CD8 T cell adjuvant effects of Salmonella FliCd flagellin in live vaccine vectors or as purified protein

Salmonella flagellin, the flagellum structural subunit, has received particular interest as a vaccine adjuvant conferring enhanced immunogenity to soluble proteins or peptides, both for activation of antibody and cellular immune responses. In the present study, we evaluated the Salmonella enterica FliCd flagellin as a T cell vaccine adjuvant using as model the 9-mer (SYVPSAEQI) synthetic H2^d-restricted CD8⁺ T cell-specific epitope (CS_280–288) derived from the Plasmodium yoelii circumsporozoite (CS) protein. The FliCd adjuvant effects were determined under two different conditions: (i) as recombinant flagella, expressed by orally delivered live S. Dublin vaccine strains expressing the target CS_280–288 peptide fused at the central hypervariable domain, and (ii) as purified protein in acellular vaccines in which flagellin was administered to mice either as a recombinant protein fused or admixed with the target CS_280–288 peptide. The results showed that CS_280–288-specific cytotoxic CD8⁺ T cells were primed when BALB/c mice were orally inoculated with the expressing the CS_280–288 epitope S. Dublin vaccine strain. In contrast, mice immunized with purified FliCd admixed with the CS_280–288 peptide and, to a lesser extent, fused with the target peptide developed specific cytotoxic CD8⁺ T cell responses without the need of a heterologous booster immunization. The CD8⁺ T cell adjuvant effects of flagellin, either fused or not with the target peptide, correlated with the in vivo activation of CD11c⁺ dendritic cells. Taken together, the present results demonstrate that Salmonella flagellins are flexible adjuvant and induce adaptative immune responses when administered by different routes or vaccine formulations.