Cytomegalovirus inhibitors of programmed cell death restrict antigen cross-presentation in the priming of antiviral CD8 T cells.

CD8 T cells are the predominant effector cells of adaptive immunity in preventing cytomegalovirus (CMV) multiple-organ disease caused by cytopathogenic tissue infection. The mechanism by which CMV-specific, naïve CD8 T cells become primed and clonally expand is of fundamental importance for our understanding of CMV immune control. For CD8 T-cell priming, two pathways have been identified: direct antigen presentation by infected professional antigen-presenting cells (pAPCs) and antigen cross-presentation by uninfected pAPCs that take up antigenic material derived from infected tissue cells. Studies in mouse models using murine CMV (mCMV) and precluding either pathway genetically or experimentally have shown that, in principle, both pathways can congruently generate the mouse MHC/H-2 class-I-determined epitope-specificity repertoire of the CD8 T-cell response. Recent studies, however, have shown that direct antigen presentation is the canonical pathway when both are accessible. This raised the question of why antigen cross-presentation is ineffective even under conditions of high virus replication thought to provide high amounts of antigenic material for feeding cross-presenting pAPCs. As delivery of antigenic material for cross-presentation is associated with programmed cell death, and as CMVs encode inhibitors of different cell death pathways, we pursued the idea that these inhibitors restrict antigen delivery and thus CD8 T-cell priming by cross-presentation. To test this hypothesis, we compared the CD8 T-cell responses to recombinant mCMVs lacking expression of the apoptosis-inhibiting protein M36 or the necroptosis-inhibiting protein M45 with responses to wild-type mCMV and revertant viruses expressing the respective cell death inhibitors. The data reveal that increased programmed cell death improves CD8 T-cell priming in mice capable of antigen cross-presentation but not in a mutant mouse strain unable to cross-present. These findings strongly support the conclusion that CMV cell death inhibitors restrict the priming of CD8 T cells by antigen cross-presentation.

Targeting the tissue factor coagulation initiation complex prevents antiphospholipid antibody development.

ABSTRACT: Antiphospholipid antibodies (aPL) in primary or secondary antiphospholipid syndrome (APS) are a major cause for acquired thrombophilia, but specific interventions preventing autoimmune aPL development are an unmet clinical need. Although autoimmune aPL cross react with various coagulation regulatory proteins, lipid-reactive aPL, including those derived from patients with COVID-19, recognize the endolysosomal phospholipid lysobisphosphatidic acid presented by the cell surface-expressed endothelial protein C receptor. This specific recognition leads to complement-mediated activation of tissue factor (TF)-dependent proinflammatory signaling and thrombosis. Here, we show that specific inhibition of the TF coagulation initiation complex with nematode anticoagulant protein c2 (NAPc2) prevents the prothrombotic effects of aPL derived from patients with COVID-19 in mice and the aPL-induced proinflammatory and prothrombotic activation of monocytes. The induction of experimental APS is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, and NAPc2 suppresses monocyte endosomal reactive oxygen species production requiring the TF cytoplasmic domain and interferon-α secretion from dendritic cells. Latent infection with murine cytomegalovirus causes TF cytoplasmic domain-dependent development of persistent aPL and circulating phospholipid-reactive B1 cells, which is prevented by short-term intervention with NAPc2 during acute viral infection. In addition, treatment of lupus prone MRL-lpr mice with NAPc2, but not with heparin, suppresses dendritic-cell activation in the spleen, aPL production and circulating phospholipid-reactive B1 cells, and attenuates lupus pathology. These data demonstrate a convergent TF-dependent mechanism of aPL development in latent viral infection and autoimmune disease and provide initial evidence that specific targeting of the TF initiation complex has therapeutic benefits beyond currently used clinical anticoagulant strategies.

Immunotherapy of cytomegalovirus infection by low-dose adoptive transfer of antiviral CD8 T cells relies on substantial post-transfer expansion of central memory cells but not effector-memory cells.

Cytomegaloviruses (CMVs) are host species-specific in their replication. It is a hallmark of all CMVs that productive primary infection is controlled by concerted innate and adaptive immune responses in the immunocompetent host. As a result, the infection usually passes without overt clinical symptoms and develops into latent infection, referred to as "latency". During latency, the virus is maintained in a non-replicative state from which it can reactivate to productive infection under conditions of waning immune surveillance. In contrast, infection of an immunocompromised host causes CMV disease with viral multiple-organ histopathology resulting in organ failure. Primary or reactivated CMV infection of hematopoietic cell transplantation (HCT) recipients in a "window of risk" between therapeutic hemato-ablative leukemia therapy and immune system reconstitution remains a clinical challenge. Studies in the mouse model of experimental HCT and infection with murine CMV (mCMV), followed by clinical trials in HCT patients with human CMV (hCMV) reactivation, have revealed a protective function of virus-specific CD8 T cells upon adoptive cell transfer (AT). Memory CD8 T cells derived from latently infected hosts are a favored source for immunotherapy by AT. Strikingly low numbers of these cells were found to prevent CMV disease, suggesting either an immediate effector function of few transferred cells or a clonal expansion generating high numbers of effector cells. In the murine model, the memory population consists of resting central memory T cells (TCM), as well as of conventional effector-memory T cells (cTEM) and inflationary effector-memory T cells (iTEM). iTEM increase in numbers over time in the latently infected host, a phenomenon known as 'memory inflation' (MI). They thus appeared to be a promising source for use in immunotherapy. However, we show here that iTEM contribute little to the control of infection after AT, which relies almost entirely on superior proliferative potential of TCM.

The herpesviral antagonist m152 reveals differential activation of STING-dependent IRF and NF-κB signaling and STING's dual role during MCMV infection.

Cytomegaloviruses (CMVs) are master manipulators of the host immune response. Here, we reveal that the murine CMV (MCMV) protein m152 specifically targets the type I interferon (IFN) response by binding to stimulator of interferon genes (STING), thereby delaying its trafficking to the Golgi compartment from where STING initiates type I IFN signaling. Infection with an MCMV lacking m152 induced elevated type I IFN responses and this leads to reduced viral transcript levels both in vitro and in vivo This effect is ameliorated in the absence of STING Interestingly, while m152 inhibits STING-mediated IRF signaling, it did not affect STING-mediated NF-κB signaling. Analysis of how m152 targets STING translocation reveals that STING activates NF-κB signaling already from the ER prior to its trafficking to the Golgi. Strikingly, this response is important to promote early MCMV replication. Our results show that MCMV has evolved a mechanism to specifically antagonize the STING-mediated antiviral IFN response, while preserving its pro-viral NF-κB response, providing an advantage in the establishment of an infection.

Eomes broadens the scope of CD8 T-cell memory by inhibiting apoptosis in cells of low affinity.

The memory CD8 T-cell pool must select for clones that bind immunodominant epitopes with high affinity to efficiently counter reinfection. At the same time, it must retain a level of clonal diversity to allow recognition of pathogens with mutated epitopes. How the level of diversity within the memory pool is controlled is unclear, especially in the context of a selective drive for antigen affinity. We find that preservation of clones that bind the activating antigen with low affinity depends on expression of the transcription factor Eomes in the first days after antigen encounter. Eomes is induced at low activating signal strength and directly drives transcription of the prosurvival protein Bcl-2. At higher signal intensity, T-bet is induced which suppresses Bcl-2 and causes a relative survival advantage for cells of low affinity. Clones activated with high-affinity antigen form memory largely independent of Eomes and have a proliferative advantage over clones that bind the same antigen with low affinity. This causes high-affinity clones to prevail in the memory pool, despite their relative survival deficit. Genetic or therapeutic targeting of the Eomes/Bcl-2 axis reduces the clonal diversity of the memory pool, which diminishes its ability to respond to pathogens carrying mutations in immunodominant epitopes. Thus, we demonstrate on a molecular level how sufficient diversity of the memory pool is established in an environment of affinity-based selection.

Coincident airway exposure to low-potency allergen and cytomegalovirus sensitizes for allergic airway disease by viral activation of migratory dendritic cells.

Despite a broad cell-type tropism, cytomegalovirus (CMV) is an evidentially pulmonary pathogen. Predilection for the lungs is of medical relevance in immunocompromised recipients of hematopoietic cell transplantation, in whom interstitial CMV pneumonia is a frequent and, if left untreated, fatal clinical manifestation of human CMV infection. A conceivable contribution of CMV to airway diseases of other etiology is an issue that so far attracted little medical attention. As the route of primary CMV infection upon host-to-host transmission in early childhood involves airway mucosa, coincidence of CMV airway infection and exposure to airborne environmental antigens is almost unavoidable. For investigating possible consequences of such a coincidence, we established a mouse model of airway co-exposure to CMV and ovalbumin (OVA) representing a protein antigen of an inherently low allergenic potential. Accordingly, intratracheal OVA exposure alone failed to sensitize for allergic airway disease (AAD) upon OVA aerosol challenge. In contrast, airway infection at the time of OVA sensitization predisposed for AAD that was characterized by airway inflammation, IgE secretion, thickening of airway epithelia, and goblet cell hyperplasia. This AAD histopathology was associated with a T helper type 2 (Th2) transcription profile in the lungs, including IL-4, IL-5, IL-9, and IL-25, known inducers of Th2-driven AAD. These symptoms were all prevented by a pre-challenge depletion of CD4+ T cells, but not of CD8+ T cells. As to the underlying mechanism, murine CMV activated migratory CD11b+ as well as CD103+ conventional dendritic cells (cDCs), which have been associated with Th2 cytokine-driven AAD and with antigen cross-presentation, respectively. This resulted in an enhanced OVA uptake and recruitment of the OVA-laden cDCs selectively to the draining tracheal lymph nodes for antigen presentation. We thus propose that CMV, through activation of migratory cDCs in the airway mucosa, can enhance the allergenic potential of otherwise poorly allergenic environmental protein antigens.

Evaluation of a laboratory-based high-throughput SARS-CoV-2 antigen assay for non-COVID-19 patient screening at hospital admission.

Several rapid antigen tests (RATs) for the detection of SARS-CoV-2 were evaluated recently. However, reliable performance data for laboratory-based, high-throughput antigen tests are lacking. Therefore and in response to a short-term shortage of PCR reagents, we evaluated DiaSorin's LIAISON SARS-CoV-2 antigen test in comparison to RT-qPCR, and concerning the application of screening non-COVID-19 patients on hospital admission. Applying the manufacturer-recommended cut-off of 200 arbitrary units (AU/mL) the specificity of the LIAISON Test was 100%, the overall analytical sensitivity 40.2%. Lowering the cut-off to 100 AU/mL increased the sensitivity to 49.7% and decreased the specificity to 98.3%. Confining the analysis to samples with an RT-qPCR result < 25 Ct resulted in a sensitivity of 91.2%. The quality of the LIAISON test is very similar to that of good RATs described in the literature with the advantage of high throughput and the disadvantage of relatively long analysis time. It passes the WHO quality criteria for rapid antigen tests and is characterized by particularly high specificity. The LIAISON test can therefore be used for the same applications as recommended for RATs by the WHO. Due to limited sensitivity, the LIAISON test should only be used for screening, if PCR-based assays are not available.

Efficient Delivery of Human Cytomegalovirus T Cell Antigens by Attenuated Sendai Virus Vectors.

Human cytomegalovirus (HCMV) represents a major cause of clinical complications during pregnancy as well as immunosuppression, and the licensing of a protective HCMV vaccine remains an unmet global need. Here, we designed and validated novel Sendai virus (SeV) vectors delivering the T cell immunogens IE-1 and pp65. To enhance vector safety, we used a replication-deficient strain (rdSeV) that infects target cells in a nonproductive manner while retaining viral gene expression. In this study, we explored the impact that transduction with rdSeV has on human dendritic cells (DCs) by comparing it to the parental, replication-competent Sendai virus strain (rcSeV) as well as the poxvirus strain modified vaccinia Ankara (MVA). We found that wild-type SeV is capable of replicating to high titers in DCs while rdSeV infects cells abortively. Due to the higher degree of attenuation, IE-1 and pp65 protein levels mediated by rdSeV after infection of DCs were markedly reduced compared to those of the parental Sendai virus recombinants, but antigen-specific restimulation of T cell clones was not negatively affected by this. Importantly, rdSeV showed reduced cytotoxic effects compared to rcSeV and MVA and was capable of mediating DC maturation as well as secretion of alpha interferon and interleukin-6. Finally, in a challenge model with a murine cytomegalovirus (MCMV) strain carrying an HCMV pp65 peptide, we found that viral replication was restricted if mice were previously vaccinated with rdSeV-pp65. Taken together, these data demonstrate that rdSeV has great potential as a vector system for the delivery of HCMV immunogens.IMPORTANCE HCMV is a highly prevalent betaherpesvirus that establishes lifelong latency after primary infection. Congenital HCMV infection is the most common viral complication in newborns, causing a number of late sequelae ranging from impaired hearing to mental retardation. At the same time, managing HCMV reactivation during immunosuppression remains a major hurdle in posttransplant care. Since options for the treatment of HCMV infection are still limited, the development of a vaccine to confine HCMV-related morbidities is urgently needed. We generated new vaccine candidates in which the main targets of T cell immunity during natural HCMV infection, IE-1 and pp65, are delivered by a replication-deficient, Sendai virus-based vector system. In addition to classical prophylactic vaccine concepts, these vectors could also be used for therapeutic applications, thereby expanding preexisting immunity in high-risk groups such as transplant recipients or for immunotherapy of glioblastomas expressing HCMV antigens.

The murine cytomegalovirus M35 protein antagonizes type I IFN induction downstream of pattern recognition receptors by targeting NF-κB mediated transcription.

The type I interferon (IFN) response is imperative for the establishment of the early antiviral immune response. Here we report the identification of the first type I IFN antagonist encoded by murine cytomegalovirus (MCMV) that shuts down signaling following pattern recognition receptor (PRR) sensing. Screening of an MCMV open reading frame (ORF) library identified M35 as a novel and strong negative modulator of IFNß promoter induction following activation of both RNA and DNA cytoplasmic PRR. Additionally, M35 inhibits the proinflammatory cytokine response downstream of Toll-like receptors (TLR). Using a series of luciferase-based reporters with specific transcription factor binding sites, we determined that M35 targets NF-κB-, but not IRF-mediated, transcription. Expression of M35 upon retroviral transduction of immortalized bone marrow-derived macrophages (iBMDM) led to reduced IFNß transcription and secretion upon activation of stimulator of IFN genes (STING)-dependent signaling. On the other hand, M35 does not antagonize interferon-stimulated gene (ISG) 56 promoter induction or ISG transcription upon exogenous stimulation of the type I IFN receptor (IFNAR). M35 is present in the viral particle and, upon MCMV infection of fibroblasts, is immediately shuttled to the nucleus where it exerts its immunomodulatory effects. Deletion of M35 from the MCMV genome and hence from the viral particle resulted in elevated type I IFN transcription and secretion in vitro and in vivo. In the absence of M35, lower viral titers are observed during acute infection of the host, and productive infection in the salivary glands was not detected. In conclusion, the M35 protein is released by MCMV immediately upon infection in order to deftly inhibit the antiviral type I IFN response by targeting NF-κB-mediated transcription. The identification of this novel viral protein reinforces the importance of timely countermeasures in the complex relationship between virus and host.

Cellular reservoirs of latent cytomegaloviruses.

Cytomegaloviruses (CMVs), members of the ß-subfamily of the herpesvirus family, have co-speciated with their respective mammalian hosts resulting in a mutual virus-host adaptation reflected by sets of 'private' viral genes that a particular CMV species does not share with other CMVs and that define the host-species specificity of CMVs. Nonetheless, based on "biological convergence" in evolution, fundamental rules in viral pathogenesis and immune control are functionally analogous between different virus-host pairs. Therefore, the mouse model of infection with murine CMV (mCMV) has revealed generally valid principles of CMV-host interactions. Specifically, the mouse model has paved the way to cellular immunotherapy of CMV disease in immunocompromised recipients of hematopoietic cell transplantation (HCT). Precisely in the context of HCT, however, current view assumes that there exists a major difference between hCMV and mCMV regarding "latent virus reservoirs" in that only hCMV establishes latency in hematopoietic lineage cells (HLCs), whereas mCMV establishes latency in endothelial cells. This would imply that only hCMV can reactivate from transplanted HLCs of a latently infected donor. In addition, as viral transcriptional activity during latency is discussed as a driver of clonal T-cell expansion over lifetime, a phenomenon known as "memory inflation", it is important to know if hCMV and mCMV establish latency in the same cell type(s) for imprinting the immune system. Here, we review the currently available evidence to propose that the alleged difference in latent virus reservoirs between hCMV and mCMV may rather relate to a difference in the focus of research. While studies on hCMV latency in HLCs likely described a non-canonical, transient type-2 latency, studies in the mouse model focussed on canonical, lifelong type-1 latency.

Function of the cargo sorting dileucine motif in a cytomegalovirus immune evasion protein.

As an immune evasion mechanism, cytomegaloviruses (CMVs) have evolved proteins that interfere with cell surface trafficking of MHC class-I (MHC-I) molecules to tone down recognition by antiviral CD8 T cells. This interference can affect the trafficking of recently peptide-loaded MHC-I from the endoplasmic reticulum to the cell surface, thus modulating the presentation of viral peptides, as well as the recycling of pre-existing cell surface MHC-I, resulting in reduction of the level of overall MHC-I cell surface expression. Murine cytomegalovirus (mCMV) was paradigmatic in that it led to the discovery of this immune evasion strategy of CMVs. Members of its m02-m16 gene family code for type-I transmembrane glycoproteins, proven or predicted, most of which carry cargo sorting motifs in their cytoplasmic, C-terminal tail. For the m06 gene product m06 (gp48), the cargo has been identified as being MHC-I, which is linked by m06 to cellular adapter proteins AP-1A and AP-3A through the dileucine motif EPLARLL. Both APs are involved in trans-Golgi network (TGN) cargo sorting and, based on transfection studies, their engagement by the dileucine motif was proposed to be absolutely required to prevent MHC-I exposure at the cell surface. Here, we have tested this prediction in an infection system with the herein newly described recombinant virus mCMV-m06AA, in which the dileucine motif is destroyed by replacing EPLARLL with EPLARAA. This mutation has a phenotype in that the transition of m06-MHC-I complexes from early endosomes (EE) to late endosomes (LE)/lysosomes for degradation is blocked. Consistent with the binding of the MHC-I α-chain to the luminal domain of m06, the m06-mediated disposal of MHC-I did not require the ß2m chain of mature MHC-I. Unexpectedly, however, disconnecting MHC-I cargo from AP-1A/3A by the motif mutation in m06 had no notable rescuing impact on overall cell surface MHC-I, though it resulted in some improvement of the presentation of viral antigenic peptides by recently peptide-loaded MHC-I. Thus, the current view on the mechanism by which m06 mediates immune evasion needs to be revised. While the cargo sorting motif is critically involved in the disposal of m06-bound MHC-I in the endosomal/lysosomal pathway at the stage of EE to LE transition, this motif-mediated disposal is not the critical step by which m06 causes immune evasion. We rather propose that engagement of AP-1A/3A by the cargo sorting motif in m06 routes the m06-MHC-I complexes into the endosomal pathway and thereby detracts them from the constitutive cell surface transport.

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 Ld. The only known second MI-driving antigenic peptide of mCMV in the murine MHC haplotype H-2d is AGPPRYSRI presented by the MHC-I molecule Dd. 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.

NKG2D stimulation of CD8+ T cells during priming promotes their capacity to produce cytokines in response to viral infection in mice.

Natural killer group 2 member D (NKG2D) is an activating receptor that is expressed on most cytotoxic cells of the immune system, including NK cells, γδ, and CD8+ T cells. It is still a matter of debate whether and how NKG2D mediates priming of CD8+ T cells in vivo, due to a lack of studies where NKG2D is eliminated exclusively in these cells. Here, we studied the impact of NKG2D on effector CD8+ T-cell formation. NKG2D deficiency that is restricted to murine CD8+ T cells did not impair antigen-specific T-cell expansion following mouse CMV and lymphocytic choriomeningitis virus infection, but reduced their capacity to produce cytokines. Upon infection, conventional dendritic cells induce NKG2D ligands, which drive cytokine production on CD8+ T cells via the Dap10 signaling pathway. T-cell development, homing, and proliferation were not affected by NKG2D deficiency and cytotoxicity was only impaired when strong T-cell receptor (TCR) stimuli were used. Transfer of antigen-specific CD8+ T cells demonstrated that NKG2D deficiency attenuated their capacity to reduce viral loads. The inability of NKG2D-deficient cells to produce cytokines could be overcome with injection of IL-15 superagonist during priming. In summary, our data show that NKG2D has a nonredundant role in priming of CD8+ T cells to produce antiviral cytokines.

Cytomegalovirus vector expressing RAE-1γ induces enhanced anti-tumor capacity of murine CD8+ T cells.

Designing CD8+ T-cell vaccines, which would provide protection against tumors is still considered a great challenge in immunotherapy. Here we show the robust potential of cytomegalovirus (CMV) vector expressing the NKG2D ligand RAE-1γ as CD8+ T cell-based vaccine against malignant tumors. Immunization with the CMV vector expressing RAE-1γ, delayed tumor growth or even provided complete protection against tumor challenge in both prophylactic and therapeutic settings. Moreover, a potent tumor control in mice vaccinated with this vector can be further enhanced by blocking the immune checkpoints TIGIT and PD-1. CMV vector expressing RAE-1γ potentiated expansion of KLRG1+ CD8+ T cells with enhanced effector properties. This vaccination was even more efficient in neonatal mice, resulting in the expansion and long-term maintenance of epitope-specific CD8+ T cells conferring robust resistance against tumor challenge. Our data show that immunomodulation of CD8+ T-cell responses promoted by herpesvirus expressing a ligand for NKG2D receptor can provide a powerful platform for the prevention and treatment of CD8+ T-cell sensitive tumors.

The Mouse Cytomegalovirus Gene m42 Targets Surface Expression of the Protein Tyrosine Phosphatase CD45 in Infected Macrophages.

The receptor-like protein tyrosine phosphatase CD45 is expressed on the surface of cells of hematopoietic origin and has a pivotal role for the function of these cells in the immune response. Here we report that following infection of macrophages with mouse cytomegalovirus (MCMV) the cell surface expression of CD45 is drastically diminished. Screening of a set of MCMV deletion mutants allowed us to identify the viral gene m42 of being responsible for CD45 down-modulation. Moreover, expression of m42 independent of viral infection upon retroviral transduction of the RAW264.7 macrophage cell line led to comparable regulation of CD45 expression. In immunocompetent mice infected with an m42 deletion mutant lower viral titers were observed in all tissues examined when compared to wildtype MCMV, indicating an important role of m42 for viral replication in vivo. The m42 gene product was identified as an 18 kDa protein expressed with early kinetics and is predicted to be a tail-anchored membrane protein. Tracking of surface-resident CD45 molecules revealed that m42 induces internalization and degradation of CD45. The observation that the amounts of the E3 ubiquitin ligases Itch and Nedd4 were diminished in cells expressing m42 and that disruption of a PY motif in the N-terminal part of m42 resulted in loss of function, suggest that m42 acts as an activator or adaptor for these Nedd4-like ubiquitin ligases, which mark CD45 for lysosomal degradation. In conclusion, the down-modulation of CD45 expression in MCMV-infected myeloid cells represents a novel pathway of virus-host interaction.

Peptide Processing Is Critical for T-Cell Memory Inflation and May Be Optimized to Improve Immune Protection by CMV-Based Vaccine Vectors.

Cytomegalovirus (CMV) elicits long-term T-cell immunity of unparalleled strength, which has allowed the development of highly protective CMV-based vaccine vectors. Counterintuitively, experimental vaccines encoding a single MHC-I restricted epitope offered better immune protection than those expressing entire proteins, including the same epitope. To clarify this conundrum, we generated recombinant murine CMVs (MCMVs) encoding well-characterized MHC-I epitopes at different positions within viral genes and observed strong immune responses and protection against viruses and tumor growth when the epitopes were expressed at the protein C-terminus. We used the M45-encoded conventional epitope HGIRNASFI to dissect this phenomenon at the molecular level. A recombinant MCMV expressing HGIRNASFI on the C-terminus of M45, in contrast to wild-type MCMV, enabled peptide processing by the constitutive proteasome, direct antigen presentation, and an inflation of antigen-specific effector memory cells. Consequently, our results indicate that constitutive proteasome processing of antigenic epitopes in latently infected cells is required for robust inflationary responses. This insight allows utilizing the epitope positioning in the design of CMV-based vectors as a novel strategy for enhancing their efficacy.

Non-redundant and redundant roles of cytomegalovirus gH/gL complexes in host organ entry and intra-tissue spread.

Herpesviruses form different gH/gL virion envelope glycoprotein complexes that serve as entry complexes for mediating viral cell-type tropism in vitro; their roles in vivo, however, remained speculative and can be addressed experimentally only in animal models. For murine cytomegalovirus two alternative gH/gL complexes, gH/gL/gO and gH/gL/MCK-2, have been identified. A limitation of studies on viral tropism in vivo has been the difficulty in distinguishing between infection initiation by viral entry into first-hit target cells and subsequent cell-to-cell spread within tissues. As a new strategy to dissect these two events, we used a gO-transcomplemented ΔgO mutant for providing the gH/gL/gO complex selectively for the initial entry step, while progeny virions lack gO in subsequent rounds of infection. Whereas gH/gL/gO proved to be critical for establishing infection by efficient entry into diverse cell types, including liver macrophages, endothelial cells, and hepatocytes, it was dispensable for intra-tissue spread. Notably, the salivary glands, the source of virus for host-to-host transmission, represent an exception in that entry into virus-producing cells did not strictly depend on either the gH/gL/gO or the gH/gL/MCK-2 complex. Only if both complexes were absent in gO and MCK-2 double-knockout virus, in vivo infection was abolished at all sites.

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.

Murine cytomegalovirus (CMV) infection via the intranasal route offers a robust model of immunity upon mucosal CMV infection.

Cytomegalovirus (CMV) is a ubiquitous virus, causing the most common congenital infection in humans, yet a vaccine against this virus is not available. Experimental studies of immunity against CMV in animal models of infection, such as the infection of mice with mouse CMV (MCMV), have relied mainly on parenteral infection protocols, although the virus naturally transmits by mucosal routes via body fluids. To characterize the biology of infections by mucosal routes, we compared the kinetics of virus replication, latent viral load and CD8 T-cell responses in lymphoid organs upon experimental intranasal (targeting the respiratory tract) and intragastric (targeting the digestive tract) infection with systemic intraperitoneal infection of two unrelated mouse strains. We observed that intranasal infection induced robust and long-term virus replication in the lungs and salivary glands but limited replication in the spleen. CD8 T-cell responses were somewhat weaker than upon intraperitoneal infection but showed similar kinetic profiles and phenotypes of antigen-specific cells. In contrast, intragastric infection resulted in abortive or poor virus replication in all tested organs and poor T-cell responses to the virus, especially at late times after infection. Consistent with the T-cell kinetics, the MCMV latent load was high in the lungs but low in the spleen of intranasally infected mice and lowest in all tested organs upon intragastric infection. In conclusion, we showed that intranasal but not intragastric infection of mice with MCMV represents a robust model to study the short- and long-term biology of CMV infection by a mucosal route.

Mast cells expedite control of pulmonary murine cytomegalovirus infection by enhancing the recruitment of protective CD8 T cells to the lungs.

The lungs are a noted predilection site of acute, latent, and reactivated cytomegalovirus (CMV) infections. Interstitial pneumonia is the most dreaded manifestation of CMV disease in the immunocompromised host, whereas in the immunocompetent host lung-infiltrating CD8 T cells confine the infection in nodular inflammatory foci and prevent viral pathology. By using murine CMV infection as a model, we provide evidence for a critical role of mast cells (MC) in the recruitment of protective CD8 T cells to the lungs. Systemic infection triggered degranulation selectively in infected MC. The viral activation of MC was associated with a wave of CC chemokine ligand 5 (CCL5) in the serum of C57BL/6 mice that was MC-derived as verified by infection of MC-deficient Kit(W-sh/W-sh) "sash" mutants. In these mutants, CD8 T cells were recruited less efficiently to the lungs, correlating with enhanced viral replication and delayed virus clearance. A causative role for MC was verified by MC reconstitution of "sash" mice restoring both, efficient CD8 T-cell recruitment and infection control. These results reveal a novel crosstalk axis between innate and adaptive immune defense against CMV, and identify MC as a hitherto unconsidered player in the immune surveillance at a relevant site of CMV disease.