Gamma-irradiated Salmonella Gallinarum oral vaccine elicits robust cellular and humoral immune responses in a chicken model.

Gamma-irradiated inactivated bacterial vaccines have emerged as a safer alternative, overcoming the safety and immunogenicity limitations of conventional live attenuated and inactivated vaccines. This study aimed to develop gamma-irradiated Salmonella Gallinarum (γ-SG) vaccine from a local field strain and evaluate its cell-mediated immune response in a chicken model, focusing on CD4+ and CD8+ T cell activation and IFN-γ production. Radiation doses ranging from 1.5 to 10 kGy were evaluated to determine the optimal level for inactivating bacterial replication while retaining metabolic activity. Dose of 7 kGy effectively inhibited replication while maintaining residual metabolic activity. In an immunization-challenge study, commercial broiler chickens (Ross 308) at 14 days of age were vaccinated twice at two-week intervals orally with γ-SG (γ-SG Oral), intramuscularly with oil-based γ-SG (γ-SG IM), oil-based formalin-inactivated SG (F-SG) and SG 9R vaccines with the concentration of each 2 × 108 CFU and SG 9R at the concentration of 2 × 107 CFU in 0.2 mL PBS. Our study showed that chickens vaccinated with γ-SG (Oral) exhibited significantly higher CD4+ T cell response (39.74 %) when compared with SG 9R (29.36 %), γ-SG (IM) (20.2 %) and F-SG (22.8 %) groups at three weeks post-vaccination (WPV). Similarly, CD8+ T cell response was highest in γ-SG (Oral) group (28.6 %) versus SG 9R (5.28 %), γ-SG (IM) (19.7 %), and F-SG (14.3 %) at 3WPV. IFN-γ concentrations were also significantly elevated in γ-SG (Oral) group (452.75 pg/mL) when compared with SG 9R (307.5 pg/mL), γ-SG (IM) (334 pg/mL), and F-SG (221.75 pg/mL) groups. In vivo efficacy study showed that γ-SG (Oral) provided 100 % protection with 0 % mortality, comparable to SG 9R while F-SG and γ-SG (IM) groups showed 40 % and 20 % mortality, respectively, compared to 70 % in unvaccinated chickens challenged with wild-type SG. These findings demonstrate that non-adjuvanted oral γ-SG vaccination is an effective strategy against fowl typhoid.