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

Experimental preemptive immunotherapy of murine cytomegalovirus disease with CD8 T-cell lines specific for ppM83 and pM84, the two homologs of human cytomegalovirus tegument protein ppUL83 (pp65)

CD8 T cells are the principal antiviral effectors controlling cytomegalovirus (CMV) infection. For human CMV, the virion tegument protein ppUL83 (pp65) has been identified as a source of immunodominant peptides and is regarded as a candidate for cytoimmunotherapy and vaccination. Two sequence homologs of ppUL83 are known for murine CMV, namely the virion protein ppM83 (pp105) expressed late in the viral replication cycle and the nonstructural protein pM84 (p65) expressed in the early phase. Here we show that ppM83, unlike ppUL83, is not delivered into the antigen presentation pathway after virus penetration before or in absence of viral gene expression, while other virion proteins of murine CMV are processed along this route. In cytokine secretion-based assays, ppM83 and pM84 appeared to barely contribute to the acute immune response and to immunological memory. Specifically, the frequencies of M83 and M84 peptide-specific CD8 T cells were low and undetectable, respectively. Nonetheless, in a murine model of cytoimmunotherapy of lethal CMV disease, M83 and M84 peptide-specific cytolytic T-cell lines proved to be highly efficient in resolving productive infection in multiple organs of cell transfer recipients. These findings demonstrate that proteins which fail to prime a quantitatively dominant immune response can nevertheless represent relevant antigens in the effector phase. We conclude that quantitative and qualitative immunodominance are not necessarily correlated. As a consequence of these findings, there is no longer a rationale for considering T-cell abundance as the key criterion for choosing specificities to be included in immunotherapy and immunoprophylaxis of CMV disease and of viral infections in general.

Allogeneic transplantation induces expression of cytomegalovirus immediate-early genes in vivo: a model for reactivation from latency

Reactivation of cytomegalovirus (CMV) from latency is a frequent complication of organ transplantation, and the molecular mechanism by which this occurs is unknown. Previous studies have shown that allogeneic stimulation induces reactivation of human CMV (HCMV) in vitro (64). We find that transplantation of vascularized allogeneic kidneys induces murine CMV (MCMV) and HCMV immediate-early (ie) gene expression. This induction is accompanied by increased expression of transcripts encoding inflammatory cytokines, including tumor necrosis factor (TNF), interleukin-2, and gamma interferon, and by activation of NF-kappaB. TNF alone can substitute for allogeneic transplantation in inducing HCMV and MCMV ie gene expression in some tissues. Our studies suggest that reactivation is a multistep process which is initiated by factors that induce ie gene expression, including TNF and NF-kappaB. Allogeneic transplantation combined with immunosuppression may be required to achieve complete reactivation in vivo.

Murine cytomegalovirus M78 protein, a G protein-coupled receptor homologue, is a constituent of the virion and facilitates accumulation of immediate-early viral mRNA

The M78 protein of murine cytomegalovirus exhibits sequence features of a G protein-coupled receptor. It is synthesized with early kinetics, it becomes partially colocalized with Golgi markers, and it is incorporated into viral particles. We have constructed a viral substitution mutant, SMsubM78, which lacks most of the M78 ORF. The mutant produces a reduced yield in cultured 10.1 fibroblast and IC21 macrophage cell lines. The defect is multiplicity dependent and greater in the macrophage cell line. Consistent with its growth defect in cultured cells, the mutant exhibits reduced pathogenicity in mice, generating less infectious progeny than wild-type virus in all organs assayed. SMsubM78 fails to efficiently activate accumulation of the viral m123 immediate-early mRNA in infected macrophages. M78 facilitates the accumulation of the immediate-early mRNA in cycloheximide-treated cells, arguing that it acts in the absence of de novo protein synthesis. We conclude that the M78 G protein-coupled receptor homologue is delivered to cells as a constituent of the virion, and it acts to facilitate the accumulation of immediate-early mRNA.

Role of nitric oxide synthase type 2 in acute infection with murine cytomegalovirus

Whether or not NO plays a critical role in murine CMV (MCMV) infection has yet to be elucidated. In this study, we examined the role of NO in acute infection with MCMV using NO synthase type 2 (NOS2)-deficient mice. NOS2(-/-) mice were more susceptible to lethal infection with MCMV than NOS2(+/+) mice and generated a much higher peak virus titer in the salivary gland after acute infection. A moderate increase in the MCMV titer was also observed in other organs of NOS2(-/-) mice such as the spleen, lung, and liver. The immune responses to MCMV infection including NK cell cytotoxicity and CTL response in NOS2(-/-) mice were comparable with those of NOS2(+/+) mice. Moreover, the ability to produce IFN-gamma is not impaired in NOS2(-/-) mice after MCMV infection. The peritoneal macrophages from NOS2(-/-) mice, however, exhibited a lower antiviral activity than those from NOS2(+/+) mice, resulting in an enhanced viral replication in macrophages themselves. Treatment of these cells from NOS2(+/+) mice with a selective NOS2 inhibitor decreased the antiviral activity to a level below that obtained with NOS2(-/-) mice. In addition, the absence of NOS2 and NOS2-mediated antiviral activity of macrophages resulted in not only an enhanced MCMV replication and a high mortality but also a consequent risk of the latency. It was thus concluded that the NOS2-mediated antiviral activity of macrophages via NO plays a protective role against MCMV infection at an early and late stage of the infection.

Random, asynchronous, and asymmetric transcriptional activity of enhancer-flanking major immediate-early genes ie1/3 and ie2 during murine cytomegalovirus latency in the lungs

The lungs are a major organ site of cytomegalovirus (CMV) pathogenesis, latency, and recurrence. Previous work on murine CMV latency has documented a high load and an even distribution of viral genomes in the lungs after the resolution of productive infection. Initiation of the productive cycle requires expression of the ie1/3 transcription unit, which is driven by the immediate-early (IE) promoter P(1/3) and generates IE1 and IE3 transcripts by differential splicing. Latency is molecularly defined by the absence of IE3 transcripts specifying the essential transactivator protein IE3. In contrast, IE1 transcripts were found to be generated focally and randomly, reflecting sporadic P(1/3) activity. Selective generation of IE1 transcripts implies molecular control of latency operating after ie1/3 transcription initiation. P(1/3) is regulated by an upstream enhancer. It is widely assumed that the viral transcriptional program is started by activation of the enhancer through the binding of transcription factors. Accordingly, stochastic transcription during latency might reflect episodes of enhancer activation by the "noise" activity of intrinsic transcription factors. In addition to ie1/3, the enhancer controls gene ie2, which has its own promoter, P(2), and is transcribed in opposite direction. We show here that ie2 is also randomly transcribed during latency. Notably, however, ie1 and ie2 were found to be expressed independently. We infer from this finding that expression of the major IE genes is regulated asymmetrically and asynchronously via the combined control unit P(1/3) -E-P(2). Our data are consistent with a stochastic nature of enhancer action as it is proposed by the "binary" or probability model.

Dominance of virus over host factors in cross-species activation of human cytomegalovirus early gene expression

Human cytomegalovirus (HCMV) exhibits a highly restricted host range. In this study, we sought to examine the relative significance of host and viral factors in activating early gene expression of the HCMV UL54 (DNA polymerase) promoter in murine cells. Appropriate activation of the UL54 promoter at early times is essential for viral DNA replication. To study how the HCMV UL54 promoter is activated in murine cells, a transgenesis system based on yeast artificial chromosomes (YACs) was established for HCMV. A 178-kb YAC, containing a subgenomic fragment of HCMV encompassing the majority of the unique long (UL) region, was constructed by homologous recombination in yeast. This HCMV YAC backbone is defective for viral growth and lacks the major immediate-early (IE) gene region, thus permitting the analysis of essential cis-acting sequences when complemented in trans. To quantitatively measure the level of gene expression, we generated HCMV YACs containing a luciferase reporter gene inserted downstream of either the UL54 promoter or, as a control for late gene expression, the UL86 promoter, which directs expression of the major capsid protein. To determine the early gene activation pathway, point mutations were introduced into the inverted repeat 1 (IR1) element of the UL54 promoter of the HCMV YAC. In the transgenesis experiments, HCMV YACs and derivatives generated in yeast were introduced into NIH 3T3 murine cells by polyethylene glycol-mediated fusion. We found that infection of YAC, but not plasmid, transgenic lines with HCMV was sufficient to fully recapitulate the UL54 expression program at early times of infection, indicating the importance of remote regulatory elements in influencing regulation of the UL54 promoter. Moreover, YACs containing a mutant IR1 in the UL54 promoter led to reduced ( approximately 30-fold) reporter gene expression levels, indicating that HCMV major IE gene activation of the UL54 promoter is fully permissive in murine cells. In comparison with HCMV, infection of YAC transgenic NIH 3T3 lines with murine cytomegalovirus (MCMV) resulted in lower (more than one order of magnitude) efficiency in activating UL54 early gene expression. MCMV is therefore not able to fully activate HCMV early gene expression, indicating the significance of virus over host determinants in the cross-species activation of key early gene promoters. Finally, these studies show that YAC transgenesis can be a useful tool in functional analysis of viral proteins and control of gene expression for large viral genomes.

Enrichment of immediate-early 1 (m123/pp89) peptide-specific CD8 T cells in a pulmonary CD62L(lo) memory-effector cell pool during latent murine cytomegalovirus infection of the lungs

Interstitial cytomegalovirus (CMV) pneumonia is a clinically relevant complication in recipients of bone marrow transplantation (BMT). Recent data for a model of experimental syngeneic BMT and concomitant infection of BALB/c mice with murine CMV (mCMV) have documented the persistence of tissue-resident CD8 T cells after clearance of productive infection of the lungs (J. Podlech, R. Holtappels, M.-F. Pahl-Seibert, H.-P. Steffens, and M. J. Reddehase, J. Virol. 74:7496-7507, 2000). It was proposed that these cells represent antiviral "standby" memory cells whose functional role might be to help prevent reactivation of latent virus. The pool of pulmonary CD8 T cells was composed of two subsets defined by the T-cell activation marker L-selectin (CD62L): a CD62L(hi) subset of quiescent memory cells, and a CD62L(lo) subset of recently resensitized memory-effector cells. In this study, we have continued this line of investigation by quantitating CD8 T cells specific for the three currently published antigenic peptides of mCMV: peptide YPHFMPTNL processed from the immediate-early protein IE1 (pp89), and peptides YGPSLYRRF and AYAGLFTPL, derived from the early proteins m04 (gp34) and M84 (p65), respectively. IE1-specific CD8 T cells dominated in acute-phase pulmonary infiltrates and were selectively enriched in latently infected lungs. Notably, most IE1-specific CD8 T cells were found to belong to the CD62L(lo) subset representing memory-effector cells. This finding is in accordance with the interpretation that IE1-specific CD8 T cells are frequently resensitized during latent infection of the lungs and may thus be involved in the maintenance of mCMV latency.

The major immediate-early gene ie3 of mouse cytomegalovirus is essential for viral growth

The significance of the major immediate-early gene ie3 of mouse cytomegalovirus (MCMV) and that of the corresponding ie2 gene of human cytomegalovirus to viral replication are not known. To investigate the function of the MCMV IE3 regulatory protein, we generated two different MCMV recombinants that contained a large deletion in the IE3 open reading frame (ORF). The mutant genomes were constructed by the bacterial artificial chromosome mutagenesis technique, and MCMV ie3 deletion mutants were reconstituted on a mouse fibroblast cell line that expresses the MCMV major immediate-early genes. The ie3 deletion mutants failed to replicate on normal mouse fibroblasts even when a high multiplicity of infection was used. The replication defect was rescued when the IE3 protein was provided in trans by a complementing cell line. A revertant virus in which the IE3 ORF was restored was able to replicate with wild-type kinetics in normal mouse fibroblasts, providing evidence that the defective growth phenotype of the ie3 mutants was due to disruption of the ie3 gene. To characterize the point of restriction in viral replication that is controlled by ie3, we analyzed the pattern of expression of selective early (beta) and late (gamma) genes. While we could detect transcripts for the immediate-early gene ie1 in cells infected with the ie3 mutants, we failed to detect transcripts for representative beta and gamma genes. These data demonstrate that the MCMV transactivator IE3 plays an indispensable role during viral replication in tissue culture, implicating a similar role for the human CMV ie2 gene product. To our knowledge, the ie3 deletion mutants represent the first MCMV recombinants isolated that contain a disruption of an essential gene.

Enhanced green fluorescent protein as a marker for localizing murine cytomegalovirus in acute and latent infection

A recombinant murine cytomegalovirus (mCMV) that expresses enhanced green fluorescent protein (EGFP) under control of the native immediate-early 1/3 promoter was constructed to detect directly sites of viral activity in latent and reactivated infections. The recombinant virus had acute and latent infection characteristics similar to those of wild-type mCMV. Rare green-fluorescing foci were observed in paraffin sections from lungs and spleens infected latently. Positive immunoperoxidase staining for EGFP in sections of the same lung tissues suggests that these cells may be sites of restricted viral gene expression. EGFP was detected easily in tissue explants reactivating from latent infection in vitro. Morphology and adhesion characteristics of fluorescing cells suggest that viral reactivation occurs in tissue macrophages in explant cultures. The observations presented in this study demonstrate the usefulness of EGFP-expressing recombinants as tools for direct tracking of mCMV activity in vivo and in vitro.

Macrophages escape inhibition of major histocompatibility complex class I-dependent antigen presentation by cytomegalovirus

The mouse cytomegalovirus (MCMV) m152- and m06-encoded glycoproteins gp40 and gp48, respectively, independently downregulate major histocompatibility complex (MHC) class I surface expression during the course of productive MCMV infection in fibroblasts. As a result, presentation of an immediate-early protein pp89-derived nonapeptide to H-2L(d)-restricted CD8(+) cytotoxic T cells is completely prevented in fibroblasts. Here we demonstrate that MCMV-infected primary bone marrow macrophages and the macrophage cell line J774 constitutively present pp89 peptides during permissive MCMV infection to cytotoxic T lymphocytes (CTL). In contrast to fibroblasts, expression of the m152 and m06 genes in macrophages does not affect surface expression of MHC class I. Assessment of pp89 synthesis and quantification of extracted peptide revealed a significantly higher efficiency of macrophages than of fibroblasts to process pp89 into finally trimmed peptide. The yield of pp89 peptide determined in MCMV-infected tissues of bone marrow chimeras confirmed that bone marrow-derived cells represent a prime source of pp89 processing in parenchymal organs. The finding that macrophages resist the viral control of MHC I-dependent antigen presentation reconciles the paradox of efficient induction of CMV-specific CD8(+) CTL in vivo despite extensive potential of CMVs to subvert MHC class I.

Murine model of interstitial cytomegalovirus pneumonia in syngeneic bone marrow transplantation: persistence of protective pulmonary CD8-T-cell infiltrates after clearance of acute infection

Interstitial pneumonia (IP) is a severe organ manifestation of cytomegalovirus (CMV) disease in the immunocompromised host, in particular in recipients of bone marrow transplantation (BMT). Diagnostic criteria for the definition of CMV-IP include clinical evidence of pneumonia together with CMV detected in bronchoalveolar lavage or lung biopsy. We have used the model of syngeneic BMT and simultaneous infection of BALB/c mice with murine CMV for studying the pathogenesis of CMV-IP by controlled longitudinal analysis. A disseminated cytopathic infection of the lungs with fatal outcome was observed only when reconstituting CD8 T cells were depleted. Neither CD8 nor CD4 T cells mediated an immunopathogenesis of acute CMV-IP. By contrast, after efficient hematolymphopoietic reconstitution, viral replication in the lungs was moderate and focal. The histopathological picture was dominated by preferential infiltration of CD8 T cells confining viral replication to inflammatory foci. Notably, after clearance of acute infection, CD62L(lo) and CD62L(hi) subsets of CD44(+) memory CD8 T cells were found to persist in lung tissue. One can thus operationally distinguish an early CMV-positive IP (phase 1) and a late CMV-negative IP (phase 2). According to the definition, phase 2 histopathology would not be diagnosed as a CMV-IP and could instead be misinterpreted as a CMV-induced immunopathology. We document here that phase 1 as well as phase 2 pulmonary CD8 T cells are capable of exerting effector functions and are effectual in protecting against productive infection. We propose that antiviral "stand-by" memory-effector T cells persist in the lungs to prevent virus recurrence from latency.

Development of a highly sensitive quantitative competitive PCR assay for the detection of murine cytomegalovirus DNA

Viral persistence and molecular latency are characteristic of infection by the human cytomegalovirus (HCMV). Using the murine cytomegalovirus (MCMV) as a model for human infection, a quantitative-competitive polymerase chain reaction (QC-PCR) assay was developed to detect and quantify MCMV-DNA in the salivary glands of infected mice. The QC-PCR detected high numbers of MCMV DNA copies in the absence of infectious virus. By comparing the DNA content and the results obtained from a standard semiquantitative plaque assay, it is concluded that 1 plaque-forming unit (pfu) is the equivalent of approximately 1500 viral genomes. By day 42-post infection (pi) 4x10(3) copies of DNA/1 mg tissue were sufficient to reactivate infectious virions after cyclophosphamide immunosupression. By day 90 pi, however, when the DNA load was decreased to <1.2x10(2), reactivation was not observed. These results indicate that viral reactivation will occur when the number of infectious DNA copies is equivalent about 2-3 pfu. This quantitative test may therefore help to detect CMV and the risk of reactivation in immunosupressed patients.

The putative natural killer decoy early gene m04 (gp34) of murine cytomegalovirus encodes an antigenic peptide recognized by protective antiviral CD8 T cells

Several early genes of murine cytomegalovirus (MCMV) encode proteins that mediate immune evasion by interference with the major histocompatibility complex class I (MHC-I) pathway of antigen presentation to cytolytic T lymphocytes (CTL). Specifically, the m152 gene product gp37/40 causes retention of MHC-I molecules in the endoplasmic reticulum (ER)-Golgi intermediate compartment. Lack of MHC-I on the cell surface should activate natural killer (NK) cells recognizing the "missing self." The retention, however, is counteracted by the m04 early gene product gp34, which binds to folded MHC-I molecules in the ER and directs the complex to the cell surface. It was thus speculated that gp34 might serve to silence NK cells and thereby complete the immune evasion of MCMV. In light of these current views, we provide here results demonstrating an in vivo role for gp34 in protective antiviral immunity. We have identified an antigenic nonapeptide derived from gp34 and presented by the MHC-I molecule D(d). Besides the immunodominant immediate-early nonapeptide consisting of IE1 amino acids 168-176 (IE1(168-176)), the early nonapeptide m04(243-251) is the second antigenic peptide described for MCMV. The primary immune response to MCMV generates significant m04-specific CD8 T-cell memory. Upon adoptive transfer into immunodeficient recipients, an m04-specific CTL line controls MCMV infection with an efficacy comparable to that of an IE1-specific CTL line. Thus, gp34 is the first noted early protein of MCMV that escapes viral immune evasion mechanisms. These data document that MCMV is held in check by a redundance of protective CD8 T cells recognizing antigenic peptides in different phases of viral gene expression.

Patchwork pattern of transcriptional reactivation in the lungs indicates sequential checkpoints in the transition from murine cytomegalovirus latency to recurrence

The lungs are a significant organ site of murine cytomegalovirus (mCMV) latency. We have shown that activity of the major immediate-early promoter (MIEP), which drives the transcription from the ie1-ie3 transcription unit, does not inevitably initiate the productive cycle (S. K. Kurz, M. Rapp, H.-P. Steffens, N. K. A. Grzimek, S. Schmalz, and M. J. Reddehase, J. Virol. 73:482-494, 1999). Thus, even though MIEP activity governed by the MIEP-enhancer is unquestionably the first condition for recurrence, regulation of the enhancer by transcription factors is not the only mechanism controlling latency. Specifically, during latency, focal and stochastic MIEP activity in lung tissue was found to selectively generate IE1 transcripts, while transactivator-specifying IE3 transcripts were missing. This suggested a control of mCMV latency that is effectual at IE1-IE3 precursor mRNA cotranscriptional processing. Here we have used this model for studying the kinetics of reactivation and recurrence in individual lung tissue pieces after hematoablative, genotoxic treatment. Notably, reactivation was triggered, but the number of transcriptionally active foci in the lungs did not increase over time. This result is not compatible with a model of spontaneous reactivations accumulating after withdrawal of immune control. Instead, the data support the idea that reactivation is an induced event. In some pieces, focal reactivation generated IE3 transcripts but not gB transcripts, while other pieces contained foci that had proceeded to gB transcription, and only a few foci actually reached the state of virus recurrence. This finding indicates the existence of several sequentially ordered control points in the transition from mCMV latency to recurrence.

In vivo replication, latency, and immunogenicity of murine cytomegalovirus mutants with deletions in the M83 and M84 genes, the putative homologs of human cytomegalovirus pp65 (UL83)

We previously identified two open reading frames (ORFs) of murine cytomegalovirus (MCMV), M83 and M84, which are putative homologs of the human cytomegalovirus (HCMV) UL83 tegument phosphoprotein pp65 (L. D. Cranmer, C. L. Clark, C. S. Morello, H. E. Farrell, W. D. Rawlinson, and D. H. Spector, J. Virol. 70:7929-7939, 1996). In this report, we show that unlike the M83 gene product, the M84 protein is expressed at early times in the infection and cannot be detected in the virion. To elucidate the functional differences between the two pp65 homologs in acute and latent MCMV infections, we constructed two MCMV K181 mutants in which either the M83 or M84 ORF was deleted. The resultant viruses, designated DeltaM83 and DeltaM84, respectively, were found to replicate in NIH 3T3 cells with kinetics identical to those of the parent strain. Western blot analysis demonstrated that except for the absence of M83 or M84 protein expression in the respective mutants, no global perturbations of protein expression were detected. When DeltaM83 and DeltaM84 were inoculated intraperitoneally (i.p.) into BALB/c mice, both viruses showed similar attenuated growth in the spleen, liver, and kidney. However, only DeltaM83 was severely growth restricted in the salivary glands, a phenotype that was abolished upon restoration of the M83 ORF. DeltaM83's growth was similarly restricted in the salivary glands of the resistant C3H/HeN or highly sensitive 129/J strain, as well as in the lungs of all three strains following intranasal inoculation. Using a nested-PCR assay, we found that both DeltaM83 and DeltaM84 established latency in BALB/c mice, with slightly decreased levels of DeltaM83 and DeltaM84 genomic DNAs, relative to K181, observed in the salivary glands and lungs. Immunization of BALB/c mice with 10(5) PFU of K181, DeltaM83, or DeltaM84 i.p. provided similar levels of protection against lethal challenge. Although immunization with 200 PFU of DeltaM83 also provided complete protection, this dose allowed both the immunizing and challenge viruses to establish latency in the spleen. Our results show that the two MCMV pp65 homologs differ in their expression kinetics, virion association, and influence on viral tropism and/or dissemination.

Unraveling immunity to gamma-herpesviruses: a new model for understanding the role of immunity in chronic virus infection

Murine gamma-herpesvirus 68 (gammaHV68) infection is a new model for understanding how immunity and chronic gamma-herpesvirus infection inter-relate. gammaHV68 is closely related to the human Epstein-Barr virus and Kaposi's sarcoma herpesvirus and is associated with tumors, vasculitis of the great elastic arteries and splenic fibrosis. Advances in the past year have provided an even stronger foundation for believing that gammaHV68 infection of normal and mutant mice will become the pre-eminent animal model for understanding gamma-herpesvirus pathogenesis and immunity. gammaHV68 latency has been characterized employing new assays for quantitating cells carrying the gammaHV68 genome and cells that reactivate gammaHV68 and for detecting the presence of preformed infectious virus in tissues. These advances have fostered the first steps towards a molecular definition of gammaHV68 latency. It appears that gammaHV68 shares latency programs with human gamma-herpesviruses - including the loci for gene 73, v-bcl-2 and the viral homolog of the G-protein coupled receptor. This provides candidate antigens for analysis of the role of T and B cells in regulating latency. Multiple cellular reservoirs for gammaHV68 latency were uncovered with the demonstration that gammaHV68 latently infects macrophages in addition to B cells. A critical role for B cells in regulating the nature of gammaHV68 latency was discovered and the mechanism was shown to be via alteration of the efficiency of reactivation. Studies of the response of CD4(+) and CD8(+) cells during acute and chronic gammaHV68 were performed. These new studies provide key building blocks for further development of this novel and interesting model system.

Host and viral genetics of chronic infection: a mouse model of gamma-herpesvirus pathogenesis

A general association of human and primate lymphotropic herpesviruses (gamma-herpesviruses) with the development of lymphomas, as well as other tumors, especially in immunocompromised hosts, has been well documented. The lack of relevant small animal models for human gamma-herpesviruses has impeded progress in understanding the role of these viruses in the development of chronic disease. Recent research characterizing infection of inbred strains of mice with a murine gamma-herpesvirus, gamma-herpesvirus 68 (gammaHV68), is providing insights into viral and host factors involved in the establishment and control of chronic gamma-herpesvirus infection.

The pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.

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.

Three distinct regions of the murine gammaherpesvirus 68 genome are transcriptionally active in latently infected mice

The program(s) of gene expression operating during murine gammaherpesvirus 68 (gammaHV68) latency is undefined, as is the relationship between gammaHV68 latency and latency of primate gammaherpesviruses. We used a nested reverse transcriptase PCR strategy (sensitive to approximately one copy of gammaHV68 genome for each genomic region tested) to screen for the presence of viral transcripts in latently infected mice. Based on the positions of known latency-associated genes in other gammaherpesviruses, we screened for the presence of transcripts corresponding to 11 open reading frames (ORFs) in the gammaHV68 genome in RNA from spleens and peritoneal cells of latently infected B-cell-deficient (MuMT) mice which have been shown contain high levels of reactivable latent gammaHV68 (K. E. Weck, M. L. Barkon, L. I. Yoo, S. H. Speck, and H. W. Virgin, J. Virol. 70:6775-6780, 1996). To control for the possible presence of viral lytic activity, we determined that RNA from latently infected peritoneal and spleen cells contained few or no detectable transcripts corresponding to seven ORFs known to encode viral gene products associated with lytic replication. However, we did detect low-level expression of transcripts arising from the region of gene 50 (encoding the putative homolog of the Epstein-Barr virus BRLF1 transactivator) in peritoneal but not spleen cells. Latently infected peritoneal cells consistently scored for expression of RNA derived from 4 of the 11 candidate latency-associated ORFs examined, including the regions of ORF M2, ORF M11 (encoding v-bcl-2), gene 73 (a homolog of the Kaposi's sarcoma-associated herpesvirus [human herpesvirus 8] gene encoding latency-associated nuclear antigen), and gene 74 (encoding a G-protein coupled receptor homolog, v-GCR). Latently infected spleen cells consistently scored positive for RNA derived from 3 of the 11 candidate latency-associated ORFs examined, including ORF M2, ORF M3, and ORF M9. To further characterize transcription of these candidate latency-associated ORFs, we examined their transcription in lytically infected fibroblasts by Northern analysis. We detected abundant transcription from regions of the genome containing ORF M3 and ORF M9, as well as the known lytic-cycle genes. However, transcription of ORF M2, ORF M11, gene 73, and gene 74 was barely detectable in lytically infected fibroblasts, consistent with a role of these viral genes during latent infection. We conclude that (i) we have identified several candidate latency genes of murine gammaHV68, (ii) expression of genes during latency may be different in different organs, consistent with multiple latency programs and/or multiple cellular sites of latency, and (iii) regions of the viral genome (v-bcl-2 gene, v-GCR gene, and gene 73) are transcribed during latency with both gammaHV68 and primate gammaherpesviruses. The implications of these findings for replacing previous operational definitions of gammaHV68 latency with a molecular definition are discussed.

Focal transcriptional activity of murine cytomegalovirus during latency in the lungs

Interstitial pneumonia is a frequent and critical manifestation of human cytomegalovirus (CMV) disease in immunocompromised patients, in particular in recipients of bone marrow transplantation. Previous work in the murine CMV infection model has identified the lungs as a major organ site of CMV latency and recurrence. It was open to question whether the viral genome is transcriptionally silent or active during latency. Transcription could be latency associated and thus be part of the latency phenotype. Alternatively, transcriptional activity could reflect episodes of reactivation. We demonstrate here that transcription of the immediate-early (IE) transcription unit ie1-ie3 selectively generates ie1-specific transcripts during latency. Notably, while the latent viral DNA was found to be evenly distributed in the lungs, transcription was focal and randomly distributed. This finding indicates that IE transcription is not a feature inherent to murine CMV latency but rather reflects foci of primordial reactivation. However, this reactivation did not initiate productive infection, since ie3 gene mRNA specifying the essential transactivator IE3 of murine CMV early gene expression was not detectable. Accordingly, transcripts encoding gB were absent during latency. By contrast, during induced virus recurrence, IE-phase transcription switched from focal to generalized and ie3-specific transcripts were generated. These data imply that latency and recurrence are regulated not only at the IE promoter-enhancer and that there exists an additional checkpoint at the level of precursor RNA splicing. We propose that focal transcription reflects random episodes of nonproductive reactivation that get terminated before IE3 is expressed and ignites the productive cycle.

Murine cytomegalovirus induces a Sjögren's syndrome-like disease in C57Bl/6-lpr/lpr mice

OBJECTIVE

To analyze Fas and tumor necrosis factor receptor I (TNFRI) apoptosis pathways in salivary gland inflammatory disease induced by murine cytomegalovirus (MCMV) infection.

METHODS

Four different strains of mice (C57BI/6 [B6]-+/+, Fas-deficient B6-lpr/lpr, TNFRI-deficient B6-tnfr1(0/0), and B6-tnfr1(0/0)-lpr/lpr mice) were infected intraperitoneally with the Smith strain of MCMV (1 x 10(5) plaque-forming units). Viral load was determined by a plaque assay, inflammation and apoptosis by immunohistochemistry and staining with terminal dUTP nickend labeling, and autoantibodies by enzyme-linked immunosorbent assay.

RESULTS

Infectious MCMV was not detectable by day 100. Although all MCMV-infected mice developed acute sialadenitis by day 28, a chronic (>100 days), severe salivary gland inflammation and anti-Ro and anti-La antibodies developed only in the B6-lpr/lpr mice. Apoptotic cells were detected during the acute, but not the chronic, phase of inflammation.

CONCLUSION

Both Fas- and TNFRI-mediated apoptosis contribute to the clearance of MCMV-infected cells in the salivary glands. However, because Fas-mediated apoptosis is necessary for the down-modulation of the immune response, a defect in this process can lead to a postinfection, chronic inflammatory response that resembles Sjögren's syndrome.

Control of cytomegalovirus in bone marrow transplantation chimeras lacking the prevailing antigen-presenting molecule in recipient tissues rests primarily on recipient-derived CD8 T cells

Cytomegalovirus (CMV) infection during the transient immunodeficiency after bone marrow transplantation (BMT) develops into disease unless antiviral CD8 T cells are restored in due course. Histoincompatibility between donor and recipient is associated with increased risk. Complications may include a rejection response against the foreign major histocompatibility complex (MHC) antigens and a lack of antiviral control resulting from a misfit between donor-derived T cells and the antigenic viral peptides presented in recipient tissues. Here we have established a murine model of CMV disease after experimental BMT performed across a single MHC class I disparity. Specifically, BALB/c bone marrow cells expressing the prevailing antigen-presenting molecule Ld were transplanted into the Ld gene deletion mutant BALB/c-H-2(dm2), an experimental setting that entails a selective risk of host-versus-graft but not graft-versus-host response. The reconstituted T-cell population proved to be chimeric in that it consisted of Ld-positive donor-derived and Ld-negative recipient-derived cells. Pulmonary infiltrates did not include cytolytic T cells directed against Ld. This finding implies that the infection did not trigger a host-versus-graft response. Notably, upon adoptive transfer, donor-derived CD8 T cells preferentially protected tissues of donor genotype, whereas recipient-derived CD8 T cells protected tissues of either genotype. We infer from these data that the focus on immunodominant antigens presented by Ld within the donor cell population distracted the donor T cells from protecting recipient tissues and that protection in the chimeras was therefore primarily based on recipient T cells. As a consequence, T-cell chimerism after BMT should give a positive prognosis with respect to control of CMV.

Apoptosis mediated by Fas but not tumor necrosis factor receptor 1 prevents chronic disease in mice infected with murine cytomegalovirus

The role of Fas- and TNF-receptor 1 (TNF-R1)-mediated apoptosis in the clearance of virally infected cells and in the regulation of the immune response was analyzed after murine cytomegalovirus (MCMV) infection of C57BL/6 (B6)-+/+ mice, Fas-mutant B6-lpr/lpr mice, TNF-R1 knockout B6-tnfr0/0 mice, and double-deficient B6-tnfr0/0 lpr/lpr mice. There was approximately equivalent clearance of MCMV in B6-+/+, B6-tnfr0/0, and B6-lpr/lpr mice, and by day 28 no infectious virus could be detected in the liver, kidney, lung, or peritoneal exudate. However, delayed virus clearance was observed in B6-tnfr0/0 lpr/lpr mice. An acute inflammatory response occurred in the liver, lung, and kidney of all mice, which was most severe 7 d after MCMV infection, but resolved by day 28 in B6-+/+ and B6-tnfr0/0 mice, but not in B6-lpr/lpr or B6-tnfr0/0 lpr/lpr mice. These results indicate that apoptosis mediated by either Fas or TNF-R1 is sufficient for rapid clearance of the virus. However, apoptosis induced by Fas, but not TNF-R1, is required for the downmodulation of the immune response to the virus and prevention of a chronic inflammatory reaction.

Occult herpes family viruses may increase mortality in critically ill surgical patients

BACKGROUND

Effects of cytomegalovirus (CMV) or herpes simplex virus (HSV) infection on surgical intensive care unit (SICU) patients' hospital outcome are unknown.

METHODS

Between July 1, 1994, and September 1, 1995, general SICU patients with persistent sepsis and no bacterial or fungal source identifiable had viral cultures obtained. Patients with positive broncho-alveolar lavage, blood, skin, or sputum cultures for CMV or HSV were studied.

RESULTS

Twenty eligible patients had positive viral cultures during the study period, and 85% of these patients developed subsequent bacterial and/or fungal infections. Mortality was significantly higher following viral infection than in chronic SICU patients (65% vs 35%, P <0.006). Patients with thrombocytopenia complicating their viral infection had significantly higher mortality than those without thrombocytopenia (92% vs 25%, P <0.004).

CONCLUSIONS

At least 14% of critically ill surgical patients have occult infection or reactivation of herpes family viruses. These viruses have known immunosuppressive effects, which may predispose chronic SICU patients to subsequent bacterial and fungal infection, and subsequent organ system failure and death.

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.

Interferon gamma regulates acute and latent murine cytomegalovirus infection and chronic disease of the great vessels

To define immune mechanisms that regulate chronic and latent herpesvirus infection, we analyzed the role of interferon gamma (IFN-gamma) during murine cytomegalovirus (MCMV) infection. Lethality studies demonstrated a net protective role for IFN-gamma, independent of IFN-alpha/beta, during acute MCMV infection. Mice lacking the IFN-gamma receptor (IFN-gammaR-/-) developed and maintained striking chronic aortic inflammation. Arteritis was associated with inclusion bodies and MCMV antigen in the aortic media. To understand how lack of IFN-gamma responses could lead to chronic vascular disease, we evaluated the role of IFN-gamma in MCMV latency. MCMV-infected IFN-gammaR-/- mice shed preformed infectious MCMV in spleen, peritoneal exudate cells, and salivary gland for up to 6 mo after infection, whereas the majority of congenic control animals cleared chronic productive infection. However, the IFN-gammaR was not required for establishment of latency. Using an in vitro explant reactivation model, we showed that IFN-gamma reversibly inhibited MCMV reactivation from latency. This was at least partly explained by IFN-gamma- mediated blockade of growth of low levels of MCMV in tissue explants. These in vivo and in vitro data suggest that IFN-gamma regulation of reactivation from latency contributes to control of chronic vascular disease caused by MCMV. These studies are the first to demonstrate that a component of the immune system (IFN-gamma) is necessary to regulate MCMV-associated elastic arteritis and latency in vivo and reactivation of a herpesvirus from latency in vitro. This provides a new model for analysis of the interrelationships among herpesvirus latency, the immune system, and chronic disease of the great vessels.

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.

Preemptive CD8 T-cell immunotherapy of acute cytomegalovirus infection prevents lethal disease, limits the burden of latent viral genomes, and reduces the risk of virus recurrence

In the immunocompetent host, primary cytomegalovirus (CMV) infection is resolved by the immune response without causing overt disease. The viral genome, however, is not cleared but is maintained in a latent state that entails a risk of virus recurrence and consequent organ disease. By using murine CMV as a model, we have shown previously that multiple organs harbor latent CMV and that reactivation occurs with an incidence that is determined by the viral DNA load in the respective organ (M. J. Reddehase, M. Balthesen, M. Rapp, S. Jonjic, I. Pavic, and U. H. Koszinowski. J. Exp. Med. 179:185-193, 1994). This predicts that a therapeutic intervention capable of limiting the load of latent viral genome should also reduce the risk of virus recurrence. Here we demonstrate the benefits and the limits of a preemptive CD8 T-cell immunotherapy of CMV infection in the immunocompromised bone marrow transplantation recipient. Antiviral CD8 T cells prevented CMV disease and accelerated the resolution of productive infection. The therapy also resulted in a lower load of latent CMV DNA in organs and consequently reduced the incidence of recurrence. The data thus provide a further supporting argument for clinical trials of preemptive cytoimmunotherapy of human CMV disease with CD8 T cells. However, CD8 T cells failed to clear the viral DNA. The therapy-susceptible portion of the DNA load differed between organs and was highest in the lungs. The existence of an invariant, therapy-resistant load suggests a role for immune system evasion mechanisms in the establishment of CMV latency.

Cellular localization of latent murine cytomegalovirus

Herpesviruses typically establish latent infection in their hosts. The cell(s) responsible for harboring latent virus, in most cases, is not known. Using immunofluorescence and PCR-in situ hybridization (PISH), a technique which combines the sensitivity of PCR with the localization and specificity of in situ hybridization, we provide the first direct evidence that endothelial cells are a major site of murine cytomegalovirus (MCMV) DNA in latently infected animals. These findings are consistent with existing knowledge of the biological behavior of CMV, in particular the transmission of latent CMV by solid organ and bone marrow transplantation, in both human and animal models. In addition, we have localized MCMV DNA in the lung alveolar macrophage and in bone marrow cells. Our findings confirm that bone marrow-derived hematopoietic cells are a site of CMV latency and further suggest that bone marrow may be a reservoir of infected progeny capable of migrating into the circulation and establishing latency in various tissues. These findings provide clearly needed insight into the site of latent infection which is central to an understanding of the mechanisms of reactivation.

Bone marrow failure by cytomegalovirus is associated with an in vivo deficiency in the expression of essential stromal hemopoietin genes

Bone marrow (BM) failure associated with cytomegalovirus (CMV) infection is a feared complication after clinical BM transplantation. Experiments in long-term BM cultures have indicated that BM stromal cells (BMSC) are targets of productive CMV infection, but an in situ infection of BM stroma remained to be documented, and the pathomechanism is open to question. Here we describe a murine in vivo model of lethal CMV aplastic anemia (CMV-AA). The reconstitution of hematopoietic progenitor cells expressing stem cell factor (SCF) receptor was found to be defective in CMV-AA. While murine CMV replication in permissive parenchymal tissues is cytolytic, the hematopoietic cord was found to be a site of very limited virus production with foci of reticular BMSC expressing the intranuclear viral IE1 protein, but with only a few BMSC positive for viral genome in the in situ hybridization. XX-XY BM chimeras were established in order to quantitate Y-chromosome-tagged BMSC by a PCR specific for the male-sex-determining gene Tdy. This approach revealed that murine CMV infection is not associated with a significant loss of BMSC. Despite the physical integrity of the stromal network, the functional integrity of the stroma was impaired. While housekeeping genes were expressed normally in BMSC of infected mice, the expression of genes encoding the essential hemopoietins SCF, granulocyte colony-stimulating factor, and interleukin-6 was markedly reduced. In conclusion, the mechanism of BM failure is not a stromal lesion but an insufficient stromal function. These findings explain CMV-AA as a manifestation of multiple hemopoietin deficiency.