Attenuated Salmonella secreting Brucella protective antigens confer dual-faceted protection against brucellosis and salmonellosis in a mouse model

We demonstrated the use of attenuated Salmonella strains secreting Brucella antigens SodC, Omp19, BLS, and PrpA as live vaccine candidates against Brucella abortus infection and presented their cross-protection against Salmonella infections using a BALB/c mice model. Here, a single immunization with each individual strain was capable of establishing significantly high (p < 0.05) Brucella-specific systemic immunoglobulin (Ig)G and secretory IgA (sIgA) responses compared to control mice. Upon stimulation of the splenocytes harvested from immunized mice with the respective antigens SodC, Omp19, BLS, and PrpA, significant increases in splenocyte proliferative responses against all four antigens versus PBS and vector controls were observed (p < 0.05). Additionally, interferon-gamma and interleukin-4 secretion clearly demonstrated an uplift of these cytokines in all four strains upon immunization compared to the control groups. However, a significantly high response was noted in the mice groups immunized with Salmonella secreting SodC and Omp19 only. Upon virulent Brucella abortus 544 challenge, all four antigens presented a significantly high protection index (PI) in the spleen, as follows: 0.85 for SodC; 0.96 for Omp19; 0.6 for BLS; and 0.66 for PrpA. In contrast, in the liver, the same antigens resulted in PI values of 1.37, 1.14, 1.12, and 1.81, respectively. Immunological profiling of immunized mice against Salmonella-specific immune responses also showed significant elicitation of both humoral and cell-mediated immune responses as measured by IgG, sIgA, splenocyte proliferation, and cytokine induction. In addition, full protection against virulent Salmonella challenge was shown with no mortality in immunized mice, whereas 100% (8/8) mortality was observed in control mice over a two-week post-Salmonella challenge. In conclusion, we show that the live attenuated Salmonella delivering Brucella protective antigens may efficiently confer dual protection against both brucellosis and salmonellosis in immunized mice.