes (1.4.10(10) M-1 s-1). However, for the other isolate (Z/SF) the rate constant of fusion at neutral pH was very small (less than 0.01 s-1), whereas the rate constant of adhesion was larger (greater than or equal to 2.10(10) M-1 s-1). Lowering the temperature decreased the fusion rate. Experiments involving competition with excess unlabeled virions indicated that not all binding sites for Sendai virus on HL-60 cells are fusion sites. The virus fusion activity towards HL-60 cells at neutral pH was not altered significantly by pre-incubation of the virus at pH 5 or 9, in contrast to earlier observations with liposomes and erythrocyte ghosts, or results based on erythrocyte hemolysis or cell-cell fusion.

Role of membrane phospholipids and glycolipids in cell-to-cell fusion by VSV

To identify membrane components of CER cells interacting with vesicular stomatitis virus (VSV) during fusion at acidic pH (fusion from without, FFWO) two different approaches have been used, i.e. (i) treating the whole cells with enzymes and (ii) testing the ability of isolated membrane molecules to interfere with FFWO. Phospholipase A2 and C digestion of cells greatly reduced syncytia formation, pointing towards the involvement of lipid structures as target sites for VSV. Cell susceptibility to FFWO was also reduced after neuraminidase, beta-galactosidase or periodate treatment, suggesting that carbohydrate residues may participate in a complex receptor structure required for virus fusion. When membrane molecules were examined separately for their ability to inhibit viral FFWO, phosphatidylserine, phosphatidylinositol, sphingomyelin, cholesterol and GM3 ganglioside were found to be active, confirming the role of membrane lipid moiety in the cell surface structures involved in the early phases of VSV 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.

A comparative study of entry modes into C6/36 cells by Semliki Forest and Japanese encephalitis viruses

The entry modes of Semliki Forest virus and Japanese encephalitis virus into C6/36 cells were compared by electron microscopic observation. At physiological pH, the two viruses showed characteristically different entry modes. Following attachment to the plasma membrane, many SF virions appeared within plasma membrane invaginations and cytoplasmic vesicles; on the other hand, JE virions remained to be found exclusively at the cell surface, with no virions appearing within cytoplasmic vesicles. Electron microscopic observation, therefore, indicated that SF virus entered C6/36 cells by receptor-mediated endocytosis, while JE virus penetrated the cells at the surface and disintegrated at or near the adsorption sites. At pH 5.8, SF virus also entered C6/36 cells by direct penetration at the cell surface. On the basis of the present and other findings, the following working hypotheses are presented for future investigations: (a) at physiological pH, the fusion protein of SF virus is in an inactive state and needs to be activated by acidic pH within the endosome in order to act on the host-cell membrane, but that of JE virus is in an active state and is capable of dissolving the host plasma membrane at the cell surface immediately after the attachment; (b) the states of viral fusion proteins (inactive or active) at the time of viral attachment to the cell surface determine which of the two entry modes these viruses follow.

Anti-glycoprotein D antibodies that permit adsorption but block infection by herpes simplex virus 1 prevent virion-cell fusion at the cell surface

Certain monoclonal antibodies specific for glycoprotein D of herpes simplex virus have potent neutralizing activity but fail to block attachment of virus to cells. Here we have investigated the fate of neutralized and infectious virus after attachment to primate cells. Infectious virions fused with the cell surface such that naked nucleocapsids were detectable in the cytoplasm near or just under the plasma membrane. Neutralized virions did not fuse with the cell. They remained attached to the cell surface and could be rendered infectious by treatment with polyethylene glycol. We conclude that some anti-glycoprotein D neutralizing antibodies can inhibit the penetration of herpes simplex virus by blocking fusion of the virion envelope with the plasma membrane. These results identify a pathway of entry that initiates successful herpes simplex virus infection and a step in this pathway that is highly sensitive to neutralizing antibodies. A role for glycoprotein D in virion-cell fusion is indicated.

An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein

During the first stage of infection, the paramyxovirus Sendai virus attaches to host cells by recognizing specific receptors on the cell surface. Productive virus-cell interactions result in membrane fusion between the viral envelope and the cell surface membrane. It has recently been shown that the ganglioside GD1a and its more complex homologs GT1b and GQ1b are cell surface receptors for Sendai virus. We report in this paper that the temperature-sensitive mutant ts271 of the Enders strain of Sendai virus lacks the viral attachment protein HN and the biological activities of hemagglutination and sialidase activity associated with it when the virus is grown at 38 degrees C. This HN- virus was unable to infect or agglutinate conventional host cells that contained receptor gangliosides and were readily infected by the parental wild-type virus. The HN- virus did, however, attach to and infect Hep G2 cells, a line of hepatoma cells that retains the asialoglycoprotein receptor (ASGP-R) upon continuous culture. This receptor is a mammalian lectin that recognizes galactose- or N-acetylgalactosamine-terminated proteins. In accordance with the known properties of this receptor, infection by the HN- virus was abolished by treatment of Hep G2 cells with sialidase, by the presence of Ca2+ chelators, and by competition with N-acetylgalactosamine, asialoorosomucoid, and antibody to the receptor. F, the only glycoprotein on the HN- virus, was shown to compete with the galactose-terminated protein asialoorosomucoid for the ASGP-R. The ability of the HN- virus to cause cell-cell fusion of Hep G2 cells indicated that attachment of this virus to the ASGP-R still permitted viral entry by its usual mode--i.e., membrane fusion at the cell surface. These results open up the possibility that enveloped viruses, which contain glycosylated proteins or lipids, may make use of naturally occurring lectins in addition to their normal receptors as a means of attachment to host cells.

Pneumotropism of Sendai virus in relation to protease-mediated activation in mouse lungs

The pneumotropism of Sendai virus in mice was studied in relation to the activation and replication of the virus in the lung. Inactive Sendai virus grown in LLC-MK(2) cells, which possessed an uncleaved precursor glycoprotein, F, and was noninfectious to tissue culture cells, neither grew nor caused pathological changes in the lung of mice. When trypsin treatment was made which cleaved F into F(1) and F(2) subunits, the virus became activated so that it could initiate replication in the bronchial epithelium of the lung. In this case, the progeny virus was produced in the activated form and multiple-cycle replication occurred successively. A parallel relationship was found between the degree of the viral replication and that of clinical signs of the respiratory disease, body weight loss, and histopathological changes in the lung. A protease mutant, TR-2, which was able to be activated only by chymotrypsin but not by trypsin, could also initiate replication in the bronchial epithelium, when activated by chymotrypsin before inoculation into mice. The progeny virus, however, remained inactive, and the replication was limited to a single cycle, which resulted in the limited lung lesion. The overall results suggest that some activating mechanism for the progeny virus of wild-type Sendai virus exists in the lung of mice and the principle (activator) responsible for this phenomenon has a character similar to trypsin. The possible location of the activator is discussed.

Fusion of Sendai viruses or subviral envelope components with chicken erythrocytes observed by freeze-fracture electron microscopy

Four different types of envelope of Sendai virus or subviral components, that is, infectious and non-infectious virions, reassembled envelope particles (REP), and Tween-ether-treated envelope fragments (TE), were studied comparatively for membrane interactions with chicken erythrocytes by freeze-fracture electron microscopy, specifically for membrane alteration by envelope fusion. The freeze-fracture replicas of the attachment of the four envelopes in the cold exhibited a common pattern of impressions with attached envelopes, although the fracture plane traversed from erythrocyte to envelope at the periphery of the contact areas of three of the envelopes but not of TE, where the fracture plane mostly cut only through erythrocyte membranes impressed with TE. The freeze-fracture replicas of the four envelopes reacting with erythrocytes after a short incubation period at 37 C exhibited distinctive features: infectious virions and REP displayed evidence of envelope fusion, but non-infectious virions and TE showed a particular pattern of envelope association without fusion. Our data demonstrate that the pattern specific for envelope fusion is the formation of a continuous membrane from envelope to cell membrane in a cross fracture of an erythrocyte.

Tobacco mosaic virus protein induces fusion of liposome membranes

The fusogenic properties of tobacco mosaic virus (TMV) coat protein were investigated. Tobacco mosaic virus protein induces membrane fusion of a population of L-alpha-dimyristoylphosphatidylcholine (DMPC) and DL-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles giving rise to larger particles as seen by a drastic absorbance increase of the liposomal solution. Differential scanning calorimetry spectra demonstrate complete mixing of the acyl chains of the lipids during fusion. Electron micrographs indicate that the fused entities are multilamellar.

Initiation of fusion and disassembly of Sendai virus membranes into liposomes

Sendai virus penetration into liposomes consists of two steps which are fusion of the viral and liposomal membranes and viral disassembly. Penetration can occur in less than one minute. The virus first causes a liposome to envelop it and then fuses with the leading edge of the developing vacuole. Viral disassembly does not follow immediately but requires release of virus-receptor binding and probably also requires changes in the association between viral proteins.

Immunoelectron microscopic study on interactions of noninfectious sendai virus and murine cells

The early interactions of LLC-MK2 cell-grown noninfectious Sendai virus and a murine cell line, P815 mastocytoma ascitic cells, were studied by electron microscopy, using the ferritin-conjugated antibody technique with anti-virus glycoprotein serum. For comparison, the interactions of egg-grown infectious Sendai virus with the same cells were also examined. When noninfectious virus was adsorbed to the cells in the cold, the cell membranes become partially invaginated at the site of contact of adsorbed virions, but ferritin-conjugated antibodies did not penetrate into the areas of envelope-cell membrane association. This pattern of virus attachment was similar to that of infectious virus attachment. Upon subsequent incubation at 37 degrees C, most of the adsorbed noninfectious virions were taken into cytoplasmic vesicles and then degraded, although a few virions remained attached to the cell membrane. No evidence of fusion of envelopes of noninfectious virions was obtained. On the other hand, envelopes of infectious virions fused with the cell membrane, and the transferred viral antigens diffused on the cell surfaces and then decreased in number.

Fusion of Semliki forest virus with the plasma membrane can be induced by low pH

When BHK-21 cells with Semliki Forest virus (SFV) bound at the plasma membrane are briefly treated with low pH medium (pH 5-6), fusion between the viral membrane and the plasma membrane occurs, releasing the viral nucleocapsid into the cytoplasm. The fusion reaction resembles that described previously for Sendai virus but with one fundamental difference; it is strictly dependent on low pH. The fusion reaction is highly efficient. Up to 86% of bound viruses fuse, and 6 X 10(6) virus spike proteins can be inserted into the plasma membrane of each cell. The process is very rapid (full activity is observed after 5 s) and it occurs over a wide temperature range and equally well with all five cell lines tested (BHK-21, HeLa B, HeLa suspension, Raji, and 3T3). Low pH-induced fusion of the virus at the plasma membrane can lead to infection of susceptible cells. The artificial nature of this infection pathway is, however, demonstrated by the facts that infection through the plasma membrane occurs only at subphysiological pH and that it is insensitive to inhibitors of the normal entry route. Nevertheless, these results indicate that low pH membrane fusion introduces the viral genome into the cytoplasm in a form suitable for replication.

Adsorption, penetration, and uncoating of murine leukemia virus studied by using its reverse transcriptase

A procedure using the virus-associated reverse transcriptase was developed for following the kinetics of adsorption, penetration, and uncoating of murine leukemia virus. Viral adsorption to cell membrane was determined by assaying this enzyme activity in isolated debris of mechanically disrupted cells after infection with murine leukemia virus in the presence of actinomycin D. At 37 degrees C, viral adsorption proceeded at a high initial rate, but after 5 min of incubation with the virus, it gradually slowed down. At 4 degrees C, viral adsorption was slower but proceeded at a linear rate. Intracellular virus was determined by centrifuging the cytoplasmic fraction of the disrupted cells at 105,000 x g for 45 min and assaying reverse-transcriptase activity in the high-speed pellet thus obtained. Sucrose gradient analysis of the enzyme activity recovered from the cytoplasm of infected cells indicated that this activity represented intact virus particles. No appreciable amount of such particles was recovered from the cytoplasm of cells infected at 4 degrees C. This indicates that the virions recovered from the cytoplasm of cells infected at 37 degrees C are indeed intracellular virus particles which penetrated into the cells and not just membrane-bound particles mechanically released to the cytoplasmic fraction during cell disruption. By this procedure intracellular virus was found to accumulate in the cytoplasm, reaching a maximal level within 20 min. The accumulated intracellular virus particles gradually disappeared from the cytoplasm, evidently due to their uncoating which was completed within 80 min.

Electron microscopic studies of tumor viruses. II. Entry and uncoating of Epstein-Barr virus

Entry of Epstein-Barr virus into human lymphoblastoid cells (Daudi cells) was studied by electron microscopy. At the site of viral attachment, two distinct interactions conducive to penetration of the virus occurred between the viral envelope and cell membrane, namely, (i) simultaneous dissolution of both the envelope and cell membrane, presumably resulting in passage of viral capsids into the cytoplasm and (ii) dissolution confined to the cell membrane with resulting penetration of enveloped virus. In the latter case envelope dissolution appears to occur subsequently in the cytoplasm with release of capsids. Fusion of the viral envelope with the cell membrane was not observed. The capsids exhibited two distinct structural forms--one dense, the other translucent or light in appearance. The former disrupted near the cell membrane with release of viral cores into the cytoplasm whereas the light capsids containing dense cores migrated toward the nucleus and accumulated in the perinuclear region. Apparently the process of releasing deoxyribonucleic acid (DNA) from the light capsid is slowed down or prevented in Daudi cells. A hypothesis is presented concerning the manner in which these two types of capsids initiate infection.

Early changes in the membrane of HeLa cells adsorbing Sendai virus under conditions of fusion

Adsorption of Sendai virus at high multiplicity (500-1,000 HAU/10(6) cells) to HeLa cells grown in monolayers causes immediate changes in the ion barrier of the cell membrane, as well as changes in the morphology of the virus-treated cells. Within minutes of adsorption the cells begin to lose potassium and an extensive influx of ions into the cells occurs. Concomitantly with these changes, the cell membrane becomes depolarized, and the resting potential across its membrane decreases. Twenty to sixty minutes post adsorption the damage to the cell membrane is repaired, and both the potassium uptake and the resting potential return to their pre-exposure values. Scanning electron-micrographs of Sendai infected cells incubated at 37 degrees C show formation of bridging microvilli in a zipper-like fashion within two to five minutes post-adsorption; 30 to 60 minutes thereafter the majority of cells in the monolayer are fused. Biochemical changes induced by virus adsorption and the role of Ca++ ions in the observed effects are discussed.

Inhibition of adsorption of West-Nile and herpes simplex viruses by procaine

The effect of the local anaesthetic drug procaine on the adsorption of two enveloped viruses was studied. Physiological concentrations of the drug (7 X 10(-3)--7 X 10(-2) M) strongly inhibited the adsorption of both West-Nile and herpes viruses as determined by plaque assay and the infective center assay.

Further evidence for entry of infectious bovine rhinotracheitis virus into bovine kidney cells

The penetration of bovine kidney cells by infectious bovine rhinotracheitis virus, a member of the herpesvirus group, was investigated using the direct immunoferritin labeling technique. Electron microscopic examination of infected cells after 10 min at 37°C revealed fusion between viral envelope and cell membrane; the former reacted with the ferritin particles conjugated with antiviral antibody. However, shortly after penetration of the nucleocapsid, viral-specific antigenic sites on the plasma membrane were not detected by the immunoferritin technique. Antigenically reactive structures in a disorganized array were frequently detected extracellularly, situated above the penetration sites as indicated by the internalized nucleocapsids.

Measles virus and its associated diseases

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Joint recognition by cytotoxic T cells of inactivated Sendai virus and products of the major histocompatibility complex

Cytotoxic T cells specific for Sendai virus were generated by culturing murine spleen cells in vitro together with UV-inactivated Sendai virus. In vivo immunization of donor mice with UV-inactivated Sendai virus resulted in an in vitro secondary response of increased magnitude. Cytotoxic activity was demonstrated in a short-term 51Cr-release assay, using syngeneic tumor cells which had been coated with inactivated Sendai virus by incubation at 4 degrees C for 30 min. The lysis of Sendai virus-coated target cells was restricted by the H-2 haplotype of the target cells, suggesting that the H-2 genes of the target cell contributed to the specificity of the lysis. Kinetic experiments showed that susceptibility to lysis by cytotoxic T cells specific for Sendai virus appeared within 30 min after coating target cells with inactivated virus. Furthermore, there was no detectable synthesis of new proteins in cells treated with UV-inactivated Sendai virus. For these reasons, we suggest that neither viral replication nor the synthesis of new proteins are necessary for the production of the antigen recognized by cytotoxic cells specific for Sendai virus. We infer that the virus-specific component on the target cells is probably a preformed virion antigen adsorbed onto or integrated into the cell membrane. These results imply that, if the cytotoxic T cell recognizes a single antigenic determinant specified both by viral and H-2 genes, this determinant is formed by the physical association of H-2 and Sendai virus antigens rather than by their alteration during the processes of synthesis.

Transmembrane phospholipid motions induced by F glycoprotein in hemagglutinating virus of Japan

Transfer of phospholipid from the envelope of hemagglutinating virus of Japan (HVJ) to erythrocyte (RBC) membrane and the virus-induced transfer of phospholipid between RBC membranes were studied using spin-labeled phosphatidylcholine (PC). The transfer of PC from membranes labeled densely with PC to unlabeled membranes was followed by the peak height increase in the electron spin resonance spectrum. The two kinds of transfer reactions took place very rapidly as reported previously. To obtain further details, the transfer reactions were studied with HVJ, HVJ inactivated by trypsin, HVJ harvested early, HVJ grown in fibroblast cells, the fibroblast HVJ activated by trypsin, influenza virus, and glutaraldehyde-treated RBCs. The results demonstrated that the viral F glycoprotein played a crucial role in the transmembrane phospholipid movements as well as in the fusion and hemolysis of RBCs. The transfer from HVJ to RBC's occurred partially through an exchange mechanism not accompanying the envelope fusion. This was shown by a decrease in the exchange broadening of the electron spin resonance spectrum of released spin-labeled HVJ (HVJ) and also by an increase in the ratio of PC to viral proteins incorporated into RBC membranes. HVJ modified RBC membrane so as to be able to exchange its phospholipids with those of inactive membranes such as fibroblast HVJ, influenza virus, glutaraldehyde-treated RBC'S, and phosphatidylcholine vesicles. HVJ affected the fluidity of RBC membranes markedly, the environments around PC being much fluidized. The virus-induced fusion was discussed based on close apposition of the membranes by HANA proteins and on the destabilization and fluidization of RBC membranes by F glycoproteins.

Persistent parainfluenza type 1 (6/94) infection of brain cells in tissue culture

A state of persistent infection with parainfluenza type 1 virus (6/94 strain) was established in cultures of human and bovine brain cells. Following primary infection of human brain cells, viral cytopathic effect (CPE) and hemadsorption (HAD) depended on the multiplicity of infection. After persistent infection was established the virus rapidly became cell-associated; no CPE occurred and no viral antigen was detectable by HAD, immunofluorescence (FA), or immunoprecipitation. Infectious virus could be recovered only by fusion or cocultivation. This was in marked contrast with infected bovine brain cells, where, following primary infection, little or no CPE occurred. A productive infection rapidly evolved and persisted without CPE, but with 100 per cent HAD and FA positive cells.

Electron microscope observations on the entry of avian infectious bronchitis virus into susceptible cells

Infectious bronchitis virus was observed to enter cells of chicken chorioallantoic membrane by viropexis. There was no support for the suggestion that entry took place by fusion of viral and plasma membranes. The results of electron microscopy showed that virus attachment occurred both at 4 degrees and at 37 degrees C. Viropexis was not observed until the preparations were warmed. Similar results were obtained using chicken kidney cells. Quantitative data obtained from a plaque counting system employing chicken kidney cells indicated that attachment was the same at both temperatures and that some virus particles were taken up at 4 degrees C. Virus uptake was triggered by attachment of the virus to the cell membrane and the subsequent process of virus entry visualised by E. M. appeared to proceed without the involvement of lysosomal enzymes. No intracellular virus was located by electron microscopy in warmed preparations when virus was treated with specific antiserum, either before or after adsorption to the cells.

Bovine parainfluenza type 3 virus infection: ultrastructural aspects of viral pathogenesis in the bovine respiratory tract

After aerosolization of a bovine strain of parainfluenza type 3 virus, the pathogenesis of the virus was followed from the trachea to the bronchioalveolar compartments of the lung of colostrum-free calves and of conventionally reared calves during a 5- to 12-day postexposure interval. By tissue titration, plaque assay, and electron microscopy, it was found that virus infection could be established in colostrum-free calves as well as in conventionally reared calves, even though sequential changes of virus replication were observed mainly in the infected colostrum-free calves during the 5- to 6-day postexposure periods. Electron microscopy demonstrations of (i) aggregates of viral nucleocapsids in the cytoplasm, (ii) alterations of cilia and basal bodies, (iii) dissolution of cytoplasmic membranes, and (iv) the shedding of virus into luminal spaces confirmed that epithelial cells of the respiratory tract were the primary target cells for the virus replication leading to cell destruction. These observations revealed further that productive infection was more efficient in the bronchioalveolar regions than in the tracheal regions, although large aggregates of viral nucleocapsids and destructive changes were more pronounced in the tracheal epithelium. The finding that parainfluenza type 3 virus replicates in the alveolar type II cells suggests that changes in surfactant production may occur during the peak of infection of these cells. The demonstration of virus budding through the basement membrane of small bronchioles and the presence of virus particles in the interstitial regions imply that one of the host defense lines, the basement membrane, may be impaired by virus invasion.

In vivo studies of parainfluenza I (6/94) virus: mononuclear cell interactions

Adsorption to and penetration of migratory mononuclear cells in vivo by Para-influenza I (6/94) virus were documented by electron microscopy during the acute phase of an experimental meningoencephalitis in ICR mice. Phagocytosis of virions was the apparent and predominant mode of viral entry into these cells. Penetration by fusion was also seen. Nucleocapsids were replicated by the mononuclear cells which became a potential reservoir for the further spread of virus.