Cytomegalovirus selectively blocks antigen processing and presentation of its immediate-early gene product

Immune Landscape of CMV Infection in Cancer Patients: From "Canonical" Diseases Toward Virus-Elicited Oncomodulation

Human Cytomegalovirus (HCMV) is an immensely pervasive herpesvirus, persistently infecting high percentages of the world population. Despite the apparent robust host immune responses, HCMV is capable of replicating, evading host defenses, and establishing latency throughout life by developing multiple immune-modulatory strategies. HCMV has coexisted with humans mounting various mechanisms to evade immune cells and effectively win the HCMV-immune system battle mainly through maintaining its viral genome, impairing HLA Class I and II molecule expression, evading from natural killer (NK) cell-mediated cytotoxicity, interfering with cellular signaling, inhibiting apoptosis, escaping complement attack, and stimulating immunosuppressive cytokines (immune tolerance). HCMV expresses several gene products that modulate the host immune response and promote modifications in non-coding RNA and regulatory proteins. These changes are linked to several complications, such as immunosenescence and malignant phenotypes leading to immunosuppressive tumor microenvironment (TME) and oncomodulation. Hence, tumor survival is promoted by affecting cellular proliferation and survival, invasion, immune evasion, immunosuppression, and giving rise to angiogenic factors. Viewing HCMV-induced evasion mechanisms will play a principal role in developing novel adapted therapeutic approaches against HCMV, especially since immunotherapy has revolutionized cancer therapeutic strategies. Since tumors acquire immune evasion strategies, anti-tumor immunity could be prominently triggered by multimodal strategies to induce, on one side, immunogenic tumor apoptosis and to actively oppose the immune suppressive microenvironment, on the other side.

Chimeric Antigen Receptors Targeting Human Cytomegalovirus

Human cytomegalovirus (CMV) is a ubiquitous pathogen that causes significant morbidity in some vulnerable populations. Individualized adoptive transfer of ex vivo expanded CMV-specific CD8+ T cells has provided proof-of-concept that immunotherapy can be highly effective, but a chimeric antigen receptor (CAR) approach would provide a feasible method for broad application. We created 8 novel CARs using anti-CMV neutralizing antibody sequences, which were transduced via lentiviral vector into primary CD8+ T cells. All CARs were expressed. Activity against CMV-infected target cells was assessed by release of cytokines (interferon-γ and tumor necrosis factor-α), upregulation of surface CD107a, proliferation, cytolysis of infected cells, and suppression of viral replication. While some CARs showed varying functional activity across these assays, 1 CAR based on antibody 21E9 was consistently superior in all measures. These results support development of a CMV-specific CAR for therapeutic use against CMV and potentially other applications harnessing CMV-driven immunotherapies.

A new clustering method identifies multiple sclerosis-specific T-cell receptors

OBJECTIVE

To characterize T-cell receptors (TCRs) and identify target epitopes in multiple sclerosis (MS).

METHODS

Peripheral blood mononuclear cells were obtained from 39 MS patients and 19 healthy controls (HCs). TCR repertoires for α/β/δ/γ chains, TCR diversity, and V/J usage were determined by next-generation sequencing. TCR β chain repertoires were compared with affectation status using a novel clustering method, Grouping of Lymphocyte Interactions by Paratope Hotspots (GLIPH). Cytomegalovirus (CMV)-IgG was measured in an additional 113 MS patients and 93 HCs. Regulatory T cells (Tregs) were measured by flow cytometry.

RESULTS

TCR diversity for all four chains decreased with age. TCRα and TCRβ diversity was higher in MS patients (P = 0.0015 and 0.024, respectively), even after age correction. TRAJ56 and TRBV4-3 were more prevalent in MS patients than in HCs (p^corr  = 0.027 and 0.040, respectively). GLIPH consolidated 208,674 TCR clones from MS patients into 1,294 clusters, among which two candidate clusters were identified. The TRBV4-3 cluster was shared by HLA-DRB1*04:05-positive patients (87.5%) and predicted to recognize CMV peptides (CMV-TCR). MS Severity Score (MSSS) was lower in patients with CMV-TCR than in those without (P = 0.037). CMV-IgG-positivity was associated with lower MSSS in HLA-DRB1*04:05 carriers (P = 0.0053). HLA-DRB1*04:05-positive individuals demonstrated higher CMV-IgG titers than HLA-DRB1*04:05-negative individuals (P = 0.017). CMV-IgG-positive patients had more Tregs than CMV-IgG-negative patients (P = 0.054).

INTERPRETATION

High TCRα/TCRβ diversity, regardless of age, is characteristic of MS. Association of a CMV-recognizing TCR with mild disability indicates CMV's protective role in HLA-DRB1*04:05-positive MS.

The cytomegalovirus UL146 gene product vCXCL1 promotes the resistance of hepatic cells to CD8+ T cells through up-regulation of PD-L1

The HCMV (human cytomegalovirus) encodes numerous proteins which function to evade the immune response, which allows the virus to replicate. Exploring the mechanisms of HCMV immune escape helps to find the strategy to inhibit HCMV replicate. CD8⁺ T cells play a critical role in the immune response to viral pathogens. However, the mechanisms of HCMV to evade the attack by CD8⁺ T cells remain largely unknown. Viral CXCL1 (vCXCL1) is the production of HCMV UL146 gene. Here, we found that vCXCL1 promoted the resistance of hepatic cells to CD8⁺ T cells. vCXCL1 increased the levels of PD-L1 protein expression and mRNA expression. VCXCL1 enhanced the binding of STAT3 transcription factor to the promoter of PD-L1 and increased the activity of PD-L1 promoter. Furthermore, down-regulation of PD-L1 reduced the effects of vCXCL1 on the resistance of hepatic cells to CD8⁺ T cells. Taken together, vCXCL1 promotes the resistance of hepatic cells to CD8⁺ T cells through up-regulation of PD-L1. This finding might provide a new mechanism of HCMV immune escape.

Herpesvirus-bacteria synergistic interaction in periodontitis

The etiopathogenesis of severe periodontitis includes herpesvirus-bacteria coinfection. This article evaluates the pathogenicity of herpesviruses (cytomegalovirus and Epstein-Barr virus) and periodontopathic bacteria (Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis) and coinfection of these infectious agents in the initiation and progression of periodontitis. Cytomegalovirus and A. actinomycetemcomitans/P. gingivalis exercise synergistic pathogenicity in the development of localized ("aggressive") juvenile periodontitis. Cytomegalovirus and Epstein-Barr virus are associated with P. gingivalis in adult types of periodontitis. Periodontal herpesviruses that enter the general circulation may also contribute to disease development in various organ systems. A 2-way interaction is likely to occur between periodontal herpesviruses and periodontopathic bacteria, with herpesviruses promoting bacterial upgrowth, and bacterial factors reactivating latent herpesviruses. Bacterial-induced gingivitis may facilitate herpesvirus colonization of the periodontium, and herpesvirus infections may impede the antibacterial host defense and alter periodontal cells to predispose for bacterial adherence and invasion. Herpesvirus-bacteria synergistic interactions, are likely to comprise an important pathogenic determinant of aggressive periodontitis. However, mechanistic investigations into the molecular and cellular interaction between periodontal herpesviruses and bacteria are still scarce. Herpesvirus-bacteria coinfection studies may yield significant new discoveries of pathogenic determinants, and drug and vaccine targets to minimize or prevent periodontitis and periodontitis-related systemic diseases.

Development of a Human Cytomegalovirus (HCMV)-Based Therapeutic Cancer Vaccine Uncovers a Previously Unsuspected Viral Block of MHC Class I Antigen Presentation

Human cytomegalovirus (HCMV) induces a uniquely high frequency of virus-specific effector/memory CD8+ T-cells, a phenomenon termed “memory inflation”. Thus, HCMV-based vaccines are particularly interesting in order to stimulate a sustained and strong cellular immune response against cancer. Glioblastoma multiforme (GBM) is the most aggressive primary brain tumor with high lethality and inevitable relapse. The current standard treatment does not significantly improve the desperate situation underlining the urgent need to develop novel approaches. Although HCMV is highly fastidious with regard to species and cell type, GBM cell lines are susceptible to HCMV. In order to generate HCMV-based therapeutic vaccine candidates, we deleted all HCMV-encoded proteins (immunoevasins) that interfere with MHC class I presentation. The aim being to use the viral vector as an adjuvant for presentation of endogenous tumor antigens, the presentation of high levels of vector-encoded neoantigens and finally the repurposing of bystander HCMV-specific CD8+ T cells to fight the tumor. As neoantigen, we exemplarily used the E6 and E7 proteins of human papillomavirus type 16 (HPV-16) as a non-transforming fusion protein (E6/E7) that covers all relevant antigenic peptides. Surprisingly, GBM cells infected with E6/E7-expressing HCMV-vectors failed to stimulate E6-specific T cells despite high level expression of E6/E7 protein. Further experiments revealed that MHC class I presentation of E6/E7 is impaired by the HCMV-vector although it lacks all known immunoevasins. We also generated HCMV-based vectors that express E6-derived peptide fused to HCMV proteins. GBM cells infected with these vectors efficiently stimulated E6-specific T cells. Thus, fusion of antigenic sequences to HCMV proteins is required for efficient presentation via MHC class I molecules during infection. Taken together, these results provide the preclinical basis for development of HCMV-based vaccines and also reveal a novel HCMV-encoded block of MHC class I presentation.

Subversion of Immune Response by Human Cytomegalovirus

Human cytomegalovirus (HCMV) is the most common cause of congenital infections and is an important pathogen in immunocompromised individuals. Despite a robust host immune system, HCMV able to replicate, evade host defenses, establish latency for life. A significant portion of HCMV genome dedicated to encode gene products for modulation of host immune response. Growing number of HCMV gene products are being recognized to play role in immune evasion. Information on viral immune evasion mechanisms by which HCMV persists in host will be useful in devising antiviral intervention strategies and development of new vaccines. This minireview provides a brief overview of immune evasion strategy adapted by HCMV by utilizing its gene products in modulation of host immune response.

Signatures of T and B Cell Development, Functional Responses and PD-1 Upregulation After HCMV Latent Infections and Reactivations in Nod.Rag.Gamma Mice Humanized With Cord Blood CD34+ Cells

Human cytomegalovirus (HCMV) latency is typically harmless but reactivation can be largely detrimental to immune compromised hosts. We modeled latency and reactivation using a traceable HCMV laboratory strain expressing the Gaussia luciferase reporter gene (HCMV/GLuc) in order to interrogate the viral modulatory effects on the human adaptive immunity. Humanized mice with long-term (more than 17 weeks) steady human T and B cell immune reconstitutions were infected with HCMV/GLuc and 7 weeks later were further treated with granulocyte-colony stimulating factor (G-CSF) to induce viral reactivations. Whole body bio-luminescence imaging analyses clearly differentiated mice with latent viral infections vs. reactivations. Foci of vigorous viral reactivations were detectable in liver, lymph nodes and salivary glands. The number of viral genome copies in various tissues increased upon reactivations and were detectable in sorted human CD14⁺, CD169⁺, and CD34⁺ cells. Compared with non-infected controls, mice after infections and reactivations showed higher thymopoiesis, systemic expansion of Th, CTL, Treg, and Tfh cells and functional antiviral T cell responses. Latent infections promoted vast development of memory CD4⁺ T cells while reactivations triggered a shift toward effector T cells expressing PD-1. Further, reactivations prompted a marked development of B cells, maturation of IgG⁺ plasma cells, and HCMV-specific antibody responses. Multivariate statistical methods were employed using T and B cell immune phenotypic profiles obtained with cells from several tissues of individual mice. The data was used to identify combinations of markers that could predict an HCMV infection vs. reactivation status. In spleen, but not in lymph nodes, higher frequencies of effector CD4⁺ T cells expressing PD-1 were among the factors most suited to distinguish HCMV reactivations from infections. These results suggest a shift from a T cell dominated immune response during latent infections toward an exhausted T cell phenotype and active humoral immune response upon reactivations. In sum, this novel in vivo humanized model combined with advanced analyses highlights a dynamic system clearly specifying the immunological spatial signatures of HCMV latency and reactivations. These signatures can be merged as predictive biomarker clusters that can be applied in the clinical translation of new therapies for the control of HCMV reactivation.

Who's Driving? Human Cytomegalovirus, Interferon, and NFκB Signaling

As essential components of the host's innate immune response, NFκB and interferon signaling are critical determinants of the outcome of infection. Over the past 25 years, numerous Human Cytomegalovirus (HCMV) genes have been identified that antagonize or modulate the signaling of these pathways. Here we review the biology of the HCMV factors that alter NFκB and interferon signaling, including what is currently known about how these viral genes contribute to infection and persistence, as well as the major outstanding questions that remain.

Heterogeneity of Specific CD4+ and CD8+ T Cells Stimulated by CMV pp65 and IE1 Antigens

Characterization of human cytomegalovirus-specific T cells (CMV-T) is of critical importance for their potential use in adoptive immunotherapy after allogeneic hematopoietic stem cell transplantation. Background frequencies of CMV-T in peripheral blood mononuclear cells (PBMCs) of CMV-seropositive healthy subjects are usually very low, hence the requirement for prolonged culture time and multiple stimulations to expand them. The evaluation of the end-culture specificity and composition has sometimes been neglected or difficult to assess in these settings. We explored the identity and the functionality of pp65-specific and IE1-specific T cells, enriched in short-term cultures from PBMCs. Antigen-specific T cells were further isolated by IFN-γ capture system and/or CD154 microbeads. Frequency of IE1-specific cytotoxic T cells in PBMCs secreting IFN-γ was higher compared with the pp65-specific one, whereas the latter cell types showed a higher median CD107a degranulation. Cell viability, rate of CMV-T increase, and multicytokine secretion profile after epitope-specific short-term cultures were heterogenous. T cells were mainly of late effector stages but they significantly dropped off upon CMV rechallenge with peptide pools. In parallel, CMV-T expansion was accompanied by a significant increase of cytotoxic naive/memory stem cells (CTLs), whereas the CD4 counterpart significantly increased only upon stimulation with IE1. Outcome was variable and showed donor and epitope dependency. Differences in human leukocyte antigen and epitope dominance and variability in the relative number of CD3 effector cells and IFN-γ/CD154 expression among healthy donors could reflect the observed individual CMV-specific cellular immunity. This heterogeneity raises points to be considered when approaching adoptive immunotherapy.

Dynamic regulatory interaction between cytomegalovirus major tegument protein pp65 and protein kinase pUL97 in intracellular compartments, dense bodies and virions

Human cytomegalovirus (HCMV) is a ubiquitous pathogen of considerable clinical importance. Understanding the processes that are important for viral replication is essential for the development of therapeutic strategies against HCMV infection. The HCMV-encoded protein kinase pUL97 is an important multifunctional regulator of viral replication. Several viral and cellular proteins are phosphorylated by pUL97. The phosphoprotein pp65 is one important substrate of pUL97. It is the most abundant tegument protein of HCMV virions, mediating the upload of other virion constituents and contributing to particle integrity. Further to that, it interferes with host innate immune defences, thereby enabling efficient viral replication. By applying different approaches, we characterized the pp65-pUL97 interaction in various compartments. Specifically, the pUL97 interaction domain of pp65 was defined (282-415). A putative cyclin bridge that enhances pUL97-pp65 interaction was identified. The impact of pUL97 mutation on virion and dense body morphogenesis was addressed using pUL97 mutant viruses. Alterations in the proteome of viral particles were seen, especially with mutant viruses expressing cytoplasmic variants of pUL97. On the basis of these data we postulate a so far poorly recognized functional relationship between pp65 and pUL97, and present a refined model of pp65-pUL97 interaction.

CRM1 Inhibitors for Antiviral Therapy

Infectious diseases are a major global concern and despite major advancements in medical research, still cause significant morbidity and mortality. Progress in antiviral therapy is particularly hindered by appearance of mutants capable of overcoming the effects of drugs targeting viral components. Alternatively, development of drugs targeting host proteins essential for completion of viral lifecycle holds potential as a viable strategy for antiviral therapy. Nucleocytoplasmic trafficking pathways in particular are involved in several pathological conditions including cancer and viral infections, where hijacking or alteration of function of key transporter proteins, such as Chromosome Region Maintenance1 (CRM1) is observed. Overexpression of CRM1-mediated nuclear export is evident in several solid and hematological malignancies. Interestingly, CRM1-mediated nuclear export of viral components is crucial in various stages of the viral lifecycle and assembly. This review summarizes the role of CRM1 in cancer and selected viruses. Leptomycin B (LMB) is the prototypical inhibitor of CRM1 potent against various cancer cell lines overexpressing CRM1 and in limiting viral infections at nanomolar concentrations in vitro. However, the irreversible shutdown of nuclear export results in high cytotoxicity and limited efficacy in vivo. This has prompted search for synthetic and natural CRM1 inhibitors that can potentially be developed as broadly active antivirals, some of which are summarized in this review.

Human cytomegalovirus phosphoproteins are hypophosphorylated and intrinsically disordered

Protein phosphorylation has important regulatory functions in cell homeostasis and is tightly regulated by kinases and phosphatases. The tegument of human cytomegalovirus (CMV) contains not only several proteins reported to be extensively phosphorylated but also cellular protein phosphatases (PP1 and PP2A). To investigate this apparent inconsistency, we evaluated the phosphorylation status of the tegument proteins pUL32 and pp65 by enzymatic dephosphorylation and MS. Enzymatic dephosphorylation with bacterial λ phosphatase, but not with PP1, shifted the pUL32-specific signal on reducing SDS-PAGE from ~150 to ~148 kDa, a mass still much larger than the ~118 kDa obtained from our diffusion studies and from the calculated protein mass of ~113 kDa. Remarkably, inhibition of phosphatases through treatment with the phosphatase inhibitors calyculin A and okadaic acid resulted in a shift to ~190 or ~180 kDa, respectively, indicating that a considerable number of potential phosphorylated residues on pUL32 are not phosphorylated under normal conditions. MS revealed a general state of hypophosphorylation of CMV phosphoproteins with only 17 phosphorylated residues detected on pUL32 and 19 on pp65, respectively. Moreover, bioinformatics analysis shows that the C-terminal two-thirds of pUL32 are intrinsically disordered and that most phosphorylations map to this region. In conclusion, we show that important CMV tegument proteins are indeed phosphorylated, though to a lesser extent than previously reported, and the difference in mobility on SDS-PAGE and calculated mass of pUL32 may not be attributed to phosphorylation but more likely due to the partially intrinsically disordered nature of pUL32.

Interaction between HCMV pUL83 and human AIM2 disrupts the activation of the AIM2 inflammasome

Background

AIM2, a cytosolic DNA sensor, plays an important role during infection caused by pathogens with double-stranded DNA; however, its role in human cytomegalovirus (HCMV) infection remains unclear. Previously, we showed an increase in AIM2 protein levels during the early stage of HCMV infection and a decrease 24 h post infection. Because HCMV has developed a variety of strategies to evade host immunity, we speculated that this decline might be attributed to a viral immune escape mechanism. The tegument protein pUL83 is an important immune evasion protein and several studies have reported that pUL83 binds to specific cellular proteins, such as AIM2-like receptor IFI16, to affect their functions. To determine whether pUL83 contributes to the variation in AIM2 levels during HCMV infection, we investigated the pUL83/AIM2 interaction and its impact on the AIM2 inflammasome activation.

Methods

We constructed plasmids expressing recombinant pUL83 and AIM2 proteins for two-hybrid and chemiluminescence assays. Using co-immunoprecipitation and immunofluorescent co-localization, we confirmed the interaction of pUL83/AIM2 in THP-1–derived macrophages infected with HCMV AD169 strain. Furthermore, by investigating the expression and cleavage of inflammasome-associated proteins in recombinant HEK293T cells expressing AIM2, apoptosis-associated speck-like protein (ASC), pro-caspase-1 and pro-IL-1β, we evaluated the effect of pUL83 on the AIM2 inflammasome.

Results

An interaction between pUL83 and AIM2 was detected in macrophages infected with HCMV as well as in transfected HEK293T cells. Moreover, transfection of the pUL83  expression vector into recombinant HEK293T cells stimulated by poly(dA:dT) resulted in reduced expression and activation of AIM2 inflammasome-associated proteins, compared with the absence of pUL83.

Conclusions

Our data indicate that pUL83 interacts with AIM2 in the cytoplasm during the early stages of HCMV infection. The pUL83/AIM2 interaction deregulates the activation of AIM2 inflammasome. These findings reveal a new strategy of immune evasion developed by HCMV, which may facilitate latent infection.

Cytomegalovirus-targeted immunotherapy and glioblastoma: hype or hope?

Malignant gliomas, including glioblastoma (GBM), are the most common primary brain tumors. Despite extensive research only modest gains have been made in long-term survival. Standard of care involves maximizing safe surgical resection followed by concurrent chemoradiation with temozolomide. Immunotherapy for GBM is an area of intense research in recent years. New immunotherapies, although promising, have not been integrated into standard practice. Human cytomegalovirus (HCMV) is a DNA virus of the family Herpesviridae. Human seroprevalence is approximately 80%, and in most cases, is associated with asymptomatic infection. HCMV may be an important agent in the initiation, promotion and/or progression of tumorigenesis. Regardless of a possible etiologic role in GBM, interest has centered on exploiting this association for development of immunomodulatory therapies.

Biology and pathogenesis of cytomegalovirus in periodontal disease

Human periodontitis is associated with a wide range of bacteria and viruses and with complex innate and adaptive immune responses. Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, Treponema denticola, cytomegalovirus and other herpesviruses are major suspected pathogens of periodontitis, and a combined herpesvirus–bacterial periodontal infection can potentially explain major clinical features of the disease. Cytomegalovirus infects periodontal macrophages and T‐cells and elicits a release of interleukin‐1β and tumor necrosis factor‐α. These proinflammatory cytokines play an important role in the host defense against the virus, but they also have the potential to induce alveolar bone resorption and loss of periodontal ligament. Gingival fibroblasts infected with cytomegalovirus also exhibit diminished collagen production and release of an increased level of matrix metalloproteinases. This article reviews innate and adaptive immunity to cytomegalovirus and suggests that immune responses towards cytomegalovirus can play roles in controlling, as well as in exacerbating, destructive periodontal disease.

Cytomegalovirus and glioma: putting the cart before the horse

In 1908, Oluf Bang and Vilhelm Ellerman laid the foundation for theory of oncoviruses by demonstrating that the avian erythroblastosis (a form of chicken leukaemia) could be transmitted by cell-free extracts. Since then, it has been shown very convincingly that viruses can directly cause several human cancers by various mechanisms. Epidemiological data imply that viruses are the second most important risk factor for cancer development in humans, exceeded only by tobacco consumption. Although the ability of certain viruses (hepatitis B and C, human papillomavirus, etc) to cause cancer has been time tested and proven scientifically, there are several other potential viral candidates whose role in oncogenesis is more controversial. One such controversial scenario involves the role of cytomegalovirus (CMV) in malignant gliomas, the most common form of primary brain tumour. CMV first attracted attention about a decade ago when CMV gene products were found in glioma tissue but not in normal brain. Since this initial observation, several different groups have shown an oncomodulatory effect of CMV; however, direct association between CMV infection and incidence of glioma is lacking. In this review, we will evaluate the evidence, both preclinical and clinical, regarding the possible role of CMV in gliomagenesis and maintenance. We will also critically evaluate the rationale for using antiviral drugs in the treatment of patients with glioma.

Human cytomegalovirus tegument protein pUL83 inhibits IFI16-mediated DNA sensing for immune evasion

Nuclear sensing of viral DNA has emerged as an essential step in innate immune responses against herpesviruses. Here, we provide mechanistic insight into host recognition of human cytomegalovirus (HCMV) and subsequent immune evasion by this prominent DNA virus. We establish that the interferon-inducible protein IFI16 acts as a nuclear DNA sensor following HCMV infection, binding viral DNA and triggering expression of antiviral cytokines via the STING-TBK1-IRF3 signaling pathway. The HCMV tegument protein pUL83 inhibits this response by interacting with the IFI16 pyrin domain, blocking its oligomerization upon DNA sensing and subsequent immune signals. pUL83 disrupts IFI16 by concerted action of its N- and C-terminal domains, in which an evolutionarily conserved N-terminal pyrin association domain (PAD) binds IFI16. Additionally, phosphorylation of the N-terminal domain modulates pUL83-mediated inhibition of pyrin aggregation. Collectively, our data elucidate the interplay between host DNA sensing and HCMV immune evasion, providing targets for restoring antiviral immunity.

The tegument protein pp65 of human cytomegalovirus acts as an optional scaffold protein that optimizes protein uploading into viral particles

The mechanisms that lead to the tegumentation of herpesviral particles are only poorly defined. The phosphoprotein 65 (pp65) is the most abundant constituent of the virion tegument of human cytomegalovirus (HCMV). It is, however, nonessential for virion formation. This seeming discrepancy has not met with a satisfactory explanation regarding the role of pp65 in HCMV particle morphogenesis. Here, we addressed the question of how the overall tegument composition of the HCMV virion depended on pp65 and how the lack of pp65 influenced the packaging of particular tegument proteins. To investigate this, we analyzed the proteomes of pp65-positive (pp65pos) and pp65-negative (pp65neg) virions by label-free quantitative mass spectrometry and determined the relative abundances of tegument proteins. Surprisingly, only pUL35 was elevated in pp65neg virions. As the abundance of pUL35 in the HCMV tegument is low, it is unlikely that it replaced pp65 as a structural component in pp65neg virions. A subset of proteins, including the third most abundant tegument protein, pUL25, as well as pUL43, pUL45, and pUL71, were reduced in pp65neg or pp65low virions, indicating that the packaging of these proteins was related to pp65. The levels of tegument components, like pp28 and the capsid-associated tegument proteins pp150, pUL48, and pUL47, were unaffected by the lack of pp65. Our analyses demonstrate that deletion of pp65 is not compensated for by other viral proteins in the process of virion tegumentation. The results are concordant with a model of pp65 serving as an optional scaffold protein that facilitates protein upload into the outer tegument of HCMV particles.

IMPORTANCE

The assembly of the tegument of herpesviruses is only poorly understood. Particular proteins, like HCMV pp65, are abundant tegument constituents. pp65 is thus considered to play a major role in tegument assembly in the process of virion morphogenesis. We show here that deletion of the pp65 gene leads to reduced packaging of a subset of viral proteins, indicating that pp65 acts as an optional scaffold protein mediating protein upload into the tegument.

Controlling cytomegalovirus: helping the immune system take the lead

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

Cytomegalovirus pp65 limits dissemination but is dispensable for persistence

The most abundantly produced virion protein in human cytomegalovirus (HCMV) is the immunodominant phosphoprotein 65 (pp65), which is frequently included in CMV vaccines. Although it is nonessential for in vitro CMV growth, pp65 displays immunomodulatory functions that support a potential role in primary and/or persistent infection. To determine the contribution of pp65 to CMV infection and immunity, we generated a rhesus CMV lacking both pp65 orthologs (RhCMVΔpp65ab). While deletion of pp65ab slightly reduced growth in vitro and increased defective particle formation, the protein composition of secreted virions was largely unchanged. Interestingly, pp65 was not required for primary and persistent infection in animals. Immune responses induced by RhCMVΔpp65ab did not prevent reinfection with rhesus CMV; however, reinfection with RhCMVΔUS2-11, which lacks viral-encoded MHC-I antigen presentation inhibitors, was prevented. Unexpectedly, induction of pp65b-specific T cells alone did not protect against RhCMVΔUS2-11 challenge, suggesting that T cells targeting multiple CMV antigens are required for protection. However, pp65-specific immunity was crucial for controlling viral dissemination during primary infection, as indicated by the marked increase of RhCMVΔpp65ab genome copies in CMV-naive, but not CMV-immune, animals. Our data provide rationale for inclusion of pp65 into CMV vaccines but also demonstrate that pp65-induced T cell responses alone do not recapitulate the protective effect of natural infection.

Cytomegalovirus following stem cell transplantation: from pharmacologic to immunologic therapy

Human cytomegalovirus is a large DNA virus that is well-equipped to evade both innate and adaptive host immune responses and to establish lifelong latency. It is a major opportunistic pathogen in immunocompromised hosts. Following allogeneic transplantation, immune responses are often inadequate to inhibit viral reactivation, resulting in progressive tissue damage, manifesting as overt human cytomegalovirus disease that usually presents as pneumonitis, colitis or hepatitis. Currently available antiviral pharmacotherapies are limited by toxicities if used prophylactically, and by a lack of efficacy in established human cytomegalovirus disease. Efforts have therefore focused on molecular diagnostic surveillance protocols that allow earlier intervention and the development of adoptive immunotherapeutic strategies to hasten host immune reconstitution.

Presentation of an immunodominant immediate-early CD8+ T cell epitope resists human cytomegalovirus immunoevasion

Control of human cytomegalovirus (HCMV) depends on CD8+ T cell responses that are shaped by an individual's repertoire of MHC molecules. MHC class I presentation is modulated by a set of HCMV-encoded proteins. Here we show that HCMV immunoevasins differentially impair T cell recognition of epitopes from the same viral antigen, immediate-early 1 (IE-1), that are presented by different MHC class I allotypes. In the presence of immunoevasins, HLA-A- and HLA-B-restricted T cell clones were ineffective, but HLA-C*0702-restricted T cell clones recognized and killed infected cells. Resistance of HLA-C*0702 to viral immunoevasins US2 and US11 was mediated by the alpha3 domain and C-terminal region of the HLA heavy chain. In healthy donors, HLA-C*0702-restricted T cells dominated the T cell response to IE-1. The same HLA-C allotype specifically protected infected cells from attack by NK cells that expressed a corresponding HLA-C-specific KIR. Thus, allotype-specific viral immunoevasion allows HCMV to escape control by NK cells and HLA-A- and HLA-B-restricted T cells, while the virus becomes selectively vulnerable to an immunodominant population of HLA-C-restricted T cells. Our work identifies a T cell population that may be of particular efficiency in HCMV-specific immunotherapy.

Human cytomegalovirus pUL97 regulates the viral major immediate early promoter by phosphorylation-mediated disruption of histone deacetylase 1 binding

Human cytomegalovirus (HCMV) is a common agent of congenital infection and causes severe disease in immunocompromised patients. Current approved therapies focus on inhibiting viral DNA replication. The HCMV kinase pUL97 contributes to multiple stages of viral infection including DNA replication, controlling the cell cycle, and virion maturation. Our studies demonstrate that pUL97 also functions by influencing immediate early (IE) gene expression during the initial stages of infection. Inhibition of kinase activity using the antiviral compound maribavir or deletion of the UL97 gene resulted in decreased expression of viral immediate early genes during infection. Expression of pUL97 was sufficient to transactivate IE1 gene expression from the viral genome, which was dependent on viral kinase activity. We observed that pUL97 associates with histone deacetylase 1 (HDAC1). HDAC1 is a transcriptional corepressor that acts to silence expression of viral genes. We observed that inhibition or deletion of pUL97 kinase resulted in increased HDAC1 and decreased histone H3 lysine 9 acetylation associating with the viral major immediate early (MIE) promoter. IE expression during pUL97 inhibition or deletion was rescued following inhibition of deacetylase activity. HDAC1 associates with chromatin by protein-protein interactions. Expression of active but not inactive pUL97 kinase decreased HDAC1 interaction with the transcriptional repressor protein DAXX. Finally, using mass spectrometry, we found that HDAC1 is uniquely phosphorylated upon expression of pUL97. Our results support the conclusion that HCMV pUL97 kinase regulates viral immediate early gene expression by phosphorylation-mediated disruption of HDAC1 binding to the MIE promoter.

Diverse immune evasion strategies by human cytomegalovirus

Members of the Herpesviridae family have the capacity to undergo both lytic and latent infection to establish a lifelong relationship with their host. Following primary infection, human cytomegalovirus (HCMV) can persist as a subclinical, recurrent infection for the lifetime of an individual. This quiescent portion of its life cycle is termed latency and is associated with periodic bouts of reactivation during times of immunosuppression, inflammation, or stress. In order to exist indefinitely and establish infection, HCMV encodes a multitude of immune modulatory mechanisms devoted to escaping the host antiviral response. HCMV has become a paradigm for studies of viral immune evasion of antigen presentation by both major histocompatibility complex (MHC) class I and II molecules. By restricting the presentation of viral antigens during both productive and latent infection, HCMV limits elimination by the human immune system. This review will focus on understanding how the virus manipulates the pathways of antigen presentation in order to modulate the host response to infection.

Human cytomegalovirus pp71 stimulates major histocompatibility complex class i presentation of IE1-derived peptides at immediate early times of infection

Suppression of major histocompatibility complex (MHC) class I-mediated presentation of human cytomegalovirus (HCMV) peptides is an important mechanism to avoid CD8 T lymphocyte recognition and killing of infected cells. Of particular interest is how MHC class I presentation of essential regulatory immediate early (IE) proteins of HCMV can be effectively compromised at times when known viral immunoevasins are not abundantly expressed. The tegument protein pp71 had been suggested to be involved in MHC class I downregulation. Intriguingly, this polypeptide is also critically engaged in the initial derepression of the major IE gene locus, leading to enhanced expression of IE proteins IE1-pp72 and IE2-pp86. Using a set of viral mutants, we addressed the role of pp71 in MHC class I presentation of IE1-pp72-derived peptides. We show that the amount of "incoming" pp71 positively correlates with IE1-pp72 protein levels and with the presentation of IE1-derived peptides. This indicates that the amount of the IE1 protein, induced by pp71, rather than a putative immunoevasive function of the tegument protein, determines MHC class I antigen presentation of IE1-derived peptides. This process proved to be independent of the presence of pp65, which had been reported to interfere with IE1 presentation. It may thus be beneficial for the success of HCMV replication to limit the level of pp71 delivered from infecting particles in order to avoid critical levels of MHC class I presentation of IE protein-derived peptides.

Nucleocytoplasmic shuttling and CRM1-dependent MHC class I peptide presentation of human cytomegalovirus pp65

The phosphoprotein 65 (pp65) of human cytomegalovirus is a prominent target of the antiviral CD8 T lymphocyte response. This study focused on investigating the properties of pp65 that render it a privileged antigen. It was found that pp65 was metabolically stable. The tegument protein was introduced into MHC class I presentation following its delivery via non-replicating dense bodies. No ubiquitination was found on particle-associated pp65. Proof was obtained that pp65 was a nucleocytoplasmic shuttle protein, using heterokaryon analyses. Based on this finding, inhibition experiments showed that presentation of particle-derived pp65 by HLA-A2 was sensitive to the impairment of the CRM1-mediated nuclear export pathway. The data support the idea that particle-derived pp65 can serve as a nuclear reservoir for proteasomal processing and MHC class I presentation, following its CRM1-dependent nuclear export. The presentation of pp65-derived peptides was also impaired by CRM1-inhibition following de novo synthesis of the tegument protein. However, pp65 protein levels were also reduced when blocking CRM1-mediated export after transient expression. This indicated that pp65 expression rather than direct interference with its own nuclear export was responsible for its reduced presentation in this case. The functionality of CRM1-mediated nuclear export is thus important for the presentation of pp65-derived peptides in the context of MHC class I on organ cells, both after exogenous uptake and after de novo synthesis of the tegument protein, but different mechanisms may account for either case.

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.

Deep sequencing of antiviral T-cell responses to HCMV and EBV in humans reveals a stable repertoire that is maintained for many years

CD8⁺ T-cell responses against latent viruses can cover considerable portions of the CD8⁺ T-cell compartment for many decades, yet their initiation and maintenance remains poorly characterized in humans. A key question is whether the clonal repertoire that is raised during the initial antiviral response can be maintained over these long periods. To investigate this we combined next-generation sequencing of the T-cell receptor repertoire with tetramer-sorting to identify, quantify and longitudinally follow virus-specific clones within the CD8⁺ T-cell compartment. Using this approach we studied primary infections of human cytomegalovirus (hCMV) and Epstein Barr virus (EBV) in renal transplant recipients. For both viruses we found that nearly all virus-specific CD8⁺ T-cell clones that appeared during the early phase of infection were maintained at high frequencies during the 5-year follow-up and hardly any new anti-viral clones appeared. Both in transplant recipients and in healthy carriers the clones specific for these latent viruses were highly dominant within the CD8⁺ T-cell receptor Vβ repertoire. These findings suggest that the initial antiviral response in humans is maintained in a stable fashion without signs of contraction or changes of the clonal repertoire.

Several viruses have found ways to evade the human immune system and cause latent infections. Examples include HIV and herpes-viruses. Most humans carry these herpes-viruses. The human immune system mounts continuous responses against these viruses to prevent them from causing disease. If this balance is disturbed, these viruses can cause extensive pathology. We do not know how the immune response against these viruses evolves over time. Understanding this response might help to understand why the immune system does not clear these viruses and might help in preventive and therapeutic strategies. Here we used a new technology that allowed us to track virus specific immune cells (CD8⁺ T cells) over time in a quantitative manner. When we used this technology to study the evolution of latent responses against herpes-viruses (from infection until 5 years later) we found that immune responses were very rigid and did not evolve over time. Collectively our data shows that – for these herpes-viruses – the initial immune response is maintained despite the fact that this does not result in clearance of the virus. Therefore, if a virus survives the initial response, it will not be cleared in the future. This is an important consideration in understanding latent infection and for vaccination-design.

Autophagy mediates transporter associated with antigen processing-independent presentation of viral epitopes through MHC class I pathway

The endogenous presentation of the majority of viral epitopes through MHC class I pathway is strictly dependent on the transporter associated with antigen processing (TAP) complex, which transfers the peptide products of proteasomal degradation into the endoplasmic reticulum. A small number of epitopes can be presented through the TAP-independent pathway, the precise mechanism for which remains largely unresolved. Here we show that TAP-independent presentation can be mediated by autophagy and that this process uses the vacuolar pathway and not the conventional secretory pathway. After macroautophagy, the antigen is processed through a proteasome-independent pathway, and the peptide epitopes are loaded within the autophagolysosomal compartment in a process facilitated by the relative acid stability of the peptide-MHC interaction. Despite bypassing much of the conventional MHC class I pathway, the autophagy-mediated pathway generates the same epitope as that generated through the conventional pathway and thus may have a role in circumventing viral immune evasion strategies that primarily target the conventional pathway.

Generation of polyclonal CMV-specific T cells for the adoptive immunotherapy of glioblastoma

Glioblastoma (GBM) is the most common primary brain cancer in adults and is virtually incurable. Recent studies have shown that cytomegalovirus (CMV) is present in majority of GBMs. To evaluate whether the CMV antigens pp65 and IE1, which are expressed in GBMs, could be targeted by CMV-specific T cells, we measured the frequency of T cells targeting pp65 and IE1 in the peripheral blood of a cohort of 11 sequentially diagnosed CMV-seropositive GBM patients, and evaluated whether it was feasible to expand autologous CMV-specific T cells for future clinical studies. All 11 CMV-seropositive GBM patients had T cells specific for pp65 and IE1 in their peripheral blood assessed by IFNγ enzyme-linked immunospot assay. However, the precursor frequency of pp65-specific T cells was decreased in comparison with healthy donors (P=0.001). We successfully reactivated and expanded CMV-specific T cells from 6 out of 6 GBM patients using antigen-presenting cells transduced with an adenoviral vector encoding pp65 and IE1. CMV-specific T-cell lines contained CD4 as well as CD8 T cells, recognized pp65 and IE1 targets and killed CMV-infected autologous GBM cells. Infusion of such CMV-specific T-cell lines may extend the benefits of T-cell therapy to patients with CMV GBMs.

[Cytomegalovirus. Pathological-anatomical manifestations and detection methods]

Human cytomegalovirus, a double-stranded DNA virus, is a member of the Herpesviridae family with high rates of transmission. Primary infection is often asymptomatic and leads to life-long latency. Reactivation may induce different organ manifestations, particularly in the setting of immunosuppression. Histopathologically, the virus can be detected by light microscopy. Different cell populations in different organs are transformed into"owl's eye" cells, which are pathognomonic. Immunohistochemistry and electron microscopy can be applied as complementary methods. Various PCR approaches in molecular pathology including nested PCR, capture probe ELISA-PCR and real time PCR confer HCMV tests high sensitivity and specificity. The present article discusses the methods of pathological diagnostic approaches and describes organ manifestations of HCMV.

Human cytomegalovirus tegument proteins (pp65, pp71, pp150, pp28)

Human cytomegalovirus (HCMV), a member of the Betaherpesvirinae sub-family of Herpesviridae family, is a widespread pathogen that infects a majority of the world's population by early adulthood. In individuals whose immune systems are immature or weakened, HCMV is a significant pathogen causing morbidity and mortality. There is no effective vaccine and only limited antiviral treatments against HCMV infection to date. A possible target for novel antiviral treatments is the HCMV proteins that localize to the tegument of the virion, since they play important roles in all stages of the viral life cycle, including, viral entry, gene expression, immune evasion, assembly, and egress. The most likely tegument protein candidates are pp65 (immune evasion), pp71 (gene expression), and pp150 and pp28 (assembly and egress). Although the subcellular localization of these proteins has been identified during HCMV infections in vitro, their localization patterns have not been determined when each protein is expressed individually in living cells. Thus, the objective of this review is elucidate the HCMV tegument as well as present current research findings concerning the subcellular localization of the tegument proteins pp65, pp71, pp150, and pp28 as fusions to one of several fluorescent proteins.

The tug-of-war between dendritic cells and human chronic viruses

The human immune system is under constant challenge from many viruses, some of which the body is successfully able to clear. Other viruses have evolved to escape the host immune responses and thus persist, leading to the development of chronic diseases. Dendritic cells are professional antigen-presenting cells that play a major role in both innate and adaptive immunity against different pathogens. This review focuses on the interaction of different chronic viruses with dendritic cells and the viruses' ability to exploit this critical cell type to their advantage so as to establish persistence within the host.

Expansion of T cells targeting multiple antigens of cytomegalovirus, Epstein-Barr virus and adenovirus to provide broad antiviral specificity after stem cell transplantation

BACKGROUND AIMS

Hematopoietic stem cell transplant (HSCT) is the treatment of choice for a proportion of patients with hematologic malignancies as well as for non-malignant diseases. However, viral infections, particularly Epstein-Barr virus (EBV), cytomegalovirus (CMV) and adenovirus (Ad), remain problematic after transplant despite the use of antiviral drugs. We have shown that cytotoxic T lymphocytes (CTL) generated against CMV-pp65, EBV and Ad antigens in a single culture are capable of controlling infections with all three viruses after HSCT. Although pp65-specific CTL have proved efficacious for the control of CMV infection, several reports highlight the importance of targeting additional CMV antigens.

METHODS

To expand multivirus-specific T cells with activity against both CMV-pp65 and CMV-IE-1, peripheral blood mononuclear cells (PBMC) were transduced with the adenoviral vector (Ad5f35-IE-1-I-pp65). After 9-12 days the CTL were restimulated with autologous EBV-transformed B cells transduced with the same Ad vector.

RESULTS

After 18 days in culture nine CTL lines expanded from less than 1.5 × 10(7) PBMC to a mean of 6.1 × 10(7) T cells that recognized CMV antigens pp65 [median 273 spot-forming cells (SFC), range 47-995] and IE-1 (median 154 SFC, range 11-505), the Ad antigens hexon (median 153 SFC, range 26-465) and penton (median 37 SFC, range 1-353), as well as EBV lymphoblastoid cell lines (median 55 SFC, range 9-301). Importantly, the T cells recognized at least two antigens per virus and lysed virus peptide-pulsed targets.

CONCLUSIONS

CTL that target at least two antigens each of CMV, EBV and Ad should have clinical benefit with broad coverage of all three viruses and enhanced control of CMV infections compared with current protocols.

[Cytomegalovirus (CMV)]

Human cytomegalovirus (HCMV) is a ubiquitous beta human herpesvirus type 5. Compared to other human herpesviruses, HCMV is the largest, with a genome of approximately 235 kb containing approximately 250 ORFs with the potential to encode proteins. Usually, HCMV asymptomatically infects the host during childhood, and establishes life-long latency. The infection is life-threatening for infants and immunocompromised individuals, because of direct cytopathicity by viral replication, causing systemic organ injuries. Intrauterine infection occasionally causes microcephaly, sensorineural hearing loss and mental retardation. HCMV genome contains a number of accessory genes. Most of them are engaged in immune evasion or inhibition of cell death, possibly, resulting in a symbiosis between virus and host. CD34-positive myeloid progenitor cells are considered as a site of latency. However, the molecular mechanisms by which HCMV establishes and maintains latency and reactivates remain poorly understood. Recently in Japan, the decline of maternal HCMV seropositivity may increase the risk of intrauterine infection. It needs to immediately establish the protection against transplacental HCMV infection, such as a new type of neutralizing antibody or vaccine, which effectively interferes viral entry specific to endothelial and epithelial cells. Furthermore, HCMV infection might be considered as the most important factor for driving immune senescence in the elderly.

Biochemical and structural characterization of the capsid-bound tegument proteins of human cytomegalovirus

Human cytomegalovirus (HCMV) is the most genetically and structurally complex human herpesvirus and is composed of an envelope, a tegument, and a dsDNA-containing capsid. HCMV tegument plays essential roles in HCMV infection and assembly. Using cryo electron tomography (cryoET), here we show that HCMV tegument compartment can be divided into two sub-compartments: an inner and an outer tegument. The inner tegument consists of densely-packed proteins surrounding the capsid. The outer tegument contains those components that are loosely packed in the space between the inner tegument and the pleomorphic glycoprotein-containing envelope. To systematically characterize the inner tegument proteins interacting with the capsid, we used chemical treatment to strip off the entire envelope and most tegument proteins to obtain a tegumented capsid with inner tegument proteins. SDS-polyacrylamide gel electrophoresis analyses show that only two tegument proteins, UL32-encoded pp150 and UL48-encoded high molecular weight protein (HMWP), remains unchanged in their abundance in the tegumented capsids as compared to their abundance in the intact particles. Three-dimensional reconstructions by single particle cryo electron microscopy (cryoEM) reveal that the net-like layer of icosahedrally-ordered tegument densities are also the same in the tegumented capsid and in the intact particles. CryoET reconstruction of the tegumented capsid labeled with an anti-pp150 antibody is consistent with the biochemical and cryoEM data in localizing pp150 within the ordered tegument. Taken together, these results suggest that pp150, a betaherpesvirus-specific tegument protein, is a constituent of the net-like layer of icosahedrally-ordered capsid-bound tegument densities, a structure lacking similarities in alpha and gammaherpesviruses.

Human cytomegalovirus decreases constitutive transcription of MHC class II genes in mature Langerhans cells by reducing CIITA transcript levels

Human cytomegalovirus (HCMV) productively infects CD34(+) progenitor-derived, mature Langerhans-type dendritic cells (matLC) and reduces surface expression of MHC class II complexes (MHC II) by increasing intracellular retention of these molecules. To determine whether HCMV also inhibits MHC II expression by other mechanisms, we assessed mRNA levels of the class II transcriptional regulator, CIITA, and several of its target genes in infected matLC. Levels of CIITA, HLA-DRA (DRA) and DRB transcripts, and new DR protein synthesis were compared in mock-infected and HCMV-infected cells by quantitative PCR and pulse-chase immunoprecipitation analyses, respectively. CIITA mRNA levels were significantly lower in HCMV-infected matLC as compared to mock-infected cells. When assessed in the presence of Actinomycin D, the stability of CIITA transcripts was not diminished by HCMV. Analysis of promoter-specific CIITA isoforms revealed that types I, III and IV all were decreased by HCMV, a result that differs from changes after incubation of these cells with lipopolysaccharide (LPS). Exposure to UV-inactivated virus failed to reduce CIITA mRNA levels, implicating de novo viral gene expression in this effect. HCMV-infected matLC also expressed lower levels of DR transcripts and reduced DR protein synthesis rates compared to mock-infected matLC. In summary, we demonstrate that HCMV infection of a human dendritic cell subset inhibits constitutive CIITA expression, most likely at the transcriptional level, resulting in reduced MHC II biosynthesis. We suggest this represents a new mechanism of modulation of mature LC by HCMV.

Immunity and immunopathology to viruses: what decides the outcome?

Many viruses infect humans and most are controlled satisfactorily by the immune system with limited damage to host tissues. Some viruses, however, do cause overt damage to the host, either in isolated cases or as a reaction that commonly occurs after infection. The outcome is influenced by properties of the infecting virus, the circumstances of infection and several factors controlled by the host. In this Review, we focus on host factors that influence the outcome of viral infection, including genetic susceptibility, the age of the host when infected, the dose and route of infection, the induction of anti-inflammatory cells and proteins, as well as the presence of concurrent infections and past exposure to cross-reactive agents.

Dendritic cells in cytomegalovirus infection: viral evasion and host countermeasures

Human cytomegalovirus (HCMV) is a beta-herpesvirus that infects the majority of the population during early childhood and thereafter establishes life-long latency. Primary infection as well as spontaneous reactivation usually remains asymptomatic in healthy hosts but can, in the context of systemic immunosuppression, result in substantial morbidity and mortality. HCMV counteracts the host immune response by interfering with the recognition of infected cells. A growing body of literature has also suggested that the virus evades the immune system by paralyzing the initiators of antiviral immune responses--the dendritic cells (DCs). In the current review, we discuss the effects of CMV (HCMV and murine CMV) on various DC subsets and the ensuing innate and adaptive immune responses. The impact of HCMV on DCs has mainly been investigated using monocyte-derived DCs, which are rendered functionally impaired by infection. In mouse models, DCs are targets of viral evasion as well, but the complex cross-talk between DCs and natural killer cells has, however, demonstrated an instrumental role for DCs in the control and clearance of viral infection. Fewer studies address the role of peripheral blood DC subsets, plasmacytoid DCs and CD11c+ myeloid DCs in the response against HCMV. These DCs, rather than being paralyzed by HCMV, are largely resistant to infection, mount a vigorous first-line defense and induce T-cell responses to the virus. This possibly provides a partial explanation for an intriguing conundrum: the highly efficient control of viral infection and reactivation in immunocompetent hosts in spite of multi-layered viral evasion mechanisms.

Immune evasion during foot-and-mouth disease virus infection of swine

Summary: The interface between successful pathogens and their hosts is often a tenuous balance. In acute viral infections, this balance involves induction and inhibition of innate responses. Foot‐and‐mouth disease virus (FMDV) is considered one of the most contagious viruses known and is characterized by rapid induction of clinical disease in cloven hoofed animals exposed to infection. Viral shedding is extensive before the equally rapid resolution of acute disease. This positive strand RNA virus is an extremely successful pathogen, due in part to the ability to interrupt the innate immune response. Previous reviews have described the inhibition of cellular innate responses in the infected cell both in vitro and in vivo. Here, we present a review of virus inhibition of cells that are a source of antiviral function in swine. Particularly in the case of dendritic cells and natural killer cells, the virus has evolved mechanisms to interrupt the normal function of these important mediators of innate function, even though these cells are not infected by the virus. Understanding how this virus subverts the innate response will provide valuable information for the development of rapidly acting biotherapeutics to use in response to an outbreak of FMDV.

Immunobiology of human cytomegalovirus: from bench to bedside

SUMMARY

Following primary infection, human cytomegalovirus (HCMV) establishes lifelong latency and periodically reactivates without causing symptoms in healthy individuals. In the absence of an adequate host-derived immune response, this fine balance of permitting viral reactivation without causing pathogenesis is disrupted, and HCMV can subsequently cause invasive disease and an array of damaging indirect immunological effects. Over the last decade, our knowledge of the immune response to HCMV infection in healthy virus carriers and diseased individuals has allowed us to translate these findings to develop better diagnostic tools and therapeutic strategies. The application of these emerging technologies in the clinical setting is likely to provide opportunities for better management of patients with HCMV-associated diseases.

Expansion of human cytomegalovirus (HCMV) immediate-early 1-specific CD8+ T cells and control of HCMV replication after allogeneic stem cell transplantation

Recovery of human cytomegalovirus (HCMV)-specific T immunity is critical for protection against HCMV disease in the early phase after allogeneic stem cell transplantation (SCT). Using an enzyme-linked immunospot assay with overlapping 15-mer peptides spanning pp65 and immediate-early 1 HCMV proteins, we investigated which HCMV-specific CD8(+) gamma interferon-positive (IFN-gamma(+)) T-cell responses against pp65 and IE-1 were associated with control of HCMV replication in 48 recipients of unmanipulated HLA-matched allografts at 3 months (M3) and 6 months (M6) after SCT and in 23 donors. At M3 after SCT, the magnitude of the pp65-specific IFN-gamma-producing CD8(+) T-cell response was greater in recipients than in donors, regardless of HCMV status. In contrast, expansion of IE-1-specific CD8(+) T cells at M3 was associated with protection against HCMV, and no patient with this expansion had HCMV replication at M3. At M6, the number of HCMV-specific CD8(+) T cells against both pp65 and IE-1 had expanded in all recipients, regardless of their previous levels of HCMV replication. The recipients' HCMV-specific CD8(+) T cells already detectable in related donors were predominantly targeting pp65. In contrast, in 40% of the cases, the HCMV-specific CD8(+) T cells in recipients involved new CD8(+) T-cell specificities undetectable in their related donors and preferentially targeting IE-1. Taken together, these results showed that the delay in reconstituting IE-1-specific CD8(+) T cells is correlated with the lack of protection against HCMV in the first 3 months after SCT. They also show that IE-1 is a major antigenic determinant of the early restoration of protective immunity to HCMV after SCT.

Tegument proteins of human cytomegalovirus

SUMMARY

Human cytomegalovirus (HCMV) is a common, medically relevant human herpesvirus. The tegument layer of herpesvirus virions lies between the genome-containing capsids and the viral envelope. Proteins within the tegument layer of herpesviruses are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active and control viral entry, gene expression, and immune evasion. Most tegument proteins accumulate to high levels during later stages of infection, when they direct the assembly and egress of progeny virions. Thus, viral tegument proteins play critical roles at the very earliest and very last steps of the HCMV lytic replication cycle. This review summarizes HCMV tegument composition and structure as well as the known and speculated functions of viral tegument proteins. Important directions for future investigation and the challenges that lie ahead are identified and discussed.

Selective modification of antigen-specific T cells by RNA electroporation

It has been observed that the efficient transfection of T cells by RNA electroporation requires prior activation of T cells with mitogens or by anti-CD3 antibody stimulation. We hypothesized that this requirement for T cell activation could be leveraged to express marker genes within activated T cells responding to antigen-pulsed dendritic cells and allow for the selective enrichment and modification of antigen-specific T cells. Using electroporation of mRNA encoding green fluorescent protein as a marker gene, we demonstrate that RNA electroporation can efficiently allow for the separation of cytomegalovirus-specific CD8+ and CD4+ T cells from bulk culture responding to cytomegalovirus pp65 antigen-pulsed dendritic cells. Furthermore, we demonstrate that cytomegalovirus-specific T cells can be functionally modified by RNA transfection of the C-X-C chemokine receptor, CXCR2, to migrate efficiently toward a variety of CXCR2-specific chemokines in vitro and in vivo. These studies demonstrate the utility of RNA transfection as a simple method by which to purify and selectively modify the function of antigen-specific T cells for use in adoptive immunotherapy, and importantly provide evidence that transient expression of proteins by RNA transfection is an efficient means of modulating the in vivo function of activated T cells.

A fusion protein of HCMV IE1 exon4 and IE2 exon5 stimulates potent cellular immunity in an MVA vaccine vector

A therapeutic CMV vaccine incorporating an antigenic repertoire capable of eliciting a cellular immune response has yet to be successfully implemented for patients who already have acquired an infection. To address this problem, we have developed a vaccine candidate derived from modified vaccinia Ankara (MVA) that expresses three immunodominant antigens (pp65, IE1, IE2) from CMV. The novelty of this vaccine is the fusion of two adjacent exons from the immediate-early region of CMV, their successful expression in MVA, and robust immunogenicity in both primary and memory response models. Evaluation of the immunogenicity of the viral vaccine in mouse models shows that it can stimulate primary immunity against all three antigens in both the CD4(+) and CD8(+) T cell subsets. Evaluation of human PBMC from healthy CMV-positive donors or patients within 6 months of receiving hematopoietic cell transplant shows robust stimulation of existing CMV-specific CD4(+) and CD8(+) T cell subsets.

Recombinant viruses as tools to study human cytomegalovirus immune modulation

Infections with cytomegaloviruses are characterized by an intricate balance between the expression of immunomodulatory viral proteins and antiviral immune defence. For human cytomegalovirus (HCMV), several proteins have been described that interfere with the recognition of infected cells by CD8 T lymphocytes. Although the modes of action of these proteins have been elucidated on the molecular level, thus rendering them useful models to understand MHC class I peptide loading and transport, their role during viral infection has remained enigmatic. We exemplify here, how HCMV mutants can help to understand the importance of individual immunomodulatory proteins in the context of viral infection.