Ability to accelerate innate and cell-mediated immune responses in meat-type chickens by in ovo vaccination with monovalent and bivalent Marek's disease vaccines

We have previously demonstrated that in ovo vaccination with Herpesvirus of Turkey (HVT) at 18 days-of-embryonation (ED) accelerates innate and cell-mediated immunity in one-day-old meat- and egg-type chickens. We also reported differences in the splenic immunophenotypes of egg-type chickens following HVT, CVI-988, and SB-1 vaccination. In this study, using meat-type chickens, we further evaluated the effect of in ovo administration of CVI-988 or SB-1 on innate and adaptive immune responses when compared to HVT. Additionally, we also assessed the synergistic immunopotentiation effect, if any, of CVI-988 or SB-1 with HVT. Results showed that all three vaccines could significantly augment splenic frequencies of γδ T cells, CD4⁺CD28⁺, CD4⁺CD44⁺, CD8⁺CD28⁺, and CD8⁺CD44⁺ T cells compared to sham-inoculated chickens. However, vaccines differed on tissue tropism, ability to increase PHA-L driven wing-web thickness, and ability to increase the frequency of some immunophenotypes. The frequencies of activated monocytes/macrophages, CD4⁺CD8⁺ double-positive (DP) cells, and CD4⁺MHC-II⁺ cells were found significantly higher in SB-1 and HVT groups compared to sham-inoculated control. Additionally, SB-1 vaccination led to significantly increased frequency of CD4⁺ T cells compared to CVI-988 and control groups. Furthermore, in comparison to HVT, SB-1 induced significantly higher frequencies of activated monocytes/macrophages and DP cells, while the CVI-988 vaccination showed a significant reduction in DP cells, CD4⁺CD28⁺, and CD4⁺CD44⁺ T cells. Finally, the synergistic effect of CVI-988 and HVT was observed in relation to augmenting splenic transcription of IFN-β and IFN-γ, while no effect on the changes in splenic immunophenotypes was observed. Collectively, all three vaccines (HVT, CVI-988 and SB-1) seem to enhance innate and/or adaptive responses in day-old meat-type chickens; however, immune-synergism of these vaccines needs further investigation.