The biology and pathogenesis of coronaviruses

Proteolytic cleavage of the E2 glycoprotein of murine coronavirus: host-dependent differences in proteolytic cleavage and cell fusion

Cell fusion induced by infection with mouse hepatitis virus strain A59 (MHV-A59) varied markedly in extent and time course in four different murine cell lines. When inoculated at a multiplicity of 3 to 5 PFU per cell, the Sac-, L2, and DBT cell lines began to fuse by 7 h, were fused into confluent syncytia by 9 to 12 h, and peeled from the substrate by 10 to 14 h. These virulent virus-cell interactions were in striking contrast to the moderate interaction of MHV-A59 with the 17 Cl 1 cell line, in which only small syncytia were observed 18 h postinoculation, and greater than 50% of the cells remained unfused by 24 h. The yield of infectious virus produced by 17 Cl 1 cells was 10-fold higher than the yields from the other three cell lines. The processing of the nucleocapsid protein, the membrane glycoprotein E1, and the peplomeric glycoprotein E2 were found to differ significantly in the four cell lines. Since the E2 glycoprotein is responsible for virus-induced cell fusion, we attempted to correlate differences in cellular processing of E2 with differences in fusion of infected cells. The predominant intracellular form of E2 in all cell lines was the 180K species. Pulse-chase experiments showed that a small portion of the 17 Cl 1 cell-associated 180K E2 was cleaved by 1 h after synthesis to yield 90K E2, shown in the preceding paper to consist of two different glycoproteins called 90A and 90B (L. S. Sturman, C. S. Ricard, and K. V. Holmes, J. Virol. 56:904-911, 1985). This cleavage occurred shortly before the release of virions from cells, as shown by pulse-chase experiments. After budding at intracellular membranes, virions released into the medium by the four cell lines contained different ratios of 180K to 90K E2. Virions from Sac- cells, which contained 100% 90K E2, fused L2 cells rapidly without requiring virus replication, whereas virions from 17 Cl 1 cells, which had 50% 90K E2, required trypsin activation to induce rapid fusion (Sturman et al., J. Virol. 56:904-911, 1985). The addition of protease inhibitors to the medium markedly delayed L2 cell fusion induced by MHV infection. The extent of coronavirus-induced cell fusion does not depend solely upon the percent cleavage of the E2 glycoprotein by cellular proteases, since extensive fusion was induced by infection of L2 and DBT cells but not 17 Cl 1 cells, although all three cell lines cleaved E2 to the same extent.(ABSTRACT TRUNCATED AT 400 WORDS)

Rotaviral and coronaviral diarrhea

A number of different viruses can be primary pathogens in the neonatal calf diarrhea complex. By far the most common viruses causing calfhood diarrhea found throughout the world are rotaviruses and coronaviruses. Primary infection of newborn calves with either one of these viruses can cause severe intestinal alterations and diarrhea. Rotaviruses can produce high-morbidity outbreaks of diarrhea in calves under 10 days of age. Morality is variable mainly owing to secondary bacterial infections and electrolyte imbalances. Rotavirus infection of the small intestinal mucosa leads to loss of enterocytes of the upper third of the intestinal villi with subsequent villous atrophy and malabsorption. There is growing evidence that different rotavirus serotypes of different pathogenicity exist. Coronavirus infections can produce high-morbidity outbreaks of diarrhea in calves under 20 days of age, with variable mortality due to secondary complications. Coronaviruses affect not only the small intestinal mucosa, producing significant villous atrophy, but also the colon, causing a very severe intestinal damage that can lead to death due to subsequent electrolyte disturbances. All coronaviruses associated with neonatal calf diarrhea appear to be of the same serotype. The etiologic diagnosis of viral diarrheas of calves requires the support of the laboratory. One of the most useful diagnostic methods is the examination of fecal extracts for the presence of virus particles by electron microscopy. Other antigen-detection procedures like enzyme immunoassays have been found to be useful in the diagnosis of rotaviral diarrheas. The sample of choice for these diagnostic tests is a fresh fecal sample collected directly from the calf as close as possible to the onset of diarrhea. Samples from more than one calf during the outbreak enhance the laboratory ability to establish a proper viral diagnosis.

Difference in sensitivity to interferon among mouse hepatitis viruses with high and low virulence for mice

Mouse hepatitis viruses (MHV) of different virulence for mice were studied with respect to interferon (IFN) sensitivity. The growth of low-virulent MHV-S and intermediately virulent MHV-JHM was significantly suppressed in IFN-treated L cells compared with untreated cells. However, a comparable suppression of the growth of highly virulent MHV-2 was not observed in IFN-treated cells. This differential effect of IFN treatment could also be demonstrated at the level of viral mRNA and viral proteins. In cells infected with MHV-S or MHV-JHM the amount of viral mRNAs was remarkably reduced by IFN treatment. Also the levels of the major intracellular viral proteins, in particular the E1 protein, were affected by IFN treatment. Similar effects could not be demonstrated in MHV-2-infected cells. These results suggest that during MHV-S or MHV-JHM infection IFN treatment suppresses virus replication at several stages. The significance of these results is discussed in terms of the pathogenecity of these viruses.

Virus-induced central positional nystagmus in mice

Geotropic direction-changing nystagmus in lateral body positions was observed in 4-week-old BALB/c mice after intracerebral injection with a temperature-sensitive mutant of mouse hepatitis virus. The positional nystagmus was detected already 2 days after infection and it lasted half a year at least. The nystagmic responses of the semicircular canals were also evaluated before and after infection. They were unaltered during the disease, which was clinically manifested by general weakness, ataxia and tremor. Histopathological examination 2 weeks after infection revealed demyelination in various parts of the CNS.

Vaccination against enteric rota and coronaviruses in cattle and pigs: enhancement of lactogenic immunity

Passive immunity against enteric viral infections is dependent upon the continual presence in the gut lumen of a protective level of specific antibodies. This article examines methods currently used to enhance the titre and duration of specific antibody in the mammary secretions of cows and pigs, with particular reference to rotavirus and coronavirus infections. In addition, some of the potential problems to be found in attempting to produce vaccines against these viral infections are outlined.

Competitive enzyme immunoassays for the rapid detection of antibodies to feline infectious peritonitis virus polypeptides

Monoclonal antibodies specific for the envelope (E1), peplomer (E2), and nucleocapsid (N) polypeptides of feline infectious peritonitis virus (FIPV) were used in rapid, competitive enzyme-linked immunosorbent assays (ELISA) to study the humoral immune response of cats to FIPV infection. Results from the competitive ELISAs were correlated with those from immunofluorescent antibody assays (IFAs) on 203 samples obtained from 64 individual cats. The IFA results correlated best with those obtained with the anti-E1 specific competitive ELISA (85.7%). In contrast, anti-N and anti-E2 competitive ELISA results correlated with IFA results only 65.5 and 2.4% of the time, respectively. The results of the anti-E1 specific competitive ELISA were not influenced by the total immunoglobulin concentration or the possible presence of free viral antigens in the serum. These results suggest that a competitive ELISA involving the use of enzyme-conjugated monoclonal antibody to the E1 glycoprotein of FIPV is a simple and rapid replacement for the more cumbersome IFA.

Characterization of leader-related small RNAs in coronavirus-infected cells: further evidence for leader-primed mechanism of transcription

Mouse hepatitis virus (MHV), a murine coronavirus, replicates in the cytoplasm and synthesizes 7 viral mRNAs containing an identical stretch of leader RNA sequences at the 5'-end of each RNA. The leader-coding sequences at the 5'-end of genomic RNA are at least 72 nucleotides in length and are joined to the viral mRNAs by a unique mechanism. Utilizing a leader-specific cDNA probe, we have detected several free leader RNA species ranging from 70 to 82 nucleotides in length. The predominant leader RNA was approximately 75 nucleotides. In addition, larger distinct leader-containing RNAs were also detected ranging from 130 to 250 nucleotides in length. The 70-82-nucleotide leader-related RNAs were present in both the cytosol and membrane fractions of infected cells. They were also detected only in the small RNA fractions but not associated with the replicative-intermediate RNA. These data suggest that the leader RNAs were associated with the membrane-bound transcription complex but at least part of them were dissociated from the RNA template. We have also identified a temperature-sensitive mutant, which synthesizes only leader RNA but not mRNAs at nonpermissive temperature, indicating that leader RNA synthesis is distinct from the transcription of mRNAs. These data support the leader-primed mechanism for coronavirus transcription and suggest that one or more free leader RNAs are used as primers of mRNA synthesis.

Antigenic variation among murine coronaviruses: evidence for polymorphism on the peplomer glycoprotein, E2

A panel of 28 monoclonal antibodies (MAb) against the structural proteins of murine hepatitis virus-4, strain JHM (MHV-4) was used in three antigen binding assays to determine the extent of antigenic homology among six strains of murine coronaviruses. The antigenic determinants studied were highly conserved on the E1 glycoproteins and nucleocapsid (N) proteins of all strains tested. In contrast, antigenic polymorphism was observed among the E2 glycoproteins. Of three previously described antigenic determinants against which neutralizing antibodies are directed, only one, termed A(E2), was conserved on all strains. Antigenic site B(E2) was found only on the strongly neurotropic MHV-4 and site C(E2) was present on the virulent MHV-4 and MHV-3 (hepatotropic) strains, but absent on the weakly pathogenic MHV-A59, MHV-1 and MHV-S strains. Four non-neutralizing antibodies against at least one topographically distinct antigenic determinant, which we previously designated D(E2), gave binding patterns consistent with two distinct sites. One of these was present on all MHV strains tested and the other was present on all strains except MHV-S. These non-neutralizing antigenic sites were redesignated E(E2) and D(E2) respectively.

Characterization of a variant virus selected in rat brains after infection by coronavirus mouse hepatitis virus JHM

The intracerebral inoculation of Lewis rats with the murine coronavirus MHV-JHM leads in the majority of animals to acute encephalitis and death within 14 days. Viral RNAs isolated from the brains of animals 5 to 7 days after infection were compared by Northern blot analysis with the RNAs produced during the lytic infection of Sac(-) or DBT cells with wild-type MHV-JHM (wt virus). Reproducibly, the subgenomic mRNAs 2 and 3 but no other viral RNAs were significantly larger in the brain-derived material. All viruses isolated from infected brain material displayed and maintained this altered mRNA profile when cultivated in Sac(-) or DBT cells. A virus isolated from the infected brain material, MHV-JHM clone 2 (cl-2 virus), has been further characterized. This isolate grew in tissue culture and induced cytopathic effects comparable to those induced by wt virus. However, the mRNAs 2 and 3 produced in cl-2 virus-infected cells had molecular weights ca. 150,000 larger than those produced in cells infected with wt virus. There was no detectable difference in genome-sized RNA (mRNA 1) or subgenomic mRNAs 4, 5, 6, and 7 as determined by electrophoresis in agarose gels. T1-resistant oligonucleotide analysis of genomic RNA revealed one additional and one missing oligonucleotide in the fingerprint of cl-2 virus compared with wt virus. The oligonucleotide fingerprints of intracellular mRNA 3 were identical for both viruses. Pulse-labeling with [35S]methionine in the presence of tunicamycin showed that the primary translation product of mRNA 3, the E2 apoprotein, was ca. 15,000 larger in molecular weight in cl-2 virus-infected cells. These data show that viruses with larger mRNAs 2 and 3 (the latter encoding an altered E2 glycoprotein) are selected for multiplication in rat brains. Mechanisms for the generation of such variants and the possible nature of their selective advantage are considered.

An enteric coronavirus of the rabbit: detection by immunoelectron microscopy and identification of structural polypeptides

The immunoelectron microscopy (IEM) technique has been used for the detection of a rabbit enteric coronavirus (RECV). Immune serum was prepared in guinea pigs; the viral antigen used for the immunization procedure was obtained from the caecum of a sick rabbit, concentrated by centrifugation and purified on Percoll gradient. In order to identify the viral particles used in the immunization procedure, the protein pattern of the particles was determined by electrophoresis and compared with the pattern of other known coronaviruses. Analysis of structural polypeptides of the purified viral particles revealed a pattern similar to that reported for other coronaviruses. These polypeptides cross reacted with two other coronavirus specific immune sera (IBV and TGE). IEM assay of fecal samples collected from healthy and sick rabbits showed the presence of immune aggregates in specimens from both sick and healthy rabbits. Those aggregates contained viral particles sharing morphological characteristics with other coronaviruses. Furthermore, IEM assay was shown to be more sensitive than a direct EM procedure to detect coronavirus particles in rabbit feces. This assay also allowed the detection of a larger number of chronic carriers.

Influence of maternal immunity on the outcome of murine coronavirus JHM infection in suckling mice

Adult C3H mice are resistant to intraperitoneal infection with murine coronavirus JHM, whereas suckling offspring of non-immune females are susceptible. Resistance can be conferred on suckling C3H mice by postnatal transmission of maternal immunity, if transfer precedes infection. Suckling mice succumb to infection even when they receive maternal antibodies within 1 day after infection. Prenatal transmission alone without subsequent postnatal transmission of maternal immunity is not sufficient to provide resistance. Persistence of virus without clinical consequences was observed when the supply of breast milk anti-JHMV antibodies was terminated 5 days before infection. Immune reactions restricted by histocompatibility antigens do not play a crucial role in bestowing resistance. As neutralizing anti-JHM serum antibody titers of adult mice only rise sharply 5 to 7 days after infection, these results indicate that infection of adults can be arrested by immunological means but that, in addition, the rate of virus dissemination must be limited by other non-immunological mechanisms.

Detection of coronavirus-like particles in homosexual men with acquired immunodeficiency and related lymphadenopathy syndrome

Coronavirus-like particles were identified by electronmicroscopy in the feces of homosexual men. The particles banded at a density of 1.21 g/ml after cesium chloride density gradient centrifugation. To determine whether the presence of this virus might be related to clinical symptoms, several patient groups were studied prospectively. In 8 of 16 (50%) homosexual males with acquired immunodeficiency syndrome (AIDS) or unexplained lymphadenopathy syndrome (LAS), coronavirus particles were found. In contrast, such particles were found in none of 18 heterosexual controls and in only 3 of 20 homosexual males without AIDS or LAS. Thus, coronavirus excretion correlated significantly (2 alpha less than 0.01) with the clinical diagnosis of AIDS or with syndromes belonging to the AIDS-related complex. In addition, such particles identified in the serum of one patient with LAS and diarrhea suggest invasion and systemic spread of the agent and underline that this virus behaves differently from "common cold" human coronaviruses.

Studies on the relationship between coronaviruses from the intestinal and respiratory tracts of calves

An immunofluorescence test on smears of nasal epithelial cells was used to detect coronavirus infection in the respiratory tract of calves. Thirteen gnotobiotic calves were infected with coronavirus isolates derived from faeces or respiratory material: virus was detected in faeces and nasal swabs from all animals. In 115 calves from a field survey, there was a significant association between coronavirus excretion from both respiratory and enteric routes in calves with diarrhoea. In a further 12 calves, at necropsy, the predilection sites for coronavirus growth were the distal small intestine, large intestine and the epithelia of the nasal cavity and trachea. Antigen was not found in lung tissue by immunofluorescence or immunoperoxidase staining. Infection with enteric coronavirus induced immunity to reinfection and to heterologous challenge with two coronavirus isolates derived from the respiratory tract. Nine coronaviruses were cultivated, cloned and antisera to three were prepared in pigs. There was complete virus neutralisation in tests with homologous sera and significant cross reactions with the eight other isolates which were of intestinal and respiratory origin. Thus, these bovine coronavirus isolates belonged to the same serotype despite the source of virus.

Feline infectious peritonitis. An immune-mediated coronaviral vasculitis

Mainly through studies inducing experimental infection of susceptible cats, significant advances have recently been made in our understanding of the pathogenesis of FIP. Much of this knowledge should not presently be directly extrapolated to field cases of FIP, because the route of infection and challenge dose and strain of virus may be significantly different. Advances in the prevention and treatment of FIP will depend greatly on clarification of the exact nature of the several coronaviruses affecting cats and the role of cell-mediated immunity in resistance to FIPV.

Antigenic relationships among proteins of bovine coronavirus, human respiratory coronavirus OC43, and mouse hepatitis coronavirus A59

Antisera prepared against each of three single and one pair of major structural proteins of the bovine coronavirus (Mebus strain) were used in immunoblotting studies to measure cross-reactivity with the structural proteins of the human coronavirus OC43 and the mouse hepatitis coronavirus A59. We conclude that the bovine coronavirus is comprised of four major structural proteins, gp190 (normally present as 120- and 100-kilodalton subunits), gp140, pp52, and gp26. The human coronavirus OC43 has an antigenically homologous counterpart of similar molecular mass to each of these proteins. The mouse hepatitis coronavirus A59 has an antigenically homologous counterpart to only three of these proteins: gp190, pp52 and gp26. There is no counterpart in the mouse virus to the 140-kilodalton glycoprotein, the apparent hemagglutinin of the bovine coronavirus.

Replication of murine coronaviruses in somatic cell hybrids between murine fibroblasts and rat schwannoma cells

The replication of the murine coronaviruses MHV3 and JHM has been studied in somatic cell hybrids formed between murine fibroblast L2 cells which support lytic infections with both these agents, and rat RN2 Schwannoma cells which support the replication of JHM in a temperature-sensitive, persistent manner but are restrictive to the replication of MHV3. The results described in this report indicate that the totally permissive state is dominant over the persistent or restricted state since the hybrid cells permit the replication of both these viral agents in a lytic manner.

Coronavirus multiplication: locations of genes for virion proteins on the avian infectious bronchitis virus genome

Six overlapping viral RNAs are synthesized in cells infected with the avian coronavirus infectious bronchitis virus (IBV). These RNAs contain a 3'-coterminal nested sequence set and were assumed to be viral mRNAs. The seven major IBV virion proteins are all produced by processing of three polypeptides of ca. 23, 51, and 115 kilodaltons. These are the core polypeptides of the small membrane proteins, the nucleocapsid protein, and the 155-kilodalton precursor to the large membrane proteins GP90 and GP84, respectively. To determine which mRNAs specify these polypeptides, we isolated RNA from infected cells and translated it in a messenger-dependent rabbit reticulocyte lysate. Proteins of 23, 51, and 110 kilodaltons were produced. Two-dimensional tryptic peptide mapping demonstrated that these proteins were closely related to the major virion proteins. Fractionation of the RNA before cell-free translation permitted the correlation of messenger activities for synthesis of the proteins with the presence of specific mRNAs. We found that the smallest RNA, RNA A, directs the synthesis of P51, the nucleocapsid protein. RNA C, which contains the sequences of RNA A, directs the synthesis of the small membrane protein P23. RNA E directs the synthesis of the large virion glycoproteins. These results supported a model in which only the unique 5'-terminal domain of each IBV mRNA is active in translation and enabled us to localize genes for virion proteins on the IBV genome.

Protection of mice against infection with mouse hepatitis virus type 3 by injection of silica

Injection of silica did not brake the resistance against MHV3 conferred to C57BL/6 mice by injection of C. parvum. However, silica itself had a marked protective effect against MHV3 infection that was maximal when injecting 1 mg 2 hrs before virus infection. The protective effect of silica was observed in a number of inbred mouse strains that differ in their relative resistance to MHV3 infection. No viral titers were observed in the spleen and liver of mice which had received MHV3 plus silica, whereas high titers were observed in the virus-infected controls. Injection of silica caused a marked decrease in the number of esterase-positive macrophages in the peritoneal wash-out population, that may be compatible with the possibility that the cause of the protection is the depletion of target cells for the viral infection. This latter effect, however, was short-lived and 24-48 hrs after injection of silica, high numbers of esterase-positive cells were again observed. This may explain why only little protection was observed when silica was administered 2 days before virus infection.

Antigenic relationships of murine coronaviruses: analysis using monoclonal antibodies to JHM (MHV-4) virus

Monoclonal antibodies were produced to JHMV-DL, a neurotropic member of the mouse hepatitis virus (MHV) or murine coronavirus group. Of 23 antibodies isolated, 10 were specific for the major envelope glycoprotein, gp180/90, 10 for the nucleocapsid protein, pp60, and 3 for the minor envelope glycoprotein, gp25. Eleven different MHV isolates were used in antibody binding assays to study antigenic relationships among the viruses. Each MHV isolate tested had a unique pattern of antibody binding, indicating that each is a distinct strain. Conservation of JHMV-DL antigenic determinants varied among the three proteins, with pp60 showing intermediate conservation, gp180/90 little conservation, and gp25 marked conservation in the different MHV strains. Monoclonal antibodies to pp60 proved most useful in delineating antigenic relationships among MHV strains. These antigenic groups correlated with pathogenic types, indicating that pp60 may be one of the gene products which mediates the distinct disease patterns manifested by different murine coronaviruses.

The effect of mouse hepatitis virus infection on the microcirculation of the liver

Mouse hepatitis virus type 3 infection results in strain-dependent liver disease. The effects of mouse hepatitis virus type 3 on the microcirculation of the liver in both fully susceptible (Balb/cJ) and fully resistant (A/J) mice were studied. In Balb/cJ mice, 6 to 12 hr following infection, abnormalities in liver blood flow were observed which consisted of granular blood flow in both terminal hepatic and terminal portal venules. In addition, sinusoidal microthrombi were present predominantly in periportal areas. By 24 to 48 hr, liver cell edema and small focal lesions were prominent. At 48 hr, thrombi and hepatocellular necrosis were widespread, and blood was shunted from damaged areas into patent sinusoids. In sharp contrast to these abnormal findings, normal streamlined blood flow was present in the resistant A/J animals at all time points following infection. Since large amounts of virus were demonstrated by immunofluorescene in and by recovery and growth from livers of both resistant and susceptible strains, the presence of the virus per se cannot explain the abnormalities observed.

Synthesis and subcellular localization of the murine coronavirus nucleocapsid protein

The synthesis and processing of the nucleocapsid protein (pp60) of the JHM strain of murine coronaviruses were examined. Pulse-chase experiments showed that pp60 was synthesized initially as a protein of approximately 57,000 in molecular weight (p57). Immunoprecipitation using mouse anti-JHMV antiserum indicated that p57 was virus specific. Immunoprecipitation with monoclonal antibodies specific for pp60 showed that p57 was antigenically related to pp60 and was not phosphorylated, while the intracellular protein that comigrated with the virion nucleocapsid protein, pp60, was phosphorylated. The p57 was found exclusively in the cytosol while the majority of pp60 was associated with the membrane fraction but pp60 was not an integral membrane protein.

Coronaviridae

The family Coronaviridae comprises a monogeneric group of 11 viruses which infect vertebrates. The main characteristics of the member viruses are: (i) Morphological: Enveloped pleomorphic particles typically 100 nm in diameter (range 60-220 nm), bearing about 20 nm long club-shaped surface projections. (ii) Structural: A single-stranded infectious molecule of genomic RNA of about (5-7) X 10(6) molecular weight. A phosphorylated nucleocapsid protein [mol. wt. (50-60) X 10(3)] complexed with the genome as a helical ribonucleoprotein; a surface (peplomer) protein, associated with one or two glycosylated polypeptides [mol. wt. (90-180) X 10(3)]; a transmembrane (matrix) protein, associated with one polypeptide which may be glycosylated to different degrees [mol. wt. (20-35) X 10(3)]. (iii) Replicative: Production in infected cells of multiple 3' coterminal subgenomic mRNAs extending for different lengths in the 5' direction. Virions bud intracytoplasmically. (iv) Antigenic: 3 major antigens, each corresponding to one class of virion protein. (v) Biological: Predominantly restricted to infection of natural vertebrate hosts by horizontal transmission via the fecal/oral route. Responsible main for respiratory and gastrointestinal disorders.

Some properties of the Epithelioma papulosum cyprini (EPC) cell line from carp cyprinus carpio

A cell line, named Epithelioma papulosum cyprini (EPC) and originating from carp epidermal herpes virus-induced hyperplastic lesions, was esta blished and has now given rise to more than 80 subcultures. It grows within a wide temperature range (15–33°C), survives between 7 and 10°C for several months without alterations in its further virus susceptibility and growth characteristics, and degenerates at 37°C. Its karyotype is 2n=96. It propagates most of the systemic infection viruses of the fish families Salmonidae, Cyprinidae, Anguillidae and Esocidae, as well as Indiana-type vesicular stomatitis virus.

Despite the origin of EPC cells, we were unable to demonstrate the presence of herpes virus in them. Their transformation was reflected in their karyotype (normally 2n=100) and by their growth ability in soft agar medium; monolayer growth, however, was inhibited at a density of 3×10⁶ cells/cm².

The temperature growth range, good splitting ratio (1/10) and virus susceptibility make EPC cells a highly suitable material both for fish pathology and for comparative virology studies.

Oligodendrocyte infection and demyelination produced in mice by the M9 mutant of Semliki Forest virus

Intraperitoneal inoculation with the M9 mutant of Semliki Forest virus caused focal demyelinating encephalomyelitis in weanling BALB/c and C57BL/6 mice. Demyelination was more severe in BALB/c than in C57BL/6 mice. Virus particles were seen in oligodendrocytes in areas of myelin vacuolation 5 and 7 days post inoculation (DPI). Oligodendrocytes containing virus in BALB/c mice showed hypertrophy and vacuolar degeneration. There was a mononuclear cell infiltrate and lymphocytes and necrotic cells were present in vacuoles in myelin sheaths. Demyelinating plaques containing macrophages laden with myelin debris were most prominent 14 DPI when virus was cleared from the brain. Remyelination of the central type occurred 28 DPI in BALB/c mice. These findings indicate that direct virus-induced injury to oligodendrocytes has a major role in the initiation of inflammation and demyelination in this model system.

Coronavirus JHM-induced demyelinating encephalomyelitis in rats: influence of immunity on the course of disease

Disease processes of the central nervous system (CNS) accompanied by demyelination may be the result of a viral infection or the consequence of an immunopathological reaction directed against myelin. In acute viral infections, the infection of oligodendroglial cells, leading to cell destruction, may be the main mechanism for inducing this neuropathological lesion. In the case of a persistent virus infection in oligodendroglia cells, however, it is conceivable that functional impairment of oligodendroglia cells, or the induction of an immune reaction to the agent that may cross-react with brain antigens, could eventually cause demyelination. Therefore, pathogenic studies on subacute or chronic demyelinating encephalomyelitides in association with viral infections may provide information on the mechanisms involved in demyelination. In rats, depending on the biological property of the virus material used, the genetic background and immune response of the host, a subacute or late demyelinating encephalomyelitis can be induced, accompanied by primary demyelination. This provides a model for analysis of the virus and host factors interacting in the pathogenesis of these diseases.

Genetic heterogeneity of murine coronaviruses

Several mouse hepatitis viruses (MHV) with different pathogenicity were studied by oligonucleotide fingerprinting. Two strains, MHV-K and MHV-D, which were isolated in Japan and, which cause anaplasia and necrosis of bone marrow and diarrhea, respectively, were found to be closely related to MHV-A59, the prototype MHV. Two other MHV strains, isolated from nude mice, were found to have diverged extensively from the known MHV strains. The MHVs isolated from separate cloned neuroblastoma cell lines persistently infected with JHM strain were also found to have diverged more markedly than the corresponding virus maintained under the conditions of lytic infection. Genetic divergence during persistent infection may be one of the mechanisms by which the MHV diverges.

Selective vulnerability of neural cells and age-related susceptibility to OC43 virus in mice

Suckling CD 1 mice infected intracerebrally or extraneurally with OC43 virus developed a lethal neurotropic infection with high titres of virus in the brain. Examination of infected brain by routine H & E staining revealed no necrosis even in extensively infected tissue. Resistance to infection developed with increasing age, and by 20 days of age mice were completely insusceptible to i.c. inoculation. Virus replication was also demonstrable by FA staining, in spinal cord, dorsal root ganglia and retina. All other tissues were insusceptible and in particular, macrophages from both susceptible and resistant mice were found to be resistant to infection both in vivo and in vitro. Immunosuppression rendered 15 day old mice more susceptible to infection but adult mice remained insusceptible. The transfer of immune or non immune spleen cells from resistant mice did not confer resistance to newborn mice. Treatment of resistant mice with anti interferon globulin (AIG) did not render them more susceptible. These results indicate that the immune response is partially responsible for the development of resistance to OC43 infection but that it is only partially protective and other factors must also be required. The basis for the unique susceptibility of neural tissues in suckling mice is being investigated.

Pleural effusion disease in rabbits. Properties of the aetiological agent

The size and heat sensitivity of Pleural effusion disease (PED) agent or virus (PEDV) propagated in rabbits were examined. The infectious particles were estimated to be between 25 and 50 nm by filtration. Residual infectivity of infectious serum was 0.1 per cent after heating at 56 degrees C for 4 hours. PEDV and the Stockholm agent appeared identical concerning pathogenic and immunogenic properties by infection experiments and protection tests in rabbits. Two of the three PEDV isolates were less pathogenic but appeared immunogenically identical to PEDV. The third isolate, obtained from the laboratory, which several years previously had supplied material for demonstration of the Stockholm agent, differed from PEDV in pathogenic and immunogenic properties. Serological examinations of paired rabbit sera did not indicate any antigenic relationship between PEDV and representative members of the two mammalian coronavirus antigenic groups. It is concluded that the aetiological agent of PED is a virus not belonging to the coronaviridae.

Antigenic relationships of murine coronaviruses

Two serological tests were used to examine the antigenic relationships between murine hepatitis viruses that cause different diseases in mice. Antisera prepared by immunization of mice with the individual viruses were tested for their ability to neutralize both the homologous immunogen and the other viruses. By a plaque reduction neutralization test, each antiserum was found to be specific for the immunizing virus; however, there was substantial cross-reactivity, indicating the viruses were closely related. By kinetic neutralization, two of the viruses tested, MHV-JHM and MHV-2, were found to be antigenically distinct. MHV-3 and MHV-A59 were found to be antigenically very similar but distinct. These data show that kinetic neutralization is a more precise method for determining the antigenic relationships between murine coronaviruses.

Characterization of small plaque mutants of mouse hepatitis virus, JHM strain

Two small plaque mutants designated as 1a and 2c were isolated from DBT cells persistently infected with the JHM strain of mouse hepatitis virus. Unlike the wild type JHM, these two mutant viruses grew more slowly with no prominent cell fusion. The buoyant densities of the mutants were slightly lower and 2c was revealed to have fewer peplomers than JHM by electron microscopy. The purified JHM contained five polypeptides with molecular weights (M.W.) of 260,000, 105,000 (GP105), 65,000, 60,000 (P60), and 23,000 (GP23). In addition to two polypeptides, P60 and GP23, which were common to JHM and the mutants, 1a was found to contain three other specific polypeptides with M.W. of 180,000 (GP180), 110,000, and 95,000 (GP95), while 2c had GP180, GP105, GP95, and one with a M.W. of 175,000. All of these polypeptides were shown to be glycosylated except for P60. After bromelain treatment, all these viruses lost the peplomers and contained P60 and another new 18,000 dalton polypeptide.

The molecular biology of coronaviruses

Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O -linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O -linked cellular glycoproteins.