Processing and presentation of murine cytomegalovirus pORFm164-derived peptide in fibroblasts in the face of all viral immunosubversive early gene functions

The CMV-encoded G protein-coupled receptors M33 and US28 play pleiotropic roles in immune evasion and alter host T cell responses

Introduction

Human cytomegalovirus (HCMV) is a global health threat due to its ubiquity and lifelong persistence in infected people. During latency, host CD8⁺ T cell responses to HCMV continue to increase in a phenomenon known as memory inflation. We used murine CMV (MCMV) as a model for HCMV to characterize the memory inflation response to wild-type MCMV (KP) and a latency-defective mutant (ΔM33_stop), which lacks M33, an MCMV chemokine receptor homolog. M33 is essential for normal reactivation from latency and this was leveraged to determine whether reactivation in vivo contributes to T cell memory inflation.

Methods

Mice were infected with wild-type or mutant MCMV and T cell responses were analyzed by flow cytometry at acute and latent time points. Ex vivo reactivation and cytotoxicity assays were carried out to further investigate immunity and virus replication. Quantitative reverse-transcriptase polymerase chain reaction (q-RTPCR) was used to examine gene expression during reactivation. MHC expression on infected cells was analyzed by flow cytometry. Finally, T cells were depleted from latently-infected B cell-deficient mice to examine the in vivo difference in reactivation between wild-type and ΔM33_stop.

Results

We found that ΔM33_stop triggers memory inflation specific for peptides derived from the immediate-early protein IE1 but not the early protein m164, in contrast to wild-type MCMV. During ex vivo reactivation, gene expression in DM33stop-infected lung tissues was delayed compared to wild-type virus. Normal gene expression was partially rescued by substitution of the HCMV US28 open reading frame in place of the M33 gene. In vivo depletion of T cells in immunoglobulin heavy chain-knockout mice resulted in reactivation of wild-type MCMV, but not ΔM33_stop, confirming the role of M33 during reactivation from latency. Further, we found that M33 induces isotype-specific downregulation of MHC class I on the cell surface suggesting previously unappreciated roles in immune evasion.

Discussion

Our results indicate that M33 is more polyfunctional than previously appreciated. In addition to its role in reactivation, which had been previously described, we found that M33 alters viral gene expression, host T cell memory inflation, and MHC class I expression. US28 was able to partially complement most functions of M33, suggesting that its role in HCMV infection may be similarly pleotropic.

Insight for Immunotherapy of HCMV Infection

Human cytomegalovirus (HCMV), a ubiquitous in humans, has a high prevalence rate. Young people are susceptible to HCMV infection in developing countries, while older individuals are more susceptible in developed countries. Most patients have no obvious symptoms from the primary infection. Studies have indicated that the virus has gradually adapted to the host immune system. Therefore, the control of HCMV infection requires strong immune modulation. With the recent advances in immunotherapy, its application to HCMV infections is receiving increasing attention. Here, we discuss the immune response to HCMV infection, the immune escape mechanism, and the different roles that HCMV plays in various types of immunotherapy, including vaccines, adoptive cell therapy, checkpoint blockade therapy, and targeted antibodies.

Stochastic Episodes of Latent Cytomegalovirus Transcription Drive CD8 T-Cell "Memory Inflation" and Avoid Immune Evasion

Acute infection with murine cytomegalovirus (mCMV) is controlled by CD8+ T cells and develops into a state of latent infection, referred to as latency, which is defined by lifelong maintenance of viral genomes but absence of infectious virus in latently infected cell types. Latency is associated with an increase in numbers of viral epitope-specific CD8+ T cells over time, a phenomenon known as "memory inflation" (MI). The "inflationary" subset of CD8+ T cells has been phenotyped as KLRG1+CD62L- effector-memory T cells (iTEM). It is agreed upon that proliferation of iTEM requires repeated episodes of antigen presentation, which implies that antigen-encoding viral genes must be transcribed during latency. Evidence for this has been provided previously for the genes encoding the MI-driving antigenic peptides IE1-YPHFMPTNL and m164-AGPPRYSRI of mCMV in the H-2d haplotype. There exist two competing hypotheses for explaining MI-driving viral transcription. The "reactivation hypothesis" proposes frequent events of productive virus reactivation from latency. Reactivation involves a coordinated gene expression cascade from immediate-early (IE) to early (E) and late phase (L) transcripts, eventually leading to assembly and release of infectious virus. In contrast, the "stochastic transcription hypothesis" proposes that viral genes become transiently de-silenced in latent viral genomes in a stochastic fashion, not following the canonical IE-E-L temporal cascade of reactivation. The reactivation hypothesis, however, is incompatible with the finding that productive virus reactivation is exceedingly rare in immunocompetent mice and observed only under conditions of compromised immunity. In addition, the reactivation hypothesis fails to explain why immune evasion genes, which are regularly expressed during reactivation in the same cells in which epitope-encoding genes are expressed, do not prevent antigen presentation and thus MI. Here we show that IE, E, and L genes are transcribed during latency, though stochastically, not following the IE-E-L temporal cascade. Importantly, transcripts that encode MI-driving antigenic peptides rarely coincide with those that encode immune evasion proteins. As immune evasion can operate only in cis, that is, in a cell that simultaneously expresses antigenic peptides, the stochastic transcription hypothesis explains why immune evasion is not operative in latently infected cells and, therefore, does not interfere with MI.

Cytomegalovirus inhibition of extrinsic apoptosis determines fitness and resistance to cytotoxic CD8 T cells

Viral immune evasion is currently understood to focus on deflecting CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) prevents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoevasins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mechanism through which vICA acts, demonstrating the central contribution of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.

Transcripts expressed in cytomegalovirus latency coding for an antigenic IE/E phase peptide that drives "memory inflation"

Roizman's definition of herpesviral latency, which applies also to cytomegaloviruses (CMVs), demands maintenance of reactivation-competent viral genomes after clearance of productive infection. It is more recent understanding that failure to complete the productive viral cycle for virus assembly and release does not imply viral gene silencing at all genetic loci and all the time. It rather appears that CMV latency is transcriptionally "noisy" in that silenced viral genes get desilenced from time to time in a stochastic manner, leading to "transcripts expressed in latency" (TELs). If a TEL happens to code for a protein that contains a CD8 T cell epitope, protein processing can lead to the presentation of the antigenic peptide and restimulation of cognate CD8 T cells during latency. This mechanism is discussed as a potential driver of epitope-selective accumulation of CD8 T cells over time, a phenomenon linked to CMV latency and known as "memory inflation" (MI). So far, expression of an epitope-encoding TEL was shown only for the major immediate-early (MIE) gene m123/ie1 of murine cytomegalovirus (mCMV), which codes for the prototypic MI-driving antigenic peptide YPHFMPTNL that is presented by the MHC class-I molecule L^d. The only known second MI-driving antigenic peptide of mCMV in the murine MHC haplotype H-2^d is AGPPRYSRI presented by the MHC-I molecule D^d. This peptide is very special in that it is encoded by the early (E) phase gene m164 and by an overlapping immediate-early (IE) transcript governed by a promoter upstream of m164. If MI is driven by presentation of TEL-derived antigenic peptides, as the hypothesis says, one should find corresponding TELs. We show here that E-phase and IE-phase transcripts that code for the MI-driving antigenic peptide AGPPRYSRI are independently and stochastically expressed in latently infected lungs.

Intratumoral infection by CMV may change the tumor environment by directly interacting with tumor-associated macrophages to promote cancer immunity

Cytomegalovirus (CMV) is a herpesvirus that induces an extremely robust and sustained immune response. For this reason, CMV has been proposed as a vaccine vector to promote immunity to both pathogens and cancer. However, exploration of CMV as a vaccine vector is at an early stage and there are many questions. Using a mouse melanoma model, we recently found that a CMV-based vaccine induced large populations of melanoma-specific T cells, but was not effective at slowing tumor growth unless it was injected directly into the tumor. These surprising results have led us to hypothesize that CMV may be adept at modulating the tumor micro-environment through its infection of macrophages. Importantly, injection of CMV into the growing tumor synergized with blockade of the PD-1 checkpoint to clear well-established tumors. Here, we discuss our results in the context of CMV-based vaccines for pathogens and cancer.

Intratumoral Infection with Murine Cytomegalovirus Synergizes with PD-L1 Blockade to Clear Melanoma Lesions and Induce Long-term Immunity

Cytomegalovirus is an attractive cancer vaccine platform because it induces strong, functional CD8(+) T-cell responses that accumulate over time and migrate into most tissues. To explore this, we used murine cytomegalovirus expressing a modified gp100 melanoma antigen. Therapeutic vaccination by the intraperitoneal and intradermal routes induced tumor infiltrating gp100-specific CD8(+) T-cells, but provided minimal benefit for subcutaneous lesions. In contrast, intratumoral infection of established tumor nodules greatly inhibited tumor growth and improved overall survival in a CD8(+) T-cell-dependent manner, even in mice previously infected with murine cytomegalovirus. Although murine cytomegalovirus could infect and kill B16F0s in vitro, infection was restricted to tumor-associated macrophages in vivo. Surprisingly, the presence of a tumor antigen in the virus only slightly increased the efficacy of intratumoral infection and tumor-specific CD8(+) T-cells in the tumor remained dysfunctional. Importantly, combining intratumoral murine cytomegalovirus infection with anti-PD-L1 therapy was synergistic, resulting in tumor clearance from over half of the mice and subsequent protection against tumor challenge. Thus, while a murine cytomegalovirus-based vaccine was poorly effective against established subcutaneous tumors, direct infection of tumor nodules unexpectedly delayed tumor growth and synergized with immune checkpoint blockade to promote tumor clearance and long-term protection.

Reconstitution of CD8 T Cells Protective against Cytomegalovirus in a Mouse Model of Hematopoietic Cell Transplantation: Dynamics and Inessentiality of Epitope Immunodominance

Successful reconstitution of cytomegalovirus (CMV)-specific CD8⁺ T cells by hematopoietic cell transplantation (HCT) gives a favorable prognosis for the control of CMV reactivation and prevention of CMV disease after hematoablative therapy of hematopoietic malignancies. In the transient immunocompromised state after HCT, pre-emptive cytoimmunotherapy with viral epitope-specific effector or memory CD8⁺ T cells is a promising option to speed up antiviral control. Despite high-coding capacity of CMVs and a broad CD8⁺ T-cell response on the population level, which reflects polymorphism in major histocompatibility complex class-I (MHC-I) glycoproteins, the response in terms of quantity of CD8⁺ T cells in any individual is directed against a limited set of CMV-encoded epitopes selected for presentation by the private repertoire of MHC-I molecules. Such epitopes are known as “immunodominant” epitopes (IDEs). Besides host immunogenetics, genetic variance in CMV strains harbored as latent viruses by an individual HCT recipient can also determine the set of IDEs, which complicates a “personalized immunotherapy.” It is, therefore, an important question if IDE-specific CD8⁺ T-cell reconstitution after HCT is critical or dispensable for antiviral control. As viruses with targeted mutations of IDEs cannot be experimentally tested in HCT patients, we employed the well-established mouse model of HCT. Notably, control of murine CMV (mCMV) after HCT was comparably efficient for IDE-deletion mutant mCMV-Δ4IDE and the corresponding IDE-expressing revertant virus mCMV-Δ4IDE-rev. Thus, antigenicity-loss mutations in IDEs do not result in loss-of-function of a polyclonal CD8⁺ T-cell population. Although IDE deletion was not associated with global changes in the response to non-IDE epitopes, the collective of non-IDE-specific CD8⁺ T-cells infiltrates infected tissue and confines infection within nodular inflammatory foci. We conclude from the model, and predict also for human CMV, that there is no need to exclusively aim for IDE-specific immunoreconstitution.

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

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

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

Comparison of multiple DNA vaccines for protection against cytomegalovirus infection in BALB/c mice

Background

Human cytomegalovirus (HCMV) causes serious HCMV-related diseases in immunocompromised people. Vaccination is the most effective measure to control infection with the pathogen, yet no vaccine has been licensed till now. We performed a head-to-head comparison of the protective abilities of multiple DNA vaccines in murine model of murine cytomegalovirus (MCMV) infection.

Methods

Five DNA vaccines were constructed. Four encoding MCMV proteins gp34 (m04), p65 (M84), DNA helicase (M105), and immediate-early 1 protein pp89 (IE-1) , respectively, which were reported to induce CD8+ T cell responses, were compared with the one expressing gB (M55), the neutralizing antibody target antigen, for immune protection in BALB/c mice. Mice were immunized with these DNA vaccines 1 to 4 times via intramuscular injection followed by electroporation, and were subsequently infected with a lethal dose (3 × LD50) of highly virulent SG-MCMV. Specific antibodies and IFN-γ secreting splenocytes were detected by immunoblotting and ELISPOT, respectively. Protective abilities in mice provided by the vaccines were evaluated by residual virus titers in organs, survival rate and weight loss.

Results

These DNA vaccines, especially m04, M84 and IE-1, could effectively reduce the virus loads in salivary glands and spleens of mice, but they couldn’t completely clear the residual virus. Survival rates of 100% in mice after a lethal dose of MCMV infection could be reached by more than one dose of M84 vaccine or two doses of m04 or IE-1 vaccine. Immunization with M55 or M105 DNA at four doses offered mice only 62.5% survival rate after the lethal challenge.

Conclusions

The study demonstrated that DNA vaccines could effectively afford mice protection against infection with a highly virulent MCMV and that the protection offered by induced CD8+ T cell immunity might be superior to that by gB-specific antibodies. These results are valuable references for development and application of HCMV vaccines.

Noncanonical expression of a murine cytomegalovirus early protein CD8 T-cell epitope as an immediate early epitope based on transcription from an upstream gene

Viral CD8 T-cell epitopes, represented by viral peptides bound to major histocompatibility complex class-I (MHC-I) glycoproteins, are often identified by “reverse immunology”, a strategy not requiring biochemical and structural knowledge of the actual viral protein from which they are derived by antigen processing. Instead, bioinformatic algorithms predicting the probability of C-terminal cleavage in the proteasome, as well as binding affinity to the presenting MHC-I molecules, are applied to amino acid sequences deduced from predicted open reading frames (ORFs) based on the genomic sequence. If the protein corresponding to an antigenic ORF is known, it is usually inferred that the kinetic class of the protein also defines the phase in the viral replicative cycle during which the respective antigenic peptide is presented for recognition by CD8 T cells. We have previously identified a nonapeptide from the predicted ORFm164 of murine cytomegalovirus that is presented by the MHC-I allomorph H-2 D^d and that is immunodominant in BALB/c (H-2^d haplotype) mice. Surprisingly, although the ORFm164 protein gp36.5 is expressed as an Early (E) phase protein, the m164 epitope is presented already during the Immediate Early (IE) phase, based on the expression of an upstream mRNA starting within ORFm167 and encompassing ORFm164.

Spatial distribution and structural arrangement of a murine cytomegalovirus glycoprotein detected by SPDM localization microscopy

Novel approaches of localization microscopy have opened new insights into the molecular nano-cosmos of cells. We applied a special embodiment called spectral position determination microscopy (SPDM) that has the advantage to run with standard fluorescent dyes or proteins under standard preparation conditions. Pointillist images with a resolution in the order of 10 nm can be obtained by SPDM. Therefore, vector pEYFP-m164, encoding the murine cytomegalovirus glycoprotein gp36.5/m164 fused to enhanced yellow fluorescent protein, was transiently transfected into COS-7 cells. This protein shows exceptional intracellular trafficking dynamics, moving within the endoplasmic reticulum (ER) and outer nuclear membrane. The molecular positions of gp36.5/m164 were visualized and determined by SPDM imaging. From the position point patterns of the protein molecules, their arrangements were quantified by next neighbour distance analyses. Three different structural arrangements were discriminated: (a) a linear distribution along the membrane, (b) a highly structured distribution in the ER, and (c) a homogenous distribution in the cellular cytoplasm. The results indicate that the analysis of next neighbour distances on the nano-scale allows the identification and discrimination of different structural arrangements of molecules within their natural cellular environment.

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.

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.

The genome of murine cytomegalovirus is shaped by purifying selection and extensive recombination

The herpesvirus lifestyle results in a long-term interaction between host and invading pathogen, resulting in exquisite adaptation of virus to host. We have sequenced the genomes of nine strains of murine cytomegalovirus (a betaherpesvirus), isolated from free-living mice trapped at locations separated geographically and temporally. Despite this separation these genomes were found to have low levels of nucleotide variation. Of the more than 160 open reading frames, almost 90% had a dN/dS ratio of amino acid substitutions of less than 0.6, indicating the level of purifying selection on the coding potential of MCMV. Examination of selection acting on individual genes at the codon level however indicates some level of positive selection, with 0.03% of codons showing strong evidence for positive selection. Conversely, 1.3% of codons show strong evidence of purifying selection. Alignments of both genome sequences and coding regions suggested that high levels of recombination have shaped the MCMV genome.

Parameters determining the efficacy of adoptive CD8 T-cell therapy of cytomegalovirus infection

Reactivation of latent cytomegalovirus (CMV) in the transient state of immunodeficiency after hematopoietic cell transplantation (HCT) is the most frequent and severe viral complication endangering leukemia therapy success. By infecting the bone marrow (BM) stroma of the transplantation recipient, CMV can directly interfere with BM repopulation by the transplanted donor-derived hematopoietic cells and thus delay immune reconstitution of the recipient. Cytopathogenic virus spread in tissues can result in CMV disease with multiple organ manifestations of which interstitial pneumonia is the most feared. There exists a 'window of risk' between hematoablative treatment and reconstitution of antiviral immunity after HCT, whereby timely reconstitution of antiviral CD8 T cells is a recognized positive prognostic parameter for the control of reactivated CMV infection and prevention of CMV disease. Supplementation of endogenous reconstitution by adoptive cell transfer of 'ready-to-go' effector and/or memory virus epitope-specific CD8 T cells is a therapeutic option to bridge the 'window of risk.' Preclinical research in murine models of CMV disease has been pivotal by providing 'proof of concept' for a benefit from CD8 T-cell therapy of HCT-associated CMV disease (reviewed in Holtappels et al. Med Microbiol Immunol 197:125-134, 2008). Here, we give an update of our previous review with focus on parameters that determine the efficacy of adoptive immunotherapy of CMV infection by antiviral CD8 T cells in the murine model.

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

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

Memory T cells persisting in the brain following MCMV infection induce long-term microglial activation via interferon-γ

Murine cytomegalovirus (MCMV) brain infection stimulates microglial cell-driven proinflammatory chemokine production which precedes the presence of brain-infiltrating systemic immune cells. Here, we show that in response to MCMV brain infection, antigen-specific CD8(+) T cells migrated into the brain and persisted as long-lived memory cells. The role of these persistent T cells in the brain is unclear because most of our understanding of antimicrobial T cell responses comes from analyses of lymphoid tissue. Strikingly, memory T cells isolated from the brain exhibited an effector phenotype and produced IFN-γ upon restimulation with viral peptide. Furthermore, we observed time-dependent and long-term activation of resident microglia, indicated by chronic MHC class II up-regulation and TNF-α production. The immune response in this immunologically restricted site persisted in the absence of active viral replication. Lymphocyte infiltrates were detected until 30 days post-infection (p.i.), with CD8(+) and CD4(+) T cells present at a 3:1 ratio, respectively. We then investigated the role of IFN-γ in chronic microglial activation by using IFN-γ-knockout (GKO) mice. At 30 days p.i., GKO mice demonstrated a similar phenotypic brain infiltrate when compared to wild-type mice (Wt), however, MHC class II expression on microglia isolated from these GKO mice was significantly lower compared to Wt animals. When IFN-γ producing CD8(+) T cells were reconstituted in GKO mice, MHC class II up-regulation on microglial cells was restored. Taken together, these results suggest that MCMV brain infection results in long-term persistence of antigen-specific CD8(+) T cells which produce IFN-γ and drive chronic microglial cell activation. This response was found to be dependent on IFN-γ production by viral Ag-specific T cells during the chronic phase of disease.

Antigen-presenting cells of haematopoietic origin prime cytomegalovirus-specific CD8 T-cells but are not sufficient for driving memory inflation during viral latency

Expansion of the CD8 T-cell memory pool, also known as 'memory inflation', for certain but not all viral epitopes in latently infected host tissues is a special feature of the immune response to cytomegalovirus. The L(d)-presented murine cytomegalovirus (mCMV) immediate-early (IE) 1 peptide is the prototype of an epitope that is associated with memory inflation. Based on the detection of IE1 transcripts in latently infected lungs it was previously proposed that episodes of viral gene expression and antigenic activity due to desilencing of a limited number of viral genes may drive epitope-specific memory inflation. This would imply direct antigen presentation through latently infected host tissue cells rather than cell death-associated cross-presentation of viral antigens derived from productively infected cells through uninfected, professional antigen-presenting cells (profAPCs). To address the role of bone marrow-derived profAPCs in CD8 T-cell priming and memory to mCMV, we have used here a combined sex-mismatched and MHC class-I mismatched dual-marker bone marrow chimera model in which presentation of the IE1 epitope is restricted to donor-derived sry(+)L(d+) cells of haematopoietic differentiation lineages. Successful CD8 T-cell priming specific for the L(d)- and D(d)-presented inflationary epitopes IE1 and m164, respectively, but selective failure in IE1 epitope-specific memory inflation in these chimeras indicates different modes of antigen presentation involved in CD8 T-cell priming and memory inflation. These data suggest that memory inflation during mCMV latency requires expression of the epitope-presenting MHC class-I molecule by latently infected non-haematopoietic host tissue cells and thus predicts a role for direct antigen presentation in memory inflation.

In vivo impact of cytomegalovirus evasion of CD8 T-cell immunity: facts and thoughts based on murine models

Cytomegaloviruses (CMVs) co-exist with their respective host species and have evolved to avoid their elimination by the hosts' immune effector mechanisms and to persist in a non-replicative state, known as viral latency. There is evidence to suggest that latency is nevertheless a highly dynamic condition during which episodes of viral gene desilencing, which can be viewed as incomplete reactivations, cause intermittent antigenic activity that stimulates CD8 memory-effector T cells and drives their clonal expansion. These T cells are supposed to terminate reactivation before completion of the productive viral cycle. In this view, CMVs do not "evade" their respective host's immune response but are actually held in check all the time, unless the host gets immunocompromised. Accordingly, CMV disease is typically a disease of the immunocompromised host only. Here we review current knowledge about the in vivo role of viral proteins involved in subverting the immune recognition of infected cells with focus on the CD8 T-cell response and viral interference with the MHC class-I pathway of antigenic peptide presentation. Whereas the intracellular functions of these "immune-evasion proteins" are known in molecular detail, knowledge of their in vivo role in CMV biology is only beginning to take shape. Experimental studies on the in vivo function of human CMV (hCMV) immune-evasion proteins prohibits, of course. Studying animal CMVs paradigmatically in the corresponding natural host is therefore used to identify principles from which the role of hCMV immune-evasion proteins can hopefully be inferred. Here we summarize recent insights gained primarily from the murine model.

Occult cytomegalovirus in vivarium-housed mice may influence transplant allograft acceptance

We have recently shown that latent murine cytomegalovirus (MCMV) can influence murine transplant allograft acceptance. During these studies we became aware that vivarium-housed control mice can acquire occult MCMV infection. The purpose of this investigation was to confirm occult MCMV transmission and determine the timing, vehicle, and possible consequences of transmission. Mice arriving from a commercial vendor were negative for MCMV both by commercial serologic testing and by our nested PCR. Mice housed in our vivarium became positive for MCMV DNA 30-60 days after arrival, but remained negative for MCMV by commercial serologic testing. To confirm MCMV we sequenced PCR products for several genes and showed >99% homology to MCMV. Further sequence analyses show that the occult MCMV is similar to a laboratory strain of MCMV, but the vehicle of transmission remains unclear. Control tissues from historical experiments with unexplained graft losses were evaluated for occult MCMV, and mice with unexplained allograft losses showed significantly higher incidence of occult MCMV than did allograft acceptors. Deliberate infection with very low titer MCMV confirmed that viral transmission can occur without measurable virus specific antibody or T-cell responses. These data suggest that vivarium-housed mice can develop occult MCMV that is missed by currently available commercial serologic testing, and that these infections may influence transplant allograft acceptance.

A novel transmembrane domain mediating retention of a highly motile herpesvirus glycoprotein in the endoplasmic reticulum

Gene m164 of murine cytomegalovirus belongs to the large group of 'private' genes that show no homology to those of other cytomegalovirus species and are thought to represent 'host adaptation' genes involved in virus-host interaction. Previous interest in the m164 gene product was based on the presence of an immunodominant CD8 T-cell epitope presented at the surface of infected cells, despite interference by viral immune-evasion proteins. Here, we provide data to reveal that the m164 gene product shows unusual features in its cell biology. A novel strategy of mass-spectrometric analysis was employed to map the N terminus of the mature protein, 107 aa downstream of the start site of the predicted open reading frame. The resulting 36.5 kDa m164 gene product is identified here as an integral type-I membrane glycoprotein with exceptional intracellular trafficking dynamics, moving within the endoplasmic reticulum (ER) and outer nuclear membrane with an outstandingly high lateral membrane motility, actually 100 times higher than those published for cellular ER-resident proteins. Notably, gp36.5/m164 does not contain any typical ER-retention/retrieval signals, such as the C-terminal motifs KKXX or KXKXX, and does not pass the Golgi apparatus. Instead, it belongs to the rare group of viral glycoproteins in which the transmembrane domain (TMD) itself mediates direct ER retention. This is the first report relating TMD usage of an ER-resident transmembrane protein to its lateral membrane motility as a paradigm in cell biology. We propose that TMD usage for ER retention facilitates free and fast floating in ER-related membranes and between ER subdomains.

Molecular mechanisms of viral immune evasion proteins to inhibit MHC class I antigen processing and presentation

Viral products inhibit MHC class I antigen processing and presentation via three major pathways: inhibition of major histocompatibility complex (MHC) class I expression on cells, blockade of peptide trafficking and loading on MHC class I molecules, and inhibition of peptide generation in host cells. Viral products also interfere with IFN-gamma -mediated JAK/STAT signal transduction in cells. These results imply that viral proteins probably inhibit the function of IFN-gamma in MHC class I antigen presentation via inactivation of JAK/STAT signal transduction in host cells. Mechanisms of viral products to inhibit IFN-gamma -mediated MHC class I antigen presentation were summarized in this literature review.

Antiviral prevention of sepsis induced cytomegalovirus reactivation in immunocompetent mice

INTRODUCTION

Immunocompetent patients can reactivate latent cytomegalovirus (CMV) during critical illness and reactivation is associated with significantly worse outcomes. Prior to clinical trials in humans to prove causality, we sought to determine an optimal antiviral treatment strategy.

METHODS

Mice latently infected with murine CMV (MCMV) received a septic reactivation trigger and were randomized to receive one of four ganciclovir regimens or saline. Lungs were evaluated for viral transcriptional reactivation and fibrosis after each regimen. Influences of ganciclovir on early sepsis-induced pulmonary inflammation and T-cell activation were studied after sepsis induction.

RESULTS

All ganciclovir regimens reduced measurable MCMV transcriptional reactivation, and 10mg/day for 7 or 21 days was most effective. Lower dose (5mg/kg/day) or delayed therapy was associated with significant breakthrough reactivation. Higher doses of ganciclovir given early were associated with the lowest incidence of pulmonary fibrosis, and delay of therapy for 1 week was associated with significantly worse pulmonary fibrosis. Although bacterial sepsis induced activation of MCMV-specific pulmonary T-cells, this activation was not influenced by ganciclovir.

CONCLUSION

These results suggest that antiviral treatment trials in humans should use 10mg/kg/day ganciclovir administered as early as possible in at-risk patients to minimize reactivation events and associated pulmonary injury.

Immune evasion proteins of murine cytomegalovirus preferentially affect cell surface display of recently generated peptide presentation complexes

For recognition of infected cells by CD8 T cells, antigenic peptides are presented at the cell surface, bound to major histocompatibility complex class I (MHC-I) molecules. Downmodulation of cell surface MHC-I molecules is regarded as a hallmark function of cytomegalovirus-encoded immunoevasins. The molecular mechanisms by which immunoevasins interfere with the MHC-I pathway suggest, however, that this downmodulation may be secondary to an interruption of turnover replenishment and that hindrance of the vesicular transport of recently generated peptide-MHC (pMHC) complexes to the cell surface is the actual function of immunoevasins. Here we have used the model of murine cytomegalovirus (mCMV) infection to provide experimental evidence for this hypothesis. To quantitate pMHC complexes at the cell surface after infection in the presence and absence of immunoevasins, we generated the recombinant viruses mCMV-SIINFEKL and mCMV-Deltam06m152-SIINFEKL, respectively, expressing the K(b)-presented peptide SIINFEKL with early-phase kinetics in place of an immunodominant peptide of the viral carrier protein gp36.5/m164. The data revealed approximately 10,000 K(b) molecules presenting SIINFEKL in the absence of immunoevasins, which is an occupancy of approximately 10% of all cell surface K(b) molecules, whereas immunoevasins reduced this number to almost the detection limit. To selectively evaluate their effect on preexisting pMHC complexes, cells were exogenously loaded with SIINFEKL peptide shortly after infection with mCMV-SIINFEKA, in which endogenous presentation is prevented by an L174A mutation of the C-terminal MHC-I anchor residue. The data suggest that pMHC complexes present at the cell surface in advance of immunoevasin gene expression are downmodulated due to constitutive turnover in the absence of resupply.

Investigation of the impact of the common animal facility contaminant murine norovirus on experimental murine cytomegalovirus infection

Murine norovirus (MNV) is a recently discovered pathogen that has become a common contaminant of specific pathogen-free mouse colonies. MNV-1 induces a robust interferon-beta response and causes histopathology in some mouse strains, suggesting that it may impact other mouse models of infection. Despite many concerns about MNV-1 contamination, there is little information about its impact on immune responses to other infections. This study addresses whether MNV-1 infection has an effect on a model of murine cytomegalovirus (MCMV) infection. Exposure to MNV-1 resulted in a decreased CD8 T cell response to immunodominant MCMV epitopes in both BALB/c and C57BL/6 mice. However, MNV-1 did not impact MCMV titers in either mouse strain, nor did it stimulate reactivation of latent MCMV. These data suggest that while MNV-1 has a mild impact on the immune response to MCMV, it is not likely to affect most experimental outcomes in immunocompetent mice in the MCMV model.

The efficacy of antigen processing is critical for protection against cytomegalovirus disease in the presence of viral immune evasion proteins

Cytomegaloviruses (CMVs) code for immunoevasins, glycoproteins that are specifically dedicated to interfere with the presentation of antigenic peptides to CD8 T cells. Nonetheless, the biological outcome is not an immune evasion of the virus, since CD8 T cells can control CMV infection even when immunoevasins are expressed. Here, we compare the processing of a protective and a nonprotective epitope derived from the same viral protein, the antiapoptotic protein M45 in the murine model. The data provide evidence to conclude that protection against CMVs critically depends on antigenic peptides generated in an amount sufficient to exhaust the inhibitory capacity of immunoevasins.

Manifestations of human cytomegalovirus infection: proposed mechanisms of acute and chronic disease

Infections with human cytomegalovirus (HCMV) are a major cause of morbidity and mortality in humans with acquired or developmental deficits in innate and adaptive immunity. In the normal immunocompetent host, symptoms rarely accompany acute infections, although prolonged virus shedding is frequent. Virus persistence is established in all infected individuals and appears to be maintained by both a chronic productive infections as well as latency with restricted viral gene expression. The contributions of the each of these mechanisms to the persistence of this virus in the individual is unknown but frequent virus shedding into the saliva and genitourinary tract likely accounts for the near universal incidence of infection in most populations in the world. The pathogenesis of disease associated with acute HCMV infection is most readily attributable to lytic virus replication and end organ damage either secondary to virus replication and cell death or from host immunological responses that target virus-infected cells. Antiviral agents limit the severity of disease associated with acute HCMV infections, suggesting a requirement for virus replication in clinical syndromes associated with acute infection. End organ disease secondary to unchecked virus replication can be observed in infants infected in utero, allograft recipients receiving potent immunosuppressive agents, and patients with HIV infections that exhibit a loss of adaptive immune function. In contrast, diseases associated with chronic or persistent infections appear in normal individuals and in the allografts of the transplant recipient. The manifestations of these infections appear related to chronic inflammation, but it is unclear if poorly controlled virus replication is necessary for the different phenotypic expressions of disease that are reported in these patients. Although the relationship between HCMV infection and chronic allograft rejection is well known, the mechanisms that account for the role of this virus in graft loss are not well understood. However, the capacity of this virus to persist in the midst of intense inflammation suggests that its persistence could serve as a trigger for the induction of host-vs-graft responses or alternatively host responses to HCMV could contribute to the inflammatory milieu characteristic of chronic allograft rejection.

MHC class I immune evasion in MCMV infection

Murine cytomegalovirus (MCMV) is a well-studied model of natural beta-herpesvirus infection. However, many questions remain regarding its control by and evasion of the immune response it generates. CD8 and CD4 T cells have both unique and redundant roles in control of the virus that differ based on the immunocompetence of the infected mice. MCMV encodes major histocompatibility complex (MHC) class I immune evasion genes that can have an impact in vitro, but their role in infection of immunocompetent mice has been difficult to identify. This review addresses the evidence for their in vivo function and suggests why they may be evolutionarily conserved.

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.

Epitope-specific in vivo protection against cytomegalovirus disease by CD8 T cells in the murine model of preemptive immunotherapy

Preclinical research in murine models as well as subsequent clinical trials have concordantly revealed a high protective potential of antiviral CD8 T cells, of donor-derived ex vivo memory CD8 T cells in particular, in the immunotherapy of cytomegalovirus (CMV) infection in immunocompromised recipients. Although it is generally held view that the observed beneficial effect of the transferred cells is viral epitope-specific, involving the recognition of MHC class-I presented peptides by cognate T cell receptors, this assumption awaits formal proof, at least with regard to the in vivo function of the CD8 T cells. This question is particularly evident for CMV, since the function of viral immune evasion proteins interferes with the MHC class-I pathway of peptide presentation. Alternatively, therefore, one has to consider the possibility that the requirement for epitope recognition may be bypassed by other ligand-receptor interactions between CD8 T cells and infected cells, which may trigger the signaling for effector functions. Clearly, such a mechanism might explain why CD8 T cells are so efficient in controlling CMV infection despite the expression of viral immune evasion proteins. Here we provide direct evidence for epitope-specificity of antiviral protection by employing a recombinant murine CMV (mCMV), namely the mutant virus mCMV-IE1-L176A, in which an immunodominant viral epitope of the regulatory immediate-early protein IE1 is functionally deleted by a point mutation replacing leucine with alanine at the C-terminal MHC anchor position of the antigenic peptide.

A flow cytometry-based method for detecting antibody responses to murine cytomegalovirus infection

An assay based on target cells infected with green fluorescent protein labeled murine cytomegalovirus (GFP-MCMV) and dual color flow cytometry for detecting antibody to MCMV is described. After optimizing conditions for this technique, kinetics of anti-MCMV IgG antibody response was tested in susceptible (BALB/c) and resistant (C57BL/6) mouse strains following primary MCMV infection. Previously published antibody kinssetics were confirmed in susceptible mice, with peak IgG response seen approximately 8 weeks after primary infection, decreasing by 20 weeks after infection. In contrast, MCMV resistant C57BL/6 mice showed significantly lower IgG antibody responses than susceptible mice. Although several techniques have been previously described to detect murine antibody responses to MCMV, including nuclear anti-complement immunofluorescence, viral immunoblotting, complement fixation, indirect immunofluorescence, indirect hemagglutination, and enzyme-liked immunosorbent assay techniques, these techniques are all time consuming and laborious. The technique presented is a simple time efficient alternative to detect previous MCMV antibody responses in experimentally infected mice.

Expression of m157, a murine cytomegalovirus-encoded putative major histocompatibility class I (MHC-I)-like protein, is independent of viral regulation of host MHC-I

A murine cytomegalovirus (MCMV)-encoded protein, m157, has a putative major histocompatibility complex class I (MHC-I) structure and is recognized by the Ly49H NK cell activation receptor. Using a monoclonal antibody against m157, in this study we directly demonstrated that m157 is a cell surface-expressed glycophosphatidylinositol-anchored protein with early viral gene kinetics. Beta-2 microglobulin and TAP1 (transporter associated with antigen processing 1) were not required for its expression. MCMV-encoded proteins that down-regulate MHC-I did not affect the expression of m157. Thus, m157 is expressed on infected cells in a manner independent of viral regulation of host MHC-I.

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

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

Coordinated function of murine cytomegalovirus genes completely inhibits CTL lysis

Murine CMV (MCMV) encodes three viral genes that interfere with Ag presentation (VIPRs) to CD8 T cells, m04, m06, and m152. Because the functional impact of these genes during normal infection of C57BL/6 mice is surprisingly modest, we wanted to determine whether the VIPRs are equally effective against the entire spectrum of H-2(b)-restricted CD8 T cell epitopes. We also wanted to understand how the VIPRs interact at a functional level. To address these questions, we used a panel of MCMV mutants lacking each VIPR in all possible combinations, and CTL specific for 15 H-2(b)-restricted MCMV epitopes. Only expression of all three MCMV VIPRs completely inhibited killing by CTL specific for all 15 epitopes, but removal of any one VIPR enabled lysis by at least some CTL. The dominant interaction between the VIPRs was cooperation: m06 increased the inhibition of lysis achieved by either m152 or m04. However, for 1 of 15 epitopes m04 functionally antagonized m152. There was little differential impact of any of the VIPRs on K(b) vs D(b), but a surprising degree of differential impact of the three VIPRs for different epitopes. These epitope-specific differences did not correlate with functional avidity, or with timing of VIPR expression in relation to Ag expression in the virus replication cycle. Although questions remain about the molecular mechanism and in vivo role of these genes, we conclude that the coordinated function of MCMV's three VIPRs results in a powerful inhibition of lysis of infected cells by CD8 T cells.

Four distinct patterns of memory CD8 T cell responses to chronic murine cytomegalovirus infection

CMVs are beta herpesviruses that establish lifelong latent infection of their hosts. Acute infection of C57BL/6 mice with murine CMV elicits a very broad CD8 T cell response, comprising at least 24 epitopes from 18 viral proteins. In contrast, we show here that the CD8 T cell response in chronically infected mice was dominated by only five epitopes. Altogether, four distinct CD8 T cell kinetic patterns were evident. Responses to some epitopes, including M45, which dominates the acute response, contracted sharply after day 7 and developed into stable long-term memory. The response to m139 underwent rapid expansion and contraction, followed by a phase of memory inflation, whereas the response to an M38 epitope did not display any contraction phase. Finally, responses against two epitopes encoded by the immediate early gene IE3 were readily detectable in chronically infected mice but near the limit of detection during acute infection. CD8 T cells specific for the noninflationary M45 epitope displayed a classic central memory phenotype, re-expressing the lymph node homing receptor CD62L and homeostatic cytokine receptors for IL-7 and IL-15, and produced low levels of IL-2. Responses to two inflationary epitopes, m139 and IE3, retained an effector memory surface phenotype (CD62L(low), IL-7Ralpha(-), IL-15Rbeta(-)) and were unable to produce IL-2. We suggest that immunological choices are superimposed on altered viral gene expression profiles to determine immunodominance during chronic murine CMV infection.

Murine cytomegalovirus interference with antigen presentation contributes to the inability of CD8 T cells to control virus in the salivary gland

Compared to other organs, murine cytomegalovirus (MCMV) replication in the salivary gland is uniquely resistant to CD8 T-cell control. The contribution of viral genes that interfere with antigen presentation (VIPRs) to this resistance was assessed using a mutant lacking MCMV's known VIPRs. Salivary gland titers of the VIPR-deficient virus were at least 10-fold lower than those of the wild type during the persistent phase of infection; the defect was reversed by depleting CD8 T cells. Thus, VIPRs contribute to CD8 T cells' inability to control virus in the salivary gland.

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

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

Elimination of ie1 significantly attenuates murine cytomegalovirus virulence but does not alter replicative capacity in cell culture

The major immediate-early (MIE) genes of cytomegaloviruses (CMV) are broadly thought to be decisive regulators of lytic replication and reactivation from latency. To directly assess the role of the MIE protein IE1 during the infection of murine CMV (MCMV), we constructed an MCMV with exon 4 of the ie1 gene deleted. We found that, independent of the multiplicity of infection, the resulting recombinant virus, MCMVdie1, which fails to express the IE1 protein, was fully competent for early gene expression and replicated in different cultured cell types with identical kinetics to those of parental or revertant virus. Immunofluorescence microscopy studies revealed that MCMVdie1 was greatly impaired in its capacity to disrupt promyelocytic leukemia bodies in NIH 3T3 cells early after infection, a process that has been proposed to increase viral transcription efficiency. We examined MCMVdie1 in the murine model using both immunocompetent BALB/c and severe combined immunodeficient (SCID) mice. When MCMVdie1 was inoculated into these two types of mice, significantly lower viral titers were detected in infected organs than in those of the wild-type virus-infected animals. Moreover, the ie1-deficient MCMV exhibited a markedly reduced virulence. While all animals infected with 5 x 10(4) PFU of parental virus died by 30 days postinfection, SCID mice infected with a similar dose of MCMVdie1 did not succumb before 60 days postinfection. The in vivo defective growth phenotype of MCMVdie1 was abrogated upon rescue of ie1. These results demonstrate the significance of the ie1 gene for promoting an acute MCMV infection and virulence yet indicate that MCMV is able to grow in vivo, although impaired, in the absence of the ie1 gene.

Highly protective in vivo function of cytomegalovirus IE1 epitope-specific memory CD8 T cells purified by T-cell receptor-based cell sorting

Reconstitution of antiviral CD8 T cells is essential for controlling cytomegalovirus (CMV) infection after bone marrow transplantation. Accordingly, polyclonal CD8 T cells derived from BALB/c mice infected with murine CMV protect immunocompromised adoptive transfer recipients against CMV disease. The protective population comprises CD8 T cells with T-cell receptors (TCRs) specific for defined and for as-yet-unknown viral epitopes, as well as a majority of nonprotective cells with unrelated specificities. Defined epitopes include IE1/m123 and m164, which are immunodominant in terms of the magnitude of the CD8 T-cell response, and a panel of subordinate epitopes (m04, m18, M45, M83, and M84). While cytolytic T-lymphocyte lines (CTLLs) were shown to be protective regardless of the immunodominance of the respective epitope, the individual contributions of in vivo resident epitope-specific CD8 T cells to the antiviral control awaited investigation. The IE1 peptide 168-YPHFMPTNL-176 is generated from the immediate-early protein 1 (IE1) (pp89/76) of murine CMV and is presented by the major histocompatibility complex class I (MHC-I) molecule Ld. To quantitate its contribution to the protective potential of a CD8-T memory (CD8-TM) cell population, IE1-TCR+ and IE1-TCR- CD8-TM cells were purified by epitope-specific cell sorting with IE1 peptide-loaded MHC-immunoglobulin G1 dimers as ligands of cognate TCRs. Of relevance for clinical approaches to an adoptive cellular immunotherapy, sorted IE1 epitope-specific CD8-TM cells were found to be exceedingly protective upon adoptive transfer. Compared with CTLLs specific for the same epitope and of comparable avidity and TCR beta-chain variable region (Vbeta)-defined polyclonality, sorted CD8-TM cells proved to be superior by more than 2 orders of magnitude.

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

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

Systemic priming-boosting immunization with a trivalent plasmid DNA and inactivated murine cytomegalovirus (MCMV) vaccine provides long-term protection against viral replication following systemic or mucosal MCMV challenge

We previously demonstrated that vaccination of BALB/c mice with a pool of 13 plasmid DNAs (pDNAs) expressing murine cytomegalovirus (MCMV) genes followed by formalin-inactivated MCMV (FI-MCMV) resulted in complete protection against viral replication in the spleen and salivary glands following sublethal intraperitoneal (i.p.) challenge. Here, we found that following intranasal (i.n.) challenge, titers of virus in the lungs of the immunized mice were reduced approximately 1,000-fold relative to those for mock-immunized controls. We next sought to extend these results and to determine whether similar protection levels could be achieved by priming with a pool of three pDNAs containing three key plasmids (IE1, M84, and gB). We found that the three-pDNA priming elicited IE1- and M84-p65-specific CD8+ T lymphocytes and, following FI-MCMV boost, high levels of virion-specific immunoglobulin G (IgG) and virus-neutralizing antibodies. When mice were i.n. challenged 4 months after the last boost, titers of virus in the lungs of immunized mice were reduced 1,000- to 2,000-fold from those for controls during the peak of viral replication. Additionally, titers of virus were either at or below the detection limits for the salivary glands, liver, and spleen of the majority of the immunized mice. Following sublethal i.p. challenge, virus was undetectable in all of the above target organs of the immunized mice. Virion-specific IgA in the lungs was consistently detected by day 6 post-i.n. challenge for the immunized mice and by day 14 for controls. These results demonstrate the immunity and high levels of protection of the priming-boosting vaccination against both systemic and mucosal challenge.

Human cytomegalovirus: clinical aspects, immune regulation, and emerging treatments

After initial infection, human cytomegalovirus remains in a persistent state with the host. Immunity against the virus controls replication, although intermitent viral shedding can still take place in the seropositive immunocompetent person. Replication of cytomegalovirus in the absence of an effective immune response is central to the pathogenesis of disease. Therefore, complications are primarily seen in individuals whose immune system is immature, or is suppressed by drug treatment or coinfection with other pathogens. Although our increasing knowledge of the host-virus relationship has lead to the development of new pharmacological strategies for cytomegalovirus-associated infections, these strategies all have limitations-eg, drug toxicities, development of resistance, poor oral bioavailability, and low potency. Immune-based therapies to complement pharmacological strategies for the successful treatment of virus-associated complications should be prospectively investigated.

Multiple epitopes in the murine cytomegalovirus early gene product M84 are efficiently presented in infected primary macrophages and contribute to strong CD8+-T-lymphocyte responses and protection following DNA immunization

We previously demonstrated that after vaccination of BALB/c mice with DNA encoding murine cytomegalovirus (MCMV) IE1 or M84, a similar level of protection against MCMV infection was achieved. However, the percentage of antigen-specific CD8(+) T cells elicited by IE1 was higher than that by M84 as measured by intracellular cytokine staining when splenocytes were stimulated with an epitope peptide (M. Ye at al., J. Virol. 76:2100-2112, 2002). We show here that after DNA vaccination with M84, a higher percentage of M84-specific CD8(+) T cells was detected when splenocytes were stimulated with J774 cells expressing full-length M84. When the defined M84 epitope 297-305 was deleted, the mutant DNA vaccine was still protective against MCMV replication and induced strong M84-specific CD8(+)-T-cell responses. The M84 gene was subsequently subcloned into three fragments encoding overlapping protein fragments. When mice were immunized with each of the M84 subfragment DNAs, at least two additional protective CD8(+)-T-cell epitopes were detected. In contrast to strong responses after DNA vaccination, M84-specific CD8(+)-T-cell responses were poorly induced during MCMV infection. The weak M84-specific response after MCMV infection was not due to poor antigen presentation in antigen-presenting cells, since both J774 macrophages and primary peritoneal macrophages infected with MCMV in vitro were able to efficiently and constitutively present M84-specific epitopes starting at the early phase of infection. These results indicate that antigen presentation by macrophages is not sufficient for M84-specific CD8(+)-T-cell responses during MCMV infection.

Stalemating a clever opportunist: lessons from murine cytomegalovirus

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

A cytomegalovirus inhibitor of gamma interferon signaling controls immunoproteasome induction

Both human and mouse cytomegaloviruses (HCMV and MCMV) avoid peptide presentation through the major histocompatibility complex (MHC) class I pathway to CD8(+) T cells. Within the MHC class I pathway, the vast majority of antigenic peptides are generated by the proteasome system, a multicatalytic protease complex consisting of constitutive subunits, three of which can be replaced by enzymatically active gamma interferon (IFN-gamma)-inducible subunits, i.e., LMP2, LMP7, and MECL1, to form the so-called immunoproteasomes. Here, we show that steady-state levels of immunoproteasomes are readily formed in response to MCMV infection in the liver. In contrast, the incorporation of immunoproteasome subunits was prevented in MCMV-infected, as well as HCMV-infected, fibroblasts in vitro. Likewise, the expression of the IFN-gamma-inducible proteasome regulator PA28 alpha beta was also impaired in MCMV-infected cells. Both MCMV and HCMV did not alter the constitutive-subunit composition of proteasomes in infected cells. Quantitative assessment of LMP2, MECL1, and LMP7 transcripts revealed that the inhibition of immunoproteasome formation occurred at a pretranscriptional level. Remarkably, a targeted deletion of the MCMV gene M27, encoding an inhibitor of STAT2 that disrupts IFN-gamma receptor signaling, largely restored transcription and protein expression of immunoproteasome subunits in infected cells. While CMV block peptide transport and MHC class I assembly by posttranslational strategies, immunoproteasome assembly, and thus the repertoire of proteasomal peptides, is controlled by pretranscriptional mechanisms. We hypothesize that the blockade of immunoproteasome formation has considerable consequences for shaping the CD8(+)-T-cell repertoire during the effector phase of the immune response.

Expansion of protective CD8+ T-cell responses driven by recombinant cytomegaloviruses

CD8(+) T cells are critical for the control of many persistent viral infections, such as human immunodeficiency virus, hepatitis C virus, Epstein-Barr virus, and cytomegalovirus (CMV). In most infections, large CD8(+)-T-cell populations are induced early but then contract and are maintained thereafter at lower levels. In contrast, CD8(+) T cells specific for murine CMV (MCMV) have been shown to gradually accumulate after resolution of primary infection. This unique behavior is restricted to certain epitopes, including an immunodominant epitope derived from the immediate-early 1 (IE1) gene product. To explore the mechanism behind this further, we measured CD8(+)-T-cell-mediated immunity induced by recombinant MCMV-expressing epitopes derived from influenza A virus or lymphocytic choriomeningitis virus placed under the control of an IE promoter. We observed that virus-specific CD8(+)-T-cell populations were induced and that these expanded gradually over time. Importantly, these CD8(+) T cells provided long-term protection against challenge without boosting. These results demonstrate a unique pattern of accumulating T cells, which provide long-lasting immune protection, that is independent of the initial immunodominance of the epitope and indicates the potential of T-cell-inducing vaccines based on persistent vectors.

A survival game of hide and seek: cytomegaloviruses and MHC class I antigen presentation pathways

Cytomegaloviruses (CMV) are members of the ubiquitous family of herpesviruses, which escape immunological clearance and persist throughout life in the infected host. Cytomegaloviruses have developed numerous strategies that permit them to co-exist with their host even as an anti-virus immune response endangers their long-term survival. A considerable number of these strategies are aimed at MHC class I presentation of viral proteins to CD8+ T cells (TCD8+ ). Although the gamut of CMV immune evasion will be briefly examined, the primary focus of this review is on the host ability to counteract the strategies developed by CMV to inhibit antigen processing and presentation. A primary mechanism used by the immune system is the recognition of very early virus proteins including recognition of the immunomodulatory proteins themselves. We further speculate that cross-presentation of antigen is an adaptive immune response to the inhibition of direct presentation. Other mechanisms, such as the evolution of pAPC subsets, may also allow the immune system to adapt to a variety of different infectious pathogens while preventing cytopathic infection of all pAPCs.

Memory inflation: continuous accumulation of antiviral CD8+ T cells over time

CD8+ T lymphocytes play an important role in the control of intracellular pathogens during both acute and persistent infections. This is particularly true in the case of persistent herpesviruses such as human CMV, which are typified by large virus-specific CD8+ T cell populations during viral latency. To understand the origin of these populations and the factors shaping them over time, we investigated the CD8+ T cell response after murine CMV (MCMV) infection. The kinetics of the acute response were characterized by rapid expansion of activated T cells, followed by a contraction phase. Thereafter, we observed a striking pattern, where MCMV-specific memory CD8+ T cells steadily accumulated over time, with 20% of all CD8+ T cells at 1 year specific for one MCMV epitope. Accumulation of MCMV-specific CD8+ T lymphocytes was seen in all organs tested and was associated with continuous activation of specific CD8+ T lymphocytes, primarily within lymph nodes. The pattern of accumulation was observed in only two of five epitopes tested, and was accompanied by a gradual restriction in usage of the variable region of the TCR beta-chain over time. This novel pattern of a virus-specific CD8+ T cell response suggests that continuous or repetitive exposure to Ag can slowly mold memory T cell populations over time. This may be relevant for understanding the evolution of the large human CMV-specific CD8+ T cell populations seen in humans.