Cited by 183Iron Metabolism and DisordersIron and Steelmaking ProcessesFerroptosis and cancer prognosis

The DOI matches its Crossref record; 267 of the 269 references it cites were found in a registry or an index; 5 of the 158 sources its claims cite were checked for support.

“Pumping iron”—how macrophages handle iron at the systemic, microenvironmental, and cellular levels

Open access
Nairz, Manfred, Theurl, Igor, Swirski, Filip K., Weiss, Guenter·2017·Pflügers Archiv - European Journal of Physiology

Abstract

Macrophages reside in virtually every organ. First arising during embryogenesis, macrophages replenish themselves in the adult through a combination of self-renewal and influx of bone marrow-derived monocytes. As large phagocytic cells, macrophages participate in innate immunity while contributing to tissue-specific homeostatic functions. Among the key metabolic tasks are senescent red blood cell recycling, free heme detoxification, and provision of iron for de novo hemoglobin synthesis. While this systemic mechanism involves the shuttling of iron between spleen, liver, and bone marrow through the concerted function of defined macrophage populations, similar circuits appear to exist within the microenvironment of other organs. The high turnover of iron is the prerequisite for continuous erythropoiesis and tissue integrity but challenges macrophages' ability to maintain cellular iron homeostasis and immune function.This review provides a brief overview of systemic, microenvironmental, and cellular aspects of macrophage iron handling with a focus on exciting and unresolved questions in the field.

Overview

AI snapshot

Written from the paper’s full text.

TL;DR

This review synthesizes how macrophages handle iron at systemic, microenvironmental, and cellular levels, emphasizing red blood cell recycling, the hepcidin–ferroportin axis, and local iron circuits in organs such as liver, spleen, muscle, heart, vessels, brain, retina, bone, and bone marrow. It argues that macrophage iron homeostasis is central to erythropoiesis, tissue integrity, and immunity, while many mechanistic questions remain unresolved.

Key findings

  • Systemic iron homeostasis depends on macrophage recycling of senescent red blood cells: 20–25 mg/day of iron is needed for new RBC production, dietary absorption supplies only 1–2 mg/day, the adult Hb iron pool is 1500–2500 mg, RBC lifespan averages 120 days, and splenic red pulp macrophages clear more than 2 × 10⁶ RBC per second in a healthy adult.
  • The hepcidin–ferroportin-1 (Fpn1) axis is central: hepatocyte-derived hepcidin targets Fpn1 for degradation, reducing iron transfer from macrophages and the diet into plasma; macrophage iron handling is further controlled by Bach1/Nrf2, SpiC, HIFs, IRPs, Hmox1, Flvcr, and Nramp1.
  • Local iron-recycling circuits exist beyond the spleen and liver: liver Kupffer cells dominate clearance during excessive RBC damage and recruit CCR2⁺ monocytes via CCL2/CCL3; muscle injury involves myoglobin (17 kDa, below the kidney’s 30–50 kDa filtration cutoff); and analogous macrophage iron handling is discussed in heart, atherosclerotic plaques, brain microglia, retina, osteoclasts, and bone marrow erythroid islands.
  • Macrophage iron overload impairs antimicrobial immunity: iron inhibits NF-IL6 and HIF-1α binding, blocks Nos2/NO formation, and promotes ROS/NF-κB, while IRP1/IRP2 deletion impairs control of Salmonella Typhimurium.

Methods

  • Narrative review; no primary experiments, systematic search strategy, or meta-analysis are described.
  • Synthesizes published literature and includes a table of selected macrophage iron-handling proteins and figures on systemic and cellular iron pathways.
  • Main analysis is conceptual and mechanistic synthesis across systemic, organ-level, and cellular contexts.

Limitations

  • Stated: many unresolved questions remain, including mechanistic insight into atherosclerosis, brain iron handling, and iron shuttling from erythroid island macrophages to erythroblasts; findings from small animal models need translation to humans.
  • Stated: some areas are controversial, such as the role of Kupffer cells in hepcidin regulation and the directionality of Nramp1 iron transport.
  • Evident: as a narrative review, it lacks systematic inclusion criteria and primary data, so conclusions depend on selected cited literature and may reflect selection bias.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1007/s00424-017-1944-8

Cites

267 of 267 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Figures

Fig. 1 a In steady state, 20–25 mg/day of iron is needed to generate new red blood cells (RBC) in the bone marrow (depicted as vertebral body). The RBC of an adult human form the body’s largest iron pool (1500–2500 mg contained in hemoglobin [Hb]). After an average lifetime of 120 days, aged RBC (aRBC) are trapped in the spleen, recognized by red pulp macrophages (RPM), and eliminated. Their Hb is recycled, and ferroportin (Fpn)-1 exports iron into sinusoidal capillaries where it is loaded onto transferrin (TF). TF-bound iron is utilized by developing erythroblasts (EB) via TF receptor (Tfr)-1. Erythroid island macrophages assist in iron uptake and differentiation. Plasma iron content is sensed by hepatocytes, presumably via Tfr1 and other iron-sensitive molecules. They affect the transcriptional activation of hepcidin, the major iron hormone, in hepatocytes. Hepcidin acts as negative feedback regulator of iron influx by targeting Fpn1 resulting in degradation of the latter and thus reduction of iron transfer from the diet and macrophages to the plasma. Kupffer cells (KC) inhibit hepcidin expression by hepatocytes and also engage in erythrophagocytosis. b The lifespan of RBC can be reduced as a consequence of autoinflammation and subsequent hemolysis, radical formation and toxin activity or based on intrinsic structural or metabolic defects. The rapid accumulation of damaged RBC (dRBC) may overwhelm the spleen’s clearance capacity. In such a scenario, KC engulf dRBC which may result in cell death. As back-up system, monocytes are recruited from the bone marrow and possibly spleen via chemokines CCL2 and CCL3 and their receptor CCR2. These monocytes encounter a niche in the liver and differentiate into KC-like cells that express Fpn1 which is induced by several mechanisms including the growth factor macrophage colony-stimulating factor (M-CSF). Fpn1-mediated iron export sustains increased erythropoiesis in the bone marrow to compensate for losses by RBC damage
Fig. 1 a In steady state, 20–25 mg/day of iron is needed to generate new red blood cells (RBC) in the bone marrow (depicted as vertebral body). The RBC of an adult human form the body’s largest iron pool (1500–2500 mg contained in hemoglobin [Hb]). After an average lifetime of 120 days, aged RBC (aRBC) are trapped in the spleen, recognized by red pulp macrophages (RPM), and eliminated. Their Hb is recycled, and ferroportin (Fpn)-1 exports iron into sinusoidal capillaries where it is loaded onto transferrin (TF). TF-bound iron is utilized by developing erythroblasts (EB) via TF receptor (Tfr)-1. Erythroid island macrophages assist in iron uptake and differentiation. Plasma iron content is sensed by hepatocytes, presumably via Tfr1 and other iron-sensitive molecules. They affect the transcriptional activation of hepcidin, the major iron hormone, in hepatocytes. Hepcidin acts as negative feedback regulator of iron influx by targeting Fpn1 resulting in degradation of the latter and thus reduction of iron transfer from the diet and macrophages to the plasma. Kupffer cells (KC) inhibit hepcidin expression by hepatocytes and also engage in erythrophagocytosis. b The lifespan of RBC can be reduced as a consequence of autoinflammation and subsequent hemolysis, radical formation and toxin activity or based on intrinsic structural or metabolic defects. The rapid accumulation of damaged RBC (dRBC) may overwhelm the spleen’s clearance capacity. In such a scenario, KC engulf dRBC which may result in cell death. As back-up system, monocytes are recruited from the bone marrow and possibly spleen via chemokines CCL2 and CCL3 and their receptor CCR2. These monocytes encounter a niche in the liver and differentiate into KC-like cells that express Fpn1 which is induced by several mechanisms including the growth factor macrophage colony-stimulating factor (M-CSF). Fpn1-mediated iron export sustains increased erythropoiesis in the bone marrow to compensate for losses by RBC damageCC BY · PubMed Central
Fig. 2 a In homeostatic conditions, iron uptake and release pathways are coordinated ensuring efficient iron recycling from aged red blood cells (aRBC) and iron delivery to sites of erythropoiesis. Apart from erythrophagocytosis, iron uptake mechanisms include (left to right) ferritin (FT) receptors such as scavenger receptor class A member (Scara)-5 and T-cell immunoglobulin and mucin domain-containing molecule (Tim)-2, transferrin (TF; depicted as monomer for simplicity) and its receptor Tfr1, the ferric reductase duodenal cytochrome B (DcytB) and divalent metal transporter (Dmt)-1, lactoferrin (LF) and its receptor LfR, lipocalin (Lcn)-2 and its receptor LcnR, haptoglobin-hemoglobin (Hb) complexes and their receptor CD163, and hemopexin (HPX)-heme complexes and their receptor CD91. Endosomal iron transporters Dmt1, natural resistance-associated macrophage protein-1 (Nramp1), and heme-regulated gene-1 (HRG1) shift ferrous iron or heme to the cytoplasm, from where the former can be exported via ferroportin (Fpn)-1 and the latter metabolized by heme oxygenase (Hmox)-1 or exported via feline leukemia virus subgroup C receptor (Flvcr). Macrophage iron homeostasis ensures proper function including the activity of pivotal transcription factors nuclear factor (NF)-IL6, NF-κB, and hypoxia-inducible factor (HIF)-1α, all of which are iron regulated. b Macrophage iron overload may result from an increase in circulating or local hepcidin concentrations, an elevated TF saturation, free iron excess, or hemolysis with subsequent accumulation of damaged RBC (dRBC), free Hb, or free heme. If severe, all these mechanisms will overwhelm the macrophage’s capacity to contain, store, and detoxify iron resulting in an increase of free iron, heme, and/or FT in cells. On the one hand, surplus intracellular iron may serve as nutrient for intraphagosomal or cytoplasmatic pathogens (e.g. for Gram-negative rods in the Tfr1-endosome and the cytoplasm). On the other hand, iron overload differentially affects distinct innate immune pathways. Surplus iron blocks the binding of transcription factors NF-IL6 and HIF-1α to their respective target promoter sequences. In parallel, iron facilitates the generation of ROS and thus NF-κB activation. Therefore, iron overload may cause a dysbalance in the transcriptional response to macrophage activation
Fig. 2 a In homeostatic conditions, iron uptake and release pathways are coordinated ensuring efficient iron recycling from aged red blood cells (aRBC) and iron delivery to sites of erythropoiesis. Apart from erythrophagocytosis, iron uptake mechanisms include (left to right) ferritin (FT) receptors such as scavenger receptor class A member (Scara)-5 and T-cell immunoglobulin and mucin domain-containing molecule (Tim)-2, transferrin (TF; depicted as monomer for simplicity) and its receptor Tfr1, the ferric reductase duodenal cytochrome B (DcytB) and divalent metal transporter (Dmt)-1, lactoferrin (LF) and its receptor LfR, lipocalin (Lcn)-2 and its receptor LcnR, haptoglobin-hemoglobin (Hb) complexes and their receptor CD163, and hemopexin (HPX)-heme complexes and their receptor CD91. Endosomal iron transporters Dmt1, natural resistance-associated macrophage protein-1 (Nramp1), and heme-regulated gene-1 (HRG1) shift ferrous iron or heme to the cytoplasm, from where the former can be exported via ferroportin (Fpn)-1 and the latter metabolized by heme oxygenase (Hmox)-1 or exported via feline leukemia virus subgroup C receptor (Flvcr). Macrophage iron homeostasis ensures proper function including the activity of pivotal transcription factors nuclear factor (NF)-IL6, NF-κB, and hypoxia-inducible factor (HIF)-1α, all of which are iron regulated. b Macrophage iron overload may result from an increase in circulating or local hepcidin concentrations, an elevated TF saturation, free iron excess, or hemolysis with subsequent accumulation of damaged RBC (dRBC), free Hb, or free heme. If severe, all these mechanisms will overwhelm the macrophage’s capacity to contain, store, and detoxify iron resulting in an increase of free iron, heme, and/or FT in cells. On the one hand, surplus intracellular iron may serve as nutrient for intraphagosomal or cytoplasmatic pathogens (e.g. for Gram-negative rods in the Tfr1-endosome and the cytoplasm). On the other hand, iron overload differentially affects distinct innate immune pathways. Surplus iron blocks the binding of transcription factors NF-IL6 and HIF-1α to their respective target promoter sequences. In parallel, iron facilitates the generation of ROS and thus NF-κB activation. Therefore, iron overload may cause a dysbalance in the transcriptional response to macrophage activationCC BY · PubMed Central

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    No regulated pathway of iron excretion exists in the mammalian organism.

    “It is generally thought that no regulated pathway of iron excretion exists in the mammalian organism.”
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Metadata

Authors
Nairz, M., Theurl, I., Swirski, F. K., & Weiss, G.
Affiliations
  • Harvard University (US)
  • Innsbruck Medical University (AT)
Corresponding author
Nairz, Manfred
Venue
Pflügers Archiv - European Journal of Physiology
Published
2017-03-01
Volume
469
Issue
3-4
Pages
397-418
ISSN
  • 0031-6768 (ISSN-L)
  • 1432-2013
OpenAlex
W2592213450
Cited by
178
References
269
Citations by year
2026: 8 · 2025: 15 · 2024: 15 · 2023: 22 · 2022: 23 · 2021: 28 · 2020: 31 · 2019: 14 · 2018: 17 · 2017: 4
FWCI
2.87
Type
journal article
Language
en
Open access
Locations
  • Pflügers Archiv - European Journal of Physiology (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Digital Access to Scholarship at Harvard (DASH) (Harvard University) (submittedVersion, cc-by)
Funders
University of Innsbruck and Medical University of Innsbruck
Topics
  • Health Sciences › Medicine › Hematology › Iron Metabolism and Disorders
  • Physical Sciences › Materials Science › Metals and Alloys › Iron and Steelmaking Processes
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Cancer Research › Ferroptosis and cancer prognosis
Keywords
macrophages, iron homeostasis, iron metabolism, heme detoxification, hemoglobin synthesis, spleen, liver, bone marrow, tissue microenvironment, innate immunity, erythropoiesis
MeSH terms
Animals, Erythropoiesis, Homeostasis, Humans, Iron, Macrophages, Cellular Microenvironment

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