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

• Prophylactic role of resiquimod (R-848) against vvIBDV was evaluated in chicken.
• R-848 apparently reduced the mortality and bursal damage.
• IBDV antigen clearance from the bursa was evident in R-848 pre-treated birds.
• R-848 conferred complete protection against the vvIBDV induced immunosuppression.
• R-848 significantly induced the transcripts of immune response genes post-challenge.

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 (50 μg/bird, intramuscular) or PBS (n = 26/group). Twenty four hour later, half of the birds from each group were challenged with 105 ELD50 of vvIBDV and observed for 10 days. 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 48 h 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.