Flagellin is an effective mucosal adjuvant for immunization against lethal respiratory challenge with Yersinia pestis

Cells of the innate immune system lack memory per se, but recognize conserved pathogen-associated molecular patterns (PAMPs) through a family of type I membrane receptors known as Toll-like receptors (TLRs). Flagella are locomotive organelles present on a wide range of bacteria and are important for the pathogenesis of many species. Flagellin, the major structural component of flagella, is a highly conserved protein recognized in hosts by TLR5. Flagellin is a potent immune activator, stimulating diverse biologic effects that mediate both innate inflammatory responses as well as the development of adaptive immunity. Binding of flagellin to the extracellular domain of TLR5 rapidly induces a signal cascade that culminates in the production of proinflammatory mediators such as cytokines, chemokines and costimulatory molecules. In the lung, flagellin induced a localized and transient innate immune response characterized by neutrophil infiltration and the production of TNF-alpha, IL-6, G-CSF and the chemokines KC, MIP-1alpha and MIP-2.; In view of the extraordinary potency of flagellin as an inducer of innate immunity and the contribution of innate responses to the development of adaptive immunity, we evaluated the potential efficacy of recombinant Salmonella flagellin as a mucosal adjuvant in an acellular plague vaccine. Mice immunized intranasally or intratracheally with the F1 antigen of Yersinia pestis and flagellin exhibited dramatic increases in F1-specific plasma IgG titers that remained stable over time. In contrast, control mice had low or undetectable antibody responses. Interferons, TNF-alpha, and IL-6 were not essential for the adjuvant effects of flagellin. Pre-existing anti-flagellin antibodies had no significant effect on the adjuvant activity of flagellin. Importantly, intranasal immunization with flagellin and the Fl antigen was protective against intranasal challenge with virulent Y. pestis CO92, with 93--100% survival of immunized mice. Vaccination of cynomolgus macaques with flagellin and the Fl and V antigens of Y. pestis induced a robust antigen-specific IgG antibody response. Lastly, passive transfer of plasma from immune cynomolgus monkeys completely protected naive mice from intranasal challenge with Y. pestis.