The impact of metal availability on immune function during infection

Abstract

Nutrient transition metals are required cofactors for many proteins to perform functions necessary for life. As such, the concentration of nutrient metals is carefully maintained to retain critical biological processes while limiting toxicity. During infection, invading bacterial pathogens must acquire essential metals, such as zinc, manganese, iron, and copper from the host to colonize and cause disease. To combat this, the host exploits the essentiality and toxicity of nutrient metals by producing factors that limit metal availability, thereby starving pathogens or accumulating metals in excess to intoxicate the pathogen in a process termed “nutritional immunity”. As a result of inflammation, a heterogeneous environment containing both metal replete and deplete niches is created, whereby nutrient metal availability may play an underappreciated role in regulating immune cell function during infection. How the host manipulates nutrient metal availability during infection, and the downstream implication that nutrient metals and metal sequestering proteins have on immune cell function is discussed in this review.

The concentration of nutrient transition metals is carefully maintained to avoid both deficiency and toxicity, as nutrient metals such as zinc, manganese, iron, and copper are required cofactors for many proteins that are critical for life. Therefore, pathogens have evolved strategies to acquire essential transition metals from the host to colonize and cause disease. To combat pathogens, the host either accumulates metals in excess to intoxicate the pathogen or produces factors that sequester and starve the pathogen of essential metals through a process termed ‘nutritional immunity’ [1, 2]. By exploiting the essentiality and toxicity of nutrient metals, the distribution of metals are altered dramatically through systemic and local changes that modulate the accessibility of metals to invading pathogens. However, immune cells must also operate in these same environments, therefore, changes in metal concentrations play an underappreciated but valuable role in regulating immune cell function during infection. This review focuses on the strategies by which the host manipulates nutrient metal availability, and the downstream implications that nutrient metal and metal sequestering proteins have on immune cell function.

Metal sequestering proteins during inflammation

S100 Proteins

S100 proteins are EF-hand calcium-binding proteins, and a subset of these proteins are released extracellularly and play a key antimicrobial role in host defense through metal sequestration. The S100 protein complex calprotectin is a heterodimer of S100A8 and S100A9 that binds and sequesters zinc, manganese, iron, and nickel [3–5]. It is highly abundant in neutrophils making up nearly 50% of the cytosolic protein content [6], and is therefore one of the most abundant immune proteins at the host-pathogen interface. As a result, calprotectin has broad antimicrobial activity toward multiple important human pathogens including Staphylococcus aureus [7, 8], Acinetobacter baumannii [9], Clostridioides difficile [10], Candida albicans [11], Aspergillus fumigatus [12] Helicobacter pylori [13], and Mycobacterium tuberculosis [14]. In addition, there is increasing evidence that other S100 proteins contribute to nutritional immunity. For example, S100A7, psoriasin, is highly expressed by keratinocytes [15] and may play a critical role in regulating metal availability among the microbiota of the skin through its capacity to bind zinc [16]. S100A7 is selectively antibacterial toward harmful bacteria such as Escherichia coli, Pseudomonas aeruginosa, and S. aureus, while apparently having no effect on commensal bacteria [17]. The human S100A12, calgranulin C [18], is expressed by monocytes, neutrophils [19], and keratinocytes [20, 21] and exhibits zinc-dependent antimicrobial activity against P. aeruginosa, C. albicans, and E. coli [22]. In addition, it has been suggested that the binding of copper to S100A12 provides antiparasitic activity through the generation of superoxide [20, 23].

Metallothioneins

Metallothioneins are a family of metal-binding proteins that bind zinc and copper in cells [24, 25], but also alleviate metal toxicity by sequestering heavy metals such as cadmium and mercury [26]. Excess zinc induces metallothionein expression, while zinc deplete conditions cause metallothioneins to release Zn as a means to balance the intracellular zinc pool in response to cellular redox and energy states [27, 28]. Metallothionein 1 (MT1) and metallothionein 2 (MT2) cooperatively sequester zinc and readily release only one zinc ion, which is distinct from metallothionein 3 (MT3) that non-cooperatively binds zinc and assumes and “open conformation” that readily releases all bound zinc [29–31]. The literature on metallothionein 4 (MT4) is scarce; however, it is postulated that MT4 may function as a copper-thionein [25]. Intracellularly, metallothioneins regulate zinc availability in the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, cytosol, and potentially zincosomes [32–36]. In addition, metallothioneins have been detected extracellularly, and evidence suggests involvement in actively modulating extracellular cues [37–39]. In support for active secretion of metallothioneins, MT3 interacts with proteins involved in multiple cellular processes including secretion, which would enable targeting to the extracellular milieu [40]. Whether metallothioneins are actively secreted or passively released requires further study.

Iron sequestering proteins

Iron is an essential trace element that is most abundant within erythrocytes, complexed to heme moieties in hemoglobin. To limit the production of free radicals in healthy individuals, iron in the plasma is sequestered by transferrin or stored within macrophages, hepatocytes, and intestinal enterocytes. During inflammation, increased production of hepcidin by the liver, neutrophils, and macrophages inhibits iron secretion thereby facilitating a precipitous drop in free extracellular iron. In addition, the immune protein lactoferrin is released by neutrophils through degranulation and neutrophil extracellular trap (NET) formation, and elicits direct antimicrobial activity by sequestering iron from bacterial pathogens such as S. aureus [41, 42], P. aeruginosa, Burkholderia cenocepacia [43], and the parasite Pneumocystis carinii [44, 45].

The importance of the struggle for iron is highlighted by the arms race between host and pathogen. A primary strategy bacterial pathogens employ to acquire iron is the secretion of an arsenal of low molecular weight iron-binding compounds called siderophores that chelate environmental iron with an extraordinarily high affinity, whereupon the iron-siderophore complex is taken up by bacterial receptors (covered in-depth in other reviews [46, 47]). In response, neutrophils secrete lipocalin-2 to sequester bacterial siderophores, preventing their uptake by bacterial cells, and thus, iron starving pathogens like E. coli during infection [48]. Some pathogens, such as Bacillus anthracis and Salmonella Typhimurium, produce ‘stealth siderophores’ that contain structural modifications to preclude lipocalin-2 binding [49, 50]. Finally, much of the iron in the host is in the form of heme. As such, activation of pathogen recognition receptors triggers the secretion of cytokines, such as interleukin (IL)-6 and IL-22, that cause the liver to produce haptoglobin and hemopexin to sequester free hemoglobin and heme during infection [51–53].

Ceruloplasmin

While during infection many nutrient metals are reduced or restricted, a gradual rise in serum copper is a common hallmark of infection, regardless of the causative agent [54, 55]. This elevation in copper is likely attributed to the cuproprotein ceruloplasmin, which accounts for 95% of the copper content in serum. Coinciding with a role during infection, ceruloplasmin is strongly induced during infection [56] with a marked increase in protein levels [57–59]. It is possible that the increased abundance of ceruloplasmin delivers copper to the site of infection, as monocytes, granulocytes, and lymphocytes contain ceruloplasmin receptors [60]. However, a study assessing ceruloplasmin-deficient mice revealed a disruption in iron homeostasis, not copper [61], which suggests that as a ferroxidase, ceruloplasmin may help mobilize iron away from infected tissues.

Metal mobilization to combat pathogens in the phagosome

Upon engaging many pathogens, professional phagocytes internalize the pathogen into phagosomal compartments. In an effort to prevent intracellular replication within the phagosome, host phagocytes mobilize the subcellular distribution of metals to both exploit the essentiality and toxicity of nutrient metals. Zinc [62, 63] and copper [64–67] are actively accumulated within the phagosomal compartment through a process referred to as the ‘brass dagger’, while iron, manganese, and magnesium are depleted by natural resistance-associated macrophage protein 1 (NRAMP1) [68–70]. The mechanisms by which the host transports metals in the phagosome is covered in-depth in a previous review [71].

The influence of nutrient metal on immune cells

Zinc

The role of zinc in regulating immune cells is complex and the functional outcomes of zinc replete or deplete conditions likely vary depending on the cell type. Zinc deplete conditions increase the overall number of granulocytes and monocytes in circulation while attenuating lymphopoiesis and erythropoiesis [72] and promoting monocyte maturation [73]. In addition, stimulation with lipopolysaccharide (LPS) decreases intracellular zinc levels within dendritic cells, which is required for dendritic cell maturation and antigen presentation [74]. These findings suggest that zinc deplete environments may heighten the inflammatory potential; but this is not always the case as zinc supplementation increases the number of peritoneal macrophages in a Trypanosoma cruzi infection model [75].

Further underlying the necessity for zinc, many pro-inflammatory signaling pathways require zinc. For example, zinc associates with LPS and influences its fluidity, causing it to more effectively induce cytokine production in human peripheral blood mononuclear cells (PBMCs) [76]. In addition, LPS causes a transient spike of intracellular free zinc in mouse and human monocytes/macrophages, and this effect is required for activation of p38 mitogen-activated protein kinase (MAPK) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) [77]. As a result, zinc supplementation enhances LPS-mediated tumor necrosis factor-α (TNF-α) production by monocytes and PBMCs [76, 77]; however, this does not apply for all conditions as zinc depletion may also increase TNF-α production by monocytes in response to LPS [73]. Zinc is also necessary for NLRP3 inflammasome activation [78], where the addition of zinc leads to enhanced levels of IL-1β [76, 78].

Zinc homeostasis has a profound effect on immune cell function that can be cell specific during infection. In response to pathogens, zinc depletion impairs phagocytosis of bacteria by neutrophils [79], but high levels of intracellular zinc amplify the respiratory burst [80]. While zinc is required for the formation of NETs [81], high levels of zinc inhibits NET formation and degranulation [82]. By contrast, zinc supplementation enhances phagocytosis of E. coli, S. aureus, [83, 84], Trypanosoma musculi [85], and Candida krusei [86] by primary macrophages with no effect on RAW264.7 macrophage-like cells [87] or bone marrow-derived macrophages [88]. Interestingly, zinc depletion also increases phagocytosis by macrophages [79]. This suggests that zinc concentrations, whether low or high, affects phagocytosis similarly; however, the effect is opposite when comparing neutrophils and macrophages. Coinciding with enhanced phagocytosis by primary macrophages following zinc supplementation, zinc replete conditions promote a stronger respiratory burst by NADPH oxidase in response to E. coli [73] and S. aureus [89]. This brings up an interesting dichotomy. Following phagocytosis, zinc is mobilized into the phagosome to intoxicate the pathogen [62, 63]; however, high levels of zinc in the phagosome could lower the capacity for the NADPH oxidase to generate reactive oxygen species [90]. Adding further complications, macrophages obtained from E. coli-infected rats that have been supplemented with zinc produce higher levels of reactive oxygen species than rats not receiving zinc [91]. But, supplementation of zinc in vitro inhibits the reactive oxygen species production by macrophages isolated from rats undergoing septicemia [91]. This suggests that the mechanisms by which zinc homeostasis is maintained in vivo during infection is cell-specific and delicately balanced to maintain optimal immune cell function.

Metallothioneins also play a critical role in regulating zinc availability and immune cell function. A majority of the work assessing metallothionein function has occurred in T cells and this has been covered in-depth in a previous review [92]. In macrophages, metallothioneins play a critical role in regulating M1/M2 skewing. Mice lacking MT1 and MT2 fail to strongly induce TNFα in response to LPS, suggesting MT1 and MT2 are required for a pro-inflammatory (M1) macrophage response [93]. Additionally, these mice exhibit gross defects in antigen-presentation, expression of MHCII and co-stimulatory molecules, and cytokine production [94]. In response to the pro-inflammatory cytokine, granulocyte macrophage colony stimulating factor (GM-CSF), a pronounced induction of MT2 and mild upregulation of MT1 is observed, which curtails the intracellular growth of Histoplasma capsulatum by sequestering zinc [34]. In contrast, the anti-inflammatory cytokine, IL-4, strongly induces MT3 [95], which is required for an anti-inflammatory (M2) macrophage response [96]. In addition, MT3 expression elevates the labile zinc pool within the macrophage and facilitates the intracellular survival of H. capsulatum [95]. An immunological role for MT4 has not been identified.

Iron/Heme

Hepcidin is a master regulator of iron homeostasis. Inflammatory cytokines, such as IL-1, IL-6, and IL-22, as well as pathogen-associated molecular patterns (PAMPs), such as LPS, induce hepcidin expression and secretion [97–99], thereby blocking cellular export of iron by ferroportin [100]. As a result, macrophages and monocytes withhold higher levels of iron intracellularly. While this iron withholding strategy is beneficial to starve extracellular pathogens of iron, it also has a significant impact on immune cell function. Accumulation of intracellular iron affects the antimicrobial functions of macrophages via inhibition of IFN-γ-inducible effector pathways [101, 102]. In addition, accumulation of intracellular iron impairs the expression of inducible nitric oxide synthase (iNOS) [29, 103, 104], which renders macrophages less capable of clearing infections with the intracellular pathogens Salmonella enterica and typhimurium [105, 106], Chlamydia pneumoniae [107], C. albicans [108], and Legionella pneumophila [109]. In addition, macrophages are critical in recycling iron to meet the metal needs for erythropoiesis; however, delivery of iron for erythropoiesis is blunted by hepcidin during inflammation. The depletion of red blood cells limits the production of the hormone erythropoietin that inhibits NF-κB activation in inflammatory macrophages and reduces expression of iNOS, TNF-α, IL-6, and IL-12, and impairs clearance of S. Typhimurium [110]. The inhibitory effect of intracellular iron accumulation is not limited to macrophages as the antibacterial activity of neutrophils are similarly impaired, including having reduced phagocytosis [111–113], reactive oxygen species production [114], and NET formation [115].

Iron exists mostly in the form of heme in the host. Heme is a necessary cofactor for NADPH oxidase [116], which is critical for producing reactive oxygen species that elicit oxidative stress upon pathogens in the phagosome. In addition, it has been postulated that heme is necessary to sense whether pathogens are alive or dead within the phagosome. Engagement of pathogens by pattern recognition receptors induces the expression of heme oxygenase-1 (HO-1) [117, 118]. The catabolism of heme by HO-1 produces carbon monoxide that diffuses across the cellular membrane and accesses the heme-containing respiratory complexes of bacteria, thereby inducing the bacteria to produce ATP [119]. As a result, the produced ATP is sensed by the P2X7 purinergic receptor of macrophages promoting NLRP3 activation and IL-1β secretion, which is not observed in response to dead bacteria [119]. Mycobacteria can sense carbon monoxide production by the macrophage and subvert this sensory mechanism by converting to a ‘dormant state’ [120].

Heme has also been demonstrated to play a critical role in inducing monocyte differentiation into erythrophagocytic macrophages [121, 122] and contributing to polarization of macrophages. Retention of iron by ferritin has been associated with the polarization of macrophages toward microbicidal function, while the polarization of macrophages toward tissue repair is associated with enhanced iron secretion and heme catabolism [123]. Specifically, heme catabolism likely contributes to a positive feedback loop where in response to LPS, catabolism by HO-1 promotes IL-10 secretion [124], thereby inducing HO-1 expression via MAPK [125] and signal transducer and activator of transcription 3 (STAT3) [126]. However, lowering intracellular heme-iron in macrophages via NRAMP1 in response to the intracellular pathogen S. Typhimurium can have the opposite effect and induce a more inflammatory state through the expression of TNF-α, IL-6, and IL-12, while introducing exogenous iron promotes expression of IL-10 [127]. Underscoring the importance of iron and heme, over 60% of the genes associated with iron homeostasis are differentially expressed between inflammatory and anti-inflammatory macrophages, which argues that the availability of heme-iron and metabolism are closely linked to macrophage polarization [123]. This suggests that iron and heme may play a central role in detecting the presence of live pathogens at the site of infection, and in the overall inflammatory state of the immune response by regulating macrophage polarization.

While essential for immune cell function, heme can be weaponized by some pathogens to suppress the immune response. The intraerythrocytic protozoan parasite Plasmodium, the causative agent of malaria, catabolizes hemoglobin as an essential source of energy [128]. However, the liberation of heme by this process is toxic to Plasmodium species. Since Plasmodium species are unable to secrete free heme and do not possess a heme oxygenase, they instead aggregate the heme into an insoluble crystal called hemozoin [129]. Monocytes and macrophages internalize but fail to degrade hemozoin, resulting in impaired functionality, even though these cells remain viable. Phagocytes that accumulate hemozoin show a significant reduction in subsequent phagocytosis [130], generation of reactive oxygen species by the respiratory burst [131], and reduced antimicrobial activity toward ingested E. coli, S. aureus, and C. albicans [132]. In addition, hemozoin accumulation severely impairs the activity of protein kinase C [133], limits antigen presentation [134], reduces secretion of IL-6 [135], and promotes the release of TNF-α [135–137], IL-1β [136], nitric oxide [138, 139], and macrophage-inhibitory proteins 1α and 1β [137].

Manganese

Compared to zinc and iron, much less is known about how manganese influences immune cell function. Increasing concentrations of manganese enhances the attachment of neutrophils to different extracellular matrixes [140, 141], suggesting that manganese availability may influence chemotaxis and cellular motility. In addition, higher levels of manganese also heighten degranulation [142], the respiratory burst, and killing of E. coli in vitro [143]. In macrophages, manganese mediates host defense against DNA viruses by heightening the sensitivity of the DNA sensor cGAS and its downstream adaptor protein STING [144], as well as promoting dendritic cell and macrophage maturation [145]. Despite the many beneficial effects of manganese in amplifying antimicrobial functions, mice fed a high manganese diet are more susceptible to systemic S. aureus infections with increased bacterial burdens in the heart; a phenotype not observed during C. difficile or A. baumannii infections [146]. While the authors demonstrate that manganese is utilized by S. aureus to detoxify reactive oxygen species to protect against neutrophil killing [146], this does not fully explain the heart-specific nature of the phenotype and suggests that manganese may alter other aspects of the immune response that are particularly important within the niche of the heart.

Copper

Similar to manganese, the role of copper in regulating immune cell function during infection is incompletely understood. Copper also plays an important role in regulating neutrophil function. A deficiency in copper substantially decreases the numbers of circulating neutrophils [147, 148] as well as reduces their capacity to generate reactive oxygen species [149–151]; however phagocytosis is unaffected by reduced copper [151]. Similarly, an impaired respiratory burst compromises the capacity for macrophages to kill C. albicans [152] and S. typhimurium [153] during copper deficiency. By contrast, heightened dietary copper leads to an unsustainable immune response with enhanced reactive oxygen species production by macrophages and neutrophils, impaired phagocytosis [154] and NET formation [44], and increased degranulation [155] and mitochondrial-mediated apoptosis [44, 154].

The intersection between DAMP activity and nutritional immunity

Many metal sequestering proteins have pleotropic roles in the immune response by acting as a damage-associated molecular pattern (DAMP) and/or opsonin during infection. For example, calprotectin not only binds nutrient metals but is also a potent DAMP that activates toll-like receptor 4 (TLR4) [156], receptor for advanced glycation end products (RAGE) [157], and CD33 [158]. As a result, calprotectin acts as a powerful chemoattractant for myeloid cells during inflammation [159, 160]. The biological effects of calprotectin are not limited to infection, as heightened expression of S100A8 and S100A9 and subsequent DAMP activity has been identified as a critical promoter of multiple cancers including acute myeloid leukemia [158, 161, 162], pancreatic ductal adenocarcinoma [163], and breast cancer [164]. In addition to responding to and regulating zinc availability, metallothioneins can act as a chemokine [165]; although, a specific receptor has not been identified. A diverse array of immunological stressors rapidly induce the expression of metallothioneins, which may support the role of extracellular metallothioneins as a DAMP that is recognized as an early indication of infection or tissues damage. Lactoferrin can exhibit iron-independent antibacterial activity by directly interacting with the cell membrane of bacteria like Bacillus subtilis [166], Klebsiella pneumoniae [167], and S. epidermidis [168], disrupting LPS in the outer membrane of Gram-negative bacteria [169], and causing damage to the cell wall of fungal pathogens like C. albicans [170, 171]. However, lactoferrin seems to support the growth of beneficial commensal bacteria such as Bifidobacteria and Lactobacillus [172]. In addition, lactoferrin possesses many anti-inflammatory functions by suppressing pro-inflammatory signaling through TL4 [173], CD14 [174], and L-selectin [175]. Even though the molecular events are less clear, lipocalin-2 seems to regulate proliferation, apoptosis [176, 177], and pyroptosis [178]. While lipocalin-2 has primarily been studied in the context of nutritional immunity during infection, metastatic cancer cells can express lipocalin-2 to acquire iron in metal deplete environments. Cancer metastases in the cerebrospinal fluid express high levels of lipocalin-2 and cognate receptor SCL22A17 as a means of acquiring iron and sustaining proliferation [179]. Ceruloplasmin levels are elevated during inflammation and infection; however, the effects on disease pathogenesis are unclear. It has been suggested that ceruloplasmin may play a critical role in preventing excessive oxidative damage to the host by binding myeloperoxidase, thereby inhibiting production of the oxidant hypochlorous acid by neutrophils [180, 181]. A better understanding of the pleiotropic functions of metal sequestering proteins is necessary to fully elucidate the complex interactions during inflammation.

Concluding Remarks

The role of metals at the host-pathogen interface is complicated. While significant progress has been made to determine how zinc and iron/heme regulate immune cell function, much less is understood regarding how other metals, such as manganese and copper, are influencing immune cell function. As such, a significant gap exists in the complete understanding of metal biology at the host-pathogen interface. Additionally, much of the work assessing how nutrient transition metal availability affects the immune cell response to pathogens occur in extreme metal excess or deplete environments. However, most healthy individuals do not experience uniform alterations in metal abundance, and during infection metal is not uniformly distributed. As a result, conclusions derived from extreme metal excess and deplete environments may not accurately reflect what is occurring at the host-pathogen interface. One intriguing possibility is that like cytokines, chemokines, metabolites, DAMPs, and PAMPs establish gradients that influence immune cell function in vivo, nutrient metal may play an underappreciated role in regulating immune cell function during infection. Furthermore, how the host mobilizes metals during infection may change depending on the pathogen and site of infection. Advancements made in characterizing the spatial and temporal distribution of metal during infection are necessary to elucidate how metals are influencing immune cell function at the host-pathogen interface.

Adding to the complexity of the host-pathogen interface, bacterial pathogens like S. aureus [8, 182–188], A. baumannii [9, 189–194], and C. difficile [195, 196] express multiple metal acquisition systems and siderophores that would also influence metal availability. While much of the focus has been on how these bacterial strategies of metal acquisition influence bacterial survival and growth within the host, it is relatively unknown how these changes in metal availability affect immune cell function. It is possible that these bacterial-induced changes in metal availability are a signal indicating infection, thereby activating the immune response, or are a strategy employed by bacteria to skew immune cell function in a manner advantageous to the pathogen by dampening or enhancing inflammation. How bacterial-induced changes in metal availability influences immune cell function at the host-pathogen interface provides an intriguing future avenue of research.

Furthermore, it is unclear how metal sequestering proteins like calprotectin, metallothionein, lactoferrin, lipocalin-2, and ceruloplasmin alter metal availability for immune cells at the host-pathogen interface. Whether immune cells can scavenge nutrient metals from metal binding proteins or sequestering of nutrient metals from pathogens also starves host-cells for those same metals is unclear. In addition, the possible synergistic effects between the diverse functions of metal-binding proteins on immune cells is not known. For example, whether metal binding by calprotectin alters or synergizes with its DAMP activity and the downstream effects this has on host cell function is unknown. Further studies assessing the full pleiotropic functions of these metal-binding proteins are necessary to truly appreciate the complex role of metals and their effects on immune cells at the host-pathogen interface.

Figure 1. The effect of nutrient metal on immune cell function.

A graphical representation showing how nutrient metal (zinc = Zn, iron = Fe, manganese = Mn, copper = Cu) excess and deplete conditions alter neutrophil and macrophage antimicrobial functions in response to pathogens. Arrows represent conditions that enhance and blocked lines represent conditions that impair the indicated immune cell function. No effect (NE) on phagocytosis by neutrophils has been observed in copper deplete conditions. Created with BioRender.com.

Figure 2. Nutrient metals modulate signal transduction in immune cells.

A graphical illustration of how intracellular levels of nutrient metals can influence signal transduction. Increased levels of cytosolic zinc (Zn) enhance IL-1β production from NLRP3 inflammasomes and LPS-mediated NF-κB and MAPK signaling. Elevated concentration of cytosolic manganese (Mn) heightens the sensitivity of cGAS and STING to double stranded DNA (dsDNA), thereby promoting the production of type-1 interferon (IFNβ). Accumulation of cytosolic heme suppresses the expression of TNF-α. LPS signaling promotes heme oxygenase (HO)-mediated degradation of heme, which reduces intracellular heme levels but produces carbon monoxide that activates IL-10. Heme degradation also increases intracellular concentrations of iron (Fe), which restricts pro-inflammatory signaling by impairing IFN-γ inducible effector pathways, including the intercellular adhesion molecule 1 (ICAM-1), and simultaneously repressing iNOS and promoting IL-10. In addition, increased intracellular storage of iron indirectly alters immune cell activation by reducing erythropoiesis, which prevents production of the hormone erythropoietin (EPO). Created with BioRender.com.

Highlights.

• During inflammation, nutrient metals are redistributed to starve and/or intoxicate invading pathogens.

• The availability of nutrient metals influences the response of innate immune cells to pathogens during infection.

• Metal sequestering proteins have pleotropic functions at the host-pathogen interface that dictate disease outcome.

Outstanding Questions.

• What role do concentration gradients of nutritional metals in the tissue play in regulating innate immune cell function during infection?

• Are immune cells capable of scavenging nutrient metals from metal binding proteins in metal deplete environment during infection or do metal binding proteins also starve host-cells for nutrient metals?

• Do synergistic effects exist between metal sequestration and the diverse function of metal-binding proteins and what implications does this have on immune cell function and disease outcome?