Acute murine cytomegalovirus infection induces lethal hepatitis

An Elastin-like Polypeptide-fusion peptide targeting capsid-tegument interface as an antiviral against cytomegalovirus infection

The tegument protein pp150 of Human Cytomegalovirus (HCMV) is known to be essential for the final stages of virus maturation and mediates its functions by interacting with capsid proteins. Our laboratory has previously identified the critical regions in pp150 important for pp150-capsid interactions and designed peptides similar in sequence to these regions, with a goal to competitively inhibit capsid maturation. Treatment with a specific peptide (PepCR2 or P10) targeted to pp150 conserved region 2 led to a significant reduction in murine CMV (MCMV) growth in cell culture, paving the way for in vivo testing in a mouse model of CMV infection. However, the general pharmacokinetic parameters of peptides, including rapid degradation and limited tissue and cell membrane permeability, pose a challenge to their successful use in vivo. Therefore, we designed a biopolymer-stabilized elastin-like polypeptide (ELP) fusion construct (ELP-P10) to enhance the bioavailability of P10. Antiviral efficacy and cytotoxic effects of ELP-P10 were studied in cell culture, and pharmacokinetics, biodistribution, and antiviral efficacy were studied in a mouse model of CMV infection. ELP-P10 maintained significant antiviral activity in cell culture, and this conjugation significantly enhanced P10 bioavailability in mouse tissues. The fluorescently labeled ELP-P10 accumulated to higher levels in mouse liver and kidneys as compared to the unconjugated P10. Moreover, viral titers from vital organs of MCMV-infected mice indicated a significant reduction of virus load upon ELP-P10 treatment. Therefore, ELP-P10 has the potential to be developed into an effective antiviral against CMV infection.

Recent Approaches and Strategies in the Generation of Anti-human Cytomegalovirus Vaccines

Human cytomegalovirus is the largest human herpesvirus and shares many core features of other herpesviruses such as tightly regulated gene expression during genome replication and latency as well as the establishment of lifelong persistence following infection. In contrast to stereotypic clinical syndromes associated with alpha-herpesvirus infections, almost all primary HCMV infections are asymptomatic and acquired early in life in most populations in the world. Although asymptomatic in most individuals, HCMV is a major cause of disease in hosts with deficits in adaptive and innate immunity such as infants who are infected in utero and allograft recipients following transplantation. Congenital HCMV is a commonly acquired infection in the developing fetus that can result in a number of neurodevelopmental abnormalities. Similarly, HCMV is a major cause of disease in allograft recipients in the immediate and late posttransplant period and is thought to be a major contributor to chronic allograft rejection. Even though HCMV induces robust innate and adaptive immune responses, it also encodes a vast array of immune evasion functions that are thought aid in its persistence. Immune correlates of protective immunity that prevent or modify intrauterine HCMV infection remain incompletely defined but are thought to consist primarily of adaptive responses in the pregnant mother, thus making congenital HCMV a potentially vaccine modifiable disease. Similarly, HCMV infection in allograft recipients is often more severe in recipients without preexisting adaptive immunity to HCMV. Thus, there has been a considerable effort to modify HCMV specific immunity in transplant recipient either through active immunization or passive transfer of adaptive effector functions. Although efforts to develop an efficacious vaccine and/or passive immunotherapy to limit HCMV disease have been underway for nearly six decades, most have met with limited success at best. In contrast to previous efforts, current HCMV vaccine development has relied on observations of unique properties of HCMV in hopes of reproducing immune responses that at a minimum will be similar to that following natural infection. However, more recent findings have suggested that immunity following naturally acquired HCMV infection may have limited protective activity and almost certainly, is not sterilizing. Such observations suggest that either the induction of natural immunity must be specifically tailored to generate protective activity or alternatively, that providing targeted passive immunity to susceptible populations could be prove to be more efficacious.

Hyperexpansion of Functional Viral-Specific CD8+ T Cells in Lymphopenia-Associated MCMV Pneumonitis

Cytomegalovirus (CMV) is a significant cause of morbidity and mortality in immunocompromised hosts, many of whom undergo significant periods of lymphopenia. However, the impact of lymphopenia and subsequent immune reconstitution on T cell responses and pulmonary pathology are poorly understood. Using a model of primary murine CMV infection in mice treated with cyclophosphamide (CY), the relationship of CD8+ T cell reconstitution to pneumonitis pathology was studied. Female BALB/c mice were infected with murine CMV (MCMV) with/without CY on day 1 post-infection. Lung pathology and viral specific T cell responses were assessed on days 7-28. T cell lymphocyte subsets, effector responses, and MCMV specificity were assessed at baseline and after in vitro stimulation of cells with immediate-early peptide pp89. CY treatment of MCMV-infected mice resulted in interstitial pneumonitis not seen with MCMV alone. Compared to MCMV alone, on day 14, MCMV/CY mice had greater number of CD8+ T cells, a fourfold increase in absolute number of pp89 tetramer-specific CD8+ cells, and an eightfold increase in MCMV specific T cell effector responses (IFN-γ; p<0.001). This expansion was preceded by transient lymphopenia, increased viral titers, and, most strikingly, a 10-fold increased proliferative capacity in MCMV/CY mice. In the setting of CY-associated lymphopenia, concurrent MCMV infection alters immune reconstitution toward a hyperexpanded MCMV-specific CD8+ effector T cell pool that correlates with significant lung immunopathology.

Recent approaches and strategies in the generation of antihuman cytomegalovirus vaccines

The development of prophylactic and to lesser extent therapeutic vaccines for the prevention of disease associated with human cytomegalovirus (HCMV) infections has received considerable attention from biomedical researchers and pharmaceutical companies over the previous 15 years, even though attempts to produce such vaccines have been described in the literature for over 40 years. Studies of the natural history of congenital HCMV infection and infection in allograft recipients have suggested that prophylaxis of disease associated with HCMV infection could be possible, particularly in hosts at risk for more severe disease secondary to the lack of preexisting immunity. Provided a substantial understanding of immune response to HCMV together with several animal models that faithfully recapitulate aspects of human infection and immunity, investigators seem well positioned to design and test candidate vaccines. Yet more recent studies of the role of a maternal immunity in the natural history of congenital HCMV infection, including the recognition that reinfection of previously immune women by genetically distinct strains of HCMV occur in populations with a high seroprevalence, have raised several questions about the nature of protective immunity in maternal populations. This finding coupled with observations that have documented a significant incidence of damaging congenital infections in offspring of women with immunity to HCMV prior to conception has suggested that vaccine development based on conventional paradigms of adaptive immunity to viral infections may be of limited value in the prevention of damaging congenital HCMV infections. Perhaps a more achievable goal will be prophylactic vaccines to modify HCMV associated disease in allograft transplant recipients. Although recent descriptions of the results from vaccine trials have been heralded as evidence of an emerging success in the quest for a HCMV vaccine, careful analyses of these studies have also revealed that major hurdles remain to be addressed by current strategies.

Sex bias in susceptibility to MCMV infection: implication of TLR9

Toll-like receptor (TLR)-dependent pathways control the activation of various immune cells and the production of cytokines and chemokines that are important in innate immune control of viruses, including mouse cytomegalovirus (MCMV). Here we report that upon MCMV infection wild-type and TLR7^−/− male mice were more resistant than their female counterparts, while TLR9^−/− male and female mice showed similar susceptibility. Interestingly, 36 h upon MCMV infection TLR9 mRNA expression was higher in male than in female mouse spleens. MCMV infection led to stronger reduction of marginal zone (MZ) B cells, and higher infiltration of plasmacytoid dendritic cells and neutrophils in wild-type male than female mice, while no such sex differences were observed in TLR9^−/− mice. In accordance, the serum levels of KC and MIP-2, major neutrophil chemoattractants, were higher in wild-type, but not in TLR9^−/−, male versus female mice. Wild-type MCMV-infected female mice showed more severe liver inflammation, necrosis and steatosis compared to infected male mice. Our data demonstrate sex differences in susceptibility to MCMV infection, accompanied by a lower activation of the innate immune system in female mice, and can be attributed, at least in a certain degree, to the lower expression of TLR9 in female than male mice.

Antiviral T cell response triggers cytomegalovirus hepatitis in mice

One common sign of human cytomegalovirus infection is altered liver function. Murine cytomegalovirus strain v70 induces a rapid and severe hepatitis in immunocompetent mice that requires the presence of T cells in order to develop. v70 exhibits approximately 10-fold-greater virulence than the commonly used strain K181, resulting in a more severe, sustained, and lethal hepatitis but not dramatically higher viral replication levels. Hepatitis and death are markedly delayed in immunodeficient SCID compared to immunocompetent BALB/c mice. Transfer of BALB/c splenocytes to SCID mice conferred rapid disease following infection, and depletion of either CD4 or CD8 T cells in BALB/c mice reduced virus-induced hepatitis. The frequency of CD8 T cells producing gamma interferon and tumor necrosis factor in response to viral antigen was higher in settings where more severe disease occurred. Thus, virus-specific effector CD8 T cells appear to contribute to lethal virus-induced hepatitis, contrasting their protective role during sublethal infection. This study reveals how protection and disease during cytomegalovirus infection depend on viral strain and dose, as well as the quality of the T cell response.

Viral Infections of Laboratory Mice

Viral infections of laboratory mice have considerable impact on research results, and prevention of such infections is therefore of crucial importance. This chapter covers infections of mice with the following viruses: herpesviruses, mousepox virus, murine adenoviruses, polyomaviruses, parvoviruses, lactate dehydrogenase-elevating virus, lymphocytic choriomeningitis virus, mammalian orthoreovirus serotype 3, murine hepatitis virus, murine norovirus, murine pneumonia virus, murine rotavirus, Sendai virus, and Theiler’s murine encephalomyelitis virus. For each virus, there is a description of the agent, epizootiology, clinical symptoms, pathology, methods of diagnosis and control, and its impact on research.

Murine cytomegalovirus displays selective infection of cells within hours after systemic administration

A distinctive feature of the cytomegaloviruses is their wide tissue tropism, demonstrated by the infection of many organs and cell types in an active infection. However, in experimental models of systemic infection, the earliest stages of infection are not well characterized, and it is unclear whether only certain cells are initially infected. Using a recombinant murine cytomegalovirus (MCMV) expressing green fluorescent protein (GFP), we tracked viral infection after systemic administration via intraperitoneal injection and showed that specific cells are infected within the first hours. We provide evidence that MCMV traffics as free virus from the peritoneal cavity into the mediastinal lymphatics, providing access to the bloodstream. We demonstrate that MCMV productively infected CD169(+) subcapsular sinus macrophages in the mediastinal lymph nodes, ER-TR7(+) CD29(+) reticular fibroblasts in the spleen and hepatocytes. Infection in the spleen followed a distinctive pattern, beginning in the marginal zone at 6 h and spreading into the red pulp by 17 h. By 48 h after infection, there was widespread infection in the spleen and liver with degeneration of infected cells. In addition, infected dendritic cells appeared in the white pulp of the spleen at 48 h post-infection. On the other hand, cowpox virus showed a different pattern of infectivity in the spleen and liver. Thus, early MCMV infection produces a distinct pattern of infection of selective cells.

Host and donor immune responses contribute to antiviral effects of amotosalen-treated donor lymphocytes following early posttransplant cytomegalovirus infection

We have previously shown that amotosalen-treated splenocytes rescued allorecipients from a lethal dose of mouse CMV (MCMV) administered on day 0 in experimental parent C57BL/6-->CB6F1 allogeneic bone marrow transplant. In this study, we investigated the mechanism of antiviral activity of amotosalen-treated donor splenocytes when sublethal MCMV infections were administered 7 days posttransplant. Recipients of 3 x 10(6) untreated splenocytes were used as control. Following MCMV infection, recipients of untreated splenocytes had 40% early mortality due to acute graft-vs-host disease compared with no deaths among recipients of 10 x 10(6) treated splenocytes. However, recipients of both types of donor splenocytes effectively cleared MCMV from their liver. Like the untreated CD8(+) T cells, amotosalen-treated CD8(+) T cells equally retained their in vivo CTL activity against MCMV early peptide-pulsed targets and expressed similar levels of granzyme B within 11 days of infection. In contrast to full donor chimerism in recipients of untreated splenocytes, recipients of amotosalen-treated splenocytes showed mixed chimerism with both donor spleen- and host-derived anti-MCMV CD8(+) T cells in their blood and lymphoid organs, with significantly higher numbers of host-derived CD4(-)CD8(-) (double negative) T cells in the spleens of recipients of treated splenocytes compared with the recipients of untreated splenocytes. Additionally, recipients of amotosalen-treated splenocytes had lower levels of serum IFN-gamma and TNF-alpha in response to MCMV infection compared with untreated recipients. Thus, adoptive immunotherapy with treated T cells is a novel therapeutic approach that facilitates hematopoietic engraftment and permits antiviral immunity of both donor and host T cells without graft-vs-host disease.

Cytokine and chemokine networks: pathways to antiviral defense

The complex interplays between cytokines and chemokines are emerging as key communication signals in the shaping of innate and adaptive immune responses against foreign pathogens, including viruses. In particular, the virus-induced expression of cytokine and chemokine profiles drives the recruitment and activation of immune effector cells to sites of tissue infection. Under the conditions of infection with murine cytomegalovirus (MCMV), a herpesvirus with pathogenic potential, early immune functions are essential in the control of virus replication and virus-induced pathology. The coordinated MCMV-induced cytokine and chemokine responses promote effective natural killer (NK) cell recruitment and function, and ultimately MCMV clearance. The studies highlighted in this chapter illustrate in vivo pathways mediated by innate cytokines in regulating chemokine responses that are vital for localized antiviral defenses.

Monocyte chemoattractant protein-1 and CCR2 interactions are required for IFN-alpha/beta-induced inflammatory responses and antiviral defense in liver

IFN-alpha/beta-mediated functions promote production of MIP-1alpha (or CCL3) by mediating the recruitment of MIP-1alpha-producing macrophages to the liver during early infection with murine CMV. These responses are essential for induction of NK cell inflammation and IFN-gamma delivery to support effective control of local infection. Nevertheless, it remains to be established if additional chemokine functions are regulated by IFN-alpha/beta and/or play intermediary roles in supporting macrophage trafficking. The chemokine MCP-1 (or CCL2) plays a distinctive role in the recruitment of macrophages by predominantly stimulating the CCR2 chemokine receptor. Here, we examine the roles of MCP-1 and CCR2 during murine CMV infection in liver. MCP-1 production preceded that of MIP-1alpha during infection and was dependent on IFN-alpha/beta effects for induction. Resident F4/80(+) liver leukocytes were identified as primary IFN-alpha/beta responders and major producers of MCP-1. Moreover, MCP-1 deficiency was associated with a dramatic reduction in the accumulation of macrophages and NK cells, as well as decreased production of MIP-1alpha and IFN-gamma in liver. These responses were also markedly impaired in mice with a targeted disruption of CCR2. Furthermore, MCP-1- and CCR2-deficient mice exhibited increased viral titers and elevated expression of the liver enzyme alanine aminotransferase in serum. These mice also had widespread virus-induced liver pathology and succumbed to infection. Collectively, these results establish MCP-1 and CCR2 interactions as factors promoting early liver inflammatory responses and define a mechanism for innate cytokines in regulation of chemokine functions critical for effective localized antiviral defenses.

Calling in the troops: regulation of inflammatory cell trafficking through innate cytokine/chemokine networks

The recruitment of immune effector cells to localized sites of infection is crucial for the effective delivery of innate immune mechanisms. Under the conditions of infections with murine cytomegalovirus (MCMV), a herpesvirus with pathogenic potential, early immune functions are essential in the control of virus replication and virus-induced pathology. Our studies have demonstrated that the chemokine macrophage inflammatory protein-1alpha (MIP-1alpha) is critical for natural killer (NK) cell inflammation and delivery of interferon (IFN)-gamma to mediate downstream protective responses against MCMV infection in liver. Moreover, IFN-alpha/beta-dependent mechanisms promote MIP-1alpha production and subsequently the accumulation of NK cells in liver. Taken together, the studies highlighted in this review define a unique in vivo pathway mediated by innate cytokines in regulating chemokine responses that are essential in the promotion of NK cell inflammation for localized antiviral defense. In addition, the downstream consequences of these events in enhancing endogenous adaptive immune responses will also be discussed. Overall, the innate cytokine/chemokine networks that are described emphasize the emerging importance of chemokine functions for protective immune responses during infection with viruses.

Pathogenesis of murine cytomegalovirus infection

Infection of mice with murine cytomegalovirus (MCMV) is an established model for studying human cytomegalovirus (HCMV) infection. Similarly to HCMV infection, pathological changes and disease manifestations during MCMV infection are mainly dependent on the immune status of the mouse host. This review focuses mainly on the pathogenesis of MCMV infection in immunocompetent and immunodeficient and/or immature mice and discusses the principles of immunosurveillance of infection and the mechanisms by which this virus evades immune control.

An animal model of neonatal cytomegalovirus infection

Cytomegalovirus (CMV) is the most common cause of congenital infection in the developed world and can lead to a life-threatening disease. We therefore developed an animal model to evaluate candidate anti-CMV drugs and to further define the pathogenesis of CMV infections. Newborn guinea pigs were infected by intraperitoneal administration of 10(6) pfu of a virulent salivary gland (SG) passaged guinea pig CMV (gpCMV) within 48 h of birth. Inoculation of animals produced 50% overall mortality. A lack of weight gain was also a hallmark of infection. By day 14 after inoculation the weight of gpCMV-infected animals was significantly less than controls (152.9+/-45 g versus 254.7+/-38.5 g, P<0.0001). The most consistent isolation and highest titers of virus were found in the liver and spleen early while lung titers were maximal at day 10. A quantitative competitive PCR (qcPCR) assay confirmed the presence of a high CMV viral load in infected organs. Antiviral treatment with cyclic HPMPC (cHPMPC) for 7 days significantly reduced mortality (1/20 versus 14/20, P<0.001) and viral replication but did not improve weight gain. This model should be useful for further evaluations of the pathogenesis of CMV infections and for evaluation of antiviral drugs and vaccines.

Loss of the perforin cytotoxic pathway predisposes mice to experimental cytomegalovirus retinitis

AIDS-related human cytomegalovirus (HCMV) retinitis continues to be a chronic ophthalmologic problem among human immunodeficiency virus type 1 (HIV-1)-infected patients who do not respond to highly active antiretroviral therapy. Although HCMV retinitis occurs during HIV-1-induced immunosuppression, the precise effector mechanism(s) that fails during the immunopathogenesis of AIDS to allow onset and progression of HCMV retinal disease remains unclear. We therefore performed a series of experiments to explore the relative roles of distinct pathways of lymphocyte-mediated cytotoxicity in either resistance or susceptibility to experimental murine cytomegalovirus (MCMV) retinitis in mice. Whereas mutant C57BL/6 mice deficient in the Fas/FasL cytotoxic pathway (gld mice) were identical to normal C57BL/6 mice and exhibited absolute resistance to retinal necrosis following subretinal MCMV inoculation, knockout C57BL/6 mice deficient in the perforin cytotoxic pathway (PKO mice) were susceptible to MCMV retinitis. Susceptibility of PKO mice to MCMV retinitis correlated with increased ocular MCMV titers when compared with ocular MCMV titers of gld and normal mice. Since mice with retrovirus-induced immunodeficiency syndrome (MAIDS) exhibited a frequency and severity of MCMV retinitis that were equivalent to those observed in PKO mice, we hypothesized that susceptibility to MCMV retinitis during MAIDS correlates with a decrease in the perforin cytotoxic pathway. To test this hypothesis, we developed a quantitative competitive reverse transcription-PCR assay to measure mouse perforin mRNA levels in the splenic T lymphocytes and MCMV-inoculated eyes of normal mice or mice with MAIDS. Perforin mRNA levels in splenic T lymphocytes were significantly decreased during MAIDS, by approximately 100-fold, from perforin mRNA levels in normal mice. Moreover, MCMV-inoculated eyes destined to develop retinitis during MAIDS also showed a significant decrease in perforin mRNA levels from the perforin mRNA levels of MCMV-inoculated eyes of normal mice destined to be resistant to retinitis. As expected, perforin mRNA could not be detected in unmanipulated and uninfected eyes of normal mice. These results provide the first evidence that the perforin cytotoxic pathway is more important than the Fas/FasL cytotoxic pathway in providing resistance to experimental MCMV retinitis and that loss of the perforin cytotoxic pathway predisposes to MCMV retinitis.

Innate immunity to cytomegalovirus: the Cmv1 locus and its role in natural killer cell function

The identification and characterization of genetic loci that contribute to patterns of susceptibility/resistance to infection provide important insights into the mechanisms of innate and adaptive immunity. Genetic heterogeneity across the population makes the characterization of such traits in humans technically difficult; however, inbred animal models represent an ideal tool for such analyses. This review illustrates the power of mouse genetics as utilized for the identification and characterization of the locus conferring early resistance to murine cytomegalovirus infection, Cmv1. This locus encodes an activating C-type lectin receptor of the Ly49 family that promotes natural killer (NK) cell cytolysis of infected cells. Although NK cells are usually able to detect and destroy virally infected cells via recognition of the downregulation of MHC class I molecules, the Cmv1 locus provides the first example of an NK receptor that is able to mediate clearance of viral infection via direct recognition of a virally encoded protein.

Genetic control of innate immune responses against cytomegalovirus: MCMV meets its match

Cytomegalovirus (CMV) is a widespread pathogen that is responsible for severe disease in immunocompromised individuals and probably, associated with vascular disease in the general population. There is increasing evidence that cells of the innate immune system play a key role in controlling this important pathogen. This is particularly evident in the experimental murine CMV (MCMV) model of infection which has revealed an important role for natural killer (NK) cells in controlling early viral replication after infection with MCMV. In this model, different strains of inbred mice exhibit striking differences in their level of susceptibility to MCMV infection. Genetic studies, performed almost 10 years ago, revealed that this pattern of susceptibility/resistance can be attributed to a single genetic locus termed Cmv1 and recently several groups that have been working on the mapping and identification of Cmv1 have met with success. Interestingly, Cmv1 is allelic to a member of the Ly49 gene family, which encode activating or inhibitory transmembrane receptors present on the surface of NK cells. All Ly49 receptors characterized to date interact with MHC class I molecules on potential target cells, resulting in the accumulation of signals to the NK to either 'kill' or 'ignore' the cell based upon the repertoire of MHC class I molecules expressed. The identification of Cmv1 as Ly49H, a stimulatory member of the Ly49 family, adds an interesting twist to the Ly49 story. Although the ligand of Ly49H is not yet known, there is already compelling evidence that the ligand is upregulated on virally infected cells, resulting in specific activation of Ly49H-expressing NK cells. This review provides an historical perspective of the MCMV infection model from its inception to the discovery of the gene responsible for the phenotype and provides a basis for further experiments aimed at understanding the role of NK cells, in general, and Ly49H, in particular, in mediating resistance to cytomegalovirus.

Experimental study of mouse cytomegalovirus infected mice

In order to investigate the human cytomegalovirus (HCMV) infection, the mouse cytomegalovirus (MCMV) infected mice were experimentally studied. 6 to 8 week old female BALB/C mice with immunosuppression were selected to undergo the MCMV inoculations: intracranial inoculation and peritoneal inoculation. MCMV of the infected mice in various organs and tissues were detected by using beta-gal staining and in situ nucleic acid hybridization assay. The pathological changes were observed in HE staining paraffin-embedded sections. It was found that all the MCMV infected mice showed the retardation of growth and development, and feather looseness. Both intracranial inoculation of 10(4) PFU viruses or peritoneal inoculation of 10(6) PFU viruses resulted in the pathological changes, to some extent, of various organs and tissues in the mice. The pathological changes in liver were consistent with the amount of beta-gal staining positive cells, indicating the liver lesions were mainly caused by viral proliferation. It was also found that the viruses in the immunosuppressed mice subjected to intracranial inoculation could spread to whole body organs, while the viruses in the immunosuppressed mice subjected to intrapeitoneal inoculation couldn't spread to the brain, suggesting blood-brain barrier could prevent the virus from spreading to the brain.

Acute murine cytomegalovirus infection: a model for determining antiviral activity against CMV induced hepatitis

Acute intraperitoneal infection of weanling BALB/c mice with murine cytomegalovirus (MCMV) resulted in an inoculum titer-dependent weight loss, mortality and elevation of plasma transaminases (ALT: alanine transaminase and AST: aspartate transaminase). Three days post infection (p.i.) with 10(4.85) plaque forming units (pfu) there was 90% mortality with a mean death day p.i. of 4.1 +/- 0.2. Plasma levels of ALT and AST were elevated 24- and 15-fold, respectively. Organ titers of virus (log10 pfu/g tissue) were 6.16 in the liver, 6.05 in the spleen, 4.0-4.7 in the lung, heart, kidney and intestine and undetectable in the muscle and brain. Organ concentrations (units/g wet-weight) of ALT were highest in the liver, whilst for AST the highest levels were found in the heart. The concentrations of ALT but not AST were reduced (35-55%) in the infected liver; the concentrations of ALT and AST were not changed in other infected organs. There were excellent correlations (r > 0.95) between viral titers in the liver, increases of plasma ALT and depletion of liver ALT. HPMPC and ganciclovir administered either p.o. or s.c. reduced mortality, increases in plasma transaminases and viral burdens in the liver and prevented depletion of liver ALT. HPMPC was approximately 10-fold more potent than ganciclovir. These results strongly suggest that intraperitoneal infection of the BALB/c mouse with MCMV represents an animal model of CMV hepatitis that can be monitored by measuring plasma ALT.

The murine cytomegalovirus M25 open reading frame encodes a component of the tegument

The murine cytomegalovirus (MCMV) monoclonal antibody 5C7:6 was used in Western analysis to probe MCMV infected murine embryo cells (MEC). This antibody recognizes three virus specific polypeptides of 130, 105, and 95 kDa and pulse-chase experiments demonstrated that these three proteins, although antigenically related, are distinct. The 105- and 95-kDa species were expressed with early kinetics, whereas the 130-kDa protein was synthesized as a true late. By screening a lambdagt11 MCMV cDNA library, the gene encoding these proteins was identified as the M25 open reading frame previously reported by Dallas et al. (Dallas, P. B., Lyons, P. A., Hudson, J. B., Scalzo, A. A., and Shellam, G. R., 1994, Virology 200, 643-650). Immunofluorescent studies monitored the location of pM25, present in the nucleus at 15 h after infection, condensing around the periphery of the nucleus at 18 h, before finally accumulating in the cytoplasm. Immunoelectron microscopy detected gold particles associated with the viral tegument of enveloped virions located in the cytoplasm and extracellular space but not with naked nucleocapsids. Western analysis of MCMV purified virions depicted the presence of the 130-kDa protein, the predominant M25 species, in mature virus particles. Together these findings provide compelling evidence that the 130-kDa M25 polypeptide is a component of the viral tegument.

Factors influencing the effects of murine cytomegalovirus on the pancreas

BACKGROUND

As human cytomegalovirus (HCMV) infections are implicated in insulin-dependent diabetes mellitus (IDDM), the effects of murine (M)CMV infection of inbred mice on the pancreas are of interest.

RESULTS

Inflammation and periacinar oedema peaked on day 3 and were replaced by a focal inflammation, but infected cells were rare. The islets were spared in C57BL mice. Insulitis normally seen in non-obese diabetic (NOD) mice was accelerated, but infected NOD mice did not become glycosuric. Isotypes of total and autoreactive antibodies suggested a shift to a Th 1 response (IgG2a) in all MCMV-infected mice. MCMV-induced pancreatitis was not affected by MHC genes but was similar or less severe in BALB/c mice. As these lack the Cmv1 gene, which provides a protective natural killer (NK) cell response in C57BL congenic mice, the C57BL background may carry a pancreatitis susceptibility gene able to counter NK-mediated restriction of viral replication. Consistently, congenic mice expressing Cmvl on a BALB/c background did not display pancreatitis, unless depleted of NK cells. In vivo treatment with soluble cytokine receptors suggested that interleukin 1 (IL-1) and/or tumour necrosis factor alpha contribute to acinar necrosis in C57BL mice.

Cytomegalovirus inhibits the engraftment of donor bone marrow cells by downregulation of hemopoietin gene expression in recipient stroma

Cytomegalovirus (CMV) disease after bone marrow (BM) transplantation is often associated with BM graft failure. There are two possible reasons for such a correlation. First, a poor hematopoietic reconstitution of unrelated etiology could promote the progression of CMV infection by the lack of immune control. Alternatively, CMV infection could interfere with the engraftment of donor BM cells in recipient BM stroma. Evidence for a causative role of CMV in BM aplasia came from studies in long-term BM cultures and from the murine in vivo model of CMV-induced aplastic anemia. A deficiency in the expression of essential stromal hemopoietins, such as stem cell factor (SCF), has indicated a functional insufficiency of the stromal microenvironment. It remained open to question whether CMV mediates a negative regulation of hemopoietin gene expression (the downregulation model) or whether it causes the default of a positive regulator (the lack-of-induction model). Further, even though implicitly assumed, it has never been formally documented that CMV directly interferes with the engraftment of a BM cell transplant. We addressed these problems in a murine model of CMV infection after experimental male-into-female BM transplantation. The data indicate that the downregulation model applies. Quantitation of the male-sex-determining gene tdy demonstrated an impaired engraftment of donor BM cells in the BM stroma of the female recipients. This graft failure was reflected by a diminished population of SCF-receptor-expressing hematopoietic progenitor cells and correlated with a reduced level of stromal SCF gene expression. Interestingly, high doses of BM cells protected against stromal insufficiency by a mechanism unrelated to control of infection.

Inhibition of hepatitis B virus replication during adenovirus and cytomegalovirus infections in transgenic mice

We have previously demonstrated that hepatitis B virus (HBV) replication and gene expression are abolished in the livers of HBV transgenic mice by cytotoxic T lymphocytes (CTLs) and during lymphocytic choriomeningitis virus (LCMV) infection, stimuli that trigger the production of alpha/beta interferon, gamma interferon, and tumor necrosis factor alpha in the liver. We now report that hepatic HBV replication and gene expression are inhibited by the local induction of these cytokines during adenovirus- and murine cytomegalovirus (MCMV)-induced hepatitis. Further, we show that MCMV also blocks HBV replication and gene expression in the proximal convoluted tubules of the kidney by causing interstitial nephritis and inducing the same cytokines in the renal parenchyma. These results suggest that inflammatory cytokines probably contribute to viral clearance during acute viral hepatitis in humans, and they imply that induction of these cytokines in the liver and other infected tissues of chronically infected patients might have therapeutic value.

Early murine cytomegalovirus (MCMV) infection induces liver natural killer (NK) cell inflammation and protection through macrophage inflammatory protein 1alpha (MIP-1alpha)-dependent pathways

Natural killer (NK) cells mediate defense against early murine cytomegalovirus (MCMV) infections in liver. The chemokine, macrophage inflammatory protein 1alpha (MIP-1alpha), can promote inflammatory responses. Our studies evaluated contributions of NK cells to early MCMV-induced liver inflammation and MIP-1alpha requirements for inflammation and delivery of antiviral defenses. NK cells were shown to be responsible for focal inflammation, and to be induced to migrate at high levels, in MCMV-infected livers. MIP-1alpha gene expression was elevated at coinciding times, and mice deficient in MIP-1alpha function were dramatically inhibited in both inflammatory and protective liver responses. The results precisely define MIP-1alpha-dependent steps required to achieve NK cell inflammation during, and mechanisms promoting defense against, viral infections in tissues.

Mechanisms for virus-induced liver disease: tumor necrosis factor-mediated pathology independent of natural killer and T cells during murine cytomegalovirus infection

The contribution of endogenous NK cells and cytokines to virus-induced liver pathology was evaluated during murine cytomegalovirus infections of mice. In immunocompetent C57BL/6 mice, the virus induced a self-limited liver disease characterized by hepatitis, with focal inflammation, and large grossly visible subcapsular necrotic foci. The inflammatory foci were most numerous and contained the greatest number of cells 3 days after infection; they colocalized with areas of viral antigen expression. The largest number of necrotic foci was found 2 days after infection. Overall hepatic damage, assessed as increased expression of liver enzymes in serum, accompanied the development of inflammatory and necrotic foci. Experiments with neutralizing antibodies demonstrated that although virus-induced tumor necrosis factor (TNF) can have antiviral effects, it also mediated significant liver pathology. TNF was required for development of hepatic necrotic foci and increased levels of liver enzymes in serum but not for increased numbers of inflammatory foci. The necrotic foci and liver enzyme indications of pathology occurred independently of NK and T cells, because mice rendered NK-cell deficient by treatment with antibodies, T- and B-cell-deficient Rag-/- mice, and NK- and T-cell-deficient E26 mice all manifested both parameters of disease. Development of necrotic foci and maximally increased levels of liver enzymes in serum also were TNF dependent in NK-cell-deficient mice. Moreover, in the immunodeficient E26 mice, virus-induced liver disease was progressive, with eventual death of the host, and neutralization of TNF significantly increased longevity. These results establish conditions separating hepatitis from significant liver damage and demonstrate a cytokine-mediated component to viral pathogenesis.

Characterization of early cytokine responses and an interleukin (IL)-6-dependent pathway of endogenous glucocorticoid induction during murine cytomegalovirus infection

Early infection with murine cytomegalovirus (MCMV) induces circulating levels of interleukin (IL)-12, interferon (IFN)-gamma, and tumor necrosis factor (TNF). Studies presented here further characterize these responses by defining kinetics and extending evaluation to include IL-1, IL-6, and glucocorticoids. IL-12 p40, IFN-gamma, TNF, IL-1alpha, and IL-6 were shown to be increased, but IL-1beta was undetectable, in serum of MCMV-infected mice. The IL-12 p40, IFN-gamma, TNF, and IL-6 responses were dramatic with peak levels reaching >150-10,000 pg/ml at 32-40 h after infection and rapidly declining thereafter. Glucocorticoid induction, peaking at 36 h and reaching 30-fold increases above control values, accompanied the cytokine responses. Mice with cytokine deficiencies or neutralized cytokine function demonstrated that IL-6 was the pivotal mediator of the glucocorticoid response, with IL-1 contributing to IL-6 production. The IL-6 requirement appeared to be specific for virus-type stimuli as the synthetic analogue of viral nucleic acid, polyinosinic-polycytidylic acid, also induced IL-6-dependent glucocorticoid release, but treatments with the bacterial product lipopolysaccharide and a non-immune physical restraint stressor elicited IL-6-independent responses. Collectively, the results identify IL-6 as a primary mediator of glucocorticoid induction, and elucidate specific pathways of interactions between immune and neuroendocrine systems during viral infection.

Requirement for natural killer cell-produced interferon gamma in defense against murine cytomegalovirus infection and enhancement of this defense pathway by interleukin 12 administration

The presence of natural killer (NK) cells contributes to early defense against murine cytomegalovirus (MCMV) infection. Although NK cells can mediate in vivo protection against MCMV, the mechanism by which they do so has not been defined. The studies presented here evaluate cytokine production by NK cells activated during MCMV infection and the role of NK cell-produced cytokines in early in vivo antiviral defenses. Experiments with normal C57BL/6, T cell-deficient C57BL/6 nude, and severe combined immunodeficient mice lacking T and B cells demonstrated that both interferon gamma (IFN-gamma) and tumor necrosis factor (TNF) production were induced at early times after infection with MCMV. Conditioned media samples prepared with cells from these mice, on day 2 after infection, produced 11-43 pg/million cells of IFN-gamma and 12-19 pg/million cells of TNF as evaluated by specific protein enzyme-linked immunosorbent assays. Studies in the NK- and T cell-deficient mouse line, E26, in mice that had been depleted in vivo of NK cells by treatment with antibodies eliminating NK cells, anti-asialo ganglio-N-tetraosylceramide or anti-NK1.1, and with populations of cells that had been depleted of NK cells by complement treatment with the anti-NK cell antibody, SW3A4, demonstrated that NK cells were solely responsible for the IFN-gamma but were not required for TNF production. The in vivo absence of NK cells was accompanied by increased viral hepatitis and viral replication in both immunocompetent and immunodeficient mice, as well as decreased survival time of immunodeficient mice. In vivo treatments with antibodies neutralizing IFN-gamma demonstrated that this factor contributed to the NK cell-mediated antiviral defense and reduced the measured parameters of viral defense to levels indistinguishable from those observed in NK cell-deficient mice. These effects appeared to be independent of cytolytic activity, as NK cells isolated from anti-IFN-gamma-treated mice mediated killing at levels comparable to those observed in control-treated mice. The consequences of interleukin 12 (IL-12) administration, a known potent inducer of IFN-gamma production by NK cells, were evaluated in MCMV-infected mice. Low IL-12 doses, i.e., 1 ng/d, increased NK cell cytotoxicity and IFN-gamma production up to twofold and resulted in improved antiviral status; virus-induced hepatitis was decreased as much as fivefold, and viral burdens were decreased to levels below detection.(ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral blood mononuclear phagocytes mediate dissemination of murine cytomegalovirus

Cytomegalovirus is transmitted with blood and organs from seropositive individuals, although the particular leukocyte population harboring latent or persistent virus remains poorly characterized. Murine cytomegalovirus, tagged with the Escherichia coli lacZ gene, was used to identify cells in which virus replicates during acute infection of immunocompetent mice. Recombinant murine cytomegaloviruses, RM461, RM460, and RM427, were constructed to express beta-galactosidase under control of the human cytomegalovirus ie1/ie2 promoter/enhancer. The lacZ gene was inserted between the ie2 and sgg1 genes in RM461 and RM460, disrupting a 0.85-kb late transcript that was found to be dispensable for replication in cultured cells as well as for infection of mice. In BALB/c mice, lacZ-tagged and wild-type viruses exhibited a similar 50% lethal dose and all had the capacity to latently infect the spleen. Peripheral blood mononuclear phagocytes were the major infected leukocyte cell type, as demonstrated by the ability of infected cells to adhere to glass and to phagocytize latex beads; however, these cells did not exhibit typical monocyte markers. Plaque assay for virus and 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining of frozen sections of organs from infected mice revealed that the major target organs included the spleen, adrenal glands, liver, and salivary glands, although tissues as diverse as brown fat and lungs were also involved. Individual blue-staining cells were readily identified in all infected tissues. These studies identified a mononuclear phagocyte, possibly a macrophage or dendritic cell precursor, as the vehicle of virus dissemination during acute infection, and demonstrate the utility of using lacZ-tagged murine cytomegalovirus for tropism, pathogenesis, and latency studies.

Cytomegalovirus and latency: an overview

Cytomegalovirus (CMV) is a member of the herpes virus group. Infection results in a variety of disorders which depend largely on the immune status of the host. A well known property of CMV is that after primary infection the virus persists in the body of the host resulting in latency. Severe immunodepression or immunodeficiency can cause reactivation of the virus from its latent state, leading to endogenous reinfection. In contrast to other herpes viruses, such as herpes simplex virus which persists in neurons, and Epstein Barr virus which persists in B lymphocytes, little is known about the localization of latent CMV. In order to obtain more insight in the organ or cell type serving as a reservoir for latent CMV, it is important to know more about the course of natural infection and the cells and organs involved. When more information is available about the localization of latent virus, studies concerning the physical state of viral DNA or the extent of viral transcription and/or translation will follow in the near future. In this review some properties of the epidemiology and transmission of human CMV, as well as data about acute infection will be given. In addition, some characteristics of the localization of latent CMV and the physical state of the virus will be discussed. Where necessary, particularly regarding insight into CMV-host interactions, knowledge of animal, particularly murine, rat and guinea pig CMV infections, will be discussed.