Induction of Immunity against Marek’s Disease using Toll-Like Receptor Ligands

Abstract

Marek’s disease virus (MDV) is a herpesvirus of chickens that causes a lymphoproliferative disease known as Marek’s disease (MD). Vaccines are available to control clinical signs of MD. However, inability of vaccines to prevent replication and transmission of MDV prompts investigation of other possible ways of enhancing host immunity against MD. Innate defense mechanisms may play an essential role in protection against MDV infection in young chicks. Toll-like receptors (TLRs) are an important component of innate defenses and recognize TLR ligands (TLR-Ls) which are primarily microbial constituents. TLR-Ls can be used as prophylactic agents or as adjuvants to enhance immunity against infectious diseases. In this regard, TLR-Ls can be encapsulated in Poly(D, L-lactic-co-glycolic)acid to further enhance host responses by controlled release of TLR-Ls for an extended period. In this thesis, the effect of encapsulated TRL-Ls to improve immunity against MD was investigated. First, TLR-Ls which inhibit MDV infection were identified as Poly(IC), Pam3CSK4, LPS and CpG. Next, encapsulated CpG and LPS were administered to embryonic day 18 (ED18) chicken embryos for investigating the initiation of innate responses. Cytokine gene expression in the spleen, lung and bursa of Fabricius demonstrated the activation of innate responses. Subsequently, the effect of administration of encapsulated TLR-Ls against MDV infection was investigated. Significant reduction of tumor incidence was observed in chickens which received encapsulated CpG (ECpG) twice at ED18 via amniotic route and at 14-day post infection via intramuscularly. Association between upregulation of interleukin (IL)-1β and IL-18 and the reduction of tumor formation was observed in response to TLR-L treatments. Finally, the adjuvant effect of ECpG on enhancing the efficacy of herpesvirus of turkey (HVT) vaccine was evaluated. Chickens which were administered ECpG and HVT, exhibited maximum reduction of tumors and significant reduction of MDV load in feathers. In conclusion, the studies described in this thesis demonstrate that administration of encapsulated TLR-Ls activates innate responses and provides protection against MD in chickens.