Search

Simple Summary The Epstein–Barr virus (EBV) is a γ-herpesvirus residing in over 90% of adults worldwide. Besides causing a benign glandular fever (infectious mononucleosis), EBV is also associated with a wide range of different types of cancers. This review will present these malignancies, current therapies, and summarize the present knowledge on an EBV-encoded oncogenic protein called BILF1. As a member of class A G protein–coupled receptors that are intrinsically successful drug targets, BILF1 will be discussed for its potential as future target in EBV-associated diseases. Finally, ongoing development of novel EBV-specific therapeutics is briefly outlined. Abstract The γ-herpesvirus Epstein–Barr Virus (EBV) establishes lifelong infections in approximately 90% of adults worldwide. Up to 1,000,000 people yearly are estimated to suffer from health conditions attributed to the infection with this virus, such as nasopharyngeal and gastric carcinomas as well as several forms of B, T and NK cell lymphoma. To date, no EBV-specific therapeutic option has reached the market, greatly reducing the survival prognoses of affected patients. Similar to other herpesviruses, EBV encodes for a G protein–coupled receptor (GPCR), BILF1, affecting a multitude of cellular signaling pathways. BILF1 has been identified to promote immune evasion and tumorigenesis, effectively ensuring a life-long persistence of EBV in, and driving detrimental health conditions to its host. This review summarizes the epidemiology of EBV-associated malignancies, their current standard-of-care, EBV-specific therapeutics in development, GPCRs and their druggability, and most importantly consolidates the findings of over 15 years of research on BILF1 in the context of EBV-specific drug development. Taken together, BILF1 constitutes a promising target for the development of novel EBV-specific therapeutics.

Background Transmission of latent human cytomegalovirus (HCMV) via organ transplantation with post-transplant viral reactivation is extremely prevalent and results in substantial adverse impact on outcomes. Therapies targeting the latent reservoir within the allograft to mitigate viral transmission would represent a major advance. Here, we delivered an immunotoxin (F49A-FTP) that targets and kills latent HCMV aiming at reducing the HCMV reservoir from donor lungs using ex-vivo lung perfusion (EVLP). Methods HCMV seropositive human lungs were placed on EVLP alone or EVLP+1mg/L of F49A-FTP for 6 hours (n=6 , each). CD14+ monocytes isolated from biopsies pre and post EVLP underwent HCMV reactivation assay designed to evaluate viral reactivation capacity. Off-target effects of F49A-FTP were studied evaluating cell death markers of CD34+ and CD14+ cells using flow cytometry. Lung function on EVLP and inflammatory cytokine production were evaluated as safety endpoints. Results We demonstrate that lungs treated ex-vivo with F49A-FTP had a significant reduction in HCMV reactivation compared to controls, suggesting successful targeting of latent virus (76% median reduction in F49A-FTP vs. 15% increase in controls, p=0.0087). Furthermore, there was comparable cell death rates of the targeted cells between both groups, suggesting no off-target effects. Ex-vivo lung function was stable over 6 hours and no differences in key inflammatory cytokines were observed demonstrating safety of this novel treatment. Conclusions Ex-vivo F49A-FTP treatment of human lungs targets and kills latent HCMV, markedly attenuating HCMV reactivation. This approach demonstrates the first experiments targeting latent HCMV in a donor organ with promising results towards clinical translation.

Purpose Post-transplant CMV reactivation from latently-infected lung allografts is extremely prevalent and results in inferior long-term outcomes. CMV latently infected cells ubiquitously express a viral surface protein, US28. We aimed to evaluate the therapeutic effects of a novel immunotoxin (F49A-FTP) that targets US28 in human lungs during EVLP, with the ultimate goal of minimizing or preventing CMV transmission and reactivation post-transplantation. Methods Human lungs rejected for transplantation were randomly placed on EVLP alone or EVLP+1mg/L of F49A-FTP (n=6 each) for 6 hours. Biopsies were collected pre and post EVLP and subjected to an in vitro assay designed to evaluate viral reactivation capacity (Fig 1A). Based on US28’s homology to the human CX3CR1 chemokine receptor, potential off-target effects of the toxin were studied evaluating cell death markers of CD34+ and CD14+ using flow cytometry, and the ratio of cells post:pre perfusion were calculated. Lung function on EVLP and inflammatory cytokine production were evaluated as safety endpoints. Results Delivery of F49A-FTP during EVLP showed no acute toxic events demonstrated by no significant differences in lung physiology and levels of IL-8, IL1-b and IFN-γ between the groups. Furthermore, while control lung tissue demonstrated a median increase from baseline of 32% (Range -16% to +112.5%) in viral reactivation, treated lungs demonstrated a significant median reduction in CMV reactivation of 76% (Range -15% to -99.9%) (Fig 1B, p=0.0087). No off-target effects were noted upon F49A-FTP delivery, demonstrated by similar amounts of cell death between the groups (Fig 1C). Conclusion We demonstrate that ex vivo F49A-FTP treatment of human lungs on EVLP markedly attenuates CMV reactivation and could potentially be used to treat donor organs prior to transplant. Ongoing in vivo reactivation experiments using the EVLP treated samples in a humanized mouse will further validate these findings.

Purpose Donor to recipient CMV mismatch leads to high incidence of CMV infection post lung transplant leading to inferior long-term patient outcomes. CMV latently infected cells ubiquitously express a surface marker, US28. We aimed to evaluate the therapeutic effects of a novel fusion toxin protein (F49A-FTP) that targets US28 in human lungs during EVLP, with the ultimate goal of minimizing or preventing CMV reactivation post transplantation. Methods Human lungs rejected for transplantation were randomly placed on EVLP alone or EVLP+1mg/L of F49A-FTP (n=6 each) for 6 hours. Biopsies were collected to evaluate viral reactivation using an in vitro assay that induces latent CMV to reactivate to an infectious phase. Since US28 has 38% homology to the human CX3CR1 chemokine receptor, potential off-target effects of the toxin were studied using flow cytometry for cell death assessment of CD34+ and CD14+ cells. Lung function on EVLP was evaluated as a safety endpoint. Results F49A-FTP was delivered through the PA on EVLP with no acute toxic events based on physiological parameters (Figure 1A). We performed an in vitro assay using tissue samples obtained after EVLP to stimulate CMV to reactivate in order to assess efficacy of the drug. After primary culture from lung tissue samples, relative reactivation events (assessed with IFF staining for IE protein of lytic CMV, Figure 1B) occurred in 100% of controls vs. only 0.04±0.02% of treated lungs. For off-target analysis, we performed flow cytometry and calculated the ratio of post:pre perfusion CD34+ and CD14+ cells (Figure 1C). There were no changes in number of cells for both groups: CD34+ ratio 0.8±0.5 vs. 0.6±0.5 (control and F49A-FTP, respectively, p=0.4974); CD14+ ratio 1.54±0.74 vs. 1.66±1.13 (control and F49A-FTP, respectively, p=0.8660). Conclusion Our study demonstrates that ex vivo F49A-FTP treatment of human lungs on EVLP markedly attenuates CMV reactivation. Ongoing ex vivo and in vivo reactivation experiments will confirm these findings.

The use of receptor-ligand interactions to direct toxins to kill diseased cells selectively has shown considerable promise for treatment of a number of cancers and, more recently, autoimmune disease. Here we move the fusion toxin protein (FTP) technology beyond cancer/autoimmune therapeutics to target the human viral pathogen, human cytomegalovirus (HCMV), on the basis of its expression of the 7TM G protein-coupled chemokine receptor US28. The virus origin of US28 provides an exceptional chemokine-binding profile with high selectivity and improved binding for the CX3C chemokine, CX3CL1. Moreover, US28 is constitutively internalizing by nature, providing highly effective FTP delivery. We designed a synthetic CX3CL1 variant engineered to have ultra-high affinity for US28 and greater specificity for US28 than the natural sole receptor for CX3CL1, CX3CR1, and we fused the synthetic variant with the cytotoxic domain of Pseudomonas Exotoxin A. This novel strategy of a rationally designed FTP provided unparalleled anti-HCMV efficacy and potency in vitro and in vivo.

Epstein-Barr virus (EBV)-induced receptor 2 (EBI2) is an orphan seven-transmembrane (7TM) receptor originally identified as the most up-regulated gene (>200-fold) in EBV-infected cells. Here we show that EBI2 signals with constitutive activity through Gαi as determined by a receptor-mediated inhibition of forskolin-induced cAMP production and an induction of the serum response element-driven transcriptional activity in a pertussis toxin-sensitive manner. Gαs and Gαq were not activated constitutively as determined by the lack of cAMP production, the lack of inositol phosphate turnover, and the lack of activities of the transcription factors: cAMP response element-binding protein and nuclear factor-κB. Immunohistochemistry and confocal microscopy of FLAG- and green fluorescent protein-tagged EBI2 revealed cell-surface expression. A putative N-terminal truncated version of EBI2, Δ4-EBI2, showed similar expression and signaling through Gαi as full-length EBI2. By using a 32P-labeled EBI2 probe we found a very high expression in lymphoid tissue (spleen and lymph node) and peripheral blood mononuclear cells and a high expression in lung tissue. Real-time PCR of EBV-infected cells showed high expression of EBI2 during latent and lytic infection, in contrast to the EBV-encoded 7TM receptor BILF1, which was induced during lytic infection. EBI2 clustered with the orphan GPR18 by alignment analysis as well as by close proximity in the chromosomal region 13q32.3. Based on the constitutive signaling and cellular expression pattern of EBI2, it is suggested that it may function in conjunction with BILF1 in the reprogramming of the cell during EBV infection.

Human cytomegalovirus (HCMV) is the causative agent of life-threatening systemic diseases in immunocompromised patients as well as a risk factor for vascular pathologies, like atherosclerosis, in immunocompetent individuals. HCMV encodes a G-protein-coupled receptor (GPCR), referred to as US28, that displays homology to the human chemokine receptor CCR1 and binds several chemokines of the CC family as well as the CX3C chemokine fractalkine with high affinity. Most importantly, following HCMV infection, US28 activates several intracellular pathways, either constitutively or in a chemokine-dependent manner. In this study, our goal was to understand the molecular interactions between chemokines and the HCMV-encoded US28 receptor. To achieve this goal, a double approach has been used, consisting in the analysis of both receptor and ligand mutants. This approach has led us to identify several amino acids located in the N terminus of US28 that differentially contribute to the high affinity binding of CC versus CX3C chemokines. Additionally, our results highlight the importance of secondary modifications occurring at US28, such as sulfation, for ligand recognition. Finally, the effects of chemokine dimerization and interaction with glycosaminoglycans (GAGs) on chemokine binding and activation of US28 were investigated as well using CCL4 as model ligand. In line with the two-state model describing chemokine/receptor interaction, we show that an aromatic residue in the N-loop region of CCL4 promotes tight binding to US28, whereas receptor activation depends on the presence of the N terminus of CCL4, as shown previously for CCR5.

Both beta- and gammaherpesviruses encode G protein-coupled receptors (GPCRs) with unique pharmacological phenotypes and important biological functions. An example is ORF74, the γ2-herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded GPCR, which is highly constitutively active and considered the key oncogene in Kaposi's sarcoma pathogenesis. In contrast, the current annotation of the Epstein-Barr virus (EBV) genome does not reveal any GPCR homolog encoded by this human oncogenic γ1-herpesvirus. However, by employing bioinformatics, we recognized that the previously established EBV open reading frame BILF1 indeed encodes a GPCR. Additionally, BILF1 is a member of a new family of related GPCRs exclusively encoded by γ1-herpesviruses. Expression of hemagglutinin-tagged BILF1 in the HEK293 epithelial cell line revealed that BILF1 is expressed as an approximately 50-kDa glycosylated protein. Immunocytochemistry and confocal microscopy revealed that BILF1 localizes predominantly to the plasma membrane, similar to the localization of KSHV ORF74. Using chimeric G proteins, we found that human and rhesus EBV-encoded BILF1 are highly potent constitutively active receptors, activating Gα i . Furthermore, BILF1 is able to inhibit forskolin-triggered CREB activation via stimulation of endogenous G proteins in a pertussis toxin-sensitive manner, verifying that BILF1 signals constitutively through Gα i . We suggest that EBV may use BILF1 to regulate Gα i -activated pathways during viral lytic replication, thereby affecting disease progression.

The human cytomegalovirus genome encodes four putative seven transmembrane domain chemokine receptor-like proteins. Although important in viral pathogenesis, little is known about the properties or functions of these proteins. We previously reported that US28 is located in endocytic vesicles and undergoes constitutive endocytosis and recycling. Here we studied the cellular distributions and trafficking of two other human cytomegalovirus chemokine receptor-like proteins, UL33 and US27, in transfected and human cytomegalovirus-infected cells. Immunofluorescence staining indicated that UL33 and US27 are located at the cell surface, although the majority of both proteins was seen in intracellular organelles located in the perinuclear region of the cell. The intracellular pools of UL33 and US27 showed overlap with markers for endocytic organelles. Antibody-feeding experiments indicated that cell surface US27 undergoes endocytosis. By immunogold labeling of cryosections and electron microscopy, UL33 was seen to localize to multivesicular bodies (MVBs or multivesicular endosomes). Electron microscopy analysis of human cytomegalovirus-infected cells showed that most virus particles wrapped individually into short membrane cisternae, although virus particles were also occasionally seen within and budding into MVBs. Electron microscopy immunolocalization of viral UL33 and US27 on ultrathin cryosections of human cytomegalovirus-infected cells showed gold particles over the membranes into which virions were wrapping, in small membrane tubules and vesicles and in MVBs. Labeling of the human cytomegalovirus glycoproteins gB and gH indicated that these proteins were also present in the same membrane structures. This first electron microscopy analysis of human cytomegalovirus assembly using immunolabeling suggests that the localization of UL33, US27 and US28 to endosomes may allow these proteins to be incorporated into the viral membrane during the final stages of human cytomegalovirus assembly.

Open reading frame 74 (ORF74) encoded by human herpesvirus 8 is a highly constitutively active seven transmembrane (7TM) receptor stimulated by angiogenic chemokines, e.g. growth-related oncogene-alpha, and inhibited by angiostatic chemokines e.g. interferon-gamma-inducible protein. Transgenic mice expressing ORF74 under control of the CD2 promoter develop highly vascularized Kaposi's sarcoma-like tumors. Through targeted mutagenesis we here create three distinct phenotypes of ORF74: a receptor with normal, high constitutive signaling through the phospholipase C pathway but deprived of binding and action of chemokines obtained through deletion of 22 amino acids from the N-terminal extension; an ORF74 with high constitutive activity but with selective elimination of stimulatory regulation by angiogenic chemokines obtained through substitution of basic residues at the extracellular ends of TM-V or TM-VI; and an ORF74 lacking constitutive activity but with preserved ability to be stimulated by agonist chemokines obtained through introduction of an Asp residue on the hydrophobic, presumed membrane-exposed face of TM-II. It is concluded that careful molecular dissection can selectively eliminate either agonist or inverse agonist modulation as well as high constitutive activity of the virally encoded oncogene ORF74 and that these mutant forms presumably can be used in transgenic animals to identify the molecular mechanism of its transforming activity.

Human cytomegalovirus (HCMV)-encoded G protein-coupled-receptor US28 is believed to participate in virus dissemination through modulation of cell migration and immune evasion. US28 binds different CC chemokines and the CX3C chemokine CX3CL1. Membrane-anchored CX3CL1 is expressed by immune-activated endothelial cells, causing redirection of CX3CR1-expressing leukocytes in the blood to sites of infection. Here, we used stable transfected cell lines to examine how US28 expression affects cell migration on immobilized full-length CX3CL1, to model how HCMV-infected leukocytes interact with inflamed endothelium. We observed that US28-expressing cells migrated more than CX3CR1-expressing cells when adhering to immobilized CX3CL1. US28-induced migration was G protein-signalling dependent and was blocked by the phospholipase Cβ inhibitor U73122 and the intracellular calcium chelator BAPTA-AM. In addition, migration was inhibited in a dose-dependent manner by competition from CCL2 and CCL5, whereas CCL3 had little effect. Instead of migrating, CX3CR1-expressing cells performed ‘dancing-on-the-spot’ movements, demonstrating that anchored CX3CL1 acts as a strong tether for these cells. At low receptor expression levels, however, no significant difference in migration potential was observed when comparing the migration of CX3CR1- and US28-expressing cells. Thus, these data showed that, in contrast to CX3CR1, which promotes efficient cell capture upon binding to anchored CX3CL1, US28 acts to increase the migration of cells upon binding to the same ligand. Overall, this indicates that infected cells probably move more than uninfected cells in inflamed tissues with high CX3CL1 expression, with soluble chemokines affecting the final migration.

Epstein-Barr virus (EBV) open reading frame BILF1 encodes a seven trans-membrane (TM) G protein-coupled receptor that signals with high constitutive activity through Galpha(i) (Beisser et al., 2005; Paulsen et al., 2005). In this paper, the transforming potential of BILF1 is investigated in vitro in a foci formation assay using retrovirally transduced NIH3T3 cells, as well as in vivo by using nude mice. BILF1 revealed a substantial transforming potential that was dependent on constitutive signaling, as a signaling-deficient mutant completely lost its ability to transform cells in vitro, and an intermediately active triple-mutated receptor possessed an intermediate transforming potential. Furthermore, BILF1 expression induced vascular endothelial growth factor secretion in a constitutively active manner. In nude mice, BILF1 promoted tumor formation in 90% of cases, ORF74 (from Kaposi's sarcoma-associated herpes virus) in 100% of cases, whereas the signaling-deficient receptor resulted in tumor establishment in 40% of cases. These data suggest that BILF1, when expressed during EBV infection, could indeed be involved in the pathogenesis of EBV-associated diseases and malignancies. Furthermore, the correlation between receptor activity and the ability to mediate cell transformation in vitro and tumor formation in vivo supports the idea that inverse agonists for BILF1 could inhibit cell transformation and be relevant therapeutic candidates.

Epstein-Barr virus (EBV)-induced receptor 2 (EBI2) is an orphan seven-transmembrane (7TM) receptor originally identified as the most up-regulated gene (200-fold) in EBV-infected cells. Here we show that EBI2 signals with constitutive activity through Galpha(i) as determined by a receptor-mediated inhibition of forskolin-induced cAMP production and an induction of the serum response element-driven transcriptional activity in a pertussis toxin-sensitive manner. Galpha(s) and Galpha(q) were not activated constitutively as determined by the lack of cAMP production, the lack of inositol phosphate turnover, and the lack of activities of the transcription factors: cAMP response element-binding protein and nuclear factor-kappaB. Immunohistochemistry and confocal microscopy of FLAG- and green fluorescent protein-tagged EBI2 revealed cell-surface expression. A putative N-terminal truncated version of EBI2, delta4-EBI2, showed similar expression and signaling through Galpha(i) as full-length EBI2. By using a 32P-labeled EBI2 probe we found a very high expression in lymphoid tissue (spleen and lymph node) and peripheral blood mononuclear cells and a high expression in lung tissue. Real-time PCR of EBV-infected cells showed high expression of EBI2 during latent and lytic infection, in contrast to the EBV-encoded 7TM receptor BILF1, which was induced during lytic infection. EBI2 clustered with the orphan GPR18 by alignment analysis as well as by close proximity in the chromosomal region 13q32.3. Based on the constitutive signaling and cellular expression pattern of EBI2, it is suggested that it may function in conjunction with BILF1 in the reprogramming of the cell during EBV infection.

Viral infections depend on an intimate relationship between the infectious agent and the host cells. Viruses need the host cells for replication, while the innate- and adaptive- immunesystem of the host is fighting to kill the infected cell in order to clear out the pathogen and survive the infection. However, since both virus and host exist, the organisms struggle must reach an ecological equilibrium. Among the best-studied interactions between viruses and the host immune system are those between herpesviruses and their hosts. Herpesviruses are known to devote a significant part of their large genomes on immuno-modulatory genes, some encoding chemokines or chemokine receptors. These genes, which may be dispensable for viral replication in vitro, are highly important for viral growth in vivo, for viral dissemination and disease progression. Indeed, all beta and gamma-herpesviruses have acquired homologs of both chemokines and chemokine receptors belonging to the 7 transmembrane (7TM) spanning, G protein-coupled receptor family. 7TM receptors are very efficient drug targets and are currently the most popular class of investigational drug targets. A notable trait for the virus encoded chemokine receptors seems to be their constitutive activity. The biological function of the constitutive activity is still unclear, but it has become clear that the receptors are involved in important parts of the viral lifecycle in vivo, and that the receptor signaling is involved in gamma-herpesvirus mediated cell transformation. Therefore, blocking the signaling of these receptors will provide an efficient and highly specific way to inhibit viral replication in vivo and disease progression in the hosts.

Prediction of protein-coding regions and other features of primary DNA sequence have greatly contributed to experimental biology. Significant challenges remain in genome annotation methods, including the identification of small or overlapping genes and the assessment of mRNA splicing or unconventional translation signals in expression. We have employed a combined analysis of compositional biases and conservation together with frame-specific G+C representation to reevaluate and annotate the genome sequences of mouse and rat cytomegaloviruses. Our analysis predicts that there are at least 34 protein-coding regions in these genomes that were not apparent in earlier annotation efforts. These include 17 single-exon genes, three new exons of previously identified genes, a newly identified four-exon gene for a lectin-like protein (in rat cytomegalovirus), and 10 probable frameshift extensions of previously annotated genes. This expanded set of candidate genes provides an additional basis for investigation in cytomegalovirus biology and pathogenesis.

The highly conserved Arg in the so-called DRY motif (Asp-Arg-Tyr) at the intracellular end of transmembrane helix 3 is in general considered as an essential residue for G protein coupling in rhodopsin-like seven transmembrane (7TM) receptors. In the open reading frame 74 (ORF74) receptor encoded by equine herpesvirus 2 (EHV2), the DRY motif is substituted with a DTW motif. Nevertheless, this receptor signaled with high constitutive activity through Gi as determined by a receptor-mediated inhibition of forskolin-induced cAMP-production and by an induction of the serum response element-driven transcriptional activity through a pertussis toxin-sensitive manner. Gs and Gq were not activated constitutively as determined by the lack of inositol phosphate turnover and activities of the three transcription factors: cAMP response element-binding protein (CREB), nuclear factor-kappaB, and nuclear factor of activated T cells. Coexpression of the ORF74-EHV2 receptor with the promiscuous G protein Gqi4myr supported the constitutive Gi activation as determined by inositol phosphate turnover and CREB activation. The constitutive activity was inhibited by nonpeptide inverse agonists with micromolar potencies, and the chemokine CXCL6 acted as a high-affinity agonist. It is noteworthy that reconstitution of the DRY motif resulted in a 4- to 5-fold decrease of the constitutive activity. Both the wild type and the receptor with the reconstituted DRY motif were expressed at the cell surface as indicated by immunohistochemistry and enzyme-linked immunosorbent assay analysis. It is concluded that the Arg of the DRY motif in transmembrane helix 3 is not essential for G protein coupling based on the constitutive as well as the ligand-mediated activity observed for ORF74-EHV2.

Fractalkine/CX3CL1 is a membrane-tethered chemokine that functions as a chemoattractant and adhesion protein by interacting with the receptor CX3CR1. To understand the molecular basis for the interaction, an extensive mutagenesis study of fractalkine's chemokine domain was undertaken. The results reveal a cluster of basic residues (Lys-8, Lys-15, Lys-37, Arg-45, and Arg-48) and one aromatic (Phe-50) that are critical for binding and/or signaling. The mutant R48A could bind but not induce chemotaxis, demonstrating that Arg-48 is a signaling trigger. This result also shows that signaling residues are not confined to chemokine N termini, as generally thought. F50A showed no detectable binding, underscoring its importance to the stability of the complex. K15A displayed unique signaling characteristics, eliciting a wild-type calcium flux but minimal chemotaxis, suggesting that this mutant can activate some, but not all, pathways required for migration. Fractalkine also binds the human cytomegalovirus receptor US28, and analysis of the mutants indicates that US28 recognizes many of the same epitopes of fractalkine as CX3CR1. Comparison of the binding surfaces of fractalkine and the CC chemokine MCP-1 reveals structural details that may account for their dual recognition by US28 and their selective recognition by host receptors.

The 7TM receptor, US28, encoded by human cytomegalovirus binds a broad spectrum of endogenous CC chemokines with sub-nanomolar affinity as determined in homologous competition binding assays. We here find that US28 also recognizes the membrane-associated CX3C chemokine, fractalkine, with sub-nanomolar affinity (IC50=0.42±0.09 nM). Importantly, although fractalkine could compete with high affinity against the binding of CC chemokines, the secreted CC chemokines were only able to compete for binding against radioactive fractalkine with very low affinity. It is concluded that US28, which is known to enhance cell-cell fusion processes through interaction with an as yet unidentified, human cell-specific factor, has been optimized by cytomegalovirus to selectively recognize the membrane-associated fractalkine. It is suggested that US28 expressed on the surface of infected cells and possibly on the envelope of the virion is involved in transfer of the virus from cell to cell.

A number of CXC chemokines competed with similar, nanomolar affinity against 125I-interleukin-8 (IL-8) binding to ORF-74, a constitutively active seven-transmembrane receptor encoded by human herpesvirus 8. However, in competition against 125I-labeled growth-related oncogene (GRO)-alpha, the ORF-74 receptor was highly selective for GRO peptides, with IL-8 being 10,000-fold less potent. The constitutive stimulating activity of ORF-74 on phosphatidylinositol turnover was not influenced by, for example, IL-8 binding. In contrast, GRO peptides acted as potent agonists in stimulating ORF-74 signaling, whereas IP-10 and stromal cell-derived factor-1alpha surprisingly acted as inverse agonists. These peptides had similar pharmacological properties with regard to enhancing or inhibiting, respectively, the stimulatory effect of ORF-74 on NIH-3T3 cell proliferation. Construction of a high affinity zinc switch through introduction of two His residues at the extracellular end of transmembrane segment V enabled Zn2+ to act as a prototype non-peptide inverse agonist, which eliminated the constitutive signaling. It is concluded that ORF-74, which is believed to be causally involved in the formation of highly vascularized tumors, has been optimized for agonist and inverse agonist modulation by the endogenous angiogenic GRO peptides and angiostatic IP-10 and stromal cell-derived factor-1alpha, respectively. ORF-74 could serve as a target for the development of non-peptide inverse agonist drugs as demonstrated by the effect of Zn2+ on the metal ion site-engineered receptor.

Kaposi's sarcoma–associated herpesvirus encodes a chemokine called vMIP-II. This protein displayed a broader spectrum of receptor activities than any mammalian chemokine as it bound with high affinity to a number of both CC and CXC chemokine receptors. Binding of vMIP-II, however, was not associated with the normal, rapid mobilization of calcium from intracellular stores; instead, it blocked calcium mobilization induced by endogenous chemokines. In freshly isolated human monocytes the virally encoded vMIP-II acted as a potent and efficient antagonist of chemotaxis induced by chemokines. Because vMIP-II could inhibit cell entry of human immunodeficiency virus (HIV) mediated through CCR3 and CCR5 as well as CXCR4, this protein may serve as a lead for development of broad-spectrum anti-HIV agents.

Genes encoding chemokine receptor-like proteins have been found in herpes and poxviruses and implicated in viral pathogenesis. Here we describe the cellular distribution and trafficking of a human cytomegalovirus (HCMV) chemokine receptor encoded by theUS28 gene, after transient and stable expression in transfected HeLa and Cos cells. Immunofluorescence staining indicated that this viral protein accumulated intracellularly in vesicular structures in the perinuclear region of the cell and showed overlap with markers for endocytic organelles. By immunogold electron microscopy US28 was seen mostly to localize to multivesicular endosomes. A minor portion of the protein (at most 20%) was also expressed at the cell surface. Antibody-feeding experiments indicated that cell surface US28 undergoes constitutive ligand-independent endocytosis. Biochemical analysis with the use of iodinated ligands showed that US28 was rapidly internalized. The high-affinity ligand of US28, the CX3C-chemokine fractalkine, reduced the steady-state levels of US28 at the cell surface, apparently by inhibiting the recycling of internalized receptor. Endocytosis and cycling of HCMV US28 could play a role in the sequestration of host chemokines, thereby modulating antiviral immune responses. In addition, the distribution of US28 mainly on endosomal membranes may allow it to be incorporated into the viral envelope during HCMV assembly.

Cellular infection by cytomegalovirus (CMV) is associated with very early G-protein-mediated signal transduction and reprogramming of gene expression. Here we investigated the involvement of human CMV (HCMV)-encoded US27, US28, and UL33 receptors as well as murine CMV-encoded M33 transmembrane (7TM) receptors in host cell signaling mechanisms. HCMV-encoded US27 did not show any constitutive activity in any of the studied signaling pathways; in contrast, US28 and M33 displayed ligand-independent, constitutive signaling through the G protein q (Gq)/phospholipase C pathway. In addition, M33 and US28 also activated the transcription factor NF-κB as well as the cyclic AMP response element binding protein (CREB) in a ligand-independent, constitutive manner. The use of specific inhibitors indicated that the p38 mitogen-activated protein (MAP) kinase but not the extracellular signal-regulated kinase 1/2-MAP kinase pathway is involved in M33- and US28-mediated CREB activation but not NF-κB activation. Interestingly, UL33—the HCMV-encoded structural homologue of M33—was only marginally constitutively active in the Gq/phospholipase C turnover and CREB activation assays and did not show any constitutive activity in the NF-κB pathway, where M33 and US28 were highly active. Hence, CMVs appear to have conserved mechanisms for regulating host gene transcription, i.e., constitutive activation of certain kinases and transcription factors through the constitutive activities of 7TM proteins. These data, together with the previous identification of the incorporation of such proteins in the viral envelope, suggest that these proteins could be involved in the very early reprogramming of the host cell during viral infection.