Leishmania chagasi: uptake of iron bound to lactoferrin or transferrin requires an iron reductase

Targeting and activation of macrophages in leishmaniasis. A focus on iron oxide nanoparticles

Macrophages play a pivotal role as host cells for Leishmania parasites, displaying a notable functional adaptability ranging from the proinflammatory, leishmanicidal M1 phenotype to the anti-inflammatory, parasite-permissive M2 phenotype. While macrophages can potentially eradicate amastigotes through appropriate activation, Leishmania employs diverse strategies to thwart this activation and redirect macrophages toward an M2 phenotype, facilitating its survival and replication. Additionally, a competition for iron between the two entities exits, as iron is vital for both and is also implicated in macrophage defensive oxidative mechanisms and modulation of their phenotype. This review explores the intricate interplay between macrophages, Leishmania, and iron. We focus the attention on the potential of iron oxide nanoparticles (IONPs) as a sort of immunotherapy to treat some leishmaniasis forms by reprogramming Leishmania-permissive M2 macrophages into antimicrobial M1 macrophages. Through the specific targeting of iron in macrophages, the use of IONPs emerges as a promising strategy to finely tune the parasite-host interaction, endowing macrophages with an augmented antimicrobial arsenal capable of efficiently eliminating these intrusive microbes.

Access and utilization of host-derived iron by Leishmania parasites

Iron is involved in many biochemical processes including oxygen transport, ATP production, DNA synthesis and antioxidant defense. The importance of iron also applies to Leishmania parasites, an intracellular protozoan pathogen causing leishmaniasis. Leishmania are heme-auxotrophs, devoid of iron storage proteins and the heme synthesis pathway. Acquisition of iron and heme from the surrounding niche is thus critical for the intracellular survival of Leishmania inside the host macrophages. Moreover, Leishmania parasites are also exposed to oxidative stress within phagolysosomes of macrophages in mammalian hosts, and they need iron superoxide dismutase for overcoming this stress. Therefore, untangling the strategy adopted by these parasites for iron acquisition and utilization can be good targets for the development of antileishmanial drugs. Here, in this review, we will address how Leishmania parasites acquire and utilize iron and heme during infection to macrophages.

Iron Uptake Controls Trypanosoma cruzi Metabolic Shift and Cell Proliferation

(1) Background: Ionic transport in Trypanosoma cruzi is the object of intense studies. T. cruzi expresses a Fe-reductase (TcFR) and a Fe transporter (TcIT). We investigated the effect of Fe depletion and Fe supplementation on different structures and functions of T. cruzi epimastigotes in culture. (2) Methods: We investigated growth and metacyclogenesis, variations of intracellular Fe, endocytosis of transferrin, hemoglobin, and albumin by cell cytometry, structural changes of organelles by transmission electron microscopy, O₂ consumption by oximetry, mitochondrial membrane potential measuring JC-1 fluorescence at different wavelengths, intracellular ATP by bioluminescence, succinate-cytochrome c oxidoreductase following reduction of ferricytochrome c, production of H₂O₂ following oxidation of the Amplex^® red probe, superoxide dismutase (SOD) activity following the reduction of nitroblue tetrazolium, expression of SOD, elements of the protein kinase A (PKA) signaling, TcFR and TcIT by quantitative PCR, PKA activity by luminescence, glyceraldehyde-3-phosphate dehydrogenase abundance and activity by Western blotting and NAD⁺ reduction, and glucokinase activity recording NADP⁺ reduction. (3) Results: Fe depletion increased oxidative stress, inhibited mitochondrial function and ATP formation, increased lipid accumulation in the reservosomes, and inhibited differentiation toward trypomastigotes, with the simultaneous metabolic shift from respiration to glycolysis. (4) Conclusion: The processes modulated for ionic Fe provide energy for the T. cruzi life cycle and the propagation of Chagas disease.

Activity of Apo-Lactoferrin on Pathogenic Protozoa

Parasites and other eventually pathogenic organisms require the ability to adapt to different environmental conditions inside the host to assure survival. Some host proteins have evolved as defense constituents, such as lactoferrin (Lf), which is part of the innate immune system. Lf in its iron-free form (apo-Lf) and its peptides obtained by cleavage with pepsin are microbicides. Parasites confront Lf in mucosae and blood. In this work, the activity of Lf against pathogenic and opportunistic parasites such as Cryptosporidium spp., Eimeria spp., Entamoeba histolytica, Giardia duodenalis, Leishmania spp., Trypanosoma spp., Plasmodium spp., Babesia spp., Toxoplasma gondii, Trichomonas spp., and the free-living but opportunistic pathogens Naegleria fowleri and Acanthamoeba castellani were reviewed. The major effects of Lf could be the inhibition produced by sequestering the iron needed for their survival and the production of oxygen-free radicals to more complicated mechanisms, such as the activation of macrophages to phagocytes with the posterior death of those parasites. Due to the great interest in Lf in the fight against pathogens, it is necessary to understand the exact mechanisms used by this protein to affect their virulence factors and to kill them.

Development of antibodies to the iron-binding proteins transferrin and ferritin in dogs and mice infected with Leishmania parasites

Most microorganisms including Leishmania parasites compete with the innate immune defenses of the infected hosts to acquire iron, an essential nutrient necessary for their growth and replication. In mammals, iron is predominantly bound to protein carriers such as transferrin and ferritin and the strategies adopted by the infected host to restrict its uptake by pathogens are still not elucidated. We compared herein the development of anti-transferrin and anti-ferritin antibodies in hosts that differs by their susceptibility to Leishmania infection. Results showed that Leishmania infantum naturally-infected dogs which have developed canine leishmaniasis (CanL) demonstrated higher titers of IgG antibodies anti-leishmanial antigens and anti-iron binding proteins than those infected without clinical signs. In the experimental mouse model, C57BL/6 mice resisted L. major infection, developed lower titers of Leishmania-specific IgG antibodies than BALB/c susceptible mice but demonstrated also the production of anti-transferrin and anti-ferritin IgG antibodies. Overall, results are in favor that mechanisms, other than the polyclonal activation of B cells associated-hypergammaglobulinemia, a characteristic of susceptible animals, are likely involved and require a replicating parasite for the limitation of iron uptake.

Hemoglobin Endocytosis and Intracellular Trafficking: A Novel Way of Heme Acquisition by Leishmania

Leishmania species are causative agents of human leishmaniasis, affecting 12 million people annually. Drugs available for leishmaniasis are toxic, and no vaccine is available. Thus, the major thrust is to identify new therapeutic targets. Leishmania is an auxotroph for heme and must acquire heme from the host for its survival. Thus, the major focus has been to understand the heme acquisition process by the parasites in the last few decades. It is conceivable that the parasite is possibly obtaining heme from host hemoprotein, as free heme is not available in the host. Current understanding indicates that Leishmania internalizes hemoglobin (Hb) through a specific receptor by a clathrin-mediated endocytic process and targets it to the parasite lysosomes via the Rab5 and Rab7 regulated endocytic pathway, where it is degraded to generate intracellular heme that is used by the parasite. Subsequently, intra-lysosomal heme is initially transported to the cytosol and is finally delivered to the mitochondria via different heme transporters. Studies using different null mutant parasites showed that these receptors and transporters are essential for the survival of the parasite. Thus, the heme acquisition process in Leishmania may be exploited for the development of novel therapeutics.

An Iron Transporter Is Involved in Iron Homeostasis, Energy Metabolism, Oxidative Stress, and Metacyclogenesis in Trypanosoma cruzi

The parasite Trypanosoma cruzi causes Chagas’ disease; both heme and ionic Fe are required for its optimal growth, differentiation, and invasion. Fe is an essential cofactor in many metabolic pathways. Fe is also harmful due to catalyzing the formation of reactive O₂ species; for this reason, all living systems develop mechanisms to control the uptake, metabolism, and storage of Fe. However, there is limited information available on Fe uptake by T. cruzi. Here, we identified a putative 39-kDa Fe transporter in T. cruzi genome, TcIT, homologous to the Fe transporter in Leishmania amazonensis and Arabidopsis thaliana. Epimastigotes grown in Fe-depleted medium have increased TcIT transcription compared with controls grown in regular medium. Intracellular Fe concentration in cells maintained in Fe-depleted medium is lower than in controls, and there is a lower O₂ consumption. Epimastigotes overexpressing TcIT, which was encountered in the parasite plasma membrane, have high intracellular Fe content, high O₂ consumption—especially in phosphorylating conditions, high intracellular ATP, very high H₂O₂ production, and stimulated transition to trypomastigotes. The investigation of the mechanisms of Fe transport at the cellular and molecular levels will assist in elucidating Fe metabolism in T. cruzi and the involvement of its transport in the differentiation from epimastigotes to trypomastigotes, virulence, and maintenance/progression of the infection.

Diverse Mechanisms of Antimicrobial Activities of Lactoferrins, Lactoferricins, and Other Lactoferrin-Derived Peptides

Lactoferrins are an iron-binding glycoprotein that have important protective roles in the mammalian body through their numerous functions, which include antimicrobial, antitumor, anti-inflammatory, immunomodulatory, and antioxidant activities. Among these, their antimicrobial activity has been the most studied, although the mechanism behind antimicrobial activities remains to be elucidated. Thirty years ago, the first lactoferrin-derived peptide was isolated and showed higher antimicrobial activity than the native lactoferrin lactoferricin. Since then, numerous studies have investigated the antimicrobial potencies of lactoferrins, lactoferricins, and other lactoferrin-derived peptides to better understand their antimicrobial activities at the molecular level. This review defines the current antibacterial, antiviral, antifungal, and antiparasitic activities of lactoferrins, lactoferricins, and lactoferrin-derived peptides. The primary focus is on their different mechanisms of activity against bacteria, viruses, fungi, and parasites. The role of their structure, amino-acid composition, conformation, charge, hydrophobicity, and other factors that affect their mechanisms of antimicrobial activity are also reviewed.

Iron in infection and immunity

Iron is an essential micronutrient for virtually all living cells. In infectious diseases, both invading pathogens and mammalian cells including those of the immune system require iron to sustain their function, metabolism and proliferation. On the one hand, microbial iron uptake is linked to the virulence of most human pathogens. On the other hand, the sequestration of iron from bacteria and other microorganisms is an efficient strategy of host defense in line with the principles of 'nutritional immunity'. In an acute infection, host-driven iron withdrawal inhibits the growth of pathogens. Chronic immune activation due to persistent infection, autoimmune disease or malignancy however, sequesters iron not only from infectious agents, autoreactive lymphocytes and neoplastic cells but also from erythroid progenitors. This is one of the key mechanisms which collectively result in the anemia of chronic inflammation. In this review, we highlight the most important interconnections between iron metabolism and immunity, focusing on host defense against relevant infections and on the clinical consequences of anemia of inflammation.

A ferric reductase of Trypanosoma cruzi (TcFR) is involved in iron metabolism in the parasite

Trypanosoma cruzi is a parasitic protozoan that infects various species of domestic and wild animals, triatomine bugs and humans. It is the etiological agent of American trypanosomiasis, also known as Chagas Disease, which affects about 17 million people in Latin America and is emerging elsewhere in the world. Iron (Fe) is a crucial micronutrient for almost all cells, acting as a cofactor for several metabolic enzymes. T. cruzi has a high requirement for Fe, using heminic and non-heminic Fe for growth and differentiation. Fe occurs in the oxidized (Fe³⁺) form in aerobic environments and needs to be reduced to Fe²⁺ before it enters cells. Fe-reductase, located in the plasma membranes of some organisms, catalyzes the Fe³⁺⇒ Fe²⁺ conversion. In the present study we found an amino acid sequence in silico that allowed us to identify a novel 35 kDa protein in T. cruzi with two transmembrane domains in the C-terminal region containing His residues that are conserved in the Ferric Reductase Domain Superfamily and are required for catalyzing Fe³⁺ reduction. Accordingly, we named this protein TcFR. Intact epimastigotes from the T. cruzi DM28c strain reduced the artificial Fe³⁺-containing substrate potassium ferricyanide in a cell density-dependent manner, following Michaelis-Menten kinetics. The TcFR activity was more than eightfold higher in a plasma membrane-enriched fraction than in whole homogenates, and this increase was consistent with the intensity of the 35 kDa band on Western blotting images obtained using anti-NOX5 raised against the human antigen. Immunofluorescence experiments demonstrated TcFR on the parasite surface. That TcFR is part of a catalytic complex allowing T. cruzi to take up Fe from the medium was confirmed by experiments in which DM28c was assayed after culturing in Fe-depleted medium: (i) proliferation during the stationary growth phase was five times slower; (ii) the relative expression of TcFR (qPCR) was 50% greater; (iii) intact cells had 120% higher Fe-reductase activity. This ensemble of results indicates that TcFR is a conserved enzyme in T. cruzi, and its catalytic properties are modulated in order to respond to external Fe fluctuations.

Iron and Heme Metabolism at the Leishmania-Host Interface

Species of the protozoan Leishmania are causative agents of human leishmaniasis, a disease that results in significant death, disability, and disfigurement around the world. The parasite is transmitted to a mammalian host by a sand fly vector where it develops as an intracellular parasite within macrophages. This process requires the acquisition of nutritional iron and heme from the host as Leishmania lacks the capacity for de novo heme synthesis and does not contain cytosolic iron-storage proteins. Proteins involved in Leishmania iron and heme transport and metabolism have been identified and shown to be crucial for the parasite's growth and replication within the host. Consequently, a detailed understanding of how these parasites harness host pathways for survival may lay the foundation for promising new therapeutic intervention against leishmaniasis.

Alkyl gallium(III) quinolinolates: A new class of highly selective anti-leishmanial agents

A series of eight alkyl gallium complexes of general formulae [GaMe₂(L)] and [Ga(Me)₂L] have been synthesised, characterised and their antimicrobial activity against bacteria, cancer cells and Leishmania assessed. All eight complexes are novel, with the solid-state structures of all complexes successfully authenticated by single crystal X-ray diffraction. The dimethyl complexes all adopt a four-coordinate tetrahedral confirmation, while the monomethyl complexes are five-coordinate trigonal bipyramidal. All complexes were screened for their anti-bacterial activity either by solution state diffusion, or a solid-state stab test. The five soluble complexes underwent testing against two differing mammalian cell controls, with excellent selectivity observed against COS-7 cells, with an IC₅₀ range of 88.5 μM to ≥100 μM. Each soluble complex was also tested for their anti-cancer capabilities, with no significant activity observed. Excellent activity was exhibited against the protozoan parasite Leishmania major (strain: V121) in both the promastigote and amastigote forms, with IC₅₀ values ranging from 1.11 μM-13.4 μM for their anti-promastigote activity and % infection values of 3.5% ± 0.65-11.5% ± 0.65 for the more clinically relevant amastigote. Selectivity indices for each were found to be in the ranges of 6.61-64.7, with significant selectivity noted for two of the complexes. At minimum, the gallium complexes show a 3-fold enhancement in activity towards the Leishmaniaamastigotes over the parent quinolinols alone.

Leishmania amazonensis ferric iron reductase (LFR1) is a bifunctional enzyme: Unveiling a NADPH oxidase activity

Leishmania amazonensis is one of leishmaniasis' causative agents, a disease that has no cure and leads to the appearance of cutaneous lesions. Recently, our group showed that heme activates a Na⁺/K⁺ ATPase in these parasites through a signaling cascade involving hydrogen peroxide (H₂O₂) generation. Heme has a pro-oxidant activity and signaling capacity, but the mechanism by which this molecule increases H₂O₂ levels in L. amazonensis has not been elucidated. Here we investigated the source of H₂O₂ stimulated by heme, ruling out the participation of mitochondria and raising the possibility of a role for a NADPH oxidase (Nox) activity. Despite the absence of a classical Nox sequence in trypanosomatid genomes, L. amazonensis expresses a surface ferric iron reductase (LFR1). Interestingly, Nox enzymes are thought to have evolved from ferric iron reductases because they share same core domain and are very similar in structure. The main difference is that Nox catalyses electron flow from NADPH to oxygen, generating reactive oxygen species (ROS), while ferric iron reductase promotes electron flow to ferric iron, generating ferrous iron. Using L. amazonensis overexpressing or knockout for LFR1 and heterologous expression of LFR1 in mammalian embryonic kidney (HEK 293) cells, we show that this enzyme is bifunctional, being able to generate both ferrous iron and H₂O₂. It was previously described that protozoans knockout for LFR1 have their differentiation to virulent forms (amastigote and metacyclic promastigote) impaired. In this work, we observed that LFR1 overexpression stimulates protozoan differentiation to amastigote forms, reinforcing the importance of this enzyme in L. amazonensis life cycle regulation. Thus, we not only identified a new source of ROS production in Leishmania, but also described, for the first time, an enzyme with both ferric iron reductase and Nox activities.

In vitro activity and mode of action of phenolic compounds on Leishmania donovani

BACKGROUND

Leishmaniasis is a disease caused by the protozoan parasite, Leishmania. The disease remains a global threat to public health requiring effective chemotherapy for control and treatment. In this study, the effect of some selected phenolic compounds on Leishmania donovani was investigated. The compounds were screened for their anti-leishmanial activities against promastigote and intracellular amastigote forms of Leishmania donovani.

METHODOLOGY/PRINCIPAL FINDINGS

The dose dependent effect and cytotoxicity of the compounds were determined by the MTT assay. Flow cytometry was used to determine the effect of the compounds on the cell cycle. Parasite morphological analysis was done by microscopy and growth kinetic studies were conducted by culturing cells and counting at 24 hours intervals over 120 hours. The cellular levels of iron in promastigotes treated with compounds was determined by atomic absorption spectroscopy and the effect of compounds on the expression of iron dependent enzymes was investigated using RT-qPCR. The IC50 of the compounds ranged from 16.34 μM to 198 μM compared to amphotericin B and deferoxamine controls. Rosmarinic acid and apigenin were the most effective against the promastigote and the intracellular amastigote forms. Selectivity indexes (SI) of rosmarinic acid and apigenin were 15.03 and 10.45 respectively for promastigotes while the SI of 12.70 and 5.21 respectively was obtained for intracellular amastigotes. Morphologically, 70% of rosmarinic acid treated promastigotes showed rounded morphology similar to the deferoxamine control. About 30% of cells treated with apigenin showed distorted cell membrane. Rosmarinic acid and apigenin induced cell arrest in the G0/G1 phase in promastigotes. Elevated intracellular iron levels were observed in promastigotes when parasites were treated with rosmarinic acid and this correlated with the level of expression of iron dependent genes.

CONCLUSIONS/SIGNIFICANCE

The data suggests that rosmarinic acid exerts its anti-leishmanial effect via iron chelation resulting in variable morphological changes and cell cycle arrest.

A MFS-like plasma membrane transporter required for Leishmania virulence protects the parasites from iron toxicity

Iron is essential for many cellular processes, but can generate highly toxic hydroxyl radicals in the presence of oxygen. Therefore, intracellular iron accumulation must be tightly regulated, by balancing uptake with storage or export. Iron uptake in Leishmania is mediated by the coordinated action of two plasma membrane proteins, the ferric iron reductase LFR1 and the ferrous iron transporter LIT1. However, how these parasites regulate their cytosolic iron concentration to prevent toxicity remains unknown. Here we characterize Leishmania Iron Regulator 1 (LIR1), an iron responsive protein with similarity to membrane transporters of the major facilitator superfamily (MFS) and plant nodulin-like proteins. LIR1 localizes on the plasma membrane of L. amazonensis promastigotes and intracellular amastigotes. After heterologous expression in Arabidopsis thaliana, LIR1 decreases the iron content of leaves and worsens the chlorotic phenotype of plants lacking the iron importer IRT1. Consistent with a role in iron efflux, LIR1 deficiency does not affect iron uptake by L. amazonensis but significantly increases the amount of iron retained intracellularly in the parasites. LIR1 null parasites are more sensitive to iron toxicity and have drastically impaired infectivity, phenotypes that are reversed by LIR1 complementation. We conclude that LIR1 functions as a plasma membrane iron exporter with a critical role in maintaining iron homeostasis and promoting infectivity in L. amazonensis.

The Gut Microbiome of the Vector Lutzomyia longipalpis Is Essential for Survival of Leishmania infantum

The vector-borne disease leishmaniasis, caused by Leishmania species protozoa, is transmitted to humans by phlebotomine sand flies. Development of Leishmania to infective metacyclic promastigotes in the insect gut, a process termed metacyclogenesis, is an essential prerequisite for transmission. Based on the hypothesis that vector gut microbiota influence the development of virulent parasites, we sequenced midgut microbiomes in the sand fly Lutzomyia longipalpis with or without Leishmania infantum infection. Sucrose-fed sand flies contained a highly diverse, stable midgut microbiome. Blood feeding caused a decrease in microbial richness that eventually recovered. However, bacterial richness progressively decreased in L. infantum-infected sand flies. Acetobacteraceae spp. became dominant and numbers of Pseudomonadaceae spp. diminished coordinately as the parasite underwent metacyclogenesis and parasite numbers increased. Importantly, antibiotic-mediated perturbation of the midgut microbiome rendered sand flies unable to support parasite growth and metacyclogenesis. Together, these data suggest that the sand fly midgut microbiome is a critical factor for Leishmania growth and differentiation to its infective state prior to disease transmission.

Leishmania infantum, a parasitic protozoan causing fatal visceral leishmaniasis, is transmitted to humans through the bite of the sand fly Lutzomyia longipalpis. Development of the parasite to its virulent metacyclic state occurs in the sand fly gut. In this study, the microbiota within the Lu. longipalpis midgut was delineated by 16S ribosomal DNA (rDNA) sequencing, revealing a highly diverse community composition that lost diversity as parasites developed to their metacyclic state and increased in abundance in infected flies. Perturbing sand fly gut microbiota with an antibiotic cocktail, which alone had no effect on either the parasite or the fly, arrested both the development of virulent parasites and parasite expansion. These findings indicate the importance of bacterial commensals within the insect vector for the development of virulent pathogens, and raise the possibility that impairing the microbial composition within the vector might represent a novel approach to control of vector-borne diseases.

Leishmania and its quest for iron: An update and overview

Parasites of genus Leishmania are the causative agents of complex neglected diseases called leishmaniasis and continue to be a significant health concern globally. Iron is a vital nutritional requirement for virtually all organisms, including pathogenic trypanosomatid parasites, and plays a crucial role in many facets of cellular metabolism as a cofactor of several enzymes. Iron acquisition is essential for the survival of parasites. Yet parasites are also vulnerable to the toxicity of iron and reactive oxygen species. The aim of this review is to provide an update on the current knowledge about iron acquisition and usage by Leishmania species. We have also discussed about host strategy to modulate iron availability and the strategies deployed by Leishmania parasites to overcome iron withholding defences and thus favour parasite growth within host macrophages. Since iron plays central roles in the host's response and parasite metabolism, a comprehensive understanding of the iron metabolism is beneficial to identify potential viable therapeutic opportunities against leishmaniasis.

Iron acquisition in Leishmania and its crucial role in infection

Iron is an essential cofactor for many basic metabolic pathways in pathogenic microbes and their hosts. It is also dangerous as it can catalyse the production of reactive free radicals. This dual character makes the host can either limit iron availability to invading microbes or exploit iron to induce toxicity to pathogens. Successful pathogens, including Leishmania species, must possess mechanisms to circumvent host's iron limitation and iron-induced toxicity in order to survive. In this review, we discuss the regulation of iron metabolism in the setting of infection and delineate the iron acquisition strategies used by Leishmania parasites and their subversions to host iron metabolism to overcome host's iron-related defences.

Dynamics of sterol synthesis during development of Leishmania spp. parasites to their virulent form

Background

The Leishmania spp. protozoa, the causative agents of the “neglected” tropical disease leishmaniasis, are transmitted to mammals by sand fly vectors. Within the sand fly, parasites transform from amastigotes to procyclic promastigotes, followed by development of virulent (metacyclic) promastigote forms. The latter are infectious to mammalian hosts. Biochemical components localized in the parasite plasma membrane such as proteins and sterols play a pivotal role in Leishmania pathogenesis. Leishmania spp. lack the enzymes for cholesterol synthesis, and the dynamics of sterol acquisition and biosynthesis in parasite developmental stages are not understood. We hypothesized that dynamic changes in sterol composition during metacyclogenesis contribute to the virulence of metacyclic promastigotes.

Methods

Sterols were extracted from logarithmic phase or metacyclic promastigotes grown in liquid culture with or without cholesterol, and analyzed qualitatively and quantitatively by gas chromatograph-mass spectrometry (GC-MS). TriTrypDB was searched for identification of genes involved in Leishmania sterol biosynthetic pathways.

Results

In total nine sterols were identified. There were dynamic changes in sterols during promastigote metacyclogenesis. Cholesterol in the culture medium affected sterol composition in different parasite stages. There were qualitative and relative quantitative differences between the sterol content of virulent versus avirulent parasite strains. A tentative sterol biosynthetic pathway in Leishmania spp. promastigotes was identified.

Conclusions

Significant differences in sterol composition were observed between promastigote stages, and between parasites exposed to different extracellular cholesterol in the environment. These data lay the foundation for further investigating the role of sterols in the pathogenesis of Leishmania spp. infections.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1470-0) contains supplementary material, which is available to authorized users.

Transferrin: Endocytosis and Cell Signaling in Parasitic Protozoa

Iron is the fourth most abundant element on Earth and the most abundant metal in the human body. This element is crucial for life because almost all organisms need iron for several biological activities. This is the case with pathogenic organisms, which are at the vanguard in the battle with the human host for iron. The latest regulates Fe concentration through several iron-containing proteins, such as transferrin. The transferrin receptor transports iron to each cell that needs it and maintains it away from pathogens. Parasites have developed several strategies to obtain iron as the expression of specific transferrin receptors localized on plasma membrane, internalized through endocytosis. Signal transduction pathways related to the activation of the receptor have functional importance in proliferation. The study of transferrin receptors and other proteins with action in the signaling networks is important because these proteins could be used as therapeutic targets due to their specificity or to differences with the human counterpart. In this work, we describe proteins that participate in signal transduction processes, especially those that involve transferrin endocytosis, and we compare these processes with those found in T. brucei, T. cruzi, Leishmania spp., and E. histolytica parasites.

Strategies of Intracellular Pathogens for Obtaining Iron from the Environment

Most microorganisms are destroyed by the host tissues through processes that usually involve phagocytosis and lysosomal disruption. However, some organisms, called intracellular pathogens, are capable of avoiding destruction by growing inside macrophages or other cells. During infection with intracellular pathogenic microorganisms, the element iron is required by both the host cell and the pathogen that inhabits the host cell. This minireview focuses on how intracellular pathogens use multiple strategies to obtain nutritional iron from the intracellular environment in order to use this element for replication. Additionally, the implications of these mechanisms for iron acquisition in the pathogen-host relationship are discussed.

Innate Humoral Defense Factors

Although innate immunity came into the research spotlight in the late 1990s when its instructive role in the adaptive immune response was recognized, innate humoral defense factors have a much older history. The exocrine secretions of the body contain a plethora of distinct soluble factors (lysozyme, lactoferrin, peroxidases, proline-rich proteins, histatins, etc.) that protect the body from mucosal microbial pathogens. More recent studies have established that the humoral arm of innate immunity contains a heterogeneous group of pattern-recognition molecules (e.g., pentraxins, collectins, and ficolins), which perform diverse host-defense functions, such as agglutination and neutralization, opsonization, control of inflammation, and complement activation and regulation. These pattern-recognition molecules, which act as functional predecessors of antibodies (“ante-antibodies”), and the classic soluble innate defense factors form an integrated system with complementary specificity, action, and tissue distribution, and they are the subject of this chapter.

The genome of Eimeria falciformis--reduction and specialization in a single host apicomplexan parasite

BACKGROUND

The phylum Apicomplexa comprises important unicellular human parasites such as Toxoplasma and Plasmodium. Eimeria is the largest and most diverse genus of apicomplexan parasites and some species of the genus are the causative agent of coccidiosis, a disease economically devastating in poultry. We report a complete genome sequence of the mouse parasite Eimeria falciformis. We assembled and annotated the genome sequence to study host-parasite interactions in this understudied genus in a model organism host.

RESULTS

The genome of E. falciformis is 44 Mb in size and contains 5,879 predicted protein coding genes. Comparative analysis of E. falciformis with Toxoplasma gondii shows an emergence and diversification of gene families associated with motility and invasion mainly at the level of the Coccidia. Many rhoptry kinases, among them important virulence factors in T. gondii, are absent from the E. falciformis genome. Surface antigens are divergent between Eimeria species. Comparisons with T. gondii showed differences between genes involved in metabolism, N-glycan and GPI-anchor synthesis. E. falciformis possesses a reduced set of transmembrane transporters and we suggest an altered mode of iron uptake in the genus Eimeria.

CONCLUSIONS

Reduced diversity of genes required for host-parasite interaction and transmembrane transport allow hypotheses on host adaptation and specialization of a single host parasite. The E. falciformis genome sequence sheds light on the evolution of the Coccidia and helps to identify determinants of host-parasite interaction critical for drug and vaccine development.

Pathways of iron acquisition and utilization in Leishmania

Iron is essential for many metabolic pathways, but is toxic in excess. Recent identification of the ferric iron reductase LFR1, the ferrous iron transporter LIT1, and the heme transporter LHR1 greatly advanced our understanding of how Leishmania parasites acquire iron and regulate its uptake. LFR1 and LIT1 have close orthologs in plants, and are required for Leishmania virulence. Consistent with the lack of heme biosynthesis in trypanosomatids, LHR1 and LABCG5, a protein involved in heme salvage from hemoglobin, seem essential for Leishmania survival. LFR1, LIT1 and LHR1 are upregulated under low iron availability, in agreement with the need to prevent excessive iron uptake. Future studies should clarify how Leishmania interacts with the iron homeostasis machinery of its host cell, the macrophage.

Efficient iron uptake via a reductive mechanism in procyclic Trypanosoma brucei

The bloodstream form of Trypanosoma brucei acquires iron from transferrin by receptor-mediated endocytosis. However, it is unknown how procyclic forms that cannot bind transferrin acquire iron. Here, we show that the procyclic form of T. brucei efficiently takes up iron from ferric complexes via a reductive mechanism and that iron obtained using this mechanism is transported to, and used in, the mitochondria. The affinity of the transport system is comparable to that of Saccharomyces cerevisiae , with an apparent K(m) of 0.85 μM.

Iron-saturated lactoferrin and pathogenic protozoa: could this protein be an iron source for their parasitic style of life?

Iron is an essential nutrient for the survival of pathogens inside a host. As a general strategy against microbes, mammals have evolved complex iron-withholding systems for efficiently decreasing the iron accessible to invaders. Pathogens that inhabit the respiratory, intestinal and genitourinary tracts encounter an iron-deficient environment on the mucosal surface, where ferric iron is chelated by lactoferrin, an extracellular glycoprotein of the innate immune system. However, parasitic protozoa have developed several mechanisms to obtain iron from host holo-lactoferrin. Tritrichomonas fetus, Trichomonas vaginalis, Toxoplasma gondii and Entamoeba histolytica express lactoferrin-binding proteins and use holo-lactoferrin as an iron source for growth in vitro; in some species, these binding proteins are immunogenic and, therefore, may serve as potential vaccine targets. Another mechanism to acquire lactoferrin iron has been reported in Leishmania spp. promastigotes, which use a surface reductase to recognize and reduce ferric iron to the accessible ferrous form. Cysteine proteases that cleave lactoferrin have been reported in E. histolytica. This review summarizes the available information on how parasites uptake and use the iron from lactoferrin to survive in hostile host environments.

LFR1 ferric iron reductase of Leishmania amazonensis is essential for the generation of infective parasite forms

The protozoan parasite Leishmania is the causative agent of serious human infections worldwide. The parasites alternate between insect and vertebrate hosts and cause disease by invading macrophages, where they replicate. Parasites lacking the ferrous iron transporter LIT1 cannot grow intracellularly, indicating that a plasma membrane-associated mechanism for iron uptake is essential for the establishment of infections. Here, we identify and functionally characterize a second member of the Leishmania iron acquisition pathway, the ferric iron reductase LFR1. The LFR1 gene is up-regulated under iron deprivation and accounts for all the detectable ferric reductase activity exposed on the surface of Leishmania amazonensis. LFR1 null mutants grow normally as promastigote insect stages but are defective in differentiation into the vertebrate infective forms, metacyclic promastigotes and amastigotes. LFR1 overexpression partially restores the abnormal morphology of infective stages but markedly reduces parasite viability, precluding its ability to rescue LFR1 null replication in macrophages. However, LFR1 overexpression is not toxic for amastigotes lacking the ferrous iron transporter LIT1 and rescues their growth defect. In addition, the intracellular growth of both LFR1 and LIT1 null parasites is rescued in macrophages loaded with exogenous iron. This indicates that the Fe(3+) reductase LFR1 functions upstream of LIT1 and suggests that LFR1 overexpression results in excessive Fe(2+) production, which impairs parasite viability after intracellular transport by LIT1.

Nutrient transport and pathogenesis in selected parasitic protozoa

Parasitic protozoa, such as malaria parasites, trypanosomes, and Leishmania, acquire a plethora of nutrients from their hosts, employing transport proteins located in the plasma membrane of the parasite. Application of molecular genetic approaches and the completion of genome projects have allowed the identification and functional characterization of a cohort of transporters and their genes in these parasites. This review focuses on a subset of these permeases that have been studied in some detail, that import critical nutrients, and that provide examples of approaches being undertaken broadly with these and other parasite transporters. Permeases reviewed include those for hexoses, purines, iron, polyamines, carboxylates, and amino acids. Topics of special emphasis include structure-function approaches, critical roles for transporters in parasite viability and physiology, regulation of transporter expression, and subcellular targeting. Investigations of parasite transporters impact a broad spectrum of basic biological problems in these protozoa.

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.

Iron metabolism in trypanosomatids, and its crucial role in infection

Iron is almost ubiquitous in living organisms due to the utility of its redox chemistry. It is also dangerous as it can catalyse the formation of reactive free radicals - a classical double-edged sword. In this review, we examine the uptake and usage of iron by trypanosomatids and discuss how modulation of host iron metabolism plays an important role in the protective response. Trypanosomatids require iron for crucial processes including DNA replication, antioxidant defence, mitochondrial respiration, synthesis of the modified base J and, in African trypanosomes, the alternative oxidase. The source of iron varies between species. Bloodstream-form African trypanosomes acquire iron from their host by uptake of transferrin, and Leishmania amazonensis expresses a ZIP family cation transporter in the plasma membrane. In other trypanosomatids, iron uptake has been poorly characterized. Iron-withholding responses by the host can be a major determinant of disease outcome. Their role in trypanosomatid infections is becoming apparent. For example, the cytosolic sequestration properties of NRAMP1, confer resistance against leishmaniasis. Conversely, cytoplasmic sequestration of iron may be favourable rather than detrimental to Trypanosoma cruzi. The central role of iron in both parasite metabolism and the host response is attracting interest as a possible point of therapeutic intervention.

Diacylglycerol-stimulated endocytosis of transferrin in trypanosomatids is dependent on tyrosine kinase activity

Small molecule regulation of cell function is an understudied area of trypanosomatid biology. In Trypanosoma brucei diacylglycerol (DAG) stimulates endocytosis of transferrin (Tf). However, it is not known whether other trypanosomatidae respond similarly to the lipid. Further, the biochemical pathways involved in DAG signaling to the endocytic system in T. brucei are unknown, as the parasite genome does not encode canonical DAG receptors (e.g. C1-domains). We established that DAG stimulates endocytosis of Tf in Leishmania major, and we evaluated possible effector enzymes in the pathway with multiple approaches. First, a heterologously expressed glycosylphosphatidylinositol phospholipase C (GPI-PLC) activated endocytosis of Tf 300% in L. major. Second, exogenous phorbol ester and DAGs promoted Tf endocytosis in L. major. In search of possible effectors of DAG signaling, we discovered a novel C1-like domain (i.e. C1_5) in trypanosomatids, and we identified protein Tyr kinases (PTKs) linked with C1_5 domains in T. brucei, T. cruzi, and L. major. Consequently, we hypothesized that trypanosome PTKs might be effector enzymes for DAG signaling. General uptake of Tf was reduced by inhibitors of either Ser/Thr or Tyr kinases. However, DAG-stimulated endocytosis of Tf was blocked only by an inhibitor of PTKs, in both T. brucei and L. major. We conclude that (i) DAG activates Tf endocytosis in L. major, and that (ii) PTKs are effectors of DAG-stimulated endocytosis of Tf in trypanosomatids. DAG-stimulated endocytosis of Tf may be a T. brucei adaptation to compete effectively with host cells for vertebrate Tf in blood, since DAG does not enhance endocytosis of Tf in human cells.

Functional characterization of LIT1, the Leishmania amazonensis ferrous iron transporter

Leishmania amazonensis LIT1 was identified based on homology with IRT1, a ferrous iron transporter from Arabidopsis thaliana. Deltalit1L. amazonensis are defective in intracellular replication and lesion formation in vivo, a virulence phenotype attributed to defective intracellular iron acquisition. Here we functionally characterize LIT1, directly demonstrating that it functions as a ferrous iron membrane transporter from the ZIP family. Conserved residues in the predicted transmembrane domains II, IV, V and VII of LIT1 are essential for iron transport in yeast, including histidines that were proposed to function as metal ligands in ZIP transporters. LIT1 also contains two regions within the predicted intracellular loop that are not found in Arabidopsis IRT1. Deletion of region I inhibited LIT1 expression on the surface of Leishmania promastigotes. Deletion of region II did not interfere with LIT1 trafficking to the surface, but abolished its iron transport capacity when expressed in yeast. Mutagenesis revealed two motifs within region II, HGHQH and TPPRDM, that are independently required for iron transport by LIT1. D263 was identified as a key residue required for iron transport within the TPPRDM motif, while P260 and P261 were dispensable. Deletion of proline-rich regions within region I and between regions I and II did not affect iron transport in yeast, but in L. amazonensis were not able to rescue the intracellular growth of Deltalit1 parasites, or their ability to form lesions in mice. These results are consistent with a potential role of the unique intracellular loop of LIT1 in intracellular regulation by Leishmania-specific factors.

Iron trafficking as an antimicrobial target

Iron is essential for the survival of most organisms. Microbial iron acquisition depends on multiple, sometimes complex steps, many of which are not shared by higher eukaryotes. Depriving pathogenic microbes of iron is therefore a potential antimicrobial strategy. The following minireview briefly describes general elements in microbial iron uptake pathways and summarizes some of the current work aiming at their medicinal inhibition.

Use and endocytosis of iron-containing proteins by Entamoeba histolytica trophozoites

Iron is essential for nearly all organisms; in mammals, it is part of proteins such as haemoglobin, and it is captured by transferrin and lactoferrin. Transferrin is present in serum, and lactoferrin is secreted by the mucosa and by neutrophils at infection sites, as a host iron-withholding response, sequestering iron away from invading microorganisms. Additionally, all cells contain ferritin, which sequesters iron when its intracellular levels are increased, detoxifying and preventing damage. Liver ferritin contains 50% of iron corporal reserves. During evolution, pathogens have evolved diverse strategies to obtain iron from their hosts in order to survive. The protozoan Entamoeba histolytica invades the intestinal mucosa, causing dysentery, and the trophozoites often travel to the liver producing hepatic abscesses; thus, intestine and liver proteins could be important iron supplies for E. histolytica. We found that E. histolytica trophozoites can grow in both ferrous and ferric iron, and that they can use haemoglobin, holo-transferrin, holo-lactoferrin, and ferritin as in vitro iron sources. These proteins supported the amoeba growth throughout consecutive passages, similarly to ferric citrate. By confocal microscopy and immunoblotting, iron-binding proteins were observed specifically bound to the amoeba surface, and they were endocytosed, trafficked through the endosomal/lysosomal route, and degraded by neutral and acidic cysteine-proteases. Transferrin and ferritin were mainly internalized through clathrin-coated vesicles, and holo-lactoferrin was mainly internalized by caveola-like structures. In contrast, apo-lactoferrin bound to membrane lipids and cholesterol, inducing cell death. The results suggest that in vivo trophozoites secrete products that can destroy enterocytes, erythrocytes, and hepatocytes, releasing transferrin, haemoglobin, ferritin, and other iron-containing proteins, which, together with lactoferrin derived from neutrophils and acinar cells, could be used as abundant iron supplies by amoebas.

Heme as a source of iron to Leishmania infantum amastigotes

Amastigotes, the mammalian stage of Leishmania, must acquire iron from molecules accessing the macrophage parasitophorous vacuole (PV) where they inhabit. These molecules likely include non-heme and heme-bound forms of iron. Here we demonstrate that, in addition to the previously documented use of ferrous iron, Leishmania amastigotes are also capable of exploiting iron from hemin and hemoglobin for nutritional purposes. Moreover, evidence is presented that a ligand at the surface of amastigotes binds hemin with high-affinity (Kd=0.044nM). This ligand may function in intracellular transport of heme while hemoglobin internalization occurs through a different molecule. The co-existence in Leishmania amastigotes of different processes to acquire iron could constitute an infective strategy, ensuring parasites a substantial advantage in situations of iron limitation.

Crusade for iron: iron uptake in unicellular eukaryotes and its significance for virulence

The effective acquisition of iron is a pre-requisite for survival of all organisms, especially parasites that have a high iron requirement. In mammals, iron homeostasis is meticulously regulated; extracellular free iron is essentially unavailable and host iron availability has a crucial role in the host-pathogen relationship. Therefore, pathogens use specialized and effective mechanisms to acquire iron. In this review, we summarize the iron-uptake systems in eukaryotic unicellular organisms with particular focus on the pathogenic species: Candida albicans, Tritrichomonas foetus, Trypanosoma brucei and Leishmania spp. We describe the diversity of their iron-uptake mechanisms and highlight the importance of the process for virulence.

Iron acquisition within host cells and the pathogenicity of Leishmania

Iron is an essential cofactor for several enzymes and metabolic pathways, in both microbes and in their eukaryotic hosts. To avoid toxicity, iron acquisition is tightly regulated. This represents a particular challenge for pathogens that reside within the endocytic pathway of mammalian cells, because endosomes and lysosomes are gradually depleted in iron by host transporters. An important player in this process is Nramp1 (Slc11a1), a proton efflux pump that translocates Fe(2+) and Mn(2+) ions from macrophage lysosomes/phagolysosomes into the cytosol. Mutations in Nramp1 cause susceptibility to infection with the bacteria Salmonella and Mycobacteria and the protozoan Leishmania, indicating that an available pool of intraphagosomal iron is critical for the intracellular survival and replication of these pathogens. Salmonella and Mycobacteria are known to express iron transporter systems that effectively compete with host transporters for iron. Until recently, however, very little was known about the molecular strategy used by Leishmania for survival in the iron-poor environment of macrophage phagolysosomes. It is now clear that intracellular residence induces Leishmania amazonensis to express LIT1, a ZIP family membrane Fe(2+) transporter that is required for intracellular growth and virulence.

Pumping iron: a potential target for novel therapeutics against schistosomes

Parasites, as with the vast majority of organisms, are dependent on iron. Pathogens must compete directly with the host for this essential trace metal, which is sequestered within host proteins and inorganic chelates. Not surprisingly, pathogenic prokaryotes and eukaryotic parasites have diverse adaptations to exploit host iron resources. How pathogenic bacteria scavenge host iron is well characterized and is reasonably well known for a few parasitic protozoa, but is poorly understood for metazoan parasites. Strategies of iron acquisition by schistosomes are examined here, with emphasis on possible mechanisms of iron absorption from host serum iron transporters or from digested haem. Elucidation of these metabolic mechanisms could lead to the development of new interventions for the control of schistosomiasis and other helminth diseases.

Protein turnover and differentiation in Leishmania

Leishmania occurs in several developmental forms and thus undergoes complex cell differentiation events during its life-cycle. Those are required to allow the parasite to adapt to the different environmental conditions. The sequencing of the genome of L. major has facilitated the identification of the parasite’s vast arsenal of proteolytic enzymes, a few of which have already been carefully studied and found to be important for the development and virulence of the parasite. This review focuses on these peptidases and their role in the cellular differentiation of Leishmania through their key involvement in a variety of degradative pathways in the lysosomal and autophagy networks.

Intracellular Leishmania: your iron or mine?

Iron is a co-factor for several essential enzymes and biochemical pathways, including those required for replication of pathogens such as Leishmania in macrophages. Iron acquisition is emerging as a key battleground in which the iron import systems of microbes are pitted against the iron withdrawal and sequestration systems of macrophages, with both competing for iron at the interface of host-pathogen interaction. The recent characterization of a ferrous iron transport system (LIT1) in Leishmania amazonensis that is induced intracellularly and is required for survival in macrophages and for virulence in vivo provides an elegant example of the adaptation of protozoa to the iron-poor phagosomal environment.

A Leishmania amazonensis ZIP family iron transporter is essential for parasite replication within macrophage phagolysosomes

Infection of mammalian hosts with Leishmania amazonensis depends on the remarkable ability of these parasites to replicate within macrophage phagolysosomes. A critical adaptation for survival in this harsh environment is an efficient mechanism for gaining access to iron. In this study, we identify and characterize LIT1, a novel L. amazonensis membrane protein with extensive similarity to IRT1, a ZIP family ferrous iron transporter from Arabidopsis thaliana. The ability of LIT1 to promote iron transport was demonstrated after expression in yeast and in L. amazonensis LIT1-null amastigotes. Endogenous LIT1 was only detectable in amastigotes replicating intracellularly, and its intracellular expression was accelerated under conditions predicted to result in iron deprivation. Although L. amazonensis lacking LIT1 grew normally in axenic culture and had no defects differentiating into infective forms, replication within macrophages was abolished. Consistent with an essential role for LIT1 in intracellular growth as amastigotes, Δlit1 parasites were avirulent. After inoculation into highly susceptible mice, no lesions were detected, even after extensive periods of time. Despite the absence of pathology, viable Δlit1 parasites were recovered from the original sites of inoculation, indicating that L. amazonensis can persist in vivo independently of the ability to grow in macrophages. Our findings highlight the essential role played by intracellular iron acquisition in Leishmania virulence and identify this pathway as a promising target for therapeutic intervention.

Human hololactoferrin: endocytosis and use as an iron source by the parasite Entamoeba histolytica

Entamoeba histolytica is an enteric protozoan that exclusively infects human beings. This parasite requires iron for its metabolic functions. Lactoferrin is a mammalian glycoprotein that chelates extracellular iron on mucosal surfaces, including the surface of the large intestine, where E. histolytica initiates infection. This work examined the interaction in vitro of E. histolytica trophozoites with human hololactoferrin (iron-saturated lactoferrin). A minimum concentration of 50 microM Fe from hololactoferrin supported growth of the amoeba. Amoebic binding sites for hololactoferrin were different from those for human apolactoferrin, holotransferrin and haemoglobin. One amoebic hololactoferrrin-binding polypeptide of 90 kDa was found, which was not observed after treatment of trophozoites with trypsin. Hololactoferrin-binding-protein levels increased in amoebas starved of iron, or grown in hololactoferrin. Internalization of hololactoferrin was inhibited by filipin. Endocytosed hololactoferrin colocalized with an anti-chick embryo caveolin mAb in amoebic vesicles, and lactoferrin was further detected in acidic vesicles; amoebic caveolin of 22 kDa was detected by Western blotting using this antibody. Cysteine proteases from amoebic extracts were able to cleave hololactoferrin. Together, these data indicate that E. histolytica trophozoites bind to hololactoferrin through specific membrane lactoferrin-binding proteins. This ferric protein might be internalized via caveolae-like microdomains, then used as an iron source, and degraded.

The therapeutic potential of lactoferrin

Lactoferrin (Lf), a natural defence iron-binding protein, is present in exocrine secretions that are commonly exposed to normal flora: milk, tears, nasal exudate, saliva, bronchial mucus, gastrointestinal fluids, cervicovaginal mucus and seminal fluid. Additionally, Lf is produced in polymorphonuclear leukocytes and is deposited by these circulating cells in septic sites. A principal function of Lf is that of scavenging non-protein-bound iron in body fluids and inflamed areas so as to suppress free radical-mediated damage and decrease accessibility of the metal to invading bacterial, fungal and neoplastic cells. Adequate sources of bovine and recombinant human Lf are now available for development of commercial applications. Among the latter are use of Lf in food preservation, fish farming, infant milk formula and oral hygiene. Other readily accessible body compartments for Lf administration include skin, throat and small intestine. Further research is needed for possible medicinal use in colon and systemic tissues. Although Lf is a natural product and should be highly biocompatible, possible hazards have been documented.