Infectious Bronchitis Virus Immune Responses in the Harderian Gland upon Initial Vaccination

In recent years, Arkansas Delmarva Poultry Industry (ArkDPI)-derived infectious bronchitis (IB) virus (IBV) vaccines have been used to characterize the immune responses of chickens subsequent to vaccination on day of hatch or beyond. Perhaps because ArkDPI vaccines display increased heterogeneity, the results on cell immune responses have shown ambiguity. In the current study, we investigated the effects of vaccination with a highly stable and homogeneous Massachusetts (Mass)-type vaccine on days 1 or 7 of age on Harderian gland (HG) responses. Confirming previous studies, both IBV serum antibodies and lachrymal IgA levels were greater upon vaccination on day 7 compared with vaccination on day 1 of age. Unlike results with ArkDPI viruses, a clear trend was detected for both B and T cells in the HG after Mass-type vaccination. Consistent with antibody responses, B- and T-helper (CD3⁺CD4⁺) cell frequencies were higher in birds vaccinated on day 7 of age. Cytotoxic T cells (CD3⁺CD8⁺) were also increased compared with chickens vaccinated on day 1 of age. Depending on the most likely age of IB outbreaks to occur in a particular region, postponing the first IBV vaccination may optimize immune responses.