In vitro and in vivo characterization of a murine cytomegalovirus with a mutation at open reading frame m166

We have recently generated a pool of murine cytomegalovirus (MCMV) mutants by using a Tn3-based transposon mutagenesis approach. In this study, one of the mutants, Rvm166, which contained the transposon sequence at open reading frame m166, was characterized both in tissue culture and in immunocompetent BALB/c mice and immunodeficient SCID mice. The viral mutant replicated as well as the wild-type Smith strain in vitro in NIH 3T3 cells, whereas the transposon insertion precluded the expression of >65% of the m166 open reading frame. Compared to the wild-type strain and a rescued virus that restored the m166 region, the viral mutant was significantly attenuated in growth in both BALB/c and SCID mice that were intraperitoneally infected with the viruses. At 21 days postinfection, the titers of the viral mutant in the salivary glands, lungs, spleens, livers, and kidneys of the infected SCID mice were lower than the titers of the Smith strain and the rescued virus by about 30000-, 10000-, 1000-, 300-, and 800-fold, respectively. Moreover, the virulence of the mutant virus appears to be severely attenuated because no death was found in SCID mice infected with the viral mutant up to 90 days postinfection, whereas all of the animals infected with the wild-type and rescued viruses died at 27 days postinfection. Our results suggest that m166 probably encodes a virulence factor and is required for MCMV virulence in killing SCID mice and for optimal viral growth in vivo.

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.

An essential role of the enhancer for murine cytomegalovirus in vivo growth and pathogenesis

The transcription of cytomegalovirus (CMV) immediate-early (IE) genes is regulated by a large and complex enhancer containing an array of binding sites for a variety of cellular transcription factors. Previously, using bacterial artificial chromosome recombinants of the virus genome, it was reported that the enhancer region of murine CMV (MCMV) is dispensable but performs a key function for viral multiplication (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998). In the present study, we defined, through the reconstitution of infectious enhancerless MCMVs, the growth requirement for the enhancer in tissue culture and explored its significance for steering a productive infection in vivo. A comparison of cis and trans complementation systems for infection of enhancerless virus in permissive fibroblasts revealed a multiplicity-dependent growth phenotype that is severely compromised in the rate of infectious-virus multiplication. The in vivo impact of viruses that have an amputated enhancer was investigated in an extremely sensitive model of MCMV infection, the SCID mouse. Histological examination of spleens, livers, lungs, and salivary glands from animals infected with enhancer-deficient MCMV demonstrated an absence of tissue damage associated with CMV infection. The lack of pathogenic lesions correlated with a defect in replication competence. Enhancerless viruses were not detectable in major target organs harvested from SCID mice. The pathogenesis and growth defect reverted upon restoration of the enhancer. Markedly, while SCID mice infected with 5 PFU of parental MCMV died within 50 days postinfection, all mice infected with enhancerless virus survived for the duration of the experiment (1 year) after infection with 5 x 10(5) PFU. Together, these results clarify the importance of the enhancer for MCMV growth in cell culture and underscore the in vivo significance of this region for MCMV virulence and pathogenesis.

High-frequency interferon-gamma-secreting splenocytes specific for murine cytomegalovirus immediate-early-1 (IE-1) peptide 168YPHFMPTNL176 are insufficient to provide complete protection from viral challenge

Infection of Balb/c mice with murine cytomegalovirus (MCMV) has been used extensively as a model system with which to study host mechanisms of immunity to cytomegaloviruses. In this model, the cytotoxic T-lymphocyte (CTL) response crucial for clearing infected cells is dominated by CTLs specific for the MCMV nonapeptide 168YPHFMPTNL176 encoded by the immediate-early 1 (IE-1) gene. The intradermal injection of plasmid pcDNA89 encoding IE-1 has been shown to offer some protection from viral challenge. In the present studies, the protective efficacy of immunisation with pcDNA89 given by intradermal injection was compared with particle-mediated DNA delivery (PMDD) and contrasted with that induced by injection with the K181 MCMV strain and with temperature-sensitive mutants (tsm) derived from the K181 strain. Modest protection was afforded by pcDNA89 immunisation given by PMDD, but none was observed after intradermal injection. PMDD immunisation induced a frequency of 168YPHFMPTNL176-specific interferon-gamma (IFN-gamma)-secreting splenocytes, which was equivalent to that after K181 infection and significantly higher than tsm immunisation. Whereas tsm-immunised mice were completely protected from MCMV challenge, PMDD-immunised mice were only weakly protected. Tsm immunisation protected mice completely against challenge with natural isolates having sequence variation in the IE-1 nonapeptide, while PMDD-immunised mice were weakly protected from isolates encoding 168YPHFMPTNL176 and were not protected against isolates encoding 168YPHFMPPSL176 or 168YLDFMPPNL176. Thus, while IE-1-specific IFN-gamma-secreting splenocytes do contribute to immunity from MCMV challenge, their presence in isolation is insufficient to provide complete protection and they may not be involved in the protection observed against MCMV isolates having IE-1 sequence variation.

Human cytomegalovirus infection induces cellular thymidylate synthase gene expression in quiescent fibroblasts

Productive infection of non-proliferating cells by cytomegalovirus (CMV) requires the coordinated stimulation of host biochemical pathways that prepare cells to synthesize DNA. Here we illustrate the ability of human CMV (HCMV) to stimulate cellular thymidylate synthase (TS) gene expression in quiescent human embryonic lung fibroblasts. TS mRNA and protein levels are nearly undetectable in quiescent cells, but are greatly increased following HCMV infection. Inhibition of TS activity was shown to impair HCMV DNA synthesis, demonstrating that TS upregulation is required for efficient HCMV replication in quiescent cells. The increase in TS gene expression was due to an increase in gene transcription, since the expression of a reporter gene driven by the human TS promoter was strongly induced by HCMV infection. Deletion analysis of the human TS promoter identified two positive elements that are important for this increased transcription. We have previously shown that murine CMV (MCMV) stimulates the mouse TS promoter by a mechanism that depends on the presence of an E2F element in the promoter region. However, deletion of the two potential E2F binding sites in the human TS promoter did not prevent the virus-induced increase in TS promoter activity. Our data suggest that HCMV activates human TS gene transcription by mechanisms that are independent of E2F and different from those used by MCMV to stimulate the mouse TS promoter.

Successful control of viruses by NK cells--a balance of opposing forces?

Natural killer (NK) cells play a crucial role in limiting the severity of diseases caused by a range of viruses. Recent data have shown that the effector functions of NK cells can be specifically stimulated when NK cell activation receptors engage cellular major histocompatibility complex (MHC) class I-like ligands induced after infection or by specific viral gene products. However, to counter this NK cell response viruses have evolved an array of strategies to subvert efficient NK cell activation. These data indicate that the balance of host NK cell responses and viral NK cell escape mechanisms can be strategically poised as each strives for survival.

Enhanced cytomegalovirus infection of developing brain independent of the adaptive immune system

Cytomegalovirus (CMV) has been suggested as the most prevalent infectious agent causing neurological dysfunction in the developing brain; in contrast, CMV infections are rare in the adult brain. One explanation generally given for the developmental susceptibility to the virus is that the developing immune system is too immature to protect the central nervous system from viral infection, but as the immune system develops it can protect the brain. We suggest an alternate view: that developing brain cells are inherently more susceptible to CMV infection, independent of the immune system. We used a recombinant mouse CMV that leads to green fluorescent protein expression in infected cells. Control experiments demonstrated a high correlation between the number of cells detected with the viral GFP reporter gene and with immunocytochemical detection of the virus. After intracerebral inoculation, the number of CMV-infected cells in neonatal brains was many times greater than in mature control or mature immunodepressed SCID mice, and the mortality rate of neonates was substantially greater than SCID or control adults. Parallel experiments with live brain slices inoculated in vitro, done in the absence of the systemic immune system, generated similar data, with immature hippocampus, hypothalamus, cortex, striatum, and cerebellum showing substantially greater numbers of infected cells (100-fold) than found in adult slices in these same regions. Interestingly, in the cerebellar cortex, CMV-infected cells were more prevalent in the postmitotic Purkinje cell layer than in the mitotic granule cell layer, suggesting a selective infection of some cell types not dependent on cell division. Together, these data support the view that CMV has an intrinsic preference for infection of developing brain cells, independent, but not mutually exclusive, of the developmental status of the systemic immune system in controlling CMV infection.

Genetic analyses of gene function and pathogenesis of murine cytomegalovirus by transposon-mediated mutagenesis

Murine cytomegalovirus (MCMV) has a linear genome of 230 kb and encodes more than 170 genes, many of which have not been extensively studied for their functions in pathogenesis in vivo. A Tn3-based transposon was constructed and used to generate MCMV mutants by disrupting viral gene targets. The functions of the mutated genes were investigated by studying the viral mutants in cultured cells and in immunocompetent Balb/c and immunodeficient SCID mice. A pool of MCMV mutants that contained the transposon sequence randomly inserted at the viral genome was generated. Studies of several mutants (e.g. a viral mutant with the transposon inserted at open reading frame m09) in cultured cells and in mice indicate that the presence of the transposon sequence per se in the viral genome does not significantly affect viral growth in vitro and in vivo. Moreover, the genome structures of the viral mutants, including the transposon insertion regions, were stable during replication in cultured cells and in animals. Several viral mutants (e.g. a viral mutant with the transposon at M27) that are attenuated in growth and virulence in animals were identified. These results suggest that the genes mutated in these viral mutants may be important for viral virulence and pathogenesis. The Tn3-based system may be a useful tool for the systematic construction of CMV mutants and for studies of CMV gene functions in viral replication in vitro and in pathogenesis in vivo.

Apoptosis in the retina during MCMV retinitis in immunosuppressed BALB/c mice

BACKGROUND

Cytomegalovirus (CMV) retinitis is the most common opportunistic ocular infection observed in immunosuppressed (IS) adult and pediatric patients. Due to the species restriction of the cytomegaloviruses, mice infected with murine CMV (MCMV) have been used to study the pathogenesis of CMV retinitis.

OBJECTIVES

The objectives of this study were to determine if retinal glial cells are the targets of MCMV infection and to determine which cells in the retina become apoptotic following inoculation of MCMV via the supraciliary route.

STUDY DESIGN

Adult female BALB/c mice were IS with methylprednisolone; one half of the mice were injected with MCMV and one half of the mice were injected with an equivalent volume of tissue culture medium via the supraciliary route. Animals were sacrificed and frozen sections of eyes were stained for MCMV early antigen, RPE65, CD45 or TUNEL; additional slides were double stained with combinations of the above reagents.

RESULTS AND CONCLUSIONS

The results indicate that most apoptotic cells in the retina were not virus infected, most apoptotic cells were not infiltrating CD45 positive leukocytes, and retinal glial cells were infected with MCMV but only late in infection. Together, these results suggest that retinal cells that undergo apoptosis during MCMV infection are neurons and that apoptosis of uninfected bystander cells is an important component of the pathogenesis of CMV retinitis.

Immunisation of Balb/c mice with severely attenuated murine cytomegalovirus mutants induces protective cellular and humoral immunity

Previously, we showed that two temperature-sensitive mutants of murine cytomegalovirus (tsm5 and tsm30) expressed immediate-early (IE-1), early (E-1), and late (gB) phase genes in the tissues of immunocompetent Balb/c mice, yet failed to produce infectious progeny virus in any tissue at any time at 1-21 days post-infection. Mice inoculated intraperitoneally with tsm5 became latently infected, but this latent virus could not be reactivated as an infectious virus after immunosuppression, although all three transcripts were produced. Immunocompetent mice infected with tsm30 did not become latently infected. In the present study, immunodeficient SCID mice supported productive infection of both mutants, suggesting that low-level viral replication does occur in immunocompetent mice, but that it is limited by the host immune response. This is supported by the observation that immunocompetent mice were protected against virulent K181 challenge even after immunisation with as few as 40 pfu of mutant virus, whereas UV-inactivated mutant or K181 virus was not immunoprotective at doses of 40,000 pfu. Immunity induced by subcutaneous inoculation was also protective, whereas that induced by intragastric immunisation was not. Protection was lifelong (18 months). Although tsm5 induced high antibody titres, there was little evidence of an antibody response to tsm30. In contrast, a significant CD8(+) CTL response to the Balb/c immunodominant IE-1 nonapeptide (YPHFMPTNL) was elicited by both mutants, as determined by an interferon-gamma ELISPOT assay, although this response was lower than that induced by K181 infection. In addition, CTLs specific for m04 (YGPSLYRRF) and M84 (AYAGLFTPL) peptides could be detected at low frequency after K181, tsm5, and tsm30 immunisation. Such protective immunity did not prevent the challenge K181 virus from entering the latent state, but it appeared to reduce the frequency of reactivation.

Mechanisms of human cytomegalovirus persistence and latency

Human cytomegalovirus (HCMV) is a ubiquitous beta-herpesvirus that causes severe disease primarily in immunosuppressed individuals. A major characteristic of HCMV with obvious clinical importance is the ability of the virus to establish lifelong infection within the host following the initial acute infection. One strategy used by HCMV to maintain itself within the host is the establishment of cellular sites of persistent infection and viral latency. Recent studies have identified endothelial cells and monocyte-derived macrophages (MDM) as sites of HCMV persistence and latency. These studies show that endothelial cell origin and MDM differentiation pathway are critical factors that influence the characteristics of HCMV replication in these cell types. The specific HCMV genes involved in endothelial cell and MDM tropism are unknown. However, studies in the closely related murine cytomegalovirus (MCMV) model have provided considerable insight into viral genes that enable replication in these cell types. This review will focus on mechanisms of HCMV replication in endothelial cells and MDM, and on the viral genes involved in regulation of viral replication in these important cell types.

Early gene m18, a novel player in the immune response to murine cytomegalovirus

The identification of all antigenic peptides encoded by a pathogen, its T cell 'immunome', is a research aim for rational vaccine design. Screening of proteome-spanning peptide libraries or computational prediction is used to identify antigenic peptides recognized by CD8 T cells. Based on their high coding capacity, cytomegaloviruses (CMVs) could specify numerous antigenic peptides. Yet, current evidence indicates that the memory CD8 T cell response in a given haplotype is actually focused on a few viral proteins. CMVs actively interfere with antigen processing and presentation by the expression of immune evasion proteins. In the case of murine CMV (mCMV), these proteins are effectual in the early (E) phase of the virus replication cycle and should thus preclude the presentation of peptides derived from E proteins. Notably, the m18 gene is here added to a growing list of mCMV E genes that encode antigenic peptides in spite of the E phase immune evasion strategies of the virus.

Murine cytomegalovirus m02 gene family protects against natural killer cell-mediated immune surveillance

The murine cytomegalovirus m02 gene family encodes putative type I membrane glycoproteins named m02 through m16. A subset of these genes were fused to an epitope tag and cloned into an expression vector. In transfected and murine cytomegalovirus-infected cells, m02, m04, m05, m06, m07, m09, m10, and m12 localized to cytoplasmic structures near the nucleus, whereas m08 and m13 localized to a filamentous structure surrounding the nucleus. Substitution mutants lacking the m02 gene (SMsubm02) or the entire m02 gene family (SMsubm02-16) grew like their wild-type parent in cultured cells. However, whereas SMsubm02 was as pathogenic as the wild-type virus, SMsubm02-16 was markedly less virulent. SMsubm02-16 produced less infectious virus in most organs compared to wild-type virus in BALB/c and C57BL/6J mice, but it replicated to wild-type levels in the organs of immunodeficient gamma(c)/Rag2 mice, lacking multiple cell types including natural killer cells, and in C57BL/6J mice depleted of natural killer cells. These results argue that one or more members of the m02 gene family antagonize natural killer cell-mediated immune surveillance.

The murine cytomegalovirus immune evasion protein m4/gp34 forms biochemically distinct complexes with class I MHC at the cell surface and in a pre-Golgi compartment

We have recently demonstrated that the murine CMV (MCMV) gene m4 is an immune evasion gene that protects MCMV-infected targets from some virus-specific CTL clones. m4 encodes m4/gp34, a 34-kDa glycoprotein that binds to major histocompatibility complex class I in the endoplasmic reticulum and forms a detergent-stable complex that is exported to the surface of the cell. To investigate how m4/gp34 promotes CTL evasion, we analyzed the assembly and export of m4/gp34-K(b) complexes. We found that 50-70% of K(b) exported over the course of MCMV infection was m4/gp34 associated. Because these complexes are present at the cell surface, it is possible that m4 mediates CTL evasion by interfering with contact between class I and receptors on the T cell. In addition, we found that K(b) retained by the MCMV immune evasion gene m152 formed a novel type of complex with Endo H-sensitive m4/gp34; these complexes are distinguished from the exported complexes by being stable in 1% digitonin and unstable in 1% Nonidet P-40. Because this association occurs in a pre-Golgi compartment, m4/gp34 might also interfere with Ag presentation by affecting some aspect of class I assembly, such as peptide loading. Although m4/gp34 requires beta(2)-microglobulin to bind class I, there was no significant binding of m4/gp34 to beta(2)-microglobulin in the absence of class I H chain, demonstrating that m4/gp34 forms Nonidet P-40-stable complexes specifically with folded conformations of class I. We conclude that m4/gp34 promotes immune evasion by a novel mechanism involving altered assembly and/or T cell recognition of class I molecules.

Inactivation of LRG-47 and IRG-47 reveals a family of interferon gamma-inducible genes with essential, pathogen-specific roles in resistance to infection

The cytokine interferon (IFN)-gamma regulates immune clearance of parasitic, bacterial, and viral infections; however, the underlying mechanisms are poorly understood. Recently, a family of IFN-gamma-induced genes has been identified that encode 48-kD GTP-binding proteins that localize to the endoplasmic reticulum of cells. The prototype of this family, IGTP, has been shown to be required for host defense against acute infections with the protozoan parasite Toxoplasma gondii, but not for normal clearance of the bacterium Listeria monocytogenes and murine cytomegalovirus (MCMV). To determine whether other members of the gene family also play important roles in immune defense, we generated mice that lacked expression of the genes LRG-47 and IRG-47, and examined their responses to representative pathogens. After infection with T. gondii, LRG-47-deficient mice succumbed uniformly and rapidly during the acute phase of the infection; in contrast, IRG-47-deficient mice displayed only partially decreased resistance that was not manifested until the chronic phase. After infection with L. monocytogenes, LRG-47-deficient mice exhibited a profound loss of resistance, whereas IRG-47-deficient mice exhibited completely normal resistance. In addition, both strains displayed normal clearance of MCMV. Thus, LRG-47 and IRG-47 have vital, but distinct roles in immune defense against protozoan and bacterial infections.

Products of US22 genes M140 and M141 confer efficient replication of murine cytomegalovirus in macrophages and spleen

Efficient replication of murine cytomegalovirus (MCMV) in macrophages is a prerequisite for optimal growth and spread of the virus in its natural host. Simultaneous deletion of US22 gene family members M139, M140, and M141 results in impaired replication of MCMV in macrophages and mice. In this study, we characterized the proteins derived from these three genes and examined the impact of individual gene deletions on viral pathogenesis. The M139, M140, and M141 gene products were identified as early proteins that localize to both the nucleus and cytoplasm in infected cells. Gene M139 encodes two proteins, of 72 and 61 kDa, while M140 and M141 each encode a single protein of 56 (pM140) and 52 (pM141) kDa, respectively. No role for the M139 proteins in MCMV replication in macrophages or mice was determined in these studies. In contrast, deletion of either M140 or M141 resulted in impaired MCMV replication in macrophages and spleen tissue. Replication of the M140 deletion mutant was significantly more impaired than that of the virus lacking M141. Further analyses revealed that the absence of the pM140 adversely affected pM141 levels by rendering the latter protein unstable. Since the replication defect due to deletion of M140 was more profound than could be explained by the reduced half-life of pM141, pM140 must exert an additional, independent function in mediating efficient replication of MCMV in macrophages and spleen tissue. These data indicate that the US22 genes M140 and M141 function both cooperatively and independently to regulate MCMV replication in a cell type-specific manner and, thus, to influence viral pathogenesis.

Susceptibility to mouse cytomegalovirus is associated with deletion of an activating natural killer cell receptor of the C-type lectin superfamily

Cytomegalovirus is the leading cause of congenital viral disease and the most important opportunistic infection in immunocompromised patients. We have used a mouse experimental infection model (MCMV) to study the genetic parameters of host/virus interaction. Susceptibility to infection with MCMV is controlled by Cmv1, a chromosome 6 locus that regulates natural killer (NK) cell activity against virally infected targets. Here, we use a positional cloning strategy to isolate the gene mutated at the Cmv1 locus. Cmv1 maps within a 0.35-cM interval defined by markers D6Ott8 and D6Ott115, which corresponds to a physical distance of 1.6 Mb (refs. 6-8). A transcript map of the region identified 19 genes, including members of the killer cell lectin-like receptor family a (Klra, formerly Ly49; refs. 9-12), which encode inhibitory or activating NK cell receptors that interact with MHC class I molecules. Klra genes have different copy numbers and genomic organization, and are highly polymorphic among inbred strains, making it difficult to distinguish between normal allelic variants and distinct Klra genes, or possible mutations associated with Cmv1. The recombinant inbred strain BXD-8/Ty (BXD-8; ref. 18), derived from Cmv1r C57BL/6 (B6, resistant) and Cmv1s DBA/2 (susceptible), is of particular interest because it is highly susceptible to MCMV infection despite having a B6 haplotype at Cmv1. We determined that MCMV susceptibility in BXD-8 is associated with the deletion of Klra8 (formerly Ly49h).

Treatment of chronic sialadenitis in a murine model of Sjögren's syndrome by local fasL gene transfer

OBJECTIVE

Infection of Fas (Fas/CD95)-mutant C57BL/6 (B6)-lpr/lpr mice with murine cytomegalovirus (MCMV) leads to a chronic sialadenitis similar to that of Sjögren's syndrome (SS). The aim of this study was to evaluate whether chronic sialadenitis would also occur in Fas ligand (FasL/CD95L)-mutant B6-gld/gld mice upon infection with MCMV and whether the expression of FasL by local gene transfer using recombinant adenoviral vectors would be an effective therapeutic strategy.

METHODS

B6-gld/gld mice were infected intraperitoneally with MCMV, and salivary glands were analyzed histologically at different time points. For treatment of sialadenitis, recombinant adenoviral vectors expressing the fasL gene (AdLoxpFasL + AxCANCre) or the lacZ gene (AdCMVLacZ) were locally injected into the salivary glands of MCMV-infected B6-gld/gld mice and uninfected B6-+/+ and B6-gld/gld mice.

RESULTS

Following MCMV infection, B6-gld/gld mice developed an acute and chronic sialadenitis characterized by multiple foci of infiltrating T cells. After local injection of adenoviral vectors, high levels of lacZ or fasL gene expression could be detected in acinar and ductal cells. Treatment of acute and chronic sialadenitis in B6-gld/gld mice with local fasL gene transfer resulted in a significant reduction in the number of inflammatory foci and tissue destruction in salivary glands compared with mice treated with AdCMVLacZ. Despite high levels of FasL expression after injection of recombinant vectors, <5% of ductal and acinar cells were TUNEL positive, demonstrating that, in this model of SS, acinar and ductal cells were not highly sensitive to FasL-mediated apoptosis.

CONCLUSION

Chronic sialadenitis similar to that of SS developed in B6-gld/gld mice after MCMV infection. FasL expression was reconstituted by local gene transfer, resulting in significant reduction of infiltrating mononuclear cells, which indicates that local gene transfer of fasL might be a novel treatment for chronic sialadenitis.

Immune responses and cytokine induction in the development of severe hepatitis during acute infections with murine cytomegalovirus

Salivary gland-derived murine cytomegalovirus (SGV) infections of mice have been widely used as models of human cytomegalovirus infections and in the study of CMV biology. Still, many aspects of SGV pathogenesis are not clearly defined. Fatal and non-fatal SGV infections were investigated to characterize pathogenetic correlates of mortality and to assess the role of the immune response in disease progression. Suppression of immune responses was observed in both lethal and sublethal infections. Depletion of immune cell populations in spleen, however, correlated with severe CMV-induced hepatitis and mortality. In addition, T cell depletion studies indicated a requirement for this immune cell subset in control of liver damage and survival of infected mice. Examination of cytokine responses revealed a previously undescribed shock-like syndrome in lethally-infected mice characterized by high levels of tumor necrosis factor alpha and interferon gamma. Furthermore, the sites of tumor necrosis factor alpha gene induction did not strictly correlate with either viral load or the sites of tissue damage during infection. Taken together, these findings define the pathogenetic progression of disease as it relates to disease outcome and suggests that organ-specific differences in cytokine induction play a significant role in the late stages of acute lethal MCMV infections.

Evasion of cytotoxic T lymphocytes by murine cytomegalovirus

Murine cytomegalovirus causes lifelong infections with little pathology in normal host animals. Control of viral replication and prevention of pathology depend on both innate and adaptive immune mechanisms, and cytolytic T lymphocytes play a key role in this process. The virus encodes a number of genes which alter the normal assembly of class I major histocompatability complex proteins, and thus interfere with the ability of infected cells to present antigen to CD8(+)T cells. This review will examine what is known about these viral genes, and present some unanswered questions regarding the role of CTL evasion in the viral infectious cycle.

Murine cytomegalovirus open reading frame M27 plays an important role in growth and virulence in mice

Using a Tn3-based transposon mutagenesis approach, we have generated a pool of murine cytomegalovirus (MCMV) mutants. In this study, one of the mutants, RvM27, which contained the transposon sequence at open reading frame M27, was characterized both in tissue culture and in immunocompetent BALB/c mice and immunodeficient SCID mice. Our results suggest that the M27 carboxyl-terminal sequence is dispensable for viral replication in vitro. Compared to the wild-type strain and a rescued virus that restored the M27 region, RvM27 was attenuated in growth in both BALB/c and SCID mice that were intraperitoneally infected with the viruses. Specifically, the titers of RvM27 in the salivary glands, lungs, spleens, livers, and kidneys of the infected SCID mice at 21 days postinfection were 50- to 500-fold lower than those of the wild-type virus and the rescued virus. Moreover, the virulence of the mutant virus appeared to be attenuated, because no deaths occurred among SCID mice infected with RvM27 for up to 37 days postinfection, while all the animals infected with the wild-type and rescued viruses died within 27 days postinfection. Our observations provide the first direct evidence to suggest that a disruption of M27 expression results in reduced viral growth and attenuated viral virulence in vivo in infected animals. Moreover, these results suggest that M27 is a viral determinant required for optimal MCMV growth and virulence in vivo and provide insight into the functions of the M27 homologues found in other animal and human CMVs as well as in other betaherpesviruses.

Atherosclerosis in apoE knockout mice infected with multiple pathogens

Cytomegalovirus (CMV) and Chlamydia pneumoniae (CP) possibly contribute to atherosclerosis. Murine CMV (MCMV) and CP increase lesion size in apoE knockout mice. In this study, apoE knockout mice were infected with MCMV and CP to determine whether infection with multiple pathogens increases lesion size to a greater extent than either pathogen alone and whether infection with MCMV changes serum cytokine levels in a manner that could increase lesion development. One group of mice received MCMV at 2 weeks of age, followed by 2 doses of CP at 6 and 8 weeks of age. Additional groups received only MCMV or CP. Animals were killed at 16 weeks of age to determine lesion area. Infection with MCMV alone, CP alone, and both MCMV and CP increased lesion size 84% (P<.001), 70% (P<.0001), and 45% (P<.01), respectively. The MCMV-induced increase in circulating levels of interferon-gamma may have contributed to this increase.

Murine cytomegalovirus containing a mutation at open reading frame M37 is severely attenuated in growth and virulence in vivo

A pool of murine cytomegalovirus (MCMV) mutants was generated by using a Tn3-based transposon mutagenesis procedure. One of the mutants, RvM37, which contained the transposon sequence at open reading frame M37, was characterized both in tissue culture and in immunocompetent BALB/c and immunodeficient SCID mice. Our results provide the first direct evidence to suggest that M37 is not essential for viral replication in vitro in NIH 3T3 cells. Compared to the wild-type strain and a rescued virus that restored the M37 region, the viral mutant was severely attenuated in growth in both BALB/c and SCID mice after intraperitoneal infection. Specifically, titers of the Smith strain and rescued virus in the salivary glands, lungs, spleens, livers, and kidneys of the SCID mice at 21 days postinfection were about 5 x 10(5), 2 x 10(5), 5 x 10(4), 5 x 10(3), and 1 x 10(4) PFU/ml of organ homogenate, respectively; in contrast, titers of RvM37 in these organs were less than 10(2) PFU/ml of organ homogenate. Moreover, the virulence of the mutant virus appeared to be significantly attenuated because none of the SCID mice infected with RvM37 had died by 120 days postinfection, while all animals infected with the wild-type and rescued viruses had died by 26 days postinfection. Our results suggest that M37 probably encodes a virulence factor and is required for MCMV virulence in SCID mice and for optimal viral growth in vivo.