Differences and similarities in viral life cycle progression and host cell physiology after infection of human dendritic cells with modified vaccinia virus Ankara and vaccinia virus

The purinergic receptor P2X7 as a modulator of viral vector-mediated antigen cross-presentation

Introduction

Modified Vaccinia Virus Ankara (MVA) is a safe vaccine vector inducing long- lasting and potent immune responses. MVA-mediated CD8+T cell responses are optimally induced, if both, direct- and cross-presentation of viral or recombinant antigens by dendritic cells are contributing.

Methods

To improve the adaptive immune responses, we investigated the role of the purinergic receptor P2X7 (P2RX7) in MVA-infected feeder cells as a modulator of cross-presentation by non-infected dendritic cells. The infected feeder cells serve as source of antigen and provide signals that help to attract dendritic cells for antigen take up and to license these cells for cross-presentation.

Results

We demonstrate that presence of an active P2RX7 in major histocompatibility complex (MHC) class I (MHCI) mismatched feeder cells significantly enhanced MVA-mediated antigen cross-presentation. This was partly regulated by P2RX7-specific processes, such as the increased availability of extracellular particles as well as the altered cellular energy metabolism by mitochondria in the feeder cells. Furthermore, functional P2RX7 in feeder cells resulted in a delayed but also prolonged antigen expression after infection.

Discussion

We conclude that a combination of the above mentioned P2RX7-depending processes leads to significantly increased T cell activation via cross- presentation of MVA-derived antigens. To this day, P2RX7 has been mostly investigated in regards to neuroinflammatory diseases and cancer progression. However, we report for the first time the crucial role of P2RX7 for antigen- specific T cell immunity in a viral infection model.

The poxvirus F17 protein counteracts mitochondrially orchestrated antiviral responses

Poxviruses are unusual DNA viruses that replicate in the cytoplasm. To do so, they encode approximately 100 immunomodulatory proteins that counteract cytosolic nucleic acid sensors such as cGAMP synthase (cGAS) along with several other antiviral response pathways. Yet most of these immunomodulators are expressed very early in infection while many are variable host range determinants, and significant gaps remain in our understanding of poxvirus sensing and evasion strategies. Here, we show that after infection is established, subsequent progression of the viral lifecycle is sensed through specific changes to mitochondria that coordinate distinct aspects of the antiviral response. Unlike other viruses that cause extensive mitochondrial damage, poxviruses sustain key mitochondrial functions including membrane potential and respiration while reducing reactive oxygen species that drive inflammation. However, poxvirus replication induces mitochondrial hyperfusion that independently controls the release of mitochondrial DNA (mtDNA) to prime nucleic acid sensors and enables an increase in glycolysis that is necessary to support interferon stimulated gene (ISG) production. To counter this, the poxvirus F17 protein localizes to mitochondria and dysregulates mTOR to simultaneously destabilize cGAS and block increases in glycolysis. Our findings reveal how the poxvirus F17 protein disarms specific mitochondrially orchestrated responses to later stages of poxvirus replication.

Armored modified vaccinia Ankara in cancer immunotherapy

Cancer immunotherapy relies on unleashing the patient´s immune system against tumor cells. Cancer vaccines aim to stimulate both the innate and adaptive arms of immunity to achieve durable clinical responses. Some roadblocks for a successful cancer vaccine in the clinic include the tumor antigen of choice, the adjuvants employed to strengthen antitumor-specific immune responses, and the risks associated with enhancing immune-related adverse effects in patients. Modified vaccinia Ankara (MVA) belongs to the family of poxviruses and is a versatile vaccine platform that combines several attributes crucial for cancer therapy. First, MVA is an excellent inducer of innate immune responses leading to type I interferon secretion and induction of T helper cell type 1 (Th1) immune responses. Second, it elicits robust and durable humoral and cellular immunity against vector-encoded heterologous antigens. Third, MVA has enormous genomic flexibility, which allows for the expression of multiple antigenic and costimulatory entities. And fourth, its replication deficit in human cells ensures a excellent safety profile. In this review, we summarize the current understanding of how MVA induces innate and adaptive immune responses. Furthermore, we will give an overview of the tumor-associated antigens and immunomodulatory molecules that have been used to armor MVA and describe their clinical use. Finally, the route of MVA immunization and its impact on therapeutic efficacy depending on the immunomodulatory molecules expressed will be discussed.

Evaluation of the Effect of CD70 Co-Expression on CD8 T Cell Response in Protein-Prime MVA-Boost Vaccination in Mice

Here, we investigate the potential of CD70 co-expression during viral vector boost vaccination to improve an antigen-specific T cell response. To determine the chance of activating antigen-specific T cells by CD70, we used the HBV core antigen as a model antigen in a heterologous protein-prime, Modified Vaccinia virus Ankara (MVA) boost vaccination scheme. Both the HBV core and a CD70 expression cassette were co-expressed upon delivery by an MVA vector under the same promoter linked by a P2A site. To compare immunogenicity with and without CD70 co-expression, HBV-naïve, C57BL/6 (wt) mice and HBV-transgenic mice were prime-vaccinated using recombinant HBV core antigen followed by the MVA vector boost. Co-expression of CD70 increased the number of vaccine-induced HBV core-specific CD8 T cells by >2-fold and improved their effector functions in HBV-naïve mice. In vaccinated HBV1.3tg mice, the number and functionality of HBV core-specific CD8 T cells was slightly increased upon CD70 co-expression in low-viremic, but not in high-viremic animals. CD70 co-expression did not impact liver damage as indicated by ALT levels in the serum, but increased the number of vaccine-induced, proliferative T cell clusters in the liver. Overall, this study indicates that orchestrated co-expression of CD70 and a vaccine antigen may be an interesting and safe means of enhancing antigen-specific CD8 T cell responses using vector-based vaccines, although in our study it was not sufficient to break immune tolerance.

Diversity of cell death signaling pathways in macrophages upon infection with modified vaccinia virus Ankara (MVA)

Regulated cell death frequently occurs upon infection by intracellular pathogens, and extent and regulation is often cell-type-specific. We aimed to identify the cell death-signaling pathways triggered in macrophages by infection with modified vaccinia virus Ankara (MVA), an attenuated strain of vaccinia virus used in vaccination. While most target cells seem to be protected by antiapoptotic proteins encoded in the MVA genome, macrophages die when infected with MVA. We targeted key signaling components of specific cell death-pathways and pattern recognition-pathways using genome editing and small molecule inhibitors in an in vitro murine macrophage differentiation model. Upon infection with MVA, we observed activation of mitochondrial and death-receptor-induced apoptosis-pathways as well as the necroptosis-pathway. Inhibition of individual pathways had a little protective effect but led to compensatory death through the other pathways. In the absence of mitochondrial apoptosis, autocrine/paracrine TNF-mediated apoptosis and, in the absence of caspase-activity, necroptosis occurred. TNF-induction depended on the signaling molecule STING, and MAVS and ZBP1 contributed to MVA-induced apoptosis. The mode of cell death had a substantial impact on the cytokine response of infected cells, indicating that the immunogenicity of a virus may depend not only on its PAMPs but also on its ability to modulate individual modalities of cell death. These findings provide insights into the diversity of cell death-pathways that an infection can trigger in professional immune cells and advance our understanding of the intracellular mechanisms that govern the immune response to a virus.

Single-cell analysis reveals divergent responses of human dendritic cells to the MVA vaccine

Modified vaccinia Ankara (MVA) is a live, attenuated human smallpox vaccine and a vector for the development of new vaccines against infectious diseases and cancer. Efficient activation of the immune system by MVA partially relies on its encounter with dendritic cells (DCs). MVA infection of DCs leads to multiple outcomes, including cytokine production, activation of costimulatory molecules for T cell stimulation, and cell death. Here, we examined how these diverse responses are orchestrated in human DCs. Single-cell analyses revealed that the response to MVA infection in DCs was limited to early viral gene expression. In response to the early events in the viral cycle, we found that DCs grouped into three distinct clusters. A cluster of infected cells sensed the MVA genome by the intracellular innate immunity pathway mediated by cGAS, STING, TBK1, and IRF3 and subsequently produced inflammatory cytokines. In response to these cytokines, a cluster of noninfected bystander cells increased costimulatory molecule expression. A separate cluster of infected cells underwent caspase-dependent apoptosis. Induction of apoptosis persisted after inhibition of innate immunity pathway mediators independently of previously described IRF-dependent or replication-dependent pathways and was a response to early MVA gene expression. Together, our study identified multiple mechanisms that underlie the interactions of MVA with human DCs.

Vaccinia Virus Infection Inhibits Skin Dendritic Cell Migration to the Draining Lymph Node

There is a paucity of information on dendritic cell (DC) responses to vaccinia virus (VACV), including the traffic of DCs to the draining lymph node (dLN). In this study, using a mouse model of infection, we studied skin DC migration in response to VACV and compared it with the tuberculosis vaccine Mycobacterium bovis bacille Calmette-Guérin (BCG), another live attenuated vaccine administered via the skin. In stark contrast to BCG, skin DCs did not relocate to the dLN in response to VACV. Infection with UV-inactivated VACV or modified VACV Ankara promoted DC movement to the dLN, indicating that interference with skin DC migration requires replication-competent VACV. This suppressive effect of VACV was capable of mitigating responses to a secondary challenge with BCG in the skin, ablating DC migration, reducing BCG transport, and delaying CD4+ T cell priming in the dLN. Expression of inflammatory mediators associated with BCG-triggered DC migration were absent from virus-injected skin, suggesting that other pathways invoke DC movement in response to replication-deficient VACV. Despite adamant suppression of DC migration, VACV was still detected early in the dLN and primed Ag-specific CD4+ T cells. In summary, VACV blocks skin DC mobilization from the site of infection while retaining the ability to access the dLN to prime CD4+ T cells.

Interference with SAMHD1 Restores Late Gene Expression of Modified Vaccinia Virus Ankara in Human Dendritic Cells and Abrogates Type I Interferon Expression

Attenuated poxviruses like modified vaccinia virus Ankara (MVA) are promising vectors for vaccines against infectious diseases and cancer. However, host innate immune responses interfere with the viral life cycle and also influence the immunogenicity of vaccine vectors. Sterile alpha motif (SAM) domain and histidine-aspartate (HD) domain-containing protein 1 (SAMHD1) is a phosphohydrolase and reduces cellular deoxynucleoside triphosphate (dNTP) concentrations, which impairs poxviral DNA replication in human dendritic cells (DCs). Human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus (SIV) encode an accessory protein called viral protein X (Vpx) that promotes proteasomal degradation of SAMHD1, leading to a rapid increase in cellular dNTP concentrations. To study the function of SAMHD1 during MVA infection of human DCs, the SIV vpx gene was introduced into the MVA genome (resulting in recombinant MVA-vpx). Infection of human DCs with MVA-vpx led to SAMHD1 protein degradation and enabled MVA-vpx to replicate its DNA genome and to express genes controlled by late promoters. Late gene expression by MVA-vpx might improve its vaccine vector properties; however, type I interferon expression was unexpectedly blocked by Vpx-expressing MVA. MVA-vpx can be used as a tool to study poxvirus-host interactions and vector safety.IMPORTANCE SAMHD1 is a phosphohydrolase and reduces cellular dNTP concentrations, which impairs poxviral DNA replication. The simian SIV accessory protein Vpx promotes degradation of SAMHD1, leading to increased cellular dNTP concentrations. Vpx addition enables poxviral DNA replication in human dendritic cells (DCs), as well as the expression of viral late proteins, which is normally blocked. SAMHD1 function during modified vaccinia virus Ankara (MVA) infection of human DCs was studied with recombinant MVA-vpx expressing Vpx. Infection of human DCs with MVA-vpx decreased SAMHD1 protein amounts, enabling MVA DNA replication and expression of late viral genes. Unexpectedly, type I interferon expression was blocked after MVA-vpx infection. MVA-vpx might be a good tool to study SAMHD1 depletion during poxviral infections and to provide insights into poxvirus-host interactions.

Modified Vaccinia Virus Ankara Can Induce Optimal CD8+ T Cell Responses to Directly Primed Antigens Depending on Vaccine Design

A variety of strains of vaccinia virus (VACV) have been used as recombinant vaccine vectors with the aim of inducing robust CD8⁺ T cell immunity. While much of the pioneering work was done with virulent strains, such as Western Reserve (WR), attenuated strains such as modified vaccinia virus Ankara (MVA) are more realistic vectors for clinical use. To unify this literature, side-by-side comparisons of virus strains are required. Here, we compare the form of antigen that supports optimal CD8⁺ T cell responses for VACV strains WR and MVA using equivalent constructs. We found that for multiple antigens, minimal antigenic constructs (epitope minigenes) that prime CD8⁺ T cells via the direct presentation pathway elicited optimal responses from both vectors, which was surprising because this finding contradicts the prevailing view in the literature for MVA. We then went on to explore the discrepancy between current and published data for MVA, finding evidence that the expression locus and in some cases the presence of the viral thymidine kinase may influence the ability of this strain to prime optimal responses from antigens that require direct presentation. This extends our knowledge of the design parameters for VACV vectored vaccines, especially those based on MVA.IMPORTANCE Recombinant vaccines based on vaccinia virus and particularly attenuated strains such as MVA are in human clinical trials, but due to the complexity of these large vectors much remains to be understood about the design parameters that alter their immunogenicity. Previous work had found that MVA vectors should be designed to express stable protein in order to induce robust immunity by CD8⁺ (cytotoxic) T cells. Here, we found that the primacy of stable antigen is not generalizable to all designs of MVA and may depend where a foreign antigen is inserted into the MVA genome. This unexpected finding suggests that there is an interaction between genome location and the best form of antigen for optimal T cell priming in MVA and thus possibly other vaccine vectors. It also highlights that our understanding of antigen presentation by even the best studied of vaccine vectors remains incomplete.

Effective mosaic-based nanovaccines against avian influenza in poultry

Avian influenza virus (AIV) is an extraordinarily diverse pathogen that causes significant morbidity in domesticated poultry populations and threatens human life with looming pandemic potential. Controlling avian influenza in susceptible populations requires highly effective, economical and broadly reactive vaccines. Several AIV vaccines have proven insufficient despite their wide use, and better technologies are needed to improve their immunogenicity and broaden effectiveness. Previously, we developed a "mosaic" H5 subtype hemagglutinin (HA) AIV vaccine and demonstrated its broad protection against diverse highly pathogenic H5N1 and seasonal H1N1 virus strains in mouse and non-human primate models. There is a significant interest in developing effective and safe vaccines against AIV that cannot contribute to the emergence of new strains of the virus once circulating in poultry. Here, we report on the development of an H5 mosaic (H5M) vaccine antigen formulated with polyanhydride nanoparticles (PAN) that provide sustained release of encapsulated antigens. H5M vaccine constructs were immunogenic whether delivered by the modified virus Ankara (MVA) strain or encapsulated within PAN. Both humoral and cellular immune responses were generated in both specific-pathogen free (SPF) and commercial chicks. Importantly, chicks vaccinated by H5M constructs were protected in terms of viral shedding from divergent challenge with a low pathogenicity avian influenza (LPAI) strain at 8 weeks post-vaccination. In addition, protective levels of humoral immunity were generated against highly pathogenic avian influenza (HPAI) of the similar H5N1 and genetically dissimilar H5N2 viruses. Overall, the developed platform technologies (MVA vector and PAN encapsulation) were safe and provided high levels of sustained protection against AIV in chickens. Such approaches could be used to design more efficacious vaccines against other important poultry infections.

Ectromelia virus induces tubulin cytoskeletal rearrangement in immune cells accompanied by a loss of the microtubule organizing center and increased α-tubulin acetylation

Ectromelia virus (ECTV) is an orthopoxvirus that productively replicates in dendritic cells (DCs), but its influence on the microtubule (MT) cytoskeleton in DCs is not known. Here, we show that ECTV infection of primary murine granulocyte-macrophage colony stimulating factor-derived bone marrow cells (GM-BM) downregulates numerous genes engaged in MT cytoskeleton organization and dynamics. In infected cells, the MT cytoskeleton undergoes dramatic rearrangement and relaxation, accompanied by disappearance of the microtubule organizing centre (MTOC) and increased acetylation and stabilization of MTs, which are exploited by progeny virions for intracellular transport. This indicates a strong ability of ECTV to subvert the MT cytoskeleton of highly specialized immune cells.

Immunogenic cell death of dendritic cells following modified vaccinia virus Ankara infection enhances CD8+ T cell proliferation

"Immunogenic cell death" (ICD) is associated with the emission of so-called damage-associated molecular patterns (DAMPs) which trigger the immune response against dead-cell associated antigens. The secretion of the DAMP, adenosine triphosphate (ATP) has been shown to be autophagy-dependent. Here, we demonstrate that Modified Vaccinia virus Ankara (MVA), a highly attenuated strain of vaccinia virus, induces both cell death and autophagy in murine bone marrow-derived dendritic cells (BMDCs), which in turn confer the (cross-)priming of OVA-specific cytotoxic T cells (OT-I cells). Additionally, we show that MVA infection leads to increased extracellular ATP (eATP) as well as intracellular ATP (iATP) levels, with the latter being influenced by the autophagy. Furthermore, we show that the increased eATP supports the proliferation of OT-I cells and inhibition of the P2RX7 receptors results in an abrogation of the proliferation. These data reveal novel mechanisms on how MVA enhances adaptive immunity in vaccine strategies.

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.

Virulent Poxviruses Inhibit DNA Sensing by Preventing STING Activation

Cytosolic recognition of DNA has emerged as a critical cellular mechanism of host immune activation upon pathogen invasion. The central cytosolic DNA sensor cGAS activates STING, which is phosphorylated, dimerizes and translocates from the endoplasmic reticulum (ER) to a perinuclear region to mediate IRF-3 activation. Poxviruses are double-stranded DNA viruses replicating in the cytosol and hence likely to trigger cytosolic DNA sensing. Here, we investigated the activation of innate immune signaling by 4 different strains of the prototypic poxvirus vaccinia virus (VACV) in a cell line proficient in DNA sensing. Infection with the attenuated VACV strain MVA activated IRF-3 via cGAS and STING, and accordingly STING dimerized and was phosphorylated during MVA infection. Conversely, VACV strains Copenhagen and Western Reserve inhibited STING dimerization and phosphorylation during infection and in response to transfected DNA and cyclic GMP-AMP, thus efficiently suppressing DNA sensing and IRF-3 activation. A VACV deletion mutant lacking protein C16, thought to be the only viral DNA sensing inhibitor acting upstream of STING, retained the ability to block STING activation. Similar inhibition of DNA-induced STING activation was also observed for cowpox and ectromelia viruses. Our data demonstrate that virulent poxviruses possess mechanisms for targeting DNA sensing at the level of the cGAS-STING axis and that these mechanisms do not operate in replication-defective strains such as MVA. These findings shed light on the role of cellular DNA sensing in poxvirus-host interactions and will open new avenues to determine its impact on VACV immunogenicity and virulence.IMPORTANCE Poxviruses are double-stranded DNA viruses infecting a wide range of vertebrates and include the causative agent of smallpox (variola virus) and its vaccine vaccinia virus (VACV). Despite smallpox eradication VACV remains of interest as a therapeutic. Attenuated strains are popular vaccine candidates, whereas replication-competent strains are emerging as efficient oncolytics in virotherapy. The successful therapeutic use of VACV depends on a detailed understanding of its ability to modulate host innate immune responses. DNA sensing is a critical cellular mechanism for pathogen detection and activation of innate immunity that is centrally coordinated by the endoplasmic reticulum-resident protein STING. Here, STING is shown to mediate immune activation in response to MVA, but not in response to virulent VACV strains or other virulent poxviruses, which prevent STING activation and DNA sensing during infection and after DNA transfection. These results provide new insights into poxvirus immune evasion and have implications in the rational design of VACV-based therapeutics.

Characterizing the effects of insertion of a 5.2 kb region of a VACV genome, which contains known immune evasion genes, on MVA immunogenicity

Modified Vaccinia virus Ankara (MVA) is an attenuated Vaccinia virus (VACV) that is a popular vaccine vector candidate against many different pathogens. Its replication-restricted nature makes it a safe vaccine. However, higher doses or multiple boosts of MVA are necessary to elicit an immune response similar to wild-type VACV. Multiple strategies have been used to create modified MVA viruses that remain safe, but have increased immunogenicity. For example, one common strategy is to delete MVA immunomodulatory proteins in hopes of increasing the host immune response. Here, we take the opposite approach and examine, for the first time, how re-introduction of a 5.2 kb region of VACV DNA (that codes for multiple immunomodulatory proteins) into MVA alters viral immunogenicity. Since antigen presenting cells (APCs) are critical connectors between the innate and adaptive immune system, we examined the effect of MVA/5.2 kb infection in these cells in vitro. MVA/5.2 kb infection decreased virus-induced apoptosis and virus-induced NF-κB activation. MVA.5.2 kb infection decreased TNF production. However, MVA/5.2 kb infection did not alter APC maturation or IL-6 and IL-8 production in vitro. We further explored MVA/5.2 kb immunogenicity in vivo. VACV-specific CD8⁺ T cells were decreased after in vivo infection with MVA/5.2 kb versus MVA, suggesting that the MVA/5.2 kb construct is less immunogenic than MVA. These results demonstrate the limitations of in vitro studies for predicting the effects of genetic manipulation of MVA on immunogenicity. Although MVA/5.2 kb did not enhance MVA's immunogenicity, this study examined an unexplored strategy for optimizing MVA, and the insight gained from these results can help direct how to modify MVA in the future.

Vaccinia Virus Encodes a Novel Inhibitor of Apoptosis That Associates with the Apoptosome

Apoptosis is an important antiviral host defense mechanism. Here we report the identification of a novel apoptosis inhibitor encoded by the vaccinia virus (VACV) M1L gene. M1L is absent in the attenuated modified vaccinia virus Ankara (MVA) strain of VACV, a strain that stimulates apoptosis in several types of immune cells. M1 expression increased the viability of MVA-infected THP-1 and Jurkat cells and reduced several biochemical hallmarks of apoptosis, such as PARP-1 and procaspase-3 cleavage. Furthermore, ectopic M1L expression decreased staurosporine-induced (intrinsic) apoptosis in HeLa cells. We then identified the molecular basis for M1 inhibitory function. M1 allowed mitochondrial depolarization but blocked procaspase-9 processing, suggesting that M1 targeted the apoptosome. In support of this model, we found that M1 promoted survival in Saccharomyces cerevisiae overexpressing human Apaf-1 and procaspase-9, critical components of the apoptosome, or overexpressing only conformationally active caspase-9. In mammalian cells, M1 coimmunoprecipitated with Apaf-1-procaspase-9 complexes. The current model is that M1 associates with and allows the formation of the apoptosome but prevents apoptotic functions of the apoptosome. The M1 protein features 14 predicted ankyrin (ANK) repeat domains, and M1 is the first ANK-containing protein reported to use this inhibitory strategy. Since ANK-containing proteins are encoded by many large DNA viruses and found in all domains of life, studies of M1 may lead to a better understanding of the roles of ANK proteins in virus-host interactions.IMPORTANCE Apoptosis selectively eliminates dangerous cells such as virus-infected cells. Poxviruses express apoptosis antagonists to neutralize this antiviral host defense. The vaccinia virus (VACV) M1 ankyrin (ANK) protein, a protein with no previously ascribed function, inhibits apoptosis. M1 interacts with the apoptosome and prevents procaspase-9 processing as well as downstream procaspase-3 cleavage in several cell types and under multiple conditions. M1 is the first poxviral protein reported to associate with and prevent the function of the apoptosome, giving a more detailed picture of the threats VACV encounters during infection. Dysregulation of apoptosis is associated with several human diseases. One potential treatment of apoptosis-related diseases is through the use of designed ANK repeat proteins (DARPins), similar to M1, as caspase inhibitors. Thus, the study of the novel antiapoptosis effects of M1 via apoptosome association will be helpful for understanding how to control apoptosis using either natural or synthetic molecules.

Functional paralysis of GM-CSF-derived bone marrow cells productively infected with ectromelia virus

Ectromelia virus (ECTV) is an orthopoxvirus responsible for mousepox, a lethal disease of certain strains of mice that is similar to smallpox in humans, caused by variola virus (VARV). ECTV, similar to VARV, exhibits a narrow host range and has co-evolved with its natural host. Consequently, ECTV employs sophisticated and host-specific strategies to control the immune cells that are important for induction of antiviral immune response. In the present study we investigated the influence of ECTV infection on immune functions of murine GM-CSF-derived bone marrow cells (GM-BM), comprised of conventional dendritic cells (cDCs) and macrophages. Our results showed for the first time that ECTV is able to replicate productively in GM-BM and severely impaired their innate and adaptive immune functions. Infected GM-BM exhibited dramatic changes in morphology and increased apoptosis during the late stages of infection. Moreover, GM-BM cells were unable to uptake and process antigen, reach full maturity and mount a proinflammatory response. Inhibition of cytokine/chemokine response may result from the alteration of nuclear translocation of NF-κB, IRF3 and IRF7 transcription factors and down-regulation of many genes involved in TLR, RLR, NLR and type I IFN signaling pathways. Consequently, GM-BM show inability to stimulate proliferation of purified allogeneic CD4+ T cells in a primary mixed leukocyte reaction (MLR). Taken together, our data clearly indicate that ECTV induces immunosuppressive mechanisms in GM-BM leading to their functional paralysis, thus compromising their ability to initiate downstream T-cell activation events.

HIV specific responses induced in nonhuman primates with ANRS HIV-Lipo-5 vaccine combined with rMVA-HIV prime or boost immunizations

We evaluated the immunogenicity of a prime/boost vaccine strategy combining 5 lipopeptides (HIV-Lipo-5) and a recombinant modified vaccinia virus Ankara (rMVA-HIV) in cynomolgus macaques. Both of these vaccine components deliver HIV LAI Gag, Pol, and Nef antigens. Systemic and local safety was excellent in all groups. Immunization with HIV-Lipo-5 alone induced significant serum anti-HIV antibody titers which were not modified by rMVA-HIV immunization. However, induction of T-cell responses, as measured by IFNγ and IL-2 producing cells upon short-term stimulation with HIV peptide pools, required combined immunization with rMVA-HIV. Responses were preferentially observed against Gag antigen. Interestingly, HIV-Lipo-5 efficiently primed HIV induced T-cell responses upon the injection of rMVA-HIV, which may help to reduce the required number of vector injections. Our results provide a rationale for the use of a strategy involving HIV-Lipo-5 priming followed by rMVA-HIV booster immunization as a prophylactic or therapeutic vaccine approach against HIV infection and AIDS.

Natural killer cell responses to dendritic cells infected by the ANRS HIV-1 vaccine candidate, MVAHIV

Innate mechanisms are critical for the development of the host immune responses to antigen. Particularly, early interaction between natural killer (NK) cells and dendritic cells (DC) greatly impacts the establishment of both innate and adaptive immune responses. In this study, using an autologous in vitro co-culture system we analyzed the NK cell response against MVAHIV-infected DC as well as the subsequent ability of these MVAHIV-primed NK cells to control HIV-1 infection in autologous DC. We found that NK cells responded early to MVAHIV- or MVAWT-infected DC in terms of degranulation and cytokine production. After a 4-day priming of NK cells by MVAHIV- or MVAWT-infected DC we observed an enhanced proliferation and modulation in the NK cell receptor repertoire expression. Interestingly, we found that MVAHIV-primed NK cells had a significant higher ability to control HIV-1 infection in autologous DC compared to MVAWT-primed NK cells; and this enhanced anti-HIV-1 activity appeared to be HIV-specific as MVAHIV-primed NK cells did not have a better ability to control other viral infections or respond against tumoral cells. Furthermore, we observed that NK cell receptors NKG2D and NKp46 modulate the priming of NK cells. This data provides evidence that in vitro NK cells can be primed by viral vector-infected DC, in the context of a NK/DC culture, to specifically target viral infected cells.

Primary human macrophages serve as vehicles for vaccinia virus replication and dissemination

Human monocytic and professional antigen-presenting cells have been reported only to exhibit abortive infections with vaccinia virus (VACV). We found that monocyte-derived macrophages (MDMs), including granulocyte macrophage colony-stimulating factor (GM-CSF)-polarized M1 and macrophage colony-stimulating factor (M-CSF)-polarized M2, but not human AB serum-derived cells, were permissive to VACV replication. The titers of infectious virions in both cell-free supernatants and cellular lysates of infected M1 and M2 markedly increased in a time-dependent manner. The majority of virions produced in permissive MDMs were extracellular enveloped virions (EEV), a secreted form of VACV associated with long-range virus dissemination, and were mainly found in the culture supernatant. Infected MDMs formed VACV factories, actin tails, virion-associated branching structures, and cell linkages, indicating that MDMs are able to initiate de novo synthesis of viral DNA and promote virus release. VACV replication was sensitive to inhibitors against the Akt and Erk1/2 pathways that can be activated by VACV infection and M-CSF stimulation. Classical activation of MDMs by lipopolysaccharide (LPS) plus gamma interferon (IFN-γ) stimulation caused no effect on VACV replication, while alternative activation of MDMs by interleukin-10 (IL-10) or LPS-plus-IL-1β treatment significantly decreased VACV production. The IL-10-mediated suppression of VACV replication was largely due to Stat3 activation, as a Stat3 inhibitor restored virus production to levels observed without IL-10 stimulation. In conclusion, our data demonstrate that primary human macrophages are permissive to VACV replication. After infection, these cells produce EEV for long-range dissemination and also form structures associated with virions which may contribute to cell-cell spread.

IMPORTANCE

Our results provide critical information to the burgeoning fields of cancer-killing (oncolytic) virus therapy with vaccinia virus (VACV). One type of macrophage (M2) is considered a common presence in tumors and is associated with poor prognosis. Our results demonstrate a preference for VACV replication in M2 macrophages and could assist in designing treatments and engineering poxviruses with special considerations for their effect on M2 macrophage-containing tumors. Additionally, this work highlights the importance of macrophages in the field of vaccine development using poxviruses as vectors. The understanding of the dynamics of poxvirus-infected foci is central in understanding the effectiveness of the immune response to poxvirus-mediated vaccine vectors. Monocytic cells have been found to be an important part of VACV skin lesions in mice in controlling the infection as well as mediating virus transport out of infected foci.

Glucopyranosyl lipid A adjuvant significantly enhances HIV specific T and B cell responses elicited by a DNA-MVA-protein vaccine regimen

Using a unique vaccine antigen matched and single HIV Clade C approach we have assessed the immunogenicity of a DNA-poxvirus-protein strategy in mice and rabbits, administering MVA and protein immunizations either sequentially or simultaneously and in the presence of a novel TLR4 adjuvant, GLA-AF. Mice were vaccinated with combinations of HIV env/gag-pol-nef plasmid DNA followed by MVA-C (HIV env/gag-pol-nef) with HIV CN54gp140 protein (+/−GLA-AF adjuvant) and either co-administered in different muscles of the same animal with MVA-C or given sequentially at 3-week intervals. The DNA prime established a population of B cells that were able to mount a statistically significant anamnestic response to the boost vaccines. The greatest antigen-specific antibody response was observed in animals that received all vaccine components. Moreover, a high proportion of the total mucosal IgG (20 – 50%) present in the vaginal vault of these vaccinated animals was vaccine antigen-specific. The potent elicitation of antigen-specific immune responses to this vaccine modality was also confirmed in rabbits. Importantly, co-administration of MVA-C with the GLA-AF adjuvanted HIV CN54gp140 protein significantly augmented the antigen-specific T cell responses to the Gag antigen, a transgene product expressed by the MVA-C vector in a separate quadriceps muscle. We have demonstrated that co-administration of MVA and GLA-AF adjuvanted HIV CN54gp140 protein was equally effective in the generation of humoral responses as a sequential vaccination modality thus shortening and simplifying the immunization schedule. In addition, a significant further benefit of the condensed vaccination regime was that T cell responses to proteins expressed by the MVA-C were potently enhanced, an effect that was likely due to enhanced immunostimulation in the presence of systemic GLA-AF.

Langerin+ dermal DC, but not Langerhans cells, are required for effective CD8-mediated immune responses after skin scarification with vaccinia virus

Skin scarification (s.s.) with Vaccinia virus (VACV) is essential for generation of an optimal protective T cell memory immune response. Dendritic Cells (DC), which are professional antigen presenting cells, are required for naïve T cell priming and activation. At least three subsets of skin resident DC have been identified: Langerhans Cells (LC), Dermal Langerin⁺ DC (Lang⁺dDC) and Dermal Langerin⁻ DC (Lang⁻dDC). Using Langerin-diphtheria toxin receptor mice and established mouse model of VACV delivered by s.s., we demonstrated that Lang⁺dDC, but not LC, are absolutely required for the induction of a rapid and robust antigen-specific CD8⁺ T cell response after s.s. with VACV. The depletion of Lang⁺dDC led to a significant delay in the priming and proliferation of antigen-specific CD8⁺ T cells. Moreover CD8⁺ T cells generated after VACV s.s. in the absence of Lang⁺dDC lacked effector cytotoxic functions both in vitro and in vivo. While s.s.-immunized WT and LC depleted mice controlled the progression of OVA_257–264 expressing T cell lymphoma EG7 (injected intradermally), the depletion of Lang⁺dDC led to rapid lymphoma progression and mortality. These data indicate that of all skin DC subsets, Lang⁺dDC the most critical for the generation of robust CD8⁺ T cell immunity after s.s. with VACV.

Deletion of the viral anti-apoptotic gene F1L in the HIV/AIDS vaccine candidate MVA-C enhances immune responses against HIV-1 antigens

Vaccinia virus (VACV) encodes an anti-apoptotic Bcl-2-like protein F1 that acts as an inhibitor of caspase-9 and of the Bak/Bax checkpoint but the role of this gene in immune responses is not known. Because dendritic cells that have phagocytosed apoptotic infected cells cross-present viral antigens to cytotoxic T cells inducing an antigen-specific immunity, we hypothesized that deletion of the viral anti-apoptotic F1L gene might have a profound effect on the capacity of poxvirus vectors to activate specific immune responses to virus-expressed recombinant antigens. This has been tested in a mouse model with an F1L deletion mutant of the HIV/AIDS vaccine candidate MVA-C that expresses Env and Gag-Pol-Nef antigens (MVA-C-ΔF1L). The viral gene F1L is not required for virus replication in cultured cells and its deletion in MVA-C induces extensive apoptosis and expression of immunomodulatory genes in infected cells. Analysis of the immune responses induced in BALB/c mice after DNA prime/MVA boost revealed that, in comparison with parental MVA-C, the mutant MVA-C-ΔF1L improves the magnitude of the HIV-1-specific CD8 T cell adaptive immune responses and impacts on the CD8 T cell memory phase by enhancing the magnitude of the response, reducing the contraction phase and changing the memory differentiation pattern. These findings reveal the immunomodulatory role of F1L and that the loss of this gene is a valid strategy for the optimization of MVA as vaccine vector.

Modified vaccinia virus Ankara-based vaccine vectors induce apoptosis in dendritic cells draining from the skin via both the extrinsic and intrinsic caspase pathways, preventing efficient antigen presentation

Dendritic cells (DC) are potent antigen-presenting cells and central to the induction of immune responses following infection or vaccination. The collection of DC migrating from peripheral tissues by cannulation of the afferent lymphatic vessels provides DC which can be used directly ex vivo without extensive in vitro manipulations. We have previously used bovine migrating DC to show that recombinant human adenovirus 5 vectors efficiently transduce afferent lymph migrating DEC-205(+) CD11c(+) CD8(-) DC (ALDC). We have also shown that recombinant modified vaccinia virus Ankara (MVA) infects ALDC in vitro, causing downregulation of costimulatory molecules, apoptosis, and cell death. We now show that in the bovine system, modified vaccinia virus Ankara-induced apoptosis in DC draining from the skin occurs soon after virus binding via the caspase 8 pathway and is not associated with viral gene expression. We also show that after virus entry, the caspase 9 pathway cascade is initiated. The magnitude of T cell responses to mycobacterial antigen 85A (Ag85A) expressed by recombinant MVA-infected ALDC is increased by blocking caspase-induced apoptosis. Apoptotic bodies generated by recombinant MVA (rMVA)-Ag85A-infected ALDC and containing Ag85A were phagocytosed by noninfected migrating ALDC expressing SIRPα via actin-dependent phagocytosis, and these ALDC in turn presented antigen. However, the addition of fresh ALDC to MVA-infected cultures did not improve on the magnitude of the T cell responses; in contrast, these noninfected DC showed downregulation of major histocompatibility complex class II (MHC-II), CD40, CD80, and CD86. We also observed that MVA-infected ALDC promoted migration of DEC-205(+) SIRPα(+) CD21(+) DC as well as CD4(+) and CD8(+) T cells independently of caspase activation. These in vitro studies show that induction of apoptosis in DC by MVA vectors is detrimental to the subsequent induction of T cell responses.

The DC receptor DNGR-1 mediates cross-priming of CTLs during vaccinia virus infection in mice

In order to prime T cells, DCs integrate signals emanating directly from pathogens and from their noxious action on the host. DNGR-1 (CLEC9A) is a DC-restricted receptor that detects dead cells. Therefore, we investigated the possibility that DNGR-1 affects immunity to cytopathic viruses. DNGR-1 was essential for cross-presentation of dying vaccinia virus-infected (VACV-infected) cells to CD8(+) T cells in vitro. Following injection of VACV or VACV-infected cells into mice, DNGR-1 detected the ligand in dying infected cells and mediated cross-priming of anti-VACV CD8(+) T cells. Loss of DNGR-1 impaired the CD8+ cytotoxic response to VACV, especially against those virus strains that are most dependent on cross-presentation. The decrease in total anti-VACV CTL activity was associated with a profound increase in viral load and delayed resolution of the primary lesion. In addition, lack of DNGR-1 markedly diminished protection from infection induced by vaccination with the modified vaccinia Ankara (MVA) strain. DNGR-1 thus contributes to anti-VACV immunity, following both primary infection and vaccination. The non-redundant ability of DNGR-1 to regulate cross-presentation of viral antigens suggests that this form of regulation of antiviral immunity could be exploited for vaccination.

Innate immune recognition of poxviral vaccine vectors

The study of poxviruses pioneered the field of vaccinology after Jenner's remarkable discovery that 'vaccination' with the phylogenetically related cowpox virus conferred immunity to the devastating disease of smallpox. The study of poxviruses continues to enrich the field of virology because the global eradication of smallpox provides a unique example of the potency of effective immunization. Other poxviruses have since been developed as vaccine vectors for clinical and veterinary applications and include modified vaccinia virus strains such as modified vaccinia Ankara and NYVAC as well as the avipox viruses, fowlpox virus and canarypox virus. Despite the empirical development of poxvirus-based vectored vaccines, it is only now becoming apparent that we need to better understand how the innate arm of the immune system drives adaptive immunity to poxviruses, and how this information is relevant to vaccine design strategies, which are the topics addressed in this article.

Systems vaccinology: learning to compute the behavior of vaccine induced immunity

The goal of systems biology is to access and integrate information about the parts (e.g., genes, proteins, cells) of a biological system with a view to computing and predicting the behavior of the system. The past decade has witnessed technological revolutions in the capacity to make high throughput measurements about the behavior of genes, proteins, and cells. Such technologies are widely used in biological research and in medicine, such as toward prognosis and therapy response prediction in cancer patients. More recently, systems biology is being applied to vaccinology, with the goal of: (1) understanding the mechanisms by which vaccines stimulate protective immunity, and (2) predicting the immunogenicity or efficacy of vaccines. Here, we review the recent advances in this area, and highlight the biological and computational challenges posed.

Regulatory T cells selectively control CD8+ T cell effector pool size via IL-2 restriction

Regulatory T cells (Treg) are key players in maintaining immune homeostasis but have also been shown to regulate immune responses against infectious pathogens. Therefore, Treg are a promising target for modulating immune responses to vaccines to improve their efficacy. Using a viral vector system, we found that Treg act on the developing immune response early postinfection by reducing the extent of dendritic cell costimulatory molecule expression. Due to this change and the lower IL-2 production that results, a substantial fraction of CD8(+) effector T cells lose CD25 expression several days after activation. Surprisingly, such Treg-dependent limitations in IL-2 signaling by Ag-activated CD8(+) T cells prevent effector differentiation without interfering with memory cell formation. In this way, Treg fine-tune the numbers of effector T cells generated while preserving the capacity for a rapid recall response upon pathogen re-exposure. This selective effect of Treg on a subpopulation of CD8(+) T cells indicates that although manipulation of the Treg compartment might not be optimal for prophylactic vaccinations, it can be potentially exploited to optimize vaccine efficacy for therapeutic interventions.

Induction of Noxa-mediated apoptosis by modified vaccinia virus Ankara depends on viral recognition by cytosolic helicases, leading to IRF-3/IFN-β-dependent induction of pro-apoptotic Noxa

Viral infection is a stimulus for apoptosis, and in order to sustain viral replication many viruses are known to carry genes encoding apoptosis inhibitors. F1L, encoded by the orthopoxvirus modified vaccinia virus Ankara (MVA) has a Bcl-2-like structure. An MVA mutant lacking F1L (MVAΔF1L) induces apoptosis, indicating that MVA infection activates and F1L functions to inhibit the apoptotic pathway. In this study we investigated the events leading to apoptosis upon infection by MVAΔF1L. Apoptosis largely proceeded through the pro-apoptotic Bcl-2 family protein Bak with some contribution from Bax. Of the family of pro-apoptotic BH3-only proteins, only the loss of Noxa provided substantial protection, while the loss of Bim had a minor effect. In mice, MVA preferentially infected macrophages and DCs in vivo. In both cell types wt MVA induced apoptosis albeit more weakly than MVAΔF1L. The loss of Noxa had a significant protective effect in macrophages, DC and primary lymphocytes, and the combined loss of Bim and Noxa provided strong protection. Noxa protein was induced during infection, and the induction of Noxa protein and apoptosis induction required transcription factor IRF3 and type I interferon signalling. We further observed that helicases RIG-I and MDA5 and their signalling adapter MAVS contribute to Noxa induction and apoptosis in response to MVA infection. RNA isolated from MVA-infected cells induced Noxa expression and apoptosis when transfected in the absence of viral infection. We thus here describe a pathway leading from the detection of viral RNA during MVA infection by the cytosolic helicase-pathway, to the up-regulation of Noxa and apoptosis via IRF3 and type I IFN signalling.

Viruses have come up with a diverse set of mechanisms to stop infected cells from committing suicide and hence secure their own propagation. In this study we use the DNA virus Modified Vaccinia virus Ankara, a highly attenuated version Vaccinia Virus, to study how cells detect viral infection and induce apoptosis. Modified Vaccinia virus Ankara is currently in clinical trials for its use in various vaccination protocols. By using a broad array of immortalized and primary cell types we observed that viral infection induced programmed cell death was controlled by proteins predominantly involved in detection of viral RNA, in particular proteins involved in the type 1 interferon response. The novelty of our findings lies on the observation that not only can RNA from DNA viruses be detected and activate the type 1 interferon response to infection, but that these responses can also directly modulate the levels of proteins regulating programmed cell death. Future treatments of infections by viral pathogens could exploit the synergistic ability of the type 1 interferon responses and programmed cell death in order to inhibit viral propagation.

Dendritic cells exposed to MVA-based HIV-1 vaccine induce highly functional HIV-1-specific CD8(+) T cell responses in HIV-1-infected individuals

Currently, MVA virus vectors carrying HIV-1 genes are being developed as HIV-1/AIDS prophylactic/therapeutic vaccines. Nevertheless, little is known about the impact of these vectors on human dendritic cells (DC) and their capacity to present HIV-1 antigens to human HIV-specific T cells. This study aimed to characterize the interaction of MVA and MVA expressing the HIV-1 genes Env-Gag-Pol-Nef of clade B (referred to as MVA-B) in human monocyte-derived dendritic cells (MDDC) and the subsequent processes of HIV-1 antigen presentation and activation of memory HIV-1-specific T lymphocytes. For these purposes, we performed ex vivo assays with MDDC and autologous lymphocytes from asymptomatic HIV-infected patients. Infection of MDDC with MVA-B or MVA, at the optimal dose of 0.3 PFU/MDDC, induced by itself a moderate degree of maturation of MDDC, involving secretion of cytokines and chemokines (IL1-ra, IL-7, TNF-α, IL-6, IL-12, IL-15, IL-8, MCP-1, MIP-1α, MIP-1β, RANTES, IP-10, MIG, and IFN-α). MDDC infected with MVA or MVA-B and following a period of 48 h or 72 h of maturation were able to migrate toward CCL19 or CCL21 chemokine gradients. MVA-B infection induced apoptosis of the infected cells and the resulting apoptotic bodies were engulfed by the uninfected MDDC, which cross-presented HIV-1 antigens to autologous CD8⁺ T lymphocytes. MVA-B-infected MDDC co-cultured with autologous T lymphocytes induced a highly functional HIV-specific CD8⁺ T cell response including proliferation, secretion of IFN-γ, IL-2, TNF-α, MIP-1β, MIP-1α, RANTES and IL-6, and strong cytotoxic activity against autologous HIV-1-infected CD4⁺ T lymphocytes. These results evidence the adjuvant role of the vector itself (MVA) and support the clinical development of prophylactic and therapeutic anti-HIV vaccines based on MVA-B.

Interplay between modified vaccinia virus Ankara and dendritic cells: phenotypic and functional maturation of bystander dendritic cells

Modified vaccinia virus Ankara (MVA) is an attenuated poxvirus strain, currently under evaluation as a vaccine vector in various clinical settings. It has been reported that human dendritic cells (DCs) mature after infection with MVA, but reports on the functionality of DCs have so far been controversial. In this work, we studied the phenotype and functionality of MVA-infected DCs. As previously reported, we found that human monocyte-derived DCs upregulated CD86 and HLA-DR in response to MVA infection. Moreover, infected DCs produced a broad array of chemokines and cytokines and were able to activate and induce gamma interferon (IFN-γ) production both in CD4(+) and in CD8(+) allogeneic T cells and in specific autologous peripheral blood lymphocytes (PBLs). Analysis of DC maturation following infection with a recombinant green fluorescent protein (GFP)-expressing MVA revealed that upregulation of CD86 expression was mainly observed in GFP(neg) (bystander) cells. While GFP(pos) (infected) DCs produced tumor necrosis factor alpha (TNF-α), they were unable to produce CXCL10 and were less efficient at inducing IFN-γ production in CEF-specific autologous PBLs. Maturation of bystander DCs could be achieved by incubation with supernatant from infected cultures or with apoptotic infected cells. Type I IFNs were partially responsible for the induction of CXCL10 on bystander DCs. Our findings demonstrate for the first time that, in MVA-infected DC cultures, the leading role with respect to functionality and maturation characteristics is achieved by the bystander DCs.

Cowpox virus inhibits human dendritic cell immune function by nonlethal, nonproductive infection

Orthopoxviruses encode multiple proteins that modulate host immune responses. We determined whether cowpox virus (CPXV), a representative orthopoxvirus, modulated innate and acquired immune functions of human primary myeloid DCs and plasmacytoid DCs and monocyte-derived DCs (MDDCs). A CPXV infection of DCs at a multiplicity of infection of 10 was nonproductive, altered cellular morphology, and failed to reduce cell viability. A CPXV infection of DCs did not stimulate cytokine or chemokine secretion directly, but suppressed toll-like receptor (TLR) agonist-induced cytokine secretion and a DC-stimulated mixed leukocyte reaction (MLR). LPS-stimulated NF-κB nuclear translocation and host cytokine gene transcription were suppressed in CPXV-infected MDDCs. Early viral immunomodulatory genes were upregulated in MDDCs, consistent with early DC immunosuppression via synthesis of intracellular viral proteins. We conclude that a nonproductive CPXV infection suppressed DC immune function by synthesizing early intracellular viral proteins that suppressed DC signaling pathways.

Reverse vaccinology: developing vaccines in the era of genomics

The sequence of microbial genomes made all potential antigens of each pathogen available for vaccine development. This increased by orders of magnitude potential vaccine targets in bacteria, parasites, and large viruses and revealed virtually all their CD4(+) and CD8(+) T cell epitopes. The genomic information was first used for the development of a vaccine against serogroup B meningococcus, and it is now being used for several other bacterial vaccines. In this review, we will first summarize the impact that genome sequencing has had on vaccine development, and then we will analyze how the genomic information can help further our understanding of immunity to infection or vaccination and lead to the design of better vaccines by diving into the world of T cell immunity.

A flow cytometry-based immuno-titration assay for rapid and accurate titer determination of modified vaccinia Ankara virus vectors

A flow cytometry-based immuno-titration titer assay was established to determine infectious unit (IU) and transducing unit (TU) of modified vaccinia Ankara (MVA) virus vectors. This titration method enumerates infected cells by measuring the expression of viral protein for IU and transgene protein for TU in individual cells after staining with fluorophore-conjugated antibodies. It presents many advantages over standard virus titration approaches, such as TCID(50) or plaque assay, for its convenience, rapidity and accuracy as illustrated by excellent assay linearity and reproducibility. Importantly, the IU and the TCID(50) assays generated similar batch-specific titer values when testing varied MVA-derived virus preparations. Assay development revealed that the post-infection time at which viral protein expression is evaluated, host cell type, and blocking the formation and release of progeny virion with nocodazole, an anti-microtubule agent or rifampin, a specific vaccinia virus assembly inhibitor, are critical parameters for the precision, robustness, and accuracy of IU titer determination. An added advantage of this assay is that it enables the concurrent determination of IU and transducing units (TU) by measuring the expression of a transgene product when testing recombinant viruses. The latter was demonstrated using a MVA vector carrying a human HER-2 gene fragment as model. Hence, this assay is very versatile in that it can be used to determine IU as well as multiple TU titers simultaneously. Furthermore, it can readily be adapted to other poxvirus vectors.

Modified vaccinia virus Ankara exerts potent immune modulatory activities in a murine model

Background

Modified vaccinia virus Ankara (MVA), a highly attenuated strain of vaccinia virus, has been used as vaccine delivery vector in preclinical and clinical studies against infectious diseases and malignancies. Here, we investigated whether an MVA which does not encode any antigen (Ag) could be exploited as adjuvant per se.

Methodology/Principal Findings

We showed that dendritic cells infected in vitro with non-recombinant (nr) MVA expressed maturation and activation markers and were able to efficiently present exogenously pulsed Ag to T cells. In contrast to the dominant T helper (Th) 1 biased responses elicited against Ags produced by recombinant MVA vectors, the use of nrMVA as adjuvant for the co-administered soluble Ags resulted in a long lasting mixed Th1/Th2 responses.

Conclusions/Significance

These findings open new ways to potentiate and modulate the immune responses to vaccine Ags depending on whether they are co-administered with MVA or encoded by recombinant viruses.

Uncovering the interplay between CD8, CD4 and antibody responses to complex pathogens

Vaccinia virus (VACV) was used as the vaccine strain to eradicate smallpox. VACV is still administered to healthcare workers or researchers who are at risk of contracting the virus, and to military personnel. Thus, VACV represents a weapon against outbreaks, both natural (e.g., monkeypox) or man-made (bioterror). This virus is also used as a vector for experimental vaccine development (cancer/infectious disease). As a prototypic poxvirus, VACV is a model system for studying host-pathogen interactions. Until recently, little was known about the targets of host immune responses, which was likely owing to VACVs large genome (>200 open reading frames). However, the last few years have witnessed an explosion of data, and VACV has quickly become a useful model to study adaptive immune responses. This review summarizes and highlights key findings based on identification of VACV antigens targeted by the immune system (CD4, CD8 and antibodies) and the complex interplay between responses.

Activation of human dendritic cells by recombinant modified vaccinia virus Ankara vectors encoding survivin and IL-2 genes in vitro

Modified vaccinia virus Ankara (MVA) has attracted significant attention as a safe, promising vector for immunotherapy. However, the precise effects of MVA infection on immune responses in humans remain largely unknown. We constructed recombinant MVA (rMVA) encoding both a human tumor-associated antigen (survivin) and the proinflammatory cytokine interleukin (IL)-2 and investigated their effects on human monocyte-derived dendritic cells (DCs). The results showed that infection with rMVA slightly impaired the upregulation of CD83 and reduced the production of IL-10 in DCs after lipopolysaccharide stimulation. However, rMVA-infected DCs were still able to express high levels of target genes and the costimulatory molecules CD80 and CD86 and to produce significant amounts of the proinflammatory cytokine tumor necrosis factor alpha. Moreover, rMVA-infected DCs exhibited a greater capacity than uninfected cells to stimulate T-cell proliferation and to reverse MVA-induced apoptosis in syngeneic T cells. Coculture of lymphocytes with rMVA-infected DCs significantly increased cytotoxic potential and interferon gamma production by cytotoxic T cells. These findings suggest that rMVA encoding survivin and IL-2 can effectively stimulate the activation of human DCs and overcome defects such as impairment of DC maturation and apoptosis of lymphocytes that are caused by vector alone. Thus, this study may provide a rational basis for further optimization of MVA vector.

Preclinical studies of a modified vaccinia virus Ankara-based HIV candidate vaccine: antigen presentation and antiviral effect

Poxvirus-based human immunodeficiency virus (HIV) vaccine candidates are currently under evaluation in preclinical and clinical trials. Modified vaccinia virus Ankara (MVA) vectors have excellent safety and immunogenicity records, but their behavior in human cell cultures remains only partly characterized. We studied here various virological and immunological aspects of the interactions of MVA-HIV, a vaccine candidate developed by the French National Agency for AIDS Research (ANRS), with primary human cells. We report that MVA-HIV infects and drives Gag expression in primary macrophages, dendritic cells (DCs), and epithelial and muscle cells. MVA-HIV-infected DCs matured, efficiently presented Gag, Pol, and Nef antigens, and activated HIV-specific cytotoxic T lymphocytes (CTLs). As expected with this type of vector, infection was cytopathic and led to DC apoptosis. Coculture of MVA-HIV-infected epithelial cells or myotubes with DCs promoted efficient Gag antigen major histocompatibility complex class I (MHC-I) cross-presentation without inducing direct infection and death of DCs. Antigen-presenting cells (APCs) infected with MVA-HIV also activated HIV-specific CD4(+) T cells. Moreover, exposure of DCs to MVA-HIV or to MVA-HIV-infected myotubes induced type I interferon (IFN) production and inhibited subsequent HIV replication and transfer to lymphocytes. Altogether, these results show that MVA-HIV promotes efficient MHC-I and MHC-II presentation of HIV antigens by APCs without facilitating HIV replication. Deciphering the immune responses to MVA in culture experiments will help in the design of innovative vaccine strategies.

Comparative analysis of MVA-CD40L and MVA-TRICOM vectors for enhancing the immunogenicity of chronic lymphocytic leukemia (CLL) cells

Adenoviral transduction with CD40L and poxviral transduction with B7-1, ICAM-1, and LFA-3 (TRICOM) have been used to enhance the antigen-presenting capacity of chronic lymphocytic leukemia (CLL) cells. This study compares the same vector (modified vaccinia virus strain Ankara (MVA)) encoding CD40L or TRICOM for its ability to enhance the immunogenicity of CLL cells. CLL cells from some patients showed differential responses to each vector in terms of induction of autologous T-cell responses. This study supports the rationale for the use of CLL cells modified ex vivo with pre-specified recombinant MVA vectors as a whole tumor-cell vaccine for immunotherapy in CLL patients.

Vaccinia virus vaccines: past, present and future

Vaccinia virus (VACV) has been used more extensively for human immunization than any other vaccine. For almost two centuries, VACV was employed to provide cross-protection against variola virus, the causative agent of smallpox, until the disease was eradicated in the late 1970s. Since that time, continued research on VACV has produced a number of modified vaccines with improved safety profiles. Attenuation has been achieved through several strategies, including sequential passage in an alternative host, deletion of specific genes or genetic engineering of viral genes encoding immunomodulatory proteins. Some highly attenuated third- and fourth-generation VACV vaccines are now being considered for stockpiling against a possible re-introduction of smallpox through bioterrorism. Researchers have also taken advantage of the ability of the VACV genome to accommodate additional genetic material to produce novel vaccines against a wide variety of infectious agents, including a recombinant VACV encoding the rabies virus glycoprotein that is administered orally to wild animals. This review provides an in-depth examination of these successive generations of VACV vaccines, focusing on how the understanding of poxviral replication and viral gene function permits the deliberate modification of VACV immunogenicity and virulence.

Harnessing the unique local immunostimulatory properties of modified vaccinia Ankara (MVA) virus to generate superior tumor-specific immune responses and antitumor activity in a diversified prime and boost vaccine regimen

Recombinant poxviruses expressing tumor-associated antigens (TAAs) are currently being evaluated in clinical trials as an approach to treat various cancers. We have previously generated poxviral vectors expressing a TAA and a TRIad of COstimulatory Molecules (B7-1, ICAM-1, and LFA-3; TRICOM) as transgenes, including replication competent recombinant vaccinia (rV) or replication-defective modified vaccinia Ankara (MVA), to prime tumor-specific immune responses, and a replication-defective recombinant fowlpox (rF) to boost these responses. MVA is a potentially safer, replication-defective form of vaccinia virus with unique immunostimulatory properties that could make it a superior priming vaccine. Here, an MVA vector encoding a tumor antigen (CEA) and TRICOM was utilized (rMVA). A single rMVA-CEA/TRICOM vaccination induced greater expression of several serum cytokines associated with enhanced T-cell immunity than that seen with vaccinia. We hypothesized that this effect might "precondition" the vaccination site for a more effective boost. An rMVA-CEA/TRICOM prime followed 7 days later (but not 30 days later) by an rF-CEA/TRICOM boost at the same injection site (but not at a distal site) induced more potent CEA-specific T-cell responses, and superior CEA-specific immunity and antitumor activity, than rV-CEA/TRICOM followed by rF-CEA/TRICOM. This preconditioning effect was also observed using a heterologous antigen model, where priming with rMVA-CEA/TRICOM followed 7 days later by rF-LacZ/TRICOM enhanced beta-gal-specific immunity compared to rF-LacZ/TRICOM only. The studies reported here show for the first time that priming with rMVA followed 7 days later by an rF boost at the same injection site, versus a distal site, generates superior tumor-specific immunity and antitumor activity.

Targeting the porcine immune system--particulate vaccines in the 21st century

During the last decade, the propagation of immunological knowledge describing the critical role of dendritic cells (DC) in the induction of efficacious immune responses has promoted research and development of vaccines systematically targeting DC. Based on the promise for the rational design of vaccine platforms, the current review will provide an update on particle-based vaccines of both viral and synthetic origin, giving examples of recombinant virus carriers such as adenoviruses and biodegradable particulate carriers. The viral carriers carry pathogen-associated molecular patterns (PAMP), used by the original virus for targeting DC, and are particularly efficient and versatile gene delivery vectors. Efforts in the field of synthetic vaccine carriers are focussing on decorating the particle surface with ligands for DC receptors such as heparan sulphate glycosaminoglycan structures, integrins, Siglecs, galectins, C-type lectins and toll-like receptors. The emphasis of this review will be placed on targeting the porcine immune system, but reference will be made to advances with murine and human vaccine delivery systems where information on DC targeting is available.

Vaccinia virus-mediated inhibition of type I interferon responses is a multifactorial process involving the soluble type I interferon receptor B18 and intracellular components

Poxviruses such as virulent vaccinia virus (VACV) strain Western Reserve encode a broad range of immune modulators that interfere with host responses to infection. Upon more than 570 in vitro passages in chicken embryo fibroblasts (CEF), chorioallantois VACV Ankara (CVA) accumulated mutations that resulted in highly attenuated modified vaccinia virus Ankara (MVA). MVA infection of mice and of dendritic cells (DC) induced significant type I interferon (IFN) responses, whereas infection with VACV alone or in combination with MVA did not. These results implied that VACV expressed an IFN inhibitor(s) that was functionally deleted in MVA. To further characterize the IFN inhibitor(s), infection experiments were carried out with CVA strains isolated after 152 (CVA152) and 386 CEF passages (CVA386). Interestingly, neither CVA152 nor CVA386 induced IFN-alpha, whereas the latter variant did induce IFN-beta. This pattern suggested a consecutive loss of inhibitors during MVA attenuation. Similar to supernatants of VACV- and CVA152-infected DC cultures, recombinantly expressed soluble IFN decoy receptor B18, which is encoded in the VACV genome, inhibited MVA-induced IFN-alpha but not IFN-beta. In the same direction, a B18R-deficient VACV variant triggered only IFN-alpha, confirming B18 as the soluble IFN-alpha inhibitor. Interestingly, VACV infection inhibited IFN responses induced by a multitude of different stimuli, including oligodeoxynucleotides containing CpG motifs, poly(I:C), and vesicular stomatitis virus. Collectively, the data presented show that VACV-mediated IFN inhibition is a multistep process involving secreted factors such as B18 plus intracellular components that cooperate to efficiently shut off systemic IFN-alpha and IFN-beta responses.

Long-term immunity against actual poxviral HLA ligands as identified by differential stable isotope labeling

Viral peptides are presented by HLA class I on infected cells to activate CD8(+) T cells. Several immunogenic peptides have been identified indirectly by epitope prediction and screening of T cell responses to poxviral vectors, including modified vaccinia virus Ankara (MVA) currently being tested as recombinant or smallpox vaccines. However, for the development of optimal vaccination and immunomonitoring strategies, it is essential to characterize the actual viral HLA ligand repertoire of infected cells. We used an innovative approach to identify naturally processed MVA HLA ligands by differential HPLC-coupled mass spectrometry. We describe 12 viral peptides presented by HLA-A*0201 and 3 by HLA-B*0702. All HLA-A*0201 ligands participated in the memory response of MVA-immune donors, and several were immunogenic in Dryvax vaccinees. Eight epitopes were novel. Viral HLA ligand presentation and viral protein abundance did not correlate. All ligands were expressed early during the viral life cycle, and a pool of three of these mediated stronger protection against a lethal challenge in mice as compared with late epitopes. This highlights the reliability of the comparative mass spectrometry-based technique to identify relevant viral CD8(+) T cell epitopes for optimizing the monitoring of protective immune responses and the development of effective peptide-based vaccines.

Human effector and memory CD8+ T cell responses to smallpox and yellow fever vaccines

To explore the human T cell response to acute viral infection, we performed a longitudinal analysis of CD8(+) T cells responding to the live yellow fever virus and smallpox vaccines--two highly successful human vaccines. Our results show that both vaccines generated a brisk primary effector CD8(+) T cell response of substantial magnitude that could be readily quantitated with a simple set of four phenotypic markers. Secondly, the vaccine-induced T cell response was highly specific with minimal bystander effects. Thirdly, virus-specific CD8(+) T cells passed through an obligate effector phase, contracted more than 90% and gradually differentiated into long-lived memory cells. Finally, these memory cells were highly functional and underwent a memory differentiation program distinct from that described for human CD8(+) T cells specific for persistent viruses. These results provide a benchmark for CD8(+) T cell responses induced by two of the most effective vaccines ever developed.

Dendritic cells are preferentially targeted among hematolymphocytes by Modified Vaccinia Virus Ankara and play a key role in the induction of virus-specific T cell responses in vivo

Background

Modified Vaccinia Ankara (MVA) is a highly attenuated strain of vaccinia virus (VV) that has lost approximately 15% of the VV genome, along with the ability to replicate in most mammalian cells. It has demonstrated impressive safety and immunogenicity profile in both preclinical and clinical studies, and is being actively explored as a promising vaccine vector for a number of infectious diseases and malignancies. However, little is known about how MVA interacts with the host immune system constituents, especially dendritic cells (DCs), to induce strong immune responses despite its inability to replicate in vivo. Using in vitro and in vivo murine models, we systematically investigated the susceptibility of murine DCs to MVA infection, and the immunological consequences of the infection.

Results

Our data demonstrate that MVA preferentially infects professional antigen presenting cells, especially DCs, among all the subsets of hematolymphoid cells. In contrast to the reported blockage of DC maturation and function upon VV infection, DCs infected by MVA undergo phenotypic maturation and produce innate cytokine IFN-α within 18 h of infection. Substantial apoptosis of MVA-infected DCs occurs after 12 h following infection and the apoptotic DCs are readily phagocytosed by uninfected DCs. Using MHC class I – deficient mice, we showed that both direct and cross-presentation of viral Ags are likely to be involved in generating viral-specific CD8⁺ T cell responses. Finally, DC depletion abrogated the T cell activation in vivo.

Conclusion

We present the first in vivo evidence that among hematolymphoid cells, DCs are the most susceptible targets for MVA infection, and DC-mediated Ag presentation is required for the induction of MVA-specific immune responses. These results provide important information concerning the mechanisms by which strong immune responses are elicited to MVA-encoded antigens and may inform efforts to further improve the immunogenicity of this already promising vaccine vector.