Bacteria have developed a variety of strategies for acquiring the essential metal, iron, from their environment, often involving interactions with their neighbors in the microbial communities they inhabit. Among the microbial communities that inhabit the upper respiratory and genitourinary tracts of vertebrate hosts are Gram-negative bacteria that utilize surface receptors capable of binding the host iron-sequestering protein, transferrin (Tf), and extracting the iron required for growth. Since the specificity of these receptors for host Tf has evolved over time as a consequence of changes in the receptor-binding regions of Tf, the presence of Tf receptors in bacteria colonizing birds and mammals suggests that the receptor proteins have been present in bacteria for over 300 million years. Unlike many of the bacteria in the microbial community that rely of siderophores produced their neighbors, bacteria expressing Tf receptors are capable of proliferating independently on the mucosal surface or within the body if they cross the mucosal surface. As a consequence, these bacteria are important pathogens of humans and food production animals, and the transferrin receptors are ideal targets for vaccine development.
We have implemented a structure-based protein engineering approach to generate antigens with improved immunological properties that include non-binding mutants of the surface lipoprotein, Tf binding protein B (TbpB), and hybrid antigens displaying surface epitopes from the integral outer membrane protein, TbpA. Notably, our antigens have been shown to not only prevent infection but to reduce or eliminate colonization by the targeted pathogens. Although there is considerable sequence diversity amongst TbpBs, the diversity is largely present in the Tf binding region and likely represents immunologically distinct binding interfaces that have evolved over long time periods. Thus, it will be possible to develop broadly cross-protective TbpB-based vaccines derived from a small number of representative variants and extend coverage against TbpA by displaying epitopes.