Functional analysis of the murine cytomegalovirus chemokine receptor homologue M33: ablation of constitutive signaling is associated with an attenuated phenotype in vivo

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

The Viral G-Protein-Coupled Receptor Homologs M33 and US28 Promote Cardiac Dysfunction during Murine Cytomegalovirus Infection

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects the majority of the world population and causes lifelong latent infection. HCMV has been shown to exacerbate cardiovascular diseases, including myocarditis, vascular sclerosis, and transplant vasculopathy. Recently, we have shown that murine CMV (MCMV) recapitulates the cardiovascular dysfunction observed in patients with HCMV-induced myocarditis. To understand the viral mechanisms involved in CMV-induced heart dysfunction, we further characterized cardiac function in response to MCMV and examined virally encoded G-protein-coupled receptor homologs (vGPCRs) US28 and M33 as potential factors that promote infection in the heart. We hypothesized that the CMV-encoded vGPCRs could exacerbate cardiovascular damage and dysfunction. Three viruses were used to evaluate the role of vGPCRs in cardiac dysfunction: wild-type MCMV, a M33-deficient virus (∆M33), and a virus with the M33 open reading frame (ORF) replaced with US28, an HCMV vGPCR (i.e., US28+). Our in vivo studies revealed that M33 plays a role in promoting cardiac dysfunction by increasing viral load and heart rate during acute infection. During latency, ΔM33-infected mice demonstrated reduced calcification, altered cellular gene expression, and less cardiac hypertrophy compared with wild-type MCMV-infected mice. Ex vivo viral reactivation from hearts was less efficient in ΔM33-infected animals. HCMV protein US28 expression restored the ability of the M33-deficient virus to reactivate from the heart. US28+ MCMV infection caused damage to the heart comparable with wild-type MCMV infection, suggesting that the US28 protein is sufficient to complement the function of M33 in the heart. Altogether, these data suggest a role for vGPCRs in viral pathogenesis in the heart and thus suggest that vGPCRs promote long-term cardiac damage and dysfunction.

The CMV-encoded G protein-coupled receptors M33 and US28 play pleiotropic roles in immune evasion and alter host T cell responses

Introduction

Human cytomegalovirus (HCMV) is a global health threat due to its ubiquity and lifelong persistence in infected people. During latency, host CD8⁺ T cell responses to HCMV continue to increase in a phenomenon known as memory inflation. We used murine CMV (MCMV) as a model for HCMV to characterize the memory inflation response to wild-type MCMV (KP) and a latency-defective mutant (ΔM33_stop), which lacks M33, an MCMV chemokine receptor homolog. M33 is essential for normal reactivation from latency and this was leveraged to determine whether reactivation in vivo contributes to T cell memory inflation.

Methods

Mice were infected with wild-type or mutant MCMV and T cell responses were analyzed by flow cytometry at acute and latent time points. Ex vivo reactivation and cytotoxicity assays were carried out to further investigate immunity and virus replication. Quantitative reverse-transcriptase polymerase chain reaction (q-RTPCR) was used to examine gene expression during reactivation. MHC expression on infected cells was analyzed by flow cytometry. Finally, T cells were depleted from latently-infected B cell-deficient mice to examine the in vivo difference in reactivation between wild-type and ΔM33_stop.

Results

We found that ΔM33_stop triggers memory inflation specific for peptides derived from the immediate-early protein IE1 but not the early protein m164, in contrast to wild-type MCMV. During ex vivo reactivation, gene expression in DM33stop-infected lung tissues was delayed compared to wild-type virus. Normal gene expression was partially rescued by substitution of the HCMV US28 open reading frame in place of the M33 gene. In vivo depletion of T cells in immunoglobulin heavy chain-knockout mice resulted in reactivation of wild-type MCMV, but not ΔM33_stop, confirming the role of M33 during reactivation from latency. Further, we found that M33 induces isotype-specific downregulation of MHC class I on the cell surface suggesting previously unappreciated roles in immune evasion.

Discussion

Our results indicate that M33 is more polyfunctional than previously appreciated. In addition to its role in reactivation, which had been previously described, we found that M33 alters viral gene expression, host T cell memory inflation, and MHC class I expression. US28 was able to partially complement most functions of M33, suggesting that its role in HCMV infection may be similarly pleotropic.

Constitutive Signaling by the Human Cytomegalovirus G Protein Coupled Receptor Homologs US28 and UL33 Enables Trophoblast Migration In Vitro

Human cytomegalovirus (HCMV) encodes four homologs of G protein coupled receptors (vGPCRs), of which two, designated UL33 and US28, signal constitutively. UL33 and US28 are also conserved with chemokine receptors: US28 binds numerous chemokine classes, including the membrane bound chemokine, fractalkine; whereas UL33 remains an orphan receptor. There is emerging data that UL33 and US28 each contribute to HCMV associated disease, although no studies to date have reported their potential contribution to aberrant placental physiology that has been detected with HCMV congenital infection. We investigated the signaling repertoire of UL33 and US28 and their potential to enable trophoblast mobilization in vitro. Results demonstrate the constitutive activation of CREB by each vGPCR in ACIM-88 and HTR-8SVneo trophoblasts; constitutive NF-kB activation was detected for US28 only. Constitutive signaling by each vGPCR enabled trophoblast migration. For US28, fractalkine exhibited inverse agonist activity and dampened trophoblast migration. UL33 stimulated expression of both p38 mitogen activated (MAP) and Jun N-terminal (JNK) kinases; while p38 MAP kinase stimulated CREB, JNK was inhibitory, suggesting that UL33 dependent CREB activation was regulated by p38/JNK crosstalk. Given that chemokines and their receptors are important for placental development, these data point to the potential of HCMV UL33 and US28 to interfere with trophoblast responses which are important for normal placental development.

The mouse cytomegalovirus G protein-coupled receptor homolog, M33, coordinates key features of in vivo infection via distinct components of its signalling repertoire

Common to all cytomegalovirus (CMV) genomes analysed to date is the presence of G protein-coupled receptors (GPCR). Animal models of CMV provide insights into their role in viral fitness. The mouse cytomegalovirus (MCMV) GPCR, M33, facilitates dendritic cell (DC)-dependent viremia, the extravasation of blood-borne infected DC to the salivary gland and the frequency of reactivation events from latently-infected tissue explants. Constitutive G protein-coupled M33 signalling is required for these phenotypes, although the contribution of distinct biochemical pathways activated by M33 is unknown. M33 engages G_q/11 to constitutively activate phospholipase C β (PLCβ) and downstream cyclic AMP response-element binding protein (CREB) in vitro. Identification of a MCMV M33 mutant (M33_ΔC38) for which CREB signalling was disabled, but PLCβ activation was preserved, provided the opportunity to investigate their relevance in vivo. Following intranasal infection with MCMV M33_ΔC38, the absence of M33 CREB G_q/11-dependent signalling correlated with reduced mobilisation of lytically-infected DC to draining lymph node high endothelial venules (HEVs) and reduced viremia compared with wild type MCMV. In contrast, M33_ΔC38-infected DC within the vascular compartment extravasated to the salivary glands via a pertussis toxin-sensitive, G_i/o-dependent and CREB-independent mechanism. In the context of MCMV latency, spleen explants from M33_ΔC38-infected mice were markedly attenuated for reactivation. Taken together, these data demonstrate that key features of the MCMV lifecycle are coordinated in diverse tissues by distinct pathways of the M33 signalling repertoire. IMPORTANCE G protein-coupled receptors (GPCRs) act as cell surface molecular "switches" which regulate the cellular response to environmental stimuli. All cytomegalovirus (CMV) genomes analysed to date possess GPCR homologs with phylogenetic evidence for independent gene capture events, signifying important in vivo roles. The mouse CMV (MCMV) GPCR homolog, designated M33, is important for cell-associated virus spread and for the establishment and/or reactivation of latent MCMV infection. The signalling repertoire of M33 is distinct from cellular GPCRs and little is known of the relevance of component signalling pathways for in vivo M33 function. In this report, we show temporal and tissue-specific M33 signalling is required facilitating in vivo infection. Understanding the relevance of the viral GPCR signalling profiles for in vivo function will provide opportunities for future targeted interventions.

A Fully Protective Congenital CMV Vaccine Requires Neutralizing Antibodies to Viral Pentamer and gB Glycoprotein Complexes but a pp65 T-Cell Response Is Not Necessary

A vaccine against congenital cytomegalovirus infection is a high priority. Guinea pig cytomegalovirus (GPCMV) is the only congenital CMV small animal model. GPCMV encodes essential glycoprotein complexes for virus entry (gB, gH/gL/gO, gM/gN) including a pentamer complex (gH/gL/GP129/GP131/GP133 or PC) for endocytic cell entry. The cohorts for protection against congenital CMV are poorly defined. Neutralizing antibodies to the viral glycoprotein complexes are potentially more important than an immunodominant T-cell response to the pp65 protein. In GPCMV, GP83 (pp65 homolog) is an evasion factor, and the GP83 mutant GPCMV has increased sensitivity to type I interferon. Although GP83 induces a cell-mediated response, a GP83-only-based vaccine strategy has limited efficacy. GPCMV attenuation via GP83 null deletion mutant in glycoprotein PC positive or negative virus was evaluated as live-attenuated vaccine strains (GP83dPC+/PC-). Vaccinated animals induced antibodies to viral glycoprotein complexes, and PC+ vaccinated animals had sterilizing immunity against wtGPCMV challenge. In a pre-conception vaccine (GP83dPC+) study, dams challenged mid-2nd trimester with wtGPCMV had complete protection against congenital CMV infection without detectable virus in pups. An unvaccinated control group had 80% pup transmission rate. Overall, gB and PC antibodies are key for protection against congenital CMV infection, but a response to pp65 is not strictly necessary.

Viral G Protein-Coupled Receptors: Attractive Targets for Herpesvirus-Associated Diseases

Herpesviruses are ubiquitous pathogens that establish lifelong, latent infections in their host. Spontaneous reactivation of herpesviruses is often asymptomatic or clinically manageable in healthy individuals, but reactivation events in immunocompromised or immunosuppressed individuals can lead to severe morbidity and mortality. Moreover, herpesvirus infections have been associated with multiple proliferative cardiovascular and post-transplant diseases. Herpesviruses encode viral G protein-coupled receptors (vGPCRs) that alter the host cell by hijacking cellular pathways and play important roles in the viral life cycle and these different disease settings. In this review, we discuss the pharmacological and signaling properties of these vGPCRs, their role in the viral life cycle, and their contribution in different diseases. Because of their prominent role, vGPCRs have emerged as promising drug targets, and the potential of vGPCR-targeting therapeutics is being explored. Overall, these vGPCRs can be considered as attractive targets moving forward in the development of antiviral, cancer, and/or cardiovascular disease treatments. SIGNIFICANCE STATEMENT: In the last decade, herpesvirus-encoded G protein-coupled receptors (GPCRs) have emerged as interesting drug targets with the growing understanding of their critical role in the viral life cycle and in different disease settings. This review presents the pharmacological properties of these viral receptors, their role in the viral life cycle and different diseases, and the emergence of therapeutics targeting viral GPCRs.

CMV-encoded GPCR pUL33 activates CREB and facilitates its recruitment to the MIE locus for efficient viral reactivation

Human cytomegalovirus (HCMV) establishes life-long latent infection in hematopoietic progenitor cells and circulating monocytes in infected individuals. Myeloid differentiation coupled with immune dysregulation leads to viral reactivation, which can cause severe disease and mortality. Reactivation of latent virus requires chromatin reorganization and the removal of transcriptional repressors in exchange for transcriptional activators. While some factors involved in these processes are identified, a complete characterization of the viral and cellular factors involved in their upstream regulation remains elusive. Herein, we show the HCMV-encoded G protein-coupled receptor (GPCR), UL33, is expressed during latency. Although this viral GPCR is not required to maintain latent infection, our data reveal UL33-mediated signaling is important for efficient viral reactivation. Additionally, UL33 signaling induces cellular cyclic AMP response element binding protein (CREB1, referred to here as CREB) phosphorylation, a transcription factor that promotes reactivation when recruited to the major immediate early (MIE) enhancer/promoter. Finally, targeted pharmacological inhibition of CREB activity reverses the reactivation phenotype of the UL33 signaling-deficient mutant. In sum, our data reveal UL33-mediated signaling functions to activate CREB, resulting in successful viral reactivation.

Human Cytomegalovirus-Encoded G Protein-Coupled Receptor UL33 Facilitates Virus Dissemination via the Extracellular and Cell-to-Cell Route

Human cytomegalovirus (HCMV) encodes four G protein-coupled receptor (GPCR) homologs. Three of these receptors, UL78, US27 and US28, are known for their roles in HCMV dissemination and latency. Despite importance of its rodent orthologs for viral replication and pathogenesis, such a function is not reported for the HCMV-encoded GPCR UL33. Using the clinical HCMV strain Merlin, we show that UL33 facilitates both cell-associated and cell-free virus transmission. A UL33-deficient virus derivative revealed retarded virus spread, formation of less and smaller plaques, and reduced extracellular progeny during multi-cycle growth analysis in fibroblast cultures compared to parental virus. The growth of UL33-revertant, US28-deficient, and US28-revertant viruses were similar to parental virus under multistep growth conditions. UL33- and US28-deficient Merlin viruses impaired cell-associated virus spread to a similar degree. Thus, the growth defect displayed by the UL33-deficient virus but not the US28-deficient virus reflects UL33's contribution to extracellular transmission. In conclusion, UL33 facilitates cell-associated and cell-free spread of the clinical HCMV strain Merlin in fibroblast cultures.

The human cytomegalovirus-encoded G protein-coupled receptor UL33 exhibits oncomodulatory properties

Herpesviruses can rewire cellular signaling in host cells by expressing viral G protein-coupled receptors (GPCRs). These viral receptors exhibit homology to human chemokine receptors, but some display constitutive activity and promiscuous G protein coupling. Human cytomegalovirus (HCMV) has been detected in multiple cancers, including glioblastoma, and its genome encodes four GPCRs. One of these receptors, US28, is expressed in glioblastoma and possesses constitutive activity and oncomodulatory properties. UL33, another HCMV-encoded GPCR, also displays constitutive signaling via Gα_q, Gα_i, and Gα_s proteins. However, little is known about the nature and functional effects of UL33-driven signaling. Here, we assessed UL33's signaling repertoire and oncomodulatory potential. UL33 activated multiple proliferative, angiogenic, and inflammatory signaling pathways in HEK293T and U251 glioblastoma cells. Notably, upon infection, UL33 contributed to HCMV-mediated STAT3 activation. Moreover, UL33 increased spheroid growth in vitro and accelerated tumor growth in different in vivo tumor models, including an orthotopic glioblastoma xenograft model. UL33-mediated signaling was similar to that stimulated by US28; however, UL33-induced tumor growth was delayed. Additionally, the spatiotemporal expression of the two receptors only partially overlapped in HCMV-infected glioblastoma cells. In conclusion, our results unveil that UL33 has broad signaling capacity and provide mechanistic insight into its functional effects. UL33, like US28, exhibits oncomodulatory properties, elicited via constitutive activation of multiple signaling pathways. UL33 and US28 might contribute to HCMV's oncomodulatory effects through complementing and converging cellular signaling, and hence UL33 may represent a promising drug target in HCMV-associated malignancies.

Murine Cytomegalovirus Spread Depends on the Infected Myeloid Cell Type

Cytomegaloviruses (CMVs) colonize blood-borne myeloid cells. Murine CMV (MCMV) spreads from the lungs via infected CD11c⁺ cells, consistent with an important role for dendritic cells (DC). We show here that MCMV entering via the olfactory epithelium, a natural transmission portal, also spreads via infected DC. They reached lymph nodes, entered the blood via high endothelial venules, and then entered the salivary glands, driven by constitutive signaling of the viral M33 G protein-coupled receptor (GPCR). Intraperitoneal infection also delivered MCMV to the salivary glands via DC. However, it also seeded F4/80⁺ infected macrophages to the blood; they did not enter the salivary glands or require M33 for extravasation. Instead, they seeded infection to a range of other sites, including brown adipose tissue (BAT). Peritoneal cells infected ex vivo then adoptively transferred showed similar cell type-dependent differences in distribution, with abundant F4/80⁺ cells in BAT and CD11c⁺ cells in the salivary glands. BAT colonization by CMV-infected cells was insensitive to pertussis toxin inhibition of the GPCR signaling through G_i/o substrate, whereas salivary gland colonization was sensitive. Since salivary gland infection required both M33 and G_i/o-coupled signaling, whereas BAT infection required neither, these migrations were mechanistically distinct. MCMV spread from the lungs or nose depended on DC, controlled by M33. Infecting other monocyte populations resulted in unpredictable new infections.IMPORTANCE Cytomegaloviruses (CMVs) spread through the blood by infecting monocytes, and this can lead to disease. With murine CMV (MCMV) we can track infected myeloid cells and so understand how CMVs spread. Previous experiments have injected MCMV into the peritoneal cavity. MCMV normally enters mice via the olfactory epithelium. We show that olfactory infection spreads via dendritic cells, which MCMV directs to the salivary glands. Peritoneal infection similarly reached the salivary glands via dendritic cells. However, it also infected other monocyte types, and they spread infection to other tissues. Thus, infecting the "wrong" monocytes altered virus spread, with potential to cause disease. These results provide a basis for understanding how the monocyte types infected by human CMV might promote different infection outcomes.

Emerging roles of cytomegalovirus-encoded G protein-coupled receptors during lytic and latent infection

Cytomegaloviruses (CMVs) have developed multiple diverse strategies to ensure their replicative success and to evade immune recognition. Given the fact that G protein-coupled receptors (GPCRs) are key regulators of numerous cellular processes and modify a variety of signaling pathways, it is not surprising that CMVs and other herpesviruses have hijacked mammalian GPCRs during their coevolution. Human cytomegalovirus (HCMV) encodes for four viral GPCR homologues (vGPCRs), termed US27, US28, UL33, and UL78. Although HCMV-encoded GPCRs were first described in 1990, the pivotal functions of these viral receptor proteins were detected only recently. Here, we summarize seminal knowledge on the functions of herpesviral vGPCRs with a focus on novel roles of cytomegalovirus-encoded vGPCRs for viral spread and the regulation of latency.

Cytomegalovirus host entry and spread

Cytomegaloviruses (CMVs) are large, complex pathogens that persistently and systemically colonize most mammals. Human cytomegalovirus (HCMV) causes congenital harm, and has proved hard to control. One problem is that key vaccine targets - virus entry and spread in naive hosts - remain ill-defined. As CMVs predate human speciation, those of other mammals can provide new insight. Murine CMV (MCMV) enters new hosts via olfactory neurons. Like HCMV it binds to heparan, which is lacking from most differentiated apical epithelia but is displayed on olfactory neuronal cilia. It then spreads via infected dendritic cells (DCs), which migrate to draining lymph nodes (LNs), rejoin the circulation by entering high endothelial venules (HEVs), and extravasate into other tissues. This migration depends quantitatively on M33, a constitutively active viral G protein-coupled receptor (GPCR). The homologous US28 GPCR of HCMV can substitute for M33 in allowing MCMV-infected DCs to leave LNs via HEVs, so HCMV could potentially use the same route. The capacity of DCs to seed MCMV to tissues, and for other DCs to collect it for redistribution, suggest that DC recirculation chronically maintains and links diverse CMV reservoirs through lytic exchange.

Roles of GP33, a guinea pig cytomegalovirus-encoded G protein-coupled receptor homolog, in cellular signaling, viral growth and inflammation in vitro and in vivo

Cytomegaloviruses (CMVs) encode cellular homologs to evade host immune functions. In this study, we analyzed the roles of GP33, a guinea pig CMV (GPCMV)-encoded G protein-coupled receptor (GPCR) homolog, in cellular signaling, viral growth and pathogenesis. The cDNA structure of GP33 was determined by RACE. The effects of GP33 on some signaling pathways were analyzed in transient transfection assays. The redET two-step recombination system for a BAC containing the GPCMV genome was used to construct a mutant GPCMV containing an early stop codon in the GP33 gene (Δ33) and a rescued GPCMV (r33). We found the following: 1) GP33 activated the CRE- and NFAT-, but not the NFκB-mediated signaling pathway. 2) GP33 was dispensable for infection in tissue cultures and in normal animals. 3) In pregnant animals, viral loads of r33 in the livers, lungs, spleens, and placentas at 6 days post-infection were higher than those of Δ33, although the viruses were cleared by 3 weeks post-infection. 4) The presence of GP33 was associated with frequent lesions, including alveolar hemorrhage in the lungs, and inflammation in the lungs, livers, and spleens of the dams. Our findings suggest that GP33 has critical roles in the pathogenesis of GPCMV during pregnancy. We hypothesize that GP33-mediated signaling activates cytokine secretion from the infected cells, which results in inflammation in some of the maternal organs and the placentas. Alternatively, GP33 may facilitate transient inflammation that is induced by the chemokine network specific to the pregnancy.

Cytomegalovirus (CMV) is a major pathogen that causes congenital diseases, including birth defects and developmental abnormalities in newborns. Better understanding of the immune evasion mechanisms may open the way to the development of new types of live attenuated vaccines for congenital CMV infection. In contrast to murine and rat CMVs, guinea pig CMV (GPCMV) causes infection in utero, which makes GPCMV animal models a useful tool for understanding the pathogenesis of congenital infection and evaluation of vaccine strategies. By constructing a GPCMV mutant lacking GP33, a viral G protein-coupled receptor homolog, this study found that GP33 was involved in the induction of significant inflammatory responses in pregnant but not in normal animals. As GP33 activated the NFAT- and CRE-, but not the NFκB-signal pathway, it is plausible that GP33 enhanced cytokine expression, which results in pathogenic outcomes in the maternal organs and placentas.

Human cytomegalovirus US28 allows dendritic cell exit from lymph nodes

Human cytomegalovirus (HCMV) colonizes blood-borne dendritic cells (DCs). They express US28, a viral G protein-coupled receptor (GPCR). In vitro functions have been described for US28, but how it contributes to host colonization has been unclear. The murine CMV (MCMV) M33 GPCR promotes DC recirculation. We show that US28 shares this function. Thus, DC recirculation is also available to HCMV via US28, and inhibiting US28 G protein-dependent signalling has the potential to reduce systemic infection. We show that M33 also promotes systemic infection through infected DC extravasation.

Murine Cytomegalovirus Spreads by Dendritic Cell Recirculation

Herpesviruses have coevolved with their hosts over hundreds of millions of years and exploit fundamental features of their biology. Cytomegaloviruses (CMVs) colonize blood-borne myeloid cells, and it has been hypothesized that systemic dissemination arises from infected stem cells in bone marrow. However, poor CMV transfer by stem cell transplantation argues against this being the main reservoir. To identify alternative pathways for CMV spread, we tracked murine CMV (MCMV) colonization after mucosal entry. We show that following intranasal MCMV infection, lung CD11c⁺ dendritic cells (DC) migrated sequentially to lymph nodes (LN), blood, and then salivary glands. Replication-deficient virus followed the same route, and thus, DC infected peripherally traversed LN to enter the blood. Given that DC are thought to die locally following their arrival and integration into LN, recirculation into blood represents a new pathway. We examined host and viral factors that facilitated this LN traverse. We show that MCMV-infected DC exited LN by a distinct route to lymphocytes, entering high endothelial venules and bypassing the efferent lymph. LN exit required CD44 and the viral M33 chemokine receptor, without which infected DC accumulated in LN and systemic spread was greatly reduced. Taken together, our studies provide the first demonstration of virus-driven DC recirculation. As viruses follow host-defined pathways, high endothelial venules may normally allow DC to pass from LN back into blood.

Human cytomegalovirus (HCMV) causes devastating disease in the unborn fetus and in the immunocompromised. There is no licensed vaccine, and preventive measures are impeded by our poor understanding of early events in host colonization. HCMV and murine CMV (MCMV) both infect blood-borne myeloid cells. HCMV-infected blood cells are thought to derive from infected bone marrow stem cells. However, infected stem cells have not been visualized in vivo nor shown to produce virus ex vivo, and hematopoietic transplants poorly transfer infection. We show that MCMV-infected dendritic cells in the lungs reach the blood via lymph nodes, surprisingly migrating into high endothelial venules. Dissemination did not require viral replication. It depended on the constitutively active viral chemokine receptor M33 and on the host hyaluronan receptor CD44. Thus, viral chemokine receptors are a possible target to limit systemic CMV infections.

Identification and functional characterisation of 5-HT4 receptor in sea cucumber Apostichopus japonicus (Selenka)

Serotonin (5-HT) is an important neurotransmitter and neuromodulator that controls a variety of sensory and motor functions through 5-HT receptors (5-HTRs). The 5-HT4R subfamily is linked to Gs proteins, which activate adenylyl cyclases (ACs), and is involved in many responses in peripheral organs. In this study, the 5-HT4R from Apostichopus japonicus (Aj5-HT4R) was identified and characterised. The cloned full-length Aj5-HT4R cDNA is 1,544 bp long and contains an open reading frame 1,011 bp in length encoding 336 amino acid proteins. Bioinformatics analysis of the Aj5-HT4R protein indicated this receptor was a member of class A G protein coupled receptor (GPCR) family. Further experiments using Aj5-HT4R-transfected HEK293 cells demonstrated that treatment with 5-HT triggered a significant increase in intracellular cAMP level in a dose-dependent manner and induced a rapid internalisation of Aj5-HT4R fused with enhanced green fluorescent protein (Aj5-HT4R-EGFP) from the cell surface into the cytoplasm. In addition, the transcriptional profiles of Aj5-HT4R in aestivating A. japonicas and phosphofructokinase (AjPFK) in 5-HT administrated A. japonicus have been analysed by real-time PCR assays. Results have led to a basic understanding of Aj5-HT4R in A. japonicus, and provide a foundation for further exploration of the cell signaling and regulatory functions of this receptor.

Luciferase-tagged wild-type and tropism-deficient mouse cytomegaloviruses reveal early dynamics of host colonization following peripheral challenge

Cytomegaloviruses (CMVs) establish persistent, systemic infections and cause disease by maternal-foetal transfer, suggesting that their dissemination is a key target for antiviral intervention. Late clinical presentation has meant that human CMV (HCMV) dissemination is not well understood. Murine CMV (MCMV) provides a tractable model. Whole mouse imaging of virus-expressed luciferase has proved a useful way to track systemic infections. MCMV, in which the abundant lytic gene M78 was luciferase-tagged via a self-cleaving peptide (M78-LUC), allowed serial, unbiased imaging of systemic and peripheral infection without significant virus attenuation. Ex vivo luciferase imaging showed greater sensitivity than plaque assay, and revealed both well-known infection sites (the lungs, lymph nodes, salivary glands, liver, spleen and pancreas) and less explored sites (the bone marrow and upper respiratory tract). We applied luciferase imaging to tracking MCMV lacking M33, a chemokine receptor conserved in HCMV and a proposed anti-viral drug target. M33-deficient M78-LUC colonized normally in peripheral sites and local draining lymph nodes but spread poorly to the salivary gland, suggesting a defect in vascular transport consistent with properties of a chemokine receptor.

The Cytoplasmic C-Tail of the Mouse Cytomegalovirus 7 Transmembrane Receptor Homologue, M78, Regulates Endocytosis of the Receptor and Modulates Virus Replication in Different Cell Types

Virus homologues of seven-transmembrane receptors (7TMR) are encoded by all beta- and gammaherpesviruses, suggesting important functional roles. M78 of mouse cytomegalovirus (MCMV) is representative of a family of 7TMR conserved in all betaherpesviruses. M78 family members have been found to exhibit cell-type specific effects upon virus replication in tissue culture and to affect virus pathogenesis in vivo. We reported previously that M78, for which no ligands are known, undergoes rapid, constitutive endocytosis. In this study, we have investigated the role of the M78 cytoplasmic C-tail in mediating endocytosis and consequences of C-tail deletion upon replication and pathogenesis. Mutations of M78 (C-tail truncations or point mutations) and CCR5-M78 chimeras identified two distinct regions affecting endocytosis. The first was a classical acidic di-leucine motif (DDxxxLL), located close to the C-terminus. The second region, the activity of which was suppressed by downstream sequences, included the putative 8^th helix, located close to the 7^th transmembrane domain. A recombinant MCMV expressing an endocytosis-deficient M78, lacking most of the C-tail (M78_CΔ155), had a cell-type specific replication phenotype. M78_CΔ155 had restricted replication in bone marrow macrophages, indistinguishable from an M78-null recombinant. In contrast, M78_CΔ155 replicated normally or with enhanced titres to wild type virus in other tested cell-types, whereas M78-null was attenuated. Distinct phenotypes for M78_CΔ155 and M78-null suggest that the C-tail deletion resulted in M78 dysfunction, rather than complete loss of function; furthermore, they highlight a cell-type specific role of M78 during replication. Infection of mice (intranasal) demonstrated that M78_CΔ155, similar to M78-null, was cleared more rapidly from the lungs than wild type virus and was severely attenuated for replication in salivary glands. It may be speculated that attenuation of both M78_CΔ155 and M78-null for replication in macrophages may have contributed to their similar pathogenic phenotypes.

The M33 G protein-coupled receptor encoded by murine cytomegalovirus is dispensable for hematogenous dissemination but is required for growth within the salivary gland

Human cytomegalovirus (HCMV) is a pathogen found worldwide and is a serious threat to immunocompromised individuals and developing fetuses. Due to the species specificity of cytomegaloviruses, murine cytomegalovirus (MCMV) has been used as a model for in vivo studies of HCMV pathogenesis. The MCMV genome, like the genomes of other beta- and gammaherpesviruses, encodes G protein-coupled receptors (GPCRs) that modulate host signaling pathways presumably to facilitate viral replication and dissemination. Among these viral receptors, the M33 GPCR carried by MCMV is an activator of CREB, NF-κB, and phospholipase C-β signaling pathways and has been implicated in aspects of pathogenesis in vivo, including persistence in the salivary glands of BALB/c mice. In this study, we used immunocompetent nonobese diabetic (NOD) and immunocompromised NOD-scid-gamma (NSG) mice to further investigate the salivary gland defect exhibited by M33 deficiency. Interestingly, we demonstrate that virus with an M33 deletion (ΔM33) can replicate in the salivary gland of immunocompromised animals, albeit with a 400-fold growth defect compared with the growth of wild-type virus. Moreover, we determined that M33 does not have a role in cell-associated hematogenous dissemination but is required for viral amplification once the virus reaches the salivary gland. We conclude that the reduced replicative capacity of the ΔM33 virus is due to a specific defect occurring within the localized environment of the salivary gland. Importantly, since the salivary gland represents a site essential for persistence and horizontal transmission, an understanding of the mechanisms of viral replication within this site could lead to the generation of novel therapeutics useful for the prevention of HCMV spread. Importance: Human cytomegalovirus infects the majority of the American people and can reside silently in infected individuals for the duration of their lives. Under a number of circumstances, the virus can reactivate, leading to a variety of diseases in both adults and developing babies, and therefore, identifying the function of viral proteins is essential to understand how the virus spreads and causes disease. We aim to utilize animal models to study the function of an important class of viral proteins termed G protein-coupled receptors with the ultimate goal of developing inhibitors to these proteins that could one day be used to prevent viral spread.

Intracellular trafficking of the human cytomegalovirus-encoded 7-trans-membrane protein homologs pUS27 and pUL78 during viral infection: a comparative analysis

Human cytomegalovirus (HCMV) encodes four G protein-coupled receptor (GPCR) homologs, termed pUS27, pUS28, pUL33, and pUL78. In contrast to the extensively characterized vGPCRs pUS28 and pUL33, knowledge concerning pUS27 and pUL78 is limited. Previous studies already demonstrated constitutive internalization of pUS27 and pUL78, as well as an association with the endosomal machinery, however, these results were mainly obtained using transiently transfected cells. To explore the subcellular localization of both receptors during viral infection, we constructed recombinant HCMVs expressing tagged vGPCRs. Colocalization analyses revealed a predominant association of pUS27 or pUL78 with the trans-Golgi network or the endoplasmic reticulum, respectively. Intriguingly, our data emphasize that protein sorting is highly regulated by viral functions as we detected dramatic changes in the colocalization of pUS27 and pUL78 with endosomal markers during progression of HCMV replication. Furthermore, we observed cell type-dependent differences in trafficking of both vGPCRs between fibroblasts and epithelial cells. Most importantly, infection experiments with a recombinant HCMV carrying tagged versions of pUS27 and pUL78 simultaneously, revealed that these two proteins do not colocalize during viral infection. This contrasts to results of transient expression experiments. In conclusion, our results highlight the importance to investigate vGPCR trafficking in a viral context.

Methods for studying the function of cytomegalovirus GPCRs

All of the cytomegaloviruses discovered to date encode two or more genes with significant homology to G-protein coupled receptors (GPCRs). The functions of these cytomegalovirus GPCRs are just beginning to be elucidated; however, it is clear that they exhibit numerous interesting activities in both in vitro and in vivo systems. In this chapter, we review the various methodologies that can be used to examine biochemical aspects of viral GPCR signaling in vitro as well as examine the biological activity of these viral GPCRs in vitro and in vivo in virus infected cells using recombinant cytomegaloviruses.

Dual analysis of the murine cytomegalovirus and host cell transcriptomes reveal new aspects of the virus-host cell interface

Major gaps in our knowledge of pathogen genes and how these gene products interact with host gene products to cause disease represent a major obstacle to progress in vaccine and antiviral drug development for the herpesviruses. To begin to bridge these gaps, we conducted a dual analysis of Murine Cytomegalovirus (MCMV) and host cell transcriptomes during lytic infection. We analyzed the MCMV transcriptome during lytic infection using both classical cDNA cloning and sequencing of viral transcripts and next generation sequencing of transcripts (RNA-Seq). We also investigated the host transcriptome using RNA-Seq combined with differential gene expression analysis, biological pathway analysis, and gene ontology analysis. We identify numerous novel spliced and unspliced transcripts of MCMV. Unexpectedly, the most abundantly transcribed viral genes are of unknown function. We found that the most abundant viral transcript, recently identified as a noncoding RNA regulating cellular microRNAs, also codes for a novel protein. To our knowledge, this is the first viral transcript that functions both as a noncoding RNA and an mRNA. We also report that lytic infection elicits a profound cellular response in fibroblasts. Highly upregulated and induced host genes included those involved in inflammation and immunity, but also many unexpected transcription factors and host genes related to development and differentiation. Many top downregulated and repressed genes are associated with functions whose roles in infection are obscure, including host long intergenic noncoding RNAs, antisense RNAs or small nucleolar RNAs. Correspondingly, many differentially expressed genes cluster in biological pathways that may shed new light on cytomegalovirus pathogenesis. Together, these findings provide new insights into the molecular warfare at the virus-host interface and suggest new areas of research to advance the understanding and treatment of cytomegalovirus-associated diseases.

Autoimmunity and infection in Sjögren's syndrome

PURPOSE OF REVIEW

To summarize the recent developments concerning the potential viral pathomechanisms and involvement of viruses in Sjögren's syndrome, and to highlight the areas for future research and therapies.

RECENT FINDINGS

Activated IFN-1 pathway plays an important part in the autoimmune disease process of Sjögren's syndrome; therefore, several therapies aiming to reduce or inhibit the IFN-1 production and its effects may be a target for future treatment plans. Activated aryl hydrocarbon receptor may interact with latent Epstein-Barr virus (EBV) infection, which in turn may predispose to the development of Sjögren's syndrome. It is estimated that the population is 95% positive for EBV serology. Microbial factors may incite autoimmune disease. Although this hypothesis is proven in a few illnesses such as rheumatic fever, there is no definitive evidence of an infectious environmental trigger in Sjögren's syndrome. However, there are circumstantial data with regard to viruses and several potential mechanisms of disease. These include antigen mimicry, polyclonal lymphocyte activation, and infection-mediated innate end-organ inflammation. In addition, hepatitis C virus infection clearly causes a Sjögren's-syndrome-like illness.

SUMMARY

Data continue to implicate viral infection in the cause of Sjögren's syndrome, but there are no definitive studies incriminating a particular virus.

Identification of common mechanisms by which human and mouse cytomegalovirus seven-transmembrane receptor homologues contribute to in vivo phenotypes in a mouse model

The mouse cytomegalovirus chemokine receptor homologue (CKR) M33 is required for salivary gland tropism and efficient reactivation from latency, phenotypes partially rescued by the human cytomegalovirus CKR US28. Herein, we demonstrate that complementation of salivary gland tropism is mediated predominantly by G protein-dependent signaling conserved with that of M33; in contrast, both G protein-dependent and -independent pathways contribute to the latency phenotypes. A novel M33-dependent replication phenotype in cultured bone marrow macrophages is also described.

US28 is a potent activator of phospholipase C during HCMV infection of clinically relevant target cells

Members of the cytomegalovirus family each encode two or more genes with significant homology to G-protein coupled receptors (GPCRs). In rodent models of pathogenesis, these viral encoded GPCRs play functionally significant roles, as their deletion results in crippled viruses that cannot traffic properly and/or replicate in virally important target cells. Of the four HCMV encoded GPCRs, US28 has garnered the most attention due to the fact that it exhibits both agonist-independent and agonist-dependent signaling activity and has been demonstrated to promote cellular migration and proliferation. Thus, it appears that the CMV GPCRs play important roles in viral replication in vivo as well as promote the development of virus-associated pathology. In the current study we have utilized a series of HCMV/US28 recombinants to investigate the expression profile and signaling activities of US28 in a number of cell types relevant to HCMV infection including smooth muscle cells, endothelial cells and cells derived from glioblastoma multiforme (GBM) tumors. The results indicate that US28 is expressed and exhibits constitutive agonist-independent signaling activity through PLC-β in all cell types tested. Moreover, while CCL5/RANTES and CX3CL1/Fractalkine both promote US28-dependent Ca⁺⁺ release in smooth muscle cells, this agonist-dependent effect appears to be cell-specific as we fail to detect US28 driven Ca⁺⁺ release in the GBM cells. We have also investigated the effects of US28 on signaling via endogenous GPCRs including those in the LPA receptor family. Our data indicate that US28 can enhance signaling via endogenous LPA receptors. Taken together, our results indicate that US28 induces a variety of signaling events in all cell types tested suggesting that US28 signaling likely plays a significant role during HCMV infection and dissemination in vivo.

Structural Diversity in Conserved Regions Like the DRY-Motif among Viral 7TM Receptors-A Consequence of Evolutionary Pressure?

Several herpes- and poxviruses have captured chemokine receptors from their hosts and modified these to their own benefit. The human and viral chemokine receptors belong to class A 7 transmembrane (TM) receptors which are characterized by several structural motifs like the DRY-motif in TM3 and the C-terminal tail. In the DRY-motif, the arginine residue serves important purposes by being directly involved in G protein coupling. Interestingly, among the viral receptors there is a greater diversity in the DRY-motif compared to their endogenous receptor homologous. The C-terminal receptor tail constitutes another regulatory region that through a number of phosphorylation sites is involved in signaling, desensitization, and internalization. Also this region is more variable among virus-encoded 7TM receptors compared to human class A receptors. In this review we will focus on these two structural motifs and discuss their role in viral 7TM receptor signaling compared to their endogenous counterparts.

Human cytomegalovirus-encoded UL33 and UL78 heteromerize with host CCR5 and CXCR4 impairing their HIV coreceptor activity

Human cytomegalovirus (HCMV) encodes four 7-transmembrane-spanning (7TM) proteins, US28, US27, UL33, and UL78, which present important sequence homology with human chemokine receptors. Whereas US28 binds a large range of chemokines and disturbs host cell signaling at different levels, the others are orphans with largely unknown functions. Assembly of 2 different 7TM proteins into hetero-oligomeric complexes may profoundly change their respective functional properties. We show that HCMV-encoded UL33 and UL78 form heteromers with CCR5 and CXCR4 chemokine receptors in transfected human embryonic kidney 293T cells and monocytic THP-1 cells. Expression of UL33 and UL78 had pleiotropic, predominantly negative, effects on CCR5 and CXCR4 cell surface expression, ligand-induced internalization, signal transduction, and migration without modifying the chemokine binding properties of CCR5 and CXCR4. Importantly, the coreceptor activity of CCR5 and CXCR4 for HIV was largely impaired in the presence of UL33 and UL78 without affecting expression of the primary HIV entry receptor CD4 and its interaction with CCR5 and CXCR4. Collectively, we identified the first molecular function for the HCMV-encoded orphan UL33 and UL78 7TM proteins, namely the regulation of cellular chemokine receptors through receptor heteromerization.

Partial functional complementation between human and mouse cytomegalovirus chemokine receptor homologues

The human cytomegalovirus (CMV) proteins US28 and UL33 are homologous to chemokine receptors (CKRs). Knockout of the mouse CMV M33 protein (UL33 homologue) results in substantial attenuation of salivary gland infection/replication and reduced efficiency of reactivation from tissue explants. M33-mediated G protein-coupled signaling is critical for the salivary gland phenotype. In this report, we demonstrate that US28 and (to a lesser degree) UL33 restore reactivation from tissue explants and partially restore replication in salivary glands (compared to a signaling-deficient M33 mutant). These studies provide a novel small animal model for evaluation of therapies targeting the human CMV CKRs.

Herpesvirus-encoded GPCRs rewire cellular signaling

Viral G-protein-coupled receptors (vGPCRs) are chemokine receptor homologues encoded by the Herpes- and Capripoxviridae. They are thought to have been hijacked from the host genome during the course of evolution. These vGPCRs play different roles in the viral lifecycle and associated pathologies. Three members of the Herpesviridae, Kaposi sarcoma-associated herpesvirus (KSHV), Epstein-Barr virus (EBV) and human cytomegalovirus (HCMV) are capable of setting up persistent latent infections in humans. Two of the herpesviruses, KSHV and EBV, are associated with cancer, while HCMV may have an oncomodulary effect. The vGPCRs may contribute to the escape of immune surveillance and (constitutively) activate signaling pathways linked to proliferation and inflammation. Some vGPCRs induce activation of autocrine and paracrine signaling, resulting in secretion of growth factors and/or cytokines. As a result, vGPCRs effectively rewire cellular signaling networks. Delineating the cellular signaling networks modulated by these vGPCRs will be crucial for treatment of virus-associated pathologies.

The carboxy-terminal tail of human cytomegalovirus (HCMV) US28 regulates both chemokine-independent and chemokine-dependent signaling in HCMV-infected cells

The human cytomegalovirus (HCMV)-encoded G-protein-coupled receptor (GPCR) US28 is a potent activator of a number of signaling pathways in HCMV-infected cells. The intracellular carboxy-terminal domain of US28 contains residues critical for the regulation of US28 signaling in heterologous expression systems; however, the role that this domain plays during HCMV infection remains unknown. For this study, we constructed an HCMV recombinant virus encoding a carboxy-terminal domain truncation mutant of US28, FLAG-US28/1-314, to investigate the role that this domain plays in US28 signaling. We demonstrate that US28/1-314 exhibits a more potent phospholipase C-beta (PLC-beta) signal than does wild-type US28, indicating that the carboxy-terminal domain plays an important role in regulating agonist-independent signaling in infected cells. Moreover, HMCV-infected cells expressing the US28/1-314 mutant exhibit a prolonged calcium signal in response to CCL5, indicating that the US28 carboxy-terminal domain also regulates agonist-dependent signaling. Finally, while the chemokine CX3CL1 behaves as an inverse agonist or inhibitor of constitutive US28 signaling to PLC-beta, we demonstrate that CX3CL1 functions as an agonist with regard to US28-stimulated calcium release. This study is the first to demonstrate that the carboxy terminus of US28 controls US28 signaling in the context of HCMV infection and indicates that chemokines such as CX3CL1 can decrease constitutive US28 signals and yet simultaneously promote nonconstitutive US28 signals.

Herpesvirus-encoded G protein-coupled receptors as modulators of cellular function

Human herpesviruses (HHVs) are widespread pathogens involved in proliferative diseases, inflammatory conditions, and cardiovascular diseases. During evolution, homologs of human chemokine receptors were integrated into the HHV genomes. In addition to binding endogenous chemokines, these viral G protein-coupled receptors (vGPCRs) have acquired the ability to signal in a constitutive manner. Ligand-induced and ligand-independent and autocrine and paracrine signaling properties of vGPCRs modify the functions of the expressing cells and lead to transformation and escape from immune surveillance. Furthermore, cross-talk or heterodimerization with endogenous chemokine receptors represent other ways for vGPCRs to modify intracellular signaling and cellular functions. As such, these viral receptors seem to play a prominent role during viral pathogenesis and life cycle and thus represent innovative antiviral therapies.

Activation of intracellular signaling pathways by the murine cytomegalovirus G protein-coupled receptor M33 occurs via PLC-{beta}/PKC-dependent and -independent mechanisms

The presence of numerous G protein-coupled receptor (GPCR) homologs within the herpesvirus genomes suggests an essential role for these genes in viral replication in the infected host. Such is the case for murine cytomegalovirus (MCMV), where deletion of the M33 GPCR or replacement of M33 with a signaling defective mutant has been shown to severely attenuate replication in vivo. In the present study we utilized a genetically altered version of M33 (termed R131A) in combination with pharmacological inhibitors to further characterize the mechanisms by which M33 activates downstream signaling pathways. This R131A mutant of M33 fails to support salivary gland replication in vivo and, as such, is an important tool that can be used to examine the signaling activities of M33. We show that M33 stimulates the transcription factor CREB via heterotrimeric G(q/11) proteins and not through promiscuous coupling of M33 to the G(s) pathway. Using inhibitors of signaling molecules downstream of G(q/11), we demonstrate that M33 stimulates CREB transcriptional activity in a phospholipase C-beta and protein kinase C (PKC)-dependent manner. Finally, utilizing wild-type and R131A versions of M33, we show that M33-mediated activation of other signaling nodes, including the mitogen-activated protein kinase family member p38alpha and transcription factor NF-kappaB, occurs in the absence of G(q/11) and PKC signaling. The results from the present study indicate that M33 utilizes multiple mechanisms to modulate intracellular signaling cascades and suggest that signaling through PLC-beta and PKC plays a central role in MCMV pathogenesis in vivo.

The M33 chemokine receptor homolog of murine cytomegalovirus exhibits a differential tissue-specific role during in vivo replication and latency

M33, encoded by murine cytomegalovirus (MCMV), is a member of the UL33 homolog G-protein-coupled receptor (GPCR) family and is conserved across all the betaherpesviruses. Infection of mice with recombinant viruses lacking M33 or containing specific signaling domain mutations in M33 results in significantly diminished MCMV infection of the salivary glands. To determine the role of M33 in viral dissemination and/or infection in other tissues, viral infection with wild-type K181 virus and an M33 mutant virus, DeltaM33B(T2), was characterized using two different routes of inoculation. Following both intraperitoneal (i.p.) and intranasal (i.n.) inoculation, M33 was attenuated for infection of the spleen and pancreas as early as 7 days after infection. Following i.p. inoculation, DeltaM33B(T2) exhibited a severe defect in latency as measured by a diminished capacity to reactivate from spleens and lungs in reactivation assays (P < 0.001). Subsequent PCR analysis revealed markedly reduced DeltaM33B(T2) viral DNA levels in the latently infected spleens, lungs, and bone marrow. Following i.n. inoculation, latent DeltaM33B(T2) viral DNA was significantly reduced in the spleen and, in agreement with results from i.p. inoculation, did not reactivate from the spleen (P < 0.001). Furthermore, in vivo complementation of DeltaM33B(T2) virus replication and/or dissemination to the salivary glands and pancreas was achieved by coinfection with wild-type virus. Overall, our data suggest a critical tissue-specific role for M33 during infection in the salivary glands, spleen, and pancreas but not the lungs. Our data suggest that M33 contributes to the efficient establishment or maintenance of long-term latent MCMV infection.

Capripoxvirus G-protein-coupled chemokine receptor: a host-range gene suitable for virus animal origin discrimination

The genus Capripoxvirus within the family Poxviridae comprises three closely related viruses, namely goat pox, sheep pox and lumpy skin disease viruses. This nomenclature is based on the animal species from which the virus was first isolated, respectively, goat, sheep and cattle. Since capripoxviruses are serologically identical, their specific identification relies exclusively on the use of molecular tools. We describe here the suitability of the G-protein-coupled chemokine receptor (GPCR) gene for use in host-range grouping of capripoxviruses. The analysis of 58 capripoxviruses showed three tight genetic clusters consisting of goat pox, sheep pox and lumpy skin disease viruses. However, a few discrepancies exist with the classical virus-host origin nomenclature: a virus isolated from sheep is grouped in the goat poxvirus clade and vice versa. Intra-group diversity was further observed for the goat pox and lumpy skin disease virus isolates. Despite the presence of nine vaccine strains, no genetic determinants of virulence were identified on the GPCR gene. For sheep poxviruses, the addition or deletion of 21 nucleic acids (7 aa) was consistently observed in the 5' terminal part of the gene. Specific signatures for each cluster were also identified. Prediction of the capripoxvirus GPCR topology, and its comparison with other known mammalian GPCRs and viral homologues, revealed not only a classical GPCR profile in the last three-quarters of the protein but also unique features such as a longer N-terminal end with a proximal hydrophobic alpha-helix and a shorter serine-rich C-tail.

Feeling manipulated: cytomegalovirus immune manipulation

No one likes to feel like they have been manipulated, but in the case of cytomegalovirus (CMV) immune manipulation, we do not really have much choice. Whether you call it CMV immune modulation, manipulation, or evasion, the bottom line is that CMV alters the immune response in such a way to allow the establishment of latency with lifelong shedding. With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. Modulation of host immunity allows survival of both the virus and the host. One way of describing it would be a kind of "mutually assured survival" (as opposed to MAD, Mutually Assured Destruction). Evaluation of this relationship provides important insights into the life cycle of CMV as well as a greater understanding of the complexity of the immune response to pathogens in general.

Viral hijacking of human receptors through heterodimerization

Epstein-Barr virus (EBV) is a human herpesvirus that primarily infects B lymphocytes and is associated with tumor development. Like other herpesviruses, EBV has pirated and modified host genes encoding important regulatory cellular proteins to take over cellular control after infection. One of these genes (BILF1) encodes a G protein-coupled receptor (GPCR). It is currently accepted that GPCRs exist and function as dimers. B lymphocyte migration and functioning is regulated by chemokines acting on their cognate receptors. In this study, we show that BILF1 heterodimerizes with various chemokine receptors using BRET, trFRET and co-immunoprecipitation. Importantly, heterodimerization of BILF1 with chemokine receptors may alter the responsiveness of B lymphocytes to chemokines thereby altering homing and homeostasis of infected B lymphocytes and might be essential for EBV dissemination and/or involved in EBV-induced pathogenesis.