The Iron age of host-microbe interactions

Abstract

Microbes exert a major impact on human health and disease by either promoting or disrupting homeostasis, in the latter instance leading to the development of infectious diseases. Such disparate outcomes are driven by the ever-evolving genetic diversity of microbes and the countervailing host responses that minimize their pathogenic impact. Host defense strategies that limit microbial pathogenicity include resistance mechanisms that exert a negative impact on microbes, and disease tolerance mechanisms that sustain host homeostasis without interfering directly with microbes. While genetically distinct, these host defense strategies are functionally integrated, via mechanisms that remain incompletely defined. Here, we explore the general principles via which host adaptive responses regulating iron (Fe) metabolism impact on resistance and disease tolerance to infection.

Keywords: anemia of chronic disease; disease tolerance; heme; iron; macrophage; nutritional immunity; tissue damage control.

Figure 1. Interrelationship of Fe and Heme metabolism

More than 80% of the bioavailable Fe in mammals exists in the form of heme contained in hemoproteins . The most abundant pool of Fe in mammals are the prosthetic heme groups of hemoglobin in red blood cells (RBC), followed by the heme groups of myoglobin in muscle cells and those of cytochromes and other ubiquitously expressed hemoproteins in all cells ,. The Fe required to sustain hemoglobin synthesis is made available by hemophagocytic Mø as these clear senescent RBC by erythrophagocytosis (top right) . The heme contained in hemoglobin is transported by heme-responsive gene-1 (HRG1) into the cytoplasm where Fe is extracted by heme-oxygnease-1 (HO-1), exported via ferroportin (FPN), and delivered to traferrin (Tf) in plasma. Surplus cytoplasmic Fe in Mø is incorporated and stored within ferritin. Tf transports and provides Fe to the erythropoietic compartment via TfR, where it is used for heme synthesis (bottom right). Heme synthesis occurs via eight successive enzymatic reactions that take place back and forward in the mitochondria (brown) and the cytosol (blue). For details, see Box 1.

Figure 2. Regulation of Fe metabolism in response to extracellular pathogens

(A) Immune responses to extracellular pathogens encompass the production of cytokines, for example, IL-1, IL-6, and IL-22, which induce the transcription of the hepcidin (HAMP) gene in hepatocytes. Cellular Fe retention by Mø can lead to the development of anemia of inflammation, which triggers the production of several hormones, such as Epo, Erfe, GDF15, PDGF-BB, or the activation of the HIF family of transcription factors, which reduce hepcidin expression. (B) Circulating hepcidin targets systemically the Fe export protein FPN for degradation, which inhibits Fe cellular retention in many cell types, including Mø, which are pivotal to the maintenance of Fe homeostasis. This is supported by the autocrine production of hepcidin by Mø. In addition, cytokines, such as IFNγ and TNF, also inhibit FPN transcription in Mø (not shown) promoting Fe retention and limiting further Fe availability to extracellular microbes. (C) Upon infection by extracellular pathogens, Mø can uptake extracellular Fe via different mechanisms involving Tf/TfR interaction, (D) the lactoferrin/lactoferrin receptor, (E) the Lcn2/Lcn2R, or (F) the divalent metal transporter DMT1. All of these contribute to scavenge extracellular Fe and prevent extracellular pathogens from accessing Fe. (G) In addition, Mø can engulf damaged RBC and prevent hemoglobin release or (H) scavenge extracellular hemoglobin/haptoglobin via the scavenger receptor CD163 as well as (I) heme/HPX complexes via the scavenger receptor CD91. (J) Uptake of heme/HPX, damaged RBC, or hemoglobin/haptoglobin by Mø is coupled to heme transport from phagolysomes to the cytoplasm, by the heme transporter HRG1, and to heme catabolism by HO-1. These pathways are induced by several cytokines in the course of an infection.

Figure 3. Regulation of Fe metabolism in response to intracellular pathogens

Mø activation in response to intracellular pathogens is associated with a reduction of intracellular Fe levels. This occurs via different mechanisms. (A) Fe or heme is exported from phagolysosomes by NRAMP1 and FPN or by HRG1, reducing Fe and heme availability to pathogens residing within this subcellular compartment. Heme catabolism by HO-1 induces the expression of the Fe-scavenging protein ferritin that stores intracellular Fe in Mø, away from intracellular pathogens. (B) Mø activation in response to PRR and/or IFNγR signaling activates the transcription factors NF-κB and STAT1/2, which induce the expression of the Fe-binding proteins ferritin, lactoferrin, Lcn2, and the Fe transporter DMT1 as well as NOX2/gp91^phox and iNOS/NOS2. (C) IFNγR signaling also inhibits Fe extracellular uptake via TfR. (D) The NO produced by iNOS/NOS2 induces, via Keap1, the activation of the transcription factor Nrf2, which induces the expression of FPN and hence Fe export from phagolysosomes and/or from the cytoplasm. (E) Lcn2 neutralizes the action of bacterial siderophores. (F) Lactoferrin binds Fe and limits its availability to bacteria.

Figure 4. Fe/heme metabolism and disease tolerance to infection

RBC are present at high numbers throughout the body . Cytotoxic products generated by pathogens can damage RBC, which are rapidly engulfed and cleared by Mø. Alternatively, damaged RBC release hemoglobin (HB) into plasma, which can exert deleterious effects, compromising disease tolerance. For example, extracellular hemoglobin can scavenge NO, promoting vasoconstriction and eventually affecting microvascular circulation (top). Oxidized extracellular hemoglobin can also release heme, which acts as a pro-inflammatory agonist in endothelial cells and Mø via TLR4, or via GPCRs in PMN cells. Labile heme sensitizes parenchyma cells to undergo programmed cell death in response to TNF, leading to tissue damage and compromising disease tolerance (right). These pathogenic effects are countered by tissue damage control mechanisms that confer disease tolerance to infection. The hemoglobin and heme scavengers HP and HPX, respectively, shuttle heme for degradation by HO-1 via the Mø scavenger receptors CD163 and CD91 (middle). This is coupled to mechanisms regulating intracellular Fe reactivity. These include ferritin, which stores intracellular Fe and FPN, which delivers Fe to Tf and supports erythropoiesis. This prevents anemia and tissue hypoxia, an effect likely to promote disease tolerance. This heme/Fe detoxifying mechanism also operates in parenchyma tissues to prevent programmed cell death and tissue damage, presumably enforcing disease tolerance (right). One of the mechanisms relies on the Fe-scavenging capacity of ferritin, which acts in an antioxidant manner and prevents sustained JNK activation from triggering programmed cell death in response to TNF. Moreover, HO-1 enzymatic activity generates CO, which promotes the secretion of IL-10 in activated Mø, impacting on MHC class II/TCR-driven activation of T_H cells and thereby contributing to tissue damage control and eventually to disease tolerance (middle). CO also binds to the heme groups of extracellular hemoglobin and prevents heme release, thus protecting tissues from the vasoactive, pro-inflammatory, and cytotoxic effects of labile heme. Finally, CO is a potent cytoprotective molecule that prevents tissue damage and as such can enforce disease tolerance to infection.