Comparison of field and vaccine strains of Australian fowlpox viruses

Characterisation of an Australian fowlpox virus carrying a near-full-length provirus of reticuloendotheliosis virus

Fowlpox virus (FWPV), which is the type member of the genus Avipoxvirus, subfamily Chordopoxvirinae, family Poxviridae, can lead to significant losses to the poultry industry. Although a large number of fowlpox virus genomes have been sequenced and characterised globally, there are no sequences available at the genomic level from Australian isolates. Here, we present the first complete genome sequence of a fowlpox virus vaccine strain (FWPV-S) containing an integrated near-full-length reticuloendotheliosis virus (REV) provirus. The genome of FWPV-S showed the highest sequence similarity to a fowlpox virus from the USA (97.74% identity). The FWPV-S genome contained 16 predicted unique genes, while a further two genes were fragmented compared to previously reported FWPV genome sequences. Subsequent phylogenetic analysis showed that FWPV-S was most closely related to other fowlpox viruses. This is the first reported genome sequence of FWPV from Australia.

Detection of T- and B-cell Target Antigens of Fowlpox Virus Isolated from Backyard Chickens in India

With the aim of assessing the antigenic characteristics of a circulating pool of fowlpox virus (FPV) that exists in the backyard poultry system in India, one of the field isolates generated was characterized by in vitro immunologic techniques. FPV was isolated from clinically positive fowlpox cases (n  =  10) from the Jhargram (West Midnapur district) and Kakdwip (South 24 Pargana district) areas of West Bengal State, India. Initially, FPV-specific PCR was performed for confirmation of the samples. Isolation of FPV was done using embryonated chicken eggs and the choreoallantoic membrane route. Subsequently, FPV antigen was prepared from chicken embryo fibroblast cell culture-adapted field isolate. Biologic transmission of FPV was performed in Rhode Island red chickens experimentally to assess humoral and cell-mediated immune (CMI) responses. High level of anti-FPV antibodies were observed in test birds as assessed by indirect ELISA. Seroreactive polypeptides (B-cell antigens) of FPV antigen with molecular weights of 44.5, 66.5, 75, 90.5, and 99 kDa were detected by western blot analysis. Significant increases in CMI responses were observed in inoculated chickens as assessed by lymphocyte proliferation assay, cytotoxicity assay, and T-cell immunoblotting. The predominant T-cell antigen of FPV detected had a molecular weight of 66.5 kDa. The present study revealed the antigenic characteristics of FPV that exists in backyard poultry system in West Bengal for the first time, thus exploring the rationality of designing future T- and B-cell vaccines against fowlpox.

Evaluation of a commercial vaccine against avian poxvirus in turkeys kept in the backyard system in the state of Yucatan, Mexico

One hundred and sixty 1-month-old turkey poults were delivered to 40 households in four communities of the State of Yucatan, Mexico. The poults were divided into two populations, one vaccinated and the other non-vaccinated against avian pox. During three months, monthly visits were carried out in order to monitor the appearance of lesions suggesting avian pox in the birds delivered. Each turkey was clinically examined, searching for characteristic avian pox lesions that were classified according to the degree of severity observed. The true incidence rate and the cumulative incidence rate of avian pox were determined and the true incidence and cumulative incidence rates of mortality were determined and the relative risks calculated. The true incidence rates for avian pox in vaccinated and non-vaccinated birds were 1.5 and 1.47 respectively. The cumulative incidence rates were 0.94 and 0.90 for vaccinated and non-vaccinated birds, respectively. The comparison for the whole period between vaccinated and non-vaccinated groups did not show a significant statistical difference for mortality. However, when mortality was compared between vaccinated and non-vaccinated turkeys for each month of the study, there was a statistically significant difference for the first month (relative risk = 0.216, confidence interval 0.069 to 0.676). In addition, when the severity of pox lesions between groups was compared, statistically significant differences were found in favour of the vaccinated birds (P < 0.0001).

Evaluation of the immune responses induced by four targeted DNA vaccines encoding the juvenile liver fluke antigen, cathepsin B in a mouse model

Background

Liver fluke can infect cattle and sheep, and is also emerging as a human pathogen in developing countries. Cathepsin B (Cat B2) is a major cysteine protease secreted by the juvenile flukes. To enhance the immune responses of Cat B2, the cDNA sequence was fused with four different DNA vaccine vectors. The induced cellular and antibody responses were compared in vaccinated mice.

Methods

The following recombinant DNA vaccine constructs were constructed: empty vector VR1012 as negative control, cytoplasmic construct pVR1012 Cat B2, secretory construct pVR1020 Cat B2, chemokine-fused construct pMCP3 Cat B2 and lymph node targeting construct pCTLA-4 Cat B2. Plasmids were constructed using standard procedures, and positive constructs screened and selected using restriction digestion analysis followed by sequence analysis. The constructs were then tested in Cos-7 cells for in vitro expression, which was analysed using immunoblotting. Subsequently, female BALB/c mice were immunised with DNA constructs as vaccines. Elicited antibody responses were measured using ELISA. The ratio between IgG1 and IgG2a antibody responses was estimated among different vaccine groups. IgG antibody avidity assay was performed and the relative avidity index was calculated. The induced cytokine production from splenocytes of vaccinated animals was estimated using ELISPOT.

Results

DNA vaccine constructs carrying Cat B2 were expressed in Cos-7 cell lines and encoded protein was recognised using western blotting using rat anti- cathepsin B antibody. DNA vaccines elicited high Cat B2- specific IgG, IgG1, IgE and also modest IgG2a antibody responses. Cat B2 specific IL-4 T cell responses were also observed in Cat B2 vaccinated mice. The comparison of immunogenic potential in each of these constructs was demonstrated as enhanced antibody responses on the lymph-node targeting vector pCTLA-4 Cat B2, the high antibody avidity of chemo-attractant pMCP3 Cat B2 and stronger T cellular responses of non-secretory DNA vaccine pVR1012 Cat B2 in vaccinated animals.

Conclusion

This study showed that the targeting DNA vaccine strategies enhanced specific immune responses to juvenile fluke Cat B2. The results of our current study have demonstrated that a gene-based vaccine as an immunotherapeutic approach to combat Fasciola infection may be feasible.

Antigen-specific T-cell responses to a recombinant fowlpox virus are dependent on MyD88 and interleukin-18 and independent of Toll-like receptor 7 (TLR7)- and TLR9-mediated innate immune recognition

Fowlpox virus (FWPV) is a double-stranded DNA virus long used as a live-attenuated vaccine against poultry diseases, but more recent interest has focused on its use as a mammalian vaccine vector. Here, in a mouse model system using FWPV encoding the nominal target antigen chicken ovalbumin (OVA) (FWPV(OVA)), we describe for the first time some of the fundamental processes by which FWPV engages both the innate and adaptive immune systems. We show that Toll-like receptor 7 (TLR7) and TLR9 are important for type I interferon secretion by dendritic cells, while TLR9 is solely required for proinflammatory cytokine secretion. Despite this functional role for TLR7 and TLR9 in vitro, only the adapter protein myeloid differentiation primary response gene 88 (MyD88) was shown to be essential for the formation of adaptive immunity to FWPV(OVA) in vivo. The dependence on MyD88 was confined only to the T-cell compartment and was not related to its contribution to TLR signaling, dendritic cell maturation, or the capture and presentation of FWPV-derived OVA antigen. We demonstrate that this is not by means of mediating T-cell-dependent interleukin-1 (IL-1) signaling, but rather, we suggest that MyD88 functions to support T-cell-specific IL-18 receptor signaling, which in turn is essential for the formation of adaptive immunity to FWPV-encoded OVA.

Type I interferons mediate the innate cytokine response to recombinant fowlpox virus but not the induction of plasmacytoid dendritic cell-dependent adaptive immunity

Type I interferons (IFNs) are considered to be important mediators of innate immunity due to their inherent antiviral activity, ability to drive the transcription of a number of genes involved in viral clearance, and their role in the initiation of innate and adaptive immune responses. Due to the central role of type I IFNs, we sought to determine their importance in the generation of immunity to a recombinant vaccine vector fowlpox virus (FPV). In analyzing the role of type I IFNs in immunity to FPV, we show that they are critical to the secretion of a number of innate and proinflammatory cytokines, including type I IFNs themselves as well as interleukin-12 (IL-12), tumor necrosis factor-alpha (TNF-alpha), IL-6, and IL-1beta, and that deficiency leads to enhanced virus-mediated antigen expression. Interestingly, however, type I IFNs were not required for adaptive immune responses to recombinant FPV even though plasmacytoid dendritic cells (pDCs), the primary producers of type I IFNs, have been shown to be requisite for this to occur. Furthermore, we provide evidence that the importance of pDCs may lie in their ability to capture and present virally derived antigen to T cells rather than in their capacity as professional type I IFN-producing cells.

Antiviral therapy with entecavir combined with post-exposure "prime-boost" vaccination eliminates duck hepatitis B virus-infected hepatocytes and prevents the development of persistent infection

Short-term antiviral therapy with the nucleoside analogue entecavir (ETV), given at an early stage of duck hepatitis B virus (DHBV) infection, restricts virus spread and leads to clearance of DHBV-infected hepatocytes in approximately 50% of ETV-treated ducks, whereas widespread and persistent DHBV infection develops in 100% of untreated ducks. To increase the treatment response rate, ETV treatment was combined in the current study with a post-exposure "prime-boost" vaccination protocol. Four groups of 14-day-old ducks were inoculated intravenously with a dose of DHBV previously shown to induce persistent DHBV infection. One hour post-infection (p.i.), ducks were primed with DNA vaccines that expressed DHBV core (DHBc) and surface (pre-S/S and S) antigens (Groups A, B) or the DNA vector alone (Groups C, D). ETV (Groups A, C) or water (Groups B, D) was simultaneously administered by gavage and continued for 14 days. Ducks were boosted 7 days p.i. with recombinant fowlpoxvirus (rFPV) strains also expressing DHBc and pre-S/S antigens (Groups A, B) or the FPV-M3 vector (Groups C, D). DHBV-infected hepatocytes were observed in the liver of all ducks at day 4 p.i. with reduced numbers in the ETV-treated ducks. Ducks treated with ETV plus the control vectors showed restricted spread of DHBV infection during ETV treatment, but in 60% of cases, infection became widespread after ETV was stopped. In contrast, at 14 and 67 days p.i., 100% of ducks treated with ETV and "prime-boost" vaccination had no detectable DHBV-infected hepatocytes and had cleared the DHBV infection. These findings suggest that ETV treatment combined with post-exposure "prime-boost" vaccination induced immune responses that eliminated DHBV-infected hepatocytes and prevented the development of persistent DHBV infection.

Retention of 1.2 kbp of 'novel' genomic sequence in two European field isolates and some vaccine strains of Fowlpox virus extends open reading frame fpv241

The emergence of variant fowlpox viruses (FWPVs) and increasing field use of recombinants against avian influenza H5N1 emphasize the need to monitor vaccines and to distinguish them from field strains. Five commercial vaccines, two laboratory viruses and two European field isolates were characterized by PCR and sequencing at 18 loci differing between attenuated FP9 and its pathogenic progenitor. PCR failed to discriminate between the viruses and sequence determination revealed no significant differences at any locus, except for a polymorphic locus encompassed by deletion 24 (9.3 kbp) in FP9. Surprisingly, 'novel' previously unreported sequence (spanning 1.2 kbp) was found in both European field isolates and three of the vaccines. It was absent from the other two vaccines, removed by a 1.2 kbp deletion identical to that surprisingly also observed in the completely sequenced genome of FPV USDA. This locus (H9) adds a potentially useful tool for discriminating between FWPV field isolates and vaccines.

Different levels of immunogenicity of two strains of Fowlpox virus as recombinant vaccine vectors eliciting T-cell responses in heterologous prime-boost vaccination strategies

The FP9 strain of F has been described as a more immunogenic recombinant vaccine vector than the Webster FPV-M (FPW) strain (R. J. Anderson et al., J. Immunol. 172:3094-3100, 2004). This study expands the comparison to include two separate recombinant antigens and multiple, rather than single, independent viral clones derived from the two strains. Dual-poxvirus heterologous prime-boost vaccination regimens using individual clones of recombinant FP9 or FPW in combination with recombinant modified V Ankara expressing the same antigen were evaluated for their ability to elicit T-cell responses against recombinant antigens from Plasmodium berghei (circumsporozoite protein) or human immunodeficiency virus type 1 (a Gag-Pol-Nef fusion protein). Gamma interferon enzyme-linked immunospot assay and fluorescence-activated cell sorting assays of the responses to specific epitopes confirmed the approximately twofold-greater cellular immunogenicity of FP9 compared to FPW, when given as the priming or boosting immunization. Equality of transgene expression in mouse cells infected with the two strains in vitro was verified by Western blotting. Directed partial sequence analysis and PCR analysis of FPW and comparison to available whole-genome sequences revealed that many loci that are mutated in the highly attenuated and culture-adapted FP9 strain are wild type in FPW, including the seven multikilobase deletions. These "passage-specific" alterations are hypothesized to be involved in determining the immunogenicity of fowlpox virus as a recombinant vaccine vector.

Evaluation of immune effects of fowlpox vaccine strains and field isolates

The immune effects of fowlpox virus (FPV) field isolates and vaccine strains were evaluated in chickens infected at the age of 1 day and 6 weeks. The field isolates and the obsolete vaccine strain (FPV S) contained integrated reticuloendotheliosis virus (REV) provirus, while the current vaccine strain (FPVST) carries only REV LTR sequences. An indirect antibody ELISA was used to measure the FPV-specific antibody response. The non-specific humoral response was evaluated by injection of two T-cell-dependent antigens, sheep red blood cells (SRBC) and bovine serum albumin (BSA). There was no significant difference in the antibody response to FPV between chickens infected with FPV various isolates and strains at either age. In contrast, antibody responses to both SRBC and BSA were significantly lower in 1-day-old chickens inoculated with field isolates and FPV S at 2-3 weeks post-inoculation. Furthermore, cell-mediated immune (CMI) responses measured by in vitro lymphocyte proliferation assay and in vivo using a PHA-P skin test were significantly depressed in chickens inoculated with field isolates and FPV S at the same periods. In addition, thymus and bursal weights were lower in infected chickens. These immunosuppressive effects were not observed in chickens inoculated with the current vaccine strain, FPVST, at any time. The results of this study suggest that virulent field isolates and FPV S have immunosuppressive effects when inoculated into young chickens, which appeared in the first 3 weeks post infection. REV integrated in the FPV field isolates and FPV S may have played a central role in the development of immunosuppression.

Use of restriction fragment length polymorphism, immunoblotting, and polymerase chain reaction in the differentiation of avian poxviruses

Restriction deoxyribonucleic acid (DNA) fragment profile analysis coupled with immunogenic protein profile analysis has provided useful information in determining the differences between vaccine strains and field isolates of fowlpox virus (FPV). The DNA of strains examined in this study clearly fell into 3 minor groups of restriction patterns similar but distinct from one another: restriction patterns exhibited by the vaccine strains except 1 vaccine strain, Vac-82; restriction profiles indicated by Vac-82 and field isolates FI-38 and FI-42; and restriction patterns indicated by field isolates FI-43, FI-51, FI-54, and FI-56. Furthermore, when the strains were analyzed and compared by immunoblotting analysis, they showed group differences similar to the differences in restriction profiles. Both techniques provided high sensitivity in verifying differences between vaccine strains and field isolates of FPV. The disparity found in restriction fragments or immunogenic protein profile between vaccine strains and field isolates does not exclude the appreciable high degree of DNA sequence conservation and homology. However, the minor disparity observed in these strains suggests a molecular basis for why vaccinated commercial flocks could have continually been infected by variant strains of FPV. A rapid and sensitive polymerase chain reaction method, which amplified a product from the 4b core protein gene of the FPV genome, was developed for identification and differentiation of members of the genus Avipoxvirus. Whereas total DNA from either vaccine strains or field isolates was used as template for amplifying a predicted product of 578 or 1409 bp, only cleavage of the amplified product (1409 bp) represented an additional detection technique for species differentiation. An attempt to distinguish between strains on the basis of amplification product was partially successful.

Presence of avipoxvirus DNA in avian dermal squamous cell carcinoma

Dermal squamous cell carcinoma (DSCC; avian keratoacanthoma) is a neoplastic skin lesion of broiler chickens of unknown aetiology. In previous studies, the possibility of the involvement of pox viruses in the cause of DSCC was considered. In this work, a sensitive and specific nested polymerase chain reaction (PCR) protocol was developed that could amplify a 419 base pair DNA fragment of fowlpox virus with a detection limit of less than one infectious unit. Fowlpox virus DNA was always detected in skin samples with fowlpox lesions while it was not detected in samples of unrelated diseases such as cowpox, Marek's disease or infectious laryngotracheitis. Some macroscopically normal skin samples from vaccinated and non-vaccinated birds also produced PCR-positive results, corroborating previous studies on the possibility that a latent or chronic form of fowlpox occurs. Fowlpox virus DNA was consistently detected from DSCC skin lesions, and this finding is discussed.

Development of a monoclonal blocking ELISA for the detection of antibodies against fowlpox virus

To provide a fast and easy method to detect antibodies against fowlpox virus (FWPV) particularly in high numbers of chicken sera we established a monoclonal blocking enzyme-linked immunosorbent assay (ELISA). We chose two different monoclonal antibodies (mAb), anti-FWPV 3D9/2B3 and anti-FWPV 8F3/2E11, which are both directed against the 39-kDa protein of FWPV strain HP-1. The blocking ELISA depends on the blocking of mAb binding to solid-phase antigen in the presence of positive serum. For an epidemiological study a total of 184 serum samples from Gambian chicken flocks were analysed against each of the mAbs. Four of the sera were shown to contain FWPV antibodies. These four sera showed a positive cut-off value of more than 50% inhibition exclusively in the test against the mAb anti-FWPV 8F3/2E11. This phenomenon can be explained by the binding of the mAbs to distinct epitopes on the same protein.

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Field and vaccine strains of fowlpox virus carry integrated sequences from the avian retrovirus, reticuloendotheliosis virus

For baculoviruses and herpesviruses, integration of transposons or retroviruses into the virus genome has been documented. We report here that field and vaccine strains of fowlpox virus (FPV) carry integrated sequences from the avian retrovirus, reticuloendotheliosis virus (REV). Using PCR and hybridization analysis we observed that vaccine and field strains of FPV carry REV sequences integrated into a previously uncharacterized region of the right 1/3 of the FPV genome. Long-range PCR, hybridization, and nucleotide sequence determination demonstrated that one vaccine strain (FPV S) and recently isolated field strains carry a near-full-length REV provirus. For another vaccine strain (FPV M) a rearranged remnant of the LTR was found at the same insertion site. By Western blotting and reverse transcriptase assays we were unable to demonstrate free REV in supernatants of FPV S cultures. The near-full-length REV provirus integrated into the FPV genome is infectious since FPV S DNA gave rise to REV upon transfection into chicken embryo fibroblasts. Upon infection of chickens with FPV S, all chickens developed high-titered antibodies to REV, and REV was isolated from the blood of half of the inoculated chickens. Our observations add to the list of targets for retrovirus integration into DNA virus genomes. The integration of a near-full-length, and apparently infectious, REV provirus into FPV provides additional transmission routes for the retrovirus by way of the infectious cycle of FPV, including the possibility of mechanical transmission by biting insects since FPV is believed to be transmitted by this route. For large DNA viruses, including the poxviruses, retrovirus integration with attendant possibilities of gene transduction may be an important mechanism for virus evolution, including the acquisition of cellular genes with the potential to modify virus virulence and pathogenicity.