ACVP &ASVCP 2014, Atlanta, GA
Mini-Symposium: Iron Biology and Anemia
Iron Maldistribution and Infection
Hal Drakesmith, PhD
Weatherall Institute of Molecular Medicine
John Radcliffe Hospital, University of Oxford, Oxford, UK
hdrakes@hammer.imm.ox.ac.uk
Introduction – (almost) everything needs iron
Fundamental cellular physiological processes require iron. Oxygen carriage, generation of
energy from oxygen, macromolecular synthesis and maintenance of genome fidelity through
DNA repair are all iron-dependent activities. Iron is utilized because its valency states endow
it with appropriate oxidoreductive properties and a capability to form bonds in multiple
orientations, allowing it to contribute to catalysis and electron shuttling in iron-sulphur
complexes, heme, and enzymes. Such basic biochemical machinations are so ancient that
they are almost$ universally shared across the Kingdoms of life, with the result that not only
do humans require iron to grow and thrive, so do the microorganisms that infect us(1).
Indeed, of all the essential micronutrients, it does appear that iron has a very
particular role in mediating host-pathogen interactions. The initial adoption of iron into newlyevolved life occurred at a (Hadean/Archean) time of high iron bioavailability, in conditions of
a relatively reductive, oxygen-poor, acidic and sulphur-rich environment, in which iron is
soluble. However since the Great Oxygenation Event of ~2.3 billion years ago, iron, although
highly abundant, has become poorly bioavailable due to its negligible solubility at neutral pH.
Therefore, because iron is both indispensable and difficult to obtain, it can be a key factor
affecting the outcome of infectious diseases. Evidence supporting the latter statement
comes from a large variety of sources. Multiple animal studies employing many different
microoganisms have shown that increasing the amount of iron available to the invading
pathogen can exacerbate infections. In humans too, correlations of high iron status and
development of disease(2, 3), observations of infections developing after iron
supplementation in both small groups and larger clinical trials(4-8), and case reports of
genetic iron overload associating with infection(9) together support the concept that host iron
matters.
Examining the role of iron from the pathogen perspective corroborates this view;
microbes have evolved a diverse array of mechanisms for scavenging iron from their hosts’
serum, hemoglobin and ferritin. For example there are more than 500 known bacterial
siderophores (small, high- affinity iron-chelating compounds). Further confirmation of the
centrality of iron for pathogen vigor is provided by the high level of genomic investment in
iron-acquiring mechanisms and by the contribution of such genes to virulence. Highly
pathogenic strains of Yersinia enterolytica, Y. pseudotuberculosis, and Y. pestis possess a
common high-pathogenicity island that encodes proteins necessary for the synthesis,
transport, and regulation of the siderophore yersiniabactin(10). Genetic detective work
based on the full sequence of Y. pestis suggests that acquisition of these improved ironacquiring capabilities allowed the organism to make a niche transition from being an enteric
to a (devastating) systemic pathogen(11).
The host response to infection, and hepcidin
In humans, iron and heme are usually tightly chaperoned, and free iron or heme in blood is
an aberrant state. Under conditions of infection, serum iron is (generally) further ‘locked
down’, becoming sequestered in reticuloendothelial macrophages and stored intracellularly
in proteinaceous cages of ferritin. The depletion of iron from serum under conditions of
$
rare exceptions include lactobacilli and the Lyme disease pathogen Borelia burgdorferi (Posey et al,
Science 2000 vol288, p1651-3), which appear to make use of manganese in place of iron
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Mini-Symposium: Iron Biology and Anemia
invasion by pathogens is termed the hypoferremia of infection, and is thought to be a critical
defence mechanism to protect against potentially fatal septicemia. The hypoferremia of
infection appears to be a common mechanism in mammals and was first observed, and
experimentally demonstrated, in the 1940s by pioneering work of Wintrobe and
colleagues(12-14). Some iron sequestering proteins, for example ferritin, lactoferrin (which
bind iron), haptoglobin and hemopexin (which bind heme) are acute-phase response
proteins that contribute to iron withholding from pathogens as an innate immune response to
infection gets underway.
The molecular mechanism of the hypoferremia of infection has been further
elucidated in recent years since the identification of the key molecules that regulate systemic
iron homeostasis, hepcidin and its target ferroportin. In brief, ferroportin mediates export of
iron from macrophages and from enterocytes (and to a lesser extent, from hepatocytes), and
hepcidin is a small peptide hormone that binds and inhibits ferroportin(15-18). Relevant to
the present context, hepcidin is ancestrally related to anti-microbial peptides called
defensins, although hepcidin itself has only limited direct microbicidal activity. However
hepcidin may be indirectly beneficial to the host in terms of protecting against infection,
because inflammatory cytokines such as IL-6, IL-22 and Type I interferon, released by the
host following sensing of infection, strongly induce transcription of hepcidin(19-21) and so
decrease iron availability to blood-borne pathogens via the blockade and degradation of
ferroportin. In addition, transcription of the ferroportin gene is itself suppressed by
inflammation, mediated at least in part by Type II interferon and TNF-alpha. The net effect is
that inflammation following infection inhibits active ferroportin and prevents synthesis of new
ferroportin; clearly these effects would be expected to synergize and result in acute
macrophage iron deposition and a ‘scorched earth’ of serum depleted of iron. Indeed, in
acute (experimental) infections in humans, transferrin saturation can rapidly fall to <5%.
Iron redistribution in specific infections and pathological sequelae
The redistribution of iron into macrophages and out of serum due to inflammation in an
infectious context is not without consequences. If an infection persists and becomes chronic,
continually raised hepcidin and hypoferremia decreases iron availability to tissues. The
major demand for iron is from the erythron where iron is incorporated into heme, and
eventually, erythropoiesis is limited by iron availability, and anemia results(22, 23). The
anemia of chronic disease, also known as the anemia of inflammation is highly prevalent in
hospitalized patients in the UK and is a substantial burden on the National Health Service.
Increased hepcidin levels may help to distinguish anemia due to inflammation and anemia
due to other causes(24). In animal models of the disease, anemia can be rescued by
prevention of hepcidin activity(25, 26), and treatments for humans based on this concept are
under development.
A second consequence of iron redistribution is the effect on predisposition to
secondary infections. Iron loading in macrophages may inhibit immunological macrophage
functions(27) and favor growth of macrophage-tropic infections (e.g. Mycobacterium
tuberculosis)(28). Iron redistribution at enrolment into a Gambian HIV cohort was predictive
of incident tuberculosis(29), and common secondary infections (Mycobacterium, Candida,
Pneumocystis) were more common in HIV-1-infected individuals with a high degree of
macrophage iron loading(30). Iron redistribution is particularly common in malaria, with
increased levels of hepcidin(31-33) and enhanced erythrophagocytosis contributing to an
increase in macrophage iron, a relative paucity of iron in hepatocytes and decreased dietary
iron absorption(34), which together lead to a lack of availability for erythropoiesis and an
increase in prevalence of anemia (for example, at the end of a malaria season(35)). This
iron redistribution has effects on both superinfection and on co-infection in the context of
malaria. Hepcidin has been shown to play a crucial role in determining the multiplicity of
malaria infections within a single host. The obligate liver stage of the malaria parasite
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Mini-Symposium: Iron Biology and Anemia
requires iron: hepcidin peptide injection or hepcidin overexpression by transgene or viral
vector can reduce parasite survival at the crucial hepatic bottleneck (36). This same effect
was observed when mice already harbouring a hepcidin-inducing blood-stage infection were
exposed to liver-tropic sporozoites; the superinfection failed to thrive and did not emerge
from the liver into the blood. Therefore it appears that the hepcidin upregulation initiated by
one blood-stage malaria infection blocks the establishment of a second, potentially
competing, Plasmodium infection (36).
The physiological redistribution of iron as a consequence of malaria may also have a
significant effect on host susceptibility to other bacterial, viral, or protozoa parasites. The
increase in macrophage iron that occurs in blood-stage malaria may benefit pathogens that
exploit the macrophage niche (37). In particular, hepcidin upregulation may help to explain
the association between malaria infections and susceptibility to non-typhoid salmonella
(NTS). The epidemiological link between malaria and NTS is well-established (38). Iron has
been implicated in the contribution of malaria to NTS susceptibility through increases in both
free heme and heme-oxygenase expression (39). By routing iron to accumulate in
macrophages, the hepcidin response to malaria may also render the host more vulnerable to
NTS directly (37).
Finally mention must be made of an important exception to the rule of the
hypoferremia of infection; we recently found that in the acute phases of both Hepatitis C
virus and Hepatitis B virus infections, during the first 10 days when detectable virus emerges
into the circulation, there was no evidence of hepcidin increase or of hypoferremia, and for
HBV there was an indication of an increase in serum iron. This is despite (or potentially
because?) the infection is localised in the organ that produces hepcidin. Chronic HCV and to
a lesser extent HBV are characterised by iron accumulation in the liver, and chronic HCV is
also associated with decreased hepcidin, even in the presence of ongoing inflammation(40,
41). Hepatic iron accumulation can contribute to organ damage in these chronic viral
diseases, and more generally is a significant health problem both in humans (where
overload may be caused due to aberrantly low levels of hepcidin and/or blood transfusion)
and in some species of animals in zoo conditions - the best characterised appears to be
browsing Rhinos (42) (where the cause is at least in part nutritional).
Conclusion
Iron is a critical nutrient required for growth and optimal function both by humans / mammals
and by the microbial organisms that can pathogenically infect us. A part of the innate
immune defence against most pathogens is the hypoferremic response, caused largely by
increased hepcidin and decreased ferroportin activity, and which leads to iron redistribution
out of serum and into macrophages. This iron relocalisation is likely protective against fatal
septicemia, but may have deleterious consequences in terms of predisposition to develop
inflammatory anemia and susceptibility to macrophage-tropic secondary infections. Hepatotropic viral infections represent interesting and important exceptions to the hyoperremia rule,
as these conditions are associated with normal or decreased hepcidin and toxic
accumulation of iron in the liver. The ability to therapeutically manipulate the hepcidin/iron
axis, as well as offering possible new treatment options for disorders of iron metabolism,
may eventually contribute to therapy for infectious diseases – because while pathogens can
and do evade antibody and T-cell immunity, and develop antibiotic resistance, they cannot
avoid their requirement for iron.
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