Effect of Marek's disease vaccines on interferon and toll like receptors when administered in ovo

The effect of two Marek's disease (MD) vaccines on the chicken embryo immune responses were evaluated. Transcription of interferon (IFN-α, IFN-β, IFN-λ, and IFN-γ) and interferon-I receptors (IFN-AR1 and IFN-AR2), as well as transcription of toll like receptors (TLR-3, TLR-7, and TLR-21) were evaluated in the bursa, thymus, spleen and lung of 1-day-old chickens that had been vaccinated with HVT, CVI988, or sham inoculated at embryonic day 18 (ED18). Each vaccine had a unique effect on the transcription of the evaluated genes and it differs among tissues. HVT increased IFN-γ and TLR-3 transcripts in the spleen and lung and IFN-β in the bursa. The immune responses elicited by CVI988 differed from that observed in the HVT inoculated group. CVI988 downregulated several of the studied genes and only upregulated IFN-β and TLR-21 in spleen. Differences in vaccine replication (53% of spleens and lungs of HVT-vaccinated embryos but only 22% of spleens of CVI988-vaccinated embryos had detectable viral gB transcripts) were detected. Previously, we have shown that intra-amniotic vaccination at ED18 with HVT but not with CVI988 rendered chickens more immunocompetent at hatch. The role of increased transcription of TLR-3 and IFN-γ in such positive effect warrant further investigations.