Immune cells
The activation of immune cells by infectious agents, auto-antigens, or neoplastic cells initiates and maintains the development of AI by several mechanisms which coexist and are cross-regulatory (Fig.
1b). The excessive production of inflammatory mediators diverts iron to the MPS, rendering it relatively unavailable for erythroid progenitors [
33]. A paradigm for such a mediator is hepcidin anti-microbial peptide (HAMP). HAMP is the hormonal negative-feedback regulator of serum iron, as it limits iron-fluxes to the circulation. Upon iron excess or inflammation, HAMP is produced by hepatocytes and, in much smaller quantities, by immune cells and other cell types. HAMP’s specific receptor is FPN1, whose only known function is to act as an export protein for ionic iron. Binding of HAMP to FPN1 tags the latter for internalization from the cell membrane and for lysosomal degradation [
34].
Activation of pattern recognition receptors such as Toll-like receptor (TLR)-4, as well as pro- and anti-inflammatory cytokines regulate HAMP expression, while similar pathways control transcriptional expression of iron transporters transferrin receptor (TFR)-1, DMT1, and FPN1, as well as the iron storage protein FT [
26].
For instance, lipopolysaccharide as a component of the Gram-negative cell wall enhances HAMP production while stimulating DMT1 expression in myeloid cells, thereby favoring iron sequestration [
35]. In parallel, interleukin (IL)-10 increases TFR1 and FT transcription, which may aggravate AI in patients with IBD [
36].
Increased HAMP levels are also well documented in infections, rheumatoid disorders, and IBD. Furthermore, in almost all patient cohorts, HAMP concentration positively correlates with disease activity linking the extent of inflammation to the severity of iron sequestration in the MPS [
37‐
43].
Liver
The liver is a key organ initiating and maintaining AI [
12]. Hepatocytes are the key source of HAMP, while Kupffer cells (KC) are a major site of inflammation-driven iron storage. Interestingly, KC dampen HAMP production in homeostatic conditions but may be required for inflammation-driven HAMP secretion [
44,
45]. IL-6 is essential for the up-regulation of HAMP upon inflammation and IL-6 blockade for the treatment of rheumatoid arthritis lowers both disease activity and circulating HAMP levels [
46,
47]. TF is a major product of hepatocytes and one of a limited number of negative acute phase reactants. IL-6 and other pro-inflammatory cytokines result in a downregulation of TF expression in the liver, thus reducing the serum’s capacity to transport iron [
48]. This mechanism may additionally contribute to iron sequestration in the MPS. Since TF-bound iron and TFR1 form the key mechanism of iron uptake for erythroid progenitors, a central role for the development of AI is implicit. TFR1 is also expressed by neoplastic cells in solid tumors and hematologic malignancies, including chronic lymphocytic leukemia, suggesting that inflammation associated with malignant diseases may also limit iron availability for cancer cells [
49,
50]. However, potential functional consequences for tumor-associated monocytes/macrophages (TAM) are not sufficiently addressed. In addition, several pathogens are able to acquire TF-bound iron [
51‐
54]. Therefore, the reduction of serum TF appears to be one of the mechanisms of microbial iron withdrawal [
54‐
57].
Serum iron (TF-bound iron), the amount of stored iron (FT-stored iron), and the iron demand for erythropoiesis are key variables that are integrated by hepatocytes to adapt HAMP production to current metabolic needs. Serum iron levels are sensed by a machinery involving TFR1, TFR2, and the hemochromatosis-associated HFE protein [
58]. However, in being the primary iron source for erythropoiesis, TF also indirectly regulates HAMP expression via erythroid progenitor-derived mediators, suggesting that the pathways of HAMP regulation are interconnected [
59‐
61].
An increase in the erythropoietic activity as observed after blood loss or erythropoietin (EPO) administration suppresses HAMP production [
62]. Part of this effect may be mediated via erythroferrone (ERFE), a lack of which delays the recovery from AI in a mouse model [
63]. Growth-differentiation factor (GDF)-15, whose levels are increased in thalassemia and AI with or without ID, also inhibits HAMP expression [
60,
64]. Hypoxia has a similar effect on HAMP that is mediated via platelet-derived growth factor isoform BB (PDGF-BB), which may enable the required increase of Hb levels at high altitude [
65].
Iron accumulation in the liver induces bone morphogenetic protein (BMP)-6, which is essential to maintain body iron homeostasis. BMP6 binds to a heterodimeric receptor complexed with hemojuvelin (HJV) and matriptase-2 (the gene product of TMPRSS6), and stimulates HAMP expression [
66,
67]. Notably, BMP6 is primarily produced by non-parenchymal liver cells and may act in a paracrine manner on adjacent hepatocytes [
68].
In the context of inflammation, IL-6 and IL-22 stimulate HAMP expression via specific receptors signaling through signal transducer and activator of transcription (STAT)-3, while alpha-1 antitrypsin may do so via HJV and matriptase-2 [
69‐
71]. However, inflammation also feeds into the BMP6 signaling pathway, adding further complexity; not only to the regulation of iron homeostasis, but also to the pathophysiology of AI and the clinical interpretation of iron indices [
72].
In their reproductive years, women have an increased iron demand. Estradiol, whose levels increase after menstrual bleeding during the first half of the menstrual cycle (follicular phase) until ovulation, inhibits HAMP transcription in hepatocytes, which may allow for higher intestinal iron absorption to compensate for the average 20–80 ml of monthly menstrual blood loss [
73,
74]. In contrast, progesterone, which rises after ovulation and dominates the second half of the cycle (luteal phase) until menstrual bleeding, rather stimulates HAMP expression [
75]. Given the resulting fluctuations of HAMP and iron indices, the last five days of the menstrual cycle have been proposed for blood sampling to allow for a more representative evaluation of iron status in women [
76].
Recently, the concept has emerged that drugs may have undesired side effects on iron homeostasis, since the mTOR inhibitor rapamycin may increase HAMP levels after heart transplantation, thus inducing a functional ID and anemia [
77].
Kidney
While hepatic HAMP formation is increased during inflammation, EPO production in the kidney is subject to inhibition by inflammatory mediators such as tumor necrosis factor (TNF) and IL-1 [
82‐
84].
CKD with a glomerular filtration rate (GFR) < 40 ml/min/m
2 results in insufficient or deregulated production of EPO and of 1, 25-dihydroxy-cholecalciferole, both of which are negative regulators of HAMP [
85,
86]. Theoretically, for the assessment of whether the renal EPO response is adequate in AI, the EPO concentration as measured should be corrected for the actual Hb level (comparable to RPI for the correction of reticulocyte counts). However, no consensus exists on a correction formula for EPO for subjects with normal renal function or for CKD patients [
84,
87].
Independently, glomerulopathy may result in proteinuria and the loss of the 80-kD serum protein TF, which is the major shuttle between compartments of iron absorption (intestine)/iron recycling (MPS) and the erythron. While isolated antibodies to TF may lead to IDA, such auto-antibodies have not yet been reported in systemic autoimmune diseases. However, it is known that a functionally distinct type of anti-TF antibodies in monoclonal gammopathies may result in hyperferritinemia and increase of hepatic iron storage [
88,
89].
Erythron
A resistance of the erythron to EPO is another mechanism underlying AI, since it reduces the erythropoietic drive even in the setting of normal or adequately increased serum EPO concentrations. Part of this may be attributed to downregulation of the EPO receptor on erythroid cells by interferon (IFN)-γ [
90]. Furthermore, a range of inflammatory mediators including TNF, IL-1, IFN-γ, and reactive intermediates inhibits the proliferation and differentiation of erythroid progenitors or induces their apoptosis [
91‐
94]. These pathways ultimately culminate in an insufficient renal EPO response and hematopoietic EPO resistance further aggravating anemia in AI [
95].
Numerous infectious agents (e. g., parvovirus B19 and human herpes virus-6) and neoplastic cells may infiltrate the bone marrow, which eventually disturbs erythropoiesis by several mechanisms, including direct damage to erythroid cells and putative negative effects on the microenvironment and the stem cell niche. In addition, there may be direct toxic effects of drugs including chemotherapeutics and of radiation therapy on hematopoietic stem/progenitor cells. Cytopenia, including anemia, is a concern of methotrexate treatment for rheumatoid arthritis [
96]. However, immunological deregulation induced by biologics such as anti-TNF therapy may also induce aplastic anemia [
97].
While in its classical form AI constitutes a hyporegenerative anemia, hemolysis may contribute to the development of AI or aggravate its degree in several settings. For instance, several bacteria including
Staphylococcus aureus produce hemolysins [
98]. These destroy RBC, liberating heme for its uptake into bacteria by specific receptors. Different mechanisms of heme iron acquisition are exploited by intraerythrocytic infectious agents such as
Plasmodium [
99]. In addition, malaria induces HAMP, suggesting that iron sequestration is a major contributing factor to malarial anemia [
100,
101]. Elevated HAMP levels have also been reported in patients with HIV (Human immunodeficiency virus) infection, in which they are associated with anemia and independently predict mortality [
102]. While auto-antibodies against RBC can be induced by acute Epstein–Barr virus and
Mycoplasma pneumoniae infections resulting in cold agglutinin disease, auto-immune hemolysis may also occur in the setting of chronic infections or as a side effect of medication [
103]. In addition, the life span of circulating RBC may be negatively affected by inflammatory mediators such as TNF and by mechanical stress [
104]. Therefore, hemolysis may also contribute to AI in conditions such as CHF associated with mechanical valve replacement or endocarditis, or when microangiopathy is present. However, due to fluid retention, the degree of anemia tends to be overestimated in CHF patients.
Others
Similar to the concurrent presence of absolute ID in the setting of AI, deficiencies in other nutrients essential to erythropoiesis, such as folate and vitamin B12, may be contributory. For instance, celiac disease may cause profound malassimilation of various nutrients or poor food intake may aggravate the anemia of the elderly. Particularly in elderly patients, anemia due to clonal hematopoietic diseases, including myelodysplastic syndromes (MDS), has to be considered as well.