Intracellular innate resistance to bacterial pathogens

Iron overload and immunity

Progress in the characterization of genes involved in the control of iron homeostasis in humans and in mice has improved the definition of iron overload and of the cells affected by it. The cell involved in iron overload with the greatest effect on immunity is the macrophage. Intriguing evidence has emerged, however, in the last 12 years indicating that parenchymal iron overload is linked to genes classically associated with the immune system. This review offers an update of the genes and proteins relevant to iron metabolism expressed in cells of the innate immune system, and addresses the question of how this system is affected in clinical situations of iron overload. The relationship between iron and the major cells of adaptive immunity, the T lymphocytes, will also be reviewed. Most studies addressing this last question in humans were performed in the clinical model of Hereditary Hemochromatosis. Data will also be reviewed demonstrating how the disruption of molecules essentially involved in adaptive immune responses result in the spontaneous development of iron overload and how they act as modifiers of iron overload.

Heme Coordination by Staphylococcus aureus IsdE

Staphylococcus aureus is a Gram-positive bacterial pathogen and a leading cause of hospital acquired infections. Because the free iron concentration in the human body is too low to support growth, S. aureus must acquire iron from host sources. Heme iron is the most prevalent iron reservoir in the human body and a predominant source of iron for S. aureus. The iron-regulated surface determinant (Isd) system removes heme from host heme proteins and transfers it to IsdE, the cognate substrate-binding lipoprotein of an ATP-binding cassette transporter, for import and subsequent degradation. Herein, we report the crystal structure of the soluble portion of the IsdE lipoprotein in complex with heme. The structure reveals a bi-lobed topology formed by an N- and C-terminal domain bridged by a single alpha-helix. The structure places IsdE as a member of the helical backbone metal receptor superfamily. A six-coordinate heme molecule is bound in the groove established at the domain interface, and the heme iron is coordinated in a novel fashion for heme transporters by Met(78) and His(229). Both heme propionate groups are secured by H-bonds to IsdE main chain and side chain groups. Of these residues, His(229) is essential for IsdE-mediated heme uptake by S. aureus when growth on heme as a sole iron source is measured. Multiple sequence alignments of homologues from several other Gram-positive bacteria, including the human pathogens pyogenes, Bacillus anthracis, and Listeria monocytogenes, suggest that these other systems function equivalently to S. aureus IsdE with respect to heme binding and transport.

Confirmation of the role of ATG16L1 as a Crohn's disease susceptibility gene

BACKGROUND

A German genome-wide nonsynonymous single nucleotide polymorphism (nsSNP) association study identified ATG16L1 as a Crohn's disease (CD) susceptibility gene. The association appeared to be confined to the nsSNP rs2,241,880 and was confirmed in 2 German independent case-control collections (combined P = 4.0 x 10(-8), odds ratio [OR] 1.45; 95% confidence interval [CI]: 1.21-1.74), a CD transmission disequilibrium test (TDT) collection, and an independent UK cohort. A weak statistical interaction with CARD15 was demonstrated. No association with ulcerative colitis (UC) was demonstrated. The aims of the study were to replicate the association with CD, examine subphenotype associations and statistical interactions with CARD15, IL23R, and the IBD5 risk haplotype, as well as explore the association with UC.

METHODS

The study included 645 CD and 676 UC rigorously phenotyped patients recruited from a single UK center. Unaffected controls comprised either spouses of patients (141) or individuals recruited from well-person clinics (1,049). The nsSNP rs2,241,880 was genotyped using MassArray (Sequenom).

RESULTS

A strong association with CD was demonstrated (P = 2.33 x 10(-7), OR 1.45 [1.25-1.67]), but no significant association was demonstrated with any subphenotype. We failed to replicate the reported interaction between rs2,241,880 and the CARD15 low-risk haplotypes dd and Dd. No significant statistical interaction with the 3 known CD susceptibility genes was seen. No association with UC susceptibility (P = 0.37, OR 1.06 [0.93-1.22]), or any UC subphenotype was identified.

CONCLUSIONS

We confirmed the findings that ATG16L1 is a CD susceptibility gene and found no evidence of interaction with CARD15, IL23R, or IBD5.

Iron metabolism at the host pathogen interface: lipocalin 2 and the pathogen-associated iroA gene cluster

The host innate immune defense protein lipocalin 2 binds bacterial enterobactin siderophores to limit bacterial iron acquisition. To counteract this host defense mechanism bacteria have acquired the iroA gene cluster, which encodes enzymatic machinery and transporters that revitalize enterobactin in the form of salmochelin. The iroB enzyme introduces glucosyl residues at the C5 site on 2,3-dihydroxybenzoylserine moieties of enterobactin and thereby prevents lipocalin 2 binding. Additional strategies to evade lipocalin 2 have evolved in other bacteria, such as Mycobacteria tuberculosis and Bacillus anthracis. Targeting these specialized bacterial evasion strategy may provide a mechanism to reinvigorate lipocalin 2 in defense against specific pathogens.

Evolution of microbial virulence: the benefits of stress

Although genome sequencing of microbial pathogens has shed light on the evolution of virulence, the drivers of the gain and loss of genes and of pathogenicity islands (gene clusters), which contribute to the emergence of new disease outbreaks, are unclear. Recent experiments with the bean pathogen Pseudomonas syringae pv. phaseolicola illustrate how exposure to resistance mechanisms acts as the driving force for genome reorganization. Here we argue that the antimicrobial conditions generated by host defences can accelerate the generation of genome rearrangements that provide selective advantages to the invading microbe. Similar exposure to environmental stress outside the host could also drive the horizontal gene transfer that has led to the evolution of pathogenicity towards both animals and plants.

The structure and function of Francisella lipopolysaccharide

A key factor in the biology of Francisella spp. is lipopolysaccharide (LPS). Francisella LPS has many unique structural properties and poorly activates proinflammatory responses due to its lack of interaction with toll-like receptor 4 (TLR4). The LPS of this organism can be modified by various carbohydrates including glucose, mannose and galactosamine, which affect various aspects of virulence. Spontaneously occurring colony variants of F. tularensis have altered LPS. This altered LPS may account for the novel phenotypes of these variants that include effects on susceptibility to killing by normal human serum, intracellular survival and animal virulence. Mutants devoid of O-antigen (directed mutants in O-antigen biosynthetic genes) show reduced intracellular survival and mouse virulence. Thus, this surface component is important in F. tularensis pathogenesis, and the inability of the LPS to alarm the immune system coupled with its frequent modification/alteration likely aid the success of this pathogen during human infection.