Local and Systemic Immune Responses in Growing Feather Pulps and Blood of Broiler Chickens Elicited by Electron Beam- and Formalin-Killed Staphylococcus aureus Vaccines

Background: Staphylococcus aureus (SA) causes septicemia and arthritis, eventually resulting in substantial economic losses and health hazards in poultry. Lethal electron beam (eBeam) treatment kills bacteria while preserving surface epitopes. Methods: This study compared broiler chicken immune responses (local and systemic) after eBeam-killed (eB) or formalin-killed (FK) SA injections. The vaccine (sham) control was endotoxin-free PBS. This study contained six treatments (trt) with five chickens/trt, with vaccine trts divided into two groups, A and B. Group A received in ovo vaccine/sham treatments (phase 1) initially. At 34 d of age, the pulps of growing feathers (GFs) received intradermal (i.d.) injections of the respective trt to elicit booster and primary responses in groups A and B, respectively. Blood was collected to analyze leukocyte populations and plasma SA-specific antibody levels. Additionally, in phase 2, GFs were collected to assess leukocyte presence in GF pulps. Two-way ANOVA was conducted to test the effects of treatment, time, and their interactions, followed by Tukey’s HSD tests at p < 0.05 for statistical significance. Results: Early in phase 1, the eB group had increased SA-specific IgM, IgA, and total lymphocyte concentrations compared with FK. In phase 2, FK and eB i.d. vaccines increased total lymphocyte and T cell proportions in GF pulps compared with sham. In blood, the recall eB vaccination increased monocytes, B cells, CD8+ T cells, and total lymphocytes, while the recall FK vaccination increased heterophils (p < 0.05). Conclusions: Improved early protection at mucosal surfaces may be reflected by higher plasma levels of SA-specific IgM and IgA following in ovo eBeam-killed-SA vaccination. Additionally, both eB-SA and FK-SA vaccines stimulated robust local inflammatory responses dominated by lymphocytes in GF pulps.