AN ELECTRON MICROSCOPE STUDY OF THE DEVELOPMENT OF A MOUSE HEPATITIS VIRUS IN TISSUE CULTURE CELLS

Samples taken at different intervals of time from suspension cultures of the NCTC 1469 line of mouse liver—derived (ML) cells infected with a mouse hepatitis virus have been studied with the electron microscope. The experiments revealed that the viruses are incorporated into the cells by viropexis within 1 hour after being added to the culture. An increasing number of particles are found later inside dense cytoplasmic corpuscles similar to lysosomes. In the cytoplasm of the cells from the samples taken 7 hours after inoculation, two organized structures generally associated and never seen in the controls are observed: one consists of dense material arranged in a reticular disposition (reticular inclusion); the other is formed by small tubules organized in a complex pattern (tubular body). No evidence has been found concerning their origin. Their significance is discussed. With the progression of the infection a system of membrane-bounded tubules and cisternae is differentiated in the cytoplasm of the ML cells. In the lumen of these tubules or cisternae, which are occupied by a dense material, numerous virus particles are observed. The virus particles which originate in association with the limiting membranes of tubules and cisternae are released into their lumen by a "budding" process. The virus particles are 75 mµ in diameter and possess a nucleoid constituted of dense particles or rods limiting an electron transparent core. The virus limiting membrane is sometimes covered by an outer layer of a dense material. In the cells from the samples taken 14 to 20 hours after inoculation, larger zones of the cell cytoplasm are occupied by inclusion bodies formed by channels or cisternae with their lumens containing numerous virus particles. In the samples taken 20 hours or more after the inoculation numerous cells show evident signs of degeneration.

Cytotoxic T cell-resistant variants are selected in a virus-induced demyelinating disease

C57BI/6 mice infected with mouse hepatitis virus, strain JHM (MHV-JHM) develop a chronic demyelinating encephalomyelitis. Infectious virus can be isolated only from symptomatic mice. In C57BI/6 mice, two CD8+ T cell epitopes within the MHV-JHM surface glycoprotein were previously identified. Here, we show that mutations in the RNA encoding the immunodominant of the epitopes are present in nearly all virus samples isolated from these mice. Mutations are not present in sequences flanking this epitope or in other CD8+ or CD4+ T cell epitopes. Furthermore, analysis of five peptides corresponding to variant epitopes in direct ex vivo cytotoxicity assays showed that each mutation caused a loss of epitope recognition. These results suggest that escape from CD8+ T cell recognition is necessary for enhanced virus replication and development of clinical disease in these MHV-JHM-infected mice.

Effect of adoptive transfer of CD4, CD8 and B cells on recovery from MHV3-induced immunodeficiencies

A chronic viral infection can occur when the host fails to mount an effective immune response to clear the virus. Mouse hepatitis virus type 3 (MHV3) appears to be an excellent model for the study of the relationship between viral-induced immunodeficiency and chronic disease development. (C57BL/6 x A/J)F1 mice surviving acute hepatitis develop a chronic disease characterized by T- and B-cell immunodeficiencies, viral persistence in various organs including the brain, spleen and thymus, and death within 3 months postinfection (p.i.). We have reported that T- or B-cell deficiencies, observed in MHV3 chronically infected (C57BL/6 x A/J)F1 mice, can be partially or totally thwarted by adoptive transfer of CD4+, CD8+ and/or B cells, at 15 days p.i. in mice surviving the acute phase of the disease. Adoptive transfer of syngeneic CD4+ and/or CD8+ allowed a partial restoration of the T-cell deficiencies, as characterized by thymic atrophy, decrease in splenic T cells, and in all thymocyte subpopulations. B-cell immunodeficiency, as defined by a decrease in splenic B cells, as well as in the bone marrow pre-B- and B-cell compartments, and the occurrence of abnormally larger forms of bone marrow pre-B and B cells, were partially thwarted by B-cell treatment only. Splenic B cells and the bone marrow B-cell compartment, respectively, returned partially or totally to normal values, whereas the pre-B-cell compartment remained depleted in infected mice treated with B cells. Levels of all immunoglobulin classes returned to normal values in MHV3 chronically infected mice when treated with CD4+ in combination with CD8+ cells. All T- and/or B-cell treatments, however, were sufficient to thwart the process of the chronic disease, and favoured the survival of mice for up to 6 months p.i.

Generation of antiviral CD11ahigh T cells in CD4+ T cell-depleted mice and adult thymectomized mice after mouse hepatitis virus infection

The mechanism of CD11ahighCD8+ T cell induction after mouse hepatitis virus infection, which has been suggested to play a key role in the elimination of infectious virus from the spleen in C57BL/6 mice, was studied. In CD4+ T cell-depleted mice, CD11ahighCD8+ T cells were induced in the spleen and spleen cells showed virus-specific cytotoxic T lymphocyte activity after mouse hepatitis virus infection. The same results were obtained in adult thymectomized mice. These results indicate that CD11ahighCD8+ T cells can be generated after mouse hepatitis virus infection in the absence of either CD4+ T cells or the thymus.

Hepatic pathology resulting from mouse hepatitis virus S infection in severe combined immunodeficiency mice

Mouse hepatitis virus (MHV) is a pervasive pathogen that causes morbidity and mortality in mouse colonies worldwide. Although it is not a major cause of mortality in immunocompetent mice, infections from MHV strains of lower virulence can be fatal to athymic nude mice. The histopathologic features and alterations of serum biochemical parameters resulting from infection with a low-virulence MHV strain in severe combined immunodeficiency (scid) mice has not been well described. Thus we recently studied the disease caused by MHV-S in scid mice after intranasal inoculation. Mouse hepatitis virus infection in scid mice, which have severe defects of B and T cells, may be highly lethal, resulting in immediate mortality. However, our results indicate that scid mice survived for an average of 12 to 14 days after infection with doses of MHV up to 10(7) PFU/mouse. The virus caused a significant increase in serum enzyme activities and bilirubin concentration associated with histologically demonstrable hepatocellular injury at postinoculation days 3, 4, and 8. Furthermore, virus was detected in mouse liver homogenates and nasal and bronchial lavage specimens. These results provide valuable information regarding the histopathologic and biochemical consequences of MHV-S infection in scid mice.

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.

Mouse hepatitis virus type 3 infection provokes a decrease in the number of sinusoidal endothelial cell fenestrae both in vivo and in vitro

Fenestrations of hepatic endothelial cells play an active role as a sieving barrier allowing extensive exchange between the blood and liver parenchyma. Alteration of these structures may be induced in the course of various pathological events and provoke important perturbations of liver function. We demonstrate here that sinusoidal endothelial cells are permissive for mouse hepatitis virus 3 (MHV3) in vivo and in vitro and that this infection leads to a striking decrease in the number of fenestrae. The disappearance of these structures observed under scanning electron microscopy or in cryofracture preparations in vivo and in vitro cannot be reversed by the action of cytochalasin B on the microfilament network. The decrease in the porosity seems to be related directly to the productive infection of the endothelial cells, because it was not observed in A/J mice resistant to the virus and in susceptible BALB/c mice immunized with a thermosensitive mutant in which no viral replication occurs. In conclusion, a viral infection of liver endothelial cells may cause extensive loss of the fenestrations and thus lead to important functional pertubations.

Interleukin-12 gene expression after viral infection in the mouse

Interleukin-12 is a lymphokine that triggers gamma interferon secretion by various cells and differentiation of T-helper lymphocytes towards the Th1 subtype. Since viruses are potent inducers of gamma interferon production and elicit immune responses most probably mediated by Th1 cells, like B-cell immunoglobulin G2a secretion, we analyzed interleukin-12 message expression after infection of mice with lactate dehydrogenase-elevating virus, mouse hepatitis virus, and mouse adenovirus. Our results indicated that the message for the p40 component of interleukin-12 was transiently increased shortly after infection. Interleukin-12 was expressed mainly by macrophages. Therefore, production of interleukin-12 might constitute the initial event that would determine the subsequent characteristics of the immune response elicited by viral infections.

Demyelination induced by murine coronavirus JHM infection of congenitally immunodeficient mice

Mouse hepatitis virus JHM (JHMV or MHV-4) induces demyelination in rodents and has been studied as a model for the human disease, multiple sclerosis (MS). As is proposed in MS, the mechanism of subacute demyelination induced by JHMV appears to be primarily immunopathological, since demyelination in JHMV-infected mice is abrogated by immunosuppressive doses of irradiation and restored by adoptive transfer of splenocytes. Thy-1+ cells play a critical role in transmitting disease to these recipient mice. To further characterize cells which may mediate JHMV-induced immunopathology, we inoculated congenitally immunodeficient mice with JHMV. By 12 days post-inoculation, both immunocompetent C57BL/6J controls and athymic nude C57BL/6 mice had severe paralysis and demyelination. In marked contrast, C57BL/6 mice with the severe combined immune deficiency (SCID) mutation had little or no paralysis or demyelination. Adoptive transfer of immune spleen cells from nude mice to infected SCID mice produced paralysis and demyelination. These findings suggest that a cell population present in immunocompetent C57BL/6J and nude mice but absent or non-functional in irradiated and SCID mice is essential for JHMV-induced demyelination. Identification of cells which mediate demyelination in this experimental system may have implications for our understanding of coronavirus pathogenesis and human demyelinating diseases.

Impairment of bone marrow pre-B and B cells in MHV3 chronically-infected mice

Mouse hepatitis virus type 3 (MHV3) appears to be an excellent model for the study of the relationship between viral-induced immunodeficiency and the development of chronic disease. Animal surviving acute hepatitis develop a chronic disease characterized by viral persistency in various organs, by a humoral immunodeficiency, and eventually die within the next three months postinfection. To verify if B cell immunodeficiency occurs during the chronic disease, percentage and absolute number of bone marrow B lineage cell subpopulations were recorded at various times postinfection (p.i.) in pathogenic L2-MHV3-infected (C57BL/6 x A/J) F1 mice. Absolute numbers of B (cmu+smu+) cells decreased as early as three days p.i. up to 15 days p.i., and then gradually returned toward normal values in L2-MHV3-infected mice during the chronic disease. In contrast, pre-B (cmu+smu-) cells were less significantly decrease during the chronic disease. In addition, abnormally enlarged cells (> 13 microns) were detected either in bone marrow pre-B or B cells from L2-MHV3-infected mice.

Effect of persistent mouse hepatitis virus infection on MHC class I expression in murine astrocytes

Neurotropic strains of mouse hepatitis virus (MHV) have been used extensively for the study of viral pathogenesis in the central nervous system (CNS), serving as models for human neurological diseases such as multiple sclerosis (MS). MHV strains A59 and JHMV both cause acute and chronic encephalomyelitis and demyelination in susceptible strains of mice and rats. In acute disease, CNS damage is most likely the result of lytic infection in neurons and oligodendrocytes, and death can be prevented by the adoptive transfer of Class I-restricted CD8+ T cells. However, in later stages of the disease induced by some MHV strains, virus tends to be restricted to astrocytes in a nonlytic infection, and the immune response appears to contribute to CNS damage. These data lead us to suggest that the astrocyte may play a central role in the neuropathogenesis of MHV infection. Consistent with this possibility, A59 has been reported to induce the expression of Class I molecules of the major histocompatibility complex (MHC) in glial cells following infection in vivo and in vitro. In this communication, we have examined the influence of persistent infection by both A59 and JHMV on MHC Class I expression in primary murine astrocytes. Persistence was characterized by the presence of intracellular viral antigen and mRNA in the absence of detectable infectious virus particles. Under these conditions, JHMV, but not A59, inhibited constitutive expression of the H-2 Kb molecule, with the magnitude of inhibition increasing with postinfection time. A59 was not able to induce Class I during persistence, presumably due to the lack of infectious virus particles. Class I expression was restored by the addition of gamma-interferon (IFN-gamma) to astrocytes persistently infected with either A59 or JHMV. Thus, Class I inhibition is not a permanent consequence of JHMV persistence, and persistence does not interfere with normal signalling pathways for Class I induction.

Transcription and translation of proinflammatory cytokines following JHMV infection

Infection with JHMV results in the transcriptional activation of two host cell genes encoding proinflammatory cytokines, tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 beta. Analysis of irradiated mice showed that IL-1 beta mRNA accumulation in the central nervous system was predominantly derived from the mononuclear infiltrate. By contrast, accumulation of TNF-alpha mRNA was unaffected by immunosuppression, suggesting that resident cells were the source of this cytokine. Infected mice were treated with anti-TNF antibody to determine if TNF-alpha contributed to either the encephalomyelitis or demyelination associated with JHMV infection. Surprisingly, neither the cellular infiltrate nor demyelination were affected. In vitro analysis showed that IL-1 beta but not TNF was secreted from JHMV infected macrophages. The absence of TNF secretion is due to a block in translation of the TNF mRNA which accumulates during infection.

Antiviral activity of interferon gamma in vivo during mouse hepatitis virus infection

A/J mice became resistant to experimental MHV3 infection after immunization with UV-inactivated MHV3 (0% mortality, 0/10). Depletion of interferon (IFN) gamma-producing CD4+ T lymphocytes with monoclonal antibodies to CD4+ led to susceptibility to virus infection (60% of mortality, 6/10). The resistance to MHV3 infection of CD4+ T lymphocyte-depleted-A/J mice was restored by treatment with 1000 U of IFN gamma on days -1, 0, 1, 2, 3 and 4 (10% of mortality, 1/10). The low virus titers observed in resistant mice (controls or CD4+ depleted plus IFN gamma treated) were cleared 6 days after infection and the virus titers observed among susceptible mice (CD4+ depleted) increased gradually and peaked on day 6, when the animals died. Previous data, taken together with the direct evidence presented in this paper, provide strong evidence supporting the concept of an in vivo antiviral role of IFN gamma through a central action on the mechanisms of resistance to MHV3 infection.

Methods for studying antiviral functions of macrophages and mononuclear phagocytes

Methods to check the antiviral activities of mononuclear phagocytes or macrophages are described. Two types of antiviral activities are defined. The intrinsic antiviral activity is determined as the outcome of virus replication in the macrophage per se whereas the extrinsic antiviral activity refers to the ability to reduce virus production in other surrounding cells that are normally permissive. The interpretation of the data are discussed.

Microcytic anemia and changes in ferrokinetics as late after-effects of glucan administration in murine hepatitis virus-infected C57BL/10ScSnPh mice

Mild microcytic anemia (without changes in mean corpuscular hemoglobin concentration, MCHC) was discovered 6-14 weeks after a single s.c. administration of 4 mg of particulate glucan to C57BL/10ScSnPh mice serologically positive for murine hepatitis (MHV). The anemia was associated with granulocytosis, decreased body weight and spleen hypertrophy. The overall intensity of erythropoiesis was measured by 59Fe-incorporation into the heme of erythropoietic organs. The localization of erythropoiesis became markedly redistributed--heme production was suppressed in the bone marrow while a several-fold increase was recorded for the spleen. A new steady state was also discovered in ferrokinetics: an iron pool localized away from the blood, erythropoietic organs and the liver was significantly elevated, and hypoferremia was detected. Anemia and wasting of mice were not observed in the same mouse strain free of MHV. A single administration of particulate glucan resulted in late impairment of red blood cell formation in the C57BL/10ScSnPh mouse strain infected with the mouse hepatitis virus. The anemia shares a number of features with those observed for the anemia of chronic diseases.

Evaluation of the role of exogenous pathogens on the incidence of embryo loss during early pregnancy in mice

The mating of CBA/j female mice (H2k) by DBA/2j male mice (H2d) typically results in an elevated incidence of spontaneous embryo loss thus providing an ideal genetically controlled laboratory model for the study of the factors causing early embryo loss during pregnancy. There is now considerable data on the cells and factors involved in fetal resorption but little is known about the events which activate this process. While the activation of the maternal response to the fetal implant could have endogenous or genetic origins, a role for exogenous factors including microbial pathogens could also be involved. In order to investigate these possibilities, the reproductive success of CBA/j female x DBA/2j male matings in a conventional animal care facility were compared with matings in a specific pathogen free (SPF) animal facility. All animals housed under these conditions were routinely screened by immunoassay and culture, for the presence of a number of viral and bacterial pathogens of mice. The incidence of spontaneous embryo loss in specific pathogen free CBA female mice mated by DBA and other male strains was found to be virtually identical to that of CBA female mice infected with multiple viral pathogens and housed under otherwise identical conditions (non-SPF). However, the numbers of implantation per pregnancy was significantly greater in an SPF facility. Therefore, exposure of mating mice to exogenous viral and bacterial pathogens did not appear to alter the overall incidence of spontaneous embryo resorption. It was concluded that the immunomodulatory effects of infection by common murine pathogens neither augmented nor reduced post-implantation embryo losses.

Critique of the toxicology and carcinogenesis studies of iodinated glycerol in F344/N rats and B6C3F1 mice

Carter-Wallace conducted a detailed audit and evaluation of the data available from the carcinogenicity studies with iodinated glycerol conducted in the B6C3F1 mouse and the F344/N rat by the National Toxicology Program (NTP). We conclude that there is no evidence of carcinogenicity of the compound in either the B6C3F1 mouse or the F344/N rat.

B cells induce apoptosis via a novel mechanism in fibroblasts infected with mouse hepatitis virus

B cells from nonimmune mice mediate the cytolysis of fibroblasts infected with the coronavirus, mouse hepatitis virus (MHV), strain A59. In this investigation, we report that splenic B cells and a B cell hybridoma induced the fragmentation of MHV-infected target cell DNA into a nucleosomal ladder pattern, characteristic of apoptosis. To determine the mechanism by which B cells mediated this killing event, we used criteria previously established for the killing of target cells by cytotoxic T lymphocytes (CTLs) and compared this B-cell-mediated killing to lymphocytic choriomeningitis virus (LCMV)-specific CTL killing of LCMV-infected target cells. Unlike CTL-mediated cytotoxicity, B cells efficiently lysed and induced the fragmentation of the DNA in their target cells in the presence of EGTA, arguing against a Ca(2+)-dependent granule exocytosis model for killing. In addition, paraformaldehyde-fixed B cells were able to kill MHV-infected targets. We were unable to detect TNF-alpha-associated cytotoxicity via bioassay with nonimmune effector B cells against the TNF-sensitive cell line, LM, or the TNF-alpha-resistant subline, L929.w, infected with MHV. Serine esterase inhibitors (benzamidine hydrochloride and N alpha-p-tosyl-L-arginine methyl ester) blocked CTL-induced 51Cr release and DNA fragmentation. In contrast, the inhibitors did not block the B-cell-induced 51Cr release, but did cause an inhibition in the fragmentation of the DNA of the target cell. These data indicate that B cells are capable of inducing the lysis and DNA fragmentation of MHV-infected target cells similar to CTL-induced apoptosis. However, we show that the mechanism(s) by which these processes are induced by B cells is distinct from CTL-mediated cytotoxicity.

In vitro and long-term in vivo immune dysfunction after infection of BALB/c mice with mouse hepatitis virus strain A59

Mouse hepatitis virus (MHV) is a pervasive pathogen in most mouse colonies worldwide. Infection with this virus, which is often inadvertent and unrecognized, has previously been correlated in numerous anecdotal reports with immune modulation seriously affecting the outcome of biomedical experiments. Studies using experimental models to examine the effects of MHV infection have demonstrated that the virus can both stimulate and depress immune function in vitro. We have used intranasal infection of MHV-susceptible BALB/c mice with MHV strain A59 to examine the effects of this virus on lymphoid tissue composition as well as immune function both in vitro and in vivo. We observed that the thymus, spleen, and lymph nodes underwent a transient period of marked cellular depletion. During that time, the percentages of T and B cells in the spleen remained normal. However, within 1 week after inoculation, splenic lymphoid cell proliferation was significantly decreased in response to the T-cell stimuli, concanavalin A and anti-CD3 monoclonal antibody. This continued through day 35 but was resolved by 102 days postinoculation. Notably, at days 35 and 102, mice infected with MHV-A59 were unable to reject skin grafts at a rate comparable to normal animals. These results support a basis for in vitro and, importantly, long-term in vivo immune dysfunction after infection with MHV.