Cytolytic activity and associated serine protease expression by skin and afferent lymph CD8+ T cells during orf virus reinfection

Immunomodulatory Strategies for Parapoxvirus: Current Status and Future Approaches for the Development of Vaccines against Orf Virus Infection

Orf virus (ORFV), the prototype species of the parapoxvirus genus, is the causative agent of contagious ecthyma, an extremely devastating skin disease of sheep, goats, and humans that causes enormous economic losses in livestock production. ORFV is known for its ability to repeatedly infect both previously infected and vaccinated sheep due to several immunomodulatory genes encoded by the virus that temporarily suppress host immunity. Therefore, the development of novel, safe and effective vaccines against ORFV infection is an important priority. Although, the commercially licensed live-attenuated vaccines have provided partial protection against ORFV infections, the attenuated viruses have been associated with major safety concerns. In addition to safety issues, the persistent reinfection of vaccinated animals warrants the need to investigate several factors that may affect vaccine efficacy. Perhaps, the reason for the failure of the vaccine is due to the long-term adaptation of the virus in tissue culture. In recent years, the development of vaccines against ORFV infection has achieved great success due to technological advances in recombinant DNA technologies, which have opened a pathway for the development of vaccine candidates that elicit robust immunity. In this review, we present current knowledge on immune responses elicited by ORFV, with particular attention to the effects of the viral immunomodulators on the host immune system. We also discuss the implications of strain variation for the development of rational vaccines. Finally, the review will also aim to demonstrate future strategies for the development of safe and efficient vaccines against ORFV infections.

Orf Virus-Based Vaccine Vector D1701-V Induces Strong CD8+ T Cell Response against the Transgene but Not against ORFV-Derived Epitopes

The potency of viral vector-based vaccines depends on their ability to induce strong transgene-specific immune response without triggering anti-vector immunity. Previously, Orf virus (ORFV, Parapoxvirus) strain D1701-V was reported as a novel vector mediating protection against viral infections. The short-lived ORFV-specific immune response and the absence of virus neutralizing antibodies enables repeated immunizations and enhancement of humoral immune responses against the inserted antigens. However, only limited information exists about the D1701-V induced cellular immunity. In this study we employed major histocompatibility complex (MHC) ligandomics and immunogenicity analysis to identify ORFV-specific epitopes. Using liquid chromatography-tandem mass spectrometry we detected 36 ORFV-derived MHC I peptides, originating from various proteins. Stimulated splenocytes from ORFV-immunized mice did not exhibit specific CD8+ T cell responses against the tested peptides. In contrast, immunization with ovalbumin-expressing ORFV recombinant elicited strong SIINFEKL-specific CD8+ T lymphocyte response. In conclusion, our data indicate that cellular immunity to the ORFV vector is negligible, while strong CD8+ T cell response is induced against the inserted transgene. These results further emphasize the ORFV strain D1701-V as an attractive vector for vaccine development. Moreover, the presented experiments describe prerequisites for the selection of T cell epitopes exploitable for generation of ORFV-based vaccines by reverse genetics.

Structural basis of GM-CSF and IL-2 sequestration by the viral decoy receptor GIF

Subversion of the host immune system by viruses is often mediated by molecular decoys that sequester host proteins pivotal to mounting effective immune responses. The widespread mammalian pathogen parapox Orf virus deploys GIF, a member of the poxvirus immune evasion superfamily, to antagonize GM-CSF (granulocyte macrophage colony-stimulating factor) and IL-2 (interleukin-2), two pleiotropic cytokines of the mammalian immune system. However, structural and mechanistic insights into the unprecedented functional duality of GIF have remained elusive. Here we reveal that GIF employs a dimeric binding platform that sequesters two copies of its target cytokines with high affinity and slow dissociation kinetics to yield distinct complexes featuring mutually exclusive interaction footprints. We illustrate how GIF serves as a competitive decoy receptor by leveraging binding hotspots underlying the cognate receptor interactions of GM-CSF and IL-2, without sharing any structural similarity with the cytokine receptors. Our findings contribute to the tracing of novel molecular mimicry mechanisms employed by pathogenic viruses.

Viruses often subvert the host immune system using molecular decoys to prevent an effective immune response. Here, the authors examine the structural details of the viral decoy receptor GIF and its antagnosim of GM-CSF and IL-2.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

The A2 gene of alcelaphine herpesvirus-1 is a transcriptional regulator affecting cytotoxicity in virus-infected T cells but is not required for malignant catarrhal fever induction in rabbits

Alcelaphine herpesvirus-1 (AlHV-1) causes malignant catarrhal fever (MCF). The A2 gene of AlHV-1 is a member of the bZIP transcription factor family. We wished to determine whether A2 is a virulence gene or not and whether it is involved in pathogenesis by interference with host transcription pathways. An A2 gene knockout (A2ΔAlHV-1) virus, revertant (A2revAlHV-1) virus, and wild-type virus (wtAlHV-1) were used to infect three groups of rabbits. A2ΔAlHV-1-infected rabbits succumbed to MCF, albeit with a delayed onset compared to the control groups, so A2 is not a critical virulence factor. Differential gene transcription analysis by RNAseq and qRT-PCR validation of a selection of these was performed in infected large granular lymphocyte (LGL) T cells obtained in culture from the MCF-affected animals. A2 was involved in the transcriptional regulation of immunological, cell cycle and apoptosis pathways. In particular, there was a bias towards γδ T cell receptor (TCR) expression and downregulation of αβ TCR. TCR signalling, apoptosis, cell cycle, IFN-γ and NFAT pathways were affected. Of particular interest was partial inhibition of the cytotoxicity-associated pathways involving perforin and the granzymes A and B in the A2ΔAlHV-1-infected LGLs compared to controls. In functional assays, A2ΔAlHV-1-infected LGLs were significantly less cytotoxic than wtAlHV-1- and A2revAlHV-1-infected LGLs using rabbit corneal epithelial cells (SIRC) as targets. This implies that A2 is involved in a pathway enhancing the expression of LGL cytotoxicity. This is important as virus-infected T cell cytotoxicity in vivo has been suggested as a potential mechanism of disease induction in MCF.

Therapeutic immunomodulation using a virus--the potential of inactivated orf virus

Viruses can manipulate the immune response against them by various strategies to influence immune cells, i.e. by over-activation leading to functional inactivation, bypassing antigen presentation or even suppression of effector functions. Little is known, however, about how these features of immune regulation and modulation could be used for therapeutic purposes. Reasons for this include the complexity of immune regulatory mechanisms under certain disease conditions and the risks that infections with viruses pose to human beings. The orf virus (ORFV), a member of the Parapoxvirus genus of the poxvirus family, is known as a common pathogen in sheep and goats worldwide. The inactivated ORFV, however, has been used as a preventative as well as therapeutic immunomodulator in veterinary medicine in different species. Here, we review the key results obtained in pre-clinical studies or clinical studies in veterinary medicine to characterise the therapeutic potential of inactivated ORFV. Inactivated ORFV has strong effects on cytokine secretion in mice and human immune cells, leading to an auto-regulated loop of initial up-regulation of inflammatory and Th1-related cytokines, followed by Th2-related cytokines that attenuate immunopathology. The therapeutic potential of inactivated ORFV has been recognised in several difficult-to-treat disease areas, such as chronic viral diseases, liver fibrosis or various forms of cancer. Further research will be required in order to evaluate the full beneficial potential of inactivated ORFV for therapeutic immunomodulation.

ORFV: a novel oncolytic and immune stimulating parapoxvirus therapeutic

Replicating viruses for the treatment of cancer have a number of advantages over traditional therapeutic modalities. They are highly targeted, self-amplifying, and have the added potential to act as both gene-therapy delivery vehicles and oncolytic agents. Parapoxvirus ovis or Orf virus (ORFV) is the prototypic species of the Parapoxvirus genus, causing a benign disease in its natural ungulate host. ORFV possesses a number of unique properties that make it an ideal viral backbone for the development of a cancer therapeutic: it is safe in humans, has the ability to cause repeat infections even in the presence of antibody, and it induces a potent T(h)-1-dominated immune response. Here, we show that live replicating ORFV induces an antitumor immune response in multiple syngeneic mouse models of cancer that is mediated largely by the potent activation of both cytokine-secreting, and tumoricidal natural killer (NK) cells. We have also highlighted the clinical potential of the virus by demonstration of human cancer cell oncolysis including efficacy in an A549 xenograft model of cancer.

A captured viral interleukin 10 gene with cellular exon structure

We have characterized a novel, captured and fully functional viral interleukin (IL)-10 homologue ((OvHV)IL-10) from the gammaherpesvirus ovine herpesvirus 2. Unlike IL-10 homologues from other gammaherpesviruses, the (OvHV)IL-10 peptide sequence was highly divergent from that of the host species. The (OvHV)IL-10 gene is unique amongst virus captured genes in that it has precisely retained the original cellular exon structure, having five exons of similar sizes to the cellular counterparts. However, the sizes of the introns are dramatically reduced. The (OvHV)IL-10 protein was shown to be a non-glycosylated, secreted protein of M(r) 21 000 with a signal peptidase cleavage site between amino acids 26 and 27 of the nascent peptide. Functional assays showed that (OvHV)IL-10, in a similar way to ovine IL-10, stimulated mast cell proliferation and inhibited macrophage inflammatory chemokine production. This is the first example of a captured herpesvirus gene retaining the full cellular gene structure.

Production and utilization of interleukin-15 in malignant catarrhal fever

Malignant catarrhal fever (MCF) is an often fatal lymphoproliferative disease of ungulates caused by either alcelaphine herpesvirus-1 (AlHV-1) or ovine herpesvirus-2 (OvHV-2). The pathogenesis of MCF is poorly understood, but appears to involve an auto-destructive pathology whereby cytotoxic lymphocytes destroy areas of a variety of tissues. The cytokine interleukin-15 (IL-15) is involved in the development and maintenance of cytotoxic lymphocytes and may therefore have a role in the pathogenesis of MCF. Virus-infected large granular lymphocytes (LGLs) were obtained from the tissues of rabbits infected with AlHV-1 or OvHV-2. These cells exhibited a similar proliferative response to IL-15 and to IL-2 in culture, but their content of the activated cytotoxic enzyme (BLT-esterase) was maintained at higher levels in the presence of IL-15 compared with IL-2. The LGLs did not express IL-15 mRNA or produce IL-15 protein. By contrast, there was abundant expression of IL-15 mRNA and protein in affected tissues. IL-15 production was associated with necrotic lesions of the mesenteric lymph node and appendix of OvHV-2-infected rabbits, but was not found in the same tissues of rabbits infected with AlHV-1 in which there were no necrotic lesions. The cellular source of the IL-15 was predominantly lymphoid cells that did not express B cell or monocyte-macrophage markers. Only a few IL-15+ cells (<10%) co-localized with pan-T cells or CD8+ T cells. The abundance of IL-15 in tissue with lesions of MCF suggests that this cytokine may have a role in the pathogenesis of MCF.

Infection with recombinant orf viruses demonstrates that the viral interleukin-10 is a virulence factor

Orf virus is the prototype parapoxvirus that causes the contagious skin disease orf. It encodes an orthologue of the cytokine interleukin (IL)-10. Recombinant orf viruses were constructed in which the viral interleukin-10 (vorfIL-10) was disabled (vorfIL-10ko) and reinserted (vorfrevIL-10) at the same locus and compared to wild-type virus for their ability to induce skin lesions in sheep. After either primary infection or reinfection, smaller less severe lesions were recorded in the vorfIL-10ko-infected animals compared with either of the vorfIL-10-intact virus-infected animals. Thus, the vorfIL-10ko virus was attenuated compared with the vorfIL-10 intact viruses, demonstrating that orf virus IL-10 is a virulence factor. The virus IL-10 is one of several virulence or immuno-modulatory factors expressed by orf virus. Removal of any one of these genes would be expected to have only a partial effect on virulence, which is what was observed in this study with vorfIL-10.

Genus Parapoxvirus

Highly contagious pustular skin infections of sheep, goats and cattle that were unwittingly transmitted to humans from close contact with infected animals, have been the scourge of shepherds, herdsmen and dairy farmers for centuries. In more recent times we recognise that these proliferative pustular lesions are likely to be caused by a group of zoonotic viruses that are classified as parapoxviruses. In addition to infecting the above ungulates, parapoxviruses have more recently been isolated from seals, camels, red deer and reindeer and most have been shown to infect man. The parapoxviruses have one of the smallest genomes of the poxvirus family (140 kb) yet share over 70% of their genes with the most virulent members. Like other poxviruses, the central core of the genomes encode factors for virus transcription and replication, and structural proteins, whereas the terminal regions encode accessory factors that give the parapoxvirus group many of its unique features. Several genes of parapoxviruses are unique to this genus and encode factors that target inflammation, the innate immune responses and the development of acquired immunity. These factors include a homologue of mammalian interleukin (IL)-10, a chemokine binding protein and a granulocyte-macrophage colony stimulating factor /IL-2 binding protein. The ability of this group to reinfect their hosts, even though a cell-mediated memory response is induced during primary infection, may be related to their epitheliotropic niche and the immunomodulators they produce. In this highly localised environment, the secreted immunomodulators only interfere with the local immune response and thus do not compromise the host’s immune system. The discovery of a vascular endothelial growth factor-like gene may explain the highly vascular nature of parapoxvirus lesions. There are many genes of parapoxviruses which do not encode polypeptides with significant matches with protein sequences in public databases, separating this genus from most other mammalian poxviruses. These genes appear to be involved in inhibiting apoptosis, manipulating cell cycle progression and degradation of cellular proteins that may be involved in the stress response, thus allowing the virus to subvert intracellular antiviral mechanisms and enhance the availability of cellular molecules required for replication. Parapoxviruses in common with Molluscum contagiosum virus lack a number of genes that are highly conserved in other poxviruses, including factors for nucleotide metabolism, serine protease inhibitors and kelch-like proteins. It is apparent that parapoxviruses have evolved a unique repertoire of genes that have allowed adaptation to the highly specialised environment of the epidermis.

A comparison of the anti-inflammatory and immuno-stimulatory activities of orf virus and ovine interleukin-10

Orf virus causes pustular skin lesions (orf) in sheep, goats and humans. The virus encodes an interleukin-10 (orfvIL-10) that is identical in amino acid composition to ovine IL-10 (ovIL-10) over the C terminal two-thirds of the polypeptide, but not in the N terminal third. The immuno-suppressive and immuno-stimulatory activities of orfvIL-10 and ovIL-10 were compared. Both orfvIL-10 and ovIL-10 inhibited TNF-alpha and IL-8 cytokine production from stimulated ovine macrophages and keratinocytes and IFN-gamma and GM-CSF production from peripheral blood lymphocytes. OrfvIL-10 and ovIL-10 co-stimulated both ovine and murine mast cell proliferation in conjunction with IL-3 (ovine) or IL-4 (murine). Isoleucine at position 87 (Ile(87)) of the mature human IL-10 (huIL-10) has been reported as essential for the immuno-stimulatory activity of huIL-10. In spite of the differences in amino acids within the N-terminal third of orfvIL-10 compared with ovIL-10 and substitution of Ile(87) with Ala(87) in ovIL-10, these variants of ovIL-10 and orfvIL-10 all co-stimulated mast cell proliferation and inhibited macrophage IL-8 production. As ovIL-10 and orfvIL-10 have a similar structure to huIL-10 and conserved receptor-binding residues, it was concluded that Ile(87) is not essential for IL-10 immuno-stimulatory activity. Finally, ovine keratinocytes do not express ovIL-10. This might explain why orf virus has evolved a viral IL-10.

Orf virus immuno-modulation and the host immune response

Orf virus encodes a range of immuno-modulatory genes that interfere with host anti-virus immune and inflammatory effector mechanisms. The function of these reflects the pathogenesis of orf. The orf virus interferon resistance protein (OVIFNR) and virus IL-10 (vIL-10) inhibit interferon production and activity. In addition the vIL-10 suppresses inflammatory cytokine production by activated macrophages and keratinocytes. The virus GM-CSF inhibitory factor (GIF) is a novel virus protein that binds to and inhibits the biological activity of GM-CSF and IL-2. Together, these immuno-modulators target key effector mechanisms of host anti-virus immunity to allow time for virus replication in epidermal cells.

Immunity and counter-immunity during infection with the parapoxvirus orf virus

Orf virus is a DNA parapoxvirus that causes orf, an acute debilitating skin disease of sheep, goats and humans. In sheep, a vigorous immune response involving neutrophils, dermal dendritic cells, T cells, B cells and antibody is generated after infection. CD4(+) T cells, IFN-gamma and to a lesser extent CD8(+) T cells are involved in partial protection against infection. In spite of this, orf virus can repeatedly infect sheep albeit with reduced lesion size and time to resolution compared to primary infection. This is due at least in part to the action of virus immuno-modulator proteins that interfere with host immune and inflammatory responses. These include: an interferon resistance protein; a viral orthologue of mammalian IL-10 (vIL-10) that is an anti-inflammatory cytokine; and a novel inhibitor of the cytokines GM-CSF and IL-2 (GIF). The virus also encodes a virulence protein that is an orthologue of mammalian vascular endothelial growth factor. The study of the immuno-modulator proteins provides an insight into disease pathogenesis and important elements of a host protective response. This information will be used to devise a rational disease control strategy.

Parapoxviruses: from the lesion to the viral genome

Viruses of the genus parapoxvirus from the family poxviridae cause widespread but localized diseases of small and large ruminants. The economically most important disease is contagious pustular dermatitis or contagious ecthyma among sheep and goats, often simply called orf. The parapoxviruses (PPV) can be transmitted to man leading to localized lesions that are named pseudocowpox or milkers' node as being mostly restricted to the hands and fingers. In cattle two forms of PPV manifestation are commonly observed, the bovine papular stomatitis in young calves and the occurrence of lesions at the udder of cows. We here report about the recent efforts in molecular characterization of orf viruses and the state of the art about the generation of orf virus recombinants. In addition the current knowledge on immune responses against orf viruses and some new data on the behaviour of orf virus recombinants under non-permissive conditions are reported.

Detection of cellular cytokine mRNA expression during orf virus infection in sheep: differential interferon-gamma mRNA expression by cells in primary versus reinfection skin lesions

In sheep infected with the parapoxvirus orf virus, primary infection orf skin lesions developed and resolved within 8 weeks. Reinfection lesions were smaller and resolved within 3 weeks. The host response in the skin was characterized by an accumulation of neutrophils, dendritic cells, CD4+ T cells, CD8+ T cells, B cells and T19+ gammadelta T cells. The magnitude of this accumulation paralleled orf virus replication in the skin. In situ hybridization was used to detect cells expressing interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and interleukin-4 (IL-4) mRNAs in orf skin. Cells expressing IL-4 mRNA were not detected at any time after infection. Cells expressing IFN-gamma mRNA were detected after reinfection but not after primary infection. Cells expressing TNF-alpha mRNA included epidermal cells, vascular endothelium and uncharacterized cells that increased more rapidly in the skin after reinfection compared to primary infection. The results are consistent with a prominent role for IFN-gamma in the host immune response controlling the severity of the disease.

Subversion and piracy: DNA viruses and immune evasion

During the co-evolution of viruses with their vertebrate hosts, the DNA viruses have acquired an impressive array of immunomodulatory genes to combat host immune responses and their hosts have developed a sophisticated immune system to contain virus infections. In order to replicate, the viruses have evolved mechanisms to inhibit key host anti-virus responses that include apoptosis, interferon production, chemokine production, inflammatory cytokine production, and the activity of cytotoxic T-cells, natural killer cells and antibody. In addition, some of the viruses encode cytokine or chemokine homologues that recruit or expand cell numbers for infection or that subvert the host cellular response from a protective response to a benign one. The specificity of the viral immunomodulatory molecules reflects the life cycle and the pathogenesis of the viruses. Herpesviruses achieve latency in host cells by inducing cell survival and protecting infected cells from immune recognition. This involves interference with cell signal transduction pathways. Many of the viral immunomodulatory proteins are homologues of host proteins that appear to have been pirated from the host and reassorted in the virus genomes. Some of these have unique functions and indicate novel or important aspects of both viral pathogenesis and host immunity to viruses. The specific example of orf virus infection of sheep is described.

Constitutive activation of Lck and Fyn tyrosine kinases in large granular lymphocytes infected with the gamma-herpesvirus agents of malignant catarrhal fever

Large granular lymphocytes (LGL) with a T or natural killer (NK) lymphoblast morphology and indiscriminate (non-major histocompatibility complex-linked) cytotoxicity for a variety of target cells can be derived in culture from the tissues of animals infected with either alcelaphine herpesvirus-1 (AlHV-1) or ovine herpesvirus-2 (OvHV-2). In this study, LGL survival in the absence of exogenous interleukin-2 was inhibited by the protein kinase inhibitor genestein, but not the p70 s6 kinase inhibitor rapamycin. Constitutive activation of the src kinases Lck and Fyn was demonstrated in a bovine LGL line infected with OvHV-2 and in two rabbit LGL lines infected with AlHV-1. The p44 erk1 and p42 erk2 mitogen-activated protein kinases (MAPK) were also constitutively activated in the LGLs but not control T cells. Lck and Fyn kinase activity in the LGLs did not increase after mitogen (concanavalin A or concanavalin A plus phorbol ester) stimulation of the cells, in contrast to control T cells. Control T cells, but not the LGLs, proliferated after mitogen stimulation. An analysis of tyrosine phosphorylated proteins in the cells indicated that the LGLs exhibited some similarities and differences to activated control T cells. The results demonstrate that the activated phenotype of the LGLs, associated with malignant catarrhal fever virus infection and in the absence of exogenous interleukin-2, involves constitutively activated Lck and Fyn kinases. These are normally crucial for the initial activation of T cells via several cell-surface receptors (e.g. the T-cell receptor and CD2). The inability of the LGLs to proliferate in response to mitogen may be due to an inability of Lck and Fyn to become further activated after mitogen stimulation.

In vivo T-cell subset depletion suggests that CD4+ T-cells and a humoral immune response are important for the elimination of orf virus from the skin of sheep

In vivo lymphocyte subset depletion offers a unique opportunity to study the roles of different cellular components of the immune system of sheep during infection with orf virus. Lambs were depleted of specific lymphocyte subsets by the intravenous administration of monoclonal antibodies against ovine lymphocyte surface markers and then challenged with orf virus. The skin lesions that developed were scored visually as to their severity. Blood samples were collected to monitor the lymphocyte depletions and to measure orf-virus-specific antibody levels. Skin biopsies were collected from the lesion site and studied to determine the course of the infection and the presence of various cell types and orf virus. All the sheep developed orf virus lesions after infection. All three of the CD4-depleted lambs were unable to clear virus from their skin and did not have an antibody response to the virus. Virus was also detected in the skin of one each of the three CD8-depleted, WC1-depleted and control sheep on the final day of the trial. CD8(+) lymphocytes did not appear to be essential for viral clearance later in the infection. Depletion of the majority of gammadelta(+) T-cells did not affect the outcome of orf virus infection. In sheep with high orf-virus-specific antibody titres at the time of infection, orf lesions healed faster than lesions in sheep with low antibody levels, and this occurred regardless of the lymphocyte depletion status of the animals. This study suggests that the presence of CD4(+) T-cells and orf-virus-specific antibodies are important for the control of viral replication in the skin of infected sheep.

Orf virus encodes a novel secreted protein inhibitor of granulocyte-macrophage colony-stimulating factor and interleukin-2

The parapoxvirus orf virus encodes a novel soluble protein inhibitor of ovine granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2). The GM-CSF- and IL-2-inhibitory factor (GIF) gene was expressed as an intermediate-late viral gene in orf virus-infected cells. GIF formed homodimers and tetramers in solution, and it bound ovine GM-CSF with a K(d) of 369 pM and ovine IL-2 with a K(d) of 1.04 nM. GIF did not bind human GM-CSF or IL-2 in spite of the fact that orf virus is a human pathogen. GIF was detected in afferent lymph plasma draining the skin site of orf virus reinfection and was associated with reduced levels of lymph GM-CSF. GIF expression by orf virus indicates that GM-CSF and IL-2 are important in host antiviral immunity.

Immunomodulation by virulence proteins of the parapoxvirus orf virus

Three orf virus putative virulence proteins are described that exhibit immunomodulatory functions. The OVIFNR gene at the left terminus of the viral genome encodes an interferon resistance protein with homology to the E3L gene of vaccinia virus. OVIFNR functions by preventing a dsRNA-dependent kinase from inhibiting virus and cell protein synthesis as part of the interferon-induced anti-viral state within infected cells. The orf virus orthologue of the ovine interleukin-10 (vIL-10) gene is located at the right terminus of the viral genome. Both vIL-10 and host (ovine) IL-10 function in vitro as inhibitors of pro-inflammatory cytokine production by keratinocytes and macrophages, and both inhibit IFN-gamma production from activated peripheral blood lymphocytes. Both the orf virus vIL-10 and ovine IL-10 stimulate mast cell and thymocyte proliferation. In this respect the orf virus IL-10 differs from Epstein Barr virus IL-10 which does not exhibit cell proliferative activity. Finally, the orf virus GM-CSF inhibitory factor gene (GIF) at the right terminus of the viral genome encodes an inhibitor of GM-CSF that also binds IL-2. Together, these viral proteins are capable of inhibiting key components of the ovine anti-virus immune and inflammatory response.

Parapoxviruses: potential alternative vectors for directing the immune response in permissive and non-permissive hosts

Parapoxvirus (PPV) represents a genus of the poxviridae, and particularly PPV ovis (Orf virus, OV) seems to offer several potential advantages for the use of vector vaccine. Therefore, we started to investigate the genome of the highly attenuated OV strain D1701, which was only poorly characterised until now. Due to recombination of non-homologous sequences, part of the right hand end of the D1701 genome was duplicated and translocated to the opposite end of the genome. As a consequence gene deletion had occurred and the inverted terminal repeat region is increased. Results are described to identify viral genes, which are non-essential for virus replication and potentially influence viral pathogenesis, virulence, and host immunity. In more detail, we analysed the expression and functional activity of the OV-specific vascular endothelial growth factor (VEGF) gene homologue. Finally the construction and production of a D1701 mutant lacking the VEGF gene homologue is reported.

Molecular genetic analyses of parapoxviruses pathogenic for humans

The current members of the genus parapoxvirus are orf virus (ORFV), bovine papular stomatitis virus (BPSV), pseudocowpoxvirus (PCPV) and parapoxvirus of red deer in New Zealand (PVNZ). BPSV and PCPV are maintained in cattle while ORFV is maintained in sheep and goats, but all three are zoonoses. Only the recently reported PVNZ has yet to be recorded as infecting humans. Tentative members of the genus are camel contagious ecthyma virus, chamois contagious ecthyma virus and sealpoxvirus. The separation of the parapoxviruses into 4 distinct groups has been based on natural host range, pathology and, more recently, on restriction endonuclease and DNA/DNA hybridisation analyses. The latter studies have shown that the parapoxviruses share extensive homology between central regions of their genomes, but much lower levels of relatedness within the genome termini. The high G + C content of parapoxvirus DNA is in contrast to most other poxviruses and suggests that a significant genetic divergence from other genera of this family has occurred. DNA sequencing of portions of the genome of ORFV, the type species of the genus, has allowed a detailed comparison with the fully sequenced genome of the orthopoxvirus, vaccinia virus (VACV). These studies have provided a genetic map of ORFV and revealed a central core of 88 kbp within which the genomic content was strikingly similar to that of VACV. This conservation is not maintained in the genome termini where insertions, deletions and translocations have occurred. The characterisation of specific ORFV genes may lead to the construction of attenuated vaccine strains in which genes such as those with the potential to interfere with the immune response of the host have been deleted. The current ORFV vaccines are living unattenuated virus and vaccination lesions produce virus which contaminates the environment in a manner similar to natural infection. The virus in scab material is relatively resistant to inactivation and this virus both perpetuates the disease in sheep and provides the most likely source of human infections. A vaccine which immunises animals without perpetuating the disease could be the best way of reducing the incidence of ORFV infection of humans. It is likely that protection against infection by ORFV is cell mediated and will require the endogenous production of relevant antigens. We have recently constructed a series of VACV recombinants each of which contains a large multigene fragment of ORFV DNA. Together the recombinants represent essentially all of the ORFV genome in an overlapping manner. Vaccination of sheep with the recombinant library provided protection against challenge with virulent ORFV. Further studies with this library may enable dominant protective antigens of ORFV to be identified and lead to their incorporation into a subunit vaccine.

Severe long-lasting contagious ecthyma infection in a goat's kid

In this report we describe an unusual orf infection in a goat's kid, which lasted for 6 months. The disease was reproduced in two susceptible goat kids, which both completely recovered within 37 days. Further clinical observations on these kids for 12 months did not reveal any signs of orf disease. The orf virus was isolated and identified from the animals which were involved in the original outbreak, from the kid that showed the long-lasting disease and from the experimentally-infected kids. The situation is discussed.

Cyclosporin A abrogates the acquired immunity to cutaneous reinfection with the parapoxvirus orf virus

The effect of cyclosporin A (CsA) on host immunity to cutaneous reinfection with the parapoxvirus orf virus was studied in 6-month-old lambs. In control reinfected animals, clinical lesions and viral replication (measured by the presence of vesicular/pustular lesions and viral antigen) in regenerating epidermal cells were at a maximum on day 4 with resolution by day 9. Lesion histology revealed recruitment of T cells, B cells and dermal dendritic cells (DDC) which increased and decreased in parallel with the clinical course of the reinfection. In animals treated with CsA (25 mg/kg/day) 1 day before and for 8 days after reinfection, more severe clinical lesions and viral replication typical of primary infections were recorded and had not resolved by 28 days following reinfection. During CsA treatment, the recruitment of T cells, B cells and DDC was inhibited. With cessation of CsA treatment there was dramatic recruitment of CD4+ T cells followed by DDC then B cells to the lesion site but rapid onset of acquired immunity was not recorded. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of cytokine mRNAs from lesion biopsies showed individual sheep variations. However, interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) mRNAs were detected in the control reinfected animals on days 3 and/or 9 after reinfection but not on these days in animals undergoing treatment with CsA. In the untreated lambs there was an inexplicable lack of IL-2 and IFN-gamma mRNAs on day 6 after reinfection. Tumour necrosis factor-alpha (TNF-alpha) and vascular endothelial growth factor (VEGF) mRNAs were unaffected by CsA treatment. The data suggest that CsA abrogates acquired immunity to orf virus reinfection by targetting T-cell lymphokine production.