MOUSE MACROPHAGES AS HOST CELLS FOR THE MOUSE HEPATITIS VIRUS AND THE GENETIC BASIS OF THEIR SUSCEPTIBILITY

Of Mice and Men: The Coronavirus MHV and Mouse Models as a Translational Approach to Understand SARS-CoV-2

The fatal acute respiratory coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Since COVID-19 was declared a pandemic by the World Health Organization in March 2020, infection and mortality rates have been rising steadily worldwide. The lack of a vaccine, as well as preventive and therapeutic strategies, emphasize the need to develop new strategies to mitigate SARS-CoV-2 transmission and pathogenesis. Since mouse hepatitis virus (MHV), severe acute respiratory syndrome coronavirus (SARS-CoV), and SARS-CoV-2 share a common genus, lessons learnt from MHV and SARS-CoV could offer mechanistic insights into SARS-CoV-2. This review provides a comprehensive review of MHV in mice and SARS-CoV-2 in humans, thereby highlighting further translational avenues in the development of innovative strategies in controlling the detrimental course of SARS-CoV-2. Specifically, we have focused on various aspects, including host species, organotropism, transmission, clinical disease, pathogenesis, control and therapy, MHV as a model for SARS-CoV and SARS-CoV-2 as well as mouse models for infection with SARS-CoV and SARS-CoV-2. While MHV in mice and SARS-CoV-2 in humans share various similarities, there are also differences that need to be addressed when studying murine models. Translational approaches, such as humanized mouse models are pivotal in studying the clinical course and pathology observed in COVID-19 patients. Lessons from prior murine studies on coronavirus, coupled with novel murine models could offer new promising avenues for treatment of COVID-19.

Efficacy and safety of aerosol inhalation of recombinant human interferon α1b (IFNα1b) injection for noninfluenza viral pneumonia, a multicenter, randomized, double-blind, placebo-controlled trial

Background

To investigate the efficacy and safety of aerosol inhalation of recombinant human interferon α1b (IFNα1b) injection for noninfluenza viral pneumonia.

Methods

One hundred sixty-four patients with noninfluenza viral pneumonia were divided into IFNα1b and control groups. The IFNα1b group received routine treatment + aerosol inhalation of recombinant human IFNα1b injection (50 μg × 2 injections, bid). The control group received routine treatment + IFN analog (two injections, bid). Overall response rate (ORR) of five kinds clinical symptoms. Further outcomes were daily average score and the response rate of each of the symptoms above.

Results

A total of 163 patients were included in the full analysis set (FAS) and 151 patients were included in the per-protocol set (PPS). After 7 days of treatment, ORR of clinical symptoms was higher in IFNα1b group than that in control group for both the FAS and PPS. Moreover, after 7 days of treatment, the daily score of three efficacy indexes including expectoration, respiratory rate, and pulmonary rales were improved. The ORRs for expectoration and pulmonary rales were higher in the IFNα1b group than in the control group (P < 0.05). There were no significant differences of the ORRs for coughing, chest pain and respiratory rate between the two groups (P > 0.05). The incidence of adverse events was 6.5% (n = 5) in IFNα1b group and 3.5% (n = 3) in control group (P > 0.05).

Conclusion

Aerosol inhalation of recombinant human IFNα1b is safe and it can improve the clinical symptoms of noninfluenza viral pneumonia.

IFN-I response timing relative to virus replication determines MERS coronavirus infection outcomes

Type 1 IFNs (IFN-I) generally protect mammalian hosts from virus infections, but in some cases, IFN-I is pathogenic. Because IFN-I is protective, it is commonly used to treat virus infections for which no specific approved drug or vaccine is available. The Middle East respiratory syndrome-coronavirus (MERS-CoV) is such an infection, yet little is known about the role of IFN-I in this setting. Here, we show that IFN-I signaling is protective during MERS-CoV infection. Blocking IFN-I signaling resulted in delayed virus clearance, enhanced neutrophil infiltration, and impaired MERS-CoV-specific T cell responses. Notably, IFN-I administration within 1 day after infection (before virus titers peak) protected mice from lethal infection, despite a decrease in IFN-stimulated gene (ISG) and inflammatory cytokine gene expression. In contrast, delayed IFN-β treatment failed to effectively inhibit virus replication, increased infiltration and activation of monocytes, macrophages, and neutrophils in the lungs, and enhanced proinflammatory cytokine expression, resulting in fatal pneumonia in an otherwise sublethal infection. Together, these results suggest that the relative timing of the IFN-I response and maximal virus replication is key in determining outcomes, at least in infected mice. By extension, IFN-αβ or combination therapy may need to be used cautiously to treat viral infections in clinical settings.

The Discovery of the Antiviral Resistance Gene Mx: A Story of Great Ideas, Great Failures, and Some Success

The discovery of the Mx gene-dependent, innate resistance of mice against influenza virus was a matter of pure chance. Although the subsequent analysis of this antiviral resistance was guided by straightforward logic, it nevertheless led us into many blind alleys and was full of surprising turns and twists. Unexpectedly, this research resulted in the identification of one of the first interferon-stimulated genes and provided a new view of interferon action. It also showed that in many species, MX proteins have activities against a broad range of viruses. To this day, Mx research continues to flourish and to provide insights into the never-ending battle between viruses and their hosts.

Lethal murine infection model for human respiratory disease-associated Pteropine orthoreovirus

Pteropine orthoreovirus (PRV) is an emerging bat-borne human pathogen causing severe respiratory illness. To date, however, the evaluation of PRV virulence has largely depended on the limited numbers of clinical cases owing to the lack of animal models. To develop an in vivo model of PRV infection, an inbred C3H mouse strain was infected intranasally with pathogenic PRV strain Miyazaki-Bali/2007. C3H mice suffered severe lung infection with significant body weight reduction and died within 7 days after intranasal infection. Infectious viruses were isolated mainly from the lungs and trachea. Histopathological examination revealed interstitial pneumonia with monocytes infiltration. Following repeated intranasal infection, mice developed antibodies to particular structural and non-structural proteins of PRV. The results of these immunological assays will help to develop laboratory protocols for sero-epidemiological studies. Our small rodent model of lethal respiratory infection will further allow investigation of the molecular mechanisms underlying the high pathogenicity of PRV.

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A lethal PRV strain Miyazaki-Bali/2007 murine infection model was established.

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Susceptibility of different mouse strains to PRV infection was investigated.

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Antibody responses to PRV proteins in C3H mice post intranasal infection were studied.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

Expression of B and T lymphocyte attenuator (BTLA) in macrophages contributes to the fulminant hepatitis caused by murine hepatitis virus strain-3

OBJECTIVES

Fulminant viral hepatitis (FH) remains a serious clinical problem for which the underlying pathogenesis remains unclear. The B and T lymphocyte attenuator (BTLA) is an immunoglobulin-domain-containing protein that has the capacity to maintain peripheral tolerance and limit immunopathological damage during immune responses. However, its precise role in FH has yet to be investigated.

DESIGN

BTLA-deficient (BTLA-/-) mice and their wild-type littermates were infected with murine hepatitis virus strain-3 (MHV-3), and the levels of tissue damage, cell apoptosis, serum liver enzymes, fibrinogen-like protein 2 (FGL2) and cytokine production were measured and compared. Survival rate was studied after MHV-3 infection with or without adoptive transferring macrophages.

RESULTS

FGL2 production, liver and spleen damage, and mortality were significantly reduced in BTLA-/- mice infected with MHV-3. This effect is due to rapid, TRAIL (TNF-related apoptosis-inducing ligand)-dependent apoptosis of MHV-3-infected macrophages in BTLA-/- mice. The early loss of macrophages resulted in reduced pathogenic tumour necrosis factor α (TNFα) and FGL2 levels and lower viral titres. The importance of TNFα in MHV-3-induced pathology was demonstrated by increased mortality in TNFα-treated MHV-3-infected BTLA-/- mice, whereas TNFα-/- mice were resistant to the infection. Moreover, adoptively transferring macrophages to BTLA-/- mice caused sensitisation, whereas blocking BTLA protected wild-type mice from virus-induced FH mortality.

CONCLUSIONS

BTLA promotes the pathogenesis of virus-induced FH by enhancing macrophage viability and function. Targeting BTLA may be a novel strategy for the treatment of FH.

Mouse Hepatitis Virus Receptor as a Determinant of the Mouse Susceptibility to MHV Infection

In this review, we report that the receptor of mouse hepatitis virus (MHV), carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), is an important determinant of mouse susceptibility to MHV infection. This finding was revealed by using mouse strains with two different allelic forms of the MHV receptor, Ceacam1a and Ceacam1b. Although previous studies indicated that susceptibility is determined by a single gene, Ceacam1, our recent work in gene-replaced mice with chimeric Ceacam1 pointed toward the involvement of other host factors (genes) in the susceptibility. Studies on mouse susceptibility to MHV, as well as the factors involved in their susceptibility, are overviewed.

Role of mouse hepatitis virus (MHV) receptor murine CEACAM1 in the resistance of mice to MHV infection: studies of mice with chimeric mCEACAM1a and mCEACAM1b

Although most inbred mouse strains are highly susceptible to mouse hepatitis virus (MHV) infection, the inbred SJL line of mice is highly resistant to its infection. The principal receptor for MHV is murine CEACAM1 (mCEACAM1). Susceptible strains of mice are homozygous for the 1a allele of mCeacam1, while SJL mice are homozygous for the 1b allele. mCEACAM1a (1a) has a 10- to 100-fold-higher receptor activity than does mCEACAM1b (1b). To explore the hypothesis that MHV susceptibility is due to the different MHV receptor activities of 1a and 1b, we established a chimeric C57BL/6 mouse (cB61ba) in which a part of the N-terminal immunoglobulin (Ig)-like domain of the mCeacam1a (1a) gene, which is responsible for MHV receptor function, is replaced by the corresponding region of mCeacam1b (1b). We compared the MHV susceptibility of these chimeric mice to that of SJL and B6 mice. B6 mice that are homozygous for 1a are highly susceptible to MHV-A59 infection, with a 50% lethal dose (LD(50)) of 10(2.5) PFU, while chimeric cB61ba mice and SJL mice homozygous for 1ba and 1b, respectively, survived following inoculation with 10(5) PFU. Unexpectedly, cB61ba mice were more resistant to MHV-A59 infection than SJL mice as measured by virus replication in target organs, including liver and brain. No infectious virus or viral RNA was detected in the organs of cB61ba mice, while viral RNA and infectious virus were detected in target organs of SJL mice. Furthermore, SJL mice produced antiviral antibodies after MHV-A59 inoculation with 10(5) PFU, but cB61ba mice did not. Thus, cB61ba mice are apparently completely resistant to MHV-A59 infection, while SJL mice permit low levels of MHV-A59 virus replication during self-limited, asymptomatic infection. When expressed on cultured BHK cells, the mCEACAM1b and mCEACAM1ba proteins had similar levels of MHV-A59 receptor activity. These results strongly support the hypothesis that although alleles of mCEACAM1 are the principal determinants of mouse susceptibility to MHV-A59, other as-yet-unidentified murine genes may also play a role in susceptibility to MHV.

Epidemiology of infectious disease: the example of measles

The situation of unacculturated Brazilian Amazon tribes is described. The isolation of these populations has been sufficiently tight that they have been free of most epidemic diseases of the cosmopolitan world, although diseases associated with persistent infection have a high prevalence. The history of measles epidemics in Amerind populations is reviewed and it is concluded that most deaths can be prevented by basic nursing care but that there is a residual excess mortality characteristic of these populations. Three Brazilian virgin-soil populations and one experienced tribe in Chile, the Mapuche, were vaccinated against measles. Elevated febrile responses were observed in the three virgin-soil populations relative to the fevers seen in the Mapuche and in cosmopolitan populations. Nutritional status, immunological experience, humoral immune response and genetic characters have been examined for an explanation of this phenomenon. The most pronounced correspondence detected so far is a high degree of homozygosity in HLA loci of the virgin populations.

Cytokine responses in severe acute respiratory syndrome coronavirus-infected macrophages in vitro: possible relevance to pathogenesis

The pathogenesis of severe acute respiratory syndrome (SARS) remains unclear. Macrophages are key sentinel cells in the respiratory system, and it is therefore relevant to compare the responses of human macrophages to infections with the SARS coronavirus (SARS-CoV) and other respiratory viruses. Primary human monocyte-derived macrophages were infected with SARS-CoV in vitro. Virus replication was monitored by measuring the levels of positive- and negative-strand RNA, by immunofluorescence detection of the SARS-CoV nucleoprotein, and by titration of the infectious virus. The gene expression profiles of macrophages infected with SARS-CoV, human coronavirus 229E, and influenza A (H1N1) virus were compared by using microarrays and real-time quantitative reverse transcriptase PCR. Secreted cytokines were measured with an enzyme-linked immunosorbent assay. SARS-CoV initiated viral gene transcription and protein synthesis in macrophages, but replication was abortive and no infectious virus was produced. In contrast to the case with human coronavirus 229E and influenza A virus, there was little or no induction of beta interferon (IFN-beta) in SARS-CoV-infected macrophages. Furthermore, SARS-CoV induced the expression of chemokines such as CXCL10/IFN-gamma-inducible protein 10 and CCL2/monocyte chemotactic protein 1. The poor induction of IFN-beta, a key component of innate immunity, and the ability of the virus to induce chemokines could explain aspects of the pathogenesis of SARS.

Interferon production by mouse leukocytes in vitro and in vivo

Vaccinia virus penetrates, or is phagocytosed by, mouse leukocytes in vitro. A cytotoxic effect is observed, but no new infectious virus is produced. Vaccinia virus, as infectious particles, is eliminated from a culture of leukocytes at a more rapid rate than can be accounted for by thermal inactivation. Leukocytes infected with vaccinia virus produce a substance with the properties of interferon. The evidence presented suggests that leukocytes also produce interferon in vivo and that this interferon is related to the observed protective effect on the outcome of intracerebral vesicular stomatitis virus challenge in mice. It is postulated that leukocytes, in this manner, may make a positive contribution to the host's defense mechanism and to the process of recovery from viral infections.

The cellular nature of genetic susceptibility to a virus

Using peritoneal macrophage cultures it was found that both PRI mice and their macrophages in culture were susceptible to mouse hepatitis virus and that C(3)H mice and macrophages were resistant. All F(1) macrophages and some back-cross cell cultures were susceptible. The degeneration of F(1) and back-cross macrophages obtained either from adult mouse peritoneal exudate or newborn mouse liver, occurred more slowly than PRI macrophages. Segregation of susceptibility occurred in the first back-cross generation. Tests of three back-cross generations from susceptible mice yielded about one-quarter of the mice shown to be susceptible either by direct test or test of their macrophages. A clear correlation between susceptibility in vivo and in vitro was established both in the test of the percentage segregation and in tests of individual back-cross mice. A small series of tests, however, indicated that 50 per cent of the back-cross mice had the genetic capacity to transmit susceptibility. Thus a hypothesis of two genes for susceptibility, although not excluded, may yield to a hypothesis of a single dominant gene, incompletely expressed. Resistant cells were converted into susceptible cells by ingestion of a relatively large particle containing a heat-stable substance. This susceptibility, although complete, was temporary. The nature of the factor causing the change has been discussed.

Coronavirus MHV-3-induced apoptosis in macrophages

Infection with mouse hepatitis virus strain 3 (MHV-3) results in lethal fulminant hepatic necrosis in fully susceptible BALB/c mice compared to the minimal disease observed in resistant strain A/J mice. Macrophages play a central role in the pathogenesis of MHV-3-induced hepatitis. In the present study we have shown that MHV-3 infection of macrophages induces these cells to undergo apoptosis. Three methods to detect apoptosis were applied: flow cytometry analysis of nuclear DNA content, fluorescence microscopic visualization of apoptotic cells labeled by the TUNEL assay, and gel electrophoresis to detect DNA laddering. Apoptosis in A/J and BALB/c macrophages was first detected at 8 h postinfection (p.i.) and reached a maximum by 12 h p.i. The degree of MHV-3-induced apoptosis was much greater in A/J-derived macrophages than in BALB/c-derived cells. Apoptosis was inversely correlated with the development of typical MHV cytopathology, namely syncytia formation. Infected macrophages from A/J mice did not form synctia in contrast to the extensive synctia formation observed in BALB/c-derived macrophages. In MHV-3-infected BALB/c macrophage cultures, apoptotic cells were not incorporated into syncytia. Apoptosis was also inversely correlated with the expression of MHV-3-induced fgl2 prothrombinase in macrophages. These results add the murine coronavirus MHV-3 to the list of RNA-containing viruses capable of inducing apoptosis.

Mutational analysis of the virus and monoclonal antibody binding sites in MHVR, the cellular receptor of the murine coronavirus mouse hepatitis virus strain A59

The primary cellular receptor for mouse hepatitis virus (MHV), a murine coronavirus, is MHVR (also referred to as Bgp1a or C-CAM), a transmembrane glycoprotein with four immunoglobulin-like domains in the murine biliary glycoprotein (Bgp) subfamily of the carcinoembryonic antigen (CEA) family. Other murine glycoproteins in the Bgp subfamily, including Bgp1b and Bgp2, also can serve as MHV receptors when transfected into MHV-resistant cells. Previous studies have shown that the 108-amino-acid N-terminal domain of MHVR is essential for virus receptor activity and is the binding site for monoclonal antibody (MAb) CC1, an antireceptor MAb that blocks MHV infection in vivo and in vitro. To further elucidate the regions of MHVR required for virus receptor activity and MAb CC1 binding, we constructed chimeras between MHVR and other members of the CEA family and tested them for MHV strain A59 (MHV-A59) receptor activity and MAb CC1 binding activity. In addition, we used site-directed mutagenesis to introduce selected amino acid changes into the N-terminal domains of MHVR and these chimeras and tested the abilities of these mutant glycoproteins to bind MAb CC1 and to function as MHV receptors. Several recombinant glycoproteins exhibited virus receptor activity but did not bind MAb CC1, indicating that the virus and MAb binding sites on the N-terminal domain of MHVR are not identical. Analysis of the recombinant glycoproteins showed that a short region of MHVR, between amino acids 34 and 52, is critical for MHV-A59 receptor activity. Additional regions of the N-terminal variable domain and the constant domains, however, greatly affected receptor activity. Thus, the molecular context in which the amino acids critical for MHV-A59 receptor activity are found profoundly influences the virus receptor activity of the glycoprotein.

Mouse susceptibility to mouse hepatitis virus infection is linked to viral receptor genotype

We have reported that the receptor for mouse hepatitis virus (MHV) expressed in MHV-susceptible BALB/c mice (MHVR1) has 10 to 30 times the virus-binding activity of the MHV receptor expressed in MHV-resistant SJL mice (MHVR2) (N. Ohtsuka, Y. K. Yamada, and F. Taguchi, J. Gen. Virol. 77:1683-1992, 1996). This fact indicates the possibility that the difference in MHV susceptibility between BALB/c and SJL mice is determined by the virus-binding activity of the receptor. To test this possibility, we have examined MHV susceptibility in mice with the homozygous MHVR1 gene (R1/R1 genotype), mice with the MHVR1 and MHVR2 genes (R1/R2 genotype), and mice with the homozygous MHVR2 gene (R2/R2 genotype) produced by cross and backcross mating between BALB/c and SJL mice. All 63 F2 and backcrossed mice with the MHVR1 gene (R1/R1 and R1/R2) were susceptible to MHV infection, and all 57 with the homozygous MHVR2 gene (R2/R2) were resistant. We have also examined the MHV receptor genotypes of several mouse strains that were reported to be susceptible to MHV infection. All of those mice had the MHVR1 gene. These results suggest the possibility that the viral receptor determines the susceptibility of the whole animal to MHV infection.

Age-related changes in susceptibility of mice to low-virulent mouse hepatitis virus (MHV-2-CC) infection

This study was performed to examine mouse age-dependent changes in susceptibility to MHV-2-CC-infection and participation of macrophages in such changes in BALB/c mice. One-week-old mice were fully susceptible (mortality, 100%), 2-week-old semi-susceptible (36%), and 3- and 4-week-old fully resistant (0%) to MHV-2-CC, respectively. Such age-dependent differences corresponded well with the differences in the virus titers in the liver, spleen and blood and in the severity of liver lesions. In 1-week-old mice with peritoneal exudate cells (PEC) transferred from 4-week-old mice and infected with MHV-2-CC, a slight prolongation of survival time was recorded, although there was no difference in mortality. In 3-week-old mice infected with MHV-2-CC after silica-treatment to suppress macrophages, there was no significant change in susceptibility. In macrophages infected with MHV-2-CC in vitro, the virus replicated better in macrophages obtained from younger mice. These results suggest that macrophages may play a small role in the age-related development of resistance to MHV-2-CC infection in BALB/c mice.

How viruses may overcome non-specific defences in the host

The pathogenicity of a virus is mainly related to its capacity to overcome the non-specific defences of a host. Fever limits the multiplication of viruses in vivo , thus allowing recovery; thermoresistant strains are virulent because they are capable of escaping the effect of hyperthermia. Resistance to the antiviral effect of interferon does not seem to exist per se but interferon may play an indirect role in virus virulence: (i) its effect may be minimized by the destruction of interferon-producing cells such as macrophages; (ii) interferon may render virus-infected target cells resistant to natural killer cell lysis. The interaction of viruses with phagocytic cells plays a predominant role; the ability to grow in macrophages or to induce macrophage blockade or paralysis may constitute a marker of virulence. This is particularly important in the liver in which the infection of Kupffer cells often represents the key event of the disease.

THE PATHOGENESIS OF HERPES VIRUS ENCEPHALITIS. II. A CELLULAR BASIS FOR THE DEVELOPMENT OF RESISTANCE WITH AGE

The resistance to herpes virus encephalitis which develops with age was studied in mice using fluorescent antibody staining. Adult mice remained susceptible to intracerebral inoculation, and the infection of the central nervous system was identical with that found in immature mice. A "barrier" to the spread of virus inoculated extraneurally developed with maturation, and the limitation of spread appeared to coincide with the infection of peritoneal and tissue macrophages. In vitro, suckling and adult mouse macrophages were infected with equal ease. However, suckling mouse macrophages infected other cells in contact with them, while infected adult mouse macrophages did not. Studies failed to reveal the nature of this change in macrophages which developed with age. The role of macrophages in the pathogenesis of herpes virus encephalitis is discussed. The hypothesis is made that an alteration in the macrophages of the maturing mouse plays an important role in its development of resistance to herpes virus encephalitis.

LEUKOCYTES AND INTERFERON IN THE HOST RESPONSE TO VIRAL INFECTIONS. I. MOUSE LEUKOCYTES AND LEUKOCYTE-PRODUCED INTERFERON IN VACCINIA VIRUS INFECTION IN VITRO

1. Investigation of the role of leukocytes in vaccinia virus infection is reported in an in vitro model, in the absence of an immune response. 2. Mouse leukocytes were shown to be capable of inhibiting the progression of vaccinia virus infection in primary mouse embryo fibroblast cultures. The degree of protection varied from slowing of spread of infection to complete control of the infection with eventual elimination of detectable virus and recovery of the culture. 3. Interferon production by leukocytes is thought to be an important factor in the observed protective effect.

Cell proteins bind specifically to West Nile virus minus-strand 3' stem-loop RNA

The first 96 nucleotides of the 5'noncoding region (NCR) of West Nile virus (WNV) genomic RNA were previously reported to form thermodynamically predicted stem-loop (SL) structures that are conserved among flaviviruses. The complementary minus-strand 3' NCR RNA, which is thought to function as a promoter for the synthesis of plus-strand RNA, forms a corresponding predicted SL structure. RNase probing of the WNV 3' minus-strand stem-loop RNA [WNV (-)3' SL RNA] confirmed the existence of a terminal secondary structure. RNA-protein binding studies were performed with BHK S100 cytoplasmic extracts and in vitro-synthesized WNV (-)3' SL RNA as the probe. Three RNA-protein complexes (complexes 1,2, and 3) were detected by a gel mobility shift assay, and the specificity of the RNA-protein interactions was confirmed by gel mobility shift and UV-induced cross-linking competition assays. Four BHK cell proteins with molecular masses of 108, 60, 50, and 42 kDa were detected by UV-induced cross-linking to the WNV (-)3' SL RNA. A preliminary mapping study indicated that all four proteins bound to the first 75 nucleotides of the WNV 3' minus-strand RNA, the region that contains the terminal SL. A flavivirus resistance phenotype was previously shown to be inherited in mice as a single, autosomal dominant allele. The efficiencies of infection of resistant cells and susceptible cells are similar, but resistant cells (C3H/RV) produce less genomic RNA than congenic, susceptible cells (C3H/He). Three RNA-protein complexes and four UV-induced cross-linked cell proteins with mobilities identical to those detected in BHK cell extracts with the WNV (-)3' SL RNA were found in both C3H/RV and C3H/He cell extracts. However, the half-life of the C3H/RV complex 1 was three times longer than that of the C3H/He complex 1. It is possible that the increased binding activity of one of the resistant cell proteins for the flavivirus minus-strand RNA could result in a reduced synthesis of plus-strand RNA as observed with the flavivirus resistance phenotype.

Thymus involution induced by mouse hepatitis virus A59 in BALB/c mice

Mouse hepatitis virus A59 (MHV-A59) infection of adult BALB/c mice induced a severe, transient atrophy of the thymus. The effect was maximal at 1 week after infection, and thymuses returned to normal size by 2 weeks after infection. There was no effect of glucocorticoids, since thymus atrophy was also found in adrenalectomized, infected mice. In infected thymus, immature CD4+ CD8+ lymphocytes were selectively depleted, and apoptosis of lymphocytes was increased. The MHV receptor glycoprotein MHVR was detected on thymus epithelial cells but not on T lymphocytes. In a small number of stromal epithelial cells, but in very few lymphocytes, the viral genome was detectable by in situ hybridization. These observations suggested that MHV-A59-induced thymic atrophy results not from a generalized lytic infection of T lymphocytes but rather from apoptosis of immature double-positive T cells that might be caused by infection of a small proportion of thymus epithelial cells or from inappropriate secretion of some factor, such as a cytokine.

Pattern of disease after murine hepatitis virus strain 3 infection correlates with macrophage activation and not viral replication

Murine hepatitis virus strain (MHV-3) produces a strain-dependent pattern of disease which has been used as a model for fulminant viral hepatitis. This study was undertaken to examine whether there was a correlation between macrophage activation and susceptibility or resistance to MHV-3 infection. Peritoneal macrophages were isolated from resistant A/J and susceptible BALB/cJ mice and, following stimulation with MHV-3 or lipopolysaccharide (LPS), analyzed for transcription of mRNA and production of interleukin-1 (IL-1), tumor necrosis factor alpha (TNF-alpha), transforming growth factor beta (TGF-beta), mouse fibrinogen-like protein (musfiblp), tissue factor (TF), leukotriene B4, and prostaglandin E2 (PGE2). Macrophages from BALB/cJ mice produced greater amounts of IL-1, TNF-alpha, TGF-beta, leukotriene B4, and musfiblp following MHV-3 infection than macrophages from resistant A/J mice, whereas in response to LPS, equivalent amounts of IL-1, TNF-alpha, TGF-beta, and TF were produced by macrophages from both strains of mice. Levels of mRNA of IL-1, TNF-alpha, and musfiblp were greater and more persistent in BALB/cJ than in A/J macrophages, whereas the levels and kinetics of IL-1, TNF-alpha, and TF mRNA following LPS stimulation were identical in macrophages from both strains of mice. Levels of production of PGE2 by MHV-3-stimulated macrophages from resistant and susceptible mice were equivalent; however, the time course for induction of PGE2, differed, but the total quantity of PGE2 produced was insufficient to inhibit induction of musfiblp, a procoagulant known to correlate with development of fulminant hepatic necrosis in susceptible mice. These results demonstrate marked differences in production of inflammatory mediators to MHV-3 infection in macrophages from resistant A/J and susceptible BALB/cJ mice, which may explain the marked hepatic necrosis and fibrin deposition and account for the lethality of MHV-3 in susceptible mice.

Low levels of poliovirus replication in primary human monocytes: possible interactions with lymphocytes

To investigate the molecular mediators of poliovirus tissue tropism, the correlation between poliovirus replication and poliovirus receptor expression was examined in a primary human tissue system. Earlier work [M. Freistadt, H. Fleit, and E. Wimmer, Virology 195: 798-803 (1993)] showed that the cellular receptor for poliovirus is present in 87% of primary human monocytes and that peripheral blood mononuclear cells support poliovirus replication. In the current work, monocytes, obtained by adherence or by a novel negative selection procedure using specific monoclonal antibodies to lymphocyte surface antigens, supported poliovirus replication. However, total virus yield was low and infectious centers assays revealed that a minority (6%) of monocytes become productively infected. Viral yield from monocytes was lower than from the heterogeneous mononuclear cells; however, when uninfected lymphocytes were added back to infected monocytes, the higher viral yield was restored. The purity of the cells did not significantly affect the number of cells infected. These results suggest that more poliovirus is produced per cell from activated rather than unactivated monocytes. Furthermore, poliovirus replication in monocytes may reflect genuine in vivo replication and comprise a system in which to determine molecular mediators of poliovirus tissue tropism.

Treatment of resistant A/J mice with methylprednisolone (MP) results in loss of resistance to murine hepatitis strain 3 (MHV-3) and induction of macrophage procoagulant activity (PCA)

BALB/cJ mice die of fulminant hepatitis within 7 days of exposure to murine hepatitis virus strain 3 (MHV-3) whereas A/J mice are fully resistant to the lethal effects of MHV-3 infection. Previous studies have implicated macrophage activation with production of a unique macrophage prothrombinase (PCA) and lymphocyte cytokine secretion in the pathogenesis of MHV-3 susceptibility and have demonstrated that immunosuppression induces susceptibility in resistant mice. This study was undertaken to determine whether macrophages, derived from resistant A/J mice and treated in vitro with methylprednisolone sodium succinate (MP), elaborated PCA following MHV-3 exposure and whether therapy with MP altered resistance of A/J mice to MHV-3 infection in vivo. Macrophages, incubated with MP in vitro, expressed dose dependent increases in PCA following infection with MHV-3. No induction of PCA occurred in macrophages treated with MHV-3 or MP alone. Analysis of mRNA transcripts for mouse fibrinogen like protein (musfiblp), the MHV-3 specific prothrombinase, in macrophages which were incubated with MP prior to exposure to MHV-3 demonstrated significantly increased mRNA levels as compared to macrophages not incubated with MP prior to MHV-3 exposure. In vivo, A/J mice treated for 3 days with 500 mg/kg/day of MP prior to infection with MHV-3 demonstrated extensive hepatocyte necrosis and fibrin deposition in hepatic sinusoids on histological examination of liver tissue, elevated serum transaminases and 100% mortality within 10 days of infection. These results therefore provide further support for the role of increased PCA in the pathogenesis of MHV-3 related liver necrosis.

B lymphocyte and macrophage expression of carcinoembryonic antigen-related adhesion molecules that serve as receptors for murine coronavirus

The expression of carcinoembryonic antigen (CEA)-related glycoproteins that have been associated with intercellular adhesion and that serve as receptors for mouse hepatitis virus (MHV) was analyzed in cells from the immune system of BALB/c mice using immunolabeling and RNA polymerase chain reaction amplification of receptor transcripts. These glycoproteins, which are called biliary glycoproteins, were highly expressed in B lymphocytes, including cells of the B-1a (CD5+) lineage, and in macrophages, but were not detectable in resting T lymphocytes. Similarly, murine cell lines of B cell and macrophage origin expressed messenger RNA encoding CEA-related molecules, while the corresponding mRNA was only slightly detectable in a T cell line. These CEA-related cell adhesion glycoproteins were also expressed in endothelial cells. Therefore, their specific interaction with their so far unknown ligand may be of functional importance in cellular interactions in the immune response. Monoclonal antibody directed against these glycoproteins blocked MHV-A59 infection of the B cell-derived SP20 cell line. Thus, the functional receptors for MHV on B lymphocytes, like those on murine fibroblasts, are isoforms of CEA-related glycoproteins. Treatment of B cells with anti-receptor antibody also blocked B cell-mediated cytotoxicity against MHV-A59-infected fibroblasts, indicating that this phenomenon is mediated by interaction of viral attachment protein on the infected target cells with specific CEA-related receptor glycoproteins on the effector B cells.

Genetic control of resistance to Mycoplasma pulmonis infection in mice

The differences in susceptibility of various inbred strains of mice to a highly pathogenic strain of Mycoplasma pulmonis CT (T2) has been known for some time. We assessed the genetic control of resistance to T2 infection. Tracheolung lavage samples and lungs of mice were assessed for T2 organisms after intratracheal injection of T2. We found that H-2b (C57BL/6 (B6) and H-2k B10.BR mice were resistant, whereas H-2b A.By, H-2k C3H/Bi, H-2k C3H/HeJ (C3H), and H-2b BALB.B mice were susceptible. We also typed individual B6C3F2 mice for H-2 and for resistance to T2 and observed that resistance to T2 infections is controlled by a single dominant gene not linked to H-2. Histologic examination revealed severe lung lesions typical of M. pulmonis infections in susceptible C3H mice, in contrast to minimal lung lesions in resistant B6 mice. No significant titers of local or systemic antimycoplasma antibodies were detected in either resistant or susceptible mice at 5 days postinfection. Macrophages taken from uninfected B6 or C3H mice failed to inhibit growth of T2 in vitro. However, macrophages from B6 mice did inhibit growth of T2 much better than C3H macrophages when harvested on day 5 of infection. Thus, there is an association between activation of macrophage bactericidal function and genetic resistance to growth of T2 organisms.

Virus specificity of the antiviral state induced by IFN gamma correlates with resistance to MHV 3 infection

A comparative study was carried out to investigate the correlation between the antiviral effect induced in macrophages by IFN gamma and the resistance of A/J and BALB/c mice to an experimental infection of MHV 3, MHV 4, and MHVA 59. Both mouse strains were resistant to intraperitoneal infection with MHV 4 or MHVA 59 and only the A/J mice showed resistance to MHV3, the BALB/c mice being fully susceptible to this virus infection. Comparable growth kinetics, for all three viruses, were observed in both mouse strains, except for the MHV3 growth in BALB/c mice, where the virus titre increased to a peak on day 2, remaining high until day 4 when the mice died of acute hepatitis. The IFN gamma titres in the peritoneum of mice preceded and correlated with the virus growth, higher titres being found in MHV 3 infected BALB/c mice. The highest titre was always observed 24 to 48 h after infection. Among viral strains grown in cultured macrophages, higher titres were always observed in cultures infected with MHVA 59, followed by MHV 3 and the lowest those infected with MHV 4. The macrophage activation by IFN gamma-induced a partial restriction of virus growth only in MHV 3 infected A/J mouse macrophages. A virus specificity of the IFN gamma-induced antiviral state was shown to be in direct correlation with the resistance of mice to MHV 3 infection.

Coronavirus species specificity: murine coronavirus binds to a mouse-specific epitope on its carcinoembryonic antigen-related receptor glycoprotein

Like most coronaviruses, the coronavirus mouse hepatitis virus (MHV) exhibits strong species specificity, causing natural infection only in mice. MHV-A59 virions use as a receptor a 110- to 120-kDa glycoprotein (MHVR) in the carcinoembryonic antigen (CEA) family of glycoproteins (G. S. Dveksler, M. N. Pensiero, C. B. Cardellichio, R. K. Williams, G. S. Jiang, K. V. Holmes, and C. W. Dieffenbach, J. Virol. 65:6881-6891, 1991; and R. K. Williams, G. S. Jiang, and K. V. Holmes, Proc. Natl. Acad. Sci. USA 88:5533-5536, 1991). The role of virus-receptor interactions in determining the species specificity of MHV-A59 was examined by comparing the binding of virus and antireceptor antibodies to cell lines and intestinal brush border membranes (BBM) from many species. Polyclonal antireceptor antiserum (anti-MHVR) raised by immunization of SJL/J mice with BALB/c BBM recognized MHVR specifically in immunoblots of BALB/c BBM but not in BBM from adult SJL/J mice that are resistant to infection with MHV-A59, indicating a major difference in epitopes between MHVR and its SJL/J homolog which does not bind MHV (7). Anti-MHVR bound to plasma membranes of MHV-susceptible murine cell lines but not to membranes of human, cat, dog, monkey, or hamster cell lines. Cell lines from these species were resistant to MHV-A59 infection, and only the murine cell lines tested were susceptible. Pretreatment of murine fibroblasts with anti-MHVR prevented binding of radiolabeled virions to murine cells and prevented virus infection. Solid-phase virus-binding assays and virus overlay protein blot assays showed that MHV-A59 virions bound to MHVR on intestinal BBM from MHV-susceptible mouse strains but not to proteins on intestinal BBM from humans, cats, dogs, pigs, cows, rabbits, rats, cotton rats, or chickens. In immunoblots of BBM from these species, both polyclonal and monoclonal antireceptor antibodies that block MHV-A59 infection of murine cells recognized only the murine CEA-related glycoprotein and not homologous CEA-related glycoproteins of other species. These results suggest that MHV-A59 binds to a mouse-specific epitope of MHVR, and they support the hypothesis that the species specificity of MHV-A59 infection may be due to the specificity of the virus-receptor interaction.

The pattern of proteins synthesized in the liver is profoundly modified upon infection of susceptible mice with mouse hepatitis virus 3

Susceptible BALB/c mice, after experimental infection with mouse hepatitis virus 3 (MHV3), revealed virus titres in the liver that increased gradually to a peak of 8 x 10(5) PFU/g of tissue after 3 days' infection, when the mice died of acute hepatitis. BALB/c mice were infected with MHV3, subsequently labelled in vivo with 35S-methionine, and then the liver preparations from both infected and non-infected animals were subjected to two-dimensional gel electrophoresis. Comparisons of the patterns by computer image analysis revealed 17 gene products which increased, and 8 gene products which decreased, upon virus infection in their two-dimensional gel spot intensity. We conclude that during MHV3 infection of a susceptible strain of mice, a major modification in protein synthesis occurs. The pattern alterations were not related to the virus gene products but were mostly endogenous mouse proteins. Whether these proteins are a result of a defence attempt by the animal, or are dictated by the virus in order to prevent a protective response from happening, remains to be shown.

Acute and late disease induced by murine coronavirus, strain JHM, in a series of recombinant inbred strains between BALB/cHeA and STS/A mice

To examine the genetic control of acute and late disease induced by a murine coronavirus, strain JHM (JHMV), BALB/cHeA, STS/A, F1 hybrids and 13 recombinant inbred (RI) strains between BALB/cHeA and STS/A mouse strains were inoculated intracerebrally with 100 pfu of JHMV. All the BALB/cHeA mice died within 2 weeks from acute encephalitis. In contrast, STS/A mice were shown to be partially resistant, with a mortality rate of 30%, longer survival times and lower rates of viral production. The mortality rates, survival times and viral titers of F1 hybrids and the RI strains varied, suggesting involvement of multiple genes. STS/A, F1 hybrid and RI mice surviving the acute infection occasionally developed severe paraparesis about 1 month post-infection. In these mice, vacuolar degeneration, astrocytosis, the absence of perivascular cuffing and minimal demyelination were found in the central nervous system. No infectious virus could be recovered from these mice. Although the paralysis of delayed onset was limited to STS/A, F1 hybrid and eight of the 13 RI strains, the incidence varied significantly among the RI strains. These results may suggest that JHMV-induced late disease is also under multifactorial control. The pathogenesis of JHMV infection is discussed.

Cloning of the mouse hepatitis virus (MHV) receptor: expression in human and hamster cell lines confers susceptibility to MHV

The cellular receptor for murine coronavirus mouse hepatitis virus (MHV)-A59 is a member of the carcinoembryonic antigen (CEA) family of glycoproteins in the immunoglobulin superfamily. We isolated a cDNA clone (MHVR1) encoding the MHV receptor. The sequence of this clone predicts a 424-amino-acid glycoprotein with four immunoglobulinlike domains, a transmembrane domain, and a short intracytoplasmic tail, MHVR1 is closely related to the murine CEA-related clone mmCGM1 (Mus musculus carcinoembryonic antigen gene family member). Western blot (immunoblot) analysis performed with antireceptor antibodies detected a glycoprotein of 120 kDa in BHK cells stably transfected with MHVR1. This corresponds to the size of the MHV receptor expressed in mouse intestine and liver. Human and hamster fibroblasts transfected with MHVR1 became susceptible to infection with MHV-A59. Like MHV-susceptible mouse fibroblasts, the MHVR1-transfected human and hamster cells were protected from MHV infection by pretreatment with monoclonal antireceptor antibody CC1. Thus, the 110- to 120-kDa CEA-related glycoprotein encoded by MHVR1 is a functional receptor for murine coronavirus MHV-A59.

Hygromycin B therapy of a murine coronaviral hepatitis

Hepatitis caused by mouse hepatitis virus (MHV-A59), a murine coronavirus, is accompanied by direct infection and replication of virus within the liver. We demonstrate here that the aminoglycoside hygromycin B is able to eliminate MHV-A59 infection from mouse peritoneal macrophages and cultured liver cells in vitro and is also able to reduce levels of virus replication and necrotic liver foci in vivo.