The human cytomegalovirus major immediate-early proteins as antagonists of intrinsic and innate antiviral host responses

The battle between host antiviral innate immunity and immune evasion by cytomegalovirus

Cytomegalovirus (CMV) has successfully established a long-lasting latent infection in humans due to its ability to counteract the host antiviral innate immune response. During coevolution with the host, the virus has evolved various evasion techniques to evade the host's innate immune surveillance. At present, there is still no vaccine available for the prevention and treatment of CMV infection, and the interaction between CMV infection and host antiviral innate immunity is still not well understood. However, ongoing studies will offer new insights into how to treat and prevent CMV infection and its related diseases. Here, we update recent studies on how CMV evades antiviral innate immunity, with a focus on how CMV proteins target and disrupt critical adaptors of antiviral innate immune signaling pathways. This review also discusses some classic intrinsic cellular defences that are crucial to the fight against viral invasion. A comprehensive review of the evasion mechanisms of antiviral innate immunity by CMV will help investigators identify new therapeutic targets and develop vaccines against CMV infection.

Mechanisms of Action of the US Food and Drug Administration-Approved Antisense Oligonucleotide Drugs

Antisense oligonucleotides (ASOs) are single stranded nucleic acids that target RNA. The US Food and Drug Administration has approved ASOs for several diseases. ASOs utilize three principal modes of action (MOA). The first MOA is initiated by base-pairing between the ASO and its target mRNA, followed by RNase H-dependent mRNA degradation. The second MOA is triggered by ASOs that occlude splice acceptor sites in pre-mRNAs leading to skipping of a mutation-bearing exon. The third MOA involves ASOs that sterically hinder mRNA function, often inhibiting translation. ASOs contain a variety of modifications to the sugar-phosphate backbone and bases that stabilize the ASO or render them resistant to RNase activity. RNase H-dependent ASOs include inotersen and eplontersen (for hereditary transthyretin amyloidosis), fomiversen (for opportunistic cytomegalovirus infection), mipomersen (for familial hypercholesterolemia), and tofersen [for amyotrophic lateral sclerosis (ALS)]. Splice modulating ASOs include nursinersen (for spinal muscular atrophy) and eteplirsen, golodirsen, viltolarsen, and casimersen (all for the treatment of Duchenne muscular dystrophy). In addition, a designer ASO, milasen, was used to treat a single individual afflicted with Batten disease. Since ASO design relies principally upon knowledge of mRNA sequence, the bench to bedside pipeline for ASOs is expedient compared with protein-directed drugs. [Graphical abstract available.].

An overview of cdc2-like kinase 1 (Clk1) inhibitors and their therapeutic indications

Over the past decade, Clk1 has been identified as a promising target for the treatment of various diseases, in which deregulated alternative splicing plays a role. First small molecules targeting Clk1 are in clinical trials for the treatment of solid cancer, where variants of oncogenic proteins derived from alternative splicing promote tumor progression. Since many infectious pathogens hi-jack the host cell's splicing machinery to ensure efficient replication, further indications in this area are under investigation, such as Influenza A, HIV-1 virus, and Trypanosoma infections, and more will likely be discovered in the future. In addition, Clk1 was found to contribute to the progression of Alzheimer's disease through causing an imbalance of tau splicing products. Interestingly, homozygous Clk1 knockout mice showed a rather mild phenotype, opposed to what might be expected in view of the profound role of Clk1 in alternative splicing. A major drawback of most Clk1 inhibitors is their insufficient selectivity; in particular, Dyrk kinases and haspin were frequently identified as off-targets, besides the other Clk isoforms. Only few inhibitors were shown to be selective over Dyrk1A and haspin, whereas no Clk1 inhibitor so far achieved selectivity over the Clk4 isoform. In this review, we carefully compiled all Clk1 inhibitors from the scientific literature and summarized their structure-activity relationships (SAR). In addition, we critically discuss the available selectivity data and describe the inhibitor's efficacy in cellular models, if reported. Thus, we provide a comprehensive overview on the current state of Clk1 drug discovery and highlight the most promising chemotypes.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

Cross-Species Analysis of Innate Immune Antagonism by Cytomegalovirus IE1 Protein

The human cytomegalovirus (CMV) immediate early 1 (IE1) protein has evolved as a multifunctional antagonist of intrinsic and innate immune mechanisms. In addition, this protein serves as a transactivator and potential genome maintenance protein. Recently, the crystal structures of the human and rat CMV IE1 (hIE1, rIE1) core domain were solved. Despite low sequence identity, the respective structures display a highly similar, all alpha-helical fold with distinct variations. To elucidate which activities of IE1 are either species-specific or conserved, this study aimed at a comparative analysis of hIE1 and rIE1 functions. To facilitate the quantitative evaluation of interactions between IE1 and cellular proteins, a sensitive NanoBRET assay was established. This confirmed the species-specific interaction of IE1 with the cellular restriction factor promyelocytic leukemia protein (PML) and with the DNA replication factor flap endonuclease 1 (FEN1). To characterize the respective binding surfaces, helix exchange mutants were generated by swapping hIE1 helices with the corresponding rIE1 helices. Interestingly, while all mutants were defective for PML binding, loss of FEN1 interaction was confined to the exchange of helices 1 and 2, suggesting that FEN1 binds to the stalk region of IE1. Furthermore, our data reveal that both hIE1 and rIE1 antagonize human STAT2; however, distinct regions of the respective viral proteins mediated the interaction. Finally, while PML, FEN1, and STAT2 binding were conserved between primate and rodent proteins, we detected that rIE1 lacks a chromatin tethering function suggesting that this activity is dispensable for rat CMV. In conclusion, our study revealed conserved and distinct functions of primate and rodent IE1 proteins, further supporting the concept that IE1 proteins underwent a narrow co-evolution with their respective hosts to maximize their efficacy in antagonizing innate immune mechanisms and supporting viral replication.

The Medicinal Chemistry of Artificial Nucleic Acids and Therapeutic Oligonucleotides

Nucleic acids play a central role in human biology, making them suitable and attractive tools for therapeutic applications. While conventional drugs generally target proteins and induce transient therapeutic effects, nucleic acid medicines can achieve long-lasting or curative effects by targeting the genetic bases of diseases. However, native oligonucleotides are characterized by low in vivo stability due to nuclease sensitivity and unfavourable physicochemical properties due to their polyanionic nature, which are obstacles to their therapeutic use. A myriad of synthetic oligonucleotides have been prepared in the last few decades and it has been shown that proper chemical modifications to either the nucleobase, the ribofuranose unit or the phosphate backbone can protect the nucleic acids from degradation, enable efficient cellular uptake and target localization ensuring the efficiency of the oligonucleotide-based therapy. In this review, we present a summary of structure and properties of artificial nucleic acids containing nucleobase, sugar or backbone modifications, and provide an overview of the structure and mechanism of action of approved oligonucleotide drugs including gene silencing agents, aptamers and mRNA vaccines.

UL34 Deletion Restricts Human Cytomegalovirus Capsid Formation and Maturation

Over 50% of the world’s population is infected with Human Cytomegalovirus (HCMV). HCMV is responsible for serious complications in the immuno-compromised and is a leading cause of congenital birth defects. The molecular function of many HCMV proteins remains unknown, and a deeper understanding of the viral effectors that modulate virion maturation is required. In this study, we observed that UL34 is a viral protein expressed with leaky late kinetics that localises to the nucleus during infection. Deletion of UL34 from the HCMV genome (ΔUL34) did not abolish the spread of HCMV. Instead, over >100-fold fewer infectious virions were produced, so we report that UL34 is an augmenting gene. We found that ΔUL34 is dispensable for viral DNA replication, and its absence did not alter the expression of IE1, MCP, gB, UL26, UL83, or UL99 proteins. In addition, ΔUL34 infections were able to progress through the replication cycle to form a viral assembly compartment; however, virion maturation in the cytoplasm was abrogated. Further examination of the nucleus in ΔUL34 infections revealed replication compartments with aberrant morphology, containing significantly less assembled capsids, with almost none undergoing subsequent maturation. Therefore, this work lays the foundation for UL34 to be further investigated in the context of nuclear organization and capsid maturation during HCMV infection.

Human Cytomegalovirus IE2 Both Activates and Represses Initiation and Modulates Elongation in a Context-Dependent Manner

Human cytomegalovirus (HCMV) immediate-early 2 (IE2) protein is a multifunctional transcription factor that is essential for lytic HCMV infection. IE2 functions as an activator of viral early genes, negatively regulates its own promoter, and is required for viral replication. The mechanisms by which IE2 executes these distinct functions are incompletely understood. Using PRO-Seq, which profiles nascent transcripts, and a recently developed DFF-chromatin immunoprecipitation (DFF-ChIP; employs chromatin digestion by the endonuclease DNA fragmentation factor prior to IP) approach that resolves occupancy and local chromatin environment, we show that IE2 controls viral gene transcription in three distinct capacities during late HCMV infection and reveal mechanisms that involve direct binding of IE2 to viral DNA. IE2 represses a subset of viral promoters by binding within their core promoter regions and blocking the assembly of preinitiation complexes (PICs). Remarkably, IE2 forms a repressive complex at the major immediate-early promoter region involving direct association of IE2 with nucleosomes and TBP. IE2 stimulates transcription by binding nearby, but not within, core promoter regions. In addition, IE2 functions as a direct roadblock to transcription elongation. At one locus, this function of IE2 appears to be important for the synthesis of a spliced viral RNA. Consistent with the minimal observed effects of IE2 depletion on host gene transcription, IE2 does not functionally engage the host genome. Our results reveal mechanisms of transcriptional control by IE2, uncover a previously unknown function of IE2 as a Pol II elongation modulator, and demonstrate that DFF-ChIP is a useful tool for probing transcription factor occupancy and interactions between transcription factors and nucleosomes at high resolution.

Differences in RNA polymerase II complexes and their interactions with surrounding chromatin on human and cytomegalovirus genomes

Interactions of the RNA polymerase II (Pol II) preinitiation complex (PIC) and paused early elongation complexes with the first downstream (+1) nucleosome are thought to be functionally important. However, current methods are limited for investigating these relationships, both for cellular chromatin and the human cytomegalovirus (HCMV) genome. Digestion with human DNA fragmentation factor (DFF) before immunoprecipitation (DFF-ChIP) precisely revealed both similarities and major differences in PICs driven by TBP on the host genome in comparison with PICs driven by TBP or the viral-specific, late initiation factor UL87 on the viral genome. Host PICs and paused Pol II complexes are frequently found in contact with the +1 nucleosome and paused Pol II can also be found in a complex involved in the initial invasion of the +1 nucleosome. In contrast, viral transcription complexes have very limited nucleosomal interactions, reflecting a relative lack of chromatinization of transcriptionally active regions of HCMV genomes.

Here the authors digested chromatin with DNA fragmentation factor (DFF) prior to chromatin immunoprecipitation (DFF-ChIP) to depict transcription complex interactions with neighboring nucleosomes in cells. Applying this method to human cytomegalovirus (HMCV)-infected cells, they find that the viral genome is underchromatinized, leading to fewer transcription complex interactions with nucleosomes.

Human MicroRNAs Attenuate the Expression of Immediate Early Proteins and HCMV Replication during Lytic and Latent Infection in Connection with Enhancement of Phosphorylated RelA/p65 (Serine 536) That Binds to MIEP

Attenuating the expression of immediate early (IE) proteins is essential for controlling the lytic replication of human cytomegalovirus (HCMV). The human microRNAs (hsa-miRs), miR-200b-3p and miR-200c-3p, have been identified to bind the 3′-untranslated region (3′-UTR) of the mRNA encoding IE proteins. However, whether hsa-miRs can reduce IE72 expression and HCMV viral load or exhibit a crosstalk with the host cellular signaling machinery, most importantly the NF-κB cascade, has not been evaluated. In this study, argonaute-crosslinking and immunoprecipitation-seq revealed that miR-200b-3p and miR-200c-3p bind the 3′-UTR of UL123, which is a gene that encodes IE72. The binding of these miRNAs to the 3′-UTR of UL123 was verified in transfected cells stably expressing GFP. We used miR-200b-3p/miR-200c-3p mimics to counteract the downregulation of these miRNA after acute HCMV infection. This resulted in reduced IE72/IE86 expression and HCMV VL during lytic infection. We determined that IE72/IE86 alone can inhibit the phosphorylation of RelA/p65 at the Ser⁵³⁶ residue and that p-Ser⁵³⁶ RelA/p65 binds to the major IE promoter/enhancer (MIEP). The upregulation of miR-200b-3p and miR-200c-3p resulted in the phosphorylation of RelA/p65 at Ser⁵³⁶ through the downregulation of IE, and the binding of the resultant p-Ser⁵³⁶ RelA/p65 to MIEP resulted in a decreased production of pro-inflammatory cytokines. Overall, miR-200b-3p and miR-200c-3p—together with p-Ser⁵³⁶ RelA/p65—can prevent lytic HCMV replication during acute and latent infection

Identification of cellular proteins associated with human cytomegalovirus (HCMV) DNA replication suggests novel cellular and viral interactions

Upon entry of Human cytomegalovirus (HCMV) into the host cell, the viral genome is transported to the nucleus where it serves as a template for transcription and genome replication. Production of new viral genomes is a coordinated effort between viral and cellular proteins. While the core replication proteins are encoded by the virus, additional cellular proteins support the process of genome synthesis. We used accelerated native isolation of proteins on nascent DNA (aniPOND) to study protein dynamics on nascent viral DNA during HCMV infection. Using this method, we identified specific viral and cellular proteins that are associated with nascent viral DNA. These included transcription factors, transcriptional regulators, DNA damage and repair factors, and chromatin remodeling complexes. The association of these identified proteins with viral DNA was confirmed by immunofluorescent imaging, chromatin-immunoprecipitation analyses, and shRNA knockdown experiments. These data provide evidence for the requirement of cellular factors involved in HCMV replication.

Mechanisms of Survival of Cytomegalovirus-Infected Tumor Cells

Human cytomegalovirus (HCMV) DNA and proteins are often detected in malignant tumors, warranting studies of the role that HCMV plays in carcinogenesis and tumor progression. HCMV proteins were shown to regulate the key processes involved in tumorigenesis. While HCMV as an oncogenic factor just came into focus, its ability to promote tumor progression is generally recognized. The review discusses the viral factors and cell molecular pathways that affect the resistance of cancer cells to therapy. CMV inhibits apoptosis of tumor cells, that not only promotes tumor progression, but also reduces the sensitivity of cells to antitumor therapy. Autophagy was found to facilitate either cell survival or cell death in different tumor cells. In leukemia cells, HCMV induces a "protective" autophagy that suppresses apoptosis. Viral factors that mediate drug resistance and their interactions with key cell death pathways are necessary to further investigate in order to develop agents that can restore the tumor sensitivity to anticancer drugs.

Development of a Vaccine against Human Cytomegalovirus: Advances, Barriers, and Implications for the Clinical Practice

Human cytomegalovirus (hCMV) is one of the most common causes of congenital infection in the post-rubella era, representing a major public health concern. Although most cases are asymptomatic in the neonatal period, congenital CMV (cCMV) disease can result in permanent impairment of cognitive development and represents the leading cause of non-genetic sensorineural hearing loss. Moreover, even if hCMV mostly causes asymptomatic or pauci-symptomatic infections in immunocompetent hosts, it may lead to severe and life-threatening disease in immunocompromised patients. Since immunity reduces the severity of disease, in the last years, the development of an effective and safe hCMV vaccine has been of great interest to pharmacologic researchers. Both hCMV live vaccines-e.g., live-attenuated, chimeric, viral-based-and non-living ones-subunit, RNA-based, virus-like particles, plasmid-based DNA-have been investigated. Encouraging data are emerging from clinical trials, but a hCMV vaccine has not been licensed yet. Major difficulties in the development of a satisfactory vaccine include hCMV's capacity to evade the immune response, unclear immune correlates for protection, low number of available animal models, and insufficient general awareness. Moreover, there is a need to determine which may be the best target populations for vaccine administration. The aim of the present paper is to examine the status of hCMV vaccines undergoing clinical trials and understand barriers limiting their development.

Prevention of Congenital Cytomegalovirus Infection with Vaccines: State of the Art

Cytomegalovirus (CMV) is the most common cause of congenital infection and non-genetic sensorineural hearing loss in childhood. Up to 2% of neonates, with the highest percentages found in developing countries, are congenitally infected with CMV. At birth, most of these infants are asymptomatic. However, approximately 10% have signs and symptoms of the disease, and 40–60% of symptomatic neonates will later develop permanent neurologic sequelae. To reduce congenital CMV (cCMV) infection, a vaccine able to prevent primary infection is essential. In this narrative review, actual ongoing research about the development of a CMV vaccine is discussed. The progressive increase in knowledge on the ways in which the host’s immune system and CMV relate has made it possible to clarify that the development of a vaccine that is certainly capable of reducing the risk of cCMV infection, and preventing both primary and nonprimary infections is extremely difficult. Many of the ways in which the virus evades the immune system and causes cCMV infection are not yet fully understood, especially in cases of nonprimary infection. Moreover, the schedule that should be recommended and that subjects must be vaccinated to obtain the greatest effect have not been precisely defined. Further studies are needed before the problem of cCMV infection and its related challenges can be totally solved.

Human cytomegalovirus infection is associated with increased expression of the lissencephaly gene PAFAH1B1 encoding LIS1 in neural stem cells and congenitally infected brains

Congenital infection of the central nervous system by human cytomegalovirus (HCMV) is a leading cause of permanent sequelae, including mental retardation or neurodevelopmental abnormalities. The most severe complications include smooth brain or polymicrogyria, which are both indicative of abnormal migration of neural cells, although the underlying mechanisms remain to be determined. To gain better insight on the pathogenesis of such sequelae, we assessed the expression levels of a set of neurogenesis-related genes, using HCMV-infected human neural stem cells derived from embryonic stem cells (NSCs). Among the 84 genes tested, we found dramatically increased expression of the gene PAFAH1B1, encoding LIS1 (lissencephaly-1), in HCMV-infected versus uninfected NSCs. Consistent with these findings, western blotting and immunofluorescence analyses confirmed the increased levels of LIS1 in HCMV-infected NSCs at the protein level. We next assessed the migratory abilities of HCMV-infected NSCs and observed that infection strongly impaired the migration of NSCs, without detectable effect on their proliferation. Moreover, we observed increased immunostaining for LIS1 in brains of congenitally infected fetuses, but not in control samples, highlighting the clinical relevance of our findings. Of note, PAFAH1B1 mutations (resulting in either haploinsufficiency or gain of function) are primary causes of hereditary neurodevelopmental diseases. Notably, mutations resulting in PAFAH1B1 haploinsufficiency cause classic lissencephaly. Taken together, our findings suggest that PAFAH1B1 is a critical target of HCMV infection. They also shine a new light on the pathophysiological basis of the neurological outcomes of congenital HCMV infection, by suggesting that defective neural cell migration might contribute to the pathogenesis of the neurodevelopmental sequelae of infection. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Regulation of the MIE Locus During HCMV Latency and Reactivation

Human cytomegalovirus (HCMV) is a ubiquitous herpesviral pathogen that results in life-long infection. HCMV maintains a latent or quiescent infection in hematopoietic cells, which is broadly defined by transcriptional silencing and the absence of de novo virion production. However, upon cell differentiation coupled with immune dysfunction, the virus can reactivate, which leads to lytic replication in a variety of cell and tissue types. One of the mechanisms controlling the balance between latency and reactivation/lytic replication is the regulation of the major immediate-early (MIE) locus. This enhancer/promoter region is complex, and it is regulated by chromatinization and associated factors, as well as a variety of transcription factors. Herein, we discuss these factors and how they influence the MIE locus, which ultimately impacts the phase of HCMV infection.

Sex-Differential Impact of Human Cytomegalovirus Infection on In Vitro Reactivity to Toll-Like Receptor 2, 4 and 7/8 Stimulation in Gambian Infants

Human cytomegalovirus (HCMV) infection rates approach 100% by the first year of life in low-income countries. It is not known if this drives changes to innate immunity in early life and thereby altered immune reactivity to infections and vaccines. Given the panoply of sex differences in immunity, it is feasible that any immunological effects of HCMV would differ in males and females. We analysed ex vivo innate cytokine responses to a panel of toll-like receptor (TLR) ligands in 108 nine-month-old Gambian males and females participating in a vaccine trial. We found evidence that HCMV suppressed reactivity to TLR2 and TLR7/8 stimulation in females but not males. This is likely to contribute to sex differences in responses to infections and vaccines in early life and has implications for the development of TLR ligands as vaccine adjuvants. Development of an effective HCMV vaccine would be able to circumvent some of these potentially negative effects of HCMV infection in childhood.

Revisiting promyelocytic leukemia protein targeting by human cytomegalovirus immediate-early protein 1

Promyelocytic leukemia (PML) bodies are nuclear organelles implicated in intrinsic and innate antiviral defense. The eponymous PML proteins, central to the self-organization of PML bodies, and other restriction factors found in these organelles are common targets of viral antagonism. The 72-kDa immediate-early protein 1 (IE1) is the principal antagonist of PML bodies encoded by the human cytomegalovirus (hCMV). IE1 is believed to disrupt PML bodies by inhibiting PML SUMOylation, while PML was proposed to act as an E3 ligase for IE1 SUMOylation. PML targeting by IE1 is considered to be crucial for hCMV replication at low multiplicities of infection, in part via counteracting antiviral gene induction linked to the cellular interferon (IFN) response. However, current concepts of IE1-PML interaction are largely derived from mutant IE1 proteins known or predicted to be metabolically unstable and globally misfolded. We performed systematic clustered charge-to-alanine scanning mutagenesis and identified a stable IE1 mutant protein (IE1cc172-176) with wild-type characteristics except for neither interacting with PML proteins nor inhibiting PML SUMOylation. Consequently, IE1cc172-176 does not associate with PML bodies and is selectively impaired for disrupting these organelles. Surprisingly, functional analysis of IE1cc172-176 revealed that the protein is hypermodified by mixed SUMO chains and that IE1 SUMOylation depends on nucleosome rather than PML binding. Furthermore, a mutant hCMV expressing IE1cc172-176 was only slightly attenuated compared to an IE1-null virus even at low multiplicities of infection. Finally, hCMV-induced expression of cytokine and IFN-stimulated genes turned out to be reduced rather than increased in the presence of IE1cc172-176 relative to wild-type IE1. Our findings challenge present views on the relationship of IE1 with PML and the role of PML in hCMV replication. This study also provides initial evidence for the idea that disruption of PML bodies upon viral infection is linked to activation rather than inhibition of innate immunity.

Killer cell proteases can target viral immediate-early proteins to control human cytomegalovirus infection in a noncytotoxic manner

Human cytomegalovirus (HCMV) is the most frequent viral cause of congenital defects and can trigger devastating disease in immune-suppressed patients. Cytotoxic lymphocytes (CD8⁺ T cells and NK cells) control HCMV infection by releasing interferon-γ and five granzymes (GrA, GrB, GrH, GrK, GrM), which are believed to kill infected host cells through cleavage of intracellular death substrates. However, it has recently been demonstrated that the in vivo killing capacity of cytotoxic T cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we demonstrate that (primary) cytotoxic lymphocytes can block HCMV dissemination independent of host cell death, and interferon-α/β/γ. Prior to killing, cytotoxic lymphocytes induce the degradation of viral immediate-early (IE) proteins IE1 and IE2 in HCMV-infected cells. Intriguingly, both IE1 and/or IE2 are directly proteolyzed by all human granzymes, with GrB and GrM being most efficient. GrB and GrM cleave IE1 after Asp³⁹⁸ and Leu⁴¹⁴, respectively, likely resulting in IE1 aberrant cellular localization, IE1 instability, and functional impairment of IE1 to interfere with the JAK-STAT signaling pathway. Furthermore, GrB and GrM cleave IE2 after Asp¹⁸⁴ and Leu¹⁷³, respectively, resulting in IE2 aberrant cellular localization and functional abolishment of IE2 to transactivate the HCMV UL112 early promoter. Taken together, our data indicate that cytotoxic lymphocytes can also employ noncytotoxic ways to control HCMV infection, which may be explained by granzyme-mediated targeting of indispensable viral proteins during lytic infection.

Human cytomegalovirus (HCMV) is the leading viral cause of congenital defects, can trigger disease in immune-compromised patients, and plays roles in cancer development. Cytotoxic lymphocytes kill HCMV-infected cells via releasing a set of five cytotoxic serine proteases called granzymes. However, the killing capacity of cytotoxic cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we show that cytotoxic lymphocytes can also use granzymes to inhibit HCMV replication in absence of cell death. All five granzymes cleave and inactivate both viral immediate-early (IE1/2) proteins, which are essential players for initiating HCMV infection. Our data support the model that cytotoxic cells employ granzymes to dampen HCMV replication prior to accumulation of sufficient hits to kill the infected cell.

Human cytomegalovirus IE2 protein regulates macrophage-mediated immune escape by upregulating GRB2 expression in UL122 genetically modified mice

Although cytomegalovirus (HCMV) infection is asymptomatic in healthy individuals, the virus can remain latent for many years due to its ability to evade host immune surveillance. However, reactivation of HCMV can lead to life-threatening disease. Recent studies have shown that HCMV infection mediates immune escape by regulating macrophage activity, although the role of the HCMV-encoded IE2 protein is unclear. A ul122 transgenic mouse model was created to stably expresses the IE2 protein, and the proportion of M1 and M2 macrophage populations in their spleen and bone marrow was compared to that in wild-type controls. In addition, the phagocytic function of the macrophages was evaluated in terms of neutral red dye uptake. Spleen and bone marrow macrophages in IE2-expressing mice were mainly of the M2 phenotype and displayed enhanced phagocytic function compared to that in control mice. The relative levels of expression of macrophage-related GRB2 and of IL-4, IFN-γ, IL-13, and TNF-α were also analyzed in the spleen and bone marrow of the two groups. The IE2-expressing mice had increased expression of GRB2 and increased expression of the M2-related cytokines IL-4 and IL-13. Taken together, the current results suggest that HCMV IE2 polarizes the host macrophages to the M2 type via a GRB2/IL-4-related pathway, which enables long-term survival of the virus in the host.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Functional Profile of Human Cytomegalovirus Genes and Their Associated Diseases: A Review

The human cytomegalovirus (HCMV), whose genome is 235 ± 1.9 kbp long, is a common herpesvirus. However, the functions of many of its genes are still unknown. HCMV is closely associated with various human diseases and infects 60-90% of the global population. It can infect various human cells, including fibroblasts, epithelial cells, endothelial cells, smooth muscle cells, and monocytes. Although HCMV infection is generally asymptomatic and causes subtle clinical symptoms, it can generate a robust immune response and establish a latent infection in immunocompromised individuals, including those with AIDS, transplant recipients, and developing fetuses. Currently available antivirals approved for the treatment of HCMV-associated diseases are limited by dose-limiting toxicity and the emergence of resistance; however, vaccines and immunoglobulins are unavailable. In this review, we have summarized the recent literature on 43 newly identified HCMV genes. We have described their novel functions on the viral replication cycle, latency, and host immune evasion. Further, we have discussed HCMV-associated diseases and current therapeutic targets. Our review may provide a foundational basis for studies aiming to prevent and develop targeted therapies for HCMV-associated diseases.

Ribosome biogenesis restricts innate immune responses to virus infection and DNA

Ribosomes are universally important in biology and their production is dysregulated by developmental disorders, cancer, and virus infection. Although presumed required for protein synthesis, how ribosome biogenesis impacts virus reproduction and cell-intrinsic immune responses remains untested. Surprisingly, we find that restricting ribosome biogenesis stimulated human cytomegalovirus (HCMV) replication without suppressing translation. Interfering with ribosomal RNA (rRNA) accumulation triggered nucleolar stress and repressed expression of 1392 genes, including High Mobility Group Box 2 (HMGB2), a chromatin-associated protein that facilitates cytoplasmic double-stranded (ds) DNA-sensing by cGAS. Furthermore, it reduced cytoplasmic HMGB2 abundance and impaired induction of interferon beta (IFNB1) mRNA, which encodes a critical anti-proliferative, proinflammatory cytokine, in response to HCMV or dsDNA in uninfected cells. This establishes that rRNA accumulation regulates innate immune responses to dsDNA by controlling HMGB2 abundance. Moreover, it reveals that rRNA accumulation and/or nucleolar activity unexpectedly regulate dsDNA-sensing to restrict virus reproduction and regulate inflammation. (145 words)

P2Y2 purinergic receptor modulates virus yield, calcium homeostasis, and cell motility in human cytomegalovirus-infected cells

Human cytomegalovirus (HCMV) manipulates many aspects of host cell biology to create an intracellular milieu optimally supportive of its replication and spread. Our study reveals that levels of several components of the purinergic signaling system, including the P2Y2 and P2X5 receptors, are elevated in HCMV-infected fibroblasts. Knockdown and drug treatment experiments demonstrated that P2Y2 enhances the yield of virus, whereas P2X5 reduces HCMV production. The HCMV IE1 protein induces P2Y2 expression; and P2Y2-mediated signaling is important for efficient HCMV gene expression, DNA synthesis, and the production of infectious HCMV progeny. P2Y2 cooperates with the viral UL37x1 protein to regulate cystolic Ca2+ levels. P2Y2 also regulates PI3K/Akt signaling and infected cell motility. Thus, P2Y2 functions at multiple points within the viral replication cycle to support the efficient production of HCMV progeny, and it may facilitate in vivo viral spread through its role in cell migration.

The Golgi sorting motifs of human cytomegalovirus UL138 are not required for latency maintenance

Human cytomegalovirus (HCMV) establishes latency within incompletely differentiated cells of the myeloid lineage. The viral protein UL138 participates in establishing and maintaining this latent state. UL138 has multiple functions during latency that include silencing productive phase viral gene transcription and modulating intracellular protein trafficking. Trafficking and subsequent downregulation of the multidrug resistance-associated protein 1 (MRP1) by UL138 is mediated by one of four Golgi sorting motifs within UL138. Here we investigate whether any of the Golgi sorting motifs of UL138 are required for the establishment and/or maintenance of HCMV latency in model cell systems in vitro. We determined that a mutant UL138 protein lacking an acidic cluster dileucine sorting motif unable to downregulate MRP1, as well as another mutant lacking all four Golgi sorting motifs still silenced viral immediate early (IE) gene expression and prevented progeny virion formation during latency. We conclude that the Golgi sorting motifs are not required for latency establishment or maintenance in model cell systems in vitro.

Breast cancer and cytomegalovirus

PURPOSE

To determine whether cytomegalovirus is causally associated with breast cancer and whether cytomegalovirus should be categorised as an oncogenic virus.

METHODS

We undertook a review of published epidemiological and laboratory studies, using established causal criteria: Bradford Hill criteria to determine whether cytomegalovirus is associated with breast cancer; and Evans/Mueller criteria to determine whether cytomegalovirus should be categorised as an oncogenic virus.

RESULTS

Although there are inconsistencies in the findings of published epidemiological and laboratory studies, these may be explained by factors such as: differences in timing of blood samples, differences in selection of cases and controls, or high cytomegalovirus seroprevalence among participants in the epidemiological studies; and, in the laboratory studies, differences in sample preparations, age of sample, whether or not paired breast cancer and normal breast tissue samples were used, differences in the tests, primers and/or antibodies used, differences in histological types of breast cancer studied, and/or features of the virus.

CONCLUSIONS

Overall, the results of published studies of cytomegalovirus and breast cancer suggest cytomegalovirus is a causal factor for at least some types of breast cancer. If the evidence for a link between cytomegalovirus and breast cancer continues to strengthen, further research could lead to: targeted screening; therapy using antiviral drugs; and, perhaps, primary prevention of a significant proportion of breast cancer. Vaccination against viruses has already been shown to be effective in preventing cervix and liver cancer; cytomegalovirus vaccines are already under development.

Human Cytomegalovirus Immediate Early 86-kDa Protein Blocks Transcription and Induces Degradation of the Immature Interleukin-1β Protein during Virion-Mediated Activation of the AIM2 Inflammasome

Secretion of interleukin-1β (IL-1β) represents a fundamental innate immune response to microbial infection that, at the molecular level, occurs following activation of proteolytic caspases that cleave the immature protein into a secretable form. Human cytomegalovirus (HCMV) is the archetypal betaherpesvirus that is invariably capable of lifelong infection through the activity of numerous virally encoded immune evasion phenotypes. Innate immune pathways responsive to cytoplasmic double-stranded DNA (dsDNA) are known to be activated in response to contact between HCMV and host cells. Here, we used clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) genome editing to demonstrate that the dsDNA receptor absent in melanoma 2 (AIM2) is required for secretion of IL-1β following HCMV infection. Furthermore, dsDNA-responsive innate signaling induced by HCMV infection that leads to activation of the type I interferon response is also shown, unexpectedly, to play a contributory role in IL-1β secretion. Importantly, we also show that rendering virus particles inactive by UV exposure leads to substantially increased IL-1β processing and secretion and that live HCMV can inhibit this, suggesting the virus encodes factors that confer an inhibitory effect on this response. Further examination revealed that ectopic expression of the immediate early (IE) 86-kDa protein (IE86) is actually associated with a block in transcription of the pro-IL-1β gene and, independently, diminishment of the immature protein. Overall, these results reveal two new and distinct phenotypes conferred by the HCMV IE86 protein, as well as an unusual circumstance in which a single herpesviral protein exhibits inhibitory effects on multiple molecular processes within the same innate immune response.IMPORTANCE Persistent infection with HCMV is associated with the operation of diverse evasion phenotypes directed at antiviral immunity. Obstruction of intrinsic and innate immune responses is typically conferred by viral proteins either associated with the viral particle or expressed immediately after entry. In line with this, numerous phenotypes are attributed to the HCMV IE86 protein that involve interference with innate immune processes via transcriptional and protein-directed mechanisms. We describe novel IE86-mediated phenotypes aimed at virus-induced secretion of IL-1β. Intriguingly, while many viruses target the function of the molecular scaffold required for IL-1β maturation to prevent this response, we find that HCMV and IE86 target the IL-1β protein specifically. Moreover, we show that IE86 impairs both the synthesis of the IL-1β transcript and the stability of the immature protein. This indicates an unusual phenomenon in which a single viral protein exhibits two molecularly separate evasion phenotypes directed at a single innate cytokine.

Expression of human miR-200b-3p and -200c-3p in cytomegalovirus-infected tissues

Human cytomegalovirus (HCMV) infection can cause inflammatory tissue-invasive end-organ diseases upon lytic replication. In humans, mature miR-200b-3p and -200c-3p suppress the synthesis of HCMV immediate early 2 (IE2) protein by binding to the 3′-UTR of the mRNA encoded by the unique long (UL) 122-123 region in human foreskin fibroblasts and pre-transplant peripheral blood mononuclear cells stimulated with HCMV. The present study aimed to quantitate the expression of Homo sapiens (hsa)-miR-200b-3p and 200c-3p in HCMV-infected tissues. We collected 240 HCMV-infected and 154 HCMV-non-infected, formalin-fixed, paraffin-embedded tissue samples of the gastrointestinal (GI) tract and bronchi/lungs. MiRNAs, HCMV, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were quantitated by quantitative reverse transcription-PCR (qRT-PCR) and quantitative PCR (qPCR) on the basis of standard curves generated using miRNA mimics, the HCMV strain from National Institute for Biological Standards and Control (NIBSC) 09/162, and GAPDH control. To avoid the effect of cell counts on the qRT-PCR and qPCR results, the data were normalized to GAPDH levels. HCMV-infected tissues had significantly lower levels of 200b-3p/GAPDH (3.03 ± 1.50 compared with 3.98 ± 1.08 log₁₀ copies/μl, P<0.001) and 200c-3p/GAPDH (4.67 ± 1.84 compared with 6.35 ± 1.47 log₁₀ copies/μl, P<0.001) than normal tissues. The values for 200b-3p/GAPDH (r = −0.51, P<0.001) and 200c-3p/GAPDH (r = −0.54, P<0.001) were significantly inversely correlated with HCMV load. Low tissue levels of 200b-3p and 200c-3p in humans are associated with cytopathic inflammation due to HCMV infection.

HCMV miRNA Targets Reveal Important Cellular Pathways for Viral Replication, Latency, and Reactivation

It is now well appreciated that microRNAs (miRNAs) play a critical role in the lifecycles of many herpes viruses. The human cytomegalovirus (HCMV) replication cycle varies significantly depending on the cell type infected, with lytic replication occurring in fully-differentiated cells such as fibroblasts, endothelial cells, or macrophages, and latent infection occurring in less-differentiated CD14+ monocytes and CD34+ hematopoietic progenitor cells where viral gene expression is severely diminished and progeny virus is not produced. Given their non-immunogenic nature and their capacity to target numerous cellular and viral transcripts, miRNAs represent a particularly advantageous means for HCMV to manipulate viral gene expression and cellular signaling pathways during lytic and latent infection. This review will focus on our current knowledge of HCMV miRNA viral and cellular targets, and discuss their importance in lytic and latent infection, highlight the challenges of studying HCMV miRNAs, and describe how viral miRNAs can help us to better understand the cellular processes involved in HCMV latency.

Role of PDGF receptor-α during human cytomegalovirus entry into fibroblasts

Human CMV (HCMV) exhibits a broad cell tropism that depends on two virion glycoprotein complexes: a trimeric complex (gH/gL/gO) that facilitates viral infection primarily in fibroblasts and a pentameric complex (gH/gL/pUL128-pUL130-pUL131A) that mediates infection in epithelial and endothelial cells. We performed genome-wide CRISPR screens in which the PDGF receptor-α (PDGFRα) was identified as the most significant cellular gene product essential for infection by HCMV virions containing only trimeric complex (trimer-only virus). Trimer-only virus did not enter PDGFRα knockout fibroblasts. By using knockout fibroblasts, the extracellular domain of PDGFRα required for virus entry was mapped, and the intracellular tyrosine kinase domain was shown to be nonessential. In addition, direct cell-to-cell spread of virus from knockout cells transfected with trimer-only viral DNA was blocked, despite the production of infectious virus in the transfected cells. In contrast to trimer-only virus, wild-type HCMV virions containing both trimeric and pentameric complexes entered PDGFRα knockout cells, reinforcing the view that fibroblasts contain a second, independent receptor for the pentameric complex. Importantly, however, wild-type virus entered the knockout fibroblasts at reduced efficiency compared with parental fibroblasts, arguing that the cellular receptor for the virion pentameric complex is limiting or that virions are produced containing different relative amounts of the two glycoprotein complexes. Finally, ectopic expression of PDGFRα in ARPE-19 epithelial cells and THP-1 monocytic cells, which have little to no endogenous PDGFRα expression, markedly enhanced their susceptibility to trimer-only virions. In sum, our data clarify several key determinants of HCMV tropism.

HCMV Infection and Apoptosis: How Do Monocytes Survive HCMV Infection?

Human cytomegalovirus (HCMV) infection of peripheral blood monocytes plays a key role in the hematogenous dissemination of the virus to multiple organ systems following primary infection or reactivation of latent virus in the bone marrow. Monocytes have a short life span of 1⁻3 days in circulation; thus, HCMV must alter their survival and differentiation to utilize these cells and their differentiated counterparts-macrophages-for dissemination and long term viral persistence. Because monocytes are not initially permissive for viral gene expression and replication, HCMV must control host-derived factors early during infection to prevent apoptosis or programmed cell death prior to viral induced differentiation into naturally long-lived macrophages. This review provides a short overview of HCMV infection of monocytes and describes how HCMV has evolved to utilize host cell anti-apoptotic pathways to allow infected monocytes to bridge the 48⁻72 h viability gate so that differentiation into a long term stable mature cell can occur. Because viral gene expression is delayed in monocytes following initial infection and only occurs (begins around two to three weeks post infection in our model) following what appears to be complete differentiation into mature macrophages or dendritic cells, or both; virally-encoded anti-apoptotic gene products cannot initially control long term infected cell survival. Anti-apoptotic viral genes are discussed in the second section of this review and we argue they would play an important role in long term macrophage or dendritic cell survival following infection-induced differentiation.

New therapies for human cytomegalovirus infections

The recent approval of letermovir marks a new era of therapy for human cytomegalovirus (HCMV) infections, particularly for the prevention of HCMV disease in hematopoietic stem cell transplant recipients. For almost 30 years ganciclovir has been the therapy of choice for these infections and by today's standards this drug exhibits only modest antiviral activity that is often insufficient to completely suppress viral replication, and drives the selection of drug-resistant variants that continue to replicate and contribute to disease. While ganciclovir remains the therapy of choice, additional drugs that inhibit novel molecular targets, such as letermovir, will be required as highly effective combination therapies are developed not only for the treatment of immunocompromised hosts, but also for congenitally infected infants. Sustained efforts, largely in the biotech industry and academia, have identified additional highly active lead compounds that have progressed into clinical studies with varying levels of success and at least two have the potential to be approved in the near future. Some of the new drugs in the pipeline inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapeutic regimens. Here, we will describe some of the unique features of HCMV biology and discuss their effect on therapeutic needs. Existing drugs will also be discussed and some of the more promising candidates will be reviewed with an emphasis on those progressing through clinical studies. The in vitro and in vivo antiviral activity, spectrum of antiviral activity, and mechanism of action of new compounds will be reviewed to provide an update on potential new therapies for HCMV infections that have progressed significantly in recent years.

Efficient Delivery of Human Cytomegalovirus T Cell Antigens by Attenuated Sendai Virus Vectors

Human cytomegalovirus (HCMV) represents a major cause of clinical complications during pregnancy as well as immunosuppression, and the licensing of a protective HCMV vaccine remains an unmet global need. Here, we designed and validated novel Sendai virus (SeV) vectors delivering the T cell immunogens IE-1 and pp65. To enhance vector safety, we used a replication-deficient strain (rdSeV) that infects target cells in a nonproductive manner while retaining viral gene expression. In this study, we explored the impact that transduction with rdSeV has on human dendritic cells (DCs) by comparing it to the parental, replication-competent Sendai virus strain (rcSeV) as well as the poxvirus strain modified vaccinia Ankara (MVA). We found that wild-type SeV is capable of replicating to high titers in DCs while rdSeV infects cells abortively. Due to the higher degree of attenuation, IE-1 and pp65 protein levels mediated by rdSeV after infection of DCs were markedly reduced compared to those of the parental Sendai virus recombinants, but antigen-specific restimulation of T cell clones was not negatively affected by this. Importantly, rdSeV showed reduced cytotoxic effects compared to rcSeV and MVA and was capable of mediating DC maturation as well as secretion of alpha interferon and interleukin-6. Finally, in a challenge model with a murine cytomegalovirus (MCMV) strain carrying an HCMV pp65 peptide, we found that viral replication was restricted if mice were previously vaccinated with rdSeV-pp65. Taken together, these data demonstrate that rdSeV has great potential as a vector system for the delivery of HCMV immunogens.IMPORTANCE HCMV is a highly prevalent betaherpesvirus that establishes lifelong latency after primary infection. Congenital HCMV infection is the most common viral complication in newborns, causing a number of late sequelae ranging from impaired hearing to mental retardation. At the same time, managing HCMV reactivation during immunosuppression remains a major hurdle in posttransplant care. Since options for the treatment of HCMV infection are still limited, the development of a vaccine to confine HCMV-related morbidities is urgently needed. We generated new vaccine candidates in which the main targets of T cell immunity during natural HCMV infection, IE-1 and pp65, are delivered by a replication-deficient, Sendai virus-based vector system. In addition to classical prophylactic vaccine concepts, these vectors could also be used for therapeutic applications, thereby expanding preexisting immunity in high-risk groups such as transplant recipients or for immunotherapy of glioblastomas expressing HCMV antigens.

Cellular responses to human cytomegalovirus infection: Induction of a mesenchymal-to-epithelial transition (MET) phenotype

Human cytomegalovirus (HCMV) is the prototypical human β-herpes virus. Here we perform a systems analysis of the HCMV host-cell transcriptome, using gene set enrichment analysis (GSEA) as an engine to globally map the host-pathogen interaction across two cell types. Our analysis identified several previously unknown signatures of infection, such as induction of potassium channels and amino acid transporters, derepression of genes marked with histone H3 lysine 27 trimethylation (H3K27me3), and inhibition of genes related to epithelial-to-mesenchymal transition (EMT). The repression of EMT genes was dependent on early viral gene expression and correlated with induction E-cadherin (CDH1) and mesenchymal-to-epithelial transition (MET) genes. Infection of transformed breast carcinoma and glioma stem cells similarly inhibited EMT and induced MET, arguing that HCMV induces an epithelium-like cellular environment during infection.

Structural determinants of TRIM protein function

Tripartite motif (TRIM) proteins constitute one of the largest subfamilies of Really Interesting New Gene (RING) E3 ubiquitin ligases and contribute to the regulation of numerous cellular activities, including innate immune responses. The conserved TRIM harbours a RING domain that imparts E3 ligase activity to TRIM family proteins, whilst a variable C-terminal region can mediate recognition of substrate proteins. The knowledge of the structure of these multidomain proteins and the functional interplay between their constituent domains is paramount to understanding their cellular roles. To date, available structural information on TRIM proteins is still largely restricted to subdomains of many TRIMs in isolation. Nevertheless, applying a combination of structural, biophysical and biochemical approaches has recently allowed important progress to be made towards providing a better understanding of the molecular features that underlie the function of TRIM family proteins and has uncovered an unexpected diversity in the link between self-association and catalytic activity.

The essential role of guinea pig cytomegalovirus (GPCMV) IE1 and IE2 homologs in viral replication and IE1-mediated ND10 targeting

Guinea pig cytomegalovirus (GPCMV) immediate early proteins, IE1 and IE2, demonstrated structural and functional homologies with human cytomegalovirus (HCMV). GPCMV IE1 and IE2 co-localized in the nucleus with each other, the viral polymerase and guinea pig ND10 components (gpPML, gpDaxx, gpSp100, gpATRX). IE1 showed direct interaction with ND10 components by immunoprecipitation unlike IE2. Additionally, IE1 protein disrupted ND10 bodies. IE1 mutagenesis mapped the nuclear localization signal to the C-terminus and identified the core domain for gpPML interaction. Individual knockout of GPCMV GP122 or GP123 (IE2 and IE1 unique exons respectively) was lethal to the virus. However, an IE1 mutant (codons 234-474 deleted), was viable with attenuated viral growth kinetics and increased susceptibility to type I interferon (IFN-I). In HCMV, the IE proteins are important T cell target antigens. Consequently, characterization of the homologs in GPCMV provides a basis for their evaluation in candidate vaccines against congenital infection.

Mechanisms Underlying T Cell Immunosenescence: Aging and Cytomegalovirus Infection

The ability of the human immune system to protect against infectious disease declines with age and efficacy of vaccination reduces significantly in the elderly. Aging of the immune system, also termed as immunosenescence, involves many changes in human T cell immunity that is characterized by a loss in naïve T cell population and an increase in highly differentiated CD28- memory T cell subset. There is extensive data showing that latent persistent human cytomegalovirus (HCMV) infection is also associated with age-related immune dysfunction in the T cells, which might enhance immunosenescence. Understanding the molecular mechanisms underlying age-related and HCMV-related immunosenescence is critical for the development of effective age-targeted vaccines and immunotherapies. In this review, we will address the role of both aging and HCMV infection that contribute to the T cell senescence and discuss the potential molecular mechanisms in aged T cells.

Convallatoxin-Induced Reduction of Methionine Import Effectively Inhibits Human Cytomegalovirus Infection and Replication

Cytomegalovirus (CMV) is a ubiquitous human pathogen that increases the morbidity and mortality of immunocompromised individuals. The current FDA-approved treatments for CMV infection are intended to be virus specific, yet they have significant adverse side effects, including nephrotoxicity and hematological toxicity. Thus, there is a medical need for safer and more effective CMV therapeutics. Using a high-content screen, we identified the cardiac glycoside convallatoxin as an effective compound that inhibits CMV infection. Using a panel of cardiac glycoside variants, we assessed the structural elements critical for anti-CMV activity by both experimental and in silico methods. Analysis of the antiviral effects, toxicities, and pharmacodynamics of different variants of cardiac glycosides identified the mechanism of inhibition as reduction of methionine import, leading to decreased immediate-early gene translation without significant toxicity. Also, convallatoxin was found to dramatically reduce the proliferation of clinical CMV strains, implying that its mechanism of action is an effective strategy to block CMV dissemination. Our study has uncovered the mechanism and structural elements of convallatoxin, which are important for effectively inhibiting CMV infection by targeting the expression of immediate-early genes.

IMPORTANCE

Cytomegalovirus is a highly prevalent virus capable of causing severe disease in certain populations. The current FDA-approved therapeutics all target the same stage of the viral life cycle and induce toxicity and viral resistance. We identified convallatoxin, a novel cell-targeting antiviral that inhibits CMV infection by decreasing the synthesis of viral proteins. At doses low enough for cells to tolerate, convallatoxin was able to inhibit primary isolates of CMV, including those resistant to the anti-CMV drug ganciclovir. In addition to identifying convallatoxin as a novel antiviral, limiting mRNA translation has a dramatic impact on CMV infection and proliferation.

Cytomegalovirus Vaccines: Current Status and Future Prospects

Congenital human cytomegalovirus (HCMV) infection can result in severe and permanent neurological injury in newborns, and vaccine development is accordingly a major public health priority. HCMV can also cause disease in solid organ transplant (SOT) and hematopoietic stem-cell transplant (HSCT) recipients, and a vaccine would be valuable in prevention of viremia and end-organ disease in these populations. Currently there is no licensed HCMV vaccine, but progress toward this goal has been made in recent clinical trials. A recombinant HCMV glycoprotein B (gB) vaccine has been shown to have some efficacy in prevention of infection in young women and adolescents, and has provided benefit to HCMV-seronegative SOT recipients. Similarly, DNA vaccines based on gB and the immunodominant T-cell target, pp65 (ppUL83), have been shown to reduce viremia in HSCT patients. This review provides an overview of HCMV vaccine candidates in various stages of development, as well as an update on the current status of ongoing clinical trials. Protective correlates of vaccine-induced immunity may be different for pregnant woman and transplant patients. As more knowledge emerges about correlates of protection, the ultimate licensure of HCMV vaccines may reflect the uniqueness of the target populations being immunized.

Cytomegalovirus vaccines under clinical development

Congenital cytomegalovirus (CMV) infection is the most common infectious cause of disability in newborn infants. CMV also causes serious disease in solid organ (SOT) and haematopoietic stem cell transplant (HSCT) recipients. In otherwise healthy children and adults, primary CMV infection rarely causes illness. However, even asymptomatic CMV infections may predispose an individual towards an increased risk of atherosclerosis, cancer and immune senescence over the life course, although such associations remain controversial. Thus, although a vaccine against congenital CMV infection would have the greatest public health impact and cost-effectiveness, arguably all populations could benefit from an effective immunisation against this virus. Currently there are no licensed CMV vaccines, but there is increased interest in developing and testing potential candidates, driven by the demonstration that a recombinant CMV glycoprotein B (gB) vaccine has some efficacy in prevention of infection in young women and adolescents, and in CMV-seronegative SOT recipients. In this review, the recent and current status of candidate CMV vaccines is discussed. Evolving concepts about proposed correlates of protective immunity in different target populations for CMV vaccination, and how these differences impact current clinical trials, are also reviewed.

Human Cytomegalovirus Immediate-Early 1 Protein Rewires Upstream STAT3 to Downstream STAT1 Signaling Switching an IL6-Type to an IFNγ-Like Response

The human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) is best known for activating transcription to facilitate viral replication. Here we present transcriptome data indicating that IE1 is as significant a repressor as it is an activator of host gene expression. Human cells induced to express IE1 exhibit global repression of IL6- and oncostatin M-responsive STAT3 target genes. This repression is followed by STAT1 phosphorylation and activation of STAT1 target genes normally induced by IFNγ. The observed repression and subsequent activation are both mediated through the same region (amino acids 410 to 445) in the C-terminal domain of IE1, and this region serves as a binding site for STAT3. Depletion of STAT3 phenocopies the STAT1-dependent IFNγ-like response to IE1. In contrast, depletion of the IL6 receptor (IL6ST) or the STAT kinase JAK1 prevents this response. Accordingly, treatment with IL6 leads to prolonged STAT1 instead of STAT3 activation in wild-type IE1 expressing cells, but not in cells expressing a mutant protein (IE1dl410-420) deficient for STAT3 binding. A very similar STAT1-directed response to IL6 is also present in cells infected with a wild-type or revertant hCMV, but not an IE1dl410-420 mutant virus, and this response results in restricted viral replication. We conclude that IE1 is sufficient and necessary to rewire upstream IL6-type to downstream IFNγ-like signaling, two pathways linked to opposing actions, resulting in repressed STAT3- and activated STAT1-responsive genes. These findings relate transcriptional repressor and activator functions of IE1 and suggest unexpected outcomes relevant to viral pathogenesis in response to cytokines or growth factors that signal through the IL6ST-JAK1-STAT3 axis in hCMV-infected cells. Our results also reveal that IE1, a protein considered to be a key activator of the hCMV productive cycle, has an unanticipated role in tempering viral replication.

Our previous work has shown that the human cytomegalovirus (hCMV) major immediate-early 1 protein (IE1) modulates host cell signaling pathways involving proteins of the signal transducer and activator of transcription (STAT) family. IE1 has also long been known to facilitate viral replication by activating transcription. In this report we demonstrate that IE1 is as significant a repressor as it is an activator of host gene expression. Many genes repressed by IE1 are normally induced via STAT3 signaling triggered by interleukin 6 (IL6) or related cytokines, whereas many genes activated by IE1 are normally induced via STAT1 signaling triggered by interferon gamma (IFNγ). Our results suggest that the repression of STAT3- and the activation of STAT1-responsive genes by IE1 are coupled. By targeting STAT3, IE1 rewires upstream STAT3 to downstream STAT1 signaling. Consequently, genes normally induced by IL6 are repressed while genes normally induced by IFNγ become responsive to IL6 in the presence of IE1. We also demonstrate that, by switching an IL6 to an IFNγ-like response, IE1 tempers viral replication. These results suggest an unanticipated dual role for IE1 in either promoting or limiting hCMV propagation and demonstrate how a key viral regulatory protein merges two central cellular signaling pathways to divert cytokine responses relevant to hCMV pathogenesis.

A single exercise bout enhances the manufacture of viral-specific T-cells from healthy donors: implications for allogeneic adoptive transfer immunotherapy

Cytomegalovirus (CMV) and Epstein-Barr virus (EBV) infections remain a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). The adoptive transfer of donor-derived viral-specific cytotoxic T-cells (VSTs) is an effective treatment for controlling CMV and EBV infections after HSCT; however, new practical methods are required to augment the ex vivo manufacture of multi-VSTs from healthy donors. This study investigated the effects of a single exercise bout on the ex vivo manufacture of multi-VSTs. PBMCs isolated from healthy CMV/EBV seropositive participants before (PRE) and immediately after (POST) 30-minutes of cycling exercise were stimulated with CMV (pp65 and IE1) and EBV (LMP2A and BMLF1) peptides and expanded over 8 days. The number (fold difference from PRE) of T-cells specific for CMV pp65 (2.6), EBV LMP2A (2.5), and EBV BMLF1 (4.4) was greater among the VSTs expanded POST. VSTs expanded PRE and POST had similar phenotype characteristics and were equally capable of MHC-restricted killing of autologous target cells. We conclude that a single exercise bout enhances the manufacture of multi-VSTs from healthy donors without altering their phenotype or function and may serve as a simple and economical adjuvant to boost the production of multi-VSTs for allogeneic adoptive transfer immunotherapy.

MicroRNAs Expressed during Viral Infection: Biomarker Potential and Therapeutic Considerations

MicroRNAs (miRNAs) are short sequences of noncoding single-stranded RNAs that exhibit inhibitory effects on complementary target mRNAs. Recently, it has been discovered that certain viruses express their own miRNAs, while other viruses activate the transcription of cellular miRNAs for their own benefit. This review summarizes the viral and/or cellular miRNAs that are transcribed during infection, with a focus on the biomarker and therapeutic potential of miRNAs (or their antagomirs). Several human viruses of clinical importance are discussed, namely, herpesviruses, polyomaviruses, hepatitis B virus, hepatitis C virus, human papillomavirus, and human immunodeficiency virus.

Suppression of RIP3-dependent necroptosis by human cytomegalovirus

Necroptosis is an alternate programmed cell death pathway that is unleashed by caspase-8 compromise and mediated by receptor-interacting protein kinase 3 (RIP3). Murine cytomegalovirus (CMV) and herpes simplex virus (HSV) encode caspase-8 inhibitors that prevent apoptosis together with competitors of RIP homotypic interaction motif (RHIM)-dependent signal transduction to interrupt the necroptosis. Here, we show that pro-necrotic murine CMV M45 mutant virus drives virus-induced necroptosis during nonproductive infection of RIP3-expressing human fibroblasts, whereas WT virus does not. Thus, M45-encoded RHIM competitor, viral inhibitor of RIP activation, sustains viability of human cells like it is known to function in infected mouse cells. Importantly, human CMV is shown to block necroptosis induced by either TNF or M45 mutant murine CMV in RIP3-expressing human cells. Human CMV blocks TNF-induced necroptosis after RIP3 activation and phosphorylation of the mixed lineage kinase domain-like (MLKL) pseudokinase. An early, IE1-regulated viral gene product acts on a necroptosis step that follows MLKL phosphorylation prior to membrane leakage. This suppression strategy is distinct from RHIM signaling competition by murine CMV or HSV and interrupts an execution process that has not yet been fully elaborated.

Cytomegalovirus and Epstein-Barr virus in breast cancer

Findings of polymerase chain reaction (PCR) studies of cytomegalovirus (CMV) and Epstein-Barr virus (EBV) and breast cancer vary, making it difficult to determine whether either, both, or neither virus is causally associated with breast cancer. We investigated CMV and EBV in paired samples of breast cancer and normal breast tissue from 70 women using quantitative PCR. A serum sample from each woman was tested for CMV and EBV IgG. To place our results in context, we reviewed the existing literature and performed a meta-analysis of our results together with previous PCR studies of EBV, CMV, and breast cancer. Of the serology samples, 67 of 70 (96%) were EBV IgG positive and 49 of 70 (70%) were CMV IgG positive. QPCR detected EBV in 24 (34%) of the tumour and 9 (13%) of the paired normal specimens and CMV in 0 (0%) of the tumour and 2 (3%) of the paired normal specimens. Our findings, together with earlier results summarised in the meta-analysis, suggest several possibilities: variable findings may be due to limitations of molecular analyses; 'hit and run' oncogenesis may lead to inconsistent results; one or both viruses has a role at a later stage in breast cancer development; infection with multiple viruses increases breast cancer risk; or neither virus has a role. Future studies should focus on ways to investigate these possibilities, and should include comparisons of breast cancer tissue samples with appropriate normal tissue samples.

Controlling cytomegalovirus: helping the immune system take the lead

Cytomegalovirus, of the Herpesviridae family, has evolved alongside humans for thousands of years with an intricate balance of latency, immune evasion, and transmission. While upwards of 70% of humans have evidence of CMV infection, the majority of healthy people show little to no clinical symptoms of primary infection and CMV disease is rarely observed during persistent infection in immunocompetent hosts. Despite the fact that the majority of infected individuals are asymptomatic, immunologically, CMV hijacks the immune system by infecting and remaining latent in antigen-presenting cells that occasionally reactivate subclinically and present antigen to T cells, eventually causing the inflation of CMV-specific T cells until they can compromise up to 10% of the entire T cell repertoire. Because of this impact on the immune system, as well as its importance in fields such as stem cell and organ transplant, the relationship between CMV and the immune response has been studied in depth. Here we provide a review of many of these studies and insights into how CMV-specific T cells are currently being used therapeutically.

Human cytomegalovirus major immediate early 1 protein targets host chromosomes by docking to the acidic pocket on the nucleosome surface

The 72-kDa immediate early 1 (IE1) protein encoded by human cytomegalovirus (hCMV) is a nuclearly localized promiscuous regulator of viral and cellular transcription. IE1 has long been known to associate with host mitotic chromatin, yet the mechanisms underlying this interaction have not been specified. In this study, we identify the cellular chromosome receptor for IE1. We demonstrate that the viral protein targets human nucleosomes by directly binding to core histones in a nucleic acid-independent manner. IE1 exhibits two separable histone-interacting regions with differential binding specificities for H2A-H2B and H3-H4. The H2A-H2B binding region was mapped to an evolutionarily conserved 10-amino-acid motif within the chromatin-tethering domain (CTD) of IE1. Results from experimental approaches combined with molecular modeling indicate that the IE1 CTD adopts a β-hairpin structure, docking with the acidic pocket formed by H2A-H2B on the nucleosome surface. IE1 binds to the acidic pocket in a way similar to that of the latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus. Consequently, the IE1 and LANA CTDs compete for binding to nucleosome cores and chromatin. Our work elucidates in detail how a key viral regulator is anchored to human chromosomes and identifies the nucleosomal acidic pocket as a joint target of proteins from distantly related viruses. Based on the striking similarities between the IE1 and LANA CTDs and the fact that nucleosome targeting by IE1 is dispensable for productive replication even in "clinical" strains of hCMV, we speculate that the two viral proteins may serve analogous functions during latency of their respective viruses.

Nuclear interferon-inducible protein 16 promotes silencing of herpesviral and transfected DNA

Mammalian cells have evolved mechanisms to silence foreign DNA introduced by viruses or by transfection. Upon herpesviral infection of cells, the viral genome is chromatinized in an attempt by the host cell to restrict expression of the viral genome. HSV ICP0 acts to counter host-intrinsic and innate responses to viral infection. We have found that nuclear interferon (IFN)-inducible protein 16 (IFI16) acts as a restriction factor against ICP0-null herpes simplex virus 1 (HSV-1) to limit viral replication and immediate-early gene expression. IFI16 promoted the addition of heterochromatin marks and the reduction of euchromatin marks on viral chromatin. IFI16 also restricted the expression of plasmid DNAs introduced by transfection but did not restrict SV40 DNA introduced into the cellular nucleus in the form of nucleosomal chromatin by viral infection. These results argue that IFI16 restricts unchromatinized DNA when it enters the cell nucleus by promoting the loading of nucleosomes and the addition of heterochromatin marks. Furthermore, these results indicate that IFI16 provides a broad surveillance role against viral and transfected DNA by promoting restriction of gene expression from the exogenous DNA and inducing innate immune responses.