Ferritin H deficiency deteriorates cellular iron handling and worsens Salmonella typhimurium infection by triggering hyperinflammation

Iron is an essential nutrient for mammals as well as for pathogens. Inflammation-driven changes in systemic and cellular iron homeostasis are central for host-mediated antimicrobial strategies. Here, we studied the role of the iron storage protein ferritin H (FTH) for the control of infections with the intracellular pathogen Salmonella enterica serovar Typhimurium by macrophages. Mice lacking FTH in the myeloid lineage (LysM-Cre+/+Fthfl/fl mice) displayed impaired iron storage capacities in the tissue leukocyte compartment, increased levels of labile iron in macrophages, and an accelerated macrophage-mediated iron turnover. While under steady-state conditions, LysM-Cre+/+Fth+/+ and LysM-Cre+/+Fthfl/fl animals showed comparable susceptibility to Salmonella infection, i.v. iron supplementation drastically shortened survival of LysM-Cre+/+Fthfl/fl mice. Mechanistically, these animals displayed increased bacterial burden, which contributed to uncontrolled triggering of NF-κB and inflammasome signaling and development of cytokine storm and death. Importantly, pharmacologic inhibition of the inflammasome and IL-1β pathways reduced cytokine levels and mortality and partly restored infection control in iron-treated ferritin-deficient mice. These findings uncover incompletely characterized roles of ferritin and cellular iron turnover in myeloid cells in controlling bacterial spread and for modulating NF-κB and inflammasome-mediated cytokine activation, which may be of vital importance in iron-overloaded individuals suffering from severe infections and sepsis

Cell-specific expression of <i>Hfe</i> determines the outcome of <i>Salmonella enterica</i> serovar Typhimurium infection in mice

Mutations in HFE cause hereditary hemochromatosis type I hallmarked by increased iron absorption, iron accumulation in hepatocytes and iron deficiency in myeloid cells. HFE encodes an MHC-I like molecule, but its function in immune responses to infection remains incompletely understood. Here, we investigated putative roles of Hfe in myeloid cells and hepatocytes, separately, upon infection with Salmonella Typhimurium, an intracellular bacterium with iron-dependent virulence. We found that constitutive and macrophage-specific deletion of Hfe protected infected mice. The propagation of Salmonella in macrophages was reduced due to limited intramacrophage iron availability for bacterial growth and increased expression of the anti-microbial enzyme nitric oxide synthase-2. By contrast, mice with hepatocyte-specific deletion of Hfe succumbed earlier to Salmonella infection because of unrestricted extracellular bacterial replication associated with high iron availability in the serum and impaired expression of essential host defense molecules such as interleukin- 6, interferon-g and nitric oxide synthase-2. Wild-type mice subjected to dietary iron overload phenocopied hepatocyte-specific Hfe deficiency suggesting that increased iron availability in the serum is deleterious in Salmonella infection and underlies impaired host immune responses. Moreover, the macrophage-specific effect is dominant over hepatocytespecific Hfe-depletion, as Hfe knockout mice have increased survival despite the higher parenchymal iron load associated with systemic loss of Hfe. We conclude that cell-specific expression of Hfe in hepatocytes and macrophages differentially affects the course of infections with specific pathogens by determining bacterial iron access and the efficacy of antimicrobial immune effector pathways. This may explain the high frequency and evolutionary conservation of human HFE mutations

Availability of Ferritin-Bound Iron to Enterobacteriaceae

The sequestration of iron in case of infection, termed nutritional immunity, is an established strategy of host defense. However, the interaction between pathogens and the mammalian iron storage protein ferritin is hitherto not completely understood. To better characterize the function of ferritin in Gram-negative infections, we incubated iron-starved cultures of Salmonella Typhimurium and knockout mutant strains defective for major iron uptake pathways or Escherichia coli with horse spleen ferritin or ionic iron as the sole iron source. Additionally, we added bovine superoxide dismutase and protease inhibitors to the growth medium to assess the effect of superoxide and bacterial proteases, respectively, on Salmonella proliferation and reductive iron release. Compared to free ionic iron, ferritin-bound iron was less available to Salmonella, but was still sufficient to significantly enhance the growth of the bacteria. In the absence of various iron acquisition genes, the availability of ferritin iron further decreased. Supplementation with superoxide dismutase significantly reduced the growth of the &Delta;entC knockout strain with holoferritin as the sole iron source in comparison with ionic ferrous iron. In contrast, this difference was not observed in the wildtype strain, suggesting that superoxide dismutase undermines bacterial iron uptake from ferritin by siderophore-independent mechanisms. Ferritin seems to diminish iron availability for bacteria in comparison to ionic iron, and its iron sequestering effect could possibly be enhanced by host superoxide dismutase activity