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Iron homeostasis and macrophage biology are closely interconnected. On the one hand, iron exerts multiple effects on macrophage polarization and functionality. On the other hand, macrophages are central for mammalian iron homeostasis. The phagocytosis of senescent erythrocytes and their degradation by macrophages enable efficient recycling of iron and the maintenance of systemic iron balance. Macrophages express multiple molecules and proteins for the acquisition and utilization of iron and many of these pathways are affected by inflammatory signals. Of note, iron availability within macrophages has significant effects on immune effector functions and metabolic pathways within these cells. This review summarizes the physiological and pathophysiological aspects of macrophage iron metabolism and highlights its relevant consequences on immune function and in common diseases such as infection and atherosclerosis., (Copyright © 2014 Elsevier GmbH. All rights reserved.)

Neutrophil gelatinase-associated lipocalin (NGAL/Lipocalin-2/Lcn-2) is a 25kDa protein which is involved in host defence against certain Gram negative bacteria upon binding of iron loaded bacterial siderophores thereby limiting the availability of this essential nutrient to bacteria resulting in inhibition of their growth and pathogenicity. As iron is important for the growth of the intracellular bacterium Chlamydia pneumoniae we questioned whether Lcn-2 affects the course of this infection. We employed primary peritoneal macrophages obtained from wildtype and Lcn-2 -/- mice and RAW 264.7 cells which were infected with C. pneumoniae. In addition, we studied C. pneumoniae multiplication in vivo in mice receiving diets with varying iron contents. We analyzed C. pneumoniae numbers by immunohistochemistry and RT-PCR and studied the expression of iron metabolism and cytokine genes by RT-PCR, Western blot or ELISA. Infection with Chlamydiae ex vivo and in vivo revealed a significantly higher bacterial growth in peritoneal macrophages of Lcn-2 -/- than of wildtype mice. These differences were significantly more pronounced upon iron challenge, which stimulated bacterial growth. Accordingly, treatment with an anti-Lnc-2 antibody increased whereas addition of recombinant Lcn-2 reduced bacterial growth in infected macrophages. When investigating the underlying mechanisms we observed partly different expression of several iron metabolism genes between Lcn-2 +/+ and Lcn-2 -/- macrophages and most strikingly an increased formation of the anti-inflammatory cytokine IL-10 by Lcn-2 -/- macrophages. Upon treatment with an anti-IL10 antibody we experienced a significant increase of Chlamydial growth within Lcn-2 -/- macrophages along with a reduction of the major iron storage protein ferritin. Herein we provide first time evidence that Lcn-2 is involved in host defence against Chlamydia presumably by limiting the availability of iron to the pathogen. In the absence of Lcn-2, increased formation of IL-10 exerts protective effects by increasing the intracellular formation of ferritin, thereby reducing the access of iron for bacteria., (Copyright © 2012 Elsevier GmbH. All rights reserved.)

In mice, the expression of the phagolysosomal protein natural-resistance associated macrophage protein 1 (Nramp1, Slc11a1) is associated with host resistance to various intracellular pathogens. Nramp1 acts as a transporter for protons, iron, and other divalent cations, and Nramp1 functionality is associated with an enhanced activity of pro-inflammatory immune pathways, including the formation of nitric oxide (NO) via transcriptional stimulation of inducible nitric oxide synthase (iNOS) expression. As iron availability also strongly influences iNOS expression, we studied the effects of Nramp1 functionality on iron homeostasis in the RAW264.7 macrophage cell line stably transfected with functional or non-functional Nramp1. We found that macrophages lacking functional Nramp1 exhibited a significantly higher iron uptake via transferrin receptor 1 and, as a consequence of this, an increased iron release which is mediated via the iron export protein ferroportin-1. RNA-bandshift experiments for determination of iron regulatory protein activity showed that, as a net effect of the altered expression of iron transporters, the overall cellular iron content was lower in macrophages bearing functional Nramp1. Since low intracellular iron availability enhances iNOS transcription, Nramp1 could exert its effect on NO formation and other pro-inflammatory immune pathways via modulation of iron homeostasis.