O-antigen-deficient, live, attenuated Salmonella typhimurium confers efficient uptake, reduced cytotoxicity, and rapid clearance in chicken macrophages and lymphoid organs and induces significantly high protective immune responses that protect chickens against Salmonella infection

In the present study, we developed an O-antigen-deficient, live, attenuated Salmonella Typhimurium (ST) strain (JOL2377) and assessed its safety, macrophage toxicity, invasion into lymphoid tissues, immunogenicity, and protection against Salmonella infection in chickens. The JOL2377 induced significantly lower cytotoxicity and higher level of cytokine response in IL-2, IL-10, IL-4, and IFN- γ than the WT strain upon macrophage uptake. It did not persist in macrophages or in chicken organs and rapidly cleared without systemic infection. None of the chicken were found to secrete Salmonella in feces into the environment exacerbating its attenuation. Interestingly JOL2377 successfully arrived in immunological hot-spots such as spleen, liver and bursa of Fabricius for an efficient antigen presentation and immune stimulation. Mucosal and parenteral immunization with JOL2377 significantly elicit antigen-specific humoral (IgY) and cell mediated responses marked by peripheral blood mononuclear cell proliferation, cytokine induction, increase in T-cell responses than non-immunized control. JOL2377 did not generate significant levels of LPS specific antibodies as compared to the WT strain due to the lack of immunogenic O-antigen component from its LPS structure. Upon virulent challenge, route dependent efficacy differences were leaving the intramuscular route is superior to the oral route on reducing splenic and liver colonization of the challenge ST. The least cytotoxicity, virulence, and superior immunogenicity of JL2377 that effectively engage both humoral and IFN- γ mediated CMI responses present an ideal scenario in host immune modulation to fight against intracellular pathogen Salmonella.