Identification and expression of human cytomegalovirus transcription units coding for two distinct Fcgamma receptor homologs

Rhesus Cytomegalovirus-encoded Fcγ-binding glycoproteins facilitate viral evasion from IgG-mediated humoral immunity

Human cytomegalovirus (HCMV) encodes four viral Fc-gamma receptors (vFcγRs) that counteract antibody-mediated activation in vitro , but their role in infection and pathogenesis is unknown. To examine the in vivo function of vFcγRs in animal hosts closely related to humans, we identified and characterized vFcγRs encoded by rhesus CMV (RhCMV). We demonstrate that Rh05, Rh152/151 and Rh173 represent the complete set of RhCMV vFcγRs, each displaying functional similarities to their respective HCMV orthologs with respect to antagonizing host FcγR activation in vitro . When RhCMV-naïve rhesus macaques were infected with vFcγR-deleted RhCMV, peak plasma viremia levels and anti-RhCMV antibody responses were comparable to wildtype infections. However, the duration of plasma viremia was significantly shortened in immunocompetent, but not in CD4+ T cell-depleted animals. Since vFcγRs were not required for superinfection, we conclude that vFcγRs delay control by virus-specific adaptive immune responses, particularly antibodies, during primary infection.

Deletion of the non-adjacent genes UL148 and UL148D impairs human cytomegalovirus-mediated TNF receptor 2 surface upregulation

Human cytomegalovirus (HCMV) is a prototypical β-herpesvirus which frequently causes morbidity and mortality in individuals with immature, suppressed, or senescent immunity. HCMV is sensed by various pattern recognition receptors, leading to the secretion of pro-inflammatory cytokines including tumor necrosis factor alpha (TNFα). TNFα binds to two distinct trimeric receptors: TNF receptor (TNFR) 1 and TNFR2, which differ in regard to their expression profiles, affinities for soluble and membrane-bound TNFα, and down-stream signaling pathways. While both TNF receptors engage NFκB signaling, only the nearly ubiquitously expressed TNFR1 exhibits a death domain that mediates TRADD/FADD-dependent caspase activation. Under steady-state conditions, TNFR2 expression is mainly restricted to immune cells where it predominantly submits pro-survival, proliferation-stimulating, and immune-regulatory signals. Based on the observation that HCMV-infected cells show enhanced binding of TNFα, we explored the interplay between HCMV and TNFR2. As expected, uninfected fibroblasts did not show detectable levels of TNFR2 on the surface. Intriguingly, however, HCMV infection increased TNFR2 surface levels of fibroblasts. Using HCMV variants and BACmid-derived clones either harboring or lacking the ULb' region, an association between TNFR2 upregulation and the presence of the ULb' genome region became evident. Applying a comprehensive set of ULb' gene block and single gene deletion mutants, we observed that HCMV mutants in which the non-adjacent genes UL148 or UL148D had been deleted show an impaired ability to upregulate TNFR2, coinciding with an inverse regulation of TACE/ADAM17.

Human immunoglobulins are transported to HCMV viral envelope by viral Fc gamma receptors-dependent and independent mechanisms

Human cytomegaloviruses (HCMVs) employ many different mechanisms to escape and subvert the host immune system, including expression of the viral IgG Fcγ receptors (vFcγRs) RL11 (gp34), RL12 (gp95), RL13 (gpRL13), and UL119 (gp68) gene products. The role of vFcγRs in HCMV pathogenesis has been reported to operate in infected cells by interfering with IgG-mediated effector functions. We found that gp34 and gp68 are envelope proteins that bind and internalize human IgGs on the surface of infected cells. Internalized IgGs are then transported on the envelope of viral particles in a vFcR-dependent mechanism. This mechanism is also responsible for the incorporation on the virions of the anti-gH neutralizing antibody MSL-109. Intriguingly, we show that gp68 is responsible for MSL-109 incorporation, but it is dispensable for other anti-HCMV antibodies that do not need this function to be transported on mature virions. HCMV-infected cells grown in presence of anti-HCMV monoclonal antibodies generate a viral progeny still infective and possible to be neutralized. This is the first example of a virus carrying neutralizing IgGs on its surface and their possible role is discussed.

Human Cytomegalovirus UL48 Deubiquitinase Primarily Targets Innermost Tegument Proteins pp150 and Itself To Regulate Their Stability and Protects Virions from Inclusion of Ubiquitin Conjugates

Viral deubiquitinases (DUBs) regulate cellular innate immunity to benefit viral replication. In human cytomegalovirus (HCMV), the UL48-encoded DUB regulates innate immune responses, including NF-κB signaling. Although UL48 DUB is known to regulate its stability via auto-deubiquitination, its impact on other viral proteins is not well understood. In this study, we investigated the role of UL48 DUB in regulating the ubiquitination of viral proteins by comparing the levels of ubiquitinated viral peptides in cells infected with wild-type virus and DUB active-site mutants using mass spectrometry. We found that ubiquitinated peptides were increased in DUB mutant virus infection for 90% of viral proteins, with the innermost tegument proteins pp150 (encoded by UL32) and pUL48 itself being most significantly affected. The highly deubiquitinated lysine residues of pUL48 were mapped within its N-terminal DUB domain and the nuclear localization signal. Among them, the arginine substitution of lysine 2 (K2R) increased pUL48 stability and enhanced viral growth at low multiplicity of infection, indicating that K2 auto-deubiquitination has a role in regulating pUL48 stability. pUL48 also interacted with pp150 and increased pp150 expression by downregulating its ubiquitination. Furthermore, we found that, unlike the wild-type virus, mutant viruses expressing the UL48 protein with the DUB domain deleted or DUB active site mutated contain higher levels of ubiquitin conjugates, including the ubiquitinated forms of pp150, in their virions. Collectively, our results demonstrate that UL48 DUB mainly acts on the innermost tegument proteins pp150 and pUL48 itself during HCMV infection and may play a role in protecting virions from the inclusion of ubiquitin conjugates. IMPORTANCE Herpesviruses encode highly conserved tegument proteins that contain deubiquitinase (DUB) activity. Although the role of viral DUBs in the regulation of host innate immune responses has been established, their roles in the stability and function of viral proteins are not well understood. In this study, we performed a comparative analysis of the levels of ubiquitinated viral peptides between wild-type and DUB-inactive HCMV infections and demonstrated that the innermost tegument proteins pp150 and pUL48 (DUB itself) are major targets of viral DUB. We also show that ubiquitinated viral proteins are effectively incorporated into the virions of DUB mutant viruses but not the wild-type virus. Our study demonstrates that viral DUBs may play important roles in promoting the stability of viral proteins and inhibiting the inclusion of ubiquitin conjugates into virions.

Insight for Immunotherapy of HCMV Infection

Human cytomegalovirus (HCMV), a ubiquitous in humans, has a high prevalence rate. Young people are susceptible to HCMV infection in developing countries, while older individuals are more susceptible in developed countries. Most patients have no obvious symptoms from the primary infection. Studies have indicated that the virus has gradually adapted to the host immune system. Therefore, the control of HCMV infection requires strong immune modulation. With the recent advances in immunotherapy, its application to HCMV infections is receiving increasing attention. Here, we discuss the immune response to HCMV infection, the immune escape mechanism, and the different roles that HCMV plays in various types of immunotherapy, including vaccines, adoptive cell therapy, checkpoint blockade therapy, and targeted antibodies.

The Zinc Finger Antiviral Protein ZAP Restricts Human Cytomegalovirus and Selectively Binds and Destabilizes Viral UL4/UL5 Transcripts

Interferon-stimulated gene products (ISGs) play a crucial role in early infection control. The ISG zinc finger CCCH-type antiviral protein 1 (ZAP/ZC3HAV1) antagonizes several RNA viruses by binding to CG-rich RNA sequences, whereas its effect on DNA viruses is less well understood. Here, we decipher the role of ZAP in the context of human cytomegalovirus (HCMV) infection, a β-herpesvirus that is associated with high morbidity in immunosuppressed individuals and newborns. We show that expression of the two major isoforms of ZAP, ZAP-S and ZAP-L, is induced during HCMV infection and that both negatively affect HCMV replication. Transcriptome and proteome analyses demonstrated that the expression of ZAP results in reduced viral mRNA and protein levels and decelerates the progression of HCMV infection. Metabolic RNA labeling combined with high-throughput sequencing (SLAM-seq) revealed that most of the gene expression changes late in infection result from the general attenuation of HCMV. Furthermore, at early stages of infection, ZAP restricts HCMV by destabilizing a distinct subset of viral mRNAs, particularly those from the previously uncharacterized UL4-UL6 HCMV gene locus. Through enhanced cross-linking immunoprecipitation and sequencing analysis (eCLIP-seq), we identified the transcripts expressed from this HCMV locus as the direct targets of ZAP. Moreover, our data show that ZAP preferentially recognizes not only CG, but also other cytosine-rich sequences, thereby expanding its target specificity. In summary, this report is the first to reveal direct targets of ZAP during HCMV infection, which strongly indicates that transcripts from the UL4-UL6 locus may play an important role for HCMV replication.IMPORTANCE Viral infections have a large impact on society, leading to major human and economic losses and even global instability. So far, many viral infections, including human cytomegalovirus (HCMV) infection, are treated with a small repertoire of drugs, often accompanied by the occurrence of resistant mutants. There is no licensed HCMV vaccine in sight to protect those most at risk, particularly immunocompromised individuals or pregnant women who might otherwise transmit the virus to the fetus. Thus, the identification of novel intervention strategies is urgently required. In this study, we show that ZAP decelerates the viral gene expression cascade, presumably by selectively handpicking a distinct set of viral transcripts for degradation. Our study illustrates the potent role of ZAP as an HCMV restriction factor and sheds light on a possible role for UL4 and/or UL5 early during infection, paving a new avenue for the exploration of potential targets for novel therapies.

The human cytomegalovirus protein UL147A downregulates the most prevalent MICA allele: MICA*008, to evade NK cell-mediated killing

Natural killer (NK) cells are innate immune lymphocytes capable of killing target cells without prior sensitization. One pivotal activating NK receptor is NKG2D, which binds a family of eight ligands, including the major histocompatibility complex (MHC) class I-related chain A (MICA). Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus causing morbidity and mortality in immunosuppressed patients and congenitally infected infants. HCMV encodes multiple antagonists of NK cell activation, including many mechanisms targeting MICA. However, only one of these mechanisms, the HCMV protein US9, counters the most prevalent MICA allele, MICA*008. Here, we discover that a hitherto uncharacterized HCMV protein, UL147A, specifically downregulates MICA*008. UL147A primarily induces MICA*008 maturation arrest, and additionally targets it to proteasomal degradation, acting additively with US9 during HCMV infection. Thus, UL147A hinders NKG2D-mediated elimination of HCMV-infected cells by NK cells. Mechanistic analyses disclose that the non-canonical GPI anchoring pathway of immature MICA*008 constitutes the determinant of UL147A specificity for this MICA allele. These findings advance our understanding of the complex and rapidly evolving HCMV immune evasion mechanisms, which may facilitate the development of antiviral drugs and vaccines.

Human cytomegalovirus (HCMV) is a common pathogen that usually causes asymptomatic infection in the immunocompetent population, but the immunosuppressed and fetuses infected in utero suffer mortality and disability due to HCMV disease. Current HCMV treatments are limited and no vaccine has been approved, despite significant efforts. HCMV encodes many genes of unknown function, and virus-host interactions are only partially understood. Here, we discovered that a hitherto uncharacterized HCMV protein, UL147A, downregulates the expression of an activating immune ligand allele named MICA*008, thus hindering the elimination of HCMV-infected cells. Elucidating HCMV immune evasion mechanisms could aid in the development of novel HCMV treatments and vaccines. Furthermore, MICA*008 is a highly prevalent allele implicated in cancer immune evasion, autoimmunity and graft rejection. In this work we have shown that UL147A interferes with MICA*008’s poorly understood, nonstandard maturation pathway, and acts additively with a functionally homologous HCMV protein, US9. Study of UL147A may enable manipulation of its expression as a therapeutic measure against HCMV.

Human cytomegalovirus antagonizes activation of Fcγ receptors by distinct and synergizing modes of IgG manipulation

Human cytomegalovirus (HCMV) is endowed with multiple highly sophisticated immune evasion strategies. This includes the evasion from antibody mediated immune control by counteracting host Fc-gamma receptor (FcγR) mediated immune control mechanisms such as antibody-dependent cellular cytotoxicity (ADCC). We have previously shown that HCMV avoids FcγR activation by concomitant expression of the viral Fc-gamma-binding glycoproteins (vFcγRs) gp34 and gp68. We now show that gp34 and gp68 bind IgG simultaneously at topologically different Fcγ sites and achieve efficient antagonization of host FcγR activation by distinct but synergizing mechanisms. While gp34 enhances immune complex internalization, gp68 acts as inhibitor of host FcγR binding to immune complexes. In doing so, gp68 induces Fcγ accessibility to gp34 and simultaneously limits host FcγR recognition. The synergy of gp34 and gp68 is compelled by the interfering influence of excessive non-immune IgG ligands and highlights conformational changes within the IgG globular chains critical for antibody effector function.

Human cytomegalovirus is a type of herpes virus that rarely causes symptoms in healthy people but can cause serious complications in unborn babies and in people with compromised immune systems, such as transplant recipients.

The virus has found ways to successfully evade the immune system, and once infected, the body retains the virus for life. It deploys an arsenal of proteins that bind to antibodies, specialized proteins the immune system uses to flag virus-infected cells for destruction. This prevents certain cells of the immune system, the natural killer cells, from recognizing and destroying virus-infected cells.

These immune-evading proteins are called viral Fc-gamma receptors, or vFcγRs. While it has been previously shown that these receptors are able to evade the immune system, it remained unknown how exactly they prevent natural killer cells from recognizing infected cells.

Now, Kolb et al. show that the cytomegalovirus deploys two vFcγRs called gp34 and gp68, which work together to block natural killer cells. The latter reduces the ability of natural killer cells to bind to antibodies on cytomegalovirus-infected cells. This paves the way for gp34 to pull virus proteins from the surface of the infected cell, making them inaccessible to the immune system. Neither protein fully protects virus-infected cells on its own, but together they are highly effective.

The experiments reveal further details about how cytomegalovirus uses two defense mechanisms simultaneously to outmaneuver the immune system. Understanding this two-part viral evasion system may help scientists to develop vaccines or new treatments that can protect vulnerable people from diseases caused by the cytomegalovirus.

The Human Cytomegalovirus pUL145 Isoforms Act as Viral DDB1-Cullin-Associated Factors to Instruct Host Protein Degradation to Impede Innate Immunity

Human cytomegalovirus (HCMV) causes diseases in individuals with immature or compromised immunity. To evade immune control, HCMV evolved numerous antagonists targeting the interferon system at multiple levels. By comparative analysis of naturally arising variants of the most widely studied HCMV strain, AD169, and a panel of targeted mutants, we uncover the UL145 gene as indispensable for STAT2 downregulation. Ribosome profiling confirms the translation of the canonical pUL145 protein (pUL145-Long) and newly identifies a shorter isoform (pUL145-Short). Both isoforms recruit DDB1-containing ubiquitin ligases to induce proteasomal degradation of STAT2. An alanine-scanning mutagenesis discloses the DDB1 interaction motif of pUL145 that resembles the DDB1-binding interface of cellular substrate receptors of DDB1-containing ubiquitin ligases. Thus, pUL145 constitutes a viral DDB1-cullin-associated factor (vDCAF), which mimics cellular DCAFs to exploit the ubiquitin-proteasome system to impede antiviral immunity. Notably, the viral exploitation of the cullins can be targeted to restore the efficacy of the host immune response.

Human Cytomegalovirus and Autoimmune Diseases: Where Are We?

Human cytomegalovirus (HCMV) is a ubiquitous double-stranded DNA virus belonging to the β-subgroup of the herpesvirus family. After the initial infection, the virus establishes latency in poorly differentiated myeloid precursors from where it can reactivate at later times to cause recurrences. In immunocompetent subjects, primary HCMV infection is usually asymptomatic, while in immunocompromised patients, HCMV infection can lead to severe, life-threatening diseases, whose clinical severity parallels the degree of immunosuppression. The existence of a strict interplay between HCMV and the immune system has led many to hypothesize that HCMV could also be involved in autoimmune diseases (ADs). Indeed, signs of active viral infection were later found in a variety of different ADs, such as rheumatological, neurological, enteric disorders, and metabolic diseases. In addition, HCMV infection has been frequently linked to increased production of autoantibodies, which play a driving role in AD progression, as observed in systemic lupus erythematosus (SLE) patients. Documented mechanisms of HCMV-associated autoimmunity include molecular mimicry, inflammation, and nonspecific B-cell activation. In this review, we summarize the available literature on the various ADs arising from or exacerbating upon HCMV infection, focusing on the potential role of HCMV-mediated immune activation at disease onset.

The Guinea pig cytomegalovirus GP119.1 gene encodes an IgG-binding glycoprotein that is incorporated into the virion

Cytomegaloviruses (CMVs) encode various immunoevasins, including viral receptors for the Fc domain of host IgG (vFcγR), to evade host immune responses. Although guinea pig CMV (GPCMV) provides a useful animal model for congenital CMV infection, the GPCMV genes encoding such receptors have not yet been characterized. In this study, we analyzed a locus that may encode gene products for the GPCMV immune evasion mechanisms and identified the following. (a) RACE analyses identified four transcripts in the GP117 to GP122 locus. One of the transcripts contained the GP119.1 ORF, which has weak homologies with human CMV UL119/UL118 encoding a viral FcγR and with guinea pig FcγR. (b) A transient transfection assay with plasmids expressing EGFP-tagged GP119.1 or its mutated forms identified its true translational initiation site, localization mainly in the endoplasmic reticulum, and N-glycosylation. (c) Importantly, GP119.1 bound to guinea pig IgG or the IgG-Fc fragment. (d) GP119.1 is present in the virion with a molecular mass of 15 and 23~30 kDa, and a portion of the GP119.1 products are N-glycosylated. (e) GP119.1 was dispensable for viral growth on guinea pig fibroblasts and epithelial cells in vitro. Taken together, our findings indicate that GP119.1 is an IgG-Fc binding glycoprotein incorporated into the virion, and this finding warrants further studies on the functions of GP119.1 in animal models.

Pathogenesis of human cytomegalovirus in the immunocompromised host

Human cytomegalovirus (HCMV) is a herpesvirus that infects ~60% of adults in developed countries and more than 90% in developing countries. Usually, it is controlled by a vigorous immune response so that infections are asymptomatic or symptoms are mild. However, if the immune system is compromised, HCMV can replicate to high levels and cause serious end organ disease. Substantial progress is being made in understanding the natural history and pathogenesis of HCMV infection and disease in the immunocompromised host. Serial measures of viral load defined the dynamics of HCMV replication and are now used routinely to allow intervention with antiviral drugs in individual patients. They are also used as pharmacodynamic read-outs to evaluate prototype vaccines that may protect against HCMV replication and to define immune correlates of this protection. This novel information is informing the design of randomized controlled trials of new antiviral drugs and vaccines currently under evaluation. In this Review, we discuss immune responses to HCMV and countermeasures deployed by the virus, the establishment of latency and reactivation from it, exogenous reinfection with additional strains, pathogenesis, development of end organ disease, indirect effects of infection, immune correlates of control of replication, current treatment strategies and the evaluation of novel vaccine candidates.

In vitro and in vivo characterization of a recombinant rhesus cytomegalovirus containing a complete genome

Cytomegaloviruses (CMVs) are highly adapted to their host species resulting in strict species specificity. Hence, in vivo examination of all aspects of CMV biology employs animal models using host-specific CMVs. Infection of rhesus macaques (RM) with rhesus CMV (RhCMV) has been established as a representative model for infection of humans with HCMV due to the close evolutionary relationships of both host and virus. However, the only available RhCMV clone that permits genetic modifications is based on the 68-1 strain which has been passaged in fibroblasts for decades resulting in multiple genomic changes due to tissue culture adaptations. As a result, 68-1 displays reduced viremia in RhCMV-naïve animals and limited shedding compared to non-clonal, low passage isolates. To overcome this limitation, we used sequence information from primary RhCMV isolates to construct a full-length (FL) RhCMV by repairing all mutations affecting open reading frames (ORFs) in the 68-1 bacterial artificial chromosome (BAC). Inoculation of adult, immunocompetent, RhCMV-naïve RM with the reconstituted virus resulted in significant viremia in the blood similar to primary isolates of RhCMV and furthermore led to high viral genome copy numbers in many tissues at day 14 post infection. In contrast, viral dissemination was greatly reduced upon deletion of genes also lacking in 68-1. Transcriptome analysis of infected tissues further revealed that chemokine-like genes deleted in 68-1 are among the most highly expressed viral transcripts both in vitro and in vivo consistent with an important immunomodulatory function of the respective proteins. We conclude that FL-RhCMV displays in vitro and in vivo characteristics of a wildtype virus while being amenable to genetic modifications through BAC recombineering techniques.

Mechanisms of Natural Killer Cell Evasion Through Viral Adaptation

The continuous interactions between host and pathogens during their coevolution have shaped both the immune system and the countermeasures used by pathogens. Natural killer (NK) cells are innate lymphocytes that are considered central players in the antiviral response. Not only do they express a variety of inhibitory and activating receptors to discriminate and eliminate target cells but they can also produce immunoregulatory cytokines to alert the immune system. Reciprocally, several unrelated viruses including cytomegalovirus, human immunodeficiency virus, influenza virus, and dengue virus have evolved a multitude of mechanisms to evade NK cell function, such as the targeting of pathways for NK cell receptors and their ligands, apoptosis, and cytokine-mediated signaling. The studies discussed in this article provide further insights into the antiviral function of NK cells and the pathways involved, their constituent proteins, and ways in which they could be manipulated for host benefit.

Animal Models of Congenital Cytomegalovirus Transmission: Implications for Vaccine Development

Although cytomegaloviruses (CMVs) are species-specific, the study of nonhuman CMVs in animal models can help to inform and direct research aimed at developing a human CMV (HCMV) vaccine. Because the driving force behind the development of HCMV vaccines is to prevent congenital infection, the animal model in question must be one in which vertical transmission of virus occurs to the fetus. Fortunately, two such animal models-the rhesus macaque CMV and guinea pig CMV-are characterized by congenital infection. Hence, each model can be evaluated in "proof-of-concept" studies of preconception vaccination aimed at blocking transplacental transmission. This review focuses on similarities and differences in the respective model systems, and it discusses key insights from each model germane to the study of HCMV vaccines.

Identification of HCMV-derived T cell epitopes in seropositive individuals through viral deletion models

In healthy individuals, immune control of persistent human cytomegalovirus (HCMV) infection is effectively mediated by virus-specific CD4+ and CD8+ T cells. However, identifying the repertoire of T cell specificities for HCMV is hampered by the immense protein coding capacity of this betaherpesvirus. Here, we present a novel approach that employs HCMV deletion mutant viruses lacking HLA class I immunoevasins and allows direct identification of naturally presented HCMV-derived HLA ligands by mass spectrometry. We identified 368 unique HCMV-derived HLA class I ligands representing an unexpectedly broad panel of 123 HCMV antigens. Functional characterization revealed memory T cell responses in seropositive individuals for a substantial proportion (28%) of these novel peptides. Multiple HCMV-directed specificities in the memory T cell pool of single individuals indicate that physiologic anti-HCMV T cell responses are directed against a broad range of antigens. Thus, the unbiased identification of naturally presented viral epitopes enabled a comprehensive and systematic assessment of the physiological repertoire of anti-HCMV T cell specificities in seropositive individuals.

Cytomegalovirus-vectored vaccines for HIV and other pathogens

: The use of cytomegalovirus (CMV) as a vaccine vector to express antigens against multiple infectious diseases, including simian immunodeficiency virus, Ebola virus, plasmodium, and mycobacterium tuberculosis, in rhesus macaques has generated extraordinary levels of protective immunity against subsequent pathogenic challenge. Moreover, the mechanisms of immune protection have altered paradigms about viral vector-mediated immunity against ectopically expressed vaccine antigens. Further optimization of CMV-vectored vaccines, particularly as this approach moves to human clinical trials will be augmented by a more complete understanding of how CMV engenders mechanisms of immune protection. This review summarizes the particulars of the specific CMV vaccine vector that has been used to date (rhesus CMV strain 68-1) in relation to CMV natural history.

Characterization of pUL5, an HCMV protein interacting with the cellular protein IQGAP1

Among the Herpesviridae, human cytomegalovirus (HCMV) owns the largest genome and displays a huge coding potential. Here, we characterized the UL5 gene product (pUL5) of the clinical isolate TR strain. The protein was predicted as a 166-amino-acid membrane protein with a theoretical mass of 19 kDa. Recombinant virus expressing pUL5 with a tag allowed the identification of two pUL5 non-glycosylated species of approximately 19 and 9 kDa, expressed with early and late kinetic respectively. Experiments in infection confirmed that the lower molecular weight species was translated from an internal ATG in the UL5 open reading frame. Confocal microscopy analysis showed that pUL5 localized within the assembly compartment, but is not incorporated in the virion, as shown by Western blot on purified viral particles. Finally, pull-down experiments coupled with mass spectrometry analysis identified IQGAP1 as a pUL5 interactor, giving new hints on possible roles of pUL5 during HCMV infection.

Modulation of innate and adaptive immunity by cytomegaloviruses

The coordinated activities of innate and adaptive immunity are critical for effective protection against viruses. To counter this, some viruses have evolved sophisticated strategies to circumvent immune cell recognition. In particular, cytomegaloviruses encode large arsenals of molecules that seek to subvert T cell and natural killer cell function via a remarkable array of mechanisms. Consequently, these 'immunoevasins' play a fundamental role in shaping the nature of the immune system by driving the evolution of new immune receptors and recognition mechanisms. Here, we review the diverse strategies adopted by cytomegaloviruses to target immune pathways and outline the host's response.

Human NK Cells and Herpesviruses: Mechanisms of Recognition, Response and Adaptation

NK cells contribute to early defenses against viruses through their inborn abilities that include sensing of PAMPs and inflammatory signals such as cytokines or chemokines, recognition, and killing of infected cells through activating surface receptors engagement. Moreover, they support adaptive responses via Ab-dependent mechanisms, triggered by CD16, and DC editing. Their fundamental role in anti-viral responses has been unveiled in patients with NK cell deficiencies suffering from severe Herpesvirus infections. Notably, these infections, often occurring as primary infections early in life, can be efficiently cleared by NK, T, and B cells in healthy hosts. Herpesviruses however, generate a complicated balance with the host immune system through their latency cycle moving between immune control and viral reactivation. This lifelong challenge has contributed to the development of numerous evasion mechanisms by Herpesviruses, many of which devoted to elude NK cell surveillance from viral reactivations rather than primary infections. This delicate equilibrium can be altered in proportions of healthy individuals promoting virus reactivation and, more often, in immunocompromised subjects. However, the constant stimulus provided by virus-host interplay has also favored NK-cell adaptation to Herpesviruses. During anti-HCMV responses, NK cells can reshape their receptor repertoire and function, through epigenetic remodeling, and acquire adaptive traits such as longevity and clonal expansion abilities. The major mechanisms of recognition and effector responses employed by NK cells against Herpesviruses, related to their genomic organization will be addressed, including those allowing NK cells to generate memory-like responses. In addition, the mechanisms underlying virus reactivation or control will be discussed.

HLA-B locus products resist degradation by the human cytomegalovirus immunoevasin US11

To escape CD8+ T-cell immunity, human cytomegalovirus (HCMV) US11 redirects MHC-I for rapid ER-associated proteolytic degradation (ERAD). In humans, classical MHC-I molecules are encoded by the highly polymorphic HLA-A, -B and -C gene loci. While HLA-C resists US11 degradation, the specificity for HLA-A and HLA-B products has not been systematically studied. In this study we analyzed the MHC-I peptide ligands in HCMV-infected cells. A US11-dependent loss of HLA-A ligands was observed, but not of HLA-B. We revealed a general ability of HLA-B to assemble with β₂m and exit from the ER in the presence of US11. Surprisingly, a low-complexity region between the signal peptide sequence and the Ig-like domain of US11, was necessary to form a stable interaction with assembled MHC-I and, moreover, this region was also responsible for changing the pool of HLA-B ligands. Our data suggest a two-pronged strategy by US11 to escape CD8+ T-cell immunity, firstly, by degrading HLA-A molecules, and secondly, by manipulating the HLA-B ligandome.

The human immune system can cover the presentation of a wide array of pathogen derived antigens owing to the three extraordinary polymorphic MHC class I (MHC-I) gene loci, called HLA-A, -B and -C in humans. Studying the HLA peptide ligands of human cytomegalovirus (HCMV) infected cells, we realized that the HCMV encoded glycoprotein US11 targeted different HLA gene products in distinct manners. More than 20 years ago the first HCMV encoded MHC-I inhibitors were identified, including US11, targeting MHC-I for proteasomal degradation. Here, we describe that the prime target for US11-mediated degradation is HLA-A, whereas HLA-B can resist degradation. Our further mechanistic analysis revealed that US11 uses various domains for distinct functions. Remarkably, the ability of US11 to interact with assembled MHC-I and modify peptide loading of degradation-resistant HLA-B was dependent on a low-complexity region (LCR) located between the signal peptide and the immunoglobulin-like domain of US11. To redirect MHC-I for proteasomal degradation the LCR was dispensable. These findings now raise the intriguing question why US11 has evolved to target HLA-A and -B differentially. Possibly, HLA-B molecules are spared in order to dampen NK cell attack against infected cells.

The Roles of Host and Viral Antibody Fc Receptors in Herpes Simplex Virus (HSV) and Human Cytomegalovirus (HCMV) Infections and Immunity

Herpesvirus infections are a leading cause of neurodevelopmental delay in newborns and end-organ disease in immunocompromised patients. One leading strategy to reduce the disease burden of herpesvirus infections such as herpes simplex virus (HSV) and human cytomegalovirus (HCMV) is to prevent primary acquisition by vaccination, yet vaccine development remains hampered by limited understanding of immune correlates of protection against infection. Traditionally, vaccine development has aimed to increase antibody titers with neutralizing function, which involves the direct binding of antibodies to viral particles. However, recent research has explored the numerous other responses that can be mediated by engagement of the antibody constant region (Fc) with Fc receptors (FcR) present on immune cells or with complement molecules. These functions include antiviral responses such as antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). Uniquely, herpesviruses encode FcR that can act as distractor receptors for host antiviral IgG, thus enabling viral evasion of host defenses. This review focuses on the relative roles of neutralizing and non-neutralizing functions antibodies that target herpesvirus antigens for HSV and HCMV, as well as the roles of Fc-FcR interactions for both host defenses and viral escape.

Host Genetics of Cytomegalovirus Pathogenesis

Human cytomegalovirus (HCMV) is a ubiquitous herpes virus (human herpes virus 5) with the highest morbidity and mortality rates compared to other herpes viruses. Risk groups include very young, elderly, transplant recipient, and immunocompromised individuals. HCMV may cause retinitis, encephalitis, hepatitis, esophagitis, colitis, pneumonia, neonatal infection sequelae, inflammatory, and age-related diseases. With an arsenal of genes in its large genome dedicated to host immune evasion, HCMV can block intrinsic cellular defenses and interfere with cellular immune responses. HCMV also encodes chemokines, chemokine receptors, and cytokines. Therefore, genes involved in human viral defense mechanisms and those encoding proteins targeted by the CMV proteins are candidates for host control of CMV infection and reactivation. Although still few in number, host genetic studies are producing valuable insights into biological processes involved in HCMV pathogenesis and HCMV-related diseases. For example, genetic variants in the immunoglobulin GM light chain can influence the antibody responsiveness to CMV glycoprotein B and modify risk of HCMV-related diseases. Moreover, CMV infection following organ transplantation has been associated with variants in genes encoding toll-like receptors (TLRs), programmed death-1 (PD-1), and interleukin-12p40 (IL-12B). A KIR haplotype (2DS4+) is proposed to be protective for CMV activation among hematopoietic stem cell transplant patients. Polymorphisms in the interferon lambda 3/4 (IFNL3/4) region are shown to influence susceptibility to CMV replication among solid organ transplant patients. Interestingly, the IFNL3/4 region is also associated with AIDS-related CMV retinitis susceptibility in HIV-infected patients. Likewise, interleukin-10 receptor 1 (IL-10R1) variants are shown to influence CMV retinitis development in patients with AIDS. Results from genome-wide association studies suggest a possible role for microtubule network and retinol metabolism in anti-CMV antibody response. Nevertheless, further genetic epidemiological studies with large cohorts, functional studies on the numerous HCMV genes, and immune response to chronic and latent states of infection that contribute to HCMV persistence are clearly necessary to elucidate the genetic mechanisms of CMV infection, reactivation, and pathogenesis.

Human cytomegalovirus evades antibody-mediated immunity through endoplasmic reticulum-associated degradation of the FcRn receptor

Human cytomegalovirus (HCMV) can persistently infect humans, but how HCMV avoids humoral immunity is not clear. The neonatal Fc receptor (FcRn) controls IgG transport from the mother to the fetus and prolongs IgG half-life. Here we show that US11 inhibits the assembly of FcRn with β₂m and retains FcRn in the endoplasmic reticulum (ER), consequently blocking FcRn trafficking to the endosome. Furthermore, US11 recruits the ubiquitin enzymes Derlin-1, TMEM129 and UbE2J2 to engage FcRn, consequently initiating the dislocation of FcRn from the ER to the cytosol and facilitating its degradation. Importantly, US11 inhibits IgG-FcRn binding, resulting in a reduction of IgG transcytosis across intestinal or placental epithelial cells and IgG degradation in endothelial cells. Hence, these results identify the mechanism by which HCMV infection exploits an ER-associated degradation pathway through US11 to disable FcRn functions. These results have implications for vaccine development and immune surveillance.

Human cytomegalovirus (HCMV) can persist for the life of a host in the face of robust immune responses owing to a wide range of immune evasion strategies. Here Liu and colleagues show that HCMV evades the IgG-mediated response by the endoplasmic reticulum-associated degradation of the neonatal Fc receptor for IgG.

Identification and Functional Characterization of a Novel Fc Gamma-Binding Glycoprotein in Rhesus Cytomegalovirus

Receptors recognizing the Fc part of immunoglobulin G (FcγRs) are key determinants in antibody-mediated immune responses. Members of the Herpesviridae interfere with this immune regulatory network by expressing viral FcγRs (vFcγRs). Human cytomegalovirus (HCMV) encodes four distinct vFcγRs that differ with respect to their IgG subtype specificity and their impact on antibody-mediated immune function in vitro The impact of vFcγRs on HCMV pathogenesis and immunomodulation in vivo is not known. The closest evolutionary animal model of HCMV is rhesus CMV (RhCMV) infection of rhesus macaques. To enable the characterization of vFcγR function in this model, we studied IgG binding by RhCMV. We show that lysates of RhCMV-infected cells contain an IgG-binding protein of 30 kDa encoded by the gene Rh05 that is a predicted type I glycoprotein belonging to the RL11 gene family. Upon deletion of Rh05, IgG-Fc binding by RhCMV strain 68-1 is lost, whereas ectopic expression of Rh05 results in IgG binding to transfected cells consistent with Rh05 being a vFcγR. Using a set of reporter cell lines stably expressing human and rhesus FcγRs, we further demonstrate that Rh05 antagonizes host FcγR activation. Compared to Rh05-intact RhCMV, RhCMVΔRh05 showed an increased activation of host FcγR upon exposure of infected cells to IgG from RhCMV-seropositive animals, suggesting that Rh05 protects infected cells from opsonization and IgG-dependent activation of host FcγRs. However, antagonizing host FcγR activation by Rh05 was not required for the establishment and maintenance of infection of RhCMV, even in a seropositive host, as shown by the induction of T cell responses to heterologous antigens expressed by RhCMV lacking the gene region encoding Rh05. In contrast to viral evasion of natural killer cells or T cell recognition, the evasion of antibody-mediated effects does not seem to be absolutely required for infection or reinfection. The identification of the first vFcγR that efficiently antagonizes host FcγR activation in the RhCMV genome will thus permit more detailed studies of this immunomodulatory mechanism in promoting viral dissemination in the presence of natural or vaccine-induced humoral immunity.IMPORTANCE Rhesus cytomegalovirus (RhCMV) offers a unique model for studying human cytomegalovirus (HCMV) pathogenesis and vaccine development. RhCMV infection of nonhuman primates greatly broadened the understanding of mechanisms by which CMVs evade or reprogram T cell and natural killer cell responses in vivo However, the role of humoral immunity and viral modulation of anti-CMV antibodies has not been studied in this model. There is evidence from in vitro studies that HCMVs can evade humoral immunity. By gene mapping and with the help of a novel cell-based reporter assay system we characterized the first RhCMV encoded IgG-Fcγ binding glycoprotein as a potent antagonist of rhesus FcγR activation. We further demonstrate that, unlike evasion of T cell immunity, this viral Fcγ receptor is not required to overcome anti-CMV immunity to establish secondary infections. These findings enable more detailed studies of the in vivo consequences of CMV evasion from IgG responses in nonhuman primate models.

The Human Cytomegalovirus Protein UL148A Downregulates the NK Cell-Activating Ligand MICA To Avoid NK Cell Attack

Natural killer (NK) cells are lymphocytes of the innate immune system capable of killing hazardous cells, including virally infected cells. NK cell-mediated killing is triggered by activating receptors. Prominent among these is the activating receptor NKG2D, which binds several stress-induced ligands, among them major histocompatibility complex (MHC) class I-related chain A (MICA). Most of the human population is persistently infected with human cytomegalovirus (HCMV), a virus which employs multiple immune evasion mechanisms, many of which target NK cell responses. HCMV infection is mostly asymptomatic, but in congenitally infected neonates and in immunosuppressed patients it can lead to serious complications and mortality. Here we discovered that an HCMV protein named UL148A whose role was hitherto unknown is required for evasion of NK cells. We demonstrate that UL148A-deficient HCMV strains are impaired in their ability to downregulate MICA expression. We further show that when expressed by itself, UL148A is not sufficient for MICA targeting, but rather acts in concert with an unknown viral factor. Using inhibitors of different cellular degradation pathways, we show that UL148A targets MICA for lysosomal degradation. Finally, we show that UL148A-mediated MICA downregulation hampers NK cell-mediated killing of HCMV-infected cells. Discovering the full repertoire of HCMV immune evasion mechanisms will lead to a better understanding of the ability of HCMV to persist in the host and may also promote the development of new vaccines and drugs against HCMV.IMPORTANCE Human cytomegalovirus (HCMV) is a ubiquitous pathogen which is usually asymptomatic but that can cause serious complications and mortality in congenital infections and in immunosuppressed patients. One of the difficulties in developing novel vaccines and treatments for HCMV is its remarkable ability to evade our immune system. In particular, HCMV directs significant efforts to thwarting cells of the innate immune system known as natural killer (NK) cells. These cells are crucial for successful control of HCMV infection, and yet our understanding of the mechanisms which HCMV utilizes to elude NK cells is partial at best. In the present study, we discovered that a protein encoded by HCMV which had no known function is important for preventing NK cells from killing HCMV-infected cells. This knowledge can be used in the future for designing more-efficient HCMV vaccines and for formulating novel therapies targeting this virus.

Detection of antibody-secreting cells specific for the cytomegalovirus and herpes simplex virus surface antigens

Infections with the herpes simplex virus (HSV) and the human cytomegalovirus (HCMV) can lead to life-threatening diseases, particularly in immunosuppressed patients. Furthermore, HSV infections at birth (herpes neonatorum) can result in a disseminated disease associated with a fatal multiorgan failure. Congenital HCMV infections can result in miscarriage, serious birth defects or developmental disabilities. Antibody-based interventions with hyperimmunoglobulins showed encouraging results in clinical studies, but clearly need to be improved. The isolation of highly neutralizing monoclonal antibodies is a promising strategy to establish potent therapy options against HSV and HCMV infections.

Monoclonal antibodies are commonly isolated from hybridomas or EBV-immortalized B-cell clones. The screening procedure to identify virus-specific cells from a cell mixture is a challenging step, since most of the highly neutralizing antibodies target complex conformational epitopes on the virus surface. Conventional assays such as ELISA are based on purified viral proteins and inappropriate to display complex epitopes.

To overcome this obstacle, we have established two full-virus based methods that allow screening for cells and antibodies targeting complex conformational epitopes on viral surface antigens. The methods are suitable to detect surface antigen-specific cells from a cell mixture and may facilitate the isolation of highly neutralizing antibodies against HSV and HCMV.

STAT2-Dependent Immune Responses Ensure Host Survival despite the Presence of a Potent Viral Antagonist

A pathogen encounter induces interferons, which signal via Janus kinases and STAT transcription factors to establish an antiviral state. However, the host and pathogens are situated in a continuous arms race which shapes host evolution toward optimized immune responses and the pathogens toward enhanced immune-evasive properties. Mouse cytomegalovirus (MCMV) counteracts interferon responses by pM27-mediated degradation of STAT2, which directly affects the signaling of type I as well as type III interferons. Using MCMV mutants lacking M27 and mice lacking STAT2, we studied the opposing relationship between antiviral activities and viral antagonism in a natural host-pathogen pair in vitro and in vivo In contrast to wild-type (wt) MCMV, ΔM27 mutant MCMV was efficiently cleared from all organs within a few days in BALB/c, C57BL/6, and 129 mice, highlighting the general importance of STAT2 antagonism for MCMV replication. Despite this effective and relevant STAT2 antagonism, wt and STAT2-deficient mice exhibited fundamentally different susceptibilities to MCMV infections. MCMV replication was increased in all assessed organs (e.g., liver, spleen, lungs, and salivary glands) of STAT2-deficient mice, resulting in mortality during the first week after infection. Taken together, the results of our study reveal the importance of cytomegaloviral interferon antagonism for viral replication as well as a pivotal role of the remaining STAT2 activity for host survival. This mutual influence establishes a stable evolutionary standoff situation with fatal consequences when the equilibrium is disturbed.IMPORTANCE The host limits viral replication by the use of interferons (IFNs), which signal via STAT proteins. Several viruses evolved antagonists targeting STATs to antagonize IFNs (e.g., cytomegaloviruses, Zika virus, dengue virus, and several paramyxoviruses). We analyzed infections caused by MCMV expressing or lacking the STAT2 antagonist pM27 in STAT2-deficient and control mice to evaluate its importance for the host and the virus in vitro and in vivo The inability to counteract STAT2 directly translates into exaggerated IFN susceptibility in vitro and pronounced attenuation in vivo Thus, the antiviral activity mediated by IFNs via STAT2-dependent signaling drove the development of a potent MCMV-encoded STAT2 antagonist which became indispensable for efficient virus replication and spread to organs required for dissemination. Despite this clear impact of viral STAT2 antagonism, the host critically required the remaining STAT2 activity to prevent overt disease and mortality upon MCMV infection. Our findings highlight a remarkably delicate balance between host and virus.

Herpesvirus Evasion of Natural Killer Cells

Natural killer (NK) cells play an important role in the host response against viral infections and cancer development. They are able to kill virus-infected and tumor cells, and they produce different important cytokines that stimulate the antiviral and antitumor adaptive immune response, particularly interferon gamma. NK cells are of particular importance in herpesvirus infections, which is illustrated by systemic and life-threatening herpesvirus disease symptoms in patients with deficiencies in NK cell activity and by the myriad of reports describing herpesvirus NK cell evasion strategies. The latter is particularly obvious for cytomegaloviruses, but increasing evidence indicates that most, if not all, members of the herpesvirus family suppress NK cell activity to some extent. This review discusses the different NK cell evasion strategies described for herpesviruses and how this knowledge may translate to clinical applications.

A Prominent Role of the Human Cytomegalovirus UL8 Glycoprotein in Restraining Proinflammatory Cytokine Production by Myeloid Cells at Late Times during Infection

Human cytomegalovirus (HCMV) persistence in infected individuals relies on a plethora of mechanisms to efficiently reduce host immune responses. To that end, HCMV uses a variety of gene products, some of which have not been identified yet. Here we characterized the UL8 gene, which consists of two exons, sharing the first with the HCMV RL11 family member UL7 UL8 is a transmembrane protein with an N-terminal immunoglobulin (Ig)-like domain in common with UL7 but with an extended stalk and a distinctive cytoplasmic tail. The UL8 open reading frame gives rise to a heavily glycosylated protein predominantly expressed on the cell surface, from where it can be partially endocytosed and subsequently degraded. Infections with UL8-tagged viruses indicated that UL8 was synthesized with late-phase kinetics. By virtue of its highly conserved Ig-like domain, this viral protein interacted with a surface molecule present on activated neutrophils. Notably, when ectopically expressed in THP-1 myeloid cells, UL8 was able to significantly reduce the production of a variety of proinflammatory cytokines. Mutations in UL8 indicated that this functional effect was mediated by the cell surface expression of its Ig-like domain. To investigate the impact of the viral protein in the infection context, we engineered HCMVs lacking the UL8 gene and demonstrated that UL8 decreases the release of a large number of proinflammatory factors at late times after infection of THP-1 cells. Our data indicate that UL8 may exert an immunosuppressive role key for HCMV survival in the host.IMPORTANCE HCMV is a major pathogen that causes life-threatening diseases and disabilities in infected newborns and immunocompromised individuals. Containing one of the largest genomes among all reported human viruses, HCMV encodes an impressive repertoire of gene products. However, the functions of a large proportion of them still remain unknown, a fact that complicates the design of new therapeutic approaches to prevent or treat HCMV-associated diseases. In this report, we have conducted an extensive study of UL8, one of the previously uncharacterized HCMV open reading frames. We found that the UL8 protein is expressed at late times postinfection and utilized by HCMV to reduce the production of proinflammatory factors by infected myeloid cells. Thus, the work presented here points to a key role of UL8 as a novel HCMV immune modulator capable of restraining host antiviral defenses.

Origins of natural killer cell memory: special creation or adaptive evolution

The few initial formative studies describing non-specific and apparently spontaneous activity of natural killer (NK) cells have since multiplied into thousands of scientific reports defining their unique capacities and means of regulation. Characterization of the array of receptors that govern NK cell education and activation revealed an unexpected relationship with the major histocompatibility molecules that NK cells originally became well known for ignoring. Proceeding true to form, NK cells continue to up-end archetypal understanding of their ever-expanding capabilities. Discovery that the NK cell repertoire is extremely diverse and can be reshaped by particular viruses into unique subsets of adaptive NK cells challenges, or at least broadens, the definition of immunological memory. This review provides an overview of studies identifying adaptive NK cells, addressing the origins of NK cell memory and introducing the heretical concept of NK cells with extensive antigenic specificity. Whether these newly apparent properties reflect adaptive utilization of known NK cell attributes and receptors or a specially creative allocation from an undefined receptor array remains to be fully determined.

Association between γ marker, human leucocyte antigens and killer immunoglobulin-like receptors and the natural course of human cytomegalovirus infection: a pilot study performed in a Sicilian population

Natural killer (NK) cells provide a major defence against human cytomegalovirus (HCMV) infection through the interaction of their surface receptors, including the activating and inhibitory killer immunoglobulin-like receptors (KIRs), and human leucocyte antigen (HLA) class I molecules. Also γ marker (GM) allotypes, able to influence the NK antibody-dependent cell-mediated cytotoxicity, appear to be involved in the immunological control of virus infections, including HCMV. In some cases, their contribution requires epistatic interaction with other genes of the immune system, such as HLA. In the present report, with the aim of gaining insight into the immune mechanisms controlling HCMV, we have studied the possible associations among humoral and NK responses, and HCMV infections. In a previous study we assessed whether the KIR and HLA repertoire might influence the risk of developing symptomatic (n = 60) or asymptomatic (n = 60) disease after primary HCMV infection in the immunocompetent host. In the present study, the immunocompetent patients with primary symptomatic HCMV infection were genotyped for GM3/17 and GM23 allotypes, along with the 60 participants with a previous asymptomatic infection as controls. Notwithstanding the presence of missing data record, advanced missing data recovery techniques were able to show that individuals carrying the GM23 allotypes, both homozygous and heterozygous, GM17/17, HLA-C2 and Bw4^T KIR-ligand groups are associated with the risk of developing symptomatic infection. Our findings on the role of both cellular and humoral immunity in the control of HCMV infection should be of value in guiding efforts to reduce HCMV-associated health complications in the elderly, including immunosenescence, and in transplantation.

Elusive Role of the CD94/NKG2C NK Cell Receptor in the Response to Cytomegalovirus: Novel Experimental Observations in a Reporter Cell System

Human cytomegalovirus (HCMV) infection promotes the differentiation and persistent expansion of a mature NK cell subset, which displays high surface levels of the activating CD94/NKG2C NK cell receptor, together with additional distinctive phenotypic and functional features. The mechanisms underlying the development of adaptive NK cells remain uncertain but some observations support the involvement of a cognate interaction of CD94/NKG2C with ligand(s) displayed by HCMV-infected cells. To approach this issue, the heterodimer and its adaptor (DAP12) were expressed in the human Jurkat leukemia T cell line; signaling was detected by transfection of a reporter plasmid encoding for Luciferase (Luc) under NFAT/AP1-dependent control. Engagement of the receptor by solid-phase bound CD94- or NKG2C-specific monoclonal antibodies (mAbs) triggered Luc expression. Moreover, reporter activation was detectable upon interaction with HLA-E+ 721.221 (.221-AEH) cells, as well as with 721.221 cells incubated with synthetic peptides, which stabilized surface expression of endogenous HLA-E; the response was specifically antagonized by soluble NKG2C- and HLA-E-specific mAbs. By contrast, activation of Jurkat-NKG2C+ was undetectable upon interaction with Human Fetal Foreskin Fibroblasts (HFFF) infected with HCMV laboratory strains (i.e., AD169, Towne), regardless of their differential ability to preserve surface HLA-E expression. On the other hand, infection with two clinical isolates or with the endotheliotropic TB40/E strain triggered Jurkat-NKG2C+ activation; yet, this response was not inhibited by blocking mAbs and was independent of CD94/NKG2C expression. The results are discussed in the framework of previous observations supporting the hypothetical existence of specific ligand(s) for CD94/NKG2C in HCMV-infected cells.

Immunoglobulin superfamily members encoded by viruses and their multiple roles in immune evasion

Pathogens have developed a plethora of strategies to undermine host immune defenses in order to guarantee their survival. For large DNA viruses, these immune evasion mechanisms frequently rely on the expression of genes acquired from host genomes. Horizontally transferred genes include members of the immunoglobulin superfamily, whose products constitute the most diverse group of proteins of vertebrate genomes. Their promiscuous immunoglobulin domains, which comprise the building blocks of these molecules, are involved in a large variety of functions mediated by ligand-binding interactions. The flexible structural nature of the immunoglobulin domains makes them appealing targets for viral capture due to their capacity to generate high functional diversity. Here, we present an up-to-date review of immunoglobulin superfamily gene homologs encoded by herpesviruses, poxviruses, and adenoviruses, that include CD200, CD47, Fc receptors, interleukin-1 receptor 2, interleukin-18 binding protein, CD80, carcinoembryonic antigen-related cell adhesion molecules, and signaling lymphocyte activation molecules. We discuss their distinct structural attributes, binding properties, and functions, shaped by evolutionary pressures to disarm specific immune pathways. We include several novel genes identified from extensive genome database surveys. An understanding of the properties and modes of action of these viral proteins may guide the development of novel immune-modulatory therapeutic tools.

Interaction of Immunoglobulin with Cytomegalovirus-Infected Cells

Intravenous immunoglobulin (IVIG) is used to treat or prevent severe viral infection, especially cytomegalovirus (CMV) infections. IVIG was characterized to understand its interaction with CMV-infected cells. IVIG retarded CMV spread and reduced virus yields depending on the neutralizing (NT) antibody titer. Immediate early protein synthesis was reduced by IVIG in 3 to 15 h, and IVIG specifically reduced the ratio of 66/68k protein synthesis among immediate early proteins in an NT antibody-dependent manner between 4 and 8 h after infection, indicating that antigenic modulation of CMV-infected cells by IVIG reduced viral protein synthesis and virus production. The half-life of antibody bound to CMV-infected cells was 3.8 h. NT antibody titers to varicella-zoster virus (VZV) and CMV in IVIG were dose dependently absorbed by cells infected with VZV and CMV, respectively, but the antibody titers to CMV and VZV, respectively, were not affected. NT antibody in 0.3 mL of IVIG (15 mg) was specifically absorbed by 10⁸ CMV-infected cells and 10⁷ VZV-infected cells, suggesting that the NT antibody in IVIG might be inactivated by one-tenth of a similar volume of CMV-infected or VZV-infected cells. Various antiviral activities of IVIG may contribute to control and alleviation of CMV infection.

Genetic markers of immunoglobulin G and immunity to cytomegalovirus in patients with breast cancer

Human cytomegalovirus (CMV), a ubiquitous herpesvirus, has been implicated in the etiology of breast cancer. It is clear that not all people exposed to CMV are equally likely to develop this malignancy, implying the presence of host genetic factors that might modulate the cancer-spurring properties of the virus. CMV has evolved sophisticated strategies for evading host immunosurveillance. One strategy involves encoding decoy Fcγ receptors (FcγR) that thwart the Fcγ-mediated effector functions, such as antibody-dependent cellular cytotoxicity. In this investigation, using an enzyme-linked immunosorbent assay (ELISA), we aimed to determine whether the decoy FcγR encoded by the CMV gene RL13 binds differentially to anti-CMV antibodies expressing different immunoglobulin GM (γ marker) allotypes, genetic markers of immunoglobulin G (IgG). Results of our ELISA binding studies showed that the absorbance values for the binding of the viral FcγR to the GM 17-expressing IgG antibodies were significantly higher than for the GM 3-expressing antibodies (0.60 vs. 0.36; p=0.0019). These findings provide mechanistic insights into the modulating role played by the genetic variants of IgG in the generation of immunity to CMV in patients with breast cancer.

The Cell Biology of Cytomegalovirus: Implications for Transplantation

Interpretation of clinical data regarding the impact of cytomegalovirus (CMV) infection on allograft function is complicated by the diversity of viral strains and substantial variability of cellular receptors and viral gene expression in different tissues. Variation also exists in nonspecific (monocytes and dendritic cells) and specific (NK cells, antibodies) responses that augment T cell antiviral activities. Innate immune signaling pathways and expanded pools of memory NK cells and γδ T cells also serve to amplify host responses to infection. The clinical impact of specific memory T cell anti-CMV responses that cross-react with graft antigens and alloantigens is uncertain but appears to contribute to graft injury and to the abrogation of allograft tolerance. These responses are modified by diverse immunosuppressive regimens and by underlying host immune deficits. The impact of CMV infection on the transplant recipient reflects cellular changes and corresponding host responses, the convergence of which has been termed the "indirect effects" of CMV infection. Future studies will clarify interactions between CMV infection and allograft injury and will guide interventions that may enhance clinical outcomes in transplantation.

In Vitro Characterization of Human Cytomegalovirus-Targeting Therapeutic Monoclonal Antibodies LJP538 and LJP539

Human cytomegalovirus (HCMV) infection is usually benign in healthy individuals but can cause life-threatening disease in those with compromised immune systems. Approved drugs available to treat HCMV disease, including ganciclovir, cidofovir, and foscarnet, have significant toxicities that limit their use in certain patient populations. LJP538 and LJP539 are human monoclonal antibodies that are being evaluated as immunoglobulin therapeutics. The antibodies target glycoproteins gB and the gH/gL/UL128/UL130/UL131a pentameric complex, respectively. Here we present an in vitro characterization of these antibodies. We show that LJP538 and LJP539 are more potent than a marketed immunoglobulin at inhibiting HCMV infection of various cell lines relevant to pathogenesis. We find that LJP538 and LJP539 are active against a panel of clinical isolates in vitro and demonstrate minor-to-moderate synergy in combination. Passage of HCMV in the presence of LJP538 or LJP539 alone resulted in resistance-associated mutations that mapped to the target genes. However, no loss of susceptibility to the combination of antibodies was observed for >400 days in culture. Finally, the binding regions of LJP538 and LJP539 are conserved among clinical isolates. Taken together, these data support the use of LJP538 and LJP539 in combination for clinical trials in HCMV patients.

Human cytomegalovirus pUL10 interacts with leukocytes and impairs TCR-mediated T-cell activation

Human cytomegalovirus (HCMV) is known to exert suppressive effects on the host immune system through expression of various viral genes, thus directly and indirectly affecting antiviral immunity of the infected individuals. We report here that HCMV UL10 encodes a protein (pUL10) with immunosuppressive properties. UL10 has been classified as a member of the HCMV RL11 gene family. Although pUL10 is known to be dispensable for viral replication in cultured cells, its amino-acid sequence is well conserved among different HCMV isolates, suggesting that the protein has a crucial role in viral survival in the host environment. We show that pUL10 is cleaved from the cell surface of fibroblasts as well as epithelial cells and interacts with a cellular receptor ubiquitously expressed on the surface of human leukocytes, demonstrated by ex vivo cell-based assays and flow cytometric analyses on both lymphoid cell lines and primary blood cells. Furthermore, preincubation of peripheral blood mononuclear cells with purified pUL10 ectodomain results in significantly impaired proliferation and substantially reduced pro-inflammatory cytokine production, in particular in CD4⁺ T cells upon in vitro T-cell stimulation. The inhibitory effect of pUL10 is also observed on antigen receptor-mediated intracellular tyrosine phosphorylation in a T-cell line. Based on these observations, we suggest that pUL10 is a newly identified immunomodulatory protein encoded by HCMV. Further elucidation of interactions between pUL10 and the host immune system during HCMV may contribute to finding ways towards new therapies for HCMV infection.

Highly individual patterns of virus-immune IgG effector responses in humans

IgG responses are fundamental to adaptive immunity and document immunological memory of previous pathogen encounter. While specific antigen recognition is mediated by the variable F(ab')2 domain of IgG, various effector functions become activated via the constant Fcγ part bridging IgG-opsonized targets to FcγR-expressing immune effector cells. Traditionally, neutralizing IgG is considered the most appropriate correlate of protective humoral immunity to viruses. However, evidence is increasing that antiviral IgG mediates protection to viruses via activation of FcγRs. Using a test system allowing quantitative detection of virus-immune IgG able to activate FcγRs, sera of healthy individuals and vaccinees were assessed with regard to two prototypical human pathogenic viruses: measles and human cytomegalovirus. Marked differences in the capacity of individuals to generate FcγRI-, FcγRII- and FcγRIII-activating responses were noted. Comparison of FcγR-activating IgG with neutralizing and ELISA IgG concentrations did not correlate for HCMV and only very poorly for MV. Since neither neutralizing IgG nor overall IgG responses faithfully predict the activation of FcγRs, only the simultaneous quantification of IgGs activating defined FcγRs will aid to delineate individual "immunograms" of virus IgG immunity. Such new multiparametric assessment of antiviral IgG qualities could be instrumental in defining correlates of protection and disease progression.

Characterization of Antibody Bipolar Bridging Mediated by the Human Cytomegalovirus Fc Receptor gp68

The human cytomegalovirus glycoprotein gp68 functions as an Fc receptor for host IgGs and can form antibody bipolar bridging (ABB) complexes in which gp68 binds the Fc region of an antigen-bound IgG. Here we show that gp68-mediated endocytosis transports ABB complexes into endosomes, after which the complex is routed to lysosomes, presumably for degradation. These results suggest gp68 contributes to evasion of IgG-mediated immune responses by mediating destruction of host IgG and viral antigens.

Islands of linkage in an ocean of pervasive recombination reveals two-speed evolution of human cytomegalovirus genomes

Human cytomegalovirus (HCMV) infects most of the population worldwide, persisting throughout the host's life in a latent state with periodic episodes of reactivation. While typically asymptomatic, HCMV can cause fatal disease among congenitally infected infants and immunocompromised patients. These clinical issues are compounded by the emergence of antiviral resistance and the absence of an effective vaccine, the development of which is likely complicated by the numerous immune evasins encoded by HCMV to counter the host's adaptive immune responses, a feature that facilitates frequent super-infections. Understanding the evolutionary dynamics of HCMV is essential for the development of effective new drugs and vaccines. By comparing viral genomes from uncultivated or low-passaged clinical samples of diverse origins, we observe evidence of frequent homologous recombination events, both recent and ancient, and no structure of HCMV genetic diversity at the whole-genome scale. Analysis of individual gene-scale loci reveals a striking dichotomy: while most of the genome is highly conserved, recombines essentially freely and has evolved under purifying selection, 21 genes display extreme diversity, structured into distinct genotypes that do not recombine with each other. Most of these hyper-variable genes encode glycoproteins involved in cell entry or escape of host immunity. Evidence that half of them have diverged through episodes of intense positive selection suggests that rapid evolution of hyper-variable loci is likely driven by interactions with host immunity. It appears that this process is enabled by recombination unlinking hyper-variable loci from strongly constrained neighboring sites. It is conceivable that viral mechanisms facilitating super-infection have evolved to promote recombination between diverged genotypes, allowing the virus to continuously diversify at key loci to escape immune detection, while maintaining a genome optimally adapted to its asymptomatic infectious lifecycle.

Essential role of protein kinase R antagonism by TRS1 in human cytomegalovirus replication

Human cytomegalovirus (HCMV) lacking TRS1 and IRS1 (HCMV[ΔI/ΔT]) cannot replicate in cell culture. Although both proteins can block the protein kinase R (PKR) pathway, they have multiple other activities and binding partners. It remains unknown which functions are essential for HCMV replication. To investigate this issue, we first identified a TRS1 mutant that is unable to bind to PKR. Like HCMV[ΔI/ΔT], a recombinant HCMV containing this mutant (HCMV[TRS1-Mut 1]) did not replicate in wild-type cells. However, HCMV[ΔI/ΔT] did replicate in cells in which PKR expression was reduced by RNA interference. Moreover, HCMV[ΔI/ΔT] and HCMV[TRS1-Mut 1] replicated to similar levels as virus containing wild-type TRS1 in cell lines in which PKR expression was knocked out by CRISPR/Cas9-mediated genome editing. These results demonstrate that the sole essential function of TRS1 is to antagonize PKR and that its other activities do not substantially enhance HCMV replication, at least in cultured human fibroblasts.

The decoy Fcγ receptor encoded by the cytomegalovirus UL119-UL118 gene has differential affinity to IgG proteins expressing different GM allotypes

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that has been implicated in many diseases. However, there is significant divergence between HCMV seroprevalence and the prevalence of HCMV-associated diseases, implying the presence of host genetic factors that might modulate immunity to this virus. HCMV deploys many sophisticated strategies to evade host immunosurveillance. One strategy involves encoding for proteins that have functional properties of the Fcγ receptor (FcγR). The aim of the present investigation was to determine whether the UL119-UL118-encoded recombinant FcγR ectodomain binds differentially to genetically disparate IgG1 proteins. Results show that mean absorbance values for binding of HCMV UL119-UL118-encoded Fcγ receptor to the immunoglobulin GM (γ marker) 1,17-expressing IgG1 were significantly higher than to the IgG1 expressing the allelic GM 3 allotype (0.225 vs. 0.151; p=0.039). These findings suggest possible mechanisms underlying the maintenance of immunoglobulin GM gene polymorphism and its putative role in the etiology of HCMV-associated diseases.

Functional differences between antiviral activities of sulfonated and intact intravenous immunoglobulin preparations toward varicella-zoster virus and cytomegalovirus

Intravenous immunoglobulin (IVIG) is used to treat severe viral infection, especially varicella-zoster virus (VZV) and cytomegalovirus (CMV) infections. The neutralization antibody titers of eleven IVIG preparations from four companies were examined using VZV and CMV with and without complement. The neutralizing antibody titers of intact IgG preparations were three to six times higher after addition of complement. The effectiveness of the sulfonated IgG preparation was not enhanced by complement, but desulfonated IgG regained enhanced neutralization activity with complement. Antibody-dependent cellular cytotoxicity (ADCC) toward VZV-infected cells was observed with both intact and sulfonated IVIG and guinea pig splenocytes, but ADCC toward CMV-infected cells was not, although NK cell activity toward cells infected with VZV or CMV was detected by splenocytes. Sulfonated IVIG had no complement-activated neutralization of VZV and CMV but retained ADCC toward VZV with less activity after dilution than with intact IVIG. Because sulfonated IVIG is converted to the intact form after intravenous administration, it would show complement-enhanced neutralization and ADCC activity similar to that of intact IVIG in vivo. In this study we showed the effects of intact and sulfonated IgG on the functional activity of IgG against VZV and CMV.

UL84-independent replication of human cytomegalovirus strains conferred by a single codon change in UL122

The UL84 gene of human cytomegalovirus (HCMV) is thought to be involved in the initiation of viral DNA replication, and is essential for replication of strains AD169 and Towne. Hence, discovery that strain TB40-BAC4 is viable in the absence of UL84 presented an enigma requiring an explanation. Data reported here show that strain TR also tolerated loss of UL84, whereas strains FIX, Merlin, Ph, and Toledo did not. UL84-independent growth required the viral replication origin. The genetic locus in TB40 that controls UL84 dependence was mapped to codon 388 of the UL122 gene, which encodes the immediate early 2 (IE2) 86kD protein. Introduction of this TB40-BAC4 variant (H388D) into FIX and Toledo clones converted these strains to UL84 independence. These results provide genetic evidence in virus-infected cells that supports the hypothesis that UL122 participates in the initiation of viral DNA replication by a mechanism involving transcription-mediated activation of the origin.

Expression of the human cytomegalovirus UL11 glycoprotein in viral infection and evaluation of its effect on virus-specific CD8 T cells

The human cytomegalovirus (CMV) UL11 open reading frame (ORF) encodes a putative type I transmembrane glycoprotein which displays remarkable amino acid sequence variability among different CMV isolates, suggesting that it represents an important virulence factor. In a previous study, we have shown that UL11 can interact with the cellular receptor tyrosine phosphatase CD45, which has a central role for signal transduction in T cells, and treatment of T cells with large amounts of a soluble UL11 protein inhibited their proliferation. In order to analyze UL11 expression in CMV-infected cells, we constructed CMV recombinants whose genomes either encode tagged UL11 versions or carry a stop mutation in the UL11 ORF. Moreover, we examined whether UL11 affects the function of virus-specific cytotoxic T lymphocytes (CTLs). We found that the UL11 ORF gives rise to several proteins due to both posttranslational modification and alternative translation initiation sites. Biotin labeling of surface proteins on infected cells indicated that only highly glycosylated UL11 forms are present at the plasma membrane, whereas less glycosylated UL11 forms were found in the endoplasmic reticulum. We did not find evidence of UL11 cleavage or secretion of a soluble UL11 version. Cocultivation of CTLs recognizing different CMV epitopes with fibroblasts infected with a UL11 deletion mutant or the parental strain revealed that under the conditions applied UL11 did not influence the activation of CMV-specific CD8 T cells. For further studies, we propose to investigate the interaction of UL11 with CD45 and the functional consequences in other immune cells expressing CD45.

IMPORTANCE

Human cytomegalovirus (CMV) belongs to those viruses that extensively interfere with the host immune response, yet the precise function of many putative immunomodulatory CMV proteins remains elusive. Previously, we have shown that the CMV UL11 protein interacts with the leukocyte common antigen CD45, a cellular receptor tyrosine phosphatase with a central role for signal transduction in T cells. Here, we examined the proteins expressed by the UL11 gene in CMV-infected cells and found that at least one form of UL11 is present at the cell surface, enabling it to interact with CD45 on immune cells. Surprisingly, CMV-expressed UL11 did not affect the activity of virus-specific CD8 T cells. This finding warrants investigation of the impact of UL11 on CD45 functions in other leukocyte subpopulations.

CMV-encoded Fcγ receptors: modulators at the interface of innate and adaptive immunity

The constant region of IgG antibodies mediates antiviral activities upon engaging host Fcγ receptors (FcγRs) expressed by a variety of immune cells, such as antibody-dependent cellullar cytotoxcity (ADCC) executed by natural killer (NK)cells. Human cytomegalovirus (HCMV) is unique among viruses by encoding also an array of several Fcγ-binding glycoproteins with cell surface disposition and concomitant incorporation into the virion. Evidence is increasing that the virus-encoded Fcγ receptors differ in their Fcγ binding mode but effectively operate as adversaries of host FcγRs since they are able to prevent IgG-mediated triggering of activating host FcγRs, i.e., FcγRI, FcγRIIA, and FcγRIIIA. Here we discuss virus-encoded FcγRs as the first known HCMV inhibitors of IgG-mediated immunity which could account for the limited efficacy of HCMV hyperimmune globulin in clinical settings. A better understanding of their molecular mode of action opens up new perspectives for improving IgG therapies against HCMV disease.