US28 actions in HCMV infection: lessons from a versatile hijacker

Advances in adoptive T-cell therapy for metastatic melanoma

Adoptive T cell therapy (ACT) is a fast developing, niche area of immunotherapy (IO), which is revolutionising the therapeutic landscape of solid tumour oncology, especially metastatic melanoma (MM). Identifying tumour antigens (TAs) as potential targets, the ACT response is mediated by either Tumour Infiltrating Lymphocytes (TILs) or genetically modified T cells with specific receptors - T cell receptors (TCRs) or chimeric antigen receptors (CARs) or more prospectively, natural killer (NK) cells. Clinical trials involving ACT in MM from 2006 to present have shown promising results. Yet it is not without its drawbacks which include significant auto-immune toxicity and need for pre-conditioning lymphodepletion. Although immune-modulation is underway using various combination therapies in the hope of enhancing efficacy and reducing toxicity. Our review article explores the role of ACT in MM, including the various modalities - their safety, efficacy, risks and their development in the trial and the real world setting.

Human Cytomegalovirus-Encoded Receptor US28 Is Expressed in Renal Allografts and Facilitates Viral Spreading In Vitro

BACKGROUND

Renal transplantation is the preferred treatment for patients with end-stage renal disease. Human cytomegalovirus (HCMV) activation is associated with decreased renal graft function and survival. Human cytomegalovirus encodes several immune modulatory proteins, including the G protein-coupled receptor US28, which scavenges human chemokines and modulates intracellular signaling.

METHODS

Our aim was to identify the expression and localization of US28 in renal allograft biopsies by immunohistochemistry and determine its role in viral spreading in vitro.

RESULTS

Immunohistochemistry revealed US28 in 31 of 34 renal transplant biopsies from HCMV-seropositive donors. Expression was independent of HCMV viremia or IgG serostatus. US28 was predominantly expressed in the cytoplasm of vascular smooth muscle cells (VSMCs) and tubular epithelial cells, with a median positivity of 20% and 40%, respectively. Also, US28-positive cells were present within arterial neointima. In contrast to US28, HCMV-encoded immediate early antigen was detected in less than 5% of VSMCs, tubular epithelial cells, interstitial endothelium, interstitial inflammatory infiltrates, and glomerular cells.Primary VSMCs were infected with green fluorescent protein-tagged wild type or US28-deficient HCMV. The viral spreading of US28-deficient HCMV, via culture medium or cell-to-cell transmission, was significantly impeded as shown by green fluorescent protein (ie, infected) cell quantification and quantitative real-time polymerase chain reaction. Additionally, the number and size of foci was smaller.

CONCLUSIONS

In summary, HCMV-encoded US28 was detected in renal allografts from HCMV-positive donors independent of viremia and serostatus. Also, US28 facilitates HCMV spreading in VSMCs in vitro. Because the vasculature is affected in chronic renal transplant dysfunction, US28 may provide a potential target for therapeutic intervention.

US28, a Virally-Encoded GPCR as an Antiviral Target for Human Cytomegalovirus Infection

Viruses continue to evolve a new strategy to take advantage of every aspect of host cells in order to maximize their survival. Due to their central roles in transducing a variety of transmembrane signals, GPCRs seem to be a prime target for viruses to pirate for their own use. Incorporation of GPCR functionality into the genome of herpesviruses has been demonstrated to be essential for pathogenesis of many herpesviruses-induced diseases. Here, we introduce US28 of human cytomegalovirus (HCMV) as the best-studied example of virally-encoded GPCRs to manipulate host GPCR signaling. In this review, we wish to summarize a number of US28-related topics including its regulation of host signaling pathways, its constitutive internalization, its structural and functional analysis, its roles in HCMV biology and pathogenesis, its proliferative activities and role in oncogenesis, and pharmacological modulation of its biological activities. This review will aid in our understanding of how pathogenic viruses usurp the host GPCR signaling for successful viral infection. This kind of knowledge will enable us to build a better strategy to control viral infection by normalizing the virally-dysregulated host GPCR signaling.

Allosteric modulation of the G protein-coupled US28 receptor of human cytomegalovirus: are the small-weight inverse agonist of US28 'camouflaged' agonists?

The highly constitutively active G protein-coupled receptor US28 of human cytomegalovirus (HCMV) is thought to camouflage agonism by mediating constitutive endocytosis. With the use of the US28Δ300 mutant, which is largely devoid of constitutive internalization, I have demonstrated that the coupling of the receptor to its downstream signaling partners is responsible for the inverse agonism to agonism efficacy switch in some small-weight ligands of US28.

Analysis of cell migration during human cytomegalovirus (HCMV) infection

Previous studies have demonstrated that dendritic cells (DC), macrophages (Mφ), and their precursors monocytes are susceptible to infection by human cytomegalovirus (HCMV) in the natural host as well as in vitro. Due to their proficient ability to take up and present antigens to the lymphocytes these cells are also called antigen presenting cells (APC) and represent a crucial component that HCMV needs to disable in order to limit the antiviral immune reaction. It is well known that cell trafficking is an essential property of APC. Monocytes and DC are usually regarded as very motile cells and their trafficking properties through the blood vessels, the peripheral tissues, and the lymphoid organs are intensively studied. On the other hand, although often considered a resident population, Mφ are also motile and can actively migrate into areas of infection, inflammation, and tissue regeneration. The movements of monocytes, DC, and Mφ require a tight control that is mainly assured by chemokines (CK) and their receptors. While it is quite common to study the expression of chemokine receptors by flow cytometry, methods for the investigation of the chemokine receptor functionality are less widespread. In this chapter, we describe different techniques that can help in the analysis of cell migration in response to CK. Cell polarization assays measure the rapid morphological changes that follow the chemokine receptors' engagement by their ligands. Actin polymerization assays measure the subsequent conversion of globular units of actin into dynamic filaments. Finally, chemotaxis assays quantify the cell movements along a CK gradient.

Development of flavonoid-based inverse agonists of the key signaling receptor US28 of human cytomegalovirus

A series of 31 chalcone- and flavonoid-based derivatives were synthesized in good overall yields and screened for their inverse agonist activity on the US28 receptor of human cytomegalovirus (HCMV). With one exception (e.g., 2-(5-bromo-2-methoxyphenyl)-3-hydroxy-4H-chromen-4-one), halogen-substituted flavonoids were typically more potent inverse agonists than their related hydro derivatives. While toxicity could be used to partially explain the inverse agonist activity of some members of the series, 5-(benzyloxy)-2-(5-bromo-2-methoxyphenyl)-4H-chromen-4-one (11b) acted on the US28 receptor as a nontoxic, inverse agonist. The full inverse agonism (efficacy, -89%) and potency (EC50 = 3.5 μM) observed with flavonoid 11b is especially important as it provides both a new tool to study US28 signaling and a potential platform for the future development of HCMV-targeting drugs.

Human cytomegalovirus encoded homologs of cytokines, chemokines and their receptors: roles in immunomodulation

Human cytomegalovirus (HCMV), the largest human herpesvirus, infects a majority of the world’s population. Like all herpesviruses, following primary productive infection, HCMV establishes a life-long latent infection, from which it can reactivate years later to produce new, infectious virus. Despite the presence of a massive and sustained anti-HCMV immune response, productively infected individuals can shed virus for extended periods of time, and once latent infection is established, it is never cleared from the host. It has been proposed that HCMV must therefore encode functions which help to evade immune mediated clearance during productive virus replication and latency. Molecular mimicry is a strategy used by many viruses to subvert and regulate anti-viral immunity and HCMV has hijacked/developed a range of functions that imitate host encoded immunomodulatory proteins. This review will focus on the HCMV encoded homologs of cellular cytokines/chemokines and their receptors, with an emphasis on how these virus encoded homologs may facilitate viral evasion of immune clearance.

Meeting of conventional and unconventional pathways at the TGN

The biosynthetic pathway allows the transport of newly synthesized proteins to the TGN via the reticulum endoplasmic and Golgi apparatus. However, many large particles reach the TGN by unconventional means. For instance, Herpes simplex virus type 1(HSV-1) capsids assemble within the nucleus, bud into the perinuclear space, fuse with the outer nuclear membrane and finally travel unenveloped towards the TGN. Given the central role of protein kinase D in the transport of small cargo from the TGN to the cell surface, we probed its potential contribution in HSV-1 egress, as a model for studying large cargo exiting from the TGN. Using a synchronized infection, we show that inactivation of protein kinase D with pharmacological inhibitors, a kinase dead mutant or siRNA all causes the retention of HSV-1 at the TGN. This highlights the role of PKD in viral exit and a dependence of the virus on the classical host cell machinery to leave the TGN, unlike its previous transport steps. Conceptually, this supports a model in which the TGN is a meeting point where conventional and unconventional routes encounter.

Human cytomegalovirus-encoded chemokine receptor homolog US28 stimulates the major immediate early gene promoter/enhancer via the induction of CREB

The major immediate early (MIE) gene of cytomegalovirus plays a key role in determining the activation and replication of cytomegalovirus, which represents the most important event signaling the onset of virus-induced disease relapse. The viral-encoded chemokine receptor homolog US28 can constitutively activate many cellular transcription factors, which can bind to the promoter/enhancer of the MIE gene and activate its transcription. Using reporter gene assays in HEK293 cells, we found that US28 enhanced the transcription efficiency of MIE and other genes via cAMP response element-binding protein (CREB). Inhibition of CREB partially blocked the effect of US28, whereas forskolin enhanced this effect. There was a direct correlation between CREB and transcription of MIE gene. These data, together with the broad-spectrum effect of cellular transcription factors, suggest that US28 may be involved in the very early transcription of the host cell during virus activation.

Virus-encoded chemokines, chemokine receptors and chemokine-binding proteins: new paradigms for future therapy

Over millions of years of coevolution with their hosts, viruses have learned the finest artifices of the immune system defense mechanisms and developed a variety of strategies for evading them. The chemokine system has been a primary target of these viral efforts because of the critical role it plays in the development of effective immune responses. Not only do chemokines control cellular recruitment at the site of infection, they also regulate the magnitude and character of the immune responses. Several viruses, and large DNA viruses in particular, have exploited the chemokine system by hijacking and reprogramming chemokine or chemokine-receptor genes, and/or secreting chemokine-binding proteins. In the past few years there has been intense investigation in this area, driven not only by the prospect of gaining a better understanding of viral-immune evasion mechanisms, but also by the possibility of targeting these molecules as part of future antiviral therapeutic approaches, as well as exploiting viral strategies of chemokine interference as novel therapies for inflammatory or neoplastic diseases.

A prospective assessment of cytomegalovirus immune evasion gene transcription profiles in transplant patients with cytomegalovirus infection

BACKGROUND

The cytomegalovirus (CMV) immune evasion genes US3, US6, and US11 may disrupt the host immune response via downregulation of major histocompatibility complex molecules. Transplant recipients with CMV infection were prospectively assessed for immune evasion gene expression.

METHODS

Seventy solid organ transplant patients with CMV infection who were given antiviral therapy were enrolled. Quantitative mRNA levels of US3, US6, and US11 were assessed using real-time polymerase chain reaction assays from peripheral blood mononuclear cells at regular time-points after starting therapy.

RESULTS

High immune evasion mRNA levels were detectable at start-of-therapy (median US3-4.5 log10 copies; US6- 3.7 log10 copies, and US11-3.3 log10 copies/10 cells). With therapy, immune evasion mRNA levels declined exponentially. For example, median calculated US3 half-life was 1.59 days (range 0.74-12.5 days). By day7, US3 mRNA was detectable in 55.7%, US6 in 38.6%, and US11 in 41.4% of patients. Early phase kinetics correlated with outcomes. When adjusted for baseline DNA level, there was a trend to higher mRNA levels in patients who relapsed. Also, detectable mRNA at day 14 after start of therapy was associated with virologic relapse after initial treatment (P<or=0.001 for US3, US6, and US11). For example, if US3 mRNA was still detectable at day 14, then risk of relapse was 84.2% vs. 29.4% if US3 mRNA not detectable at day 14 (P<0.001). This correlation was independent of the DNA viral load.

CONCLUSION

CMV immune evasion gene expression is detectable at high levels in patients with CMV infection and declines exponentially with therapy. Expression levels can be independently correlated with outcomes.

Primate cytomegalovirus US12 gene family: a distinct and diverse clade of seven-transmembrane proteins

Human cytomegalovirus (HCMV; Human herpesvirus 5) and the other betaherpesviruses encode a number of distinct gene families, including the US12 family, which is represented only in the cytomegaloviruses of higher primates, and is comprised of a set of 10 contiguous genes (US12 through US21), each encoding a seven-transmembrane (7TM) protein. Nonessential for replication in cell culture but well-conserved among clinical isolates, little is known of possible US12 family member functions, other than a previously identified amino acid sequence similarity between US21 and a group of 7TM proteins that include known inhibitors of apoptosis, and a very limited description of similarity between US12 family members and G-protein-coupled receptors (GPCR). As a prelude to biochemical analysis, we have conducted a detailed analysis of the relationships among US12 family members and between these proteins and other proteins, particularly GPCR and other 7TM molecules. In most cases, the closest relatives of individual genes are their colinear counterparts in the other viruses. Thus, the initial duplication and divergence events that resulted in the current version of the US12 family preceded divergence of the rhesus and hominoid lineages. Our phylogenetic analysis indicates that the US12 family represents a distinct branch of the 7TM superfamily. Although they are distantly related, at least some of the US12 family members may have GPCR-related properties, but they are also likely to embody functions and mechanisms that differ from more conventional GPCRs. Our analyses suggest that the 7TM structure of US12 family members constitutes a functionally flexible structural scaffold that can be readily adapted to diverse functional ends. This strategy may be the driving force in the emergence of the several families of duplicated and diverged betaherpesvirus genes.

The human cytomegalovirus-encoded receptor US28 increases the activity of the major immediate-early promoter/enhancer

The activation of the major immediate-early promoter (MIEP) is a key event in the cytomegalovirus replication cycle and is dependent on cellular transcription factors which are partially activated by viral proteins. Expression of the viral chemokine receptor homolog US28 results in constitutive activation of pro-inflammatory transcription factors that may be involved in the activation of the major immediate-early promoter/enhancer. Using reporter gene assays in human embryonic kidney cells, we found that US28 signaling was responsible for increased major immediate-early promoter/enhancer activity which was independent of beta-chemokine binding. Inhibition of nuclear factor-kappaB (NF-kappaB) only partially blocked the effect of US28, whereas treatment with a specific p38 mitogen activated kinase (MAPK) inhibitor fully abrogated the US28-induced enhancement of promoter activity. Our results suggest that during human cytomegalovirus (HCMV) infection, US28 in epithelial cells transactivates the major immediate-early promoter/enhancer via the activation of p38 MAPK and downstream signaling that partially involves NF-kappaB.