FliC adjuvant augments protective efficacy of recombinant T2544 antigen against Salmonella Typhiand Paratyphi A by increasing serum antibodies, intestinal secretory immunoglobulin A and T cell memory

Background

The need for safe, effective, and affordable vaccines against Salmonella Typhi and Paratyphi A remains urgent, particularly in endemic regions of Asia and Africa. Contrary to earlier beliefs, parenteral vaccines can induce mucosal immunity, especially when combined with mucosal trafficking signals. Recent findings show that systemic delivery of soluble flagellin (FliC) activates CD103⁺ dendritic cells via TLR5, promoting mucosal IgA and regulatory T cell responses. Building on this, we evaluated recombinant FliC (rFliC) from S. Typhimurium as an adjuvant to enhance both systemic and mucosal immunity of rT2544, a subunit vaccine candidate.

Method

Female BALB/c mice were immunized subcutaneously with varying doses of rT2544 and rFliC to identify an optimal formulation. The selected dose (5 μg rT2544 + 2.5 μg rFliC) was tested for humoral and cellular responses, including serum IgG/IgA titers, antibody-secreting cells, cytokines (IFN-γ, IL-4, IL-17), and CTL activity. Protective efficacy was evaluated in an iron-overload mouse model challenged with S. Typhiand S. Paratyphi A, by assessing survival and bacterial loads in the intestine, liver, and spleen.

Result

Co-administration of rFliC with rT2544 enhanced systemic IgG and intestinal IgA responses, with increased ASCs and strong bactericidal activity against S. Typhiand S. Paratyphi A. The vaccine induced robust Th1, Th2, and Th17 cytokines, and improved CTL activity. Immunized mice showed 83 % and 91 % survival against S. Typhiand S. Paratyphi A, respectively, with significantly reduced bacterial loads in the intestine, liver, and spleen. Importantly, subcutaneous rFliC delivery effectively induced intestinal sIgA, eliminating the need for mucosal administration.

Conclusion

Subcutaneous co-administration of rFliC induced strong antigen-specific mucosal sIgA, overcoming limitations of conventional mucosal adjuvants. The rFliC-adjuvanted rT2544 elicited robust humoral, mucosal, and cellular responses, providing significant protection against S. Typhiand S. Paratyphi A. These findings support its potential as a promising injectable subunit vaccine for enteric fever.