Natural killer cell activity during mouse hepatitis virus infection: response in the absence of interferon

Protective and pathologic roles of the immune response to mouse hepatitis virus type 1: implications for severe acute respiratory syndrome

Intranasal mouse hepatitis virus type 1 (MHV-1) infection of mice induces lung pathology similar to that observed in severe acute respiratory syndrome (SARS) patients. However, the severity of MHV-1-induced pulmonary disease varies among mouse strains, and it has been suggested that differences in the host immune response might account for this variation. It has also been suggested that immunopathology may represent an important clinical feature of SARS. Little is known about the host immune response to MHV-1 and how it might contribute to some of the pathological changes detected in infected mice. In this study we show that an intact type I interferon system and the adaptive immune responses are required for controlling MHV-1 replication and preventing morbidity and mortality in resistant C57BL/6J mice after infection. The NK cell response also helps minimize the severity of illness following MHV-1 infection of C57BL/6J mice. In A/J and C3H/HeJ mice, which are highly susceptible to MHV-1-induced disease, we demonstrate that both CD4 and CD8 T cells contribute to morbidity during primary infection, and memory responses can enhance morbidity and mortality during subsequent reexposure to MHV-1. However, morbidity in A/J and C3H/HeJ mice can be minimized by treating them with immune serum prior to MHV-1 infection. Overall, our findings highlight the role of the host immune response in contributing to the pathogenesis of coronavirus-induced respiratory disease.

Intrahepatic endothelial and Kupffer cells involved in immunosuppressive cytokines and natural killer (NK)/NK T cell disorders in viral acute hepatitis

During acute viral hepatitis, the intrahepatic tolerance sustained by immunosuppressive cytokines such as interleukin (IL)-4, IL-10, transforming growth factor (TGF)-beta and prostaglandin E2 (PGE2), produced by Kupffer cells (KC), liver sinusoidal endothelial cells (LSEC), natural killer (NK) T cells and natural regulatory T cells may be disturbed. NK cells are recruited normally in the liver and produce interferon (IFN)-gamma to control viral replication. The use of mouse hepatitis virus type 3 (MHV3) attenuated variants showing selected tropisms for KC or LSEC have allowed determining their roles in the disturbances of immune tolerance during viral hepatitis. Groups of C57BL/6 mice were infected with the pathogenic L2-MHV3 (KC+, LSEC+), low attenuated 51.6-MHV3 (KC+, LSEC-) or high attenuated CL12-MHV3 (KC-, LSEC-) variants for the first 3 days. Results showed that IL-10, TGF-beta and PGE2 production in the liver decreased in L2-MHV3-infected mice and increased in 51.6-MHV3- and CL12-MHV3-infected mice. The ratio of IFN-gamma/IL-4 in liver decreased in L2-MHV3-infected mice, while it was not (or low) altered in mice infected with the attenuated MHV3 variant mice. Phenotypic analysis of intrahepatic mononuclear cells revealed that apoptotic NK and NK T cells increased in mice infected with the L2-MHV3, but were minor in 51.6-MHV3- and CL12-MHV3-infected mice. The numbers of CD4+ forkhead box P3+ cells increased in the livers from low pathogenic CL12-MHV3 and YAC-MHV3-infected mice. These results indicate that viral permissivity of KC and LSEC is involved in the decrease of IL-10 and PGE2, while KC may play an additional role in the apoptosis of NK and NK T cells during acute viral hepatitis.

Murine viral hepatitis involves NK cell depletion associated with virus-induced apoptosis

Mouse hepatitis virus type 3 (MHV3), a coronavirus, is an excellent animal model for the study of immunological disorders related to acute and chronic hepatitis. In this study, we have verified if the fulminant hepatitis induced by MHV3 could be related to an impairment of innate immunity. Groups of three C57BL/6 mice were infected with the pathogenic L2-MHV3 or attenuated YAC-MHV3 viruses, and the natural killer (NK) cell populations from liver, spleen and bone marrow were analysed. The percentage of intrahepatic NK1.1(+)T cell receptor (TCR)- cells did not increase while NK1.1(+)TCR(inter) cells decreased in both L2-MHV3- and YAC-MHV3-infected mice. Concurrently, splenic and myeloid NK1.1+ cells decreased in L2-MHV3-infected mice. However, the cytotoxic activity of NK cells increased in liver and decreased in bone marrow from pathogenic L2-MHV3-infected mice while no modification was detected in YAC-MHV3-infected mice. Flow cytometric analysis revealed that both normal and larger splenic or myeloid NK cells decreased more in pathogenic L2-MHV3-infected mice than in attenuated YAC-MHV3-infected mice. In vitro viral infections of interleukin (IL)-15-stimulated lymphoid cells from liver and bone marrow revealed that L2-MHV3 induced higher decreases in cell viability of NK1.1+ cells than the YAC-MHV3 variant. The NK cell decreases were due to the viral permissivity leading to cytopathic effects characterized by cell rounding, syncytia formation and apoptosis. Larger NK+ syncytia were observed in L2-MHV3-infected cells than in YAC-MHV3-infected cells. These results suggest that NK cell production is impaired by viral infection favouring fulminant hepatitis.

CXC chemokine ligand 10 controls viral infection in the central nervous system: evidence for a role in innate immune response through recruitment and activation of natural killer cells

How chemokines shape the immune response to viral infection of the central nervous system (CNS) has largely been considered within the context of recruitment and activation of antigen-specific lymphocytes. However, chemokines are expressed early following viral infection, suggesting an important role in coordinating innate immune responses. Herein, we evaluated the contributions of CXC chemokine ligand 10 (CXCL10) in promoting innate defense mechanisms following coronavirus infection of the CNS. Intracerebral infection of RAG1(-/-) mice with a recombinant CXCL10-expressing murine coronavirus (mouse hepatitis virus) resulted in protection from disease and increased survival that correlated with a significant increase in recruitment and activation of natural killer (NK) cells within the CNS. Accumulation of NK cells resulted in a reduction in viral titers that was dependent on gamma interferon secretion. These results indicate that CXCL10 expression plays a pivotal role in defense following coronavirus infection of the CNS by enhancing innate immune responses.

MHV infection of the CNS: mechanisms of immune-mediated control

Mice infected with neurotropic strains of mouse hepatitis virus (MHV) clear infectious virus; nevertheless, viral persistence in the central nervous system (CNS) is associated with ongoing primary demyelination. Acute infection induces a potent regional CD8+ T-cell response. The high prevalence of virus specific T cells correlates with ex vivo cytolytic activity, interferon-gamma (IFN-gamma) secretion and efficient reduction in virus. Viral clearance from most cell types is controlled by a perforin dependent mechanism. However, IFN-gamma is essential for controlling virus replication in oligodendrocytes. Furthermore, CD4+ T cells enhance CD8+ T-cell survival and effectiveness. Clearance of infectious virus is associated with a gradual decline of CNS T cells; nevertheless, activated T cells are retained within the CNS. The loss of cytolytic activity, but retention of IFN-gamma secretion during viral clearance suggests stringent regulation of CD8+ T-cell effector function, possibly as a means to minimize CNS damage. However, similar CD8+ T-cell responses to demyelinating and non demyelinating JHMV variants support the notion that CD8+ T cells do not contribute to the demyelinating process. Although T-cell retention is tightly linked to the presence of persisting virus, contributions to regulating the latent state are unknown. Studies in B-cell-deficient mice suggest that antibodies are required to prevent virus recrudescence. Although acute JHMV infection is thus primarily controlled by CD8+ T cells, both CD4+ T cells and B cells make significant contributions in maintaining the balance between viral replication and immune control, thus allowing host and pathogen survival.

Innate immune response of the human host to exposure with herpes simplex virus type 1: in vitro control of the virus infection by enhanced natural killer activity via interleukin-15 induction

Infections with herpes simplex virus type 1 (HSV-1) in humans and in animal models are accompanied by enhanced natural killer (NK) activity. In vitro, HSV-1 also enhances the NK activity of human peripheral blood mononuclear cells (PBMC). The molecular basis of this enhanced NK activity, however, is not well characterized. We investigated the role of human interleukin-15 (IL-15) in this phenomenon and report here that HSV-1-mediated enhanced NK activity was abrogated by neutralizing antibodies for IL-15 but not for other cytokines (i.e., IL-2, IL-12, gamma interferon [IFN-gamma], tumor necrosis factor alpha, or IFN-alpha). Anti-CD122 antibodies which block signaling through IL-2 receptor beta chain, and therefore neutralize the effects of IL-15 (and IL-2), also abrogated this enhancement. Furthermore, HSV-1 increased the levels of IL-15 mRNA and the production of IL-15 in HSV-1-infected PBMC cultures. The neutralization of IL-15 in cocultures of PBMC with HSV-1-infected cells significantly increased HSV-1 production. These results strongly suggest a role for IL-15 in the HSV-1-mediated in vitro enhancement of NK activity and in the PBMC-mediated suppression of HSV-1 replication.

Mouse hepatitis virus-specific cytotoxic T lymphocytes protect from lethal infection without eliminating virus from the central nervous system

Acute infection of the central nervous system by the neurotropic JHM strain of mouse hepatitis virus (JHMV) induces nucleocapsid protein specific cytotoxic T lymphocytes (CTL) not found in the periphery (S. Stohlman, S. Kyuwa, J. Polo, D. Brady, M. Lai, and C. Bergmann, J. Virol. 67:7050-7059, 1993). Peripheral induction of CTL specific for the nucleocapsid protein of JHMV by vaccination with recombinant vaccinia viruses was unable to provide significant protection to a subsequent lethal virus challenge. By contrast, the transfer of nucleoprotein-specific CTL protected mice from a subsequent lethal challenge by reducing virus replication within the central nervous system, demonstrating the importance of the CTL response to this epitope in JHMV infection. Transfer of these CTL directly into the central nervous system was at least 10-fold more effective than peripheral transfer. Histological analysis indicated that the CTL reduced virus replication in ependymal cells, astrocytes, and microglia. Although the CTL were relatively ineffective at reducing virus replication in oligodendroglia, survivors showed minimal evidence of virus persistence within the central nervous system and no evidence of chronic ongoing demyelination.

Interleukin-6 induction in vitro in mouse brain endothelial cells and astrocytes by exposure to mouse hepatitis virus (MHV-4, JHM)

Interleukin-6 (IL-6) induction, as detected by bioassay and Northern analysis, was examined in vitro in endothelial cells or astrocytes derived from BALB/c (susceptible) or SJL (resistant) mice following exposure to mouse hepatitis virus (MHV-4) or UV inactivated MHV-4 (UV-MHV-4). In BALB/c endothelial cells, up to 16-fold more IL-6 (> 640 U/ml) was induced, compared to SJL cells which showed a minimal response (40 U/ml), relative to basal levels (< 20 U/ml). In contrast, both BALB/c and SJL astrocytes showed a substantial IL-6 response to MHV-4 and UV-MHV-4 exposure, although a strain difference persisted. Despite strain and cell specific differences in released IL-6, equivalent levels of IL-6 mRNA were induced in all cell types following exposure to MHV-4 or UV-MHV-4.

Immune response to a murine coronavirus: identification of a homing receptor-negative CD4+ T cell subset that responds to viral glycoproteins

The lymphocyte proliferative response to mouse hepatitis virus, strain JHM (MHV-JHM), a well-described cause of chronic and acute neurological infections, has been studied using vaccinia virus recombinants expressing individual MHV proteins. The surface (S) and transmembrane (M) glycoproteins were the most active proteins in causing proliferation of lymphocytes isolated from immunized adult mice, whereas lymphocytes from persistently infected mice proliferated only in response to the S protein. The cells from immunized mice which proliferated most actively in response to MHV were positive for the CD4 antigen and secreted interferon-gamma. In addition, the most responsive subset of cells did not express gp90MEL-14, the lymph node-specific homing receptor. The results identify a subpopulation of CD4+ T cells that may be an important component of the cell-mediated immune response to this virus. The data also suggest that response to the M protein is important in preventing disease progression in C57BL/6 mice since cells which recognize this protein are absent from persistently infected mice.

Characterization of brain-infiltrating mononuclear cells during infection with mouse hepatitis virus strain JHM

The eradication of infectious virus from the central nervous system (CNS) following infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) is thought to be immune-mediated. Furthermore, a significant decrease of infectious virus coincides with the appearance of prominent inflammatory infiltrates in the brain and spinal cord. In the present study, mononuclear cells infiltrating the brain during JHMV infection were isolated and characterized. While all subsets of immune cells were present, there appeared to be a temporal relationship between the peak incidence of CD8+ T cells (40% of total isolated cells) and reduction of virus at day 7 post-infection. Cells with the natural killer (NK) phenotype (at least 30%) were also present throughout infection. These data suggest that CD8+ T cells and NK cells are prominent among cells which infiltrate the brain during JHM virus infection and may have important roles in reduction of virus within the CNS.

Effective clearance of mouse hepatitis virus from the central nervous system requires both CD4+ and CD8+ T cells

Both CD4+ and CD8+ T cells are required for the clearance of virus from the central nervous system following infection with the JHM strain of mouse hepatitis virus. Development of antiviral antibodies requires the presence of CD4+ T cells but appears to play a minimal role in the reduction of virus. The data presented are consistent with the hypothesis that clearance of JHM virus is mediated by virus-specific CD8+ T cells, which appear to require the presence of CD4+ T cells.

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.

T-cell-mediated clearance of mouse hepatitis virus strain JHM from the central nervous system

Clearance of the neurotropic JHM strain of mouse hepatitis virus from the central nervous system was examined by the transfer of spleen cells from immunized donors. A T cell with the surface phenotype of Thy1.2+ CD4+ CD8- asialo-GM1+ Mac-1- was found to be necessary for viral clearance. The surface phenotype and adherence to nylon wool suggest that these cells are activated helper-inducer T cells. Adoptive transfer to congenic histocompatibility strains demonstrated the necessity for compatibility at the D locus of the major histocompatibility complex. The expression of the CD4 surface marker and the requirement for major histocompatibility complex class I were further studied by the transfer of cells to recipients treated with anti-CD4 or anti-CD8 monoclonal antibodies. Treatment of recipients with either the anti-CD8 or the anti-CD4 antibodies inhibited virus clearance from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system. This suggests that the CD4+ cell acts as a helper and that virus is cleared from the central nervous system by CD8+ cells that recognize viral antigen in the context of the H-2Db gene product.

Studies on the mechanism of protection from acute viral encephalomyelitis by delayed-type hypersensitivity inducer T cell clones

Previous studies have shown that mice can be protected from a lethal infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) by the adoptive transfer of delayed-type hypersensitivity (DTH)-inducer T cell clones specific for the virus. Protection does not involve the suppression of virus replication in the central nervous system (CNS) or via augmentation of the antiviral antibody response. In the present report we have compared the CNS lesions induced by JHMV in lethally infected and T cell clone protected mice. The presence of virus-specific T cell clones induced a transient increase in mononuclear cell infiltration into the parenchyma of the brains of protected mice, consistent with previous data suggesting that a DTH response was responsible for protection. Immunohistochemical studies suggested further that virus was not replicating in the ependyma or cellular infiltrate, but that the presence of the T cell clone prevented neuronal infection. While the mechanism of effectively altering the in vivo cellular tropism is unknown, survival is accompanied by increased specific destruction of target tissues with fulminant CNS demyelination and an increased incidence of persistent infection.

Splenic natural killer-cell activity in mice infected with Leishmania donovani

Several strains of inbred mice were infected with the protozoan parasite Leishmania donovani, and, at several points during the infection, spleens of groups of these mice were tested for natural killer (NK)-cell activity vs lymphoma target cells in vitro and were evaluated for parasite burdens. Generally, elevated followed by normal (compared to uninfected control mice) or subnormal NK responses occurred as the result of infection. Elevated NK responses were not accompanied by high circulating levels of interferon, yet infected mice responded to an injection of an interferon inducer with interferon production as great as control mice. No consistent correlations among susceptibility phenotype to L. donovani infection, spontaneous NK activity phenotype, and infection-induced NK activation/depression patterns were detected among the various strains of mice.

Induction of natural killer cells and interferon during mouse hepatitis virus infection of resistant and susceptible inbred mouse strains

Following infection by mouse hepatitis virus (JHM strain), an induction of natural killer (NK) cell activity was observed in C3H mice, which are considered to be sensitive to JHM virus infection. In contrast, mice of the resistant SJL strain did not show any increase of NK cell activity after JHM virus infection. However, infection of both SJL and C3H mice with mouse hepatitis virus type 3 (MHV3) resulted in an increase of NK level, comparable to that observed with the JHM virus infection in the C3H strain. No significant differences were observed in the NK cell activity of the peritoneal exudate or spleen cells of infected mice. Low levels of interferon were detected in serum or peritoneal exudate of C3H mice infected with JHM virus 18 or 24 hours before, but no detectable early interferon production was found. Also no interferon could be detected in the resistant SJL mice. After JHM virus infection, the number of peritoneal exudate cells (PEC) was increased significantly in C3H mice but not in SJL mice. Macrophages obtained from the C3H mice supported virus replication, whereas SJL macrophages did not. Our data suggest that NK cells do not play a role in the resistance of SJL mice against JHM virus infection but may participate in the defence mechanisms against this virus in C3H mice.

The biological relationship of mouse hepatitis virus (MHV) strains and interferon: in vitro induction and sensitivities

Five prototype strains of mouse hepatitis virus (MHV) -1, -3, -S, -A59 and -JHM were analyzed for their ability to induce interferon (IFN) in seven cell lines of rodent origin. Induction of IFN by all of the prototype MHV strains was infrequent and unpredictable, while IFN was produced consistently by five cell lines treated with known inducers. Priming and/or aging of cells did not enhance IFN induction by the MHV strains except in the case of MHV-A59 which consistently induced moderate levels of IFN on L-cells which were both primed and aged. Kinetic studies of MHV-A59-induced IFN on primed and aged L-cells demonstrated that detectable levels of IFN were not produced until 24 hours post-inoculation (p.i.). Peak levels were attained at 30 hours p.i. with no additional IFN produced through 48 hours p.i. MHV-induced IFN was similar in composition and properties to Newcastle disease virus-induced IFN. The sensitivities of the five MHV strains to eight concentrations of preformed L-cell IFN were also assessed. All strains except MHV-S fit a linear model with MHV-3, MHV-A59 and MHV-JHM having similar slopes. At most concentrations MHV-3 was less sensitive than MHV-1, -A59 or -JHM to IFN. The response curve for MHV-S was non-linear. This strain was more sensitive to the antiviral effects of the pre-formed IFN except at the highest concentrations of IFN used.