A common adaptation among several highly host-adapted Gram-negative species from the Pasteurellaceae, Neisseriaceae, and Moraxellaceae families that exclusively reside in the upper respiratory tract is the ability to directly bind host transferrin (Tf) and use it as a source of iron for growth (1, 2). Iron-loaded transferrin is captured by surface receptors that remove iron from Tf and transport the iron across the outer membrane, where it is subsequently transported into the cell through a periplasm binding protein-dependent ABC (ATP binding cassette) transport system. Early observations that the interaction of the bacterial receptors with Tf was highly host specific (3-5) provided a rational explanation for the strict host specificity of these bacterial pathogens.The initial capture of iron-loaded Tf is mediated by a surface lipoprotein, Tf binding protein B (TbpB), which consists of two structurally equivalent lobes preceded by a relatively long anchoring peptide that would allow the protein to extend far from the outer membrane surface (6). The role of TbpB is to capture the iron-loaded form of Tf and deliver it to Tf binding protein A (TbpA), the integral outer membrane protein that serves as the channel for transporting iron across the outer membrane. The structure of a Tf-TbpB complex has recently been determined (7), revealing that that the process of binding iron-loaded Tf does not involve substantial changes in the conformation of the TbpB Nlobe or the Tf C-lobe and effectively traps the Tf C-lobe in the iron-loaded state. In contrast, binding of Tf to TbpA involves substantial conformational changes in the TbpB C-lobe, resulting in substantial separation of the C1 and C2 domains that both contribute ligands for coordination of iron (8). In the absence of structural information for TbpA alone one can only speculate on the conformational changes that occur in the surface loop structures of TbpA upon binding Tf.The process by which TbpB mediates the initial capture of iron-loaded Tf and transfers it to TbpA is only partly understood. The variable association of the anchoring peptide with the C-lobe (9) and its requirement for formation of the ternary complex (10) may indicate that modulation of the anchor peptide may be involved. Although structural models can be developed for the ternary complex (2, 8), these are not based on high-resolution structural information for the actual complex, and how TbpB maintains an interaction with Tf upon domain separation is still not resolved. Similarly, the process by which iron is released and transported across the outer membrane and the degree to which different regions of TbpB participate in this process is uncertain.Since the first discovery of the bacterial Tf receptors (11, 12) and the demonstration of their exquisite host specificity (4), they were postulated to be essential for survival in the native host and thus potentially ideal vaccine targets. The importance of the receptor proteins has been confirmed in a male gonococcal infection

Nutrient acquisition systems are often crucial for pathogen growth and survival during infection, and represent attractive therapeutic targets. Here, we study the protein machinery required for heme uptake in the opportunistic pathogen Acinetobacter baumannii. We show that the hemO locus, which includes a gene encoding the heme-degrading enzyme, is required for high-affinity heme acquisition from hemoglobin and serum albumin. The hemO locus includes a gene coding for a heme scavenger (HphA), which is secreted by a Slam protein. Furthermore, heme uptake is dependent on a TonB-dependent receptor (HphR), which is important for survival and/or dissemination into the vasculature in a mouse model of pulmonary infection. Our results indicate that A. baumannii uses a two-component receptor system for the acquisition of heme from host heme reservoirs.

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The surface transferrin receptor proteins from Neisseria gonorrhoeae have been recognized as ideal vaccine targets due to their critical role in survival in the human male genitourinary tract. Recombinant forms of the surface lipoprotein component of the receptor, transferrin binding protein B (TbpB), can be readily produced at high levels in the Escherichia coli cytoplasm and is suitable for commercial vaccine production. In contrast, the integral outer membrane protein, transferrin binding protein A (TbpA), is produced at relatively low levels in the outer membrane and requires detergents for solubilization and stabilization, processes not favorable for commercial applications. Capitalizing on the core β-barrel structural feature common to the lipoprotein and integral outer membrane protein we engineered the lipoprotein as a scaffold for displaying conserved surface epitopes from TbpA. A stable version of the C-terminal domain of TbpB was prepared by replacing four larger exposed variable loops with short linking peptide regions. Four surface regions from the plug and barrel domains of Neisseria TbpA were transplanted onto this TbpB C-lobe scaffold, generating stable hybrid antigens. Antisera generated in mice and rabbits against the hybrid antigens recognized TbpA at the surface of Neisseria meningitidis and inhibited transferrin-dependent growth at levels comparable or better than antisera directed against the native TbpA protein. Two of the engineered hybrid antigens each elicited a TbpA-specific bactericidal antibody response comparable to that induced by TbpA. A hybrid antigen generated using a foreign scaffold (TbpB from the pig pathogen Haemophilus parasuis) displaying neisserial TbpA loop 10 was evaluated in a model of lower genital tract colonization by N. gonorrhoeae and a model of invasive infection by N. meningitidis . The loop 10 hybrid antigen was as effective as full length TbpA in eliminating N. gonorrhoeae from the lower genital tract of female mice and was protective against the low dose invasive infection by N. meningitidis . These results demonstrate that TbpB or its derivatives can serve as an effective scaffold for displaying surface epitopes of integral outer membrane antigens and these antigens can elicit protection against bacterial challenge.