Major histocompatibility complex class I allele-specific cooperative and competitive interactions between immune evasion proteins of cytomegalovirus

Engineered HCMV-infected APCs enable the identification of new immunodominant HLA-restricted epitopes of anti-HCMV T-cell immunity

Complications due to HCMV infection or reactivation remain a challenging clinical problem in immunocompromised patients, mainly due to insufficient or absent T-cell functionality. Knowledge of viral targets is crucial to improve monitoring of high-risk patients and optimise antiviral T-cell therapy. To expand the epitope spectrum, genetically-engineered dendritic cells (DCs) and fibroblasts were designed to secrete soluble (s)HLA-A*11:01 and infected with an HCMV mutant lacking immune evasion molecules (US2-6 + 11). More than 700 HLA-A*11:01-restricted epitopes, including more than 50 epitopes derived from a broad range of HCMV open-reading-frames (ORFs) were identified by mass spectrometry and screened for HLA-A*11:01-binding using established prediction tools. The immunogenicity of the 24 highest scoring new candidates was evaluated in vitro in healthy HLA-A*11:01+/HCMV+ donors. Thus, four subdominant epitopes and one immunodominant epitope, derived from the anti-apoptotic protein UL36 and ORFL101C (A11SAL), were identified. Their HLA-A*11:01 complex stability was verified in vitro. In depth analyses revealed highly proliferative and cytotoxic memory T-cell responses against A11SAL, with T-cell responses comparable to the immunodominant HLA-A*02:01-restricted HCMVpp65NLV epitope. A11SAL-specific T cells were also detectable in vivo in immunosuppressed transplant patients and shown to be effective in an in vitro HCMV-infection model, suggesting their crucial role in inhibiting viral replication and improvement of patient's outcome. The developed in vitro pipeline is the first to utilise genetically-engineered DCs to identify naturally presented immunodominant HCMV-derived epitopes. It therefore offers advantages over in silico predictions, is transferable to other HLA alleles, and will significantly expand the repertoire of viral targets to improve therapeutic options.

Murine cytomegalovirus downregulates ERAAP and induces an unconventional T cell response to self

The endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP) plays a crucial role in shaping the peptide-major histocompatibility complex (MHC) class I repertoire and maintaining immune surveillance. While murine cytomegalovirus (MCMV) has multiple strategies for manipulating the antigen processing pathway to evade immune responses, the host has also developed ways to counter viral immune evasion. In this study, we find that MCMV modulates ERAAP and induces an interferon γ (IFN-γ)-producing CD8+ T cell effector response that targets uninfected ERAAP-deficient cells. We observe that ERAAP downregulation during infection leads to the presentation of the self-peptide FL9 on non-classical Qa-1b, thereby eliciting Qa-1b-restricted QFL T cells to proliferate in the liver and spleen of infected mice. QFL T cells upregulate effector markers upon MCMV infection and are sufficient to reduce viral load after transfer to immunodeficient mice. Our study highlights the consequences of ERAAP dysfunction during viral infection and provides potential targets for anti-viral therapies.

Host-Adapted Gene Families Involved in Murine Cytomegalovirus Immune Evasion

Cytomegaloviruses (CMVs) are host species-specific and have adapted to their respective mammalian hosts during co-evolution. Host-adaptation is reflected by “private genes” that have specialized in mediating virus-host interplay and have no sequence homologs in other CMV species, although biological convergence has led to analogous protein functions. They are mostly organized in gene families evolved by gene duplications and subsequent mutations. The host immune response to infection, both the innate and the adaptive immune response, is a driver of viral evolution, resulting in the acquisition of viral immune evasion proteins encoded by private gene families. As the analysis of the medically relevant human cytomegalovirus by clinical investigation in the infected human host cannot make use of designed virus and host mutagenesis, the mouse model based on murine cytomegalovirus (mCMV) has become a versatile animal model to study basic principles of in vivo virus-host interplay. Focusing on the immune evasion of the adaptive immune response by CD8⁺ T cells, we review here what is known about proteins of two private gene families of mCMV, the m02 and the m145 families, specifically the role of m04, m06, and m152 in viral antigen presentation during acute and latent infection.

Control of Viral Infection by Natural Killer Cell Inhibitory Receptors

Major histocompatibility complex class I (MHC-I)-restricted immune responses are largely attributed to cytotoxic T lymphocytes (CTLs). However, natural killer (NK) cells, as predicted by the missing-self hypothesis, have opposing requirements for MHC-I, suggesting that they may also demonstrate MHC-I-restricted effects. In mice, the Ly49 inhibitory receptors prevent NK cell killing of missing-self targets in effector responses, and they have a proposed second function in licensing or educating NK cells via self-MHC-I in vivo. Here we show MHC-I-restricted control of murine cytomegalovirus (MCMV) infection in vivo that is NK cell dependent. Using mice lacking specific Ly49 receptors, we show that control of MCMV requires inhibitory Ly49 receptors and an inhibitory signaling motif and the capacity for MCMV to downregulate MHC-I. Taken together, these data provide definitive evidence that the inhibitory receptors are required for missing-self rejection and are relevant to MHC-I-restricted NK cell control of a viral infection in vivo.

Parikh et al. show that major histocompatibility complex class I (MHC-I)-restricted control of viral infection is due to natural killer (NK) cells rather than cytotoxic T cells. Inhibitory NK cell receptors are essential for protection, requiring NK cell licensing (education) by self-MHC-I and missing-self recognition due to virus-induced MHC-I downregulation.

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.

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.

Hematopoietic cell-mediated dissemination of murine cytomegalovirus is regulated by NK cells and immune evasion

Cytomegalovirus (CMV) causes clinically important diseases in immune compromised and immune immature individuals. Based largely on work in the mouse model of murine (M)CMV, there is a consensus that myeloid cells are important for disseminating CMV from the site of infection. In theory, such dissemination should expose CMV to cell-mediated immunity and thus necessitate evasion of T cells and NK cells. However, this hypothesis remains untested. We constructed a recombinant MCMV encoding target sites for the hematopoietic specific miRNA miR-142-3p in the essential viral gene IE3. This virus disseminated poorly to the salivary gland following intranasal or footpad infections but not following intraperitoneal infection in C57BL/6 mice, demonstrating that dissemination by hematopoietic cells is essential for specific routes of infection. Remarkably, depletion of NK cells or T cells restored dissemination of this virus in C57BL/6 mice after intranasal infection, while dissemination occurred normally in BALB/c mice, which lack strong NK cell control of MCMV. These data show that cell-mediated immunity is responsible for restricting MCMV to hematopoietic cell-mediated dissemination. Infected hematopoietic cells avoided cell-mediated immunity via three immune evasion genes that modulate class I MHC and NKG2D ligands (m04, m06 and m152). MCMV lacking these 3 genes spread poorly to the salivary gland unless NK cells were depleted, but also failed to replicate persistently in either the nasal mucosa or salivary gland unless CD8⁺ T cells were depleted. Surprisingly, CD8⁺ T cells primed after intranasal infection required CD4⁺ T cell help to expand and become functional. Together, our data suggest that MCMV can use both hematopoietic cell-dependent and -independent means of dissemination after intranasal infection and that cell mediated immune responses restrict dissemination to infected hematopoietic cells, which are protected from NK cells during dissemination by viral immune evasion. In contrast, viral replication within mucosal tissues depends on evasion of T cells.

Cytomegalovirus (CMV) is a common cause of disease in immune compromised individuals as well as a common cause of congenital infections leading to disease in newborns. The virus is thought to enter primarily via mucosal barrier tissues, such as the oral and nasal mucosa. However, it is not clear how the virus escapes these barrier tissues to reach distant sites. In this study, we used a mouse model of CMV infection. Our data illustrate a complex balance between the immune system and viral infection of “myeloid cells”, which are most commonly thought to carry the virus around the body after infection. In particular, our data suggest that robust immune responses at the site of infection force the virus to rely on myeloid cells to escape the site of infection. Moreover, viral genes designed to evade these immune responses were needed to protect the virus during and after its spread to distant sites. Together, this work sheds light on the mechanisms of immune control and viral survival during CMV infection of mucosal tissues and spread to distant sites of the body.

Positive Role of the MHC Class-I Antigen Presentation Regulator m04/gp34 of Murine Cytomegalovirus in Antiviral Protection by CD8 T Cells

Murine cytomegalovirus (mCMV) codes for MHC class-I trafficking modulators m04/gp34, m06/gp48, and m152/gp40. By interacting with the MHC class-Iα chain, these proteins disconnect peptide-loaded MHC class-I (pMHC-I) complexes from the constitutive vesicular flow to the cell surface. Based on the assumption that all three inhibit antigen presentation, and thus the recognition of infected cells by CD8 T cells, they were referred to as “immunoevasins.” Improved antigen presentation mediated by m04 in the presence of m152 after infection with deletion mutant mCMV-Δm06^W, compared to mCMV-Δm04m06 expressing only m152, led us to propose renaming these molecules “viral regulators of antigen presentation” (vRAP) to account for both negative and positive functions. In accordance with a positive function, m04-pMHC-I complexes were found to be displayed on the cell surface, where they are primarily known as ligands for Ly49 family natural killer (NK) cell receptors. Besides the established role of m04 in NK cell silencing or activation, an anti-immunoevasive function by activation of CD8 T cells is conceivable, because the binding site of m04 to MHC class-Iα appears not to mask the peptide binding site for T-cell receptor recognition. However, functional evidence was based on mCMV-Δm06^W, a virus of recently doubted authenticity. Here we show that mCMV-Δm06^W actually represents a mixture of an authentic m06 deletion mutant and a mutant with an accidental additional deletion of a genome region encompassing also gene m152. Reanalysis of previously published experiments for the authentic mutant in the mixture confirms the previously concluded positive vRAP function of m04.

Revisiting CD8 T-cell 'Memory Inflation': New Insights with Implications for Cytomegaloviruses as Vaccine Vectors

Murine models of cytomegalovirus (CMV) infection have revealed an exceptional kinetics of the immune response. After resolution of productive infection, transient contraction of the viral epitope-specific CD8 T-cell pool was found to be followed by a pool expansion specific for certain viral epitopes during non-productive ‘latent’ infection. This phenomenon, known as ‘memory inflation’ (MI), was found to be based on inflationary KLRG1⁺CD62L⁻ effector-memory T cells (iTEM) that depend on repetitive restimulation. MI gained substantial interest for employing CMV as vaccine vector by replacing MI-driving CMV epitopes with foreign epitopes for generating high numbers of protective memory cells specific for unrelated pathogens. The concept of an MI-driving CMV vector is questioned by human studies disputing MI in humans. A bias towards MI in experimental models may have resulted from systemic infection. We have here studied local murine CMV infection as a route that is more closely matching routine human vaccine application. Notably, KLRG1⁻CD62L⁺ central memory T cells (TCM) and conventional KLRG1⁻CD62L⁻ effector memory T cells (cTEM) were found to expand, associated with ‘avidity maturation’, whereas the pool size of iTEM steadily declined over time. The establishment of high avidity CD8 T-cell central memory encourages one to pursue the concept of CMV vector-based vaccines.

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.

Insufficient Antigen Presentation Due to Viral Immune Evasion Explains Lethal Cytomegalovirus Organ Disease After Allogeneic Hematopoietic Cell Transplantation

Reactivation of latent cytomegalovirus (CMV) poses a clinical problem in transiently immunocompromised recipients of hematopoietic cell (HC) transplantation (HCT) by viral histopathology that results in multiple organ manifestations. Compared to autologous HCT and to syngeneic HCT performed with identical twins as HC donor and recipient, lethal outcome of CMV infection is more frequent in allogeneic HCT with MHC/HLA or minor histocompatibility loci mismatch between donor and recipient. It is an open question if a graft-vs.-host (GvH) reaction exacerbates CMV disease, or if CMV exacerbates GvH disease (GvHD), or if interference is mutual. Here we have used a mouse model of experimental HCT and murine CMV (mCMV) infection with an MHC class-I mismatch by gene deletion, so that either HCT donor or recipient lack a single MHC class-I molecule, specifically H-2 Ld. This particular immunogenetic disparity has the additional advantage that it allows to experimentally separate GvH reaction of donor-derived T cells against recipient's tissues from host-vs.-graft (HvG) reaction of residual recipient-derived T cells against the transplanted HC and their progeny. While in HvG-HCT with Ld-plus donors and Ld-minus recipients almost all infected recipients were found to control the infection and survived, almost all infected recipients died of uncontrolled virus replication and consequent multiple-organ viral histopathology in case of GvH-HCT with Ld-minus donors and Ld-plus recipients. Unexpectedly, although anti-Ld-reactive CD8+ T cells were detected, mortality was not found to be associated with GvHD histopathology. By comparing HvG-HCT and GvH-HCT, investigation into the mechanism revealed an inefficient reconstitution of antiviral high-avidity CD8+ T cells, associated with lack of formation of protective nodular inflammatory foci (NIF) in host tissue, selectively in GvH-HCT. Most notably, mice infected with an immune evasion gene deletion mutant of mCMV survived under otherwise identical GvH-HCT conditions. Survival was associated with enhanced antigen presentation and formation of protective NIF by antiviral CD8+ T cells that control the infection and prevent viral histopathology. This is an impressive example of lethal viral disease in HCT recipients based on a failure of the immune control of CMV infection due to viral immune evasion in concert with an MHC class-I mismatch.

Eomes broadens the scope of CD8 T-cell memory by inhibiting apoptosis in cells of low affinity

The memory CD8 T-cell pool must select for clones that bind immunodominant epitopes with high affinity to efficiently counter reinfection. At the same time, it must retain a level of clonal diversity to allow recognition of pathogens with mutated epitopes. How the level of diversity within the memory pool is controlled is unclear, especially in the context of a selective drive for antigen affinity. We find that preservation of clones that bind the activating antigen with low affinity depends on expression of the transcription factor Eomes in the first days after antigen encounter. Eomes is induced at low activating signal strength and directly drives transcription of the prosurvival protein Bcl-2. At higher signal intensity, T-bet is induced which suppresses Bcl-2 and causes a relative survival advantage for cells of low affinity. Clones activated with high-affinity antigen form memory largely independent of Eomes and have a proliferative advantage over clones that bind the same antigen with low affinity. This causes high-affinity clones to prevail in the memory pool, despite their relative survival deficit. Genetic or therapeutic targeting of the Eomes/Bcl-2 axis reduces the clonal diversity of the memory pool, which diminishes its ability to respond to pathogens carrying mutations in immunodominant epitopes. Thus, we demonstrate on a molecular level how sufficient diversity of the memory pool is established in an environment of affinity-based selection.

This study shows that the diversity of the memory CD8 T-cell pool is regulated by the transcription factor Eomes, which drives transcription of the pro-survival protein Bcl-2. Genetic or therapeutic targeting of the Eomes/Bcl-2 axis reduces the clonal diversity of the memory pool.

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.

The complex of MCMV proteins and MHC class I evades NK cell control and drives the evolution of virus-specific activating Ly49 receptors

Železnjak et al. demonstrate that two MCMV-encoded proteins interact with MHC I molecules, forming an altered-self complex that prevents missing self recognition by increasing specificity for inhibitory Ly49 receptors. This led to the evolution of CMV-specific activating Ly49s.

CMVs efficiently target MHC I molecules to avoid recognition by cytotoxic T cells. However, the lack of MHC I on the cell surface renders the infected cell susceptible to NK cell killing upon missing self recognition. To counter this, mouse CMV (MCMV) rescues some MHC I molecules to engage inhibitory Ly49 receptors. Here we identify a new viral protein, MATp1, that is essential for MHC I surface rescue. Rescued altered-self MHC I molecules show increased affinity to inhibitory Ly49 receptors, resulting in inhibition of NK cells despite substantially reduced MHC I surface levels. This enables the virus to evade recognition by licensed NK cells. During evolution, this novel viral immune evasion mechanism could have prompted the development of activating NK cell receptors that are specific for MATp1-modified altered-self MHC I molecules. Our study solves a long-standing conundrum of how MCMV avoids recognition by NK cells, unravels a fundamental new viral immune evasion mechanism, and demonstrates how this forced the evolution of virus-specific activating MHC I–restricted Ly49 receptors.

Molluscum contagiosum virus MC80 sabotages MHC-I antigen presentation by targeting tapasin for ER-associated degradation

The human specific poxvirus molluscum contagiosum virus (MCV) produces skin lesions that can persist with minimal inflammation, suggesting that the virus has developed robust immune evasion strategies. However, investigations into the underlying mechanisms of MCV pathogenesis have been hindered by the lack of a model system to propagate the virus. Herein we demonstrate that MCV-encoded MC80 can disrupt MHC-I antigen presentation in human and mouse cells. MC80 shares moderate sequence-similarity with MHC-I and we find that it associates with components of the peptide-loading complex. Expression of MC80 results in ER-retention of host MHC-I and thereby reduced cell surface presentation. MC80 accomplishes this by engaging tapasin via its luminal domain, targeting it for ubiquitination and ER-associated degradation in a process dependent on the MC80 transmembrane region and cytoplasmic tail. Tapasin degradation is accompanied by a loss of TAP, which limits MHC-I access to cytosolic peptides. Our findings reveal a unique mechanism by which MCV undermines adaptive immune surveillance.

Immunometabolic phenotype of BV-2 microglia cells upon murine cytomegalovirus infection

Microglia are resident brain macrophages with key roles in development and brain homeostasis. Cytomegalovirus (CMV) readily infects microglia cells, even as a possible primary target of infection in development. Effects of CMV infection on a cellular level in microglia are still unclear; therefore, the aim of this research was to assess the immunometabolic changes of BV-2 microglia cells following the murine cytomegalovirus (MCMV) infection. In light of that aim, we established an in vitro model of ramified BV-2 microglia (BV-2^∅FCS, inducible nitric oxide synthase (iNOS^low), arginase-1 (Arg-1^high), mannose receptor CD206^high, and hypoxia-inducible factor 1α (HIF-1α^low)) to better replicate the in vivo conditions by removing FCS from the cultivation media, while the cells cultivated in 10% FCS DMEM displayed an ameboid morphology (BV-2^{FCS high}, iNOS^high, Arg-1^low, CD206^low, and HIF-1α^high). Experiments were performed using both ramified and ameboid microglia, and both of them were permissive to productive viral infection. Our results indicate that MCMV significantly alters the immunometabolic phenotypic properties of BV-2 microglia cells through the manipulation of iNOS and Arg-1 expression patterns, along with an induction of a glycolytic shift in the infected cell cultures.

Function of the cargo sorting dileucine motif in a cytomegalovirus immune evasion protein

As an immune evasion mechanism, cytomegaloviruses (CMVs) have evolved proteins that interfere with cell surface trafficking of MHC class-I (MHC-I) molecules to tone down recognition by antiviral CD8 T cells. This interference can affect the trafficking of recently peptide-loaded MHC-I from the endoplasmic reticulum to the cell surface, thus modulating the presentation of viral peptides, as well as the recycling of pre-existing cell surface MHC-I, resulting in reduction of the level of overall MHC-I cell surface expression. Murine cytomegalovirus (mCMV) was paradigmatic in that it led to the discovery of this immune evasion strategy of CMVs. Members of its m02-m16 gene family code for type-I transmembrane glycoproteins, proven or predicted, most of which carry cargo sorting motifs in their cytoplasmic, C-terminal tail. For the m06 gene product m06 (gp48), the cargo has been identified as being MHC-I, which is linked by m06 to cellular adapter proteins AP-1A and AP-3A through the dileucine motif EPLARLL. Both APs are involved in trans-Golgi network (TGN) cargo sorting and, based on transfection studies, their engagement by the dileucine motif was proposed to be absolutely required to prevent MHC-I exposure at the cell surface. Here, we have tested this prediction in an infection system with the herein newly described recombinant virus mCMV-m06AA, in which the dileucine motif is destroyed by replacing EPLARLL with EPLARAA. This mutation has a phenotype in that the transition of m06-MHC-I complexes from early endosomes (EE) to late endosomes (LE)/lysosomes for degradation is blocked. Consistent with the binding of the MHC-I α-chain to the luminal domain of m06, the m06-mediated disposal of MHC-I did not require the β2m chain of mature MHC-I. Unexpectedly, however, disconnecting MHC-I cargo from AP-1A/3A by the motif mutation in m06 had no notable rescuing impact on overall cell surface MHC-I, though it resulted in some improvement of the presentation of viral antigenic peptides by recently peptide-loaded MHC-I. Thus, the current view on the mechanism by which m06 mediates immune evasion needs to be revised. While the cargo sorting motif is critically involved in the disposal of m06-bound MHC-I in the endosomal/lysosomal pathway at the stage of EE to LE transition, this motif-mediated disposal is not the critical step by which m06 causes immune evasion. We rather propose that engagement of AP-1A/3A by the cargo sorting motif in m06 routes the m06-MHC-I complexes into the endosomal pathway and thereby detracts them from the constitutive cell surface transport.

The herpesviral antagonist m152 reveals differential activation of STING-dependent IRF and NF-κB signaling and STING's dual role during MCMV infection

Cytomegaloviruses (CMVs) are master manipulators of the host immune response. Here, we reveal that the murine CMV (MCMV) protein m152 specifically targets the type I interferon (IFN) response by binding to stimulator of interferon genes (STING), thereby delaying its trafficking to the Golgi compartment from where STING initiates type I IFN signaling. Infection with an MCMV lacking m152 induced elevated type I IFN responses and this leads to reduced viral transcript levels both in vitro and in vivo. This effect is ameliorated in the absence of STING. Interestingly, while m152 inhibits STING‐mediated IRF signaling, it did not affect STING‐mediated NF‐κB signaling. Analysis of how m152 targets STING translocation reveals that STING activates NF‐κB signaling already from the ER prior to its trafficking to the Golgi. Strikingly, this response is important to promote early MCMV replication. Our results show that MCMV has evolved a mechanism to specifically antagonize the STING‐mediated antiviral IFN response, while preserving its pro‐viral NF‐κB response, providing an advantage in the establishment of an infection.

Viral MHCI inhibition evades tissue-resident memory T cell formation and responses

Lauron et al. demonstrate that viral MHCI inhibition within infected cells reduces local antigen-driven generation of resident memory CD8⁺ T cells. Additionally, resident memory CD8⁺ T cells are insufficient in controlling peripheral infection in the context of viral MHCI evasion.

Tissue-resident memory CD8⁺ T cells (T_RMs) confer rapid protection and immunity against viral infections. Many viruses have evolved mechanisms to inhibit MHCI presentation in order to evade CD8⁺ T cells, suggesting that these mechanisms may also apply to T_RM-mediated protection. However, the effects of viral MHCI inhibition on the function and generation of T_RMs is unclear. Herein, we demonstrate that viral MHCI inhibition reduces the abundance of CD4⁺ and CD8⁺ T_RMs, but its effects on the local microenvironment compensate to promote antigen-specific CD8⁺ T_RM formation. Unexpectedly, local cognate antigen enhances CD8⁺ T_RM development even in the context of viral MHCI inhibition and CD8⁺ T cell evasion, strongly suggesting a role for in situ cross-presentation in local antigen-driven T_RM differentiation. However, local cognate antigen is not required for CD8⁺ T_RM maintenance. We also show that viral MHCI inhibition efficiently evades CD8⁺ T_RM effector functions. These findings indicate that viral evasion of MHCI antigen presentation has consequences on the development and response of antiviral T_RMs.

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

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.

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.

Murine cytomegalovirus degrades MHC class II to colonize the salivary glands

Cytomegaloviruses (CMVs) persistently and systemically infect the myeloid cells of immunocompetent hosts. Persistence implies immune evasion, and CMVs evade CD8⁺ T cells by inhibiting MHC class I-restricted antigen presentation. Myeloid cells can also interact with CD4⁺ T cells via MHC class II (MHC II). Human CMV (HCMV) attacks the MHC II presentation pathway in vitro, but what role this evasion might play in host colonization is unknown. We show that Murine CMV (MCMV) down-regulates MHC II via M78, a multi-membrane spanning viral protein that captured MHC II from the cell surface and was necessary although not sufficient for its degradation in low pH endosomes. M78-deficient MCMV down-regulated MHC I but not MHC II. After intranasal inoculation, it showed a severe defect in salivary gland colonization that was associated with increased MHC II expression on infected cells, and was significantly rescued by CD4⁺ T cell loss. Therefore MCMV requires CD4⁺ T cell evasion by M78 to colonize the salivary glands, its main site of long-term shedding.

Human cytomegalovirus is the commonest infectious cause of harm to unborn children. Vaccines have not stopped it establishing chronic, systemic infections. Murine cytomegalovirus (MCMV) provides an accessible model to understand why. We show that MCMV evades CD4⁺ T cells via its M78 protein, and that this helps infection to spread despite the immune response. Thus while CD4⁺ T cells are important for host defence, viral evasion limits their capacity to act alone in controlling infection.

CMV immune evasion and manipulation of the immune system with aging

Human cytomegalovirus (HCMV) encodes numerous proteins and microRNAs that function to evade the immune response and allow the virus to replicate and disseminate in the face of a competent innate and acquired immune system. The establishment of a latent infection by CMV, which if completely quiescent at the level of viral gene expression would represent an ultimate in immune evasion strategies, is not sufficient for lifelong persistence and dissemination of the virus. CMV needs to reactivate and replicate in a lytic cycle of infection in order to disseminate further, which occurs in the face of a fully primed secondary immune response. Without reactivation, latency itself would be redundant for the virus. It is also becoming clear that latency is not a totally quiescent state, but is characterized by limited viral gene expression. Therefore, the virus also needs immune evasion strategies during latency. An effective immune response to CMV is required or viral replication will cause morbidity and ultimately mortality in the host. There is clearly a complex balance between virus immune evasion and host immune recognition over a lifetime. This poses the important question of whether long-term evasion or manipulation of the immune response driven by CMV is detrimental to health. In this meeting report, three groups used the murine model of CMV (MCMV) to examine if the contribution of the virus to immune senescence is set by the (i) initial viral inoculum, (ii) inflation of T cell responses, (iii) or the balance between functionally distinct effector CD4+ T cells. The work of other groups studying the CMV response in humans is discussed. Their work asks whether the ability to make immune responses to new antigens is compromised by (i) age and HCMV carriage, (ii) long-term exposure to HCMV giving rise to an overall immunosuppressive environment and increased levels of latent virus, or (iii) adapted virus mutants (used as potential vaccines) that have the capacity to elicit conventional and unconventional T cell responses.

Mouse cytomegalovirus encoded immunoevasins and evolution of Ly49 receptors - Sidekicks or enemies?

Cytomegaloviruses (CMVs) have dedicated a large portion of their genome towards immune evasion targeting many aspects of the host immune system, particularly NK cells. However, the host managed to cope with the infection by developing multiple mechanisms to recognize viral threat and counterattack it, thus illustrating never-ending evolutionary interplay between CMV and its host. In this review, we will focus on several mechanisms of NK cell evasion by mouse CMV (MCMV), the role of host inhibitory and activating Ly49 receptors involved in the virus control and acquisition of adaptive features by NK cells as a consequence of MCMV infection.

Optimal conjugation of catechol group onto hyaluronic acid in coronary stent substrate coating for the prevention of restenosis

Although endovascular stenting has been used as an interventional therapy to treat cardio- and cerebro-vascular diseases, it is associated with recurrent vascular diseases following stent thrombosis and in-stent restenosis. In this study, a metallic stent was coated with dopamine-conjugated hyaluronic acid with different ratios of catechol group to improve hemocompatibility and re-endothelialization. Especially, we were interested in how much amount of catechol group is appropriate for the above-mentioned purposes. Therefore, a series of dopamine-conjugated hyaluronic acid conjugates with different ratios of catechol group were synthesized via a carbodiimide coupling reaction. Dopamine-conjugated hyaluronic acid conjugates were characterized with 1H-nuclear magnetic resonance and Fourier transform infrared spectroscopy, and the amount of catechol group in dopamine-conjugated hyaluronic acid was measured by ultraviolet spectrometer. Co-Cr substrates were polished and coated with various dopamine-conjugated hyaluronic acid conjugates under pH 8.5. Dopamine-conjugated hyaluronic acid amounts on the substrate were quantified by micro-bicinchoninic acid assay. Surface characteristics of dopamine-conjugated hyaluronic-acid-coated Co-Cr were evaluated by water contact angle, scanning electron microscopy, and atomic force microscopy. The hemocompatibility of the surface-modified substrates was assessed by protein adsorption and platelet adhesion tests. Adhesion and activation of platelets were confirmed with scanning electron microscopy and lactate dehydrogenase assay. Human umbilical vein endothelial cells were cultured on the substrates, and the viability, adhesion, and proliferation were investigated through cell counting kit-8 assay and fluorescent images. Obtained results demonstrated that optimal amounts of catechol group (100 µmol) in the dopamine-conjugated hyaluronic acid existed in terms of various properties such as hemocompatibility and cellular responses.

A Novel MHC-I Surface Targeted for Binding by the MCMV m06 Immunoevasin Revealed by Solution NMR

As part of its strategy to evade detection by the host immune system, murine cytomegalovirus (MCMV) encodes three proteins that modulate cell surface expression of major histocompatibility complex class I (MHC-I) molecules: the MHC-I homolog m152/gp40 as well as the m02-m16 family members m04/gp34 and m06/gp48. Previous studies of the m04 protein revealed a divergent Ig-like fold that is unique to immunoevasins of the m02-m16 family. Here, we engineer and characterize recombinant m06 and investigate its interactions with full-length and truncated forms of the MHC-I molecule H2-L(d) by several techniques. Furthermore, we employ solution NMR to map the interaction footprint of the m06 protein on MHC-I, taking advantage of a truncated H2-L(d), "mini-H2-L(d)," consisting of only the α1α2 platform domain. Mini-H2-L(d) refolded in vitro with a high affinity peptide yields a molecule that shows outstanding NMR spectral features, permitting complete backbone assignments. These NMR-based studies reveal that m06 binds tightly to a discrete site located under the peptide-binding platform that partially overlaps with the β2-microglobulin interface on the MHC-I heavy chain, consistent with in vitro binding experiments showing significantly reduced complex formation between m06 and β2-microglobulin-associated MHC-I. Moreover, we carry out NMR relaxation experiments to characterize the picosecond-nanosecond dynamics of the free mini-H2-L(d) MHC-I molecule, revealing that the site of interaction is highly ordered. This study provides insight into the mechanism of the interaction of m06 with MHC-I, suggesting a structural manipulation of the target MHC-I molecule at an early stage of the peptide-loading pathway.

The evolution of natural killer cell receptors

Natural killer (NK) cells are immune cells that play a crucial role against viral infections and tumors. To be tolerant against healthy tissue and simultaneously attack infected cells, the activity of NK cells is tightly regulated by a sophisticated array of germline-encoded activating and inhibiting receptors. The best characterized mechanism of NK cell activation is “missing self” detection, i.e., the recognition of virally infected or transformed cells that reduce their MHC expression to evade cytotoxic T cells. To monitor the expression of MHC-I on target cells, NK cells have monomorphic inhibitory receptors which interact with conserved MHC molecules. However, there are other NK cell receptors (NKRs) encoded by gene families showing a remarkable genetic diversity. Thus, NKR haplotypes contain several genes encoding for receptors with activating and inhibiting signaling, and that vary in gene content and allelic polymorphism. But if missing-self detection can be achieved by a monomorphic NKR system why have these polygenic and polymorphic receptors evolved? Here, we review the expansion of NKR receptor families in different mammal species, and we discuss several hypotheses that possibly underlie the diversification of the NK cell receptor complex, including the evolution of viral decoys, peptide sensitivity, and selective MHC-downregulation.

Evaluating Human T-Cell Therapy of Cytomegalovirus Organ Disease in HLA-Transgenic Mice

Reactivation of human cytomegalovirus (HCMV) can cause severe disease in recipients of hematopoietic stem cell transplantation. Although preclinical research in murine models as well as clinical trials have provided 'proof of concept' for infection control by pre-emptive CD8 T-cell immunotherapy, there exists no predictive model to experimentally evaluate parameters that determine antiviral efficacy of human T cells in terms of virus control in functional organs, prevention of organ disease, and host survival benefit. We here introduce a novel mouse model for testing HCMV epitope-specific human T cells. The HCMV UL83/pp65-derived NLV-peptide was presented by transgenic HLA-A2.1 in the context of a lethal infection of NOD/SCID/IL-2rg^-/- mice with a chimeric murine CMV, mCMV-NLV. Scenarios of HCMV-seropositive and -seronegative human T-cell donors were modeled by testing peptide-restimulated and T-cell receptor-transduced human T cells, respectively. Upon transfer, the T cells infiltrated host tissues in an epitope-specific manner, confining the infection to nodular inflammatory foci. This resulted in a significant reduction of viral load, diminished organ pathology, and prolonged survival. The model has thus proven its potential for a preclinical testing of the protective antiviral efficacy of HCMV epitope-specific human T cells in the evaluation of new approaches to an immunotherapy of CMV disease.

Pre-emptive CD8 T-cell therapy of human cytomegalovirus (HCMV) disease in immunocompromised recipients of hematopoietic stem cell transplantation gave promising results in clinical trials, but limited efficacy and the need of HCMV-seropositive memory cell donors has so far prevented adoptive cell transfer from becoming clinical routine. Further development is currently hampered by the lack of experimental animal models that allow preclinical testing of the protective efficacy of human T cells in functional organs. While humanized mouse models with human tissue implants are technically and statistically demanding, and are limited to studying human T-cell activation and local virus control in the implants, a more feasible model for control of systemic infection and prevention of multiple-organ CMV disease is regrettably missing. Here we introduce such a model based on infection of genetically immunocompromised, HLA-A2.1-transgenic NOD/SCID/IL-2rg^-/- mice with a chimeric murine CMV engineered to express the HCMV NLV-peptide epitope. Mimicking the scenario of HCMV-unexperienced donors, human T cells transduced with a human T-cell receptor specific for HLA-A.2.1-presented NLV peptide controlled systemic infection and moderated organ disease resulting in a survival benefit. The model promises to become instrumental in defining T-cell properties that determine their protective efficacy for a further development of adoptive immunotherapy of post-transplantation CMV infection.

Modulation of Clr Ligand Expression and NKR-P1 Receptor Function during Murine Cytomegalovirus Infection

Viruses are known to induce pathological cellular states that render infected cells susceptible or resistant to immune recognition. Here, we characterize an MHC-I-independent natural killer (NK) cell recognition mechanism that involves modulation of inhibitory NKR-P1B:Clr-b receptor-ligand interactions in response to mouse cytomegalovirus (MCMV) infection. We demonstrate that mouse Clr-b expression on healthy cells is rapidly lost at the cell surface and transcript levels in a time- and dose-dependent manner upon MCMV infection. In addition, cross-species infections using rat cytomegalovirus (RCMV) infection of mouse fibroblasts and MCMV infection of rat fibroblasts suggest that this response is conserved during host-pathogen interactions. Active viral infection appears to be necessary for Clr-b loss, as cellular stimulation using UV-inactivated whole virus or agonists of many innate pattern recognition receptors failed to elicit efficient Clr-b downregulation. Notably, Clr-b loss could be partially blocked by titrated cycloheximide treatment, suggesting that early viral or nascent host proteins are required for Clr-b downregulation. Interestingly, reporter cell assays suggest that MCMV may encode a novel Clr-b-independent immunoevasin that functionally engages the NKR-P1B receptor. Together, these data suggest that Clr-b modulation is a conserved innate host cell response to virus infection that is subverted by multiple CMV immune evasion strategies.

An endocytic YXXΦ (YRRF) cargo sorting motif in the cytoplasmic tail of murine cytomegalovirus AP2 'adapter adapter' protein m04/gp34 antagonizes virus evasion of natural killer cells

Viruses have evolved proteins that bind immunologically relevant cargo molecules at the cell surface for their downmodulation by internalization. Via a tyrosine-based sorting motif YXXΦ in their cytoplasmic tails, they link the bound cargo to the cellular adapter protein-2 (AP2), thereby sorting it into clathrin-triskelion-coated pits for accelerated endocytosis. Downmodulation of CD4 molecules by lentiviral protein NEF represents the most prominent example. Based on connecting cargo to cellular adapter molecules, such specialized viral proteins have been referred to as 'connectors' or 'adapter adapters.' Murine cytomegalovirus glycoprotein m04/gp34 binds stably to MHC class-I (MHC-I) molecules and suspiciously carries a canonical YXXΦ endocytosis motif YRRF in its cytoplasmic tail. Disconnection from AP2 by motif mutation ARRF should retain m04-MHC-I complexes at the cell surface and result in an enhanced silencing of natural killer (NK) cells, which recognize them via inhibitory receptors. We have tested this prediction with a recombinant virus in which the AP2 motif is selectively destroyed by point mutation Y248A, and compared this with the deletion of the complete protein in a Δm04 mutant. Phenotypes were antithetical in that loss of AP2-binding enhanced NK cell silencing, whereas absence of m04-MHC-I released them from silencing. We thus conclude that AP2-binding antagonizes NK cell silencing by enhancing endocytosis of the inhibitory ligand m04-MHC-I. Based on a screen for tyrosine-based endocytic motifs in cytoplasmic tail sequences, we propose here the new hypothesis that most proteins of the m02-m16 gene family serve as 'adapter adapters,' each selecting its specific cell surface cargo for clathrin-assisted internalization.

Cytomegalovirus immune evasion by perturbation of endosomal trafficking

Cytomegaloviruses (CMVs), members of the herpesvirus family, have evolved a variety of mechanisms to evade the immune response to survive in infected hosts and to establish latent infection. They effectively hide infected cells from the effector mechanisms of adaptive immunity by eliminating cellular proteins (major histocompatibility Class I and Class II molecules) from the cell surface that display viral antigens to CD8 and CD4 T lymphocytes. CMVs also successfully escape recognition and elimination of infected cells by natural killer (NK) cells, effector cells of innate immunity, either by mimicking NK cell inhibitory ligands or by downregulating NK cell-activating ligands. To accomplish these immunoevasion functions, CMVs encode several proteins that function in the biosynthetic pathway by inhibiting the assembly and trafficking of cellular proteins that participate in immune recognition and thereby, block their appearance at the cell surface. However, elimination of these proteins from the cell surface can also be achieved by perturbation of their endosomal route and subsequent relocation from the cell surface into intracellular compartments. Namely, the physiological route of every cellular protein, including immune recognition molecules, is characterized by specific features that determine its residence time at the cell surface. In this review, we summarize the current understanding of endocytic trafficking of immune recognition molecules and perturbations of the endosomal system during infection with CMVs and other members of the herpesvirus family that contribute to their immune evasion mechanisms.

Kaposi's sarcoma-associated herpesvirus K3 and K5 ubiquitin E3 ligases have stage-specific immune evasion roles during lytic replication

The downregulation of immune synapse components such as major histocompatibility complex class I (MHC-I) and ICAM-1 is a common viral immune evasion strategy that protects infected cells from targeted elimination by cytolytic effector functions of the immune system. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes two membrane-bound ubiquitin E3 ligases, called K3 and K5, which share the ability to induce internalization and degradation of MHC-I molecules. Although individual functions of K3 and K5 outside the viral genome are well characterized, their roles during the KSHV life cycle are still unclear. In this study, we individually introduced the amino acid-coding sequences of K3 or K5 into a ΔK3 ΔK5 recombinant virus, at either original or interchanged genomic positions. Recombinants harboring coding sequences within the K5 locus showed higher K3 and K5 protein expression levels and more rapid surface receptor downregulation than cognate recombinants in which coding sequences were introduced into the K3 locus. To identify infected cells undergoing K3-mediated downregulation of MHC-I, we employed a novel reporter virus, called red-green-blue-BAC16 (RGB-BAC16), which was engineered to harbor three fluorescent protein expression cassettes: EF1α-monomeric red fluorescent protein 1 (mRFP1), polyadenylated nuclear RNA promoter (pPAN)-enhanced green fluorescent protein (EGFP), and pK8.1-monomeric blue fluorescent protein (tagBFP), marking latent, immediate early, and late viral gene expression, respectively. Analysis of RGB-derived K3 and K5 deletion mutants showed that while the K5-mediated downregulation of MHC-I was concomitant with pPAN induction, the reduction of MHC-I surface expression by K3 was evident in cells that were enriched for pPAN-driven EGFP(high) and pK8.1-driven blue fluorescent protein-positive (BFP(+)) populations. These data support the notion that immunoreceptor downregulation occurs by a sequential process wherein K5 is critical during the immediately early phase and K3 plays a significant role during later stages.

IMPORTANCE

Although the roles of K3 and K5 outside the viral genome are well characterized, the function of these proteins in the context of the KSHV life cycle has remained unclear, particularly in the case of K3. This study examined the relative contributions of K3 and K5 to the downregulation of MHC-I during the lytic replication of KSHV. We show that while K5 acts immediately upon entry into the lytic phase, K3-mediated downregulation of MHC-I was evident during later stages of lytic replication. The identification of distinctly timed K3 and K5 activities significantly advances our understanding of KSHV-mediated immune evasion. Crucial to this study was the development of a novel recombinant KSHV, called RGB-BAC16, which facilitated the delineation of stage-specific phenotypes.

Human cytomegalovirus Fcγ binding proteins gp34 and gp68 antagonize Fcγ receptors I, II and III

Human cytomegalovirus (HCMV) establishes lifelong infection with recurrent episodes of virus production and shedding despite the presence of adaptive immunological memory responses including HCMV immune immunoglobulin G (IgG). Very little is known how HCMV evades from humoral and cellular IgG-dependent immune responses, the latter being executed by cells expressing surface receptors for the Fc domain of IgG (FcγRs). Remarkably, HCMV expresses the RL11-encoded gp34 and UL119-118-encoded gp68 type I transmembrane glycoproteins which bind Fcγ with nanomolar affinity. Using a newly developed FcγR activation assay, we tested if the HCMV-encoded Fcγ binding proteins (HCMV FcγRs) interfere with individual host FcγRs. In absence of gp34 or/and gp68, HCMV elicited a much stronger activation of FcγRIIIA/CD16, FcγRIIA/CD32A and FcγRI/CD64 by polyclonal HCMV-immune IgG as compared to wildtype HCMV. gp34 and gp68 co-expression culminates in the late phase of HCMV replication coinciding with the emergence of surface HCMV antigens triggering FcγRIII/CD16 responses by polyclonal HCMV-immune IgG. The gp34- and gp68-dependent inhibition of HCMV immune IgG was fully reproduced when testing the activation of primary human NK cells. Their broad antagonistic function towards FcγRIIIA, FcγRIIA and FcγRI activation was also recapitulated in a gain-of-function approach based on humanized monoclonal antibodies (trastuzumab, rituximab) and isotypes of different IgG subclasses. Surface immune-precipitation showed that both HCMV-encoded Fcγ binding proteins have the capacity to bind trastuzumab antibody-HER2 antigen complexes demonstrating simultaneous linkage of immune IgG with antigen and the HCMV inhibitors on the plasma membrane. Our studies reveal a novel strategy by which viral FcγRs can compete for immune complexes against various Fc receptors on immune cells, dampening their activation and antiviral immunity.

Herpes viruses persist lifelong continuously alternating between latency and virus production and transmission. The latter events occur despite the presence of immune IgG antibodies. IgG acts by neutralization of virions and activation of immune cells bearing one or more surface receptors, called FcγRs, recognizing the constant Fc domain of IgG. Activating FcγRs induce a wide range of immune responses, including antibody dependent cellular cytotoxicity (ADCC) of virus-infected cells by natural killer (NK) cells, cytokine secretion and the uptake of immune complexes to enhance antigen presentation to T cells. We demonstrate that the HCMV glycoproteins RL11/gp34 and UL119-118/gp68 block IgG-mediated activation of FcγRs. A novel reporter cell-based assay was used to test FcγRs individually and assess their relative susceptibility to each antagonist. This approach revealed that gp34 and gp68 block triggering of activating FcγRs, i.e. FcγRI (CD64), FcγRII (CD32A) and FcγRIII (CD16). Co-immunoprecipitation showed the formation of ternary complexes containing IgG, IgG-bound antigen and the viral antagonists on the cell surface. Assigning the redundant abilities of HCMV to hinder IgG effector responses to the viral Fc binding proteins, we discuss gp34 and gp68 as potential culprits which might contribute to the limited efficacy of therapeutic IgG against HCMV.

Know Thyself: NK-Cell Inhibitory Receptors Prompt Self-Tolerance, Education, and Viral Control

Natural killer (NK) cells provide essential protection against viral infections. One of the defining features of this lymphocyte population is the expression of a wide array of variable cell surface stimulatory and inhibitory NK receptors (sNKR and iNKR, respectively). The iNKR are particularly important in terms of NK-cell education. As receptors specific for MHC class I (MHC I) molecules, they are responsible for self-tolerance and adjusting NK-cell reactivity based on the expression level of self-MHC I. The end result of this education is twofold: (1) inhibitory signaling tunes the functional capacity of the NK cell, endowing greater potency with greater education, and (2) education on self allows the NK cell to detect aberrations in MHC I expression, a common occurrence during many viral infections. Many studies have indicated an important role for iNKR and MHC I in disease, making these receptors attractive targets for manipulating NK-cell reactivity in the clinic. A greater understanding of iNKR and their ability to regulate NK cells will provide a basis for future attempts at translating their potential utility into benefits for human health.

Cytomegalovirus m154 hinders CD48 cell-surface expression and promotes viral escape from host natural killer cell control

Receptors of the signalling lymphocyte-activation molecules (SLAM) family are involved in the functional regulation of a variety of immune cells upon engagement through homotypic or heterotypic interactions amongst them. Here we show that murine cytomegalovirus (MCMV) dampens the surface expression of several SLAM receptors during the course of the infection of macrophages. By screening a panel of MCMV deletion mutants, we identified m154 as an immunoevasin that effectively reduces the cell-surface expression of the SLAM family member CD48, a high-affinity ligand for natural killer (NK) and cytotoxic T cell receptor CD244. m154 is a mucin-like protein, expressed with early kinetics, which can be found at the cell surface of the infected cell. During infection, m154 leads to proteolytic degradation of CD48. This viral protein interferes with the NK cell cytotoxicity triggered by MCMV-infected macrophages. In addition, we demonstrate that an MCMV mutant virus lacking m154 expression results in an attenuated phenotype in vivo, which can be substantially restored after NK cell depletion in mice. This is the first description of a viral gene capable of downregulating CD48. Our novel findings define m154 as an important player in MCMV innate immune regulation.

Use of recombinant approaches to construct human cytomegalovirus mutants

To fully understand the function of cytomegalovirus (CMV) genes, it is imperative that they be studied in the context of infection. Therefore, the targeted deletion of individual viral genes and the comparison of loss of function viral mutants to the wild-type virus allow the identification of the relevance and role for a particular gene in the viral replication cycle. Targeted CMV mutagenesis has made huge advances over the past 15 years. The cloning of CMV genomes into (E. coli) as bacterial artificial chromosomes (BAC) allows not only quick and efficient deletion of viral genomic regions, individual genes, or single nucleotide exchanges in the viral genome but also the insertion of heterologous genetic sequences for gain of function approaches. The conceptual advantage of this strategy is that it overcomes the restrictions of recombinant technologies in cell culture systems. Namely, recombination in infected cells occurs only in a few clones, and their selection is not possible if the targeted genes are relevant for virus replication and are not able to compete for growth against the unrecombined viruses. On the other hand, BAC mutagenesis enables the selection for antibiotic resistance in E. coli, allowing a selective growth advantage to the recombined genomes. Here we describe the methods used for the generation of a CMV BAC, targeted mutagenesis of BAC clones, and transfection of human cells with CMV BAC DNA in order to reconstitute the viral infection process.

The p36 isoform of murine cytomegalovirus m152 protein suffices for mediating innate and adaptive immune evasion

The MHC-class I (MHC-I)-like viral (MHC-Iv) m152 gene product of murine cytomegalovirus (mCMV) was the first immune evasion molecule described for a member of the β-subfamily of herpesviruses as a paradigm for analogous functions of human cytomegalovirus proteins. Notably, by interacting with classical MHC-I molecules and with MHC-I-like RAE1 family ligands of the activatory natural killer (NK) cell receptor NKG2D, it inhibits presentation of antigenic peptides to CD8 T cells and the NKG2D-dependent activation of NK cells, respectively, thus simultaneously interfering with adaptive and innate immune recognition of infected cells. Although the m152 gene product exists in differentially glycosylated isoforms whose individual contributions to immune evasion are unknown, it has entered the scientific literature as m152/gp40, based on the quantitatively most prominent isoform but with no functional justification. By construction of a recombinant mCMV in which all three N-glycosylation sites are mutated (N61Q, N208Q, and N241Q), we show here that N-linked glycosylation is not essential for functional interaction of the m152 immune evasion protein with either MHC-I or RAE1. These data add an important functional detail to recent structural analysis of the m152/RAE1γ complex that has revealed N-glycosylations at positions Asn61 and Asn208 of m152 distant from the m152/RAE1γ interface.

Nodular inflammatory foci are sites of T cell priming and control of murine cytomegalovirus infection in the neonatal lung

Neonates, including mice and humans, are highly susceptible to cytomegalovirus (CMV) infection. However, many aspects of neonatal CMV infections such as viral cell tropism, spatio-temporal distribution of the pathogen as well as genesis of antiviral immunity are unknown. With the use of reporter mutants of the murine cytomegalovirus (MCMV) we identified the lung as a primary target of mucosal infection in neonatal mice. Comparative analysis of neonatal and adult mice revealed a delayed control of virus replication in the neonatal lung mucosa explaining the pronounced systemic infection and disease in neonates. This phenomenon was supplemented by a delayed expansion of CD8⁺ T cell clones recognizing the viral protein M45 in neonates. We detected viral infection at the single-cell level and observed myeloid cells forming “nodular inflammatory foci” (NIF) in the neonatal lung. Co-localization of infected cells within NIFs was associated with their disruption and clearance of the infection. By 2-photon microscopy, we characterized how neonatal antigen-presenting cells (APC) interacted with T cells and induced mature adaptive immune responses within such NIFs. We thus define NIFs of the neonatal lung as niches for prolonged MCMV replication and T cell priming but also as sites of infection control.

Neonates are highly susceptible to a number of infections that usually cause disease only in immunocompromised individuals, most likely because of their incompletely developed immune system. Although this phenomenon has been frequently observed, immune responses of neonates remain largely undefined upon infections with viruses. There is lack of knowledge about the spatio-temporal dynamics of host-virus interaction, especially in comparative infection models of neonates and adults. In this study, with the use of virus reporter mutants, we provide elaborate insight into these aspects in the mouse model of CMV infection. We define hallmarks of virus tropism, early cellular immune responses and general infection dynamics, findings that are fundamental to understand neonatal antiviral immunity. Furthermore, we found that neonatal APCs induce T cell responses in nodular inflammatory foci of the lung, a process which was supposed to be restricted to lymphoid organs. However, the MCMV-specific T cell response was qualitatively different in neonates from that in adults, possibly explaining - in part - the higher susceptibility of newborns. These observations expand our understanding of where adaptive immunity can be initiated, highlights the importance of early local cellular immune responses and sheds more light on neonatal antiviral immunity.

Superior induction and maintenance of protective CD8 T cells in mice infected with mouse cytomegalovirus vector expressing RAE-1γ

Due to a unique pattern of CD8 T-cell response induced by cytomegaloviruses (CMVs), live attenuated CMVs are attractive candidates for vaccine vectors for a number of clinically relevant infections and tumors. NKG2D is one of the most important activating NK cell receptors that plays a role in costimulation of CD8 T cells. Here we demonstrate that the expression of CD8 T-cell epitope of Listeria monocytogenes by a recombinant mouse CMV (MCMV) expressing the NKG2D ligand retinoic acid early-inducible protein 1-gamma (RAE-1γ) dramatically enhanced the effectiveness and longevity of epitope-specific CD8 T-cell response and conferred protection against a subsequent challenge infection with Listeria monocytogenes. Unexpectedly, the attenuated growth in vivo of the CMV vector expressing RAE-1γ and its capacity to enhance specific CD8 T-cell response were preserved even in mice lacking NKG2D, implying additional immune function for RAE-1γ beyond engagement of NKG2D. Thus, vectors expressing RAE-1γ represent a promising approach in the development of CD8 T-cell-based vaccines.

A new reporter mouse cytomegalovirus reveals maintained immediate-early gene expression but poor virus replication in cycling liver sinusoidal endothelial cells

Background

The MCMV major immediate early promoter/enhancer (MIEP) is a bidirectional promoter that drives the expression of the three immediate early viral genes, namely ie1, ie2 and ie3. The regulation of their expression is intensively studied, but still incompletely understood.

Methods

We constructed a reporter MCMV, (MCMV-MIEP^r) expressing YFP and tdTomato under the control of the MIEP as proxies of ie1 and ie2, respectively. Moreover, we generated a liver sinusoidal endothelial cell line (LSEC-uniLT) where cycling is dependent on doxycycline. We used these novel tools to study the kinetics of MIEP-driven gene expression in the context of infection and at the single cell level by flow cytometry and by live imaging of proliferating and G₀-arrested cells.

Results

MCMV replicated to higher titers in G₀-arrested LSEC, and cycling cells showed less cytopathic effect or YFP and tdTomato expression at 5 days post infection. In the first 24 h post infection, however, there was no difference in MIEP activity in cycling or G₀-arrested cells, although we could observe different profiles of MIEP gene expression in different cell types, like LSECs, fibroblasts or macrophages. We monitored infected LSEC-uniLT in G₀ by time lapse microscopy over five days and noticed that most cells survived infection for at least 96 h, arguing that quick lysis of infected cells could not account for the spread of the virus. Interestingly, we noticed a strong correlation between the ratio of median YFP and tdTomato expression and length of survival of infected cells.

Conclusion

By means of our newly developed genetic tools, we showed that the expression pattern of MCMV IE1 and IE2 genes differs between macrophages, endothelial cells and fibroblasts. Substantial and cell-cycle independent differences in the ie1 and ie2 transcription could also be observed within individual cells of the same population, and marked ie2 gene expression was associated with longer survival of the infected cells.

Murine cytomegalovirus downregulates interleukin-17 in mice with retrovirus-induced immunosuppression that are susceptible to experimental cytomegalovirus retinitis

Interleukin-17 (IL-17), a pro-inflammatory cytokine produced by CD4+ Th17 cells, has been associated with the pathogenesis of several autoimmune diseases including uveitis. The fate of IL-17 during HIV/AIDS, however, remains unclear, and a possible role for IL-17 in the pathogenesis of AIDS-related diseases has not been investigated. Toward these ends, we performed studies using a well-established animal model of experimental murine cytomegalovirus (MCMV) retinitis that develops in C57/BL6 mice with retrovirus-induced immunosuppression (MAIDS). After establishing baseline levels for IL-17 production in whole splenic cells of healthy mice, we observed a significant increase in IL-17 mRNA levels in whole splenic cells of mice with MAIDS of 4-weeks (MAIDS-4), 8-weeks (MAIDS-8), and 10-weeks (MAIDS-10) duration. In contrast, enriched populations of splenic CD4+ T cells, splenic macrophages, and splenic neutrophils exhibited a reproducible decrease in levels of IL-17 mRNA during MAIDS progression. To explore a possible role for IL-17 during the pathogenesis of MAIDS-related MCMV retinitis, we first demonstrated constitutive IL-17 expression in retinal photoreceptor cells of uninfected eyes of healthy mice. Subsequent studies, however, revealed a significant decrease in intraocular levels of IL-17 mRNA and protein in MCMV-infected eyes of MAIDS-10 mice during retinitis development. That MCMV infection might cause a remarkable downregulation of IL-17 production was supported further by the finding that systemic MCMV infection of healthy, MAIDS-4, or MAIDS-10 mice also significantly decreased IL-17 mRNA production by splenic CD4+ T cells. Based on additional studies using IL-10 -/- mice infected systemically with MCMV and IL-10 -/- mice with MAIDS infected intraocularly with MCMV, we propose that MCMV infection downregulates IL-17 production via stimulation of suppressor of cytokine signaling (SOCS)-3 and interleukin-10.

MHC class I cross-presentation by dendritic cells counteracts viral immune evasion

DCs very potently activate CD8(+) T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8(+) T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8(+) T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8(+) T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α(+) DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α(+) DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

Mechanisms of virus immune evasion lead to development from chronic inflammation to cancer formation associated with human papillomavirus infection

Human papillomavirus (HPV) has developed strategies to escape eradication by innate and adaptive immunity. Immune response evasion has been considered an important aspect of HPV persistence, which is the main contributing factor leading to HPV-related cancers. HPV-induced cancers expressing viral oncogenes E6 and E7 are potentially recognized by the immune system. The major histocompatibility complex (MHC) class I molecules are patrolled by natural killer cells and CD8+ cytotoxic T lymphocytes, respectively. This system of recognition is a main target for the strategies of immune evasion deployed by viruses. The viral immune evasion proteins constitute useful tools to block defined stages of the MHC class I presentation pathway, and in this way HPV avoids the host immune response. The long latency period from initial infection to persistence signifies that HPV evolves mechanisms to escape the immune response. It has now been established that there are oncogenic mechanisms by which E7 binds to and degrades tumor suppressor Rb, while E6 binds to and inactivates tumor suppressor p53. Therefore, interaction of p53 and pRb proteins can give rise to an increased immortalization and genomic instability. Overexpression of NF-κB in cervical and penile cancers suggests that NF-κB activation is a key modulator in driving chronic inflammation to cancer. HPV oncogene-mediated suppression of NF-κB activity contributes to HPV escape from the immune system. This review focuses on the diverse mechanisms of the virus immune evasion with HPV that leads to chronic inflammation and cancer.

Murine cytomegalovirus immune evasion proteins operative in the MHC class I pathway of antigen processing and presentation: state of knowledge, revisions, and questions

Medical interest in cytomegalovirus (CMV) is based on lifelong neurological sequelae, such as sensorineural hearing loss and mental retardation, resulting from congenital infection of the fetus in utero, as well as on CMV disease with multiple organ manifestations and graft loss in recipients of hematopoietic cell transplantation or solid organ transplantation. CMV infection of transplantation recipients occurs consequent to reactivation of virus harbored in a latent state in the transplanted donor cells and tissues, or in the tissues of the transplantation recipient herself or himself. Hence, CMV infection is a paradigm for a viral infection that causes disease primarily in the immunocompromised host, while infection of the immunocompetent host is associated with only mild and nonspecific symptoms so that it usually goes unnoticed. Thus, CMV is kept under strict immune surveillance. These medical facts are in apparent conflict with the notion that CMVs in general, human CMV as well as animal CMVs, are masters of 'immune evasion', which during virus-host co-speciation have convergently evolved sophisticated mechanisms to avoid their recognition by innate and adaptive immunity of their respective host species, with viral genes apparently dedicated to serve just this purpose (Reddehase in Nat Rev Immunol 2:831-844, 2002). With focus on viral interference with antigen presentation to CD8 T cells in the preclinical model of murine CMV infection, we try here to shed some more light on the in vivo balance between host immune surveillance of CMV infection and viral 'immune evasion' strategies.

Antigen presentation under the influence of 'immune evasion' proteins and its modulation by interferon-gamma: implications for immunotherapy of cytomegalovirus infection with antiviral CD8 T cells

Cytomegalovirus (CMV) disease with multiple organ manifestations is the most feared viral complication limiting the success of hematopoietic cell transplantation as a therapy of hematopoietic malignancies. A timely endogenous reconstitution of CD8 T cells controls CMV infection, and adoptive transfer of antiviral CD8 T cells is a therapeutic option to prevent CMV disease by bridging the gap between an early CMV reactivation and delayed endogenous reconstitution of protective immunity. Preclinical research in murine models has provided 'proof of concept' for CD8 T-cell therapy of CMV disease. Protection by CD8 T cells appears to be in conflict with the finding that CMVs encode proteins that inhibit antigen presentation to CD8 T cells by interfering with the constitutive trafficking of peptide-loaded MHC class I molecules (pMHC-I complexes) to the cell surface. Here, we have systematically explored antigen presentation in the presence of the three currently noted immune evasion proteins of murine CMV in all possible combinations and its modulation by pre-treatment of cells with interferon-gamma (IFN-γ). The data reveal improvement in antigen processing by pre-treatment with IFN-γ can almost overrule the inhibitory function of immune evasion molecules in terms of pMHC-I expression levels capable of triggering most of the specific CD8 T cells, though the intensity of stimulation did not retrieve their full functional capacity. Notably, an in vivo conditioning of host tissue cells with IFN-γ in adoptive cell transfer recipients constitutively overexpressing IFN-γ (B6-SAP-IFN-γ mice) enhanced the antiviral efficiency of CD8 T cells in this transgenic cytoimmunotherapy model.

A novel bat herpesvirus encodes homologues of major histocompatibility complex classes I and II, C-type lectin, and a unique family of immune-related genes

Herpesviruses or herpesviral sequences have been identified in various bat species. Here, we report the isolation, cell tropism, and complete genome sequence of a novel betaherpesvirus from the bat Miniopterus schreibersii (MsHV). In primary cell culture, MsHV causes cytopathic effects (CPE) and reaches peak virus production 2 weeks after infection. MsHV was found to infect and replicate less efficiently in a feline kidney cell, CRFK, and failed to replicate in 13 other cell lines tested. Sequencing of the MsHV genome using the 454 system, with a 224-fold coverage, revealed a genome size of 222,870 bp. The genome was extensively analyzed in comparison to those of related viruses. Of the 190 predicted open reading frames (ORFs), 40 were identified as herpesvirus core genes. Among 93 proteins with identifiable homologues in tree shrew herpesvirus (THV), human cytomegalovirus (HCMV), or rat cytomegalovirus (RCMV), most had highest sequence identities with THV counterparts. However, the MsHV genome organization is colinear with that of RCMV rather than that of THV. The following unique features were discovered in the MsHV genome. One predicted protein, B125, is similar to human herpesvirus 6 (HHV-6) U94, a homologue of the parvovirus Rep protein. For the unique ORFs, 7 are predicted to encode major histocompatibility complex (MHC)-related proteins, 2 to encode MHC class I homologues, and 3 to encode MHC class II homologues; 4 encode the homologues of C-type lectin- or natural killer cell lectin-like receptors;, and the products of a unique gene family, the b149 family, of 16 members, have no significant sequence identity with known proteins but exhibit immunoglobulin-like beta-sandwich domains revealed by three-dimensional (3D) structural prediction. To our knowledge, MsHV is the first virus genome known to encode MHC class II homologues.

Human cytomegalovirus US3 modulates destruction of MHC class I molecules

Human cytomegalovirus (HCMV), a member of the Herpesviridae family, is proficient at establishing lifelong persistence within the host in part due to immune modulating genes that limit immune recognition. HCMV encodes at least five glycoproteins within its unique short (US) genomic region that interfere with MHC class I antigen presentation, thus hindering viral clearance by cytotoxic T lymphocytes (CTL). Specifically, US3 retains class I within the endoplasmic reticulum (ER), while US2 and US11 induce class I heavy chain destruction. A cooperative effect on class I down-regulation during stable expression of HCMV US2 and US3 has been established. To address the impact of US3 on US11-mediated MHC class I down-regulation, the fate of class I molecules was examined in US3/US11-expressing cells and virus infection studies. Co-expression of US3 and US11 resulted in a decrease of surface expression of class I molecules. However, the class I molecules in US3/US11 cells were mostly retained in the ER with an attenuated rate of proteasome destruction. Analysis of class I levels from virus-infected cells using HCMV variants either expressing US3 or US11 revealed efficient surface class I down-regulation upon expression of both viral proteins. Cells infected with both US3 and US11 expressing viruses demonstrate enhanced retention of MHC class I complexes within the ER. Collectively, the data suggests a paradigm where HCMV-induced surface class I down-regulation occurs by diverse mechanisms dependent on the expression of specific US genes. These results validate the commitment of HCMV to limiting the surface expression of class I levels during infection.

The human cytomegalovirus-specific UL1 gene encodes a late-phase glycoprotein incorporated in the virion envelope

We have investigated the previously uncharacterized human cytomegalovirus (HCMV) UL1 open reading frame (ORF), a member of the rapidly evolving HCMV RL11 family. UL1 is HCMV specific; the absence of UL1 in chimpanzee cytomegalovirus (CCMV) and sequence analysis studies suggest that UL1 may have originated by the duplication of an ancestor gene from the RL11-TRL cluster (TRL11, TRL12, and TRL13). Sequence similarity searches against human immunoglobulin (Ig)-containing proteins revealed that HCMV pUL1 shows significant similarity to the cellular carcinoembryonic antigen-related (CEA) protein family N-terminal Ig domain, which is responsible for CEA ligand recognition. Northern blot analysis revealed that UL1 is transcribed during the late phase of the viral replication cycle in both fibroblast-adapted and endotheliotropic strains of HCMV. We characterized the protein encoded by hemagglutinin (HA)-tagged UL1 in the AD169-derived HB5 background. UL1 is expressed as a 224-amino-acid type I transmembrane glycoprotein which becomes detectable at 48 h postinfection. In infected human fibroblasts, pUL1 colocalized at the cytoplasmic site of virion assembly and secondary envelopment together with TGN-46, a marker for the trans-Golgi network, and viral structural proteins, including the envelope glycoprotein gB and the tegument phosphoprotein pp28. Furthermore, analyses of highly purified AD169 UL1-HA epitope-tagged virions revealed that pUL1 is a novel constituent of the HCMV envelope. Importantly, the deletion of UL1 in HCMV TB40/E resulted in reduced growth in a cell type-specific manner, suggesting that pUL1 may be implicated in regulating HCMV cell tropism.