The luminal part of the murine cytomegalovirus glycoprotein gp40 catalyzes the retention of MHC class I molecules

Targeting MHC-I inhibitory pathways for cancer immunotherapy

The MHC-I antigen presentation (AP) pathway is key to shaping mammalian CD8+ T cell immunity, with its aberrant expression closely linked to low tumor immunogenicity and immunotherapy resistance. While significant attention has been given to genetic mutations and downregulation of positive regulators that are essential for MHC-I AP, there is a growing interest in understanding how tumors actively evade MHC-I expression and/or AP through the induction of MHC-I inhibitory pathways. This emerging field of study may offer more viable therapeutic targets for future cancer immunotherapy. Here, we explore potential mechanisms by which cancer cells evade MHC-I AP and function and propose therapeutic strategies that might target these MHC-I inhibitors to restore impaired T cell immunity within the tumor microenvironment (TME).

The Quest for Immunity: Exploring Human Herpesviruses as Vaccine Vectors

Herpesviruses are large DNA viruses that have long been used as powerful gene therapy tools. In recent years, the ability of herpesviruses to stimulate both innate and adaptive immune responses has led to their transition to various applications as vaccine vectors. This vaccinology branch is growing at an unprecedented and accelerated rate. To date, human herpesvirus-based vectors have been used in vaccines to combat a variety of infectious agents, including the Ebola virus, foot and mouth disease virus, and human immunodeficiency viruses. Additionally, these vectors are being tested as potential vaccines for cancer-associated antigens. Thanks to advances in recombinant DNA technology, immunology, and genomics, numerous steps in vaccine development have been greatly improved. A better understanding of herpesvirus biology and the interactions between these viruses and the host cells will undoubtedly foster the use of herpesvirus-based vaccine vectors in clinical settings. To overcome the existing drawbacks of these vectors, ongoing research is needed to further advance our knowledge of herpesvirus biology and to develop safer and more effective vaccine vectors. Advanced molecular virology and cell biology techniques must be used to better understand the mechanisms by which herpesviruses manipulate host cells and how viral gene expression is regulated during infection. In this review, we cover the underlying molecular structure of herpesviruses and the strategies used to engineer their genomes to optimize capacity and efficacy as vaccine vectors. Also, we assess the available data on the successful application of herpesvirus-based vaccines for combating diseases such as viral infections and the potential drawbacks and alternative approaches to surmount them.

The show and tell of cross-presentation

Cross-presentation is the culmination of complex subcellular processes that allow the processing of exogenous proteins and the presentation of resultant peptides on major histocompatibility class I (MHC-I) molecules to CD8 T cells. Dendritic cells (DCs) are a cell type that uniquely specializes in cross-presentation, mainly in the context of viral or non-viral infection and cancer. DCs have an extensive network of endovesicular pathways that orchestrate the biogenesis of an ideal cross-presentation compartment where processed antigen, MHC-I molecules, and the MHC-I peptide loading machinery all meet. As a central conveyor of information to CD8 T cells, cross-presentation allows cross-priming of T cells which carry out robust adaptive immune responses for tumor and viral clearance. Cross-presentation can be canonical or noncanonical depending on the functional status of the transporter associated with antigen processing (TAP), which in turn influences the vesicular route of MHC-I delivery to internalized antigen and the cross-presented repertoire of peptides. Because TAP is a central node in MHC-I presentation, it is targeted by immune evasive viruses and cancers. Thus, understanding the differences between canonical and noncanonical cross-presentation may inform new therapeutic avenues against cancer and infectious disease. Defects in cross-presentation on a cellular and genetic level lead to immune-related disease progression, recurrent infection, and cancer progression. In this chapter, we review the process of cross-presentation beginning with the DC subsets that conduct cross-presentation, the signals that regulate cross-presentation, the vesicular trafficking pathways that orchestrate cross-presentation, the modes of cross-presentation, and ending with disease contexts where cross-presentation plays a role.

Exploring the Potential of Cytomegalovirus-Based Vectors: A Review

Viral vectors have emerged as powerful tools for delivering and expressing foreign genes, playing a pivotal role in gene therapy. Among these vectors, cytomegalovirus (CMV) stands out as a promising viral vector due to its distinctive attributes including large packaging capacity, ability to achieve superinfection, broad host range, capacity to induce CD8+ T cell responses, lack of integration into the host genome, and other qualities that make it an appealing vector candidate. Engineered attenuated CMV strains such as Towne and AD169 that have a ~15 kb genomic DNA deletion caused by virus passage guarantee human safety. CMV's large genome enables the efficient incorporation of substantial foreign genes as demonstrated by CMV vector-based therapies for SIV, tuberculosis, cancer, malaria, aging, COVID-19, and more. CMV is capable of reinfecting hosts regardless of prior infection or immunity, making it highly suitable for multiple vector administrations. In addition to its broad cellular tropism and sustained high-level gene expression, CMV triggers robust, virus-specific CD8+ T cell responses, offering a significant advantage as a vaccine vector. To date, successful development and testing of murine CMV (MCMV) and rhesus CMV (RhCMV) vectors in animal models have demonstrated the efficacy of CMV-based vectors. These investigations have explored the potential of CMV vectors for vaccines against HIV, cancer, tuberculosis, malaria, and other infectious pathogens, as well as for other gene therapy applications. Moreover, the generation of single-cycle replication CMV vectors, produced by deleting essential genes, ensures robust safety in an immunocompromised population. The results of these studies emphasize CMV's effectiveness as a gene delivery vehicle and shed light on the future applications of a CMV vector. While challenges such as production complexities and storage limitations need to be addressed, ongoing efforts to bridge the gap between animal models and human translation continue to fuel the optimism surrounding CMV-based vectors. This review will outline the properties of CMV vectors and discuss their future applications as well as possible limitations.

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.

The murine cytomegalovirus immunoevasin gp40/m152 inhibits NKG2D receptor RAE-1γ by intracellular retention and cell surface masking

NKG2D (also known as KLRK1) is a crucial natural killer (NK) cell-activating receptor, and the murine cytomegalovirus (MCMV) employs multiple immunoevasins to avoid NKG2D-mediated activation. One of the MCMV immunoevasins, gp40 (m152), downregulates the cell surface NKG2D ligand RAE-1γ (also known as Raet1c) thus limiting NK cell activation. This study establishes the molecular mechanism by which gp40 retains RAE-1γ in the secretory pathway. Using flow cytometry and pulse-chase analysis, we demonstrate that gp40 retains RAE-1γ in the early secretory pathway, and that this effect depends on the binding of gp40 to a host protein, TMED10, a member of the p24 protein family. We also show that the TMED10-based retention mechanism can be saturated, and that gp40 has a backup mechanism as it masks RAE-1γ on the cell surface, blocking the interaction with the NKG2D receptor and thus NK cell activation.

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.

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.

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.

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.

Present Yourself! By MHC Class I and MHC Class II Molecules

Since the discovery of MHC molecules, it has taken 40 years to arrive at a coherent picture of how MHC class I and MHC class II molecules really work. This is a story of the proteases and MHC-like chaperones that support the MHC class I and II molecules in presenting peptides to the immune system. We now understand that the MHC system shapes both the repertoire of presented peptides and the subsequent T cell response, with important implications ranging from transplant rejection to tumor immunotherapies. Here we present an illustrated review of the ins and outs of MHC class I and MHC class II antigen presentation.

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.

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.

The murine cytomegalovirus immunoevasin gp40 binds MHC class I molecules to retain them in the early secretory pathway

In the presence of the murine cytomegalovirus (mCMV) gp40 (m152) protein, murine major histocompatibility complex (MHC) class I molecules do not reach the cell surface but are retained in an early compartment of the secretory pathway. We find that gp40 does not impair folding or high-affinity peptide binding of class I molecules but binds to them to retain them in the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment (ERGIC), and the cis-Golgi, most likely by retrieval from the cis-Golgi to the ER. We identify a sequence in gp40 that is required for both its own retention in the early secretory pathway and for that of class I molecules.

A novel murine cytomegalovirus vaccine vector protects against Mycobacterium tuberculosis

Tuberculosis remains a global health problem so that a more effective vaccine than bacillus Calmette–Guérin is urgently needed. Cytomegaloviruses persist lifelong in vivo and induce powerful immune and increasing (“inflationary”) responses, making them attractive vaccine vectors. We have used an m1–m16-deleted recombinant murine CMV (MCMV) expressing Mycobacterium tuberculosis Ag 85A to show that infection of mice with this recombinant significantly reduces the mycobacterial load after challenge with M. tuberculosis, whereas control empty virus has a lesser effect. Both viruses induce immune responses to H-2^d–restricted epitopes of MCMV pp89 and M18 Ags characteristic of infection with other MCMVs. A low frequency of 85A-specific memory cells could be revealed by in vivo or in vitro boosting or after challenge with M. tuberculosis. Kinetic analysis of M. tuberculosis growth in the lungs of CMV-infected mice shows early inhibition of M. tuberculosis growth abolished by treatment with NK-depleting anti–asialo ganglio-N-tetraosylceramide Ab. Microarray analysis of the lungs of naive and CMV-infected mice shows increased IL-21 mRNA in infected mice, whereas in vitro NK assays indicate increased levels of NK activity. These data indicate that activation of NK cells by MCMV provides early nonspecific protection against M. tuberculosis, potentiated by a weak 85A-specific T cell response, and they reinforce the view that the innate immune system plays an important role in both natural and vaccine-induced protection against M. tuberculosis.

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.

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.

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.

Manipulation of NKG2D ligands by cytomegaloviruses: impact on innate and adaptive immune response

NKG2D is a potent activating receptor expressed on NK cells, NKT cells, γδ T cells, and CD8 T cells. NKG2D recognizes cell surface molecules structurally related to MHC class I proteins induced by infection or other type of cellular stress. The engagement of NKG2D leads to NK cell cytotoxicity and cytokine secretion or to a co-stimulation of CD8 T cells. Both human and mouse cytomegalovirus (CMV) have evolved numerous mechanisms to evade NKG2D-mediated immune response. This review describes the mechanisms used by CMV to inhibit NKG2D ligand expression and the recent advances in exploiting the NKG2D recognition pathway for mounting efficient and long-lasting immune response.

All is fair in virus-host interactions: NK cells and cytomegalovirus

The infection of mice with mouse cytomegalovirus (MCMV) as a model of human cytomegalovirus (HCMV) infection has been particularly informative in elucidating the role of innate and adaptive immune response mechanisms during infection. Millions of years of co-evolution between cytomegaloviruses (CMV) and their hosts has resulted in numerous attempts to overwhelm each other. CMVs devote many genes to modulating the host natural killer (NK) cell response and NK cells employ many strategies to cope with CMV infection. While focusing on these attack-counterattack measures, this review will discuss several novel mechanisms of immune evasion by MCMV, the role of Ly49 receptors in mediating resistance to MCMV, and the impact of the initial NK cell response on the shaping of adaptive immunity.

Temporal profiling of the coding and noncoding murine cytomegalovirus transcriptomes

The global transcriptional program of murine cytomegalovirus (MCMV), involving coding, noncoding, and antisense transcription, remains unknown. Here we report an oligonucleotide custom microarray platform capable of measuring both coding and noncoding transcription on a genome-wide scale. By profiling MCMV wild-type and immediate-early mutant strains in fibroblasts, we found rapid activation of the transcriptome by 6.5 h postinfection, with absolute dependency on ie3, but not ie1 or ie2, for genomic programming of viral gene expression. Evidence is also presented to show, for the first time, genome-wide noncoding and bidirectional transcription at late stages of MCMV infection.

Direct interaction of the mouse cytomegalovirus m152/gp40 immunoevasin with RAE-1 isoforms

Cytomegaloviruses (CMVs) are ubiquitous species-specific viruses that establish acute, persistent, and latent infections. Both human and mouse CMVs encode proteins that inhibit the activation of natural killer (NK) cells by downregulating cellular ligands for the NK cell activating receptor, NKG2D. The MCMV glycoprotein m152/gp40 downregulates the surface expression of RAE-1 to prevent NK cell control in vivo. So far, it is unclear if there is a direct interaction between m152 and RAE-1 and, if so, if m152 interacts differentially with the five identified RAE-1 isoforms, which are expressed as two groups in MCMV-susceptible or -resistant mouse strains. To address these questions, we expressed and purified the extracellular domains of RAE-1 and m152 and performed size exclusion chromatography binding assays as well as analytical ultracentrifugation and isothermal titration calorimetry to characterize these interactions quantitatively. We further evaluated the role of full-length and naturally glycosylated m152 and RAE-1 in cotransfected HEK293T cells. Our results confirmed that m152 binds RAE-1 directly, relatively tightly (K(d) < 5 microM), and with 1:1 stoichiometry. The binding is quantitatively different depending on particular RAE-1 isoforms, corresponding to the susceptibility to downregulation by m152. A PLWY motif found in RAE-1beta, although contributing to its affinity for m152, does not influence the affinity of RAE-1gamma or RAE-1delta, suggesting that other differences contribute to the RAE-1-m152 interaction. Molecular modeling of the different RAE-1 isoforms suggests a potential site for the m152 interaction.

Structure and function of murine cytomegalovirus MHC-I-like molecules: how the virus turned the host defense to its advantage

The mouse cytomegalovirus (CMV), a beta-herpesvirus, exploits its large (~230 kb) double-stranded DNA genome for both essential and non-essential functions. Among the non-essential functions are those that offer the virus a selective advantage in eluding both the innate and adaptive immune responses of the host. Several non-essential genes of MCMV are thought to encode MHC-I-like genes and to function as immunoevasins. To understand further the evolution and function of these viral MHC-I (MHC-Iv) molecules, X-ray structures of several of them have been determined, not only confirming the overall MHC-I-like structure, but also elucidating features unique to this family. Future efforts promise to clarify the nature of the molecular ligands of these molecules, their evolution in the context of the adapting immune response of the murine host, and by analogy the evolution of the host response to human CMV as well.

Differential susceptibility of RAE-1 isoforms to mouse cytomegalovirus

The NKG2D receptor is one of the most potent activating natural killer cell receptors involved in antiviral responses. The mouse NKG2D ligands MULT-1, RAE-1, and H60 are regulated by murine cytomegalovirus (MCMV) proteins m145, m152, and m155, respectively. In addition, the m138 protein interferes with the expression of both MULT-1 and H60. We show here that one of five RAE-1 isoforms, RAE-1delta, is resistant to downregulation by MCMV and that this escape has functional importance in vivo. Although m152 retained newly synthesized RAE-1delta and RAE-1gamma in the endoplasmic reticulum, no viral regulator was able to affect the mature RAE-1delta form which remains expressed on the surfaces of infected cells. This differential susceptibility to downregulation by MCMV is not a consequence of faster maturation of RAE-1delta compared to RAE-1gamma but rather an intrinsic property of the mature surface-resident protein. This difference can be attributed to the absence of a PLWY motif from RAE-1delta. Altogether, these findings provide evidence for a novel mechanism of host escape from viral immunoevasion of NKG2D-dependent control.

Rat cytomegalovirus infection depletes MHC II in bone marrow derived dendritic cells

While cytomegalovirus (CMV) infects and replicates in a multitude of cell types, the ability of the virus to replicate in antigen presenting cells (APCs) is believed to play a critical role in the viral dissemination and latency. CMV infection of APCs and manipulation of their function are important areas of investigation. CMV down regulation of MHC II is reportedly mediated by the HCMV proteins US2, US3, UL83, UL111a (vIL10) or through the induction of cellular IL10. In this study, we demonstrate that rat CMV (RCMV) significantly reduces MHC II expression neither by mechanisms that do not involve orthologues of the known HCMV genes nor by an increase in cellular IL10. Rat bone marrow derived dendritic cells (BMDC) were highly susceptible to infection with RCMV and a recombinant RCMV expressing eGFP. RCMV infection of BMDCs depleted both surface and intracellular MHC II to nearly undetectable levels as well as reduced surface expression of MHC I. The effect on MHC II only occurred in the infected GFP positive cells and is mediated by an immediate early or early viral gene product. Furthermore, treatment of uninfected immature DCs with virus-free conditioned supernatants from infected cells failed to down regulate MHC II. RCMV depletion of MHC II was sensitive to treatment with lysosomal inhibitors but not proteasomal inhibitors suggesting that the mechanism of RCMV-mediated down regulation of MHC II occurs through endocytic degradation. Since RCMV does not encode homologues of US2, US3, UL83 or UL111a, these data indicate a novel mechanism for RCMV depletion of MHC II.

Feline herpesvirus-1 down-regulates MHC class I expression in an homologous cell system

Cytotoxic T lymphocytes (CTLs) are an essential component of the immune defense against many virus infections. CTLs recognize viral peptides in the context of the major histocompatibility complex (MHC) class I molecules on the surface of infected cells. Many viruses have evolved mechanisms to interfere with MHC class I expression as a means of evading the host immune response. In the present research we have studied the effect of in vitro Feline Herpesvirus 1 (FeHV-1) infection on MHC class I expression. The results of this study demonstrate that FeHV-1 down regulates surface expression of MHC class I molecules on infected cells, presumably to evade cytotoxic T-cell recognition and, perhaps, attenuate induction of immunity. Sensitivity to UV irradiation and insensitivity to a viral DNA synthesis inhibitor, like phosphonacetic acid, revealed that immediate early or early viral gene(s) are responsible. Use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible.

Targeting the retinoblastoma protein by MC007L, gene product of the molluscum contagiosum virus: detection of a novel virus-cell interaction by a member of the poxviruses

The human pathogenic poxvirus molluscum contagiosum virus (MCV) is the causative agent of benign neoplasm, with worldwide incidence, characterized by intraepidermal hyperplasia and hypertrophy of cells. Here, we present evidence that the MC007L protein of MCV targets retinoblastoma protein (pRb) via a conserved LxCxE motif, which is present in many viral oncoproteins. The deregulation of the pRb pathway plays a central role in tumor pathogenesis. The oncoproteins of small DNA viruses contain amino acid sequences that bind to and inactivate pRb. Isolated expression of these oncoproteins induces apoptosis, cell proliferation, and cellular transformation. The MC007L gene displays no homology to other genes within the poxvirus family. The protein anchors into the outer mitochondrial membrane via an N-terminal mitochondrial targeting sequence. Through the LxCxE motifs, MC007L induces a cytosolic sequestration of pRb at mitochondrial membranes, leading to the inactivation of the protein by mislocalization. MC007L precipitates the endogenous pRb/E2F-1 complex. Moreover, MC007L is able to cooperate to transform primary rat kidney cells. The interaction between MC007L and pRb provides a novel mechanism by which a virus can perturb the cell cycle.

Immune modulation by the human cytomegalovirus-encoded molecule UL18, a mystery yet to be solved

Human cytomegalovirus infects human populations at a high frequency worldwide. During the long coevolution of virus and host, a fine balance has developed between viral immune evasion strategies and defense mechanisms of the immune system. Human cytomegalovirus encodes multiple proteins involved in the evasion of immune recognition, among them UL18, a MHC class I homologue. Despite almost 20 years of research and the discovery of a broadly expressed inhibitory receptor for this protein, its function in immune modulation is not clear yet. Recent data suggest that besides inhibitory effects on various immune cells, UL18 may also act as an activating component during CMV infection. In this review, we provide an overview of the biology of UL18 and discuss several attempts to shed light on its function.

Quantitative membrane proteomics reveals new cellular targets of viral immune modulators

Immunomodulators of pathogens frequently affect multiple cellular targets, thus preventing recognition by different immune cells. For instance, the K5 modulator of immune recognition (MIR2) from Kaposi sarcoma-associated herpesvirus prevents activation of cytotoxic T cells, natural killer cells, and natural killer T cells by downregulating major histocompatibility complex (MHC) class I molecules, the MHC-like molecule CD1, the cell adhesion molecules ICAM-1 and PECAM, and the co-stimulatory molecule B7.2. K5 belongs to a family of viral- and cellular-membrane-spanning RING ubiquitin ligases. While a limited number of transmembrane proteins have been shown to be targeted for degradation by this family, it is unknown whether additional targets exist. We now describe a quantitative proteomics approach to identify novel targets of this protein family. Using stable isotope labeling by amino acids, we compared the proteome of plasma, Golgi, and endoplasmic reticulum membranes in the presence and absence of K5. Mass spectrometric protein identification revealed four proteins that were consistently underrepresented in the plasma membrane of K5 expression cells: MHC I (as expected), bone marrow stromal antigen 2 (BST-2, CD316), activated leukocyte cell adhesion molecule (ALCAM, CD166) and Syntaxin-4. Downregulation of each of these proteins was independently confirmed by immunoblotting with specific antibodies. We further demonstrate that ALCAM is a bona fide target of both K5 and the myxomavirus homolog M153R. Upon exiting the endoplasmic reticulum, ALCAM is ubiquitinated in the presence of wild-type, but not RING-deficient or acidic motif-deficient, K5, and is targeted for lysosomal degradation via the multivesicular body pathway. Since ALCAM is the ligand for CD6, a member of the immunological synapse of T cells, its removal by viral immune modulators implies a role for CD6 in the recognition of pathogens by T cells. The unbiased global proteome analysis therefore revealed novel immunomodulatory functions of pathogen proteins.

The interplay between host and viral factors in shaping the outcome of cytomegalovirus infection

Cytomegalovirus (CMV) remains a major human pathogen causing significant morbidity and mortality in immunosuppressed or immunoimmature individuals. Although significant advances have been made in dissecting out certain features of the host response to human CMV (HCMV) infection, the strict species specificity of CMVs means that most aspects of antiviral immunity are best assessed in animal models. The mouse model of murine CMV (MCMV) infection is an important tool for analysis of in vivo features of host-virus interactions and responses to antiviral drugs that are difficult to assess in humans. Important studies of the contribution of host resistance genes to infection outcome, interplays between innate and adaptive host immune responses, the contribution of virus immune evasion genes and genetic variation in these genes to the establishment of persistence and in vivo studies of resistance to antiviral drugs have benefited from the well-developed MCMV model. In this review, we discuss recent advances in the immunobiology of host-CMV interactions that provide intriguing insights into the complex interplay between host and virus that ultimately facilitates viral persistence. We also discuss recent studies of genetic responses to antiviral therapy, particularly changes in DNA polymerase and protein kinase genes of MCMV and HCMV.

Cytomegalovirus encodes a positive regulator of antigen presentation

Murine cytomegalovirus encodes three regulators of antigen presentation to antiviral CD8 T cells. According to current paradigms, all three regulators are committed to the inhibition of the presentation of antigenic peptides. Whereas m152/gp40 catalyzes the retention of peptide-loaded major histocompatibility complex (MHC) class I molecules in a cis-Golgi compartment, m06/gp48 binds stably to class I molecules and directs them into the cellular cargo-sorting pathway of lysosomal degradation. Regulator m04/gp34 also binds stably to class I molecules, but unlike m152 and m06, it does not downmodulate MHC class I cell surface expression. It has entered the literature as a direct inhibitor of T-cell recognition of the MHC-peptide complex at the cell surface. In this work, we have studied the presentation of antigenic viral peptides in cells infected with a comprehensive set of mutant viruses expressing the three regulators separately as well as in all possible combinations. The results redefine m04 as a positive regulator dedicated to the facilitation of antigen presentation. When expressed alone, it did not inhibit T-cell recognition, and when expressed in the presence of m152, it restored antigen presentation by antagonizing the inhibitory function of m152. Its intrinsic positive function, however, was antagonized and even slightly overcompensated for by the negative regulator m06. In an adoptive cell transfer model, the opposing forces of the three regulators were found to govern immune surveillance in the infected host. While negative regulators, also known as immunoevasins, are common, the existence of a positive regulator is without precedent and indicates an intriguing genetic potential of this virus to influence antigen presentation.

Human cytomegalovirus protein pp71 disrupts major histocompatibility complex class I cell surface expression

The human cytomegalovirus tegument protein pp71 is the product of the UL82 gene. Roles for pp71 in stimulating gene transcription, increasing infectivity of viral DNA, and the degradation of retinoblastoma family proteins have been described. Here we report a novel function for pp71 in limiting accumulation of cell surface major histocompatibility complex (MHC) class I complexes. MHC molecules were analyzed in glioblastoma cells exposed to a replication-defective adenovirus expressing UL82 (Adpp71) or after transient transfection of the UL82 gene. Accumulation of cell surface MHC class I levels diminished in a specific and dose-dependent manner after exposure to Adpp71 but not after exposure to an adenovirus expressing beta-galactosidase (Adbeta gal). UL82 expression did not interfere with accumulation of either MHC class I heavy-chain transcript or protein, nor did UL82 expression correlate with markers of apoptosis. Rather, UL82 expression correlated with an increased proportion of MHC class I molecules exhibiting sensitivity to endoglycosidase H treatment. Finally, we show that, in cells infected with recombinant virus strain missing all of the unique short region MHC class I evasion genes, disruption of UL82 expression by short, interfering RNAs led to increased accumulation of cell surface MHC class I complexes. These findings support a novel role for HCMV pp71 in disruption of the MHC class I antigen presentation pathway.

Molecular mechanisms of HLA class I antigen abnormalities following viral infection and transformation

In humans as in other animal species, CD8+ cytotoxic T lymphocytes (CTLs) play an important if not the major role in controlling virus-infected and malignant cell growth. The interactions between CD8+ T cells and target cells are mediated by human leukocyte antigen (HLA) class I antigens loaded with viral and tumor antigen-derived peptides along with costimulatory receptor/ligand stimuli. Thus, to escape from CD8+ T-cell recognition and destruction, viruses and tumor cells have developed strategies to inhibit the expression and/or function of HLA class I antigens. In contrast, cells with downregulated MHC class I surface expression can be recognized by NK cells, although NK cell-mediated lysis could be abrogated by the expression of inhibiting NK cell receptors. This review discusses the molecular mechanisms utilized by viruses to inhibit the formation, transport and/or expression of HLA class I antigen/peptide complexes on the cell surface. The knowledge about viral interference with MHC class I antigen presentation is not only crucial to understand the pathogenesis of viral diseases, but contributes also to the design of novel strategies to counteract the escape mechanisms utilized by viruses. These investigations may eventually lead to the development of effective immunotherapies to control viral infections and virus-associated malignant diseases.

Rat Cytomegalovirus Induces a Temporal Downregulation of Major Histocompatibility Complex Class I Cell Surface Expression

Herpesviruses are known to influence expression of major histocompatibility complex (MHC) class I molecules on the surface of infected cells using a variety of mechanisms. Downregulation of MHC class I expression prohibits detection and elimination of infected cells by cytotoxic T lymphocytes. To investigate the effect of rat cytomegalovirus (RCMV) infection on MHC class I expression, we infected immortalized and primary rat fibroblasts with RCMV and monitored surface expression of MHC class I molecules at various time-points postinfection. These experiments revealed a downregulation of MHC class I surface expression by RCMV, a phenomenon that has also been reported for human and murine CMV. However, in contrast to the other cytomegaloviruses, RCMV causes only a temporal downregulation of MHC class I, with a maximal decrease at 12 h postinfection. Unlike murine and human CMV, RCMV does not induce proteolytic degradation of MHC class I molecules. In RCMV-infected cells, the MHC class I molecules are stable, but their exit from the ER is delayed.

Viral modulation of antigen presentation: manipulation of cellular targets in the ER and beyond

Viruses that establish long-term infections in their hosts have evolved a number of methods to interfere with the activities of the innate and adaptive immune systems. Control of viral infections is achieved in part through the action of cytotoxic T lymphocytes (CTLs) that recognize cytosolically derived antigenic peptides in the context of class I major histocompatibility complex (MHC) molecules. Viral replication within host cells produces abundant proteinaceous fodder for proteasomal digestion and display by class I MHC products. Tactics that disrupt antigen-presentation pathways and prevent the display of peptides to CD8(+) CTLs have been favored during the course of host-virus co-evolution. Viral immunoevasins exploit diverse cellular processes to interfere with host antiviral functions. The study of such viral factors has uncovered novel host proteins that assist these viral factors in their task and that themselves perform important cellular functions. Here, we focus on viral immunoevasins that, together with their cellular targets, interfere with antigen-presentation pathways. In particular, we emphasize the intersection of the cellular quality-control machinery in the endoplasmic reticulum with the herpesvirus proteins that have co-opted it.

Inhibition of MHC class I is a virulence factor in herpes simplex virus infection of mice

Herpes simplex virus (HSV) has a number of genes devoted to immune evasion. One such gene, ICP47, binds to the transporter associated with antigen presentation (TAP) 1/2 thereby preventing transport of viral peptides into the endoplasmic reticulum, loading of peptides onto nascent major histocompatibility complex (MHC) class I molecules, and presentation of peptides to CD8 T cells. However, ICP47 binds poorly to murine TAP1/2 and so inhibits antigen presentation by MHC class I in mice much less efficiently than in humans, limiting the utility of murine models to address the importance of MHC class I inhibition in HSV immunopathogenesis. To address this limitation, we generated recombinant HSVs that efficiently inhibit antigen presentation by murine MHC class I. These recombinant viruses prevented cytotoxic T lymphocyte killing of infected cells in vitro, replicated to higher titers in the central nervous system, and induced paralysis more frequently than control HSV. This increase in virulence was due to inhibition of antigen presentation to CD8 T cells, since these differences were not evident in MHC class I-deficient mice or in mice in which CD8 T cells were depleted. Inhibition of MHC class I by the recombinant viruses did not impair the induction of the HSV-specific CD8 T-cell response, indicating that cross-presentation is the principal mechanism by which HSV-specific CD8 T cells are induced. This inhibition in turn facilitates greater viral entry, replication, and/or survival in the central nervous system, leading to an increased incidence of paralysis.

While animal models are often instructive in understanding human diseases, many factors that influence disease differ between mouse and man. Although mice can be experimentally infected with HSV-1, this virus has evolved as a human pathogen. One facet of this evolution is HSV's mechanisms to evade the immune response, allowing the virus to persist for the lifetime of the human host. This evasion includes preventing CD8 T cells from recognizing and killing infected cells by inhibiting the expression of the molecule that presents viral peptides to CD8 T cells: major histocompatibility complex (MHC) class I. HSV is unable to inhibit mouse MHC class I, thus rendering this immune-evasion strategy inoperative in the mouse. To better understand the biology of HSV infection and the immune response to this virus in humans, the authors corrected this deficiency by inserting a gene which inhibits murine MHC class I. This recombinant virus demonstrates that MHC class I inhibition is an important determinant of disease progression. The authors found that the recombinant HSV still effectively elicits a CD8 T-cell response, but this response is ineffective in controlling the infection. This finding reveals the important distinction between the size of the immune response and the effectiveness of the response, which may be important to HSV vaccine studies.

Viral Interference with Antigen Presentation to CD8+T Cells: Lessons from Cytomegalovirus

Cytomegaloviruses (CMV), in common with other Herpesviruses, establish lifelong persistence in their hosts. These highly host-specific viruses each encode viral genes that interfere with antigen presentation to CD8+ T cells, although the molecular mechanisms by which this end is achieved differ for human and murine CMVs. In each case, there has been a presumption that these genes are necessary for virus persistence in the host. However, recent data in the murine model casts doubt on that presumption. Here, we review the molecular mechanisms of interference with the class I pathway, and the published data regarding functional significance, with a focus on the murine model.

Viral evasion of the MHC class I antigen-processing machinery

In their adaptation to the immune system in vertebrates, viruses have been forced to evolve elaborate strategies for evading the host's immune response. To ensure life-long persistence in the host, herpes viruses, adenoviruses and retroviruses have exploited multiple cellular pathways for their purpose, including the class I antigen-processing machinery. Attractive and prominent targets for viral attacks are the proteasome complex, the transporter associated with antigen processing, and MHC class I molecules. This review briefly outlines the different mechanisms of viral interference with the antigen-presentation pathway.

Role of tumor cell immune escape mechanisms in cytomegalovirus-mediated oncomodulation

It has been known for a long time that cytomegalovirus (CMV) has evolved mechanisms that allow the escape from the host immune surveillance. In the past, many efforts have been done to elucidate the molecular mechanisms underlying this virus-mediated immune escape and thus virus persistence. However, it is unknown, whether CMV may also impair immune responses directed against tumor cells. This might have severe consequences on tumor progression and may explain the growing evidence for CMV-mediated oncomodulation. This review summarizes recent work on CMV-mediated immune escape mechanisms of tumor cells and oncomodulation and proposes novel aspects that may be important for understanding the CMV-associated tumor progression.

Viral modulation of NK cell immunity

Natural killer (NK) cells have been implicated in innate immune responses against viruses such as herpesviruses, which cause persistent infections in the host. In response to the selective pressure that is exerted by NK cells, many viruses have evolved strategies either to evade detection by NK cells or to modulate the activity of NK cells. Here, we review the unique relationship that exists between NK cells and viruses, with a focus on herpesviruses.

NK cell activation through the NKG2D ligand MULT-1 is selectively prevented by the glycoprotein encoded by mouse cytomegalovirus gene m145

The NK cell–activating receptor NKG2D interacts with three different cellular ligands, all of which are regulated by mouse cytomegalovirus (MCMV). We set out to define the viral gene product regulating murine UL16-binding protein-like transcript (MULT)-1, a newly described NKG2D ligand. We show that MCMV infection strongly induces MULT-1 gene expression, but surface expression of this glycoprotein is nevertheless completely abolished by the virus. Screening a panel of MCMV deletion mutants defined the gene m145 as the viral regulator of MULT-1. The MCMV m145-encoded glycoprotein turned out to be necessary and sufficient to regulate MULT-1 by preventing plasma membrane residence of MULT-1. The importance of MULT-1 in NK cell regulation in vivo was confirmed by the attenuating effect of the m145 deletion that was lifted after NK cell depletion. Our findings underline the significance of escaping MULT-1/NKG2D signaling for viral survival and maintenance.

Coactivators p300 and PCAF physically and functionally interact with the foamy viral trans-activator

Background

Foamy virus Bel1/Tas trans-activators act as key regulators of gene expression and directly bind to Bel1 response elements (BRE) in both the internal and the 5'LTR promoters leading to strong transcriptional trans-activation. Cellular coactivators interacting with Bel1/Tas are unknown to date.

Results

Transient expression assays, co-immunoprecipitation experiments, pull-down assays, and Western blot analysis were used to demonstrate that the coactivator p300 and histone acetyltransferase PCAF specifically interact with the retroviral trans-activator Bel1/Tas in vivo. Here we show that the Bel1/Tas-mediated trans-activation was enhanced by the coactivator p300, histone acetyltransferases PCAF and SRC-1 based on the crucial internal promoter BRE. The Bel1/Tas-interacting region was mapped to the C/H1 domain of p300 by co-immunoprecipitation and pull-down assays. In contrast, coactivator SRC-1 previously reported to bind to the C-terminal domain of p300 did not directly interact with the Bel1 protein but nevertheless enhanced Bel1/Tas-mediated trans-activation. Cotransfection of Bel1/Tas and p300C with an expression plasmid containing the C/H1domain partially inhibited the p300C-driven trans-activation.

Conclusions

Our data identify p300 and PCAF as functional partner molecules that directly interact with Bel1/Tas. Since the acetylation activities of the three coactivators reside in or bind to the C-terminal regions of p300, a C/H1 expression plasmid was used as inhibitor. This is the first report of a C/H1 domain-interacting retroviral trans-activator capable of partially blocking the strong Bel1/Tas-mediated activation of the C-terminal region of coactivator p300. The potential mechanisms and functional roles of the three histone and factor acetyltransferases p300, PCAF, and SRC-1 in Bel1/Tas-mediated trans-activation are discussed.

Stalemating a clever opportunist: lessons from murine cytomegalovirus

Cytomegaloviruses and their specific hosts have come to an arrangement that avoids disease but allows the viruses to persist in the individual host and to spread in the host species. Recent work has uncovered some of the molecular details of this evolutionary "contract for mutual survival." Cytomegaloviruses encode proteins, referred to as "immunoevasins," which are specifically committed to subvert the immune defense of the host for evading virus elimination. In reply, the hosts have evolved countermeasures to overcome the viral immunoevasins and present antigenic peptides to an extent that is sufficient for confining virus replication to below a harmful level. Accordingly, cytomegalic inclusion disease is a disease only of the immunocompromised. Although the details of the contract differ between the various cytomegalovirus host pairs, the general principles are strikingly analogous. Paradigmatic findings were made in the murine model, which adds the advantage of providing proof of principle by in vivo studies. With the focus on CD8 T cells and the major histocompatibility complex class I pathway of antigen presentation, we will discuss our view on the immune surveillance of cytomegalovirus in the murine model.

Bovine aortic endothelial cells are susceptible to hantavirus infection; a new aspect in hantavirus ecology

Hantaviruses are enveloped RNA viruses that belong to the family Bunyaviridae. They are the causative agents of hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Hantaviruses show a worldwide distribution with specific rodent species as natural hosts. It is known that rodents can transmit the virus via feces, urine, saliva, or bites to humans. Additionally, antibodies against different hantaviruses were also found in domestic animals, For example, Danes et al. documented hantavirus-specific IgG titers in 2% of examined cattle [Ceskoslov. Epidemiol. Mikrobiol. Imunol. 41 (1992) 15]. In order to clarify the possibility of a nonrodent and nonhuman hantavirus infection, the susceptibility of bovine aortic endothelial cells (BAEC) to Hantavirus serotype Puumala infection was investigated. The hantaviral nucleocapsid protein was detected in 95% of infected BAEC at the fourth cell culture passage 12 weeks after initial infection by immunofluorescence assay (IFA). The presence of Puumala virus (PUU) nucleocapsid protein and the viral glycoproteins G1 and G2 in infected cells were additionally confirmed by Western blot analysis. The viral RNA genome was identified in infected BAEC cultures and in cell-free culture medium at the fourth passage by reverse transcription polymerase chain reaction (RT-PCR), verified by cDNA nucleotide sequence analysis, showing a 98-100% homology to the input virus. The infected BAEC cultures were shown to express alpha(V)beta(3)-integrin surface receptors that are known to mediate virus entry in human cells and revealed no major cytopathic effects (CPEs) as assayed by immunofluorescence staining of the cytoskeletal components actin and microtubules. In the present study, we documented for the first time that a nonrodent and nonhuman aortic endothelial cell culture of bovine origin (BAEC) can be efficiently infected with a hantavirus. This finding is of particular importance because it adds new aspects to questions dealing with host species barrier, viral reservoir, virus transmission, and ecology of hantaviruses.

Human cytomegalovirus glycoprotein UL16 causes intracellular sequestration of NKG2D ligands, protecting against natural killer cell cytotoxicity

The activating receptor, NKG2D, is expressed on a variety of immune effector cells and recognizes divergent families of major histocompatibility complex (MHC) class I-related ligands, including the MIC and ULBP proteins. Infection, stress, or transformation can induce NKG2D ligand expression, resulting in effector cell activation and killing of the ligand-expressing target cell. The human cytomegalovirus (HCMV) membrane glycoprotein, UL16, binds to three of the five known ligands for human NKG2D. UL16 is retained in the endoplasmic reticulum and cis-Golgi apparatus of cells and causes MICB to be similarly retained and stabilized within cells. Coexpression of UL16 markedly reduces cell surface levels of MICB, ULBP1, and ULBP2, and decreases susceptibility to natural killer cell-mediated cytotoxicity. Domain swapping experiments demonstrate that the transmembrane and cytoplasmic domains of UL16 are important for intracellular retention of UL16, whereas the ectodomain of UL16 participates in down-regulation of NKG2D ligands. The intracellular sequestration of NKG2D ligands by UL16 represents a novel HCMV immune evasion mechanism to add to the well-documented viral strategies directed against antigen presentation by classical MHC molecules.

Strategies for the identification and analysis of viral immune-evasive genes--cytomegalovirus as an example

Co-evolution of herpesviruses with their hosts has resulted in multiple interactions between viral genes and cellular functions. Some interactions control genomic maintenance and replication in specific tissues, other affect the immune control at various stages. Few immunomodulatory functions of genes can be predicted by sequence homology. The majority of genes with immunomodulatory properties only become apparent in functional assays. This chapter reviews procedures which have been used for successful identification of immunomodulatory genes in the past and deals with recent methods which may be applicable for the identification of additional immunomodulatory functions unknown so far.

The HCMV gene products US2 and US11 target MHC class I molecules for degradation in the cytosol

Over millions of years of coevolution with their hosts, viruses have developed highly effective strategies to elude the host immune system. The degradation of major histocompatibility complex (MHC) class I heavy chains by human cytomegalovirus (HCMV) is an example of this. Two HCMV proteins, US2 and US11, target newly synthesized MHC class I heavy chains for destruction via a pathway that involves ubiquitin-dependent retrograde transport, or "dislocation", of the heavy chains from the ER to the cytosol, where the proteins are degraded by proteasomes. In this review, US2- and US11-mediated degradation of MHC class I heavy chains is discussed in relation to data concerning the degradation of other ER luminal proteins. A new, unified model for translocon-facilitated dislocation and degradation of MHC class I heavy chains is presented.

Fatal attraction: cytomegalovirus-encoded chemokine homologs

Members of the cytomegalovirus (CMV) subfamily of betaherpesviruses infecting primates and rodents encode divergent proteins with sequence characteristics and activities of chemokines, a class of small, secreted proteins that control leukocyte migration and trafficking behavior. Human CMV genes UL146 and UL147 encode proteins with sequence characteristics of CXC chemokines, whereas, murine CMV encodes a CC chemokine homolog (MCK-2). Human CMV UL146 encodes a neutrophil-attracting chemokine denoted viral CXC chemokine-1 (vCXCL1) that is as potent as host IL-8 and functions via the CXCR2 receptor, one of two human IL-8 receptors. Murine CMV MCK-2 is composed of a chemokine domain derived from open reading frame (ORF) m131 (and denoted MCK-1) as well as a domain derived from m129 that does not have sequence similarity to any known class of proteins. A synthetic version of murine CMV m131 (MCK-1) protein carries out many of the activities of a positive-acting chemokine, including transient release of intracellular calcium stores and cell adhesion of peritoneal macrophage populations. In the context of the viral genome and infection of the mouse host, the m131-m129 (MCK-2) gene product confers increased inflammation, higher levels of viremia, and higher titers of virus in salivary glands, consistent with a role in promoting dissemination by attracting an important mononuclear leukocyte population. Other characterized primate CMVs, but not other primate betaherpesviruses, encode gene products similar to human UL146 and UL147. Other characterized rodent CMVs encode a gene product similar to the murine CMV chemokine homolog, although not as a spliced gene product. Thus chemokines, like viral proteins that downmodulate MHC class I expression or have sequence homology to host MHC class I proteins, have evolved in primate and rodent CMVs to carry out an analogous set of immunomodulatory functions during infection of the host even though they arise from distinct origins.