The Maastricht strain and England strain of rat cytomegalovirus represent different betaherpesvirus species rather than strains

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

Rat cytomegalovirus (RCMV) English isolate and a newly identified Berlin isolate share similarities with but are separate as an anciently diverged clade from Mouse CMV and the Maastricht isolate of RCMV

The genome of the rat cytomegalovirus (RCMV) English isolate (MuHV-8) differs significantly from the RCMV Maastricht isolate (MuHV-2) and other cytomegaloviruses (CMVs) in its size, base composition and genomic content. Analysis of the RCMV-Berlin isolate, MuHV-8, revealed that the two MuHV-8 isolates are highly similar in genome size and content, indicating that the smaller genome size (202 946 bp) compared to other known CMVs was not the result of an accidental deletion during passage in tissue culture. Surprisingly, the proteins encoded in MuHV-8 shared more overall similarity with their orthologues from mouse CMV (MuHV-1) compared to their orthologues in rat CMV (MuHV-2). Phylogenetic analyses of conserved viral genes showed that the two MuHV-8 isolates are from the same species and represent a unique clade that is distinct from other rodent CMVs.

Complete genome sequence of the english isolate of rat cytomegalovirus (Murid herpesvirus 8)

The complete genome of the English isolate of rat cytomegalovirus (RCMV-E) was determined. RCMV-E has a 202,946-bp genome with noninverting repeats but without terminal repeats. Thus, it differs significantly in size and genomic arrangement from closely related rodent cytomegaloviruses (CMVs). To account for the differences between the rat CMV isolates of Maastricht and England, RCMV-E was classified as Murid herpesvirus 8 by the International Committee on Taxonomy of Viruses.

Identification of a novel betaherpesvirus in Mus musculus

Rodent betaherpesviruses vary considerably in genomic content, and these variations can result in a distinct pathogenicity. Therefore, the identification of unknown betaherpesviruses in house mice (Mus musculus), the most important rodent host species in basic research, is of importance. During a search for novel herpesviruses in house mice using herpesvirus consensus PCR and attempts to isolate viruses in tissue culture, we identified a previously unknown betaherpesvirus. The primary PCR search in mouse organs revealed the presence of known strains of murine cytomegalovirus (Murid herpesvirus 1) and of Mus musculus rhadinovirus 1 only. However, the novel virus was detected after incubation of organ pieces in fibroblast tissue culture and subsequent PCR analysis of the supernatants. Long-distance PCR amplification including the DNA polymerase and glycoprotein B genes revealed a 3.4 kb sequence that was similar to sequences of rodent cytomegaloviruses. Pairwise sequence comparisons and phylogenetic analyses showed that this newly identified murine virus is most similar to the English isolate of rat cytomegalovirus, thereby raising the possibility that two distinct CMV lineages have evolved in both Mus musculus and Rattus norvegicus.

Evolution of the G+C Content Frontier in the Rat Cytomegalovirus Genome

Within the 230138 bp of the rat cytomegalovirus (RCMV) genome, the G+C content changes abruptly at position 142644, constituting a G+C content frontier. To the left of this point, overall G+C content is 69.2%, and to the right it is only 47.6%. A region of extremely low G+C content (33.8%) is found in the 5 kb immediately to the right of the frontier, in which there are no predicted coding sequences. To the right of position 147501, the G+C content rises and predicted coding sequences reappear. However, these genes are much shorter (average 848 bp, 50% G+C) than those in the left two-thirds of the genome (average 1462 bp, 70% G+C). Whole genome alignment of several viruses indicates that the initial ultra-low G+C region appeared in the common ancestor of the genera Cytomegalovirus and Muromegalovirus, and that the lowering of G+C in the right third has been a subsequent process in the lineage leading to RCMV. The left two-thirds of RCMV has stop codon occurrences at 67.5% of their expected level, based on a modified Markov chain model of stop codon distribution, and the corresponding figure for the right third is 78%. Therefore, despite heavy mutation pressure, selective constraint has operated in the right third of the RCMV genome to maintain a degree of gene length unusual for such low G+C sequences.

Role of the cytomegalovirus major immediate early enhancer in acute infection and reactivation from latency

The cytomegalovirus (CMV) major immediate early (MIE) enhancer-containing promoter regulates the expression of the downstream MIE genes, which have critical roles in reactivation from latency and acute infection. The enhancer consists of binding sites for cellular transcription factors that are repeated multiple times. The primate and nonprimate CMV enhancers can substitute for one another. The enhancers are not functionally equivalent, but they do have overlapping activities. The CMV MIE enhancers are located between divergent promoters where the leftward genes are critical and essential for reactivation from latency and acute infection and the rightward gene is nonessential. The rightward transcription unit is controlled by an enhancer for murine CMV. In contrast, human CMV has a set of repressor elements that prevents enhancer effects on the rightward viral promoter. The human CMV enhancer that controls the leftward transcription unit has a distal component that is nonessential at high multiplicity of infection (MOI), but has a significant impact on the MIE gene expression at low MOI. The proximal enhancer influences directly the level of transcription of the MIE genes and contains an essential Sp-1 site. The MIE promoter has a site adjacent to the transcription start site that is essential at the earliest stage of infection. The MIE enhancer-containing promoter responds to signal transduction events and to cellular differentiation. The role of the CMV MIE enhancer-containing promoter in acute infection and reactivation from latency are reviewed.

Identification of novel rodent herpesviruses, including the first gammaherpesvirus of Mus musculus

Rodent herpesviruses such as murine cytomegalovirus (host, Mus musculus), rat cytomegalovirus (host, Rattus norvegicus), and murine gammaherpesvirus 68 (hosts, Apodemus species) are important tools for the experimental study of human herpesvirus diseases. However, alphaherpesviruses, roseoloviruses, and lymphocryptoviruses, as well as rhadinoviruses, that naturally infect Mus musculus (house mouse) and other Old World mice are unknown. To identify hitherto-unknown rodent-associated herpesviruses, we captured M. musculus, R. norvegicus, and 14 other rodent species in several locations in Germany, the United Kingdom, and Thailand. Samples of trigeminal ganglia, dorsal root ganglia, brains, spleens, and other organs, as well as blood, were analyzed with a degenerate panherpesvirus PCR targeting the DNA polymerase (DPOL) gene. Herpesvirus-positive samples were subjected to a second degenerate PCR targeting the glycoprotein B (gB) gene. The sequences located between the partial DPOL and gB sequences were amplified by long-distance PCR and sequenced, resulting in a contiguous sequence of approximately 3.5 kbp. By DPOL PCR, we detected 17 novel betaherpesviruses and 21 novel gammaherpesviruses but no alphaherpesvirus. Of these 38 novel herpesviruses, 14 were successfully analyzed by the complete bigenic approach. Most importantly, the first gammaherpesvirus of Mus musculus was discovered (Mus musculus rhadinovirus 1 [MmusRHV1]). This virus is a member of a novel group of rodent gammaherpesviruses, which is clearly distinct from murine herpesvirus 68-like rodent gammaherpesviruses. Multigenic phylogenetic analysis, using an 8-kbp locus, revealed that MmusRHV1 diverged from the other gammaherpesviruses soon after the evolutionary separation of Epstein-Barr virus-like lymphocryptoviruses from human herpesvirus 8-like rhadinoviruses and alcelaphine herpesvirus 1-like macaviruses.

Identification and characterization of two antisense transcripts from the major immediate early region of rat cytomegalovirus

We have identified and characterized two antisense transcripts from the rat cytomegalovirus (RCMV) major immediate early (MIE) region. These transcripts, designated IE-AS1 and IE-AS2, are complementary to part of the sense IE1 transcript. The IE-AS transcripts were first detected in peripheral blood leukocytes (PBL) of RCMV-infected rats at 7 days post-infection (pi) in the absence of IE1 transcription. Nevertheless, both the IE1 and IE-AS transcripts were found at the same time in the salivary glands of RCMV-infected rats at 7 and 120 days pi as well as in RCMV-infected rat embryo fibroblasts (REFs) at 48 h pi.

Herpesviral Fcgamma receptors: culprits attenuating antiviral IgG?

Production of IgG in response to virus infection is central to antiviral immune effector functions and a hallmark of B cell memory. Antiviral antibodies (Abs) recognising viral glycoproteins or protein antigen displayed on the surface of virions or virus-infected cells are crucial in rendering the virus noninfectious and in eliminating viruses or infected cells, either acting alone or in conjunction with complement. In many instances, passive transfer of Abs is sufficient to protect from viral infection. Herpesviruses (HV) are equipped with a large array of immunomodulatory functions which increase the efficiency of infection by dampening the antiviral immunity. Members of the α- and β-subfamily of the Herpesviridae are distinct in encoding transmembrane glycoproteins which selectively bind IgG via its Fc domain. The Fc-binding proteins constitute viral Fcγ receptors (vFcγRs) which are expressed on the cell surface of infected cells. Moreover, vFcγRs are abundantly incorporated into the envelope of virions. Despite their molecular and structural heterogeneity, the vFcγRs generally interfere with IgG-mediated effector functions like antibody (Ab)-dependent cellular cytolysis, complement activation and neutralisation of infectivity of virions. vFcγRs may thus contribute to the limited therapeutic potency of antiherpesviral IgG in clinical settings. A detailed molecular understanding of vFcγRs opens up the possibility to design recombinant IgG molecules resisting vFcγRs. Engineering IgG with a better antiviral efficiency represents a new therapeutic option against herpesviral diseases.

The English strain of rat cytomegalovirus (CMV) contains a novel captured CD200 (vOX2) gene and a spliced CC chemokine upstream from the major immediate-early region: further evidence for a separate evolutionary lineage from that of rat CMV Maastricht

Sequence data for eight genes, together with time-course Northern blotting and 3′- and 5′-RACE (rapid amplification of cDNA ends) analysis for some mRNAs from a 12 kb region upstream from the major immediate-early (MIE) genes of the English isolate of rat cytomegalovirus (RCMV), are presented. The results identified important differences compared to both murine cytomegalovirus (MCMV) and the Maastricht isolate of RCMV. A striking finding is the presence of a highly conserved, rightwards-oriented homologue of the rat cellular CD200 (OX2) gene immediately to the right of the MIE region, which replaces either the leftwards-oriented AAV REP gene of RCMV (Maastricht) or the upstream spliced portions of the immediate-early 2 gene (ie2) in MCMV. From the presence of other homologues of MCMV- and RCMV-specific genes, such as the β-chemokine MCK-2, SGG1 and an Fcγ receptor gene, as reported here, the basic architecture of the MIE region (reported previously) and the level of IE2 and DNA polymerase (POL) protein conservation in phylogenetic analyses, it is clear that the English strain of RCMV is also a member of the genus Muromegalovirus, but is a β-herpesvirus species that is very distinct from both MCMV and RCMV (Maastricht). Both the lack of a CD200 homologue in the other two rodent viruses and the depth of sequence divergence of the rodent CMV IE2 and POL proteins suggest that these three viruses have evolved as separate species in the genus Muromegalovirus since very early in the host rodent lineage.

Isolates of cytomegalovirus (CMV) from the black rat Rattus rattus form a distinct group of rat CMV

Two different betaherpesviruses, the English and Maastricht species of rat cytomegalovirus (CMV), have previously been isolated from Rattus norvegicus. CMVs were isolated from both the brown rat, R. norvegicus, and the black rat, R. rattus, within Australia. The viruses isolated from R. norvegicus appeared to be genetically related to the English species of rat CMV by PCR, RFLP, and sequencing, but the viruses isolated from R. rattus were distinct from both prototype virus species, although more closely genetically related to the Maastricht virus. This is the first genetic characterization of cytomegaloviruses from R. rattus, and the first isolation of CMVs from Australian rats.

Prevention of cytomegalovirus infection-enhanced experimental obliterative bronchiolitis by antiviral prophylaxis or immunosuppression in rat tracheal allografts

In this study, the prevention of rat cytomegalovirus (RCMV) infection-enhanced experimental obliterative bronchiolitis in rat tracheal allografts was investigated. RCMV infection markedly enhanced cell proliferation and histological changes of obliterative bronchiolitis, a form of chronic rejection after lung transplantation. These alterations were linked to increased interleukin (IL)-2 and tumor necrosis factor-alpha (TNF-alpha) immunoreactivity, and reduction of IL-10 expression. In recipient rats with acute RCMV infection, prophylaxis with either ganciclovir (DHPG) or hyperimmune serum (HIS) totally prevented RCMV infection-enhanced tracheal occlusion. DHPG treatment initiated during acute RCMV infection also reduced lesion development but markedly less than DHPG prophylaxis. Treatment of acute RCMV infection with HIS alone or in combination with DHPG had no significant effect on tracheal occlusion. Inhibition of the transcription of cytokines by high doses of cyclosporine A significantly reduced RCMV infection-enhanced tracheal obliteration. In rats with chronic RCMV infection, obliterative alterations were prevented by DHPG prophylaxis initiated at the time of transplantation. Prophylaxis either with DHPG or HIS did not affect the amount of infectious RCMV recovered from host salivary glands, nor were there differences seen in RCMV major immediate early DNA expression in tracheal allografts between different antiviral drug regimens. Immunohistochemical analysis of allografts revealed that inhibition of tracheal occlusion by antiviral prophylaxis was associated with a reduction in the number of ED1(+) macrophages and cells staining for Th1 cytokines and TNF-alpha, while immune modulation by cyclosporine A up-regulated IL-10 production. In conclusion, the results of the present study suggest that the CMV infection-enhanced chronic rejection develops independently of viral load but requires both immune activation and simultaneous CMV gene expression beyond immediate early genes.

Rat cytomegalovirus major immediate-early enhancer switching results in altered growth characteristics

It has been hypothesized that the major immediate-early (MIE) enhancer of cytomegalovirus (CMV) is important in determining virus tropism and latency because of its essential role in initiating the cascade of early gene expression necessary for virus replication. Although rat CMV (RCMV) and murine CMV (MCMV) exhibit extreme species specificity in vivo, they differ in their ability to replicate in tissue culture. MCMV can replicate in a rat embryo fibroblast (REF) cell line while RCMV does not grow in murine fibroblasts. The tropism is not due to a block in virus entry into the cell. We have constructed a recombinant RCMV in which the RCMV MIE enhancer has been replaced with that of MCMV. Growth of the recombinant virus in tissue culture remains restricted to rat cells, suggesting that other viral and/or host factors are more important in determining in vitro tropism. Unlike findings using recombinant MCMV in which the human CMV (HCMV) MIE enhancer substitutes for the native one (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998), infection with our recombinant virus at a low multiplicity of infection resulted in a substantial decrease in virus replication. This occurred despite comparable or increased MIE transcription from the recombinant virus. In vivo experiments showed that the recombinant virus replicates normally in the spleen during acute infection. Notably, the recombinant virus appears to be deficient in spreading to the salivary gland, suggesting a role for the MIE enhancer in tropism for certain tissues involved in virus dissemination. Four months after infection, recombinant virus with the foreign MIE enhancer was reactivated from spleen explants.

Cytomegalovirus infection, viral DNA, and immediate early-1 gene expression in rejecting rat liver allografts

BACKGROUND

Cytomegalovirus (CMV) infection has been linked to acute and chronic rejection. We have previously shown that concomitant rat cytomegalovirus (RCMV) infection increases portal inflammation and bile duct destruction in rejecting rat liver allografts. Many of the pro-inflammatory effects of CMV have been attributed to the immediate early (IE) proteins of CMV. We wanted to investigate whether RCMV and IE-1 gene expression persist in the liver graft in our model.

METHODS

Liver transplantations were performed from PVG (RT1c) into BN (RT1n) rats. One day after transplantation, the rats were infected with RCMV. No immunosuppression was given. The graft infection was studied by viral culture, immunofluorescence, DNA in situ hybridization and RT-PCR for the detection of IE-1 mRNA at various time points.

RESULTS

RCMV caused an active infection from 5 days to 2 weeks after transplantation, during which infectious virus was found in the graft. Thereafter the cultures were negative. RCMV antigens and DNA were found in hepatocytes, endothelial, inflammatory, and bile duct cells during the active infection. At 4 weeks, RCMV DNA positive hepatocytes, endothelial, inflamma tory, and bile duct cells could still be found, but in much smaller quantities. IE-1 mRNA expression was, however, only detected during the active infection, not at 4 weeks postinfection.

CONCLUSIONS

RCMV IE-1 expression does not persist in the graft after the active infection, although some viral DNA can be detected in the graft up to 4 weeks. In our model, the CMV-induced increase in graft damage does not seem to require the continued expression of IE-1.

Identification and characterization of a spliced C-type lectin-like gene encoded by rat cytomegalovirus

The English isolate of rat cytomegalovirus (RCMV) encodes a 20-kDa protein with a C-type lectin-like domain that is expressed in the delayed-early and late phases of the viral replication cycle. Genomic sequence analysis of the restriction fragment KpnR of RCMV revealed significant homology to several C-type lectin-containing molecules implicated in natural killer (NK) and T-cell interactions, as well as genes from four poxviruses and African swine fever virus. The gene is spliced into five exons and shows a splicing pattern with exon boundaries similar to those observed in the human differentiation antigen CD69. The cap site of the gene was mapped by RNase protection, 5' rapid amplification of cDNA ends, and primer extension experiments. This analysis demonstrated that the core promoter of the RCMV lectin-like gene contains a GATA rather than a TATA box. Splicing patterns were confirmed with isolates from an infected-cell cDNA library. A unique aspect of the protein is that its translation is not initiated by the canonical methionine but rather by alanine. To study its role in virus replication and pathogenesis, a recombinant virus was constructed in which the gene is interrupted. Replication in tissue culture was similar to that of wild-type virus.

Rat cytomegalovirus R89 is a highly conserved gene which expresses a spliced transcript

In all sequenced herpesvirus genomes, a homolog of the herpes simplex virus type 1 UL15 gene has been identified. This gene encodes a protein that is involved in viral genome maturation. Although transcription of the alphaherpesvirus UL15 gene has been analyzed in detail, not much is known about the expression of its betaherpesvirus homologs. We therefore set out to characterize transcription of the rat cytomegalovirus counterpart of UL15, R89. Here we report that R89 consists of two exons separated by a 4.7-kb intron. The spliced R89 transcript, which is expressed at late times postinfection (p.i.), has the capacity to encode a protein of 670 amino acids with a calculated molecular mass of 77.1 kDa. The predicted amino acid sequence of this protein is highly similar to that of the proteins predicted to be encoded by the human cytomegalovirus UL89 and murine cytomegalovirus M89 genes (64.3 and 84.5% overall identity, respectively). The region between R89 exon 1 and exon 2 was found to contain five additional genes, r90, R91, R92, R93 and R94, the latter two of which are conserved among all herpesviruses. We show that these genes are transcribed in a highly complex fashion, resulting in numerous mono- and polycistronic mRNAs.

Complete DNA sequence of the rat cytomegalovirus genome

We have determined the complete genome sequence of the Maastricht strain of rat cytomegalovirus (RCMV). The RCMV genome has a length of 229,896 bp and is arranged as a single unique sequence flanked by 504-bp terminal direct repeats. RCMV was found to have counterparts of all but one of the open reading frames (ORFs) that are conserved between murine CMV (MCMV) and human CMV (HCMV). Like HCMV, RCMV lacks homologs of the genes belonging to the MCMV m02 glycoprotein gene family. However, RCMV contains 15 ORFs with homology to members of the MCMV m145 glycoprotein gene family. Four ORFs are predicted to encode homologs of host proteins; R33 and R78 both putatively encode G protein-coupled receptors, whereas r144 and r131 encode homologs of major histocompatibility class I heavy chains and CC chemokines, respectively. An intriguing feature of the RCMV genome is the presence of an ORF, r127, with similarity to the rep gene of parvoviruses as well as ORF U94 of human herpesvirus 6A (HHV-6A) and HHV-6B. Counterparts of these ORFs have not been found in the other sequenced herpesviruses.

The relationship between acute rejection and chronic rejection is highly dependent on specific MHC matching: a multi-strain rat heterotopic heart transplant study

BACKGROUND

The impact of acute rejection, immunosuppression, and infection, specifically cytomegalovirus infection, on the development of chronic rejection in the cardiac allograft, has been the subject of a large number of investigations. One of the difficulties in finding associations has been the marked immunologic heterogeneity of the patient population coupled with the lack of the ability to HLA match. Furthermore, the ideal animal model, which duplicates as well as controls for this immunologic heterogeneity, is lacking.

METHODS

To try to simulate differences in HLA matching, immunosuppression regiments and cytomegalovirus infection, heterotopic heart transplantation was performed in two separate, complete MHC mismatch, rat strain combinations (WF-LEW, BN-LEW) requiring chronic immunosuppression and employing four separate immunosuppression/infection protocols. Animals were followed for 6 months, killed, and rejection and vascular changes were scored blinded to the group.

RESULTS

The mean vascular and acute rejection scores were not significantly different between treatment regiments for either specific strain combination. There was a trend for the subtherapeutic groups to have higher vascular scores. Overall, there were no significant differences in vascular scores between the WF-LEW and BN-LEW groups (1.25+/-0.18 vs. 1.13+/-0.20, P=NS). Similar numbers of WF-LEW and BN-LEW exhibited cellular infiltration and necrosis of the allograft, but the intensity of the response (rejection score) was more severe in the WF-LEW combination (4.54+/-0.22 vs. 3.92+/-0.21, P=0.052) when limiting the analysis to those with myocyte necrosis. There was no significant correlation between acute rejection and vascular lesion severity in the WF-LEW combination (r=0.22, P=NS) but a high correlation between these parameters in the BN-LEW combination (r=0.74, P<0.0001).

CONCLUSIONS

These data suggest that, although acute rejection and chronic rejection are related, MHC matching may influence their interdependence. These data also may explain why the clinical association between acute and chronic rejection is difficult to demonstrate.

Production of high-titer stocks of the English strain of rat cytomegalovirus

A simple technique to increase dramatically the yield of rat cytomegalovirus (RCMV) from infected monolayers of a rat embryo fibroblast-derived cell line is described. The method, which involves daily changes of the cell culture medium, can result in a 50000-fold amplification of virus from cell monolayers inoculated with only a few RCMV particles. This modification of the standard in vitro culture technique to amplify viral yield can be used to increase the sensitivity of the plaque assay for detecting very small amounts of infectious virus in organ homogenates of RCMV infected animals.

The r144 major histocompatibility complex class I-like gene of rat cytomegalovirus is dispensable for both acute and long-term infection in the immunocompromised host

The rat cytomegalovirus (RCMV) r144 gene encodes a polypeptide homologous to major histocompatibility complex class I heavy chains. To study the role of r144 in virus replication, an RCMV r144 null mutant strain (RCMVDeltar144) was generated. This strain replicated with efficiency similar to that of wild-type (WT) RCMV in vitro. Additionally, WT RCMV and RCMVDeltar144 were found not to differ in their replication characteristics in vivo. First, the survival rate was similar among groups of immunosuppressed rats infected with either RCMVDeltar144 or WT RCMV. Second, the dissemination of virus did not differ in either RCMVDeltar144- or WT RCMV-infected, immunosuppressed rats, either in the acute phase of infection or approximately 1 year after infection. These data indicate that the RCMV r144 gene is essential neither for virus replication in the acute phase of infection nor for long-term infection in immunocompromised rats. Interestingly, in a local infection model in which footpads of immunosuppressed rats were inoculated with virus, a significantly higher number of infiltrating macrophage cells as well as of CD8(+) T cells was observed in WT RCMV-infected paws than in RCMVDeltar144-infected paws. This suggests that r144 might function in the interaction with these leukocytes in vivo.

The rat cytomegalovirus R32 gene encodes a virion-associated protein that elicits a strong humoral immune response in infected rats

A gene of rat cytomegalovirus (RCMV), designated R32, has been identified that encodes a homologue of the human cytomegalovirus (HCMV) pp150 (ppUL32) major tegument phosphoprotein. The R32 ORF has the capacity to encode a 667 amino acid polypeptide (pR32) with a calculated molecular mass of 73 kDa. The predicted amino acid sequence of pR32 shows similarity to that of polypeptides predicted to be encoded by the HCMV UL32, murine cytomegalovirus M32 and human herpesvirus types 6 and 7 U11 genes. The R32 gene is transcribed as a 2.5 kb mRNA during the late phase of RCMV infection in rat embryo fibroblasts in vitro. To study expression of the pR32 protein in vitro and in vivo, a rabbit polyclonal antiserum was raised against a recombinant protein that comprised amino acids 252-522 of pR32. By using this antiserum, pR32 could be detected predominantly in the cytoplasm of RCMV-infected fibroblasts at 24 and 48 h post-infection in vitro. The pR32 protein was also detected within virions isolated from the culture medium of RCMV-infected cells. Expression of pR32 in vivo was observed within the cytoplasm of salivary gland epithelial cells of RCMV-infected rats. In addition, recombinant pR32 was found to react with sera from rats that were previously infected with RCMV, whereas reactivity was not seen with sera from mock-infected rats. Together, these findings indicate that RCMV pR32 represents the homologue of HCMV ppUL32, both in primary structure and in function.

Deletion of the R78 G protein-coupled receptor gene from rat cytomegalovirus results in an attenuated, syncytium-inducing mutant strain

The rat cytomegalovirus (RCMV) R78 gene belongs to an uncharacterized class of viral G protein-coupled receptor (GCR) genes. The predicted amino acid sequence of the R78 open reading frame (ORF) shows 25 and 20% similarity with the gene products of murine cytomegalovirus M78 and human cytomegalovirus UL78, respectively. The R78 gene is transcribed throughout the early and late phases of infection in rat embryo fibroblasts (REF) in vitro. Transcription of R78 was found to result in three different mRNAs: (i) a 1.8-kb mRNA containing the R78 sequence, (ii) a 3.7-kb mRNA containing both R77 and R78 sequences, and (iii) a 5.7-kb mRNA containing at least ORF R77 and ORF R78 sequences. To investigate the function of the R78 gene, we generated two different recombinant virus strains: an RCMV R78 null mutant (RCMVDeltaR78a) and an RCMV mutant encoding a GCR from which the putative intracellular C terminus has been deleted (RCMVDeltaR78c). These recombinant viruses replicated with a 10- to 100-fold-lower efficiency than wild-type (wt) virus in vitro. Interestingly, unlike wt virus-infected REF, REF infected with the recombinants develop a syncytium-like appearance. A striking difference between wt and recombinant viruses was also seen in vivo: a considerably higher survival was seen among recombinant virus-infected rats than among RCMV-infected rats. We conclude that the RCMV R78 gene encodes a novel GCR-like polypeptide that plays an important role in both RCMV replication in vitro and the pathogenesis of viral infection in vivo.