Abstract
Marek’s Disease Virus (MDV) is the causative agent of a lymphoproliferative disease, Marek’s disease (MD) in chickens. MD is only controlled by mass vaccination; however, immunity induced by MD vaccines is unable to prevent MDV replication and transmission. The herpesvirus of turkey (HVT) vaccine is one of the most widely used MD vaccines in poultry industry. Vaccines can be adjuvanted with Toll-like receptor ligands (TLR-Ls) to enhance their efficacy. In this study, we examined whether combining TLR-Ls with HVT can boost host immunity against MD and improve its efficacy. Results demonstrated that HVT alone or HVT combined with encapsulated CpG-ODN partially protected chickens from tumor incidence and reduced virus replication compared to the control group. However, encapsulated CpG-ODN only moderately, but not significantly, improved HVT efficacy and reduced tumor incidence from 53% to 33%. Further investigation of cytokine gene profiles in spleen and bursa of Fabricius revealed an inverse association between interleukin (IL)-10 and IL-18 expression and protection conferred by different treatments. In addition, the results of this study raise the possibility that interferon (IFN)-β and IFN-γ induced by the treatments may exert anti-viral responses against MDV replication in the bursa of Fabricius at early stage of MDV infection in chickens.
Highlights
Since Marek’s Disease Virus (MDV) was first identified more than 50 years ago, several vaccines have been developed to control clinical signs of the disease, none of them can fully prevent replication or transmission of MDV
To enhance host responses against MDV, administration of encapsulated CpG (ECpG) along with herpesvirus of turkey (HVT) was investigated in the current study
Among the HVT-administered groups, a significant reduction in both tumor incidence and MDV load in feathers was observed in Group 2 (G2) and Group 4 (G4) chickens
Summary
Since MDV was first identified more than 50 years ago, several vaccines have been developed to control clinical signs of the disease, none of them can fully prevent replication or transmission of MDV. The activation of innate responses orchestrates the induction of adaptive responses which in turn require several days to provide protective immunity against an infection. This is not well understood in regard to MD vaccination. Increased IFN-γ expression observed with in ovo HVT vaccination in chickens[16] as well as increased NK like activity observed with HVT vaccination at hatch and post-hatch[17,18] revealed that innate responses can be elevated to significant level with appropriate immune modulations[19]. TLR3 ligand, polyinosinic:polycytidylic acid (Poly(IC)), has been shown to induce innate responses in chickens[25] and its administration to chickens combined with HVT could lead to reduction of tumor incidence after challenge with a very virulent strain of MDV26. Administration of encapsulated CpG (ECpG) as a prophylactic agent demonstrated considerable effects on the outcome of MD in our previous study[29], which prompted further investigation of the adjuvant effect of ECpG with MDV vaccine in chickens
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