Inhibition of the MHC class II antigen presentation pathway by human cytomegalovirus

Immune-checkpoint expression in antigen-presenting cells (APCs) of cytomegaloviruses infection after transplantation: as a diagnostic biomarker

Cytomegalovirus (CMV), a member of the Herpesviridae family, mostly causes only slight feverish symptoms or can be asymptomatic in immunocompetent individuals. However, it is known to be particularly a significant cause of morbidity in immunocompromised patients, including transplant recipients, whose immune system has been weakened due to the consumption of immunosuppressor drugs. Therefore, the diagnosis of CMV infection after transplantation is crucial. New diagnostic methods for the quick detection of CMV have been developed as a result of understanding the clinical importance of invasive CMV. Antigen-presenting cells (APCs) and T cells are important components of the immune system and it may be possible to diagnose viral infections using immunological markers, such as lymphocytosis, cytotoxic T lymphocytes (CTL), and serum cytokine levels. Moreover, PD-1, CTLA 4, and TIGIT, which are expressed on certain T cells and antigen-presenting cells, are over-expressed during the infection. The assessment of CMV infection based on T cell and APC activity, and the expression of immunological checkpoints, can be helpful for the diagnosis of transplant patients at risk for CMV infection. In this review, we will investigate how immune checkpoints affect immune cells and how they impair organ transplantation after CMV infection.

Human cytomegalovirus in cancer: the mechanism of HCMV-induced carcinogenesis and its therapeutic potential

Cancer is one of the leading causes of death worldwide. Human cytomegalovirus (HCMV), a well-studied herpesvirus, has been implicated in malignancies derived from breast, colorectal muscle, brain, and other cancers. Intricate host-virus interactions are responsible for the cascade of events that have the potential to result in the transformed phenotype of normal cells. The HCMV genome contains oncogenes that may initiate these types of cancers, and although the primary HCMV infection is usually asymptomatic, the virus remains in the body in a latent or persistent form. Viral reactivation causes severe health issues in immune-compromised individuals, including cancer patients, organ transplants, and AIDS patients. This review focuses on the immunologic mechanisms and molecular mechanisms of HCMV-induced carcinogenesis, methods of HCMV treatment, and other studies. Studies show that HCMV DNA and virus-specific antibodies are present in many types of cancers, implicating HCMV as an important player in cancer progression. Importantly, many clinical trials have been initiated to exploit HCMV as a therapeutic target for the treatment of cancer, particularly in immunotherapy strategies in the treatment of breast cancer and glioblastoma patients. Taken together, these findings support a link between HCMV infections and cellular growth that develops into cancer. More importantly, HCMV is the leading cause of birth defects in newborns, and infection with HCMV is responsible for abortions in pregnant women.

Overview of how HCMV manipulation of host cell intracellular trafficking networks can promote productive infection

Human cytomegalovirus (HCMV) is a significant cause of morbidity and mortality in the immunocompromised and developing fetuses. Infection has also been linked to chronic inflammatory diseases, cardiovascular disease, and the development of certain cancers. The wide range of pathologies associated with HCMV infection is attributable to the broad cellular tropism of the virus where infection affects every organ system. Like other viruses, HCMV must tailor host cells to support productive infection. In particular, HCMV dedicates many resources and various strategies to manipulate host intracellular trafficking networks to facilitate various aspects of infection across all infected cell types. The dysregulation of host intracellular trafficking networks allows the virus to translocate to the host cell nucleus for genome replication, facilitate nuclear import/export of viral proteins and immature virions, subvert the host immune response, form new organelles for progeny virion assembly, maturation and egress, and promote cellular migration and viral spread. However, due to their complex nature, many aspects of these processes are not well-studied. New research and omics-based technologies have recently begun to elucidate the extent to which HCMV dysregulates host cell trafficking machinery. Here we review the variety of strategies HCMV utilizes to dysregulate intracellular trafficking networks to promote productive infection.

Targeting the latent human cytomegalovirus reservoir for T-cell-mediated killing with virus-specific nanobodies

Latent human cytomegalovirus (HCMV) infection is characterized by limited gene expression, making latent HCMV infections refractory to current treatments targeting viral replication. However, reactivation of latent HCMV in immunosuppressed solid organ and stem cell transplant patients often results in morbidity. Here, we report the killing of latently infected cells via a virus-specific nanobody (VUN100bv) that partially inhibits signaling of the viral receptor US28. VUN100bv reactivates immediate early gene expression in latently infected cells without inducing virus production. This allows recognition and killing of latently infected monocytes by autologous cytotoxic T lymphocytes from HCMV-seropositive individuals, which could serve as a therapy to reduce the HCMV latent reservoir of transplant patients.

Epigenetic reprogramming of host and viral genes by Human Cytomegalovirus infection in Kasumi-3 myeloid progenitor cells at early times post-infection

HCMV establishes latency in myeloid cells. Using the Kasumi-3 latency model, we previously showed that lytic gene expression is activated prior to establishment of latency in these cells. The early events in infection may have a critical role in shaping establishment of latency. Here, we have used an integrative multi-omics approach to investigate dynamic changes in host and HCMV gene expression and epigenomes at early times post infection. Our results show dynamic changes in viral gene expression and viral chromatin. Analyses of Pol II, H3K27Ac and H3K27me3 occupancy of the viral genome showed that 1) Pol II occupancy was highest at the MIEP at 4 hours post infection. However, it was observed throughout the genome; 2) At 24 hours, H3K27Ac was localized to the major immediate early promoter/enhancer and to a possible second enhancer in the origin of replication OriLyt; 3) viral chromatin was broadly accessible at 24 hpi. In addition, although HCMV infection activated expression of some host genes, we observed an overall loss of de novo transcription. This was associated with loss of promoter-proximal Pol II and H3K27Ac, but not with changes in chromatin accessibility or a switch in modification of H3K27.Importance.HCMV is an important human pathogen in immunocompromised hosts and developing fetuses. Current anti-viral therapies are limited by toxicity and emergence of resistant strains. Our studies highlight emerging concepts that challenge current paradigms of regulation of HCMV gene expression in myeloid cells. In addition, our studies show that HCMV has a profound effect on de novo transcription and the cellular epigenome. These results may have implications for mechanisms of viral pathogenesis.

Chlamydia and Its Many Ways of Escaping the Host Immune System

The increasing number of new cases of Chlamydia infection worldwide may be attributed to the pathogen's ability to evade various host immune responses. Summarized here are means of evasion utilized by Chlamydia enabling survival in a hostile host environment. The pathogen's persistence involves a myriad of molecular interactions manifested in a variety of ways, e.g., formation of membranous intracytoplasmic inclusions and cytokine-induced amino acid synthesis, paralysis of phagocytic neutrophils, evasion of phagocytosis, inhibition of host cell apoptosis, suppression of antigen presentation, and induced expression of a check point inhibitor of programmed host cell death. Future studies could focus on the targeting of these molecules associated with immune evasion, thus limiting the spread and tissue damage caused by this pathogen.

Herpes Simplex Virus 1 Specifically Targets Human CD1d Antigen Presentation To Enhance Its Pathogenicity

Herpes simplex virus 1 (HSV-1) is one of the most prevalent herpesviruses in humans and represents a constant health threat to aged and immunocompromised populations. How HSV-1 interacts with the host immune system to efficiently establish infection and latency is only partially known. CD1d-restricted NKT cells are a critical arm of the host innate immune system and play potent roles in anti-infection and antitumor immune responses. We discovered previously that upon infection, HSV-1 rapidly and efficiently downregulates CD1d expression on the cell surface and suppresses the function of NKT cells. Furthermore, we identified the viral serine/threonine protein kinase US3 as a major viral factor downregulating CD1d during infection. Interestingly, neither HSV-1 nor its US3 protein efficiently inhibits mouse CD1d expression, suggesting that HSV-1 has coevolved with the human immune system to specifically suppress human CD1d (hCD1d) and NKT cell function for its pathogenesis. This is consistent with the fact that wild-type mice are mostly resistant to HSV-1 infection. On the other hand, in vivo infection of CD1d-humanized mice (hCD1d knock-in mice) showed that HSV-1 can indeed evade hCD1d function and establish infection in these mice. We also report here that US3-deficient viruses cannot efficiently infect hCD1d knock-in mice but infect mice lacking all NKT cells at a higher efficiency. Together, these studies supported HSV-1 evasion of human CD1d and NKT cell function as an important pathogenic factor for the virus. Our results also validated the potent roles of NKT cells in antiherpesvirus immune responses and pointed to the potential of NKT cell ligands as adjuvants for future vaccine development.IMPORTANCE Herpes simplex virus 1 (HSV-1) is among the most common human pathogens. Little is known regarding the exact mechanism by which this virus evades the human immune system, particularly the innate immune system. We reported previously that HSV-1 employs its protein kinase US3 to modulate the expression of the key antigen-presenting molecule, CD1d, so as to evade the antiviral function of NKT cells. Here we demonstrated that the virus has coevolved with the human CD1d and NKT cell system and that NKT cells indeed play potent roles in anti-HSV immune responses. These studies point to the great potential of exploring NKT cell ligands as adjuvants for HSV vaccines.

Cytomegalovirus and HIV Persistence: Pouring Gas on the Fire

The inherent stability of a small population of T cells that are latently infected with HIV despite antiretroviral therapy (ART) remains a stubborn obstacle to an HIV cure. By exploiting the memory compartment of our immune system, HIV maintains persistence in a small subset of quiescent cells with varying phenotypes, thus evading immune surveillance and clinical detection. Understanding the molecular and immunological mechanisms that maintain the latent reservoir will be critical to the success of HIV eradication strategies. Human cytomegalovirus (CMV), another chronic viral infection, frequently co-occurs with HIV and occupies an oversized proportion of memory T cell responses. CMV and HIV have both evolved complex strategies to manipulate our immune system for their own advantage. Given the increasingly clear links between CMV replication, chronic immune activation, and increased HIV reservoirs, we present a closer examination of the interplay between these two chronic coinfections. Here we review the effects of CMV on the immune system and show how they may affect persistence of the latent HIV reservoir during ART. The studies described herein suggest that hijacking of cytokine and chemokine signaling, manipulation of cell development pathways, and transactivation of HIV expression by CMV might be pouring gas on the fire of HIV persistence. Future interventional studies are required to formally determine the extent to which CMV is causally associated with inflammation and HIV reservoir expansion.

Genetic, Transcriptome, Proteomic, and Epidemiological Evidence for Blood-Brain Barrier Disruption and Polymicrobial Brain Invasion as Determinant Factors in Alzheimer's Disease

Diverse pathogens are detected in Alzheimer's disease (AD) brains. A bioinformatics survey showed that AD genome-wide association study (GWAS) genes (localized in bone marrow, immune locations and microglia) relate to multiple host/pathogen interactomes (Candida albicans, Cryptococcus neoformans, Bornavirus, Borrelia burgdorferri, cytomegalovirus, Ebola virus, HSV-1, HERV-W, HIV-1, Epstein-Barr, hepatitis C, influenza, Chlamydia pneumoniae, Porphyrymonas gingivalis, Helicobacter pylori, Toxoplasma gondii, Trypanosoma cruzi). These interactomes also relate to the AD hippocampal transcriptome and to plaque or tangle proteins. Upregulated AD hippocampal genes match those upregulated by multiple bacteria, viruses, fungi, or protozoa in immunocompetent cells. AD genes are enriched in GWAS datasets reflecting pathogen diversity, suggesting selection for pathogen resistance, as supported by the old age of AD patients, implying resistance to earlier infections. APOE4 is concentrated in regions of high parasitic burden and protects against childhood tropical infections and hepatitis C. Immune/inflammatory gain of function applies to APOE4, CR1, and TREM2 variants. AD genes are also expressed in the blood-brain barrier (BBB), which is disrupted by AD risk factors (age, alcohol, aluminum, concussion, cerebral hypoperfusion, diabetes, homocysteine, hypercholesterolemia, hypertension, obesity, pesticides, pollution, physical inactivity, sleep disruption, smoking) and by pathogens, directly or via olfactory routes to basal-forebrain BBB control centers. The BBB benefits from statins, NSAIDs, estrogen, melatonin, memantine, and the Mediterranean diet. Polymicrobial involvement is supported by upregulation of bacterial, viral, and fungal sensors/defenders in the AD brain, blood, or cerebrospinal fluid. AD serum amyloid-β autoantibodies may attenuate its antimicrobial effects favoring microbial survival and cerebral invasion leading to activation of neurodestructive immune/inflammatory processes, which may also be augmented by age-related immunosenescence. AD may thus respond to antibiotic, antifungal, or antiviral therapy.

Partners in Crime: The Role of CMV in Immune Dysregulation and Clinical Outcome During HIV Infection

In the current era of combination antiretroviral therapy (ART), human immunodeficiency virus (HIV)-infected individuals are living longer and healthier lives. Nevertheless, HIV-infected persons are at greater risk for age-related disorders, which have been linked to residual immune dysfunction and inflammation. HIV-infected individuals are almost universally co-infected with cytomegalovirus (CMV) and both viruses are associated with inflammation-related morbidities. Therefore, a detailed investigation of the relationship between CMV and aging-related morbidities emerging during chronic HIV infection is warranted. Here, we review the literature on how CMV co-infection affects HIV infection and host immunity and we discuss the gaps in our knowledge that need elucidation.

A Simple Proteomics-Based Approach to Identification of Immunodominant Antigens from a Complex Pathogen: Application to the CD4 T Cell Response against Human Herpesvirus 6B

Most of humanity is chronically infected with human herpesvirus 6 (HHV-6), with viral replication controlled at least in part by a poorly characterized CD4 T cell response. Identification of viral epitopes recognized by CD4 T cells is complicated by the large size of the herpesvirus genome and a low frequency of circulating T cells responding to the virus. Here, we present an alternative to classical epitope mapping approaches used to identify major targets of the T cell response to a complex pathogen like HHV-6B. In the approach presented here, extracellular virus preparations or virus-infected cells are fractionated by SDS-PAGE, and eluted fractions are used as source of antigens to study cytokine responses in direct ex vivo T cell activation studies. Fractions inducing significant cytokine responses are analyzed by mass spectrometry to identify viral proteins, and a subset of peptides from these proteins corresponding to predicted HLA-DR binders is tested for IFN-γ production in seropositive donors with diverse HLA haplotypes. Ten HHV-6B viral proteins were identified as immunodominant antigens. The epitope-specific response to HHV-6B virus was complex and variable between individuals. We identified 107 peptides, each recognized by at least one donor, with each donor having a distinctive footprint. Fourteen peptides showed responses in the majority of donors. Responses to these epitopes were validated using in vitro expanded cells and naturally expressed viral proteins. Predicted peptide binding affinities for the eight HLA-DRB1 alleles investigated here correlated only modestly with the observed CD4 T cell responses. Overall, the response to the virus was dominated by peptides from the major capsid protein U57 and major antigenic protein U11, but responses to other proteins including glycoprotein H (U48) and tegument proteins U54 and U14 also were observed. These results provide a means to follow and potentially modulate the CD4 T-cell immune response to HHV-6B.

Human cytomegalovirus and autoimmune disease

Human cytomegalovirus (HCMV) represents a prototypic pathogenic member of the β-subgroup of the herpesvirus family. A range of HCMV features like its lytic replication in multiple tissues, the lifelong persistence through periods of latency and intermitting reactivation, the extraordinary large proteome, and extensive manipulation of adaptive and innate immunity make HCMV a high profile candidate for involvement in autoimmune disorders. We surveyed the available literature for reports on HCMV association with onset or exacerbation of autoimmune disease. A causative linkage between HCMV and systemic lupus erythematosus (SLE), systemic sclerosis (SSc), diabetes mellitus type 1, and rheumatoid arthritis (RA) is suggested by the literature. However, a clear association of HCMV seroprevalence and disease could not be established, leaving the question open whether HCMV could play a coresponsible role for onset of disease. For convincing conclusions population-based prospective studies must be performed in the future. Specific immunopathogenic mechanisms by which HCMV could contribute to the course of autoimmune disease have been suggested, for example, molecular mimicry by UL94 in SSc and UL83/pp65 in SLE patients, as well as aggravation of joint inflammation by induction and expansion of CD4+/CD28− T-cells in RA patients. Further studies are needed to validate these findings and to lay the grounds for targeted therapeutic intervention.

Suppression of CD8+ T-cell recognition in the immediate-early phase of human cytomegalovirus infection

Human cytomegalovirus (HCMV) interferes with MHC class I-restricted antigen presentation and thereby reduces recognition by CD8(+) T-cells. This interference is mediated primarily by endoplasmic reticulum-resident glycoproteins that are encoded in the US2-11 region of the viral genome. Such a suppression of recognition would be of particular importance immediately after infection, because several immunodominant viral antigens are already present in the cell in this phase. However, which of the evasion proteins gpUS2-11 interfere(s) with antigen presentation to CD8(+) T-cells at this time of infection is not known. Here we address this question, using recombinant viruses (RV) that express only one of the immunoevasins gpUS2, gpUS3 or gpUS11. Infection with RV-US3 had only a limited impact on the presentation of peptides from the CD8(+) T-cell antigens IE1 and pp65 under immediate-early (IE) conditions imposed by cycloheximide/actinomycin D blocking. Unexpectedly, both RV-US2 and RV-US11 considerably impaired the recognition of IE1 and pp65 by CD8(+) T-cells, and both US2 and, to a lesser extent, US11 were transcribed under IE conditions. Thus, gpUS2 and gpUS11 are key effectors of MHC class I immunoevasion immediately after HCMV infection.

Herpes simplex virus 1 glycoprotein B and US3 collaborate to inhibit CD1d antigen presentation and NKT cell function

Herpes simplex viruses (HSVs) are prevalent human pathogens that establish latency in human neuronal cells and efficiently evade the immune system. It has been a major medical challenge to eradicate them and, despite intensive efforts, an effective vaccine is not available. We previously showed that upon infection of antigen-presenting cells, HSV type 1 (HSV-1) rapidly and efficiently downregulates the major histocompatibility complex class I-like antigen-presenting molecule, CD1d, and potently inhibits its recognition by CD1d-restricted natural killer T (NKT) cells. It suppresses CD1d expression primarily by inhibiting its recycling to the cell surface after endocytosis. We identify here the viral glycoprotein B (gB) as the predominant CD1d-interacting protein. gB initiates the interaction with CD1d in the endoplasmic reticulum and stably associates with it throughout CD1d trafficking. However, an additional HSV-1 component, the serine-threonine kinase US3, is required for optimal CD1d downregulation. US3 expression in infected cells leads to gB enrichment in the trans-Golgi network (TGN) and enhances the relocalization of both gB and CD1d to this compartment, suggesting that following internalization CD1d is translocated from the endocytic pathway to the TGN by its association with gB. Importantly, both US3 and gB are required for efficient inhibition of CD1d antigen presentation and NKT cell activation. In summary, our results suggest that HSV-1 uses gB and US3 to rapidly inhibit NKT cell function in the initial antiviral response.

Human cytomegalovirus pUL83 stimulates activity of the viral immediate-early promoter through its interaction with the cellular IFI16 protein

The human cytomegalovirus (HCMV) virion protein pUL83 (also termed pp65) inhibits the expression of interferon-inducible cellular genes. In this work we demonstrate that pUL83 is also important for efficient induction of transcription from the viral major immediate-early promoter. Infection with a mutant virus containing a premature translation termination codon in the UL83 open reading frame (ORF) (UL83Stop) resulted in decreased transcription from the major immediate-early promoter in a time- and multiplicity-dependent manner. Expression of pUL83 alone is capable of transactivating the promoter in a reporter assay, and pUL83 associates with the promoter in infected cells. To investigate the mechanism by which the protein regulates the major immediate-early promoter, we utilized a mutant virus expressing an epitope-tagged pUL83 from its own promoter to identify protein binding partners for pUL83 during infection. We identified and confirmed the interaction of pUL83 with cellular IFI16 family members throughout the course of HCMV infection. pUL83 recruits IFI16 to the major immediate-early promoter, and IFI16 binding at the promoter is dependent upon the presence of pUL83. Consistent with the results obtained with the UL83Stop virus, infection of IFI16 knockdown cells with wild-type virus resulted in decreased levels of immediate-early transcripts compared to those of control cells. These data identify a previously unknown role for pUL83 in the initiation of the human cytomegalovirus gene expression cascade.

Natural killer cells and lung transplantation, roles in rejection, infection, and tolerance

Despite improvements in surgical technique, organ preservation, immunosuppression, and management of infection, the long term survival following lung transplantation remains low, mainly due to immune mediated complications such as acute and chronic rejection. Almost all immunosuppressive agents used in the prophylaxis and treatment of rejection following lung transplantation are targets of T cell maturation, function or proliferation, which in theory should cause sufficient disruption of the adaptive immune system to prevent graft rejection. However the five year survival rate of only 50% suggests this is not the case. More recent evidence suggests that NK cells may play a significant role in immune processes following lung transplantation. This article reviews the literature on the potential function of NK cells in rejection, infection, malignancy and tolerance following lung transplantation.

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.

Optimization of CD4+ T lymphocyte response to human cytomegalovirus nuclear IE1 protein through modifications of both size and cellular localization

We have previously reported that the CD4+ T lymphocyte response against nuclear human CMV IE1 protein depends in part on endogenous MHC class II presentation. To optimize presentation by HLA-DR of the nuclear IE1 protein and increase the response by CD4+ T cells, we have constructed two different adenovirus vectors containing mutant versions of IE1, containing a HLA-DR3 epitope, fused to GFP. The first construct consisted of a sequence of 46 aa encoded by exon 4, called GFP-IE1 (86-131). The second construct consisted of the whole IE1 mutated on exon 4 nuclear localization signals, identified in this study, and deleted of already known exon 2 nuclear localization signals (GFP-IE1M). Both of these IE1 vectors expressed proteins with cytoplasmic localization, as evidenced by GFP expression, as opposed to control GFP-IE1, which was nuclear. GFP-IE1 (86-131) induced IE1-specific CD4+ T cell clone response that was >30-fold more potent than that against GFP-IE1 and GFP-IE1M. The CD4+ T cell response was due to endogenous presentation followed by exogenous presentation at later time points. Presentation was dependent on both proteasome and acidic compartments. GFP-IE1 (86-131) was rapidly degraded by the APC, which may account for better presentation. Our data show potentiation of the CD4+ T cell response to a specific epitope through shortening and relocation of an otherwise nuclear protein and suggest applications in vaccination.

Identification of cytomegalovirus-specific cytotoxic T lymphocytes in vitro is greatly enhanced by the use of recombinant virus lacking the US2 to US11 region or modified vaccinia virus Ankara expressing individual viral genes

Cytomegalovirus (CMV) elicits a potent T-cell response in humans that appears to protect the host from virus-associated disease. Despite facing strong host defense mechanisms, CMV remains as a lifelong infection that may reactivate and cause life-threatening disease in immunocompromised individuals. This persistence is probably assisted by expression of immune subversion proteins of the virus encoded by genes belonging to the US gene family. These proteins modulate major histocompatibility complex expression in infected cells and bias in vitro experiments toward the detection of only certain specificities. We have combined the use of recombinant CMV, lacking the US2 to US11 region genes, and cytoplasmic gamma interferon staining to define a more accurate assessment of CMV-specific responses in vivo. Recombinant CMV stimulation reveals a CD8 response much larger than that of parental virus in all donors tested. In some cases, this represented up to 10-fold increases in the number of cells detected. Responses were directed mainly against pp65, IE-1, and pp50 in the majority of donors. In addition, previously unreported IE-2-specific T-cell responses could be detected in a minority of cases. Furthermore, we observed a less marked increase in the response to mutant CMV by CD4 T cells in some donors. This suggests that a much broader T-cell response to CMV exists in vivo than is revealed by restimulation with wild-type virus and adds to the evidence that the efficacy of immune evasion strategies may not be as absolute as previously believed.

Latency, chromatin remodeling, and reactivation of human cytomegalovirus in the dendritic cells of healthy carriers

Human cytomegalovirus (HCMV) persists as a subclinical, lifelong infection in the normal human host, but reactivation from latency in immunocompromised subjects results in serious disease. Latency and reactivation are defining characteristics of the herpesviruses and are key to understanding their biology; however, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation during natural infection remain poorly understood. Here we present evidence, based entirely on direct analysis of material isolated from healthy virus carriers, to show that myeloid dendritic cell (DC) progenitors are sites of HCMV latency and that their ex vivo differentiation to a mature DC phenotype is linked with reactivation of infectious virus resulting from differentiation-dependent chromatin remodeling of the viral major immediate-early promoter. Thus, myeloid DC progenitors are a site of HCMV latency during natural persistence, and there is a critical linkage between their differentiation to DC and transcriptional reactivation of latent virus, which is likely to play an important role in the pathogenesis of HCMV infection.

Human cytomegalovirus US2 causes similar effects on both major histocompatibility complex class I and II proteins in epithelial and glial cells

The human cytomegalovirus (HCMV) glycoprotein US2 specifically binds to major histocompatibility complex (MHC) class I heavy chain (HC) and class II proteins DRalpha and DMalpha, triggering their degradation by proteasomes. Effects of US2 on class II proteins were originally characterized in HCMV- or adenovirus vector-infected U373 astroglioma cells. Here, we have extended characterization of US2-mediated degradation of class II DRalpha to two other cell lines, including biologically relevant epithelial cells. Comparison of the effects of US2 in cells expressing both class I and II proteins demonstrated only a slight preference for class I HC. Moreover, US2 caused degradation of DRalpha and DMalpha when these proteins were expressed by transfection without DRbeta, invariant chain (Ii), or DMbeta. Therefore, US2 binds to alpha chains of DR and DM and triggers endoplasmic reticulum degradation without formation of class II DR alphabeta/Ii or DM alphabeta complexes. Similar levels of degradation of class II alpha were observed in cells expressing vastly different amounts of class II, suggesting that cellular factors, other than class II, were limiting. We concluded that US2 has broad effects in a variety of cells that express both class I and II proteins and is relevant to HCMV infection in vivo.

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.

Human cytomegalovirus infection inhibits tumor necrosis factor alpha (TNF-alpha) signaling by targeting the 55-kilodalton TNF-alpha receptor

Infection with human cytomegalovirus (HCMV) results in complex interactions between viral and cellular factors which perturb many cellular functions. HCMV is known to target the cell cycle, cellular transcription, and immunoregulation, and it is believed that this optimizes the cellular environment for viral DNA replication during productive infection or during carriage in the latently infected host. Here, we show that HCMV infection also prevents external signaling to the cell by disrupting the function of TNFRI, the 55-kDa receptor for tumor necrosis factor alpha (TNF-alpha), one of the receptors for a potent cytokine involved in eliciting a wide spectrum of cellular responses, including antiviral responses. HCMV infection of fully permissive differentiated monocytic cell lines and U373 cells resulted in a reduction in cell surface expression of TNFRI. The reduction appeared to be due to relocalization of TNFRI from the cell surface and was reflected in the elimination of TNF-alpha-induced Jun kinase activity. Analysis of specific phases of infection suggested that viral early gene products were responsible for this relocalization. However, a mutant HCMV in which all viral gene products known to be involved in down-regulation of major histocompatibility complex (MHC) class I were deleted still resulted in relocalization of TNFRI. Consequently, TNFRI relocalization by HCMV appears to be mediated by a novel viral early function not involved in down-regulation of cell surface MHC class I expression. We suggest that upon infection, HCMV isolates the cell from host-mediated signals, forcing the cell to respond only to virus-specific signals which optimize the cell for virus production and effect proviral responses from bystander cells.

Inhibition of HLA-DR assembly, transport, and loading by human cytomegalovirus glycoprotein US3: a novel mechanism for evading major histocompatibility complex class II antigen presentation

Human cytomegalovirus (HCMV) establishes persistent lifelong infections and replicates slowly. To withstand robust immunity, HCMV utilizes numerous immune evasion strategies. The HCMV gene cassette encoding US2 to US11 encodes four homologous glycoproteins, US2, US3, US6, and US11, that inhibit the major histocompatibility complex class I (MHC-I) antigen presentation pathway, probably inhibiting recognition by CD8(+) T lymphocytes. US2 also inhibits the MHC-II antigen presentation pathway, causing degradation of human leukocyte antigen (HLA)-DR-alpha and -DM-alpha and preventing recognition by CD4(+) T cells. We investigated the effects of seven of the US2 to US11 glycoproteins on the MHC-II pathway. Each of the glycoproteins was expressed by using replication-defective adenovirus vectors. In addition to US2, US3 inhibited recognition of antigen by CD4(+) T cells by a novel mechanism. US3 bound to class II alpha/beta complexes in the endoplasmic reticulum (ER), reducing their association with Ii. Class II molecules moved normally from the ER to the Golgi apparatus in US3-expressing cells but were not sorted efficiently to the class II loading compartment. As a consequence, formation of peptide-loaded class II complexes was reduced. We concluded that US3 and US2 can collaborate to inhibit class II-mediated presentation of endogenous HCMV antigens to CD4(+) T cells, allowing virus-infected cells to resist recognition by CD4(+) T cells.

Binding of human cytomegalovirus US2 to major histocompatibility complex class I and II proteins is not sufficient for their degradation

Human cytomegalovirus (HCMV) glycoprotein US2 causes degradation of major histocompatibility complex (MHC) class I heavy-chain (HC), class II DR-alpha and DM-alpha proteins, and HFE, a nonclassical MHC protein. In US2-expressing cells, MHC proteins present in the endoplasmic reticulum (ER) are degraded by cytosolic proteasomes. It appears that US2 binding triggers a normal cellular pathway by which misfolded or aberrant proteins are translocated from the ER to cytoplasmic proteasomes. To better understand how US2 binds MHC proteins and causes their degradation, we constructed a panel of US2 mutants. Mutants truncated from the N terminus as far as residue 40 or from the C terminus to amino acid 140 could bind to class I and class II proteins. Nevertheless, mutants lacking just the cytosolic tail (residues 187 to 199) were unable to cause degradation of both class I and II proteins. Chimeric proteins were constructed in which US2 sequences were replaced with homologous sequences from US3, an HCMV glycoprotein that can also bind to class I and II proteins. One of these US2/US3 chimeras bound to class II but not to class I, and a second bound class I HC better than wild-type US2. Therefore, US2 residues involved in the binding to MHC class I differ subtly from those involved in binding to class II proteins. Moreover, our results demonstrate that the binding of US2 to class I and II proteins is not sufficient to cause degradation of MHC proteins. The cytosolic tail of US2 and certain US2 lumenal sequences, which are not involved in binding to MHC proteins, are required for degradation. Our results are consistent with the hypothesis that US2 couples MHC proteins to components of the ER degradation pathway, enormously increasing the rate of degradation of MHC proteins.