A Bacterial Flagellin, <i>Vibrio vulnificus</i> FlaB, Has a Strong Mucosal Adjuvant Activity To Induce Protective Immunity

TLDR

Flagellin is a ligand for Toll‑like receptor 5 (TLR5) that activates innate and adaptive immunity, making TLR ligands attractive vaccine adjuvants. The study demonstrates that Vibrio vulnificus FlaB is a highly potent mucosal adjuvant. Intranasally delivered FlaB reaches cervical lymph nodes and systemic circulation, binds TLR5 on epithelial cells, activates NF‑κB and IL‑8, and localizes to dendritic cells, thereby stimulating TLR5‑mediated immune activation. Co‑administration of FlaB with tetanus toxoid via intranasal immunization markedly increased mucosal IgA and systemic IgG responses and conferred complete protection against a 200‑fold lethal dose of tetanus toxin.

Abstract

ABSTRACT Flagellin, the structural component of flagellar filament in various locomotive bacteria, is the ligand for Toll-like receptor 5 (TLR5) of host cells. TLR stimulation by various pathogen-associated molecular patterns leads to activation of innate and subsequent adaptive immune responses. Therefore, TLR ligands are considered attractive adjuvant candidates in vaccine development. In this study, we show the highly potent mucosal adjuvant activity of a Vibrio vulnificus major flagellin (FlaB). Using an intranasal immunization mouse model, we observed that coadministration of the flagellin with tetanus toxoid (TT) induced significantly enhanced TT-specific immunoglobulin A (IgA) responses in both mucosal and systemic compartments and IgG responses in the systemic compartment. The mice immunized with TT plus FlaB were completely protected from systemic challenge with a 200× minimum lethal dose of tetanus toxin. Radiolabeled FlaB administered into the nasal cavity readily reached the cervical lymph nodes and systemic circulation. FlaB bound directly to human TLR5 expressed on cultured epithelial cells and consequently induced NF-κB and interleukin-8 activation. Intranasally administered FlaB colocalized with CD11c as patches in putative dendritic cells and caused an increase in the number of TLR5-expressing cells in cervical lymph nodes. These results indicate that flagellin would serve as an efficacious mucosal adjuvant inducing protective immune responses through TLR5 activation.