Molecular genetic analysis of cytomegalovirus gene regulation in growth, persistence and latency

Modulation of cytomegalovirus immune evasion identifies direct antigen presentation as the predominant mode of CD8 T-cell priming during immune reconstitution after hematopoietic cell transplantation

Cytomegalovirus (CMV) infection is the most critical infectious complication in recipients of hematopoietic cell transplantation (HCT) in the period between a therapeutic hematoablative treatment and the hematopoietic reconstitution of the immune system. Clinical investigation as well as the mouse model of experimental HCT have consistently shown that timely reconstitution of antiviral CD8 T cells is critical for preventing CMV disease in HCT recipients. Reconstitution of cells of the T-cell lineage generates naïve CD8 T cells with random specificities among which CMV-specific cells need to be primed by presentation of viral antigen for antigen-specific clonal expansion and generation of protective antiviral effector CD8 T cells. For CD8 T-cell priming two pathways are discussed: "direct antigen presentation" by infected professional antigen-presenting cells (pAPCs) and "antigen cross-presentation" by uninfected pAPCs that take up antigenic material derived from infected tissue cells. Current view in CMV immunology favors the cross-priming hypothesis with the argument that viral immune evasion proteins, known to interfere with the MHC class-I pathway of direct antigen presentation by infected cells, would inhibit the CD8 T-cell response. While the mode of antigen presentation in the mouse model of CMV infection has been studied in the immunocompetent host under genetic or experimental conditions excluding either pathway of antigen presentation, we are not aware of any study addressing the medically relevant question of how newly generated naïve CD8 T cells become primed in the phase of lympho-hematopoietic reconstitution after HCT. Here we used the well-established mouse model of experimental HCT and infection with murine CMV (mCMV) and pursued the recently described approach of up- or down-modulating direct antigen presentation by using recombinant viruses lacking or overexpressing the central immune evasion protein m152 of mCMV, respectively. Our data reveal that the magnitude of the CD8 T-cell response directly reflects the level of direct antigen presentation.

Persistent humoral immune responses in the CNS limit recovery of reactivated murine cytomegalovirus

Background

Experimental infection of the mouse brain with murine CMV (MCMV) elicits neuroimmune responses that terminate acute infection while simultaneously preventing extensive bystander damage. Previous studies have determined that CD8⁺ T lymphocytes are required to restrict acute, productive MCMV infection within the central nervous system (CNS). In this study, we investigated the contribution of humoral immune responses in control of MCMV brain infection.

Methodology/Principal Findings

Utilizing our MCMV brain infection model, we investigated B-lymphocyte-lineage cells and assessed their role in controlling the recovery of reactivated virus from latently infected brain tissue. Brain infiltrating leukocytes were first phenotyped using markers indicative of B-lymphocytes and plasma cells. Results obtained during these studies showed a steady increase in the recruitment of B-lymphocyte-lineage cells into the brain throughout the time-course of viral infection. Further, MCMV-specific antibody secreting cells (ASC) were detected within the infiltrating leukocyte population using an ELISPOT assay. Immunohistochemical studies of brain sections revealed co-localization of CD138⁺ cells with either IgG or IgM. Additional immunohistochemical staining for MCMV early antigen 1 (E1, m112–113), a reported marker of viral latency in neurons, confirmed its expression in the brain during latent infection. Finally, using B-cell deficient (Jh^−/−) mice we demonstrated that B-lymphocytes control recovery of reactivated virus from latently-infected brain tissue. A significantly higher rate of reactivated virus was recovered from the brains of Jh^−/− mice when compared to Wt animals.

Conclusion

Taken together, these results demonstrate that MCMV infection triggers accumulation and persistence of B-lymphocyte-lineage cells within the brain, which produce antibodies and play a significant role in controlling reactivated virus.

Serum proteomics with SELDI-TOF-MS in congenital human cytomegalovirus hepatitis

Human cytomegalovirus (HCMV) is a widespread pathogen, the most common congenital viral infection, and the leading cause of infant hepatitis syndrome. In this study, serum samples were collected from 20 HCMV-infected infants with hepatitis and 25 controls. Of the 25 infants in the control group, 5 were infected with HCMV but without hepatitis, 10 had hepatitis but no HCMV infection, and 10 were healthy. Proteomic expression in the serum was detected by WCX2 chips and surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF-MS), to identify serum protein biomarkers in infants with hepatitis syndrome resulting from HCMV. Fifteen protein peaks were distinctly different among the four groups in the mass range from 2,000 to 20,000 Da. Of these 15 peaks, 4 at 4,349.8, 5,808.7, 7,935.6, and 8,885.9 Da were significantly different between the congenital HCMV-infected infants with hepatitis and the controls. Five peaks were distinctly up-regulated in the infants with HCMV infection (3,266.8, 5,638.5, 5,909.1, 7,771.4, and 15,835.6 Da) compared to those without HCMV infection. Two proteins at 4,600.1 and 5,704.3 were up-regulated in infants with HMCV infection but no hepatitis. Four protein peaks were markedly different (7,567.0, 13,744.8, 15,100.7, and 15,915.0 Da) between the infants with hepatitis and the other controls. Comparison of the differentially expressed proteins' properties with those available on an international database suggest that specific serum proteins such as the augmenter of liver regeneration, pre-albumin, and haptoglobin closely related to liver function, and cytokines such as beta-defensins 31 and 8, and macrophage-derived chemokine, among others, are involved in HMCV infection and the pathogenesis of HMCV-induced hepatitis in infants.

The ribonucleotide reductase R1 homolog of murine cytomegalovirus is not a functional enzyme subunit but is required for pathogenesis

Ribonucleotide reductase (RNR) is the key enzyme in the biosynthesis of deoxyribonucleotides. Alpha- and gammaherpesviruses express a functional enzyme, since they code for both the R1 and the R2 subunits. By contrast, betaherpesviruses contain an open reading frame (ORF) with homology to R1, but an ORF for R2 is absent, suggesting that they do not express a functional RNR. The M45 protein of murine cytomegalovirus (MCMV) exhibits the sequence features of a class Ia RNR R1 subunit but lacks certain amino acid residues believed to be critical for enzymatic function. It starts to be expressed independently upon the onset of viral DNA synthesis at 12 h after infection and accumulates at later times in the cytoplasm of the infected cells. Moreover, it is associated with the virion particle. To investigate direct involvement of the virally encoded R1 subunit in ribonucleotide reduction, recombinant M45 was tested in enzyme activity assays together with cellular R1 and R2. The results indicate that M45 neither is a functional equivalent of an R1 subunit nor affects the activity or the allosteric control of the mouse enzyme. To replicate in quiescent cells, MCMV induces the expression and activity of the cellular RNR. Mutant viruses in which the M45 gene has been inactivated are avirulent in immunodeficient SCID mice and fail to replicate in their target organs. These results suggest that M45 has evolved a new function that is indispensable for virus replication and pathogenesis in vivo.

Synergy of type I interferon-A6 and interferon-B naked DNA immunotherapy for cytomegalovirus infection

Delivery of type I IFN transgenes by naked DNA immunization can protect against cytomegalovirus infection and myocarditis. Here, we investigate IFN transgene expression, antiviral efficacy, and immunomodulation of myocarditis using various treatment regimes in a mouse CMV model. In vivo expression of the IFN transgene was observed in the sera for 35 days post-DNA inoculation. Prophylactic IFN-A6 and IFN-B DNA treatment for 14 days prior to murine cytomegalovirus (MCMV) infection was more efficacious in significantly reducing viral titres, than 2 days prior to or 2 days post-virus infection. Similarly, IFN-A6 DNA treatment commencing 14 days prior to virus infection was superior in suppressing both acute and chronic myocarditis. Furthermore, reduction of autoantibody titres was more pronounced when IFN was administered 14 days prior to viral infection. Combinational IFN gene therapy was assessed for synergy between IFN subtypes. Combination treatment with either IFN-A6/A9 or IFN-A6/B greatly reduced spleen viral titres while IFN-A6/B and IFN-A9/B reduced virus replication in the liver. Only IFN-A6/A9 and IFN-A9/B reduced acute viral myocarditis, whereas IFNA6/B treatment was most efficacious for autoimmune chronic myocarditis. Finally, treatment with IFN-A6 DNA 2 weeks post-MCMV infection proved effective at inhibiting the development of chronic autoimmune myocarditis. These findings suggest that immunomodulation of both antiviral and autoimmune responses by IFN DNA immunization may be an avenue for improved viral immunotherapy.

Cytomegalovirus-encoded beta chemokine promotes monocyte-associated viremia in the host

Chemokine homologs are encoded by many large DNA viruses, suggesting that they contribute to control of host leukocyte transmigration and trafficking during viral infection. Murine cytomegalovirus carries a CC (beta) chemokine homolog gene giving rise to two related proteins, murine cytomegalovirus chemokine 1 and 2 (MCK-1 and MCK-2). MCK-1 peptide was found to induce calcium signaling and adherence in murine peritoneal macrophages. Cells bearing human chemokine receptor CCR3 and the human macrophage THP1 cell line were responsive to MCK-1. This pattern suggested that MCK-1 might act as an agonist, promoting leukocyte trafficking during viral infection. Consistent with this prediction, MCK-1/MCK-2 mutant viruses exhibit dramatically reduced peak levels of monocyte-associated viremia in experimentally infected mice. Thus, MCK-1/MCK-2 appears to promote host leukocyte migration to initial sites of infection and may be responsible for attracting monocytes or macrophages that efficiently disseminate virus in the host.

In vivo replication of recombinant murine cytomegalovirus driven by the paralogous major immediate-early promoter-enhancer of human cytomegalovirus

Transcription of the major immediate-early (MIE) genes of cytomegaloviruses (CMV) is driven by a strong promoter-enhancer (MIEPE) complex. Transactivator proteins encoded by these MIE genes are essential for productive infection. Accordingly, the MIEPE is a crucial control point, and its regulation by activators and repressors is pertinent to virus replication. Since the MIEPE contains multiple regulatory elements, it was reasonable to assume that specific sequence motifs are irreplaceable for specifying the cell-type tropism and replication pattern. Recent work on murine CMV infectivity (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998) has documented the proposed enhancing function of the enhancer in that its resection or its replacement by a nonregulatory stuffer sequence resulted in a significant reduction of infectivity, even though replication competence was maintained by a basal activity of the spared authentic MIE promoter. Notably, full capacity for productive in vitro infection of fibroblasts was restored in recombinant viruses by the human CMV enhancer. Using two-color in situ hybridization with MIEPE-specific polynucleotide probes, we demonstrated that a murine CMV recombinant in which the complete murine CMV MIEPE is replaced by the paralogous human CMV core promoter and enhancer (recombinant virus mCMVhMIEPE) retained the potential to replicate in vivo in all tissues relevant to CMV disease. Notably, mCMVhMIEPE was also found to replicate in the liver, a site at which transgenic hCMV MIEPE is silenced. We conclude that productive in vivo infection with murine CMV does not strictly depend on a MIEPE type-specific regulation.

Role of macrophages in acute murine cytomegalovirus infection

It has been recognized that macrophages play an important role in controlling virus infection in experimental animal models. To evaluate the role of macrophages in acute murine cytomegalovirus infection, macrophages in the spleen and the liver were eliminated by an intravenous injection of liposomes containing a cytolytic agent, dichloromethylene diphosphonate. The depletion of macrophages led to a significant increase of virus titer in the spleen and lungs in both susceptible BALB/c and resistant C57BL/6 mice during the first three days after intravenous infection. In the spleen, the increase of virus titer in macrophage-depleted BALB/c mice was much greater than that in NK cell-depleted mice. These results suggest that macrophages contribute to protection mainly by the mechanisms which are independent of NK cells during the first three days after infection. The increase of virus titer in macrophage-depleted C57BL/6 mice was as great as that in NK cell-depleted mice because of the high contribution of NK cells to protection in C57BL/6 mice. In the liver in both strains of mice, the effects of macrophage depletion on virus titer were not as much as those in the spleen and lungs. Furthermore, the local depletion of peritoneal macrophages resulted in a great increase of virus titer in the spleen at three days after intraperitoneal infection. We conclude that macrophages greatly contribute to decreasing the virus load in some organs possibly through either or both intrinsic and extrinsic mechanisms in the early phase of primary infection with murine cytomegalovirus.

Hierarchical and redundant lymphocyte subset control precludes cytomegalovirus replication during latent infection

Reactivation from latent cytomegalovirus (CMV) infection is often associated with conditions of immunosuppression and can result in fatal disease. Whether the maintenance of systemic CMV latency is mainly governed by factors of the infected cell or by immune control functions is unknown. Likewise, the putative immune control mechanisms which could prevent the induction and spread of recurrent CMV infection are not clearly identified. We took advantage of latently infected B cell-deficient mice and a sensitive method for virus detection to study CMV reactivation after ablation of lymphocyte subsets. A crucial role of both T lymphocytes and natural killer (NK) cells was demonstrated. Within 5 d after depletion of lymphocytes, productive infection occurred in 50% of mice, and 14 d later 100% of mice exhibited recurrent infection. A hierarchy of immune control functions of CD8(+), NK, and CD4(+) cells was established. Reactivation was rare if only one of the lymphocyte subsets was depleted, but was evident after removal of a further subset, indicating a functional redundancy of control mechanisms. The salivary glands were identified as the site of most rapid virus shedding, followed by the detection of recurrent virus in the lungs, and eventually in the spleen. Our findings document a previously unknown propensity of latent CMV genomes to enter productive infection immediately and with a high frequency after immune cell depletion. The data indicate that only the sustained cellular immune control prevents CMV replication and restricts the viral genome to a systemic state of latency.

[Cardiotropic DNA viruses and bacteria in the pathogenesis of dilated cardiomyopathy with or without inflammation]

In the report of the 1995 WHO/ISFC task force on the definition and classification of cardiomyopathies a new entity within the dilated cardiomyopathies was introduced as "inflammatory cardiomyopathy". It is defined as myocarditis associated with cardiac dysfunction. Idiopathic, autoimmune and infectious forms of inflammatory cardiomyopathy are now recognized through this definition. Dilated cardiomyopathy with inflammation (DCMi, chronic myocarditis) was also defined by a recent ISFC task force as > 14 lymphocytes/macrophages/mm3. Enteroviruses, adenoviruses and cytomegaloviruses are considered as main etiopathogenetic factors in the pathogenesis of inflammatory heart disease and have been demonstrated as important trigger for inflammatory cardiac disease. They may also cause dilated cardiomyopathy by viral persistence or secondary immunopathogenesis due to antigenic or molecular mimicry. For the detection of viral persistence the investigation of endomyocardial biopsies in patients with cardiomyopathy by the use of polymerase chain reaction and southern blot analysis is an important step for the standardization of diagnostic criteria on virally induced inflammatory cardiomyopathy. Present studies indicate an incidence of cytomegalovirus-DNA in patients with inflammatory cardiomyopathy in 10%, adenoviral-DNA in 17% and borreliosis only in rare cases (< 1%). In dilated cardiomyopathy without inflammation the respective incidences were for cytomegalovirus 12%, 15% for adenovirus and only 0.5% of cases for borreliosis. In addition the results of immunohistochemical analysis and molecular biological investigations of endomyocardial biopsies may have implications for future therapeutic studies. Depending on the etiology of the disease, immunosuppression may have benefit for patients with virus-negative cardiomyopathy with inflammation in contrast to patients with cytomegalo-, adenovirus-DNA or enteroviral persistence, in whom immunomodulation with hyperimmunoglobulins or immunoglobulins may be a feasible therapeutic option. Patients with a positive PCR for Borrelia burgdorferi should be treated with 3rd generation cephalosporines and/or sublactam.

Latency versus persistence or intermittent recurrences: evidence for a latent state of murine cytomegalovirus in the lungs

The state of cytomegalovirus (CMV) after the resolution of acute infection is an unsolved problem in CMV research. While the term "latency" is in general use to indicate the maintenance of the viral genome, a formal exclusion of low-level persistent productive infection depends on the sensitivity of the assay for detecting infectious virus. We have improved the method for detecting infectivity by combining centrifugal infection of permissive indicator cells in culture, expansion to an infectious focus, and sensitive detection of immediate-early RNA in the infected cells by reverse transcriptase PCR. A limiting-dilution approach defined the sensitivity of this assay. Infectivity was thereby found to require as few as 2 to 9 virion DNA molecules of murine CMV, whereas the standard measure of infectivity, the PFU, is the equivalent of ca. 500 viral genomes. Since murine CMV forms multicapsid virions in most infected tissues, the genome-to-infectivity ratio is necessarily >1. This assay thus sets a new standard for investigating CMV latency. In mice in which acute infection was resolved, the viral DNA load in the lungs, a known organ site of CMV latency and recurrence, was found to be 1 genome per 40 lung cells, or a total of ca. 1 million genomes. Despite this high load of CMV DNA, infectious virus was not detected with the improved assay, but recurrence was inducible. These data provide evidence against a low-level persistent productive infection and also imply that intermittent spontaneous recurrence is not a frequent event in latently infected lungs.

Murine cytomegalovirus with a deletion of genes spanning HindIII-J and -I displays altered cell and tissue tropism

Murine cytomegalovirus (MCMV) gene products dispensable for growth in cell culture are likely to have important functions within the infected host, influencing tissue tropism, dissemination, or immunological responses against the virus. To identify such genes, our strategy was to delete large regions of the MCMV genome likely to contain genes nonessential for virus replication in NIH 3T3 cells. Mutant virus RV7 contained a deletion of 7.7 kb spanning portions of MCMV HindIII-J and -I. This virus grew comparably to wild-type (WT) virus in NIH 3T3 fibroblasts, primary embryo fibroblasts, and bone marrow macrophages. However, RV7 failed to replicate in target organs of immunocompetent BALB/c mice and severe combined immunodeficient mice, which are exquisitely sensitive to MCMV infection. This defect in vivo growth may be related to the observation that RV7 grew poorly in the peritoneal macrophage cell line IC-21, which is highly permissive for growth of WT MCMV. Two other mutant viruses with an insertion or smaller deletion in the region common to the RV7 deletion grew comparably to WT virus in the macrophage cell line and replicated in salivary gland tissue. The poor growth of RV7 in IC-21 cells was due to a block in immediate-early gene expression, as levels of RNA from immediate-early gene IE1 were reduced eightfold compared with levels for WT virus in macrophages infected with RV7. Consequently, levels of RNA from early and late genes were also reduced. The lower expression of IE1 in RV7-infected IC-21 macrophages was not due to defective entry of virus into the cells, as equal amounts of viral DNA were present in cells 3 h after infection with RV7 or WT MCMV. These studies demonstrate that deletion of sequences in HindIII-J and -I confer altered cell and tissue tropism.

Immunosuppression induces transcription of murine cytomegalovirus glycoprotein H in the eye and at non-ocular sites

In these studies, DNA PCR was used to identify sites of murine cytomegalovirus (MCMV) latency after inoculation of virus into the supraciliary space of the eye. Reverse transcription (RT) PCR for an immediate early gene and a late gene was used to identify putative sites of virus reactivation after methylprednisolone (steroid)-induced immunosuppression. Ten weeks after inoculation of 5 x 10(2) PFU of MCMV, BALB/c mice were immunosuppressed by intramuscular injection of steroid. Control mice were infected but not immunosuppressed. Two weeks after initiation of immunosuppression, mice were sacrificed. DNA and RNA extracted from homogenized tissues were amplified for immediate early gene 1 (IE1) and late gene, glycoprotein H (gH), DNA and mRNA by PCR and RT-PCR, respectively. Replicating virus was detected in homogenized ocular and non-ocular tissues by plaque assay. In the latently infected PBS-treated control group, viral DNA was detected in the inoculated eye and in several non-ocular tissues; IE1 mRNA was found in most of the DNA-positive tissues, while gH mRNA was amplified only in a few of the MCMV DNA-positive tissues from a single mouse. After immunosuppression, viral DNA and IE1 mRNA were detected at a higher frequency in various tissues of steroid-treated mice. gH mRNA was detected in a significantly higher number of the inoculated eyes, salivary glands and other non-ocular tissues of steroid-treated mice. After immunosuppression, low titers of infectious virus were recovered from the salivary glands of steroid-treated mice, but infectious virus was not recovered from the inoculated eye of either steroid-treated of non-immunosuppressed mice. The DNA PCR results suggest that after inoculation of 5 x 10(2) PFU of MCMV into the supraciliary space of euthymic BALB/c mice, virus becomes latent in the inoculated eye, salivary gland and other extraocular tissues. The RT-PCR results suggest that latent MCMV can be reactivated in multiple tissues by immunosuppression.

Cell types involved in replication and distribution of human cytomegalovirus

As the number of patients suffering from severe HCMV infections has steadily increased, there is a growing need to understand the molecular mechanisms by which the virus causes disease. The factors that control infection at one time and the events leading to virus multiplication at another time are only beginning to be understood. The interaction of HCMV with different host cells is one key for elucidating these processes. Through modern techniques, much has been learned about the biology of HCMV infections in culture systems. In addition to endothelial cells, epithelial cells, and smooth muscle cells, fibroblasts are one cell population preferentially infected in solid tissues in vivo. From these sites of multiplication, the virus may be carried by peripheral monocytes and circulating endothelial cells to reach distant sites of the body. This would explain the multiorgan involvement in acute HCMV infection and the modes of viral transmission. From what has been learned mainly from human fibroblast culture systems, future studies will focus on how HCMV regulates the expression of its putative 200 genes in different host cells at different stages of cell differentiation and activation to result in viral latency and pathogenesis.

Human cytomegalovirus (HCMV) immediate-early enhancer/promoter specificity during embryogenesis defines target tissues of congenital HCMV infection

Congenital human cytomegalovirus (HCMV) infection is a common cause of deafness and neurological disabilities. Many aspects of this prenatal infection, including which cell types are infected and how infection proceeds, are poorly understood. Transcription of HCMV immediate-early (IE) genes is required for expression of all other HCMV genes and is dependent on host cell transcription factors. Cell type-specific differences in levels of IE transcription are believed to underlie differences in infection permissivity. However, DNA transfection experiments have paradoxically suggested that the HCMV major IE enhancer/promoter is a broadly active transcriptional element with little cell type specificity. In contrast, we show here that expression of a lacZ gene driven by the HCMV major IE enhancer/promoter -524 to +13 segment is restricted in transgenic mouse embryos to sites that correlate with known sites of congenital HCMV infection in human fetuses. This finding suggests that the IE enhancer/promoter is a major determinant of HCMV infection sites in humans and that transcription factors responsible for its regulation are cell type-specifically conserved between humans and mice. The lacZ expression patterns of these transgenic embryos yield insight into congenital HCMV pathogenesis by providing a spatiotemporal map of the sets of vascular, neural, and epithelial cells that are likely targets of infection. These transgenic mice may constitute a useful model system for investigating IE enhancer/promoter regulation in vivo and for identifying factors that modulate active and latent HCMV infections in humans.

Characterization of a conserved gene block in the murine cytomegalovirus genome

The nucleotide sequence between the lytic origin and the gH gene of the murine cytomegalovirus genome (map units 0.416-0.455) was determined. Five of the 15 open reading frames identified encode polypeptides, which exhibit significant homology to polypeptides of human cytomegalovirus (HCMV) (UL69, UL71, and UL73) and to polypeptides of several other herpesviruses (dUTPase and helicase/primase). The location in the genome of the five open reading frames and their direction of transcription is perfectly conserved between murine cytomegalovirus (MCMV) and HCMV. These data suggest that MCMV and HCMV have a highly related genome organization. Thus, MCMV offers a good model for molecular and functional analysis of cytomegalovirus genes in vitro and in vivo.

Lungs are a major organ site of cytomegalovirus latency and recurrence

Recurrence of infectious virus from the latent viral genomes is the initiating event in the pathogenesis of cytomegalovirus (CMV) disease during states of immunodeficiency. Interstitial pneumonia is a frequent manifestation of posttransplantation CMV disease, in particular after bone marrow transplantation and heart and lung transplantations. Recurrence can occur within the transplant derived from a latent infected donor as well as within latently infected organs of the transplant recipient. The reason for a predilection of the lungs as a site of CMV pathology is so far unknown. In a murine model of CMV latency, the lungs were identified as an authentic site of latent infection, since the viral genome remained detectable in lung tissue even after it was cleared to an undetectable level in blood and bone marrow. A comparison between the lungs and the spleen, the previously most thoroughly investigated site of murine CMV latency, revealed a 10-fold-higher burden of latent viral genome for the lungs. Most important, the organ-specific risk of in vivo recurrence was found to correlate with the organ-specific viral genomic load. This new finding thus characterizes the lungs as a high-risk organ for CMV recurrence, and this fact may explain in part why interstitial pneumonia is a frequent manifestation of recurrent CMV infection.

The human cytomegalovirus US3 immediate-early protein lacking the putative transmembrane domain regulates gene expression

Through alternative transcript splicing, the human cytomegalovirus (HCMV) US3 immediate-early (IE) locus encodes multiple products including potential membrane-bound glycoproteins. To characterize the US3 products and determine which encode regulatory activity, individual cDNAs were cloned and expressed. Three transcript species were confirmed through the isolation of cDNAs; an unspliced transcript, a transcript spliced once from exon 3 to exon 5 and a transcript spliced both at exon 1 to exon 3 and at exon 3 to exon 5. The predicted signal sequences and N-linked glycosylation sites in the US3 products were confirmed using expression in reticulocyte lysates containing microsomal membranes. Regulatory activity of the individual US3 products was demonstrated using transient transfection assays. The unspliced cDNA and the cDNA containing the exon 3 to exon 5 splice, encoded products which increased expression of the human heat shock protein 70 (hsp70) promoter, while the product of the doubly-spliced US3 cDNA did not. Transactivation was synergistically increased by coexpression with the HCMV UL37 protein. We conclude that the first 132 amino acids common to the unspliced and the singly-spliced US3 gene products are sufficient for hsp70 transactivation; while the amino-terminal 28 amino acids, encoded by the doubly-spliced US3 cDNA, are not. These results demonstrate that a US3 IE protein lacking the putative transmembrane domain has regulatory activity.

Cytomegalovirus determinant of replication in salivary glands

Murine cytomegalovirus carrying a deletion mutation disrupting the expression of a gene dispensable for growth in cultured cells was found to disseminate poorly in the mouse. The mutation resulted in a dramatic decrease in the expression of a 1.5-kb major and a 1.8-kb minor beta transcript from a region adjacent to the ie2 gene in the viral genome. Nucleotide sequence determination indicated that 323 bp, including a predicted polyadenylation signal, was deleted from this beta gene. In cultured cells, the plaque morphology and growth characteristics of the mutant were similar to those of parental or rescued wild-type viruses. Following intraperitoneal inoculation of BALB/c mice, growth of the mutant in the salivary gland was dramatically reduced 10,000-fold, while growth in the liver and spleen was not dramatically affected. The beta gene was thus denoted sgg1 (salivary gland growth gene 1). Neither intranasal infection nor direct inoculation into the salivary glands completely overcame the restriction of growth in this organ, suggesting that the sgg1 gene encoded a determinant of tissue tropism. To investigate the impact of the sgg1 mutation on virus dissemination via the blood, the virus titer in peripheral blood leukocytes was determined. No difference was found between the sgg1 mutant and rescued wild-type virus. Thus, murine cytomegalovirus sgg1 gene products appear to be involved in entry or replication of virus in salivary gland cells.

Transactivation of the cytomegalovirus ICP36 gene promoter requires the alpha gene product TRS1 in addition to IE1 and IE2

Very little is known about the human cytomegalovirus functions that activate gamma (late) gene expression. We have investigated the regulation of the human cytomegalovirus gamma gene encoding the ICP36 major late DNA-binding protein family (UL44). Transactivation of the ICP36 gene promoter was found to be absolutely dependent on the trs1 gene product when expressed in cells in conjunction with ie1 and ie2 gene products. Transactivation occurred poorly or not at all when any one of these three transactivators was omitted. TRS1 is a member of the US22 family of proteins and is encoded by a region near the L-S junction of the viral genome within the c repeat and adjacent Us sequences. TRS1 is highly homologous to IRS1, which is encoded from the other copy of the c repeat, and plasmid constructs carrying the irs1 gene were also able to mediate transactivation of the ICP36 promoter. RNA blot analysis of steady-rate RNA throughout infection showed that the trs1 transcript was expressed with the kinetics of an alpha gene but its accumulation was delayed relative to that of ie1 and ie2 transcripts. On the basis of these experiments, TRS1 and IRS1 are proposed to be important intermediaries in the cascade of cytomegalovirus gene expression.

A possible role for nonsense suppression in the synthesis of a human cytomegalovirus 58-kDa virion protein

A 1.6-kb late mRNA originating from the HindIII R fragment of human cytomegalovirus (HCMV) encodes a 58-kDa virion phosphoprotein. Data presented support the hypothesis that this protein may be synthesized via the translational readthrough of an opal termination codon separating two open reading frames located in tandem. To our knowledge this is the first report of nonsense suppression as a means of regulating gene expression in HCMV.

Structure and expression of murine cytomegalovirus immediate-early gene 2

The immediate-early gene ie2 of murine cytomegalovirus was characterized. The 1.75-kb ie2 transcript is spliced from three exons, of 78, 124, and 1,283 nucleotides, which are separated by introns of 1,245 and 364 nucleotides. An ATG codon located in the third exon leads into an open reading frame of 391 codons. Immediate-early expression of the predicted polypeptide was confirmed by immunoprecipitation of a 43-kDa protein by using an antiserum raised against a bacterial fusion protein. The predicted IE2 amino acid sequence has regions with similarity to amino acid sequences of members of the human cytomegalovirus US22 family.

Human cytomegalovirus ie2 negatively regulates alpha gene expression via a short target sequence near the transcription start site

Repression of human cytomegalovirus alpha (immediate-early) gene expression is under the control of the viral ie2 gene. Here we show that ie2 negatively regulates gene expression directed by the strong cytomegalovirus enhancer via a specific 15-bp target sequence (which we term cis repression signal [crs]). This crs is located between -14 and +1 relative to the transcription start site and will function in an orientation-independent fashion, consistent with repression occurring at the transcriptional level. Repression is dominant over transactivation by ie1 gene products. The crs (5'-CGTTTAGTGAACCGT-3') does not contain previously recognized binding sites for cellular transcription factors, and a precise copy is not found elsewhere in the human cytomegalovirus genome. The position of the signal near the transcription start site appears to be important in function; addition of the crs near the transcription start site of a heterologous promoter, from the thymidine kinase gene of herpes simplex virus type 1, conferred cytomegalovirus ie2-dependent repression upon this promoter. Thus, we propose that an ie2 gene product or an induced cellular protein mediates repression by binding to crs. Negative regulation of alpha gene expression may be important during viral replication or latency.