Multiple mechanisms mediate enhanced immunity generated by mAb-inactivated F. tularensis immunogen.

We have previously demonstrated that immunization with the inactivated Francisella tularensis, a Category A intracellular mucosal pathogen, combined with IgG2a anti-F. tularensis monoclonal antibody (Ab), enhances protection against subsequent F. tularensis challenge. To understand the mechanism(s) involved, we examined the binding, internalization, presentation, and in vivo trafficking of inactivated F. tularensis in the presence and absence of opsonizing monoclonal Ab. We found that when inactivated F. tularensis is combined with anti-F. tularensis monoclonal Ab, presentation to F. tularensis-specific T cells is enhanced. This enhancement is Fc receptor (FcR)-dependent, and requires a physical linkage between the monoclonal Ab and the inactivated F. tularensis immunogen. This enhanced presentation is due, in part, to enhanced binding and internalization of inactivated F. tularensis by antigen(Ag)-presenting cells, and involves interactions with multiple FcR types. Furthermore, targeting inactivated F. tularensis to FcRs enhances dendritic cell maturation and extends the time period over which Ag-presenting cells stimulate T cells. In vivo trafficking studies reveal enhanced transport of inactivated F. tularensis immunogen to the nasal-associated lymphoid tissue in the presence of monoclonal Ab, which is FcRn-dependent. In summary, these are the first comprehensive studies using a single-vaccine protection model/immunogen to establish the array of mechanisms involved in enhanced immunity/protection mediated by an FcR-targeted mucosal immunogen. These results demonstrate that multiple cellular/immune mechanisms contribute to FcR-enhanced immunity.