Modulation of Protective Efficacy of Fracisella tularensis Immunogens by Differential Cultivation in Mueler Hinton Broth Versus Brain Heart Infusion Medium

Abstract

Abstract Francisella tularensis (Ft) can survive in a variety of habitats with a remarkable ability to adapt to changing environmental conditions. For example, Ft expresses a distinct set of antigens (Ags) in the macrophage, which are not expressed in Ft grown in vitro with Mueler Hinton Broth (MHB) medium. In contrast, Ft grown in Brain Heart Infusion (BHI) medium exhibits an antigenic profile more closely resembling that of in vivo growth conditions. In addition, studies also suggest an alteration in immunological properties of Ft when grown in BHI versus MHB medium. Thus, we investigated the protective efficacy of BHI and MHB-grown Ft, when inactivated and used as an immunogen to protect against Ft LVS challenge. Inactivated Ft (iFt) generated in MHB (iFt-MHB) was found to have superior protective efficacy as compared to that of the iFt generated in BHI (iFt-BHI). The enhanced protection generated by iFt- MHB-immunized mice was also associated with lower bacterial burden and sepsis-like conditions in the lungs and spleens of iFt-MHB versus iFt-BHI-immunized mice. Further evaluation of early immunological events revealed, in the case of iFt-MHB immunogen, superior Ag binding to Ag-presenting dendritic cells (DCs), increased MHC class II expression by DCs, and enhanced Ag presentation by DCs, as apposed to the use of iFt-BHI. Also, iFt-MHB induced increased levels of Ft-specific IgG as well as IFNg and IL17 production versus that of iFt-BHI. These studies thus demonstrate the critical importance of considering bacterial growth medium in Ft vaccine development.