Volume 5, Number 3—June 1999
Iron Loading and Disease Surveillance
Iron is an oxidant as well as a nutrient for invading microbial and neoplastic cells. Excessive iron in specific tissues and cells (iron loading) promotes development of infection, neoplasia, cardiomyopathy, arthropathy, and various endocrine and possibly neurodegenerative disorders. To contain and detoxify the metal, hosts have evolved an iron withholding defense system, but the system can be compromised by numerous factors. An array of behavioral, medical, and immunologic methods are in place or in development to strengthen iron withholding. Routine screening for iron loading could provide valuable information in epidemiologic, diagnostic, prophylactic, and therapeutic studies of emerging infectious diseases.
Excessive iron in specific tissues (iron loading) promotes infection, neoplasia, cardiomyopathy, arthropathy, and a profusion of endocrine and possibly neurodegenerative disorders (1-5). An array of behavioral, medical, and immunologic methods are being developed to decrease iron loading or its detrimental effects. Routine screening for iron loading in populations exposed to certain diseases can provide valuable epidemiologic, diagnostic, prophylactic, and therapeutic information.
Hazards of Iron Loading
Iron can contribute to disease development in several ways. Excessive amounts of the metal in specific tissues and cells can hinder the ability of proteins, such as transferrin and ferritin, to prevent accretion of free iron. Moreover, in infectious diseases, inflammatory diseases, and illnesses that involve ischemia and reperfusion, iron causes reactions that produce superoxide radicals (6). Nonprotein bound ferric ions are reduced by superoxide, and the ferrous product is reoxidized by peroxide to regenerate ferric ions and yield hydroxyl radicals, which attack all classes of biologic macromolecules. Hydroxyl radicals can depolymerize polysaccharides, cause DNA strand breaks, inactivate enzymes, and initiate lipid peroxidation (6).
Iron can also increase disease risk by functioning as a readily available essential nutrient for invading microbial and neoplastic cells. To survive and replicate in hosts, microbial pathogens must acquire host iron. Highly virulent strains possess exceptionally powerful mechanisms for obtaining host iron from healthy hosts (7). In persons whose tissues and cells contain excessive iron, pathogens can much more readily procure iron from molecules of transferrin that are elevated in iron saturation. In such cases, even microbial strains that are not ordinarily dangerous can cause illness. Markedly invasive neoplastic cell strains can glean host iron more easily than less malignant strains or normal host cells (3). Moreover, iron-loaded tissues are especially susceptible to growth of malignant cells (Table 1).
How Microbes Acquire Iron: A Determinant of Host Range and of Tissue Localization
The number of infectious disease agents whose virulence is enhanced by iron continues to increase (Table 2). To obtain host iron, successful pathogens use one or more of four strategies: binding of ferrated siderophilins with extraction of iron at the cell surface; erythrocyte lysis, digestion of hemoglobin, and heme assimilation; use of siderophores that withdraw iron from transferrin; and procurement of host intracellular iron.
Microbial strains that use siderophilin binding often have a very narrow host range (7). Bacterial receptors recognize siderophilins generally from a single or closely related host species. Strains of Haemophilus somnus, for example, form receptors for bovine but not for human transferrin; these bacteria are virulent for cattle but not for humans (9). The human pathogen, Neisseria meningitidis, can bind ferrated transferrins from humans and such hominids as chimpanzees, gorillas, and orangutans, but not from monkeys or nonprimate mammals (10,11). Actinobacillus pleuropneumoniae synthesizes a swine-specific transferrin receptor and causes pneumonia only in hogs (12).
Each of the above three pathogens, as well as other organisms that use siderophilin binding, can often obtain iron from heme. Helicobacter pylori, for instance, first obtains iron from human ferrated lactoferrin in the gastric lumen. Then, as it migrates into intercellular junctions of epithelial cells in the gastric wall, its sole source of iron is heme. This pathogen binds neither bovine ferrated lactoferrin nor human, bovine, or equine ferrated transferrin (13).
However, not every pathogen that uses siderophilin binding has a narrow host range. For example, Staphylococcus aureus can be virulent for a variety of mammalian species. Strains of this organism can bind human, rat, and rabbit transferrins and, much less efficiently, bovine, porcine, and avian transferrins (14). Moreover, isolates of S. aureus also may produce siderophores (15,16). These small molecules can withdraw iron from transferrins synthesized by a variety of host species. The siderophore, staphyloferrin A, removes iron from both human and porcine transferrin; thus, the metal can be available to invading cells in humans and in hogs. Erythrocyte lysis, digestion of hemoglobin, and heme assimilation are available to strains of S. aureus. Bacterial hemolysins generally are active against erythrocytes from several, although not from all, potential host species.
Virulent streptococci are examples of bacteria that neither bind siderophilins nor produce siderophores yet proficiently invade and replicate in many tissues in diverse host species. The cellulytic activities of these pathogens enable them to access such intracellular sources of host iron as hemoglobin, myoglobin, catalase, and ferritin (17).
The remarkable versatility for host species shown by Listeria monocytogenes illustrates the adeptness of this organism in procuring iron. Although mainly a saprophyte that lives in the plant-soil environment, L. monocytogenes can be acquired by humans and other mammals through ingestion of undercooked tissue of other mammals, birds, fish, and Crustacea, as well as from raw vegetables. Unable to bind siderophilins or form siderophores, L. monocytogenes obtains iron by using either exogenous siderophores of other microorganisms or natural catechols, such as dopamine and norepinephrine, in host tissues. The pathogen expresses a cell surface ferric reductase that recognizes the siderophoric chelated iron site; the metal is then reduced and assimilated (18). Furthermore, in contrast to saprophytic strains, systemic pathogenic strains of L. monocytogenes are hemolytic.
To grow within host cells, pathogens apparently are not required to synthesize siderophilin binding sites or form siderophores. For instance, unlike the wild type, siderophore-minus mutants of Salmonella Typhimurium cannot grow in extracellular compartments of the host. However, both the wild and mutant strains replicate within host cells (19). Possible sources of intracellular iron are heme, iron released from transferrin at pH 5.5-6, and ferritin.
For at least two pathogens, Francisella tularensis and Legionella pneumophila, the host intracellular niche is obligatory. Like the mutant strain of S. Typhimurium, these organisms are unable to access iron in extracellular fluids and tissues. Culturing these bacteria in laboratory media requires markedly elevated concentrations of iron (20,21).
In host intracellular niches, growth of microbial pathogens is stimulated by elevation and depressed by decrease of iron. Indeed, at least one bacterial pathogen, Ehrlichia chaffeensis, induces elevation of iron in its host cells; intracellular inclusions of the organism cause the host cell to upregulate expression of the transferrin receptor mRNA (22).
Iron Withholding Defense System
Hosts use several mechanisms (Table 3) to withhold iron from invading microbial and neoplastic cells: stationing of potent iron binding proteins at sites of impending microbial invasion; lowering iron levels in body fluids, diseased tissues, and invaded cells during invasion; and synthesizing immunoglobulins to the iron acquisition antigens of microbes.
High concentrations of iron not only benefit invading cells, they may also mediate antimicrobial activities of defense cells. In in vitro studies, 150 µM iron augmented macrophage killing of Brucella abortus (24) and, without altering phagocytosis, 250 µM iron enhanced anti-Candida activity of microglia (25). In the latter system, the metal suppressed synthesis of nitric oxide but not of tumor necrosis factor A. By generating oxidant-sensitive mediators, iron may focus influx of neutrophils to sites of infection (26). Iron loading of staphylococci increased their killing by peroxide, macrophages, and neutrophil-derived cytoplasts but not by neutrophils (27). Certain conditions can impair iron withholding (Table 4); numerous studies have presented evidence that risk for infection or neoplasia is increased significantly in persons with these conditions.
Detection of Iron Loading
Screening of large populations for iron loading can be accomplished with inexpensive, noninvasive methods. A useful indicator of iron loading is marked elevation of serum ferritin (sFt). However, sole reliance on this measurement can be misleading because sFt increases moderately during inflammatory episodes. Accordingly, concurrent determination of the percentage of iron saturation of serum transferrin (%TS) provides useful information (29). In iron loaded persons, hyperferritinemia generally is accompanied by an elevation in %TS. In contrast, in patients with an inflammatory process, hyperferritinemia generally is accompanied by a reduction in %TS.
Iron loading is associated also with moderate depression of a third variable, serum transferrin receptor (sTfR). The ratio of sTfR/sFt, apparently independent of inflammation, is significantly reduced in persons with high levels of iron (5).
Strengthening the Iron Withholding Defense
A considerable array of behavioral, medical, and immunologic methods are in place or in development for strengthening iron withholding (Table 5) (3). Additional precautions are indicated for persons who are known to be (or have a tendency to become) iron loaded. For example, persons with elevated iron due to either hemochromatosis or alcoholism are cautioned to avoid eating raw oysters, which may contain Vibrio vulnificus (30). Another pathogen that likewise causes severe systemic infection in hosts with elevated iron is Capnocytophaga canimorsis. Accordingly, persons who have hemochromatosis, alcoholism, or asplenia are advised to receive prompt antibiotic therapy if they are exposed to a dog bite (31).
De-ironing by phlebotomy is effective in lowering risk for cardiovascular diseases (32,33) and various neoplasms (34), as well as in therapy for hepatitis C (35). Interfering with iron metabolism by administering gallium can be useful in suppressing growth of lymphoma and bladder cancer cells (36). The antineoplastic action of monoclonal antibodies against ferrated transferrin receptors has been examined (37). Combinations of the iron chelator, deferoxamine, with gallium or with antibodies against ferrated transferrin receptors increase effectiveness against tumor cells.
The natural iron scavenger, lactoferrin, has been shown to remove free iron from synovial fluid aspirated from joints of rheumatoid arthritic patients (38). Recombinant human lactoferrin, which is indistinguishable from native breast milk lactoferrin with respect to its iron binding properties, is now available (39) and could become a very useful addition to our array of de-ironing pharmaceutical products.
A recently discovered integral membrane phosphoglycoprotein, Nrampl, is expressed exclusively in macrophages and is localized to phagolysosomes. The protein suppresses replication of intramacrophage microbial invaders apparently by altering iron availability (23). A second protein, Nramp2, is involved in enhancement of intestinal iron absorption (40). Future research might develop useful medical procedures for modulation of the actions of these proteins.
Potential vaccines that incorporate iron acquisition antigens of pathogens in the families Neisseriaceae and Pasteurellaceae are being developed by several research groups. For example, in Moraxella catarrhalis, the recombinant transferrin binding protein B (TbpB) has been shown to elicit bactericidal antibodies (41) In N. meningitidis, antisera to TbpA and TbpB were bactericidal for both homologous and heterologous strains (42,43). Because the antigenic proteins function at the cell surfaces of the pathogens, the receptors are potentially ideal vaccine candidates. For synthesis of the receptors, the organisms must be cultured in iron-restricted media.
Perspectives and Conclusions
There is growing awareness that transmissible agents are involved in diseases not earlier suspected of being infectious (44-46). A recent review contains a list of 34 degenerative, inflammatory, and neoplastic diseases associated in various ways with specific infectious agents (44). Other chronic inflammatory diseases, such as sarcoidosis, inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus, Wegener granulomatosis, diabetes mellitus, primary biliary cirrhosis, tropical sprue, and Kawasaki disease may also have infectious etiologies (45). Excessive iron is correlated with synovial damage in rheumatoid arthritis (47) and with impaired glucose metabolism in diabetes (48). The association of Chlamydia pneumoniae (49) and excessive iron (5) with cardiovascular disease is well established. Growth of this pathogen is strongly suppressed by iron restriction (50).
Proving the role of infection in chronic inflammatory diseases and cancer presents challenges (46). The means by which pathogens suppress, subvert, or evade host defenses to establish chronic or latent infection have received little attention. However, the association and causal role of infectious agents in chronic inflammatory diseases and cancer have major implications for public health, treatment, and prevention (44-46).
Iron loading is a risk factor in these illnesses, as well as in classic infectious diseases. Because the prevalence of iron loading in various populations can be remarkably high, routine screening of iron values in host populations could provide valuable information in epidemiologic, diagnostic, prophylactic, and therapeutic studies of emerging infectious diseases.
Dr. Weinberg is professor emeritus of microbiology in both the College of Arts and Sciences and the School of Medicine at Indiana University, Bloomington, IN. His studies on iron were initiated in 1952. Since retiring from teaching in 1992, he has devoted full time to research.
Dedicated to Jerome L. Sullivan, pioneer and leader in our awareness of the role of iron in cardiovascular disease.
Support for this review was provided by the Office of Research and the University Graduate School, Indiana University, Bloomington, IN, USA.
- Kontoghiorghes GJ, Weinberg ED. Iron: mammalian defense systems, mechanisms of disease, and chelation therapy approaches. Blood Rev. 1995;9:33–45.
- Weinberg ED, Weinberg GA. The role of iron in infection. Curr Opin Infect Dis. 1995;8:164–9.
- Weinberg ED. The role of iron in cancer. Eur J Cancer Prev. 1996;5:19–36.
- Connor JR, Beard JL. Dietary iron supplements in the elderly: to use or not to use? Nutr Today. 1997;32:102–9.
- Tuomainen T-P, Punnonen K, Nyyssonen K, Salonen JT. Association between body iron stores and the risk of acute myocardial infarction in men. Circulation. 1998;97:1461–6.
- McCord JM. Effects of positive iron status at a cellular level. Nutr Rev. 1996;54:85–8.
- Weinberg ED. Patho-ecological implications of microbial acquisition of host iron. Reviews in Medical Microbiology. 1998;9:171–8.
- Weinberg ED. Acquisition of iron and other nutrients in vivo. In: Roth JA, Bolin CA, Brogdon KA, Wannemuehler MJ, editors. Virulence mechanisms of bacterial pathogens. Washington: American Society for Microbiology; 1995. p. 79-94.
- Ogunnariwo JA, Cheng C, Ford J, Schryvers AB. Response of Haemophilus somnus to iron limitation: expression and identification of a bovine-specific transferrin receptor. Microb Pathog. 1990;9:397–406.
- Arko RJ. Animal models for Neisseria species. Clin Microbiol Rev. 1989;2:S56–9.
- Gray-Owen SD, Schryvers AB. The interaction of primate transferrins with receptors on bacteria pathogenic to humans. Microb Pathog. 1993;14:389–98.
- Gonzalez GC, Casmano OL, Schryvers AB. Identification and characterization of a porcine-specific transferrin receptor in Actinobacillus pleuropneumoniae. Mol Microbiol. 1990;4:1173–9.
- Worst DJ. Iron acquisition by Helicobacter pylori. Ph.D. thesis. Amsterdam: Vrije Universiteit; 1997; p. 109-16.
- Modun B, Evans RW, Joannou CL, Williams P. Receptor-mediated recognition and uptake of iron from human transferrin by Staphylococcus aureus and Staphylococcus epidermidis. Infect Immun. 1998;65:1944–8.
- Lindsay JA, Riley TV. Staphylococcal iron requirements, siderophore production, and iron-regulated protein expression. Infect Immun. 1994;62:2309–14.
- Courcol RJ, Trivier D, Bissinger M-C, Martin GR, Brown MRW. Siderophore production by Staphylococcus aureus and identification of iron-regulated proteins. Infect Immun. 1997;65:1944–8.
- Eichenbaum Z, Muller E, Morse SA, Scott JR. Acquisition of iron from host proteins by the group A streptococcus. Infect Immun. 1996;64:5428–9.
- Coulanges V, Andre P, Vidon DJ-M. Effect of siderophores, catecholamines, and catechol compounds on Listeria spp. growth in iron-complexed medium. Biochem Biophys Res Commun. 1998;24:526–30.
- Tsolis RM, Baumler AJ, Heffron F, Stojikovic I. Contributions of TonB- and feo-mediated iron uptake to growth of Salmonella typhimurium in the mouse. Infect Immun. 1996;64:4549–56.
- Fortier AH, Leiby DA, Narayanan RB, Asafoadjei E, Crawford RM, Nacy CA, . Growth of Francisella tularensis LVS in macrophages: the acidic intracellular compartment provides essential iron required for growth. Infect Immun. 1995;63:1478–83.
- Byrd TF. Cytokines and legionellosis. Biotherapy. 1994;7:179–86.
- Barnewall RE, Rikihisa Y, Lee EH. Ehrlichia chaffeensis inclusions are early endosomes which selectively accumulate transferrin receptor. Infect Immun. 1997;65:1455–61.
- Gomes MS, Appelberg R. Evidence for a link between iron metabolism and Nrampl gene function in innate resistance against Mycobacterium avium. Immunology. 1998;95:165–8.
- Jiang X, Baldwin CL. Iron augments macrophage-mediated killing of Brucella abortus alone and in conjunction with interferon. Cell Immunol. 1993;148:397–407.
- Saleppico S, Mazzolla R, Boelaert JR, Puliti M, Barluzzi R, Bistoni F, Iron regulates microglial cell-mediated secretory and effector functions. Cell Immunol. 1996;170:251–9.
- Ghio AJ, Piantadosi CA, Crumbliss AL. Hypothesis: iron chelation plays a vital role in neutrophilic inflammation. Biometals. 1997;19:135–42.
- Hoepelman IM, Bezemer WA, Vandenbroucke-Grauls CMJE, Marx JJM, Verhoef J. Bacterial iron enhances oxygen radical-mediated killing of Staphylococcus aureus by phagocytes. Infect Immun. 1990;58:26–31.
- Delanghe JR, Langlois MR, Boelaert Jr, Van Acker J, Van Wanzeele F, van der Groen G, . Haptoglobin polymorphism, iron metabolism and mortality in HIV infection. AIDS. 1998;12:1027–32.
- Witte DL, Crosby WH, Edwards CQ, Fairbanks VF, Mitros FA. Hereditary hemochromatosis. Clin Chim Acta. 1996;245:139–200.
- Shapiro RL, Altekruse S, Hutwagner L, Bishop R, Hammond R, Wilson S, The role of Gulf Coast oysters harvested in warmer months in Vibrio vulnificus infections in the United States, 1988-1996. J Infect Dis. 1998;178:752–9.
- Weinberg ED. DF-2 sepsis: a sequela of sideremia? Med Hypotheses. 1987;24:287–9.
- Meyers DG, Strickland D, Maloley PA, Seburg JK, Wilson JE, McManus BF. Possible association of a reduction in cardiovascular events with blood donation. Heart. 1997;78:188–93.
- Kiechl S, Willeit J, Egger G, Poewe W, Oberhollenzer F. Body iron stores and the risk of carotid atherosclerosis. Prospective results from the Bruuneck study. Circulation. 1997;96:3300–7.
- Merk K, Mattson B, Mattson A, Holm G, Gullbring B, Bjorkholm M. The incidence of cancer among blood donors. Int J Epidemiol. 1990;19:505–9.
- Bonkovsky HL, Banner BF, Rothman AL. Iron and chronic viral hepatitis. Hepatology. 1997;26:759–68.
- Chitambar CR, Narasimhan J. Targeting iron-dependent DNA synthesis with gallium and transferrin-gallium. Pathobiology. 1991;59:3–10.
- Kemp JD. Iron deprivation and cancer: a view beginning with studies of monoclonal antibodies against the transferrin receptor. Histol Histopathol. 1997;12:291–6.
- Guillen C, McInnes IB, Kruger H, Brock JH. Iron, lactoferrin-and iron regulatory protein activity in the synovium; relative importance of iron loading and the inflammatory response. Ann Rheum Dis. 1998;57:309–14.
- Ward PP, Piddington CS, Cunningham GA, Zhou X, Wyatt RD, Conneely OM. A system for production of commercial quantities of human lactoferrin, a broad spectrum natural antibiotic. Biotechnology. 1995;13:498–503.
- Andrews NC, Levy JE. Iron is hot: an update on the pathophysiology of hemochromatosis. Blood. 1998;92:1845–51.
- Myers LE, Yang Y-P, Du R-P, Wang Q, Harkness RE, Schryvers AB, The transferrin binding protein B of Moraxella catarrhalis elicits bactericidal antibodies and is a potential vaccine antigen. Infect Immun. 1998;66:4183–92.
- Lissolo L, Maitre-Wilmotte G, Dumas P, Mignon M, Danve B. QuentinMillet M-J. Evaluation of transferrin-binding protein 2 within the transferrin-binding complex as a potential antigen for future meningococcal vaccines. Infect Immun. 1995;63:884–90.
- Pintor M, Ferron L, Gomez JA, Powell NBL, Ala `Aldeen DAA, Boriello SP, . Blocking of iron uptake from transferrin by antibodies against the transferrin binding proteins in Neisseria meningitidis. Microb Pathog. 1996;20:127–39.
- Lorber B. Are all diseases infectious? Ann Intern Med. 1996;125:844–51.
- Relman DA. Detection and identification of previously unrecognized microbial pathogens. Emerg Infect Dis. 1998;4:382–9.
- Cassell GH. Infectious causes of chronic inflammatory diseases and cancer. Emerg Infect Dis. 1998;4:475–87.
- Morris CJ, Earl JR, Trenam CW, Bkaje DR. Reactive oxygen species and irona dangerous partnership in inflammation. Int J Biochem Cell Biol. 1995;27:109–22.
- Tuomainen T-P, Nyyssonen K, Salonen R, Tervahauta A, Korpela H, Lakka T, Body iron stores are associated with serum insulin and blood glucose concentrations. Diabetes Care. 1997;20:426–8.
- Campbell LA, Kuo C-C, Grayston JT. Chlamydia pneumoniae and cardiovascular disease. Emerg Infect Dis. 1998;4:571–9.
- Freidank HM, Billing H. Influence of iron restriction on the growth of Chlamydia pneumoniae TWAR and Chlamydia trachomatis. Clinical Microbiology and Infection 1997;3 Suppl 2:193.23
Suggested citation: Weinberg ED. Iron Loading and Disease Surveillance. Emerg Infect Dis [serial on the Internet]. 1999, Jun [date cited]. Available from http://wwwnc.cdc.gov/eid/article/5/3/99-0305.htm
Comments to the Authors
West Nile Virus RNA
in Tissues from Donor
Transmission to Organ