Aptamer-based Salmonella enteritidis-inactivated vaccine injected intradermally elicits local leukocyte recruitment and mucosal immunity in broiler chickens

On-farm non-adapted Salmonella enterica ser. Enteritidis (S. Enteritidis) control in poultry requires vaccines that elicit rapid local immune responses maturing into long-lasting systemic and mucosal protective immunity. To assess the responses to primary and secondary intradermal (i.d.) immunization with CD40-agonistic DNA aptamer-based S. Enteritidis-inactivated vaccine (CD40-AP), thirty chicks were randomly assigned to six treatment groups (n = 5 per group) at hatch. Groups g1 - g3 received only a primary i.d. immunization on day 32, while groups g4 - g6 received a subcutaneous primary immunization on day 14 and an i.d. booster immunization on day 32. Vaccine treatment groups were: phosphate-buffered saline (PBS) control (g1, g4), mannosylated chitosan-based vaccine (MCA) (g2, g5), or CD40-AP (g3, g6). Blood and cloacal swabs were collected before immunization and 7 days after each immunization to determine relative levels of S. Enteritidis-specific plasma IgY and mucosal IgA. Growing feather (GF)-pulps were collected before (0 h) and at 0.25 (6 h), 1, 2, 3, 5, and 7 days after i.d. primary and booster immunization to assess the dynamics of the local leukocyte response. Both MCA and CD40-AP-inactivated vaccines induced macrophage and heterophil infiltration after primary and booster immunization (P < 0.05), while the secondary CD40-AP immunization promoted higher and prolonged presence of T cells, including both CD4+ and CD8+ subsets, and B cells compared to MCA (P < 0.05). The CD40-AP primary immunization induced higher levels of S. Enteritidis-specific mucosal IgA on day 7 compared to MCA (P < 0.05); whereas the MCA primary and booster immunization induced higher plasma levels of S. Enteritidis IgY than CD40-AP (P < 0.05). The more pronounced local T and B cell- and S. Enteritidis-specific mucosal-immune responses with CD40-AP, support the potential of the CD40-agonistic DNA aptamer-based vaccine platform to promote cell-mediated and mucosal immune responses while preserving the safety of inactivated vaccines in chickens.