Iron Acquisition by Uropathogenic Escherichia coli: ChuA and Hma Heme Receptors as Virulence Determinants and Vaccine Targets.

Abstract

In Gram negative bacteria, specific outer membrane receptors facilitate the import of iron-chelating siderophores and iron from host organisms. Uropathogenic Escherichia coli (UPEC), the predominant cause of uncomplicated urinary tract infection (UTI), utilizes a wide array of these receptors to acquire iron, an essential nutrient, from within the iron-limited urinary tract. However, the contributions of specific sources of host iron remain unknown. Furthermore, efforts to induce protective immunity against UPEC by targeting known virulence determinants have been largely unsuccessful, suggesting that novel targets are required. To address this, an immunoproteomics approach was used here to identify 23 outer membrane proteins that elicited an immune response during murine UTI. Among the most prevalent of these were TonB-dependent siderophore and iron compound receptors, including the putative receptor c2482 (heme acquisition protein, Hma). Intranasal immunization with Hma or the siderophore receptors IreA or IutA significantly protected mice from UPEC challenge, while vaccination against the heme receptor ChuA did not confer protection from UTI. Additionally, genetic and biochemical approaches were used to demonstrate that the vaccine candidate Hma functions as a TonB-dependent heme receptor. Hma shares limited homology with known heme receptors and our structure-function studies identified a unique residue required for Hma-mediated heme utilization. We further establish that heme is an essential source of iron for UPEC in the kidney and present evidence for the importance of heme acquisition within the context of siderophore-mediated iron uptake. The findings presented here provide a foundation for the development of an outer membrane protein-based vaccine against UPEC and suggest that iron receptor antigens may represent putative vaccine targets in other Gram negative bacteria. Furthermore, this work offers novel insight into the distinct contributions of specific UPEC receptors and highlights the necessity of iron acquisition for bacterial survival within the mammalian host.