Assessing an O-antigen deficient, live attenuated Salmonella Gallinarium strain that is DIVA compatible, environmentally safe, and protects chickens against fowl typhoid

The objective of the present study was to create a highly attenuated, safe Salmonella Gallinarium (SG) vaccine strain for chicken vaccination against fowl typhoid (FT) diseases. The SG vaccine strain (SGVS) consists of three virulence-related gene deletions, namely, lon, cpxR, and rfaL. The parent strain (SGPS) with Δlon ΔcpxR genotype was utilized as the host strain for in-frame rfaL gene deletion by lambda red recombination. The SGVS was highly attenuated with improved environmental safety by zero fecal contamination beyond seven days for both oral and intramuscular immunization routes. Upon inoculation into 1-month-old young chicken, no vaccine-induced adverse behaviors were observed and did not cause a chronic state of infection as the SG wild-type strain did. Immunization of chicken elicited both humoral and cell-mediated immune responses demarcated by, IgY antibody assessment, T-cell responses in peripheral blood mononuclear cells, and the induction of immunomodulatory cytokines, IFN-γ, IL-2, IL-12, and IL-4 to resemble both Th1 and Th2 type of immune responses. The immunological assessment revealed a high level of efficacy of the SGVS when inoculated via the IM route than the oral route. The strain was less cytotoxic with reduced cytotoxicity on chicken macrophages and was DIVA capable with minimum reactivity of immunized serum with purified SG lipopolysaccharides. The challenge study could generate 70% protection in chicken for SGVS, whereas no birds were protected in the PBS challenged group. The protection levels were evident in histopathological assessment of spleen and liver specimens and also the external appearance of the spleen with reduced lesions on immunized groups. Further experiments may be warranted to dose and route optimization for further increase in the protection level derived by present SGVS.