Viral interference with B7-1 costimulation: a new role for murine cytomegalovirus fc receptor-1

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

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

Structural and functional insights into the modulation of T cell costimulation by monkeypox virus protein M2

The rapid spread of monkeypox in multiple countries has resulted in a global public health threat and has caused international concerns since May 2022. Poxvirus encoded M2 protein is a member of the poxvirus immune evasion family and plays roles in host immunomodulation via the regulation of innate immune response mediated by the NF-κB pathway and adaptive immune response mediated by B7 ligands. However, the interaction of monkeypox virus (MPXV) M2 with B7 ligands and structural insight into poxviral M2 function have remained elusive. Here we reveal that MPXV M2, co-existing as a hexamer and a heptamer, recognizes human B7.1 and B7.2 (hB7.1/2) with high avidities. The binding of oligomeric MPXV M2 interrupts the interactions of hB7.1/2 with CD28 and CTLA4 and subverts T cell activation mediated by B7.1/2 costimulatory signals. Cryo-EM structures of M2 in complex with hB7.1/2 show that M2 binds to the shallow concave face of hB7.1/2 and displays sterically competition with CD28 and CTLA4 for the binding to hB7.1/2. Our findings provide structural mechanisms of poxviral M2 function and immune evasion deployed by poxviruses.

Cytomegalovirus restricts ICOSL expression on antigen-presenting cells disabling T cell co-stimulation and contributing to immune evasion

Viral infections are controlled, and very often cleared, by activated T lymphocytes. The inducible co-stimulator (ICOS) mediates its functions by binding to its ligand ICOSL, enhancing T-cell activation and optimal germinal center (GC) formation. Here, we show that ICOSL is heavily downmodulated during infection of antigen-presenting cells by different herpesviruses. We found that, in murine cytomegalovirus (MCMV), the immunoevasin m138/fcr-1 physically interacts with ICOSL, impeding its maturation and promoting its lysosomal degradation. This viral protein counteracts T-cell responses, in an ICOS-dependent manner, and limits virus control during the acute MCMV infection. Additionally, we report that blockade of ICOSL in MCMV-infected mice critically regulates the production of MCMV-specific antibodies due to a reduction of T follicular helper and GC B cells. Altogether, these findings reveal a novel mechanism evolved by MCMV to counteract adaptive immune surveillance, and demonstrates a role of the ICOS:ICOSL axis in the host defense against herpesviruses.

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

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

Cytomegalovirus protein m154 perturbs the adaptor protein-1 compartment mediating broad-spectrum immune evasion

Cytomegaloviruses (CMVs) are ubiquitous pathogens known to employ numerous immunoevasive strategies that significantly impair the ability of the immune system to eliminate the infected cells. Here, we report that the single mouse CMV (MCMV) protein, m154, downregulates multiple surface molecules involved in the activation and costimulation of the immune cells. We demonstrate that m154 uses its cytoplasmic tail motif, DD, to interfere with the adaptor protein-1 (AP-1) complex, implicated in intracellular protein sorting and packaging. As a consequence of the perturbed AP-1 sorting, m154 promotes lysosomal degradation of several proteins involved in T cell costimulation, thus impairing virus-specific CD8⁺ T cell response and virus control in vivo. Additionally, we show that HCMV infection similarly interferes with the AP-1 complex. Altogether, we identify the robust mechanism employed by single viral immunomodulatory protein targeting a broad spectrum of cell surface molecules involved in the antiviral immune response.

Cowpox virus encodes a protein that binds B7.1 and B7.2 and subverts T cell costimulation

Costimulation is required for optimal T cell activation, yet it is unclear whether poxviruses dedicatedly subvert costimulation during infection. Here, we report that the secreted M2 protein encoded by cowpox virus (CPXV) specifically interacts with human and murine B7.1 (CD80) and B7.2 (CD86). We also show that M2 competes with CD28 and CTLA4 for binding to cell surface B7 ligands, with stronger efficacy against CD28. Functionally, recombinant M2 and culture supernatants from wild-type (WT) but not M2-deficient (∆M2) CPXV-infected cells can potently suppress B7 ligand-mediated T cell proliferation and interleukin-2 (IL-2) production. Furthermore, we observed increased antiviral CD4 and CD8 T cell responses in C57BL/6 mice challenged by ∆M2 CPXV compared with WT virus. These differences in immune responses to ∆M2 and WT CPXV were not observed in CD28-deficient mice. Taken together, our findings define a mechanism of viral sabotage of T cell activation that highlights the role of CD28 costimulation in host defense against poxvirus infections.

Cytomegalovirus: Shape-Shifting the Immune System

Systems-based based approaches have begun to shed light on extrinsic factors that contribute to immune system variation. Among these, CMV (HHV-5, a β-herpesvirus) imposes a surprisingly profound impact. Most of the world's population is CMV⁺, and the virus goes through three distinct infection phases en route to establishing lifelong détente with its host. Immune control of CMV in each phase recruits unique arms of host defense, and in turn the virus employs multiple immune-modulatory strategies that help facilitate the establishment of lifelong persistence. In this review, we explain how CMV shapes immunity and discuss the impact it may have on overall health.

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

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

Murine CMV Expressing the High Affinity NKG2D Ligand MULT-1: A Model for the Development of Cytomegalovirus-Based Vaccines

The development of a vaccine against human cytomegalovirus (CMV) has been a subject of long-term medical interest. The research during recent years identified CMV as an attractive vaccine vector against infectious diseases and tumors. The immune response to CMV persists over a lifetime and its unique feature is the inflationary T cell response to certain viral epitopes. CMV encodes numerous genes involved in immunoevasion, which are non-essential for virus growth in vitro. The deletion of those genes results in virus attenuation in vivo, which enables us to dramatically manipulate its virulence and the immune response. We have previously shown that the murine CMV (MCMV) expressing RAE-1γ, one of the cellular ligands for the NKG2D receptor, is highly attenuated in vivo but retains the ability to induce a strong CD8⁺ T cell response. Here, we demonstrate that recombinant MCMV expressing high affinity NKG2D ligand murine UL16 binding protein-like transcript (MULT-1) (MULT-1MCMV) inserted in the place of its viral inhibitor is dramatically attenuated in vivo in a NK cell-dependent manner, both in immunocompetent adult mice and in immunologically immature newborns. MULT-1MCMV was more attenuated than the recombinant virus expressing RAE-1γ. Despite the drastic sensitivity to innate immune control, MULT-1MCMV induced an efficient CD8⁺ T cell response to viral and vectored antigens. By using in vitro assay, we showed that similar to RAE-1γMCMV, MULT-1 expressing virus provided strong priming of CD8⁺ T cells. Moreover, MULT-1MCMV was able to induce anti-viral antibodies, which after passing the transplacental barrier protect offspring of immunized mothers from challenge infection. Altogether, this study further supports the concept that CMV expressing NKG2D ligand possesses excellent characteristics to serve as a vaccine or vaccine vector.

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

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

The viral context instructs the redundancy of costimulatory pathways in driving CD8(+) T cell expansion

Signals delivered by costimulatory molecules are implicated in driving T cell expansion. The requirements for these signals, however, vary from dispensable to essential in different infections. We examined the underlying mechanisms of this differential T cell costimulation dependence and found that the viral context determined the dependence on CD28/B7-mediated costimulation for expansion of naive and memory CD8⁺ T cells, indicating that the requirement for costimulatory signals is not imprinted. Notably, related to the high-level costimulatory molecule expression induced by lymphocytic choriomeningitis virus (LCMV), CD28/B7-mediated costimulation was dispensable for accumulation of LCMV-specific CD8⁺ T cells because of redundancy with the costimulatory pathways induced by TNF receptor family members (i.e., CD27, OX40, and 4-1BB). Type I IFN signaling in viral-specific CD8⁺ T cells is slightly redundant with costimulatory signals. These results highlight that pathogen-specific conditions differentially and uniquely dictate the utilization of costimulatory pathways allowing shaping of effector and memory antigen-specific CD8⁺ T cell responses.

DOI:http://dx.doi.org/10.7554/eLife.07486.001

When the immune system detects a virus in the body it mounts a response to eliminate it. Immune cells called CD8⁺ T cells detect fragments of virus proteins that are presented on the surface of other immune cells. The CD8⁺ T cells then rapidly divide to form populations that roam the body to kill cells that are infected with the virus. Afterwards, some of the CD8⁺ T cells become ‘memory T cells’, which allow the immune system to respond more rapidly if the virus returns. This means that a subsequent infection of the same virus is usually stopped before it can become severe enough for an individual to feel unwell.

Vaccines take advantage of the activities of CD8⁺ T cells to enable a person to become ‘immune’ to a virus without having to experience the disease. Vaccines contain dead or weakened viruses that can't spread in the body, but are able to activate the CD8⁺ T cells. However, a vaccine may not be as effective in activating the T cells as the live virus, perhaps because it fails to trigger the production of other molecules in the host that promote T cell activation. There are many of these ‘co-stimulatory molecules’ in the body, but it is not clear exactly how they work.

Now, Welten et al. show that the role of co-stimulatory molecules in the activation of CD8⁺ T cells depends on the type of virus and how it affects cells. Mice that were genetically engineered to lack two co-stimulatory molecules called CD80 and CD86 failed to accumulate active CD8⁺ T cells in response to infection with a herpes-like virus. However, if these mice were infected with a different virus called LCMV—which causes swelling of the brain and spinal cord—they produced many active CD8⁺ T cells to fight the infection.

Welten et al. found that other co-stimulatory molecules are able to compensate for the loss of CD80 and CD86 to boost the activation of T cells in response to LCMV, but not the herpes-like virus. Further experiments showed that LCMV triggers a lot more inflammation in infected cells than the other virus. This leads to the production of many different types of co-stimulatory molecules, which ensures that if one fails to boost the activation of CD8⁺ T cells, another molecule can do so instead. Better understanding of how these co-stimulatory molecules work could help scientists to develop more effective vaccines in future.

DOI:http://dx.doi.org/10.7554/eLife.07486.002

Cytomegalovirus immune evasion of myeloid lineage cells

Cytomegalovirus (CMV) evades the immune system in many different ways, allowing the virus to grow and its progeny to spread in the face of an adverse environment. Mounting evidence about the antiviral role of myeloid immune cells has prompted the research of CMV immune evasion mechanisms targeting these cells. Several cells of the myeloid lineage, such as monocytes, dendritic cells and macrophages, play a role in viral control, but are also permissive for CMV and are naturally infected by it. Therefore, CMV evasion of myeloid cells involves mechanisms that qualitatively differ from the evasion of non-CMV-permissive immune cells of the lymphoid lineage. The evasion of myeloid cells includes effects in cis, where the virus modulates the immune signaling pathways within the infected myeloid cell, and those in trans, where the virus affects somatic cells targeted by cytokines released from myeloid cells. This review presents an overview of CMV strategies to modulate and evade the antiviral activity of myeloid cells in cis and in trans.

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

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

Cytomegalovirus immune evasion by perturbation of endosomal trafficking

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

Deciphering the role of DC subsets in MCMV infection to better understand immune protection against viral infections

Infection of mice with murine cytomegalovirus (MCMV) recapitulates many physiopathological characteristics of human CMV infection and enables studying the interactions between a virus and its natural host. Dendritic cells (DC) are mononuclear phagocytes linking innate and adaptive immunity which are both necessary for MCMV control. DC are critical for the induction of cellular immunity because they are uniquely efficient for the activation of naïve T cells during their first encounter with a pathogen. DC are equipped with a variety of innate immune recognition receptors (I2R2) allowing them to detect pathogens or infections and to engulf molecules, microorganisms or cellular debris. The combinatorial engagement of I2R2 during infections controls DC maturation and shapes their response in terms of cytokine production, activation of natural killer (NK) cells and functional polarization of T cells. Several DC subsets exist which express different arrays of I2R2 and are specialized in distinct functions. The study of MCMV infection helped deciphering the physiological roles of DC subsets and their molecular regulation. It allowed the identification and first in vivo studies of mouse plasmacytoid DC which produce high level of interferons-α/β early after infection. Despite its ability to infect DC and dampen their functions, MCMV induces very robust, efficient and long-lasting CD8 T cell responses. Their priming may rely on the unique ability of uninfected XCR1⁺ DC to cross-present engulfed viral antigens and thus to counter MCMV interference with antigen presentation. A balance appears to have been reached during co-evolution, allowing controlled replication of the virus for horizontal spread without pathological consequences for the immunocompetent host. We will discuss the role of the interplay between the virus and DC in setting this balance, and how advancing this knowledge further could help develop better vaccines against other intracellular infectious agents.

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

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

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

Human herpesvirus 7 U21 tetramerizes to associate with class I major histocompatibility complex molecules

The U21 gene product from human herpesvirus 7 binds to and redirects class I major histocompatibility complex (MHC) molecules to a lysosomal compartment. The molecular mechanism by which U21 reroutes class I MHC molecules to lysosomes is not known. Here, we have reconstituted the interaction between purified soluble U21 and class I MHC molecules, suggesting that U21 does not require additional cellular proteins to interact with class I MHC molecules. Our results demonstrate that U21, itself predicted to contain an MHC class I-like protein fold, interacts tightly with class I MHC molecules as a tetramer, in a 4:2 stoichiometry. These observations have helped to elucidate a refined model describing the mechanism by which U21 escorts class I MHC molecules to the lysosomal compartment.

IMPORTANCE

In this report, we show that the human herpesvirus 7 (HHV-7) immunoevasin U21, itself a class I MHC-like protein, binds with high affinity to class I MHC molecules as a tetramer and escorts them to lysosomes, where they are degraded. While many class I MHC-like molecules have been described in detail, this unusual viral class I-like protein functions as a tetramer, associating with class I MHC molecules in a 4:2 ratio, illuminating a functional significance of homooligomerization of a class I MHC-like protein.

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

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

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.

Priming of CD8+ T cells against cytomegalovirus-encoded antigens is dominated by cross-presentation

CMV can infect dendritic cells (DCs), and direct Ag presentation could, therefore, lead to the priming of CMV-specific CD8(+) T cells. However, CMV-encoded immune evasins severely impair Ag presentation in the MHC class I pathway; thus, it is widely assumed that cross-presentation drives the priming of antiviral T cells. We assessed the contribution of direct versus cross priming in mouse CMV (MCMV) infection using recombinant viruses. DCs infected with an MCMV strain encoding the gB498 epitope from HSV-1 were unable to stimulate in vitro naive gB498-specific CD8(+) T cells from TCR transgenic mice. Infection of C57BL/6 mice with this recombinant virus led, however, to the generation of abundant numbers of gB498-specific T cells in vivo. Of the DC subsets isolated from infected mice, only CD8α(+) DCs were able to stimulate naive T cells, suggesting that this DC subset cross-presents MCMV-encoded Ag in vivo. Upon infection of mice with MCMV mutants encoding Ag that can either be well or hardly cross-presented, mainly CD8(+) T cells specific for cross-presented epitopes were generated. Moreover, even in the absence of immune evasion genes interfering with MHC class I-mediated Ag presentation, priming of T cells to Ag that can only be presented directly was not observed. We conclude that the host uses mainly DCs capable of cross-presentation to induce the CMV-specific CD8(+) T cell response during primary, acute infection and discuss the implications for the development of a CMV vaccine.

Stimulation of human EBV- and CMV-specific cytolytic effector function using allogeneic HLA molecules

Viral infection is a major cause of morbidity and mortality, and there are few therapeutic options available to augment a virus-specific T cell response. Although allo-HLA cross-reactivity from virus-specific memory T cells is common, it is unclear whether priming with specific allogeneic cells could conversely elicit a viral peptide/self-HLA restricted cytotoxic T cell response in humans. First, we used the previously described allo-HLA-B*44:02 cross-reactivity of EBV peptide/HLA-B8 restricted T cells, to determine whether allogeneic HLA stimulation can elicit a cytolytic immune response against EBV. HLA-B8(+) HLA-B44(-) EBV-seropositive PBMCs were stimulated with either HLA-B*44:02(+) or HLA-B*44:03(+) mismatched irradiated PBMCs in a 7-10 d MLR. The allo-HLA stimulated responder cells were then evaluated for cytotoxicity using EBV peptide loaded autologous target cells and unloaded HLA-B8(+) EBV LCL target cells. PBMCs from EBV-seropositive donors gained EBV-specific cytolytic effector function following specific allo-HLA stimulation. Finally, we also elicited cytolytic CMV-specific responses using specific allogeneic cell stimulation, to confirm that this technique can be used to elicit viral peptide/self-HLA restricted responses even from nonpublic TCR responses. Allogeneic cell stimulation used as a cell therapy may be a potential tool to augment an antiviral T cell response in patients with EBV or CMV infection.

Antiviral T cell response triggers cytomegalovirus hepatitis in mice

One common sign of human cytomegalovirus infection is altered liver function. Murine cytomegalovirus strain v70 induces a rapid and severe hepatitis in immunocompetent mice that requires the presence of T cells in order to develop. v70 exhibits approximately 10-fold-greater virulence than the commonly used strain K181, resulting in a more severe, sustained, and lethal hepatitis but not dramatically higher viral replication levels. Hepatitis and death are markedly delayed in immunodeficient SCID compared to immunocompetent BALB/c mice. Transfer of BALB/c splenocytes to SCID mice conferred rapid disease following infection, and depletion of either CD4 or CD8 T cells in BALB/c mice reduced virus-induced hepatitis. The frequency of CD8 T cells producing gamma interferon and tumor necrosis factor in response to viral antigen was higher in settings where more severe disease occurred. Thus, virus-specific effector CD8 T cells appear to contribute to lethal virus-induced hepatitis, contrasting their protective role during sublethal infection. This study reveals how protection and disease during cytomegalovirus infection depend on viral strain and dose, as well as the quality of the T cell response.

Immune senescence: relative contributions of age and cytomegalovirus infection

Immune senescence, defined as the age-associated dysregulation and dysfunction of the immune system, is characterised by impaired protective immunity and decreased efficacy of vaccines. Recent clinical, epidemiological and immunological studies suggest that Cytomegalovirus (CMV) infection may be associated with accelerated immune senescence, possibly by restricting the naïve T cell repertoire. However, direct evidence whether and how CMV-infection is implicated in immune senescence is still lacking. In this study, we have investigated whether latent mouse CMV (MCMV) infection with or without thymectomy (Tx) alters antiviral immunity of young and aged mice. After infection with lymphocytic choriomeningitis virus (LCMV) or Vaccinia virus, specific antiviral T cell responses were significantly reduced in old, old MCMV-infected and/or Tx mice compared to young mice. Importantly, control of LCMV replication was more profoundly impaired in aged MCMV-infected mice compared to age-matched MCMV-naïve or young mice. In addition, latent MCMV infection was associated with slightly reduced vaccination efficacy in old Tx mice. In contrast to the prevailing hypothesis of a CMV-mediated restriction of the naïve T cell repertoire, we found similar naïve T cell numbers in MCMV-infected and non-infected mice, whereas ageing and Tx clearly reduced the naïve T cell pool. Instead, MCMV-infection expanded the total CD8⁺ T cell pool by a massive accumulation of effector memory T cells. Based on these results, we propose a new model of increased competition between CMV-specific memory T cells and any ‘de novo’ immune response in aged individuals. In summary, our results directly demonstrate in a mouse model that latent CMV-infection impairs immunity in old age and propagates immune senescence.

Cytomegalovirus (CMV) persistently infects 50–90% of the human population. After primary infection, constant immune surveillance is required to prevent CMV-related disease. During ageing, increasing T cell resources are expended to keep CMV under control. Recent human studies have suggested that this investment may come at the cost of accelerated immune senescence, a condition describing the age-associated decline of the immune system's functionality. In the present study, we have developed a mouse model to directly investigate whether and how CMV-infection might impair immunity of aged individuals. We demonstrate that old mice with long-lasting CMV-infection are more susceptible to viral infections than old mice without CMV since their virus specific T cell response is suppressed. Contrary to the prevailing hypothesis we found no indication for a CMV-associated shrinking of the naïve T cell compartment. Instead, CMV-infection precipitated a massive expansion of memory T cells. Thus, we propose an alternative mechanism of CMV-enhanced immune senescence based on T cell competition between CMV-specific memory T cells and de novo generated T cell responses. In summary, we provide the first direct evidence that CMV-infection is indeed a propagating factor for poor immunity in the elderly.

Herpesvirus exploitation of host immune inhibitory pathways

Herpesviruses employ a plethora of mechanisms to circumvent clearance by host immune responses. A key feature of mammalian immune systems is the employment of regulatory pathways that limit immune responsiveness. The primary functions of these mechanisms are to control autoimmunity and limit exuberant responses to harmless antigen in mucosal surfaces. However, such pathways can be exploited by viral pathogens to enable acute infection, persistence and dissemination. Herein, we outline the current understanding of inhibitory pathways in modulating antiviral immunity during herpesvirus infections in vivo and discuss strategies employed by herpesviruses to exploit these pathways to limit host antiviral immunity.

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

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

How the virus outsmarts the host: function and structure of cytomegalovirus MHC-I-like molecules in the evasion of natural killer cell surveillance

Natural killer (NK) cells provide an initial host immune response to infection by many viral pathogens. Consequently, the viruses have evolved mechanisms to attenuate the host response, leading to improved viral fitness. One mechanism employed by members of the β-herpesvirus family, which includes the cytomegaloviruses, is to modulate the expression of cell surface ligands recognized by NK cell activation molecules. A novel set of cytomegalovirus (CMV) genes, exemplified by the mouse m145 family, encode molecules that have structural and functional features similar to those of host major histocompatibility-encoded (MHC) class I molecules, some of which are known to contribute to immune evasion. In this review, we explore the function, structure, and evolution of MHC-I-like molecules of the CMVs and speculate on the dynamic development of novel immunoevasive functions based on the MHC-I protein fold.

Absence of vasoactive intestinal peptide expression in hematopoietic cells enhances Th1 polarization and antiviral immunity in mice

Vasoactive intestinal peptide (VIP) induces regulatory dendritic cells (DC) in vitro that inhibit cellular immune responses. We tested the role of physiological levels of VIP on immune responses to murine CMV (mCMV) using VIP-knockout (VIP-KO) mice and radiation chimeras engrafted with syngenic VIP-KO hematopoietic cells. VIP-KO mice had less weight loss and better survival following mCMV infection compared with wild-type (WT) littermates. mCMV-infected VIP-KO mice had lower viral loads, faster clearance of virus, with increased numbers of IFN-γ(+) NK and NKT cells, and enhanced cytolytic activity of NK cells. Adaptive antiviral cellular immunity was increased in mCMV-infected VIP-KO mice compared with WT mice, with more Th1/Tc1-polarized T cells, fewer IL-10(+) T cells, and more mCMV-M45 epitope peptide MHC class I tetramer(+) CD8(+) T cells (tetramer(+) CD8 T cells). mCMV-immune VIP-KO mice had enhanced ability to clear mCMV peptide-pulsed target cells in vivo. Enhanced antiviral immunity was also seen in WT transplant recipients engrafted with VIP-KO hematopoietic cells, indicating that VIP synthesized by neuronal cells did not suppress immune responses. Following mCMV infection there was a marked upregulation of MHC-II and CD80 costimulatory molecule expression on DC from VIP-KO mice compared with DC from WT mice, whereas programmed death-1 and programmed death ligand-1 expression were upregulated in activated CD8(+) T cells and DC, respectively, in WT mice, but not in VIP-KO mice. Because the absence of VIP in immune cells increased innate and adaptive antiviral immunity by altering costimulatory and coinhibitory pathways, selective targeting of VIP signaling represents an attractive therapeutic target to enhance antiviral immunity.

Differential B7-CD28 costimulatory requirements for stable and inflationary mouse cytomegalovirus-specific memory CD8 T cell populations

CMV establishes a lifelong persistent infection, and viral immune-modulating strategies are important in facilitating this. A particularly diverse CD8 T cell response develops as a result of this host-virus détente, with the CMV-specific memory T cell pool displaying unique functions and phenotypes. To gain insight into the factors that regulate CMV-specific CD8 T cell responses, we examined the influence of the B7-CD28 costimulatory pathway on magnitude, kinetics, and phenotype. Initial expansion of mouse CMV-specific CD8 T cells that establish stable memory pools was severely lower in mice lacking B7-CD28 signaling, and the resulting memory levels also remained reduced during persistent/latent infection. In contrast, expansion of CD8 T cells that undergo memory inflation during chronic infection was less affected in the absence of B7-CD28 costimulatory signals, eventually reaching the levels seen in wild-type mice at later times. Regardless of their differential requirements for B7-CD28 signals, both stable and inflationary memory T cell populations showed normal cytotoxic capacity. These results reveal that B7-CD28 costimulation differentially regulates the magnitude and kinetics of the multifaceted CD8 T cell response that develops during CMV infection.

The herpes simplex virus 1 IgG fc receptor blocks antibody-mediated complement activation and antibody-dependent cellular cytotoxicity in vivo

Herpes simplex virus 1 (HSV-1) glycoprotein E (gE) mediates cell-to-cell spread and functions as an IgG Fc receptor (FcγR) that blocks the Fc domain of antibody targeting the virus or infected cell. Efforts to assess the functions of the HSV-1 FcγR in vivo have been hampered by difficulties in preparing an FcγR-negative strain that is relatively intact for spread. Here we report the FcγR and spread phenotypes of NS-gE264, which is a mutant strain that has four amino acids inserted after gE residue 264. The virus is defective in IgG Fc binding yet causes zosteriform disease in the mouse flank model that is only minimally reduced compared with wild-type and the rescue strains. The presence of zosteriform disease suggests that NS-gE264 spread functions are well maintained. The HSV-1 FcγR binds the Fc domain of human, but not murine IgG; therefore, to assess FcγR functions in vivo, mice were passively immunized with human IgG antibody to HSV. When antibody was inoculated intraperitoneally 20 h prior to infection or shortly after virus reached the dorsal root ganglia, disease severity was significantly reduced in mice infected with NS-gE264, but not in mice infected with wild-type or rescue virus. Studies of C3 knockout mice and natural killer cell-depleted mice demonstrated that the HSV-1 FcγR blocked both IgG Fc-mediated complement activation and antibody-dependent cellular cytotoxicity. Therefore, the HSV-1 FcγR promotes immune evasion from IgG Fc-mediated activities and likely contributes to virulence at times when antibody is present, such as during recurrent infections.

B7-mediated costimulation of CD4 T cells constrains cytomegalovirus persistence

Cytomegalovirus (CMV) utilizes multiple strategies to modulate immunity and promote lifelong, persistent/latent infection, including suppressing T cell activation pathways. Here we examined the role of B7 costimulatory ligands in establishing immune détente from both the host and virus perspectives. Mice lacking both B7.1 and B7.2 showed reduced early expansion of CMV-specific CD4 T cells, consequently allowing for enhanced levels of persistent virus replication. In turn, a CMV mutant lacking expression of the m138 and m147.5 gene products, which restrict B7.1 and B7.2 expression in infected antigen-presenting cells, induced a more robust CD4 T cell response and showed decreased persistence. Together, these data reveal a requirement for B7-mediated signaling in regulating the CMV-specific CD4 T cell response and establishing host-virus equilibrium.

The Natural Selection of Herpesviruses and Virus-Specific NK Cell Receptors

During the co-evolution of cytomegalovirus (CMV) and natural killer (NK) cells, each has evolved specific tactics in an attempt to prevail. CMV has evolved multiple immune evasion mechanisms to avoid detection by NK cells and other immune cells, leading to chronic infection. Meanwhile, the host has evolved virus-specific receptors to counter these evasion strategies. The natural selection of viral genes and host receptors allows us to observe a unique molecular example of “survival of the fittest”, as virus and immune cells try to out-maneuver one another or for the virus to achieve détente for optimal dissemination in the population.

CD28/B7-mediated co-stimulation is critical for early control of murine cytomegalovirus infection

Control of acute murine cytomegalovirus (MCMV) infection is dependent upon both innate and adaptive immune responses, relying primarily upon natural killer (NK) and T-cell responses for control. Although CD28/B7 plays a clear role in T-cell responses in many antigen systems including some viral infections, the importance of co-stimulation during MCMV infection is unconfirmed. In addition, recent data suggest that CD28/B7 co-stimulation might also be important to Ly49H+ NK-cell expansion. We therefore hypothesized that CD28/B7 co-stimulation is critical to viral control after MCMV infection, and further that CD28/B7 co-stimulation plays a role in MCMV-specific T- and NK-cell responses. To test these hypotheses, we utilized C57BL/6 mice lacking the co-stimulatory molecules B7-1 and B7-2 or CD28. After primary infection with MCMV, viral titers are significantly elevated in mice lacking CD28 or B7 compared with wild-type mice. Impaired viral control is associated with significant defects in peripheral T-cell responses to MCMV, which appear to be dependent upon CD28/B7 co-stimulation. Abnormal hepatic T-cell responses in CD28(-/-) mice are preceded by impaired MCMV-specific Ly49H+ NK-cell responses. Cytokine evaluations confirm that CD28/B7 co-stimulation is not required for non-specific antiviral responses. We conclude that CD28-mediated co-stimulation is critical for early viral control during acute MCMV infection.

Preferential use of B7.2 and not B7.1 in priming of vaccinia virus-specific CD8 T cells

Recent studies have demonstrated that CD28 provides critical costimulatory signals required for optimal CD8 T cell expansion and effector function in response to several viruses, including influenza, HSV, and vaccinia virus (VACV). CD28 has two ligands expressed largely on professional APC, named B7.1 (CD80) and B7.2 (CD86). Although some results suggest that these ligands are equivalent and both promote CD28 signaling, it is not clear whether they are equally important for priming of antiviral T cells. Herein we show that B7.2 is critical for early CD8 T cell responses to both dominant and subdominant VACV epitopes, correlating with its strong induction on CD8alpha(+) dendritic cells. In contrast, B7.1 plays no significant role. Signals from an exogenously applied adjuvant can recruit B7.1 activity and lead to further enhanced priming of VACV-reactive CD8 T cells. However, during a natural infection, B7.1 is not functional, likely related to inefficient up-regulation or active suppression by VACV. These studies provide evidence that B7.2 is the major ligand for the CD28 receptor on VACV-specific CD8 T cells, that B7.2 can promote efficient CD8 T cell priming without B7.1, and that B7.1 and B7.2 can be differentially utilized during antiviral responses.

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

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

Selected Toll-like receptor ligands and viruses promote helper-independent cytotoxic T cell priming by upregulating CD40L on dendritic cells

CD40L (CD154) on CD4(+) T cells has been shown to license dendritic cells (DCs) via CD40 to prime cytotoxic T lymphocyte (CTL) responses. We found that the converse (CD40L on DCs) was also important. Anti-CD40L treatment decreased endogenous CTL responses to both ovalbumin and influenza infection even in the absence of CD4(+) T cells. DCs expressed CD40L upon stimulation with agonists to Toll-like receptor 3 (TLR3) and TLR9. Moreover, influenza infection, which stimulates CTLs without help, upregulated CD40L on DCs, but herpes simplex infection, which elicits CTLs through help, did not. CD40L-deficient (Cd40lg(-/-)) DCs are suboptimal both in vivo in bone marrow chimera experiments and in vitro in mixed lymphocyte reactions. In contrast, Cd40lg(-/-) CD8(+) T cells killed as effectively as wild-type cells. Thus, CD40L upregulation on DCs promoted optimal priming of CD8(+) T cells without CD4(+) T cells, providing a mechanism by which pathogens may elicit helper-independent CTL immunity.

Feeling manipulated: cytomegalovirus immune manipulation

No one likes to feel like they have been manipulated, but in the case of cytomegalovirus (CMV) immune manipulation, we do not really have much choice. Whether you call it CMV immune modulation, manipulation, or evasion, the bottom line is that CMV alters the immune response in such a way to allow the establishment of latency with lifelong shedding. With millions of years of coevolution within their hosts, CMVs, like other herpesviruses, encode numerous proteins that can broadly influence the magnitude and quality of both innate and adaptive immune responses. These viral proteins include both homologues of host proteins, such as MHC class I or chemokine homologues, and proteins with little similarity to any other known proteins, such as the chemokine binding protein. Although a strong immune response is launched against CMV, these virally encoded proteins can interfere with the host's ability to efficiently recognize and clear virus, while others induce or alter specific immune responses to benefit viral replication or spread within the host. Modulation of host immunity allows survival of both the virus and the host. One way of describing it would be a kind of "mutually assured survival" (as opposed to MAD, Mutually Assured Destruction). Evaluation of this relationship provides important insights into the life cycle of CMV as well as a greater understanding of the complexity of the immune response to pathogens in general.

Viral inhibitors of NKG2D ligands: friends or foes of immune surveillance?

NKG2D is a potent activating receptor on NK cells and a co-stimulatory receptor on CD8+ T cells. Through its ability to recognize a panel of ligands inducible by stress or infection, it plays an important role in the control of viral infections. The viruses have evolved robust mechanisms to counteract NKG2D-dependent immune response. The functions of these viral inhibitors are well characterized during the early days post-infection but less so during the chronic viral infection.

Dendritic cells in viral infections

Antigen presenting cells (APCs) are recognized as key initiators of adaptive immunity, particularly to pathogens, by eliciting a rapid and potent immune attack on infected cells. Amongst APCs, dendritic cells (DCs) are specially equipped to initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. To achieve this, they are equipped with highly efficient mechanisms that allow them to detect pathogens, to capture, process and present antigens, and to activate and guide the differentiation of T cells into effector and memory cells. DCs can no longer be considered as a homogeneous cell type performing a single function, but are heterogeneous both in phenotype, function and dependence on inflammatory stimuli for their formation and responsiveness. Recent studies of DC subtypes have highlighted the contrasting roles of different professional APCs in activating divergent arms of the immune response towards pathogens. In this review, we discuss the progress that has been made in dissecting the attributes of different DC subsets that migrate into, or reside permanently, within lymphoid tissues and their putative roles in the induction of the anti-viral immune response.

Dendritic cell programming by cytomegalovirus stunts naive T cell responses via the PD-L1/PD-1 pathway

Early during infection, CMV targets dendritic cells (DC) and alters their functions. Herein we show that CMV-infected DC maintain the ability to present both virus-derived and exogenous Ags, but that they actively induce tolerance or anergy in Ag-specific T cells. CMV accomplishes this by selectively maintaining high-level expression of the negative costimulatory molecule programmed death ligand-1 (PD-L1), while commensurately down-regulating positive costimulatory molecules and MHC on the DC surface. Consequently, CD4 and CD8 T cells activated by these infected DC have a stunted phenotype, characterized by poor proliferation, effector function. and recall responses. Blocking PD-L1, but not PD-L2, during direct priming of naive T cells by infected DC significantly restores Ag-specific T cell functions. Using systems where direct and cross-priming of T cells can be distinguished revealed that PD-L1/PD-1 signaling contributes only when naive T cells are primed directly by infected DC, and not upon cross-presentation of viral Ags by uninfected DC. These data suggest that murine CMV programs infected DC during acute infection to inhibit early host adaptive antiviral responses by tipping the balance between negative and positive cosignals.

Immune evasion of natural killer cells by viruses

Natural killer (NK) cells are important in the host resistance to viral infections. They are among the first cells to sense the release of proinflammatory cytokines, as well as the downregulation of surface MHC class I molecules and molecules induced by viral invasion of cells. Various viral functions have evolved to counter NK cell responses illustrating the evolutionary struggles between viruses and NK cells. Ligands for NK cell receptors are primary targets for viral immunoevasion. In order to counteract NK cell activation via the 'missing self'-axis, viruses encode proteins which serve as ligands for inhibitory NK cell receptors. Viruses also downmodulate the ligands for the activating NK cell receptors and encode soluble ligands which block these receptors. In addition to viral immunoregulatory proteins, regulatory RNAs can also inhibit the expression of ligands for NK cell receptors. Improving our understanding of viral regulation of NK cell function could be essential for designing more efficient measures in the prophylaxis and treatment of virus-induced pathology.

Murine cytomegalovirus regulation of NKG2D ligands

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that causes morbidity risk in immunologically suppressed and immunodeficient patients including congenital infections. Approaches to curb the consequences of HCMV infections are restricted by a lack of complete understanding of viral pathogenesis. The infection of mice with murine cytomegalovirus (MCMV) as a model of HCMV infection has been particularly useful in elucidating the role of innate and adaptive immune response mechanisms. A large number of cytomegalovirus genes modulate the innate and the adaptive host immune response. The products of several MCMV genes are involved in subverting the natural killer (NK) cell response by down-modulating cellular ligands for the NKG2D receptor expressed on NK cells and CD8(+) T cells. Mutant viruses lacking these immunoevasion genes are attenuated with respect to virus growth in vivo. Given the importance of the NKG2D receptor in controlling both NK- and T cell-mediated immunity, it is of tremendous importance to understand the molecular mechanisms and consequences of viral regulation of the NKG2D ligands.

The human cytomegalovirus Fc receptor gp68 binds the Fc CH2-CH3 interface of immunoglobulin G

Recognition of immunoglobulin G (IgG) by surface receptors for the Fc domain of immunoglobulin G (Fcgamma), FcgammaRs, can trigger both humoral and cellular immune responses. Two human cytomegalovirus (HCMV)-encoded type I transmembrane receptors with Fcgamma-binding properties (vFcgammaRs), gp34 and gp68, have been identified on the surface of HCMV-infected cells and are assumed to confer protection against IgG-mediated immunity. Here we show that Fcgamma recognition by both vFcgammaRs occurs independently of N-linked glycosylation of Fcgamma, in contrast with the properties of host FcgammaRs. To gain further insight into the interaction with Fcgamma, truncation mutants of the vFcgammaR gp68 ectodomain were probed for Fcgamma binding, resulting in localization of the Fcgamma binding site on gp68 to residues 71 to 289, a region including an immunoglobulin-like domain. Gel filtration and biosensor binding experiments revealed that, unlike host FcgammaRs but similar to the herpes simplex virus type 1 (HSV-1) Fc receptor gE-gI, gp68 binds to the C(H)2-C(H)3 interdomain interface of the Fcgamma dimer with a nanomolar affinity and a 2:1 stoichiometry. Unlike gE-gI, which binds Fcgamma at the slightly basic pH of the extracellular milieu but not at the acidic pH of endosomes, the gp68/Fcgamma complex is stable at pH values from 5.6 to pH 8.1. These data indicate that the mechanistic details of Fc binding by HCMV gp68 differ from those of host FcgammaRs and from that of HSV-1 gE-gI, suggesting distinct functional and recognition properties.

Cytomegalovirus immune evasion

Human cytomegalovirus (HCMV) has become a paradigm for viral immune evasion due to its unique multitude of immune-modulatory strategies. HCMV modulates the innate as well as adaptive immune response at every step of its life cycle. It dampens the induction of antiviral interferon-induced genes by several mechanisms. Further striking is the multitude of genes and strategies devoted to modulating and escaping the cellular immune response. Several genes are independently capable of inhibiting antigen presentation to cytolytic T cells by downregulating MHC class I. Recent data revealed an astounding variety of methods in triggering or inhibiting activatory and inhibitory receptors found on NK cells, NKT cells, T cells as well as auxiliary cells of the immune system. The multitude and complexity of these mechanisms is fascinating and continues to reveal novel insights into the host-pathogen interaction and novel cell biological and immunological concepts.

B7 costimulation molecules expressed from the herpes simplex virus 2 genome rescue immune induction in B7-deficient mice

The interaction between B7 costimulation molecules on antigen-presenting cells and CD28 on antigen-responsive T cells is essential for T-cell activation and maturation of immune responses to herpes simplex virus (HSV) infection. Vaccine-induced immune responses also depend upon adequate upregulation of B7 costimulation molecules, but this signal may be limiting for replication-defective virus vaccines. We investigated whether expression of B7 costimulation molecules by a prototypical replication-defective antiviral vaccine could enhance immune responses to the vaccine and whether B7-1 and B7-2 would be similarly effective. We altered an ICP8(-) replication-defective strain of HSV type 2 (HSV-2), 5BlacZ, to encode either murine B7-1 or B7-2. B7 molecule expression was detected on the surface of cells infected in vitro and at the RNA level in tissue of immunized mice. Immunization of B7-1/B7-2 knockout mice with B7-encoding virus modestly expanded the number of gamma interferon-producing T cells and significantly augmented class-switched HSV-specific antibody responses compared with the parental virus. Mice immunized with either B7-expressing virus showed less replication of challenge virus in the genital mucosa than mice immunized with 5BlacZ, markedly fewer signs of genital and neurological disease, and little weight loss. Virtually all mice immunized with B7-encoding virus survived challenge with a large dose of HSV-2, whereas most 5BlacZ-immunized mice succumbed to infection. These results indicate that protective immune responses can be enhanced by the inclusion of host B7 costimulation molecules in a prototypical replication-defective HSV vaccine against HSV-2 genital infection and that B7-1 and B7-2 induce immune responses with similar capacities to fight HSV-2 infection.