Effects of challenge with very virulent infectious bursal disease virus reassortants in commercial chickens

Differential immunomodulatory effect of vitamin D (1,25 (OH)2 D3) on the innate immune response in different types of cells infected in vitro with infectious bursal disease virus

It has been demonstrated that vitamin D (Vit D) included in diets offers a beneficial effect by improving innate immune responses in chickens. However, its mechanisms of action and the effect on immunosuppressive pathogens, such as infectious bursal disease virus, are not yet known. In the present study, we have studied the immunomodulatory effect of Vit D on the innate immune response in 3 cell lines: fibroblast cells (DF-1), macrophages (HD11), and B cells (DT-40) infected with IBDV (intermediate vaccine) at 2 multiplicity of infections (MOI) (1 and 0.1). Genes associated with innate immune responses (TLR-3, TLR-21, MDA-5, MyD88, TRIF, IRF-7, INF-α, INF-β, PKR, OAS, viperin, IL-1β, IL-6, and IL-12) were evaluated at different time points (3, 6, 12, 24, and 36 h after infection, h.p.i). Virus production reached a maximum at 24 h.p.i., which was significantly (P < 0.05) higher in DF-1 cells, followed by HD-11 and DT-40 cells. Mainly in HD-11 cells, there was a significant (P < 0.05) effect of Vit D supplementation on receptors TLR-3, TLR-21, and MDA-5 after 12 h.p.i, independent of MOI. DT-40 cells showed the highest antiviral activity, with a significant (P < 0.05) effect on IRF-7, IFN-β, OAS, and PKR gene expression, where expression of IRF-7 and IFN-β correlated positively with Vit D supplementation, while OAS and PKR were independent of Vit D. Proinflammatory cytokines were significantly (P < 0.05) upregulated and found to be Vit D and MOI dependent. In conclusion, this study demonstrated the capacity of IBDV to trigger a strong innate immune response in chicken cells and contributes to the understanding of the activation pathways of innate immunity induced by IBDV and further shows the benefitial effect of Vit D supplementation as an immunomodulator.

Alleviation of infectious-bursal-disease-virus-induced bursal injury by betaine is associated with DNA methylation in IL-6 and interferon regulatory factor 7 promoter

Infectious bursal disease virus (IBDV) often infects young chickens and causes severe immunosuppression and inflammatory injury. Betaine is an antiviral and anti-inflammatory ingredient that may exert functions through epigenetic regulation. However, the effects of betaine on an IBDV-induced bursal injury and their underlying mechanisms have not been investigated. In this study, betaine was supplemented to the drinking water of newly hatched commercial broilers for 3 wk. Afterward, the chickens were infected with the IBDV. After 5 D of infection, the bursal lesions were examined. The mRNA expression levels of IBDV VP2 gene, pro-inflammatory cytokines, and interferons were detected. Furthermore, the 5-methylcytosine level of the CpG island in the promoter region of IL-6 and interferon regulatory factor 7 (IRF7) were determined. The IBDV induced the depletion of lymphocytes and inflammation in the bursal follicles. IBDV infection considerably elevated the mRNA levels of VP2, IL-6, types I (IFNα and IFNβ) and II (IFNγ) interferons, and IRF7. The CpG island methylation in the promoter regions of IL-6 and IRF7 were substantially decreased after IBDV infection. Betaine administration attenuated the IBDV-induced bursal lesions. Meanwhile, the IBDV-induced mRNA expression levels of IL-6, IFNβ, and IRF7 were suppressed by betaine consumption. Furthermore, the hypomethylation effects of IBDV infection to the promoter regions of IL-6 and IRF7 genes were eliminated and relieved by betaine administration. Our results indicated that the IBDV-induced expression levels of IL-6 and IRF7 genes are associated with the suppression of methylation in the promoter region. Betaine administration through drinking water may alleviate the IBDV-induced bursal injury via epigenetic regulation.

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.

gga-miR-142-5p attenuates IRF7 signaling and promotes replication of IBDV by directly targeting the chMDA5's 3' untranslated region

Chicken melanoma differentiation-associated gene 5 (chMDA5) is a key pattern recognition receptor (PRR) that recognizes RNA viral infections and initiates an antiviral innate immune response in chickens. MicroRNAs (miRNAs) are involved in the regulation of chMDA5 to sense RNA virus infection, but how it exerts antiviral activity against infectious bursal disease virus (IBDV) infection and regulates chMDA5 in chicken cells is unclear. Thus, we measured the expression of chMDA5 in IBDV-infected DT40 cells and found it significantly increased. Overexpression of chMDA5 activated the IFN-β and Mx promoters via IRF7-dependent pathways and inhibited replication of IBDV in DT40 cells. The opposite effect occurred after chMDA5 knockdown using siRNA. Also, gga-miR-142-5p regulated chMDA5 according to bioinformatic analysis and data from a dual-luciferase reporter system. Overexpression of gga-miR-142-5p reduced the expression of the chMDA5 protein, promoting IBDV replication, and decreased the activity of the IFN-β and Mx promoters via an IRF7-dependent pathway; however, it had no effect on the NF-κB-dependent pathway in DT40 cells. Thus, gga-miR-142-5p is a negative regulator of chMDA5 and promotes IBDV replication in DT40 cells through an IRF7-dependent pathway.

Pathogenicity of Genome Reassortant Infectious Bursal Disease Viruses in Chickens and Turkeys

Infectious bursal disease virus (IBDV) contains two genome segments (segment A/segment B) that can reassort among the viruses. Reassortant IBDVs have been identified in several countries including the United States. These reassortant viruses usually include at least one genome segment from a very virulent (vv)IBDV strain. In vivo virulence of six reassortant IBDV from the United States was assessed relative to the virulence of three frequently described IBDV pathotypes: vvIBDV (rB strain), classic virulent (cv)IBDV (STC strain), and subclinical (sc)IBDV (Del-E strain). Morbidity and mortality in 4-wk-old specific-pathogen-free (SPF) leghorns indicated that reassortant IBDV with a vv genome segment A and non-vv segment B were less pathogenic than the vv/vv rB strain but more pathogenic than the cv/cv STC strain. The sc/vv IBDV strain D6337 (sc/vv) was comparable to the STC strain in pathogenicity. Viruses with a serotype 2 (ser2) genome segment A, regardless of the type of genome segment B, did not cause clinical disease in SPF chickens or turkeys. None of the reassorted viruses caused morbidity, mortality, or gross lesions in SPF turkeys. Histopathologic lesions in the bursa of turkeys were not observed in any group except those challenged with the serotype 2 OH strain, which had a mild lymphocytic depletion. No mortality was observed in maternally immune broilers inoculated with any of the IBDV pathotypes at 1, 2, 3, and 4 wk of age. No bursal lesions were observed in any of the broiler chicken groups at 1 wk of age except for the D2712 (ser2/cv)-inoculated birds that had mild lymphocyte depletion. Based on evaluation of bursal lesion scores and IBDV reverse transcriptase-PCR on broilers challenged at 2 wk of age, the K669 (vv/ser2) virus broke through the maternal immunity while the STC, Del-E, rB, D2712 (ser2/cv), and 7741 (vv/cv) viruses did not. All viruses broke through maternal immunity in the broilers at 3 wk of age except the Del-E scIBDV and D2712 (ser2/cv) reassortant IBDVs. At 4 wk of age, maternal antibodies were very low and bursal lesions were observed in all broilers challenged with the viruses. The data indicate that genome reassortant IBDVs are less pathogenic than is the rB (vv/vv) IBDV. However, the reassortant viruses with a vv genome segment A can still cause morbidity and mortality in SPF chickens, and they were able to break through maternal immunity produced via use of commercial classic and variant vaccines at an early age. This suggests that current breeder vaccination programs may not adequately protect against the reassortant vv/ser2 and vv/cv IBDV strains.

Comparison of infectious bursal disease live vaccines and a HVT-IBD vector vaccine and their effects on the immune system of commercial layer pullets

Infectious bursal disease (IBD) is an economically important disease affecting poultry production worldwide. Previous experimental studies indicated that IBD live vaccination may induce transient immunosuppression, leading to suboptimal vaccine responses and therefore insufficient protection against other pathogens. Layer pullets are commonly not only vaccinated against IBD within their rearing period, but also against a variety of other pathogens. Therefore, it is of interest to investigate the effects of different IBD vaccination regimes on conventionally applied vaccines against other pathogens, and possible protection against widely spread very virulent IBD-virus (vvIBDV). A commercially available Herpesvirus of turkey vector vaccine (vHVT-IBD) expressing viral protein 2 of IBDV, and two IBD live vaccines were compared in commercial pullets for their effects on circulating B cell numbers, the ability of vaccinated birds to mount a humoral immune response against different antigens as well as their ability to induce protection against vvIBDV challenge. The results of this study demonstrate a clear immunosuppressive effect of the intermediate plus IBD live vaccine on the humoral branch of the immune system. On the other hand, no detectable effects of vHVT-IBD vaccination on these parameters were observed. All tested IBD vaccines protected against clinical IBD, although none induced sterile immunity in commercial layer pullets. vHVT-IBD-vaccinated birds showed significantly less lesions after vvIBDV challenge than IBD live-vaccinated or non-vaccinated birds (P < 0.05). Therefore, vHVT-IBD may be a suitable alternative to conventional IBD live vaccines, and may be applied even in the presence of maternally derived IBD antibodies without induction of detectable humoral immunosuppression.

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.

Screening scFv antibodies against infectious bursal disease virus by co-expression of antigen and antibody in the bacteria display system

We have previously reported an antigen and antibody co-expression (AAC) technology to demonstrate the interaction between a known antigen and antibody. To validate the co-expression system for screening antibody libraries, a single chain fragment variable(scFv)antibody library was constructed from chickens immunized with the VP2 protein of infectious bursal disease virus (IBDV). The genes of both VP2 and scFv antibodies were inserted into the pBFD-Ab-Ag vector. The co-expression library was subjected to three rounds of screening by flow cytometry (FCM) using a polyclonal antibody against VP2 through a bacteria display system. We achieved enrichment of scFv specific for IBDV. 110 individual clones were initially selected and sequenced. Twenty clones were selected based on fluorescence intensity by FCM. The scFv antibodies were expressed by pET-27b in E.coli and purified. The specificity and affinity of the selected scFv antibodies were confirmed by western blotting assay and ELISA analysis. What's more, the neutralizing capacity was measured with IBDV-B87(100 TCID₅₀) in vitro. Four scFvs (clone 8(1), Y8, L10 and L7) showed significant neutralizing capacity. Two of the four scFvs (clone 8(1) and Y8) demonstrated a higher neutralizing activity to IBDV-B87 and the titers were 16,384 and 8,192, respectively. The two scFvs had higher neutralizing capacity than those obtained in previous studies. We demonstrated that the AAC technology could be applied to screen antibody libraries without baiting antigen to make antibody screening process easier and obtain scFvs with higher neutralizing capacity.