Pivotal role of ChIFNgamma in the pathogenesis and immunosuppression of infectious bursal disease

Glycoprotein Production by Bursal Secretory Dendritic Cells in Normal, Vaccinated, and Infectious Bursal Disease Virus (IBDV)-Infected Chickens

The aim of this study is to follow the gp production in IBDV-vaccinated and challenged birds. The progress of IBDV infection was monitored using anti-VP2 immunocytochemistry, light and transmission electron microscopy. In the medulla of the bursal follicle, the Movat pentachrome staining discovered an extracellular glycoprotein (gp) produced by bursal secretory dendritic cells (BSDCs). The secretory granules of BSDCs either discharge resulting in extracellular gp or fuse together forming intracellular corpuscles. The double fate of granules suggests a dual function of BSDCs: (a.) For the discharged granules, gp contributes to the medullary microenvironment (ME). (b.) The intracellular corpuscles may be the sign of BSDC transformation to a macrophage-like cell (Mal). Infectious bursal disease virus (IBDV) infection accelerates the BSDC transformation to Mal. The decreased number of BSDCs is feedback for the precursor cells of BSDCs lodging in the cortico-medullary epithelial arches (CMEA), where they proliferate. Opening the CMEA, the precursor cells enter the medulla, and differentiate to immature BSDCs. The virus uptake in the corpuscles prevents the granular discharge resulting in the absence of gp and alteration in ME. In vaccine-take birds, the mitotic rate of BSDC precursor cells cannot restore the precursor pool; therefore, in the case of IBDV challenge, the number of newly formed BSDCs is too low for outbreak of clinical disease. The BSDCs, as a primary target of IBDV, may contribute to the long-lasting immunosuppressive status of IBDV-infected chickens.

Infectious bursal disease virus' interferences with host immune cells: What do we know?

AbstractInfectious bursal disease virus (IBDV) induces one of the most important immunosuppressive diseases in chickens leading to high economic losses due increased mortality and condemnation rates, secondary infections and the need for antibiotic treatment. Over 400 publications have been listed in PubMed.gov in the last five years pointing out the research interest in this disease and the development of improved preventive measures. While B cells are the main target cells of the virus, also other immune and non-immune cell populations are affected leading a multifaceted impact on the normally well orchestrated immune system in IBDV-infected birds. Recent studies clearly revealed the contribution of innate immune cells as well as T cells to a cytokine storm and subsequent death of affected birds in the acute phase of the disease. Transcriptomics identified differential regulation of immune related genes between different chicken genotypes as well as virus strains, which may be associated with a variable disease outcome. The recent availability of primary B cell culture systems allowed a closer look into virus-host interactions during IBDV-infection. The new emerging field of research with transgenic chickens will open up new opportunities to understand the impact of IBDV on the host also under in vivo conditions, which will help to understand the complex virus-host interactions further.

Comparative Pathogenicity of Malaysian variant and very virulent infectious bursal disease viruses in chickens

Variant infectious bursal disease virus (vaIBDV) has been identified in various countries with significant economic losses. Recently, the first identification of variant strain in Malaysia was reported. The pathogenicity of the Malaysian variant, UPM1432/2019 and very virulent infectious bursal disease virus (vvIBDV), UPM1056/2018 strains were comparatively evaluated in specific-pathogen-free (SPF) chickens based on gross and histopathological examinations and viral load. Four-week-old SPF chickens were randomly divided into 3 groups; Group 1 served as the control, while groups 2 and 3 birds were challenged with the vaIBDV and vvIBDV, respectively. Three birds from each group were weighed, euthanised and necropsied at 2-, 3-, 4-, 5-, 7- and 21-days post-challenge (dpc). Unlike UPM1056/2018 group, birds from UPM1432/2019 group did not show clinical signs or death. UPM1056/2018 strain caused 11% mortality rate in the infected chickens. The bursal body index (BBIX) for UPM1432/2019- and UPM1056/2018 groups was < 0.7 from 2 dpc and continued to decrease to 0.49 and 0.45, respectively at 21 dpc. UPM1432/2019 strain was more persistent in the bursa than UPM1056/2018 strain. Both strains induced similar pathological lesions in SPF chicks. These results indicate that the Malaysian vaIBDV severely damaged the immune organs of chickens and was more persistent in bursal tissue than vvIBDV. The study provides insight into the pathogenicity of the variant strain as further study may be required to evaluate the efficacy of the current available IBD vaccines in Malaysia against the strain.

Very virulent infectious bursal disease virus-induced immune injury is involved in inflammation, apoptosis, and inflammatory cytokines imbalance in the bursa of fabricius

Infectious bursal disease virus (IBDV) can cause a highly contagious disease in young chickens, resulting in bursal necrosis that causes severe damage to the immune system. The effects of various IBDV strains on the bursa of Fabricius (BF) have been extensively studied; however, few studies have investigated the effects of IBDV strain LJ-5, a newly discovered very virulent IBDV (vvIBDV), infection on young chicken BF. In this study, three-week-old specific pathogen-free (SPF) chickens were infected with vvIBDV for one to five days. LJ-5 decreased the bursa index, B lymphocyte viability and immunoglobulin (Ig) levels, including IgM and IgA in the bursa and IgY in the sera. Histopathological analysis revealed necrosis and depletion of the lymphoid cells and complete loss of bursal architecture in the BF, and transmission electron microscopy revealed mitochondrial vacuoles, cristae breaks, and nuclear damage in vvIBDV-infected bursa tissue. The number of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling-positive nuclei significantly increased following IBDV infection. Cytokine levels increased in the bursa after IBDV infection, promoting inflammation and causing an inflammatory imbalance. Apoptotic gene expression confirmed that vvIBDV infection promotes the apoptosis of bursal cells. These results suggest that vvIBDV infection attenuate immune responses by reducing B lymphocyte activity of secretion Ig in the bursa or sera and triggers inflammation, apoptosis, and an imbalance of inflammatory cytokines in the BF, resulting in immune injury in SPF chickens, which offered basic data for further study of vvIBDV pathogenesis.

Current knowledge about interactions between avian dendritic cells and poultry pathogens

In poultry production conditions today, birds are surrounded by viral, bacterial, and parasitic agents. DCs are the main antigen-presenting cells located in tissues of the body, and their role involves recognizing antigen structures, engulfing and processing them, and subsequently presenting antigen peptides on their surface by major histocompatibility complex, where T cells and B cells are stimulated and can begin appropriate cellular and antibody immune response. This unique function indicates that these cells can be used in producing vaccines, but first it is necessary to culture DCs in vitro to identify the principles of their interactions with pathogens. The following review summarizes our current knowledge about avian dendritic cells and their interactions with pathogens. It provides a basis for future studies of these unique cells and their use in vaccine development.

Genotype-associated differences in bursal recovery after infectious bursal disease virus (IBDV) inoculation

T-cell immune responses were shown to play an important role in the regulation of infectious bursal disease virus (IBDV) replication and development of lesions in the bursa of Fabricius (BF) (bursal lesions) but also in the recovery from the infection. Studies suggested that the host-genotype influences T-cell responses during the acute phase of infection. Genotype-related differences in the recovery phase were not investigated so far. The present study used commercial broiler- (BT), layer- (LT), dual-purpose type (DT) chicken lines as well as a specific pathogen free (SPF) LT chicken as a reference for comparison of T-cell related differences in IBDV-immunopathogenesis not only in the early phase post inoculation (pi) but also in the recovery phase. The Deventer formula was used to determine the optimal time point of inoculation with an intermediate plus IBDV strain when maternally derived antibody (MDA) titers were below the calculated breakthrough level of the virus for all genotypes. Differences in the bursal lesion development, intrabursal CD4+ and CD8+ T-cell accumulation and numbers of IBDV-positive cells were determined. In addition, anti-IBDV antibody development and the relative amount of anti-inflammatory cytokine mRNA were recorded until 28 days post IBDV inoculation. Differences between the genotypes were observed in the duration and magnitude of bursal lesions, CD4+ and CD8+ T-cell infiltration as well as the presence of anti-inflammatory Interleukin (IL)-10 and Transforming growth factor (TGF) β4 cytokine mRNA (P < 0.05). While the investigated immune parameters were comparable between the genotypes at seven days pi, during 14, 21 and 28 days pi a delayed recovery process in LT and DT chickens compared to BT chickens was observed (P < 0.05). Furthermore, the age and residual MDA levels had a genotype-dependent influence on the onset of the anti-IBDV specific humoral and T-cell mediated immune responses. This study suggests, that the impact of T-cell immunity on the recovery process after IBDV infection may need to be considered further for the development of new breeding programs for disease resistant chicken lines.

Immune responses upon in ovo HVT-IBD vaccination vary between different chicken lines

The genotype of chickens is assumed to be associated with variable immune responses. In this study a modern, moderate performing dual-purpose chicken line (DT) was compared with a high-performing layer-type (LT) as well as a broiler-type (BT) chicken line. One group of each genotype was vaccinated in ovo with a recombinant herpesvirus of turkeys expressing the virus protein VP2 of the infectious bursal disease virus (HVT-IBD) while one group of each genotype was left HVT-IBD unvaccinated (control group). Genotype associated differences in innate and adapted immune responses between the groups were determined over five weeks post hatch. HVT-IBD vaccination significantly enhanced humoral immune responses against subsequently applied live vaccines compared to non-HVT-IBD vaccinated groups at some of the investigated time points (P < 0.05). In addition HVT-IBD vaccination had depending on the genotype a significant impact on splenic macrophage as well as bursal CD4⁺ T-cell numbers (P < 0.05). On the other hand, the detectable genotype influence on Interferon (IFN) γ and nitric oxide (NO) release of ex vivo stimulated spleen cells was independent of HVT-IBD vaccination. The results of our study suggest considering a genotype specific vaccination regime in the field.

Virus-Induced Immunosuppression in Chickens

A healthy immune system is a cornerstone for poultry production. Any factor diminishing the immune responses will affect production parameters and increase cost. There are numerous factors, infectious and noninfectious, causing immunosuppression (IS) in chickens. This paper reviews the three viral diseases that most commonly induce IS or subclinical IS in chickens: Marek's disease virus (MDV), chicken infectious anemia virus (CIAV), and infectious bursal disease virus (IBDV), as well as the interactions among them. MDV-induced IS (MDV-IS) affects both humoral and cellular immune responses. It is very complex, poorly understood, and in many cases underdiagnosed. Vaccination protects against some but not all aspects of MDV-IS. CIAV induces apoptosis of the hemocytoblasts resulting in anemia, hemorrhages, and increased susceptibility to bacterial infections. It also causes apoptosis of thymocytes and dividing T lymphocytes, affecting T helper functions, which are essential for antibody production and cytotoxic T lymphocyte (CTL) functions. Control of CIAV is based on vaccination of breeders and maternal antibodies (MAbs). However, subclinical IS can occur after MAbs wane. IBDV infection affects the innate immune responses during virus replication and humoral immune responses as a consequence of the destruction of B-cell populations. Vaccines with various levels of attenuation are used to control IBDV. Interactions with MAbs and residual virulence of the vaccines need to be considered when designing vaccination plans. The interaction between IBDV, CIAV, and MDV is critical although underestimated in many cases. A proper control of IBDV is a must to have proper humoral immune responses needed to control CIAV. Equally, long-term control of MDV is not possible if chickens are coinfected with CIAV, as CIAV jeopardizes CTL functions critical for MDV control.

Combination of TLR2 and TLR3 agonists derepress infectious bursal disease virus vaccine-induced immunosuppression in the chicken

Live intermediate plus infectious bursal disease virus (IBDV) vaccines (hot vaccines) are used for protection against the virulent IBDV strains in young chickens. We evaluated the potential of Toll-like receptor (TLR) agonists to alleviate hot vaccine-induced immunosuppression. The combination of Pam3CSK4 and poly I:C synergistically upregulated IFN-β, IFN-γ, IL-12, IL-4, and IL-13 transcripts and cross-inhibited IL-1β, IL-10, and iNOS transcripts in the chicken peripheral blood mononuclear cells (PBMCs) as analyzed by quantitative real-time PCR. Further, four-week old specific pathogen free White Leghorn chickens (n = 60) were randomly divided into six groups and either immunized with hot IBDV vaccine with or without Pam3CSK4 and/or poly I:C or not vaccinated to serve as controls. The results indicated that poly I:C alone and in combination with Pam3CSK4 alleviated vaccine-induced immunosuppression, as evidenced by greater weight gain, increased overall antibody responses to both sheep erythrocytes and live infectious bronchitis virus vaccine, upregulated IFN-γ transcripts and nitric oxide production by PBMCs (P < 0.05), and lower bursal lesion score in the experimental birds. In conclusion, poly I:C alone and its combination with Pam3CSK4 reduced the destruction of B cells as well as bursal damage with restoration of function of T cells and macrophages when used with a hot IBDV vaccine.

Study of the underlying mechanisms and consequences of pathogenicity differences between two in vitro selected G1-H9N2 clones originating from a single isolate

The G1-H9N2 avian influenza virus (AIV) has caused significant economic losses in the commercial poultry industry due to reduced egg production and increased mortality. The field observations have shown that H9N2 viruses circulate and naturally mix with other pathogens and these simultaneous infections can exacerbate disease. To avoid an incorrect virus characterization, due to co-infection, isolates were purified by in vitro plaque assays. Two plaque purified G1-H9N2 clones, selected on different cell types, named MDCK-and CEF-clone in regards to the cell culture used, were studied in vivo, revealing two different virulence phenotypes. Subsequently, the underlying mechanisms were studied. Specifically, the phenotypical outcome of SPF bird infection by the two clones resulted in completely different clinical outcomes. These differences in clinical outcome were used to study the factors behind this output in more detail. Further studies demonstrated that the more severe disease outcome associated with the MDCK-clone involves a strong induction of pro-inflammatory cytokines and a lack of type I interferon production, whereas the mild disease outcome associated with the CEF-clone is related to a greater antiviral cytokine response. The immunosuppressive effect of the MDCK-clone on splenocytes was further demonstrated via ChIFN-γ lack production after ex vivo mitogenic stimulation. Genome sequencing of the two clones identified only four amino acid differences including three in the HA sequence (HA-E198A, HA-R234L, HA-E502D-H9 numbering) and one in the NA sequence (NA-V33M). In the present study, valuable insights on the mechanisms responsible for AI pathogenicity and molecular mechanisms of H9N2 infections in chicken were obtained while highlighting the impact of the cells viruses are grown on their virulence.

Exacerbated Apoptosis of Cells Infected with Infectious Bursal Disease Virus upon Exposure to Interferon Alpha

Infectious bursal disease virus (IBDV) belongs to the Birnaviridae family and is the etiological agent of a highly contagious and immunosuppressive disease (IBD) that affects domestic chickens (Gallus gallus). IBD or Gumboro disease leads to high rates of morbidity and mortality of infected animals and is responsible for major economic losses to the poultry industry worldwide. IBD is characterized by a massive loss of IgM-bearing B lymphocytes and the destruction of the bursa of Fabricius. The molecular bases of IBDV pathogenicity are still poorly understood; nonetheless, an exacerbated cytokine immune response and B cell depletion due to apoptosis are considered main factors that contribute to the severity of the disease. Here we have studied the role of type I interferon (IFN) in IBDV infection. While IFN pretreatment confers protection against subsequent IBDV infection, the addition of IFN to infected cell cultures early after infection drives massive apoptotic cell death. Downregulation of double-stranded RNA (dsRNA)-dependent protein kinase (PKR), tumor necrosis factor alpha (TNF-α), or nuclear factor κB (NF-κB) expression drastically reduces the extent of apoptosis, indicating that they are critical proteins in the apoptotic response induced by IBDV upon treatment with IFN-α. Our results indicate that IBDV genomic dsRNA is a major viral factor that contributes to the triggering of apoptosis. These findings provide novel insights into the potential mechanisms of IBDV-induced immunosuppression and pathogenesis in chickens.IMPORTANCE IBDV infection represents an important threat to the poultry industry worldwide. IBDV-infected chickens develop severe immunosuppression, which renders them highly susceptible to secondary infections and unresponsive to vaccination against other pathogens. The early dysregulation of the innate immune response led by IBDV infection and the exacerbated apoptosis of B cells have been proposed as the main factors that contribute to virus-induced immunopathogenesis. Our work contributes for the first time to elucidating a potential mechanism driving the apoptotic death of IBDV-infected cells upon exposure to type I IFN. We provide solid evidence about the critical importance of PKR, TNF-α, and NF-κB in this phenomenon. The described mechanism could facilitate the early clearance of infected cells, thereby aiding in the amelioration of IBDV-induced pathogenesis, but it could also contribute to B cell depletion and immunosuppression. The balance between these two opposing effects might be dramatically affected by the genetic backgrounds of both the host and the infecting virus strain.

Detection of different genotypes of infectious bronchitis virus and of infectious bursal disease virus in European broilers during an epidemiological study in 2013 and the consequences for the diagnostic approach

To gather recent data regarding the infectious bronchitis (IB) and infectious bursal disease (IBD) situation in Europe, a large-scale field epidemiological survey using diagnostic samples has been implemented in 2013 for about six months in several European countries: France, Germany, Greece, Italy, the Netherlands, Poland, Portugal, the Republic of Ireland, Spain and the UK. In 234 flocks that were sampled, strains from 10 different IBV genotypes were detected: the 793B genotype was detected most frequently, followed by QX, Massachusetts (Mass) and the Xindadi-like strains. Strains belonging to the Q1, Ark, D274, D1466, Italy-02 and B1648 genotypes were detected as well, although less frequently. The separate sampling of tracheas and kidneys for IBV detection using reverse transcriptase PCR was very useful, as different genotypes or significant differences in sequences of the same genotype were detected between both organs. The data of this survey also provided valuable information about the replication of IBD vaccines and subsequent infectious bursal disease virus (IBDV) antibody responses under field conditions. The detection of five non-vvIBDV field strains of two different genotypes shows the presence of non-vvIBDV non-vaccine strains, which can easily be undetected in Europe due to the focus on sampling of clinically ill birds. Detection of vaccine virus in the bursa and antibody response to the IBD vaccination in flocks that had been vaccinated by the drinking water with a live attenuated vaccine compared to a vaccination in the hatchery using an immune-complex vaccine showed a delayed replication of the vaccines that had been applied by the drinking water, indicating mistakes in the timing and/or application of the vaccines.

Characterization of Chicken Splenic-Derived Dendritic Cells Following Vaccine and Very Virulent Strains of Infectious Bursal Disease Virus Infection

Studies have shown that infectious bursal disease virus (IBDV) infects lymphoid cells, mainly B cells and macrophages. This study was aimed to examine the involvement of chicken splenic-derived dendritic cells (ch-sDCs) in specific-pathogen-free chickens following inoculation with IBDV vaccine strain (D78) and a very virulent (vv) strain (UPM0081). Following IBDV infection, enriched activated ch-sDCs were collected by using the negative selection method and were examined based on morphology and immunophenotyping to confirm the isolation method for dendritic cells (DCs). The presence of IBDV on enriched activated ch-sDCs was analyzed based on the immunofluorescence antibody test (IFAT), flow cytometry, and quantitative real-time PCR (RT-qPCR) while the mRNAs of several cytokines were detected using RT-qPCR. The isolated ch-sDCs resembled typical DC morphologies found in mammals by having a veiled shape and they grew in clusters. Meanwhile, the expression of DC maturation markers, namely CD86 and MHCII, were increased at day 2 and day 3 following vvIBDV and vaccine strain inoculation, respectively, ranging from 10% to 40% compared to the control at 2.55% (P < 0.05). At day 3 postinfection, IBDV VP3 proteins colocalized with CD86 were readily detected via IFAT and flow cytometry in both vaccine and vvIBDV strains. In addition, enriched activated ch-sDCs were also detected as positive based on the VP4 gene by RT-qPCR; however, a higher viral load was detected on vvIBDV compared to the vaccine group. Infection with vaccine and vvIBDV strains induced the enriched activated ch-sDCs to produce proinflammatory cytokines and Th1-like cytokines from day 3 onward; however, the expressions were higher in the vvIBDV group (P < 0.05). These data collectively suggest that enriched activated ch-sDCs were permissive to IBDV infection and produced a strong inflammatory and Th1-like cytokine response following vvIBDV infection as compared to the vaccine strain.

Transcription profiles of the responses of chicken bursae of Fabricius to IBDV in different timing phases

Background

Infectious bursal disease virus (IBDV) infection causes immunosuppression in chickens and increases their susceptibility to secondary infections. To explore the interaction between host and IBDV, RNA-Seq was applied to analyse the transcriptional profiles of the responses of chickens’ bursas of Fabricius in the early stage of IBDV infection.

Results

The results displayed that a total of 15546 genes were identified in the chicken bursa libraries. Among the annotated genes, there were 2006 and 4668 differentially expressed genes in the infection group compared with the mock group on day 1 and day 3 post inoculation (1 and 3 dpi), respectively. Moreover, there were 676 common up-regulated and 83 common down-regulated genes in the bursae taken from the chickens infected with IBDV on both 1 and 3 dpi. Meanwhile, there were also some characteristic differentially expressed genes on 1 and 3 dpi. On day 1 after inoculation with IBDV, host responses mainly displayed immune response processes, while metabolic pathways played an important role on day three post infection. Six genes were confirmed by quantitative reverse transcription-PCR.

Conclusions

In conclusion, the differential gene expression profile demonstrated with RNA-Seq might offer a better understanding of the molecular interactions between host and IBDV during the early stage of infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-017-0757-x) contains supplementary material, which is available to authorized users.

Chemokine receptor CCR5 and CXCR4 might influence virus replication during IBDV infection

Both CCR5 and CXCR4 are important chemokine receptors and take vital role in migration, development and distribution of T cells, however, whether they will influence the process of T cell infiltration into bursa of Fabricius during infectious bursal disease virus (IBDV) infection is unclear. In the current study, CCR5 and CXCR4 antagonists, Maraviroc and AMD3100, were administrated into chickens inoculated with IBDV, and the gene levels of IBDV VP2, CCR5, CXCR4 and related cytokines were determined by real-time PCR. The results showed that large number of T cells began to migrate into the bursae on Day 3 post infection with IBDV and the mRNA of chemokine receptors CCR5 and CXCR4 began to increase on Day 1. Moreover, antagonist treatments have increased the VP2, CCR5 and CXCR4 gene transcriptions and influenced on the gene levels of IL-2, IL-6, IL-8, IFN-γ, TGF-β4, MHC-I and MDA5. In conclusion, the chemokine receptors CCR5 and CXCR4 might influence virus replication during IBDV infection and further study would focus on the interaction between chemokine receptors and their ligands.

Infectious Bursal Disease Virus-Host Interactions: Multifunctional Viral Proteins that Perform Multiple and Differing Jobs

Infectious bursal disease (IBD) is an acute, highly contagious and immunosuppressive poultry disease caused by IBD virus (IBDV). The consequent immunosuppression increases susceptibility to other infectious diseases and the risk of subsequent vaccination failure as well. Since the genome of IBDV is relatively small, it has a limited number of proteins inhibiting the cellular antiviral responses and acting as destroyers to the host defense system. Thus, these virulence factors must be multifunctional in order to complete the viral replication cycle in a host cell. Insights into the roles of these viral proteins along with their multiple cellular targets in different pathways will give rise to a rational design for safer and effective vaccines. Here we summarize the recent findings that focus on the virus-cell interactions during IBDV infection at the protein level.

In vitro characterization of chicken bone marrow-derived dendritic cells following infection with very virulent infectious bursal disease virus

Infectious bursal disease is caused by infectious bursal disease virus (IBDV), an immunosuppressive virus that targets immune cells such as B cells and macrophages. However, the involvement of dendritic cells (DCs) during IBDV infection is not well understood. In this study the in vitro effects of live and inactivated very virulent IBDV (vvIBDV) UPM0081 on bone marrow-derived DCs (BM-DC) were characterized and compared with BM-DC treated with lipopolysaccharide (LPS). Morphologically, BM-DC treated with LPS and vvIBDV showed stellate shape when compared to immature BM-DC. In addition, LPS-treated and both live and inactivated vvIBDV-infected BM-DC expressed high levels of double positive CD86 and major histocompatibility complex class II antigens (>20%). vvIBDV-infected BM-DC showed significantly higher numbers of apoptotic cells compared to LPS. Replication of vvIBDV was detected in the infected BM-DC as evidenced by the increased expression of VP3 and VP4 IBDV antigens based on flow cytometry, real-time polymerase chain reaction and immunofluorescence tests. Levels of different immune-related genes such as interleukin-1β (IL-1β), CXCLi2 (IL-8), IL-18, interferon gamma (IFN-γ, IL-12α, CCR7 and Toll-like receptor-3 (TLR3) were measured after LPS and vvIBDV treatments. However, marked differences were noticed in the onset and intensity of the gene expression between these two treatment groups. LPS was far more potent than live and inactivated vvIBDV in inducing the expression of IL-1β, IL-18 and CCR7 while expression of Th1-like cytokines, IFN-γ and IL-12α were significantly increased in the live vvIBDV treatment group. Meanwhile, the expression of TLR3 was increased in live vvIBDV-infected BM-DC as compared to control. Inactivated vvIBDV-treated BM-DC failed to stimulate IFN-γ, IL-12α and TLR3 expressions. This study suggested that BM-DC may serve as another target cells during IBDV infection which require further confirmation via in vivo studies.

Prophylactic potential of resiquimod against very virulent infectious bursal disease virus (vvIBDV) challenge in the chicken

The study evaluated the prophylactic potential of resiquimod (R-848), a synthetic TLR7 agonist, against very virulent infectious bursal disease virus (vvIBDV) infection in chicken. Specific pathogen free White Leghorn chicks of three week age were treated with R-848 (50μg/bird, intramuscular) or PBS (n=26/group). Twenty four hour later, half of the birds from each group were challenged with 10(5) ELD50 of vvIBDV and observed for 10days. To understand the effect of R-848, immune response genes such as interferon (IFN)-β, IFN-γ, IL-1β, IL-4, iNOS and TLR7 were analyzed at 24 and 48h post-challenge in PBMCs ex vivo by real-time PCR (n=6/group). On day 4 post-challenge, representative birds (n=3/group) were sacrificed to study the bursal damage and IBDV antigen clearance. Immunosuppression was assessed by antibody response against live Newcastle disease virus (NDV) vaccine, which was administered on day 10 post-challenge. R-848 pre-treatment significantly upregulated the transcripts of each immune response gene studied (P<0.05). There was 50% mortality on vvIBDV challenge in control birds, while it was only 20% with R-848 group. R-848 pre-treatment reduced the bursal damage as indicated by lower bursal lesion score in histopathology, reduced IBDV antigen signal in immunohistochemistry and improved antigen clearance in agar gel immunodiffusion test. Further, it protected significantly against vvIBDV induced immunosuppression as indicated by HI antibody titre. It is concluded that pre-treatment of R-848 conferred partial protection from mortality and bursal damage while complete protection against immunosuppression in chicken when challenged with vvIBDV, which could be due to the upregulation of immune response genes.

Cyclophilin A Interacts with Viral VP4 and Inhibits the Replication of Infectious Bursal Disease Virus

Nonstructural protein VP4, a serine protease of infectious bursal disease virus (IBDV) that catalyzes the hydrolysis of polyprotein pVP2-VP4-VP3 to form the viral proteins VP2, VP4, and VP3, is essential to the replication of IBDV. However, the interacting partners of VP4 in host cells and the effects of the interaction on the IBDV lifecycle remain incompletely elucidated. In this study, using the yeast two-hybrid system, the putative VP4-interacting partner cyclophilin A (CypA) was obtained from a chicken embryo fibroblast (CEF) expression library. CypA was further confirmed to interact with VP4 of IBDV using co-immunoprecipitation (CO-IP), GST pull-down, and confocal microscopy assays. Moreover, we found that the overexpression of CypA suppressed IBDV replication, whereas the knock-down of CypA by small interfering RNAs promoted the replication of IBDV. Taken together, our findings indicate that the host cell protein CypA interacts with viral VP4 and inhibits the replication of IBDV.

Avian Immunosuppressive Diseases and Immunoevasion

Subclinical immunosuppression in chickens is an important but often underestimated factor in the subsequent development of clinical disease. Immunosuppression can be caused by pathogens such as chicken infectious anemia virus, infectious bursal disease virus, reovirus, and some retroviruses (e.g., reticuloendotheliosis virus). Mycotoxins and stress, often caused by poor management practices, can also cause immunosuppression. The effects on the innate and acquired immune responses and the mechanisms by which mycotoxins, stress and infectious agents cause immunosuppression are discussed. Immunoevasion is a common ploy by which viruses neutralize or evade immune responses. DNA viruses such as herpesvirus and poxvirus have multiple genes, some of them host-derived, which interfere with effective innate or acquired immune responses. RNA viruses may escape acquired humoral and cellular immune responses by mutations in protective antigenic epitopes (e.g., avian influenza viruses), while accessory non-structural proteins or multi-functional structural proteins interfere with the interferon system (e.g., Newcastle disease virus).

In vitro rapid clearance of infectious bursal disease virus in peripheral blood mononuclear cells of chicken lines divergent for antibody response might be related to the enhanced expression of proinflammatory cytokines

Infectious bursal disease (IBD) is an acute and highly contagious viral disease of young chickens caused by infectious bursal disease virus (IBDV). An effective way to control IBDV would be to breed chickens with a reduced susceptibility to IBDV infection. In the present work, we used chickens selected for high and low specific responses to sheep red blood cells (SRBC) (H and L, respectively) to assess the susceptibility of differential immune competent animals to IBDV infection. The peripheral blood mononuclear cells (PBMCs) of high SRBC line (HL) and low SRBC line (LL) were infected with IBDV and viral RNA loads were determined at different time post-IBDV infection. Chicken orthologues of the T helper 1 (Th1) cytokines, interferon-γ (IFN-γ) and interleukin-2 (IL-2); a Th2 cytokine, IL-10; a pro inflammatory cytokine, IL-6; the CCL chemokines, chCCLi2, chCCLi4 and chCCLi7; colony stimulating factor, GM-CSF; and a anti-inflammatory cytokine, transforming growth factor β-2 (TGFβ-2) were quantified. The expression of chCCLi2, chCCLi4 and chCCLi7 was significantly higher in L line as compared to H line. However, in H line the viral RNA loads were significantly lower than in L line. Therefore, the upregulated chemokines might be associated with the susceptibility to IBDV. The expression of IFN-γ, IL-2 and IL-6 was significantly higher in H line as compared to L line. We assume that the higher proinflammatory cytokines expression in H line might be related to the rapid clearance of virus from PBMCs. Significantly higher levels of IL-10 and TGFβ-2 mRNAs in L line might be related to the pathogenesis of IBDV. In conclusion, selection for antibody responses appears to influence the expression profiles of chemokines and cytokines against IBDV. Further, the selection for high SRBC response might improve the immuno-competence of chickens against IBDV.

Infectious Bursal Disease: a complex host-pathogen interaction

Infectious Bursal Disease (IBD) is caused by a small, non-enveloped virus, highly resistant in the outside environment. Infectious Bursal Disease Virus (IBDV) targets the chicken's immune system in a very comprehensive and complex manner by destroying B lymphocytes, attracting T cells and activating macrophages. As an RNA virus, IBDV has a high mutation rate and may thus give rise to viruses with a modified antigenicity or increased virulence, as emphasized during the last decades. The molecular basis of pathogenicity and the exact cause of clinical disease and death are still poorly understood, as it is not clearly related to the severity of the lesions and the extent of the bursal damage. Recent works however, pointed out the role of an exacerbated innate immune response during the early stage of the infection with upregulated production of promediators that will induce a cytokine storm. In the case of IBDV, immunosuppression is both a direct consequence of the infection of specific target immune cells and an indirect consequence of the interactions occurring in the immune network of the host. Recovery from disease or subclinical infection will be followed by immunosuppression with more serious consequences if the strain is very virulent and infection occurs early in life. Although the immunosuppression caused by IBDV is principally directed towards B-lymphocytes, an effect on cell-mediated immunity (CMI) has also been demonstrated therefore increasing the impact of IBDV on the immunocompetence of the chicken. In addition to its zootechnical impact and its role in the development of secondary infections, it may affect the immune response of the chicken to subsequent vaccinations, essential in all types of intensive farming. Recent progress in the field of avian immunology has allowed a better knowledge of the immunological mechanisms involved in the disease but also should give improved tools for the measurement of immunosuppression in the field situation. Although satisfactory protection may be provided by the induction of high neutralizing antibody titres, interference from parental antibodies with vaccination has become the most important obstacle in the establishment of control programs. In this context, recombinant HVT and immune complex vaccines show promising results.

Dual infections with low virulent chicken infectious anaemia virus (lvCIAV) and intermediate infectious bursal disease virus (iIBDV) in young chicks increase lvCIAV in thymus and bursa while decreasing lymphocyte disorders induced by iIBDV

The use of attenuated vaccines or the occurrence of low virulent T-lymphotropic or B-lymphotropic viruses in flocks may alter the immune responses of young chicks in spite of the absence of clinical signs. Infections with a low virulent T-lymphotropic chicken infectious anaemia virus (lvCIAV) followed by infection with an intermediate B-lymphotropic infectious bursal disease virus (iIBDV) were conducted in specific pathogen free chicks. Thirty-six 1-day-old chicks were infected with the lvCIAV strain (CAV-VAC®) and a similar number of chicks were inoculated with phosphate-buffered saline. At 14 days after lvCIAV infection, one group of 18 lvCIAV-infected chicks and one group of 18 uninfected chicks were infected with an iIBDV strain. At 4, 7 and 14 days post infection with iIBDV, six chicks from each group were euthanized and lymphoid organs were collected. Detection of lvCIAV and iIBDV genomes was conducted by polymerase chain reaction and reverse transcriptase-polymerase chain reaction, respectively. Double-labelled lymphoid subsets from the thymus, spleen and bursa were studied by cytofluorometric analysis. The results reveal that previous infection with lvCIAV increases the occurrence of the lvCIAV and iIBDV genome in thymus and/or bursa without the occurrence of clinical signs in dually lvCIAV/iIBDV-infected chicks. However, the decreases of B cells in spleen and bursa and increases of T-cell subsets in bursa observed in chicks infected with iIBDV did not occur in chicks previously infected with lvCIAV. Taken together, these results suggest that previous infection of young chicks with lvCIAV decreases lymphoid disorders induced by iIBDV while subsequent iIBDV infection increases the lvCIAV genome in lymphoid organs.

Expression of interferon gamma by a highly virulent strain of Newcastle disease virus decreases its pathogenicity in chickens

The role of interferon gamma (IFN-γ) expression during Newcastle disease virus (NDV) infection in chickens is unknown. Infection of chickens with highly virulent NDV results in rapid death, which is preceded by increased expression of IFN-γ in target tissues. IFN-γ is a cytokine that has pleiotropic biological effects including intrinsic antiviral activity and immunomodulatory effects that may increase morbidity and mortality during infections. To better understand how IFN-γ contributes to NDV pathogenesis, the coding sequence of the chicken IFN-γ gene was inserted in the genome of the virulent NDV strain ZJ1 (rZJ1-IFNγ), and the effects of high levels of IFN-γ expression during infection were determined in vivo and in vitro. IFN-γ expression did not significantly affect NDV replication in fibroblast or in macrophage cell lines. However, it affected the pathogenesis of rZJ1-IFNγ in vivo. Relative to the virus expressing the green fluorescent protein (rZJ1-GFP) or lacking the IFN-γ insert (rZJ1-rev), expression of IFN-γ by rZJ1-IFNγ produced a marked decrease of pathogenicity in 4-week-old chickens, as evidenced by lack of mortality, decreased disease severity, virus shedding, and antigen distribution. These results suggest that early expression of IFN-γ had a significant protective role against the effects of highly virulent NDV infection in chickens, and further suggests that the level and timing of expression of this cytokine may be critical for the disease outcome. This is the first description of an in vivo attenuation of a highly virulent NDV by avian cytokines, and shows the feasibility to use NDV for cytokine delivery in chicken organs. This approach may facilitate the study of the role of other avian cytokines on the pathogenesis of NDV.

Both genome segments contribute to the pathogenicity of very virulent infectious bursal disease virus

Infectious bursal disease virus (IBDV) causes an economically significant disease of chickens worldwide. Very virulent IBDV (vvIBDV) strains have emerged and induce as much as 60% mortality. The molecular basis for vvIBDV pathogenicity is not understood, and the relative contributions of the two genome segments, A and B, to this phenomenon are not known. Isolate 94432 has been shown previously to be genetically related to vvIBDVs but exhibits atypical antigenicity and does not cause mortality. Here the full-length genome of 94432 was determined, and a reverse genetics system was established. The molecular clone was rescued and exhibited the same antigenicity and reduced pathogenicity as isolate 94432. Genetically modified viruses derived from 94432, whose vvIBDV consensus nucleotide sequence was restored in segment A and/or B, were produced, and their pathogenicity was assessed in specific-pathogen-free chickens. We found that a valine (position 321) that modifies the most exposed part of the capsid protein VP2 critically modified the antigenicity and partially reduced the pathogenicity of 94432. However, a threonine (position 276) located in the finger domain of the virus polymerase (VP1) contributed even more significantly to attenuation. This threonine is partially exposed in a hydrophobic groove on the VP1 surface, suggesting possible interactions between VP1 and another, as yet unidentified molecule at this amino acid position. The restored vvIBDV-like pathogenicity was associated with increased replication and lesions in the thymus and spleen. These results demonstrate that both genome segments influence vvIBDV pathogenicity and may provide new targets for the attenuation of vvIBDVs.

Conjugated linoleic acids alleviate the immunosuppression of peripheral blood T lymphocytes in broiler chickens exposed to cyclosporin A

The immunoregulatory actions of conjugated linoleic acids (CLA) of relevance immunosuppression were investigated. To test the hypothesis that CLA ameliorate immunosuppression, we developed the immunosuppressive model of peripheral blood T lymphocytes in broiler chickens induced by cyclosporin A. Peripheral blood T lymphocytes of broiler chickens were cultured with media containing various concentrations (25, 50, 100, and 200 μmol/L) of c9, t11-CLA and t10, c12-CLA to investigate the effects of CLA isomers on peripheral blood T lymphocyte proliferation, interleukin-2, the activity of phospholipase C, and protein kinase C production. Results suggested that CLA alleviated the immunosuppression of T lymphocytes in broiler chickens exposed to cyclosporin A through increasing of peripheral blood T lymphocyte proliferation and interleukin-2. The 2 CLA isomers enhanced T lymphocyte proliferation at low concentration and inhibited T lymphocyte proliferation at high concentration. In addition, the effect of c9, t11-CLA was better than that of t10, c12-CLA. At the cellular level, the effects of CLA on the alleviation of immunosuppression in T lymphocytes are mainly attributable to increasing the signaling molecules, such as phospholipase C and protein kinase C.

Differences in genetic background influence the induction of innate and acquired immune responses in chickens depending on the virulence of the infecting infectious bursal disease virus (IBDV) strain

Previous studies and field observations have suggested that genetic background influences infectious bursal disease virus (IBDV) pathogenesis. However, the influence of the virulence of the infecting IBDV strain and the mechanisms underlying the differences in susceptibility are not known. In the present study IBDV pathogenesis was compared between specific-pathogen-free layer-type (LT) chickens, which are the most susceptible chicken for IBDV and have been used as the model for pathogenesis studies, and broiler-type (BT) chickens, which are known to be less susceptible to clinical infectious bursal disease (IBD). The innate and acquired immune responses were investigated after inoculation of an intermediate (i), virulent (v) or very virulent (vv) strain of IBDV. IBDV pathogenesis was comparable among genetic backgrounds after infection with iIBDV. After infection with vIBDV and vvIBDV, LT birds showed severe clinical disease and mortality, higher bursal lesion scores and IBDV-antigen load relative to BT birds. Circulating cytokine induction varied significantly in both timing and quantity between LT and BT birds and among virus strains (P<0.05). Evaluation of different immune cell populations by flow-cytometric analysis in the bursa of Fabricius provided circumstantial evidence of a stronger local T cell response in BT birds vs. LT birds after infection with the virulent strain. On the other hand, LT birds showed a more significant increase in circulating macrophage-derived immune mediators such as total interferon (IFN) and serum nitrite than BT birds on days 2 and 3 post-vIBDV infection (P<0.05). Stronger stimulation of innate immune reactions especially after vIBDV infection in the early phase may lead to faster and more severe lesion development accompanied by clinical disease and death in LT chickens relative to BT chickens. Interestingly, no significant differences were seen between genetic backgrounds in induction of the IBDV-specific humoral response: timing of IBDV-antibody induction and antibody levels were comparable between BT and LT birds. This study clearly demonstrates a significant influence of chickens' genetic background on disease outcome. The difference between backgrounds in IBDV susceptibility is further influenced by the virulence of the infecting virus strain.