Enterotropic mouse hepatitis virus infection in nude mice

The cause of emaciation and diarrhea in athymic nude mice was found to be hyperplastic typhlocolitis resulting from infection with enterotropic mouse hepatitis virus (MHV). The disease was reproduced in experimentally-inoculated nude mice using intestinal homogenates from affected mice and cell culture-derived virus. Material derived from an experimental mouse was passed into neonatal Swiss mice and caused acute typhlocolitis. Virus failed to grow in NCTC-1469 cells and 17Cl-1 cells, which are normally permissive for MHV, but grew to low titer in a mouse rectal carcinoma cell line, CMT 93. These results show that an enterotropic strain of MHV can cause chronic enteric disease in athymic nude mice. The pattern of infection differs markedly from the more common MHV wasting syndrome in nude mice caused by non-enteric strains of MHV.     

Study of camelpox virus pathogenesis in athymic nude mice

Camelpox virus (CMLV) is the closest known orthopoxvirus genetically related to variola virus. So far, CMLV was restricted to camelids but, recently, three human cases of camelpox have been described in India, highlighting the need to pursue research on its pathogenesis, which has been hampered by the lack of small animal models. Here, we confirm that NMRI immunocompetent mice are resistant to intranasal (i.n.) CMLV infection. However, we demonstrate that CMLV induced a severe disease following i.n. challenge of athymic nude mice, which was accompanied with a failure in gaining weight, leading to euthanasia of the animals. On the other hand, intracutaneous (i.c.) infection resulted in disease development without impacting the body weight evolution. CMLV replication in tissues and body fluids was confirmed in the two models. We further analyzed innate immune and B cell responses induced in the spleen and draining lymph nodes after exposure to CMLV. In both models, strong increases in CD11b(+)F4/80(+) macrophages were seen in the spleen, while neutrophils, NK and B cell responses varied between the routes of infection. In the lymph nodes, the magnitude of CD11c(+)CD8α(+) lymphoid and CD11c(+)CD11b(+) myeloid dendritic cell responses increased in i.n. challenged animals. Analysis of cytokine profiles revealed significant increases of interleukin (IL)-6 and IL-18 in the sera of infected animals, while those of other cytokines were similar to uninfected controls. The efficacy of two antivirals (cidofovir or HPMPC, and its 2, 6-diaminopurine analog) was evaluated in both models. HPMPC was the most effective molecule affording 100% protection from morbidity. It appeared that both treatments did not affect immune cell responses or cytokine expression. In conclusion, we demonstrated that immunodeficient mice are permissive for CMLV propagation. These results provide a basis for studying the pathogenesis of CMLV, as well as for evaluating potential antiviral therapies in an immunodeficiency context.     

Respiratory disease and wasting in athymic mice infected with pneumonia virus of mice

Although pneumonia virus of mice (PVM) is ubiquitous among rodent colonies in the United States, it has not been reported to cause clinically apparent disease in euthymic mice. However, PVM has been reported to cause respiratory disease and death in experimentally infected euthymic and athymic mice. A group of nu/nu mice, housed in quarantine in a Trexler-type isolator, had weight loss and dyspnea. Gross necropsy findings included cachexia and diffuse pulmonary edema or lobar consolidation. Histologically there was diffuse interstitial pneumonia. Electron microscopy revealed filamentous virions budding from plasma membranes, and immunohistochemical staining of lung tissue was positive for PVM antigen. PVM was isolated from affected lung tissue in BHK 21 cells and mouse antibody production tests resulted in seroconversion to PVM. Experimental inoculation of athymic mice with lung homogenate from spontaneously infected mice resulted in clinically apparent respiratory disease and histologic lung changes similar to those in naturally infected mice. Inoculation of athymic mice with infected BHK 21 cell culture fluid resulted in pneumonia which was qualitatively similar to, but less severe than, that observed in mice with spontaneous disease. These findings indicate that naturally occurring PVM infection in athymic mice may cause respiratory disease and wasting.     

Modification of immune response in nude mice infected with mouse hepatitis virus.

In nude mice experimentally infected with mouse hepatitis virus (MHV), the numbers of early and later plaque forming cells (PFC) to sheep red blood cells (SRBC) generated in the spleen were 7 to 20 times and 2 to 163 times, respectively, greater than those in non-infected nude mice, when SRBC were given at day 0 to day 21 postinfection. Splenic theta-positive lymphocytes in infected nude mice were shown to increase only at day 10 or more postinfection. PFC response to bacterial lipopolysaccharide, a T cell-independent antigen, was not modified in MHV-infected nude mice.     

Persistent infection with mouse hepatitis virus of low virulence in nude mice.

A persisting type of infection with wasting syndrome was established in congenitally athymic nude mice after intraperitoneal inoculation with a mouse hepatitis virus which was not fully pathogenic for heterozygous haired littermates. From the liver, spleen, lymph nodes, and brain of most infected nude mice, the virus was detected at high titers during aperiod from 6 to 35 days postinfection, occurrence of degenerative and necrotic lesions being correlated with virus titers in these organs. The titer of serum neutralizing antibody remained undetectable or very low in most diseases nude mice, whereas some animals resisting the infection could produce antibody at a later stage. In heterozygous haired mice, some lesions were detectable at a very early stage of infection in the spleen and liver, but they seemed to disappear with a marked elevation of the neutralizing antibody titer. Nude mice were able to resist the virus infection when they had previously received transfer of thymocytes from weanling heterozygous littermates.     

Restricted virus replication in the spinal cords of nude mice infected with a Theiler''s virus variant.

The Daniels strain of Theiler's murine encephalomyelitis produces a chronic disease which is an animal model for human demyelinating disorders. Previously, we selected a neutralization-resistant virus variant producing an altered and diminished central nervous system disease in immunocompetent mice which was evident during the later stage of infection (after 4 weeks) (A. Zurbriggen and R. S. Fujinami, J. Virol. 63:1505-1513, 1989). The exact epitope determining neurovirulence was precisely mapped to a capsid protein, VP-1, and represents a neutralizing region (A. Zurbriggen, J. M. Hogle, and R. S. Fujinami, J. Exp. Med. 170:2037-2049, 1989). Here, we present experiments with immunoincompetent animals to determine viral replication, spread, and targeting to the central nervous system in the absence of detectable antibodies or functional T cells. Nude mice were infected orally, and the virus was monitored by plaque assay, immunohistochemistry, and in situ hybridization. Early during the infection (1 week), the variant virus induced an acute disease comparable to that induced by the wild-type virus in these nude mice. Alterations in tropism in the central nervous system were not apparent when wild-type parental Daniels strain virus was compared with the variant virus. Moreover, variant virus replicated in tissue culture (BHK-21 cells) to similarly high titers in a time course identical to that of the wild-type virus (A. Zurbriggen and R. S. Fujinami, J. Virol. 63:1505-1513, 1989). However, replication of the variant virus versus the wild-type virus within the spinal cord of athymic nude mice infected per os was substantially restricted by 6 weeks postinfection. Therefore, the reduced neurovirulence in the later stage (6 weeks) of the disease is most likely due to a diminished growth rate or spread of the variant virus in the central nervous system rather than to marked differences in viral tropism.     

Immunopathology of chronic progressive hepatitis in nude mice infected with low-virulent mouse hepatitis virus

Progressive hepatitis in athymic nude (nu/nu) mice due to a low-virulent mouse hepatitis virus, MHV-2 cc, was examined for involvement of immunocytes and serum antibodies. At 3 to 6 weeks postinoculation (p.i.) a considerable number of Mac 1- and asialo GM1-positive cells were accumulated in the affected liver and spleen. There were also some Thy-1-positive cells. Later than 2 weeks p.i., serum IgG and IgM antibodies were detected in parallel with virus-neutralizing activity, while the IgG levels were lower than those of infected euthymic (nu/+) littermates. By transfer of the infected nu/nu mouse serum, the recipient euthymic mice acquired resistance to lethal challenge infection with a virulent virus, MHV-2.     

Role of T cells in virus control and disease after infection with pneumonia virus of mice

Infection of mice with pneumonia virus of mice (PVM) is used as a natural host experimental model for studying the pathogenesis of infection with the closely related human respiratory syncytial virus. We analyzed the contribution of T cells to virus control and pathology after PVM infection. Control of a sublethal infection with PVM strain 15 in C57BL/6 mice was accompanied by a 100-fold increase in pulmonary cytotoxic T lymphocytes, 20% of which were specific for PVM. T-cell-deficient mice failed to eliminate PVM and became virus carriers in the absence of the clinical or histopathological signs of pneumonia that occurred after infection of control mice. Mice with limited T-cell numbers did not achieve virus control without weight loss, indicating that T-cell-mediated virus control was closely linked to immunopathology. Both CD4 and CD8 T cells independently contributed to virus elimination and disease. Virus control and disease were similar in the absence of perforin, gamma interferon, or tumor necrosis factor alpha. Interestingly, disease and mortality after lethal high-dose PVM infection were independent of T cells. These data illustrate a key role for T cells in control of PVM infection and demonstrate that both T-cell-dependent and -independent pathways contribute to disease in a viral dose-dependent fashion.     

Protection of mice against Sendai virus pneumonia by non-neutralizing anti-F monoclonal antibodies

Nine monoclonal antibodies (MAbs) directed to F protein of Sendai virus were obtained and characterized for their protective ability against Sendai virus infection in mice. None of the MAbs showed hemagglutination-inhibition (HI), hemolysis-inhibition (HLI), or neutralization (NT) activities in vitro when assayed by standard methods. Some of the MAbs, however, showed complement-requiring NT (C-NT) and complement-requiring hemolysis (C-HL) activities when assayed in the presence of complement. Passive immunization experiments revealed that the MAbs with higher C-NT and C-HL activities showed protective activity against Sendai virus pneumonia in mice, and that some MAbs with IgG1 isotype having neither C-NT nor C-HL activity also showed the protective activity. Digestion of the MAbs with pepsin which split immunoglobulin molecules into F(ab')2 and Fc fragments greatly suppressed the protective activity. These results suggest that not only complement-mediated immunological responses such as immune virolysis but also antibody-dependent cellular cytotoxicity (ADCC) and/or immune phagocytosis, in which complement system is not necessarily involved, play an important role in the protection of mice from Sendai virus infection.     

Persistent infection of BHK-21 cells with pneumonia virus of mice.

Trypsinization of BHK-21 cells 72 h after primary infection with pneumonia virus of mice yielded clones of persistently infected cells which specifically adsorbed murine erythrocytes. We describe one clone of cells, the progeny of which, after more than 100 passages, still bore viral antigen demonstrable by immunofluorescence and immune electron microscopy, but produced little or no detectable infectious virus.     

Study of prophylaxis by didecyl dimethyl ammonium chloride against herpes simplex virus infection in nude mice

Primary or recurrent viral infections, especially by herpetoviridae, with viraemias--after organ transplants, for example--are a serious medical problem which can affect the prognosis for survival. An experimental protocol for the prevention of infection by a herpes simplex virus type 1 was applied to Nude mice infected via inhalation. It suggests the possibility of appreciably limiting viral infection by the parenteral use of didecyl dimethyl ammonium chloride. The results are encouraging and could lead to further experimental studies to prove the efficiency of this molecule whose level of toxicity can be disassociated from its antiviral action to limit the infections by other enveloped viruses.     

Cells with natural killer activity in the cerebrospinal fluid of normal mice and athymic nude mice with acute Sindbis virus encephalitis

Sindbis virus causes an acute, nonfatal inflammatory encephalitis in weanling BALB/c mice. Mononuclear inflammatory cells are present in the cerebrospinal fluid (CSF) as well as in the parenchyma of the brain. Both aspects of this inflammatory response were eliminated by treatment with cyclophosphamide. Athymic nude (nu/nu) mice developed no inflammation in the brain, but did develop a CSF pleocytosis that peaked on day 2 after infection. The time course of the appearance of cells in the CSF was earlier in nu/nu mice than their heterozygote (nu/+) littermates. The pleocytosis in nu/nu mice reached a peak on day 2, whereas in nu/+ mice the peak was on day 4, as it is in normal BALB/c mice. To determine whether some of the CSF cells in nu/nu mice may be natural killer (NK) cells, NK activity was measured in a 4-hr assay by using a YAC-1 target cell. NK cell activity in the spleen and peripheral blood was induced by infection with Sindbis virus in nu/nu mice with a similar time course to that of nu/+ mice (peak 1 day after infection). CSF from nu/nu mice had NK activity present 2 days after infection that was greater than that present in either the peripheral blood or spleen. BALB/c and nu/+ mice had insufficient cells present for assay at day 2, but BALB/c mice had NK activity present in the CSF 3 and 5 days after infection that exceeded that in the peripheral blood or spleen. Brain interferon was detectable on day 1 in nu/nu mice, but not until day 2 in nu/+ mice even though the amounts of brain virus were the same in the two groups at all time points. It is concluded that cells with NK activity contribute to the CSF pleocytosis induced by acute Sindbis virus encephalitis.