Intradermal immune response after infection with Vaccinia virus

The parapoxvirus Orf virus inhibits dsDNA-mediated type I IFN expression via STING-dependent and STING-independent signalling pathways

Type I interferons (IFNs) are critical in the host defence against viruses. They induce hundreds of interferon-stimulated genes (ISGs) many of which have an antiviral role. Poxviruses induce IFNs via their pathogen-associated molecular patterns, in particular, their genomic DNA. In a majority of cell types, dsDNA is detected by a range of cytoplasmic DNA sensors that mediate type I IFN expression via stimulator of interferon genes (STING). Orf virus (ORFV) induces cutaneous pustular skin lesions and is the type species of the Parapoxvirus genus within the Poxviridae family. The aim of this study was to investigate whether ORFV modulates dsDNA-induced type I IFN expression via STING-dependent signalling pathways in human dermal fibroblasts (hNDF) and THP-1 cells. We showed that ORFV infection of these cell types treated with poly(dA:dT) resulted in strong inhibition of expression of IFN-β. In hNDFs, we showed using siRNA knock-down that STING was essential for type I IFN induction. IFN-β expression was further reduced when both STING and RIG-I were knocked down. In addition, HEK293 cells that do not express STING or Toll-like receptors also produce IFN-β following stimulation with poly(dA:dT). The 5' triphosphate dsRNA produced by RNA polymerase III specifically results in the induction of type I IFNs through the RIG-I receptor. We showed that ORFV infection resulted in strong inhibition of IFN-β expression in HEK293 cells stimulated with poly(dA:dT). Overall, this study shows that ORFV potently counteracts the STING-dependent and STING-independent IFN response by antagonizing dsDNA-activated IFN signalling pathways.

Transcriptional reprogramming of natural killer cells by vaccinia virus shows both distinct and conserved features with mCMV

Natural killer (NK) cells have an established role in controlling poxvirus infection and there is a growing interest to exploit their capabilities in the context of poxvirus-based oncolytic therapy and vaccination. How NK cells respond to poxvirus-infected cells to become activated is not well established. To address this knowledge gap, we studied the NK cell response to vaccinia virus (VACV) in vivo, using a systemic infection murine model. We found broad alterations in NK cells transcriptional activity in VACV-infected mice, consistent with both direct target cell recognition and cytokine exposure. There were also alterations in the expression levels of specific NK surface receptors (NKRs), including the Ly49 family and SLAM receptors, as well as upregulation of memory-associated NK markers. Despite the latter observation, adoptive transfer of VACV-expercienced NK populations did not confer protection from infection. Comparison with the NK cell response to murine cytomegalovirus (MCMV) infection highlighted common features, but also distinct NK transcriptional programmes initiated by VACV. Finally, there was a clear overlap between the NK transcriptional response in humans vaccinated with an attenuated VACV, modified vaccinia Ankara (MVA), demonstrating conservation between the NK response in these different host species. Overall, this study provides new data about NK cell activation, function, and homeostasis during VACV infection, and may have implication for the design of VACV-based therapeutics.

DNA-PK Is Targeted by Multiple Vaccinia Virus Proteins to Inhibit DNA Sensing

Virus infection is sensed by pattern recognition receptors (PRRs) detecting virus nucleic acids and initiating an innate immune response. DNA-dependent protein kinase (DNA-PK) is a PRR that binds cytosolic DNA and is antagonized by vaccinia virus (VACV) protein C16. Here, VACV protein C4 is also shown to antagonize DNA-PK by binding to Ku and blocking Ku binding to DNA, leading to a reduced production of cytokines and chemokines in vivo and a diminished recruitment of inflammatory cells. C4 and C16 share redundancy in that a double deletion virus has reduced virulence not seen with single deletion viruses following intradermal infection. However, non-redundant functions exist because both single deletion viruses display attenuated virulence compared to wild-type VACV after intranasal infection. It is notable that VACV expresses two proteins to antagonize DNA-PK, but it is not known to target other DNA sensors, emphasizing the importance of this PRR in the response to infection in vivo.

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DNA-PK is a pattern recognition receptor that binds cytosolic DNA

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Vaccinia virus proteins C4 and C16 antagonize DNA-PK by blocking DNA binding

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C4 and C16 inhibit IRF3 signaling, cytokine production, and immune cell recruitment

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C4 and C16 share redundant and non-redundant functions in vivo

Modulating Vaccinia Virus Immunomodulators to Improve Immunological Memory

The increasing frequency of monkeypox virus infections, new outbreaks of other zoonotic orthopoxviruses and concern about the re-emergence of smallpox have prompted research into developing antiviral drugs and better vaccines against these viruses. This article considers the genetic engineering of vaccinia virus (VACV) to enhance vaccine immunogenicity and safety. The virulence, immunogenicity and protective efficacy of VACV strains engineered to lack specific immunomodulatory or host range proteins are described. The ultimate goal is to develop safer and more immunogenic VACV vaccines that induce long-lasting immunological memory.

Deletion of the K1L Gene Results in a Vaccinia Virus That Is Less Pathogenic Due to Muted Innate Immune Responses, yet Still Elicits Protective Immunity

All viruses strategically alter the antiviral immune response to their benefit. The vaccinia virus (VACV) K1 protein has multiple immunomodulatory effects in tissue culture models of infection, including NF-κB antagonism. However, the effect of K1 during animal infection is poorly understood. We determined that a K1L-less vaccinia virus (vΔK1L) was less pathogenic than wild-type VACV in intranasal and intradermal models of infection. Decreased pathogenicity was correlated with diminished virus replication in intranasally infected mice. However, in intradermally inoculated ears, vΔK1L replicated to levels nearly identical to those of VACV, implying that the decreased immune response to vΔK1L infection, not virus replication, dictated lesion size. Several lines of evidence support this theory. First, vΔK1L induced slightly less edema than vK1L, as revealed by histopathology and noninvasive quantitative ultrasound technology (QUS). Second, infiltrating immune cell populations were decreased in vΔK1L-infected ears. Third, cytokine and chemokine gene expression was decreased in vΔK1L-infected ears. While these results identified the biological basis for smaller lesions, they remained puzzling; because K1 antagonizes NF-κB in vitro, antiviral gene expression was expected to be higher during vΔK1L infection. Despite these diminished innate immune responses, vΔK1L vaccination induced a protective VACV-specific CD8⁺ T cell response and protected against a lethal VACV challenge. Thus, vΔK1L is the first vaccinia virus construct reported that caused a muted innate immune gene expression profile and decreased immune cell infiltration in an intradermal model of infection yet still elicited protective immunity.IMPORTANCE The vaccinia virus (VACV) K1 protein inhibits NF-κB activation among its other antagonistic functions. A virus lacking K1 (vΔK1L) was predicted to be less pathogenic because it would trigger a more robust antiviral immune response than VACV. Indeed, vΔK1L was less pathogenic in intradermally infected mouse ear pinnae. However, vΔK1L infection unexpectedly elicited dramatically reduced infiltration of innate immune cells into ears. This was likely due to decreased expression of cytokine and chemokine genes in vΔK1L-infected ears. As such, our finding contradicted observations from cell culture systems. Interestingly, vΔK1L conferred protective immunity against lethal VACV challenge. This suggests that the muted immune response triggered during vΔK1L infection remained sufficient to mount an effective protective response. Our results highlight the complexity and unpredictable nature of virus-host interactions, a relationship that must be understood to better comprehend virus pathogenesis or to manipulate viruses for use as vaccines.

A systemic macrophage response is required to contain a peripheral poxvirus infection

The goal of the innate immune system is to reduce pathogen spread prior to the initiation of an effective adaptive immune response. Following an infection at a peripheral site, virus typically drains through the lymph to the lymph node prior to entering the blood stream and being systemically disseminated. Therefore, there are three distinct spatial checkpoints at which intervention to prevent systemic spread of virus can occur, namely: 1) the site of infection, 2) the draining lymph node via filtration of lymph or 3) the systemic level via organs that filter the blood. We have previously shown that systemic depletion of phagocytic cells allows viral spread after dermal infection with Vaccinia virus (VACV), which infects naturally through the skin. Here we use multiple depletion methodologies to define both the spatial checkpoint and the identity of the cells that prevent systemic spread of VACV. Subcapsular sinus macrophages of the draining lymph node have been implicated as critical effectors in clearance of lymph borne viruses following peripheral infection. We find that monocyte populations recruited to the site of VACV infection play a critical role in control of local pathogenesis and tissue damage, but do not prevent dissemination of virus. Following infection with virulent VACV, the subcapsular sinus macrophages within the draining lymph node become infected, but are not exclusively required to prevent systemic spread. Rather, small doses of VACV enter the bloodstream and the function of systemic macrophages, but not dendritic cells, is required to prevent further spread. The results illustrate that a systemic innate response to a peripheral virus infection may be required to prevent widespread infection and pathology following infection with virulent viruses, such as poxviruses.

IL-1R Type 1-Deficient Mice Demonstrate an Impaired Host Immune Response against Cutaneous Vaccinia Virus Infection

The IL-1 superfamily of cytokines and receptors has been studied extensively. However, the specific roles of IL-1 elements in host immunity to cutaneous viral infection remain elusive. In this study, we applied vaccinia virus (VACV) by scarification to IL-1R1 knockout mice (IL-1R1^-/-) and found that these mice developed markedly larger lesions with higher viral genome copies in skin than did wild-type mice. The phenotype of infected IL-1R1^-/- mice was similar to eczema vaccinatum, a severe side effect of VACV vaccination that may develop in humans with atopic dermatitis. Interestingly, the impaired cutaneous response of IL-1R1^-/- mice did not reflect a systemic immune deficiency, because immunized IL-1R1^-/- mice survived subsequent lethal VACV intranasal challenge, or defects of T cell activation or T cell homing to the site of inoculation. Histologic evaluation revealed that VACV infection and replication after scarification were limited to the epidermal layer of wild-type mice, whereas lack of IL-1R1 permitted extension of VACV infection into dermal layers of the skin. We explored the etiology of this discrepancy and determined that IL-1R1^-/- mice contained significantly more macrophages and monocyte-derived dendritic cells in the dermis after VACV scarification. These cells were vulnerable to VACV infection and may augment the transmission of virus to adjacent skin, thus leading to larger skin lesions and satellite lesions in IL-1R1^-/- mice. These results suggest new therapeutic strategies for treatment of eczema vaccinatum and inform assessment of risks in patients receiving IL-1 blocking Abs for treatment of chronic inflammatory disorders.

Both CD8+ and CD4+ T Cells Contribute to Corneal Clouding and Viral Clearance following Vaccinia Virus Infection in C57BL/6 Mice

Vaccinia virus (VACV) keratitis is a serious complication following smallpox vaccination and can lead to blindness. The pathological mechanisms involved in ocular VACV infection are poorly understood. Previous studies have used rabbits, but the lack of immune reagents and transgenic or knockout animals makes them less suitable for mechanistic studies. We report that infection of C57BL/6 mice with 1 × 10(7) PFU of vaccinia virus strain WR results in blepharitis, corneal neovascularization, and stromal keratitis. The DryVax strain of VACV was completely attenuated. Infection required corneal scarification and replication-competent virus, and the severity of ocular disease was similar in 4- to 6-week-old and 1-year-old mice. Viral titers peaked at approximately 1 × 10(6) PFU on day 5 postinfection, and virus had not cleared by day 13 postinfection. Neutrophils were found in the peripheral cornea on day 1 after infection and then declined, followed by infiltration of both CD4(+) and CD8(+) T cells, which remained peripheral throughout the infection. Blood vessel growth extended 2 to 5 mm into the cornea from the limbus. Infection of CD4(-/-), CD8(-/-), or antibody-depleted mice resulted in similar disease severity and corneal clouding, indicating that both T-cell subsets were involved in the immunopathological response. Depletion of both CD4(+) and CD8(+) T cells resulted in significantly more severe disease and failure to clear the virus. On the basis of our results, the pathology of VACV keratitis is significantly different from that of herpes simplex virus keratitis. Further studies are likely to reveal novel information regarding virulence and immune responses to viral ocular infection.

IMPORTANCE

Potentially blinding eye infections can occur after vaccination for smallpox. Very little is known about the pathological mechanisms that are involved, and the information that is available was generated using rabbit models. The lack of immunological reagents for rabbits makes such studies difficult. We characterized a mouse model of vaccinia virus ocular disease using C57BL/6 mice and strain WR and show that both CD4(+) and CD8(+) T-cell subsets play a role in the blinding eye disease and in controlling virus replication. On the basis of these results, vaccinia virus keratitis is significantly different from herpes simplex virus keratitis, and further studies using this model should generate novel insights into immunopathological responses to viral ocular infection.

Vaccinia virus immune evasion: mechanisms, virulence and immunogenicity

Virus infection of mammalian cells is sensed by pattern recognition receptors and leads to an innate immune response that restricts virus replication and induces adaptive immunity. In response, viruses have evolved many countermeasures that enable them to replicate and be transmitted to new hosts, despite the host innate immune response. Poxviruses, such as vaccinia virus (VACV), have large DNA genomes and encode many proteins that are dedicated to host immune evasion. Some of these proteins are secreted from the infected cell, where they bind and neutralize complement factors, interferons, cytokines and chemokines. Other VACV proteins function inside cells to inhibit apoptosis or signalling pathways that lead to the production of interferons and pro-inflammatory cytokines and chemokines. In this review, these VACV immunomodulatory proteins are described and the potential to create more immunogenic VACV strains by manipulation of the gene encoding these proteins is discussed.

Vaccinia virus protein K7 is a virulence factor that alters the acute immune response to infection

Vaccinia virus (VACV) encodes many proteins that antagonize the innate immune system including a family of intracellular proteins with a B-cell lymphoma (Bcl)-2-like structure. One of these Bcl-2 proteins called K7 binds Toll-like receptor-adaptor proteins and the DEAD-box RNA helicase DDX3 and thereby inhibits the activation of NF-κB and interferon regulatory factor 3. However, the contribution of K7 to virus virulence is not known. Here a VACV lacking the K7R gene (vΔK7) was constructed and compared with control viruses that included a plaque purified wt (vK7), a revertant with the K7R gene reinserted (vK7-rev) and a frame-shifted virus in which the translational initiation codon was mutated to prevent K7 protein expression (vK7-fs). Data presented show that loss of K7 does not affect virus replication in cell culture or in vivo; however, viruses lacking the K7 protein were less virulent than controls in murine intradermal (i.d.) and intranasal (i.n.) infection models and there was an altered acute immune response to infection. In the i.d. model, vΔK7 induced smaller lesions than controls, and after i.n. infection vΔK7 induced a reduced weight loss and signs of illness, and more rapid clearance of virus from infected tissue. Concomitantly, the intrapulmonary innate immune response to infection with vΔK7 showed increased infiltration of NK cells and CD8⁺ T-cells, enhanced MHC class II expression by macrophages, and enhanced cytolysis of target cells by NK cells and VACV-specific CD8⁺ T-cells. Thus protein K7 is a virulence factor that affects the acute immune response to infection.

Vaccinia virus protein C4 inhibits NF-κB activation and promotes virus virulence

Vaccinia virus (VACV) strain Western Reserve protein C4 has been characterized and its function and contribution to virus virulence assessed. Bioinformatic analysis showed that C4 is conserved in six orthopoxvirus species and shares 43 % amino acid identity with VACV protein C16, a known virulence factor. A recombinant VACV expressing a C-terminally tagged version of C4 showed that, like C16, this 37 kDa protein is expressed early during infection and localizes to both the cytoplasm and the nucleus. Functional assays using a firefly luciferase reporter plasmid under the control of a nuclear factor kappa B (NF-κB)-dependent promoter demonstrated that C4 inhibits NF-κB activation at, or downstream of, the inhibitor of kappa kinase (IKK) complex. Consistent with this, C4 inhibited interleukin-1β-induced translocation of p65 into the nucleus. A VACV lacking the C4L gene (vΔC4) showed no significant differences from wild-type virus in growth kinetics or spread in cell culture, but had reduced virulence in a murine intranasal model of infection. vΔC4-infected mice exhibited fewer symptoms, lost less weight and recovered 7 days earlier than animals infected with control viruses expressing C4. Furthermore, bronchoalveolar lavage fluid from vΔC4-infected mice had increased cell numbers at day 5 post-infection, which correlated with reduced lung virus titres from this time onward. C4 represents the ninth VACV protein to inhibit NF-κB activation and remarkably, in every case examined, loss of each protein individually caused an alteration in virus virulence, despite the presence of other NF-κB inhibitors.

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.

Skin mast cells protect mice against vaccinia virus by triggering mast cell receptor S1PR2 and releasing antimicrobial peptides

Mast cells (MCs) are well-known effectors of allergic reactions and are considered sentinels in the skin and mucosa. In addition, through their production of cathelicidin, MCs have the capacity to oppose invading pathogens. We therefore hypothesized that MCs could act as sentinels in the skin against viral infections using antimicrobial peptides. In this study, we demonstrate that MCs react to vaccinia virus (VV) and degranulate using a membrane-activated pathway that leads to antimicrobial peptide discharge and virus inactivation. This finding was supported using a mouse model of viral infection. MC-deficient (Kit(wsh-/-)) mice were more susceptible to skin VV infection than the wild type animals, whereas Kit(wsh-/-) mice reconstituted with MCs in the skin showed a normal response to VV. Using MCs derived from mice deficient in cathelicidin antimicrobial peptide, we showed that antimicrobial peptides are one important antiviral granule component in in vivo skin infections. In conclusion, we demonstrate that MC presence protects mice from VV skin infection, MC degranulation is required for protecting mice from VV, neutralizing Ab to the L1 fusion entry protein of VV inhibits degranulation apparently by preventing S1PR2 activation by viral membrane lipids, and antimicrobial peptide release from MC granules is necessary to inactivate VV infectivity.

CD11b⁺, Ly6G⁺ cells produce type I interferon and exhibit tissue protective properties following peripheral virus infection

The goal of the innate immune system is containment of a pathogen at the site of infection prior to the initiation of an effective adaptive immune response. However, effector mechanisms must be kept in check to combat the pathogen while simultaneously limiting undesirable destruction of tissue resulting from these actions. Here we demonstrate that innate immune effector cells contain a peripheral poxvirus infection, preventing systemic spread of the virus. These innate immune effector cells are comprised primarily of CD11b⁺Ly6C⁺Ly6G^- monocytes that accumulate initially at the site of infection, and are then supplemented and eventually replaced by CD11b⁺Ly6C⁺Ly6G⁺ cells. The phenotype of the CD11b⁺Ly6C⁺Ly6G⁺ cells resembles neutrophils, but the infiltration of neutrophils typically occurs prior to, rather than following, accumulation of monocytes. Indeed, it appears that the CD11b⁺Ly6C⁺Ly6G⁺ cells that infiltrated the site of VACV infection in the ear are phenotypically distinct from the classical description of both neutrophils and monocyte/macrophages. We found that CD11b⁺Ly6C⁺Ly6G⁺ cells produce Type I interferons and large quantities of reactive oxygen species. We also observed that depletion of Ly6G⁺ cells results in a dramatic increase in tissue damage at the site of infection. Tissue damage is also increased in the absence of reactive oxygen species, although reactive oxygen species are typically thought to be damaging to tissue rather than protective. These data indicate the existence of a specialized population of CD11b⁺Ly6C⁺Ly6G⁺ cells that infiltrates a site of virus infection late and protects the infected tissue from immune-mediated damage via production of reactive oxygen species. Regulation of the action of this population of cells may provide an intervention to prevent innate immune-mediated tissue destruction.

During a natural virus infection, small doses of infectious virus are deposited at a peripheral infection site, and then a “race” ensues, in which the replicating virus attempts to “outpace” the responding immune system of the host. In the early phases of infection, the innate immune system must contain the infection prior to the development of an effective adaptive response. Here we have characterized the cells of the innate immune system that move to a site of peripheral virus infection, and we find that a subset of these cells display atypical expression of cell surface molecules, timing of infiltration, and function. These cells protect the infected tissue from damage by producing reactive oxygen molecules, which are widely accepted to increase tissue damage. Therefore our findings indicate that during a peripheral virus infection, the typical rules governing the function of the innate immune system are altered to prevent tissue damage.

The Vaccinia virus complement control protein modulates adaptive immune responses during infection

Complement activation is an important component of the innate immune response against viral infection and also shapes adaptive immune responses. Despite compelling evidence that complement activation enhances T cell and antibody (Ab) responses during viral infection, it is unknown whether inhibition of complement by pathogens alters these responses. Vaccinia virus (VACV) modulates complement activation by encoding a complement regulatory protein called the vaccinia virus complement control protein (VCP). Although VCP has been described as a virulence factor, the mechanisms by which VCP enhances VACV pathogenesis have not been fully defined. Since complement is necessary for optimal adaptive immune responses to several viruses, we hypothesized that VCP contributes to pathogenesis by modulating anti-VACV T cell and Ab responses. In this study, we used an intradermal model of VACV infection to compare pathogenesis of wild-type virus (vv-VCPwt) and a virus lacking VCP (vv-VCPko). vv-VCPko formed smaller lesions in wild-type mice but not in complement-deficient mice. Attenuation of vv-VCPko correlated with increased accumulation of T cells at the site of infection, enhanced neutralizing antibody responses, and reduced viral titers. Importantly, depleting CD8(+) T cells together with CD4(+) T cells, which also eliminated T helper cell-dependent Ab responses, restored vv-VCPko to wild-type levels of virulence. These results suggest that VCP contributes to virulence by dampening both antibody and T cell responses. This work provides insight into how modulation of complement by poxviruses contributes to virulence and demonstrates that a pathogen-encoded complement regulatory protein can modulate adaptive immunity.

Dose sparing enabled by skin immunization with influenza virus-like particle vaccine using microneedles

To address the limitations of conventional influenza vaccine manufacturing and delivery, this study investigated administration of virus-like particle (VLP) influenza vaccine using a microneedle patch. The goal was to determine if skin immunization with influenza VLP vaccine using microneedles enables dose sparing. We found that low-dose influenza (A/PR/8/34 H1N1) VLP vaccination using microneedles was more immunogenic than low-dose intramuscular (IM) vaccination and similarly immunogenic as high-dose IM vaccination in a mouse model. With a 1μg dose of vaccine, both routes showed similar immune responses and protective efficacy, with microneedle vaccination being more effective in inducing recall antibody responses in lungs and antibody secreting cells in bone marrow. With a low dose of vaccine (0.3μg), microneedle vaccination induced significantly superior protective immunity, which included binding and functional antibodies as well as complete protection against a high dose lethal infection with A/PR/8/34 virus, whereas IM immunization provided only partial (40%) protection. Therefore, this study demonstrates that microneedle vaccination in the skin confers more effective protective immunity at a lower dose, thus providing vaccine dose-sparing effects.

Kinetics of immune cell infiltration in vaccinia virus keratitis

PURPOSE

Vaccinia virus keratitis leading to blindness is a severe complication of smallpox vaccination. The clinical manifestations of vaccinia virus keratitis are similar to those of herpes simplex virus keratitis, a well-studied immunopathologic disease. Vaccinia virus keratitis is likely to involve an immunopathologic component, but little is known about the pathogenesis of the disease. The goal of this study was to determine type and kinetics of immune cell infiltration in the cornea during vaccinia virus keratitis.

METHODS

Rabbit eyes were trephined and inoculated with 1x10(5) pfu of the Dryvax strain of the vaccinia virus. On days 2, 4, 7, 10, 14, and 28 after infection, the animals were scored for clinical disease and eye sections were stained for B cells, CD4+ cells, CD8+ cells, and neutrophils. The eyelid, ciliary body, cornea, iris, iridocorneal angle, and choroid were examined.

RESULTS

Corneal vaccinia virus challenge resulted in the infiltration of B cells, CD4+ cells, CD8+ cells, and neutrophils into the cornea and eyelids. Neutrophils were the predominant cell type on days 2 and 3 after infection, whereas CD4+ cells were the predominant cell type detected in corneas on days 4 through 10. CD8+ cells and B cells peaked on day 10, but at lower levels than CD4+ cells and neutrophils.

CONCLUSIONS

These results suggest that sequential migration of neutrophils, then CD4+ cells, plays an important role in vaccinia virus keratitis.

Gamma interferon-induced interferon regulatory factor 1-dependent antiviral response inhibits vaccinia virus replication in mouse but not human fibroblasts

Vaccinia virus (VACV) replicates in mouse and human fibroblasts with comparable kinetics and efficiency, yielding similar titers of infectious progeny. Here we demonstrate that gamma interferon (IFN-gamma) but not IFN-alpha or IFN-beta pretreatment of mouse fibroblasts prior to VACV infection induces a long-lasting antiviral state blocking VACV replication. In contrast, high doses of IFN-gamma failed to establish an antiviral state in human fibroblasts. In mouse fibroblasts, IFN-gamma impeded the viral replication cycle at the level of late gene transcription and blocked the multiplication of VACV genomes. The IFN-gamma-induced antiviral state invariably prevented the growth of different VACV strains but was not effective against the replication of ectromelia virus. The IFN-gamma effect required intact IFN-gamma receptor signaling prior to VACV infection through Janus kinase 2 (Jak2) and signal transducer and activator of transcription 1 (STAT1). The permissive state of IFN-gamma-treated human cells was unrelated to the VACV-encoded IFN decoy receptors B8 and B18 and associated with a complete disruption of STAT1 homodimer formation and DNA binding. Unlike human fibroblasts, mouse cells responded with long-lasting STAT1 activation which was preserved after VACV infection. The deletion of the IFN regulatory factor 1 (IRF-1) gene from mouse cells rescued efficient VACV replication, demonstrating that IRF-1 target genes have a critical role in VACV control. These data have implications for the understanding of VACV pathogenesis and identify an incongruent IFN-gamma response between the human host and the mouse model.

The immunologic aspects of poxvirus oncolytic therapy

The concept of using replicating oncolytic viruses in cancer therapy dates to the beginning of the twentieth century. However, in the last few years, an increasing number of pre-clinical and clinical trials have been carried out with promising preliminarily results. Novel, indeed, is the suggestion that viral oncolytic therapy might not operate exclusively through an oncolysis-mediated process but additionally requires the "assistance" of the host's immune system. Originally, the host's immune response was believed to play a predominant obstructive role against viral replication, hence limiting the anti-tumor efficacy of viral vectors. Recent data, however, suggest that the immune response may also play a key role in promoting tumor destruction in association with the oncolytic process. In fact, immune effector pathways activated during oncolytic virus-induced tumor rejection seem to follow a similar pattern to those observed when the broader phenomenon of immune-mediated tissue-specific rejection occurs in other immune-related pathologies. We recently formulated the "Immunologic Constant of Rejection" hypothesis, emphasizing commonalties in transcriptional patterns observed when tissue-destruction occurs: whether with a favorable outcome, such as in tumor rejection and pathogen clearance; or a destructive one, such as in allograft rejection or autoimmunity. Here, we propose that a similar mechanism induces clearance of virally infected tumors and that such a mechanism is primarily dependent on innate immune functions.

Vaccinia virus lacking the Bcl-2-like protein N1 induces a stronger natural killer cell response to infection

The vaccinia virus (VACV) N1 protein is an intracellular virulence factor that has a Bcl-2-like structure and inhibits both apoptosis and signalling from the interleukin 1 receptor, leading to nuclear factor kappa B activation. Here, we investigated the immune response to intranasal infection with a virus lacking the N1L gene (vΔN1L) compared with control viruses expressing N1L. Data presented show that deletion of N1L did not affect the proportion of CD4⁺ and CD8⁺ T cells infiltrating the lungs or the cytotoxic T-cell activity of these cells. However, vΔN1L induced an increased local natural killer cell activity between days 4 and 6 post-infection. In addition, in the absence of N1 the host inflammatory infiltrate was characterized by a reduced proportion of lymphocytes bearing the early activation marker CD69. Notably, there was a good correlation between the level of CD69 expression and weight loss. The implications of these findings are discussed.

Role for CCR5 in dissemination of vaccinia virus in vivo

In an earlier report, we provided evidence that expression of CCR5 by primary human T cells renders them permissive for vaccinia virus (VACV) replication. This may represent a mechanism for dissemination throughout the lymphatic system. To test this hypothesis, wild-type CCR5(+/+) and CCR5 null mice were challenged with VACV by intranasal inoculation. In time course studies using different infective doses of VACV, we identified viral replication in the lungs of both CCR5(+/+) and CCR5(-/-) mice, yet there were diminished viral loads in the spleens and brains of CCR5(-/-) mice compared with CCR5(+/+) mice. Moreover, in association with VACV infection, we provide evidence for CD4+ and CD8+ T-cell as well as CD11c+ and F4/80+ cell infiltration into the lungs of CCR5(+/+) but not CCR5(-/-) mice, and we show that the CCR5-expressing T cells harbor virus. We demonstrate that this CCR5 dependence is VACV specific, since CCR5(-/-) mice are as susceptible to intranasal influenza virus (A/WSN/33) infection as CCR5(+/+) mice. In a final series of experiments, we provide evidence that adoptive transfer of CCR5(+/+) bone marrow leukocytes into CCR5(-/-) mice restores VACV permissiveness, with evidence of lung and spleen infection. Taken together, our data suggest a novel role for CCR5 in VACV dissemination in vivo.

Vaccinia virus protein C16 acts intracellularly to modulate the host response and promote virulence

The vaccinia virus (VACV) strain Western Reserve C16 protein has been characterized and its effects on virus replication and virulence have been determined. The C16L gene is present in the inverted terminal repeat and so is one of the few VACV genes that are diploid. The C16 protein is highly conserved between different VACV strains, and also in the orthopoxviruses variola virus, ectromelia virus, horsepox virus and cowpox virus. C16 is a 37.5 kDa protein, which is expressed early during infection and localizes to the cell nucleus and cytoplasm of infected and transfected cells. The loss of the C16L gene had no effect on virus growth kinetics but did reduce plaque size slightly. Furthermore, the virulence of a virus lacking C16L (vDeltaC16) was reduced in a murine intranasal model compared with control viruses and there were reduced virus titres from 4 days post-infection. In the absence of C16, the recruitment of inflammatory cells in the lung and bronchoalveolar lavage was increased early after infection (day 3) and more CD4(+) and CD8(+) T cells expressed the CD69 activation marker. Conversely, late after infection with vDeltaC16 (day 10) there were fewer T cells remaining, indicating more rapid clearance of infection. Collectively, these data indicate that C16 diminishes the immune response and is an intracellular immunomodulator.

Virulence in murine model shows the existence of two distinct populations of Brazilian Vaccinia virus strains

Brazilian Vaccinia virus had been isolated from sentinel mice, rodents and recently from humans, cows and calves during outbreaks on dairy farms in several rural areas in Brazil, leading to high economic and social impact. Some phylogenetic studies have demonstrated the existence of two different populations of Brazilian Vaccinia virus strains circulating in nature, but little is known about their biological characteristics. Therefore, our goal was to study the virulence pattern of seven Brazilian Vaccinia virus strains. Infected BALB/c mice were monitored for morbidity, mortality and viral replication in organs as trachea, lungs, heart, kidneys, liver, brain and spleen. Based on the virulence potential, the Brazilian Vaccinia virus strains were grouped into two groups. One group contained GP1V, VBH, SAV and BAV which caused disease and death in infected mice and the second one included ARAV, GP2V and PSTV which did not cause any clinical signals or death in infected BALB/c mice. The subdivision of Brazilian Vaccinia virus strains into two groups is in agreement with previous genetic studies. Those data reinforce the existence of different populations circulating in Brazil regarding the genetic and virulence characteristics.

Vaccinia virus infection modulates the hematopoietic cell compartments in the bone marrow

Successful proliferation and differentiation of hematopoietic progenitor cells in bone marrow (BM) is essential to generate all mature blood cell types, including those involved in the immune response. Although vaccinia virus (VV) is known to induce a strong immune response, the effect of VV infection on hematopoiesis remains largely unknown. Here, we showed that in vivo VV infection results in the expansion of c-Kit(hi)Sca-1+Lin- (KSL) hematopoietic stem cells. The in vivo expansion of the KSL population requires MyD88 that is a critical adaptor for Toll-like receptor-mediated signaling. Moreover, in BM of VV-infected mice, common myeloid progenitors (CMP) was decreased because of the rapid differentiation of CMP to more mature cells. However, the CMP compartment was not affected by VV infection in the absence of MyD88. The common lymphoid progenitor (CLP) cell population was increased regardless of MyD88 status, suggesting the independent regulation of CMP and CLP compartments by VV infection. VV infection also enhanced the potential of progenitors that preferentially induce the programming of dendritic cell (DC) development toward plasmacytoid DC. Therefore, the host immune response is gearing toward antiviral responses as early as at the precursor level upon VV infection.

Initiation of primary anti-vaccinia virus immunity in vivo

The primary focus of our work is the initiation of an antiviral immune response. While we employ many experimental systems to address this fundamental issue, much of our work revolves around the use of vaccinia virus. Concerns over the negative effects of vaccination have prevented the return of the smallpox immunization program to the general population and underscored the importance of understanding the primary immune response to vaccinia virus. This response is comprised of a complex symphony of immune system components employing a variety of different mechanisms. In this review, we will both highlight the roles of many of these components and touch on the applications of vaccinia virus in the laboratory and the clinic.

Vaccinia virus gene F3L encodes an intracellular protein that affects the innate immune response

The Vaccinia virus BTB/kelch protein F3 has been characterized and its effects on virus replication in vitro and virus virulence in vivo have been determined. The loss of the F3L gene had no effect on virus growth, plaque phenotype or cytopathic effect in cell culture under the conditions tested. However, the virulence of a virus lacking F3L in an intradermal model was reduced compared with controls, and this was demonstrated by a significantly smaller lesion and alterations to the innate immune response to infection. The predicted molecular mass of the F3 protein is 56 kDa; however, immunoblotting of infected cell lysates using an antibody directed against recombinant F3 revealed two proteins of estimated sizes 37 and 25 kDa.

Vaccinia virus strain Western Reserve protein B14 is an intracellular virulence factor

A characterization of the B14R gene from Vaccinia virus (VACV) strain Western Reserve (WR) is presented. Computational analyses of the B14R gene indicated high conservation in orthopoxviruses but no orthologues outside the Poxviridae. To characterize the B14 protein, the B14R gene was expressed in Escherichia coli and recombinant protein was purified and used to generate a rabbit polyclonal antiserum. This antiserum recognized a 15 kDa cytoplasmic protein in mammalian cells that were transfected with the B14R gene or infected with VACV WR, but not from cells infected with a VACV mutant (vdeltaB14) from which the B14R gene was deleted. Compared to wild-type and revertant virus controls, vdeltaB14 had normal growth kinetics in cell culture. The virulence of vdeltaB14 was assessed in two in vivo models. Mice infected intranasally with vdeltaB14 had similar weight loss compared to the controls, but in C57BL/6 mice infected intradermally vdeltaB14 induced a smaller lesion size compared with controls. Moreover, intradermal infection with vdeltaB14 caused an increased infiltration of cells into the infected lesion despite the smaller lesion size. Therefore, B14 is an intracellular protein that is non-essential for virus replication in cell culture but contributes to virus virulence in vivo and affects the host response to infection.