In ovo vaccination with herpesvirus of turkey enhances innate and cellular responses in meat-type chickens: Effect of vaccine dose and strain.

In ovo vaccination with herpesvirus of turkey (HVT) or recombinant HVT (rHVT) is commonly used in meat-type chickens. Previous studies showed that in ovo vaccination with HVT enhances innate, cellular, and humoral immune responses in egg-type chicken embryos. This study evaluated if in ovo vaccination with HVT hastens immunocompetence of commercial meat-type chickens and optimized vaccination variables (dose and strain of HVT) to accelerate immunocompetence. A conventional HVT vaccine was given at recommended dose (RD), HVT-RD = 6080 plaque forming units (PFU), double-dose (2x), half-dose (1/2), or quarter-dose (1/4). Two rHVTs were given at RD: rHVT-A = 7380 PFU, rHVT-B = 8993 PFU. Most, if not all, treatments enhanced splenic lymphoproliferation with Concanavalin A and increased the percentage of granulocytes at day of age. Dose had an effect and HVT-RD was ideal. An increase of wing-web thickness after exposure to phytohemagglutinin-L was only detected after vaccination with HVT-RD. Furthermore, compared to sham-inoculated chickens, chickens in the HVT-RD had an increased percentage of CD3+ T cells and CD4+ T-helper cells, and increased expression of major histocompatibility complex (MHC)-II on most cell subsets (CD45+ cells, non-T leukocytes, T cells and the CD8+ and T cell receptor γδ T-cell subsets). Other treatments (HVT-1/2 and rHVT-B) share some of these features but differences were not as remarkable as in the HVT-RD group. Expression of MHC-I was reduced, compared to sham-inoculated chickens, in most of the cell phenotypes evaluated in the HVT-RD, HVT-2x and rHVT-A groups, while no effect was observed in other treatments. The effect of in ovo HVT on humoral immune responses (antibody responses to keyhole limpet hemocyanin and to a live infectious bronchitis/Newcastle disease vaccine) was minimal. Our study demonstrates in ovo vaccination with HVT in meat-type chickens can accelerate innate and adaptive immunity and we could optimize such effect by modifying the vaccine dose.