Rodney R. Dietert, Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, U.S.A.
Jerome Nriagu, School of Public Health, University of Michigan, Ann Arbor, MI 48109, U.S.A.
Lothar Rink, Institute of Immunology, Medical Faculty, RWTH Aachen University, 52074 Aachen, Germany
Anthony Schryvers, Department of Microbiology, Immunology and Infectious Diseases, University of Calgary Health Science, Calgary, Canada
Eric P. Skaar, Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine, Nashville, TN 37232-2363, U.S.A.
To understand the linkage between heavy metals1 and the pathogenesis of infectious diseases and to address the underlying mechanisms that moderate the outcome of infection.
1 Heavy metals are defined as metals with a density higher than 5 g/cm3. Of 90 naturally occurring elements, 53 can be regarded as heavy metals. Based on their solubility under physiological conditions, about 20 of them are available to living cells and among these, only eight are known to be essential in human biology (iron, molybdenum, manganese, zinc, nickel, copper, vanadium, cobalt, and selenium), while the others appear in cells as a result of their wide distribution in various ecosystems. Regardless of their essentiality to life, however, most heavy metals can be toxic to cells and invading pathogens when their concentrations increase above a certain level.Top of page
There is growing concern that human beings may inadvertently be contributing to the epidemic of trace metal deficiency -- a problem which affects billions of people worldwide but is especially prevalent in the developing countries (Graham et al., 2012). The venerated Green Revolution depended on selective breeding of high-yielding crops (mainly wheat, rice, and corn) that would tolerate the extremes of stress (such as heat, cold, drought, flooding, and pests and diseases) better than the crops (especially the pulses or grain legumes) they replaced. It had the collateral effect, however, of inducing or aggravating low concentrations of zinc and other trace metals in the human food chain. Features of the Green Revolution that would diminish the levels of trace metals in the high-yielding cereal crops and hence the staple human diet include (Graham et al., 2012):
(i) use of phosphate fertilizers that tend to decrease the uptake of trace metals by plants;
(ii) use of nitrogenous fertilizers that tend to reduce the translocation of trace metals from leaves to seeds in metal-deficient soils;
(iii) the low trace metal status of many soils has been aggravated further – of 190 major agricultural soils of the world, 49% are deficient in zinc, 31% deficient in boron, 15% deficient in molybdenum, 14% deficient in copper, 10% deficient in manganese, and only 3% deficient in iron;
(iv) expansion of agriculture to higher-pH, lower-rainfall soils as a consequence of soil degradation and population growth;
(v) global loss of diet diversity through increasing reliance on more refined cereal-based diets that are lower in trace metals and other micronutrients especially provitamin A carotenoids.
Given that more than 2 billion people worldwide suffer from iron deficiency and that iron overload is one of the most common single-gene inherited disease, the idea that much of the nutrition iron deficiency in humans globally may be due to the underlying zinc and other trace metal deficiencies has recently been advanced and is a matter of concern (Graham et al., 2012). The unintended consequences of trace metal imbalances that arose out of the Green Revolution raise concerns as to whether new efforts to increase food production in the developing countries will further exacerbate the disparities in the global burden of infectious diseases.
Another recent development of growing public health concern is the worldwide contamination of the environment from toxic but non-essential trace metals, such as arsenic, cadmium, lead, mercury, platinum, silver, and thallium (Nriagu and Pacyna, 1988). The bioaccumulation and biomagnifications of these heavy metals increase the threat of toxicity further up the food chain. Since these heavy metals have become so widely dispersed, they have the potential to pose a continuous risk for human and animal health for a long time because they are non-degradable. In many parts of the world, high levels of heavy metal pollution co-exist with high prevalence of trace metal deficiencies, and the potential for interaction of these two health risks cannot be ignored. Research during the last half century has clearly established that heavy metals, whether essential or non-essential, play important roles in a wide variety of biological processes of living systems and can be a defining factor in the outcome of parasitic infections (Winans et al., 2011; Crichton, 2012). Regardless of their essentiality to life, most heavy metals are toxic to host cells and invading pathogens when their concentrations increase above a certain level.
Infectious diseases remain the leading cause of morbidity and mortality worldwide, accounting for over 11 million deaths in 2004 (WHO, 2012). Three of the five leading causes of death and morbidity, as measured by disease adjusted life years (DALYs), in 2004 were due to infectious diseases (WHO, 2012). Many parts of the world that are endemic for the most common infectious diseases of our time, including malaria, HIV/AIDS, diarrhea, upper respiratory tract infections, and tuberculosis, also have the highest prevalence rates of trace metal deficiencies. Parasites responsible for these diseases can impose strong evolutionary pressure on their host, and the co-clustering of major infectious diseases with deficiency status in trace metals has inevitably created a complex web of interactions with serious but poorly understood health repercussions (Winans et al., 2011). The widespread occurrence of deficiencies in trace metals (especially copper, zinc, or iron) in humans has served as the impetus for recent efforts to determine whether nutritional supplementation with trace metals alone or as adjuvants has the potential to prevent, attenuate, and eventually treat infectious diseases (Sazawal et al., 2006; Chasapis et al., 2012). The traditional wisdom is that host defense mechanisms are primarily a function of the immune system that can be deployed to detect and eliminate invading parasites. This paradigm has recently been challenged by studies on the differential ability of hosts to tolerate an infective pathogen, which show that tolerance to infection can be considered as a distinct strategy of host defense; i.e., a new phenotype (Medzhitov et al., 2012). Thus, the human host can protect itself from infectious disease using two distinct strategies: resistance and tolerance. Tolerance reduces the negative impact of an infection on host fitness without directly affecting the pathogen burden. On the other hand, resistance reduces pathogen burden once the infection is established. The role that trace metals can play in the host’s ability to tolerate the presence of a pathogen is huge but has largely been overlooked in animal and human studies. An improvement of our knowledge in this field would be instrumental in generating novel countermeasures against disease infections.
A related, important development is the growing convergence in the study of metallomics (metal-containing biomolecules) and proteomics into the emerging field of metalloproteomics which has given rise to new developments in analytical methods and technology for studying the distribution, trafficking and fate of trace metals in biological systems. These developments have placed scientists at a threshold for major paradigm shift in our understanding of the relationships between homeostatic mechanisms of trace metals and infectious diseases. This timely Forum will draw on expertise in several fields including analytical science, biogeochemistry, bioinformatics, biological catalysis, biological environmental science, cell biology, clinical chemistry, environmental health, medicine, metallobiochemistry, microbiology, nutritional chemistry, pharmacology, plant biochemistry and physiology and toxicology.
New Frontiers in the Study of Heavy Metals and Disease Infection
Trace metals command a central position at the host–pathogen interface because mammalian and pathogenic cells have an essential demand for metals which are required for many metabolic processes (Failla, 2003). Mammals have evolved complex strategies aimed at restricting the supply of the essential nutrient metals to pathogens, which represents an effective strategy of host defense sometimes termed “nutritional immunity.” On the other hand, pathogens can evoke multiple strategies to acquire the essential elements metals from their hosts especially since sufficient supply of trace metals is linked to their proliferation, virulence, and persistence (Failla, 2004; Haase et al., 2008). The control over the homeostatic cycle of essential trace metals is a critical battlefield in host–pathogen interplay—one that influences the course of an infectious disease in favor of either the mammalian host or the pathogenic invader. A plethora of new insights in recent years has revealed a complex control network of proteins and other molecules involved in the competition between the host cells and invading pathogens for essential trace elements with emphasis on the host's mechanisms of metal restriction and on the counteracting metal-acquisition strategies employed by pathogens (Inadera, 2006; Jamola and Valco, 2011). An interesting evolutionary perspective is that iron deficiency may be a protective adaptive response in areas of the world where there is a higher burden of infectious disease (Denic and Agarwal, 2007; Johnson and Wessling- Resnick, 2012), but whether this is also true for deficiencies in other trace metals is a matter of an ongoing debate.
Every human pathogen is characterized by a certain degree of virulence or the ability to cause disease in its host; to acquire a critical trace element for its survival, for instance, the pathogen may disrupt the normal homeostatic process or destroy some cells or a tissue. Disease resistance is driven by the immune system and is aimed at detecting, neutralizing, destroying, and expelling the invading pathogens (Chaturvedi et al., 2004). The moderating effects of heavy metals on both innate and adaptive immune systems, which contribute to host resistance to infections, have been documented in a number of studies (Dietert, 2009; Festa and Thiele, 2011; Johnson and Wessling- Resnick, 2012). Immune responses leading to the destruction and elimination of pathogens can be a double-edged sword, either resolving an infection or leading to over-exuberant inflammation, tissue damage, and/or impairments in normal tissue function; metabolic cycles of trace metals are strongly implicated in these phenomena. Collectively, the negative and unavoidable consequence of immune defenses on host fitness is referred to as immunopathology (Medzhitov et al., 2012). An invading parasite can impose two types of tissue damage on the host, namely, direct damage by the pathogen and immunopathology. In response, the human host has evolved multiple mechanisms within and outside the immune system to reduce the pathogen-induced damage and minimize the immunopathology (Osman and Cavet, 2011). In general, the degree of immunopathology is positively correlated with the magnitude and duration of the immune response and often with metal status. One can therefore surmise that the dedicated mechanisms primed to restore trace metal homeostasis and normative tissue function (regardless of what caused their dysregulation) are also engaged in the reduction of the impacts of infections.
In contrast to resistance, tolerance decreases the host’s susceptibility to tissue damage, or other fitness costs, caused by the pathogens or by the immune response against them (Medzhitov et al., 2012). The term tolerance is not to be confused with immunological tolerance, which is defined as unresponsiveness to self antigens (Medzhitov et al., 2012). However, since immune cells require an adequate supply of trace elements to express and preserve the structure and function of key metalloproteins that participate in housekeeping processes, the immunological tolerance can be one of the mechanisms employed by the human host to protect itself from immune- or pathogeninflicted damage under some circumstances. The idea of host tolerance is conceptually related to the “Typhoid Mary” phenomenon – healthy carriers remain asymptomatic and tolerant despite being infected. Possible mechanisms by which metal metabolism increases the host’s tolerance to invading pathogen include (a) the production of reactive oxygen species and toxic free radicals; (b) inducible mechanisms such as the expression of antimicrobial factors, (for instance,the iron chelator deferoxamine (DFO) has been demonstrated to kill Plasmodium falciparum in vitro and enhance the clearance of P. falciparum in human infections); and (c) phagocytosis-dependent microbial containment (Deretic and Levine, 2009; Chifman et al., 2012; Rashed, 2011). When these and related mechanisms are sufficient to prevent major disruptions of physiological functions, infections remain asymptomatic.
In spite of the highly elaborate immune defense mechanisms, the human host often succumbs to infectious diseases when defenses fail because resistance mechanisms are insufficient, overpowered, or evaded by the pathogen (Medzhitov et al., 2012). Morbidity and mortality may also result from the failure of tolerance mechanism the underlying factor, boosting immunity and reducing pathogen burden (using drugs) may be ineffective, whereas enhancing tolerance (for example, with trace metal intervention) may have salutary effects. Drug interventions that target the tolerance pathways may also be more desirable when immune defenses are either inefficient, compromised, or cause excessive immunopathology. Boosting tissue tolerance could be a particularly useful strategy in the case of pandemic diseases that cause morbidity and mortality worldwide, such as malaria, tuberculosis, and HIV – the infectious diseases for which pathogen control through vaccination or antimicrobial drugs is currently unattainable (Medzhitov et al., 2012). Trace metal-related intervention holds some promise in this regard.
Exposure to many (non-essential and toxic) heavy metals found in the environment may trigger autoimmunity (overactive immune system) or result in immunotoxicity (Dietert, 2009). The manifestation of autoimmune diseases includes production of autoantibodies, inflammation and cytokines in various target organs, and deposition of immune complexes in vascular sites (i.e., immunopathology). On the other hand, exposure to metals, especially at elevated levels, can exert direct toxicity on the immune system through suppression of the system as a whole or by the disruption of the regulatory systems, and hence result in exaggerated responses infections (Failla, 2004). While the influence of toxic metals on pathogenesis of disease after infection is widely viewed as an immunodeficiency problem, defects in tolerance mechanisms may also be a determining factor. The trade-off between metal-related tolerance and resistance of the host to an infective pathogen can clearly be an important contributor to the disease outcome as well. The widely inconsistent and contradictory results from various laboratory experiments, human trials, and epidemiological studies on effects of exposure to toxic metals (lead, mercury, or cadmium) in populations with iron, zinc, or selenium deficiency reflect our limited understanding of the underlying mechanisms of how heavy metals moderate the outcome of an infection.
Although only a few elements are known to be biologically essential (iron, molybdenum, manganese, zinc, nickel, copper, vanadium, cobalt, and selenium), almost every element in the Periodic Table can appear in human cells and tissues as a result of their wide distribution in various environmental media. Recently, there is growing realization that many elements found in the environment (including arsenic, bismuth, boron, cadmium, chromium, cobalt, germanium, gold, silver, lead, mercury, nickel, manganese, molybdenum, platinum, palladium, rhodium, ruthenium, thallium, tin, titanium, and vanadium) can moderate the host’s response to infective pathogens. Many of these metals and metalloids have been reported in human and animal experiments to display anti-viral, anti-fungal, anti-bacterial, and/or anti-protozoa properties but whether the effects depend on the moderation of resistance or tolerance defense strategies is unknown at this time. The anti-parasitic properties may also be related to interactions with the key metals involved in the activation, maturation, and function of immune cells. It is conceivable for the tolerance mechanism for Metal A (MA) to amplify the tolerance mechanism for Metal B (MB), leading to increased tolerance against infections. Indeed, in vivo and in vitro studies have reported reduced host vulnerability in response to co-exposure to two or more trace metals. On the other hand, the tolerance mechanism from MA can be incompatible with the tolerance mechanism for MB, which then can lead to a net negative tolerance. Examples of metal co-exposures that have been implicated in enhanced virulence or morbidity following an infection have likewise been reported in the scientific literature. Competition phenomena between zinc and several cations (such as cadmium, lead, calcium, iron, manganese, and copper) have been documented in zinc supplementation trials; however, the results are inconsistent because of failure to consider the basal zinc status during the experiments. Although interactions in terms of host infection are usually explained in terms of the synergistic or antagonistic effects on the immune system, it is also possible that they are the result of compromised tolerance mechanisms of the metals. An understanding of such interactions and metal-induced tolerance mechanisms can provide evidence-based strategies for reducing the morbidity and mortality of infections by targeting the compromised protective pathways.Top of page
The following themes will guide the Forum:
Group 1: Metal Ions Host Microbe Interactions: The Microbe Perspective
This group will address the relationship between the trace metal status in different host niches (upper respiratory tract, gastrointestinal tract, urogenital tract, blood stream) and the capabilities of microbes to compete for trace metals ions with other microbes and the host. It will seek to understand the role of trace metals in the proliferation, virulence and persistence of pathogens and to determine whether pathogenic microbes have unique capabilities related to trace metal ion acquisition and homeostasis. The group will compare metal ion homeostasis in different microbes including the regulation of uptake, accumulation and metabolism; interactions within the metallome; and molecular, genetic and epigenetic regulatory factors that may be related to the outcome of pathogenic infection; and the differences in metals needs between the pathogens and commensals.
Group 2: Metals in Host Microbe Interactions: The Host Perspective
This group will define the physiological metal ion homeostasis, the changes during infection and the impact of supplementation. Specific goals are the distribution of metal ions in different parts of the body and the functional consequences of this distribution as well as the changes during life stages. Furthermore, the role of metal ion concentration on host resistance/tolerance/susceptibility at the barriers/contact surfaces should be described. Another task is to evaluate vulnerable subpopulations in terms of risks, benefits and limitations of immune-boosting by metals covering all parts of the immune system. Additionally anti-microbial metallic drugs will be addressed. Lastly different types of intervention trials with metals – their successes, limitations, and possible areas for improvement will be discussed. A particular point in this task will be the bioavailability as well as the cross talk of different forms of metal ions supplied.
Group 3: Metals in the Environment as Risk Factors for Infectious Diseases
This group will focus on heavy metals of environmental concern that can exercise some moderating influence on infectious diseases, including the emerging trace metals (such the platinum and palladium group elements and others with increasing demand for semi-conductor and high-tech industries); the pervasive pollutants including lead, cadmium, arsenic, and mercury; and the biometals (zinc, copper and iron). It will address the environmental exposure-disease relationships; association of heavy metal deficiency/toxicity and prevalence of disease infection; biogeochemistry and interactions among metals; impact of the Green Revolution/biofortification, and lifestyleassociated risks.
Group 4: Novel methods for analysis of trace metals in biologic systems
This group will focus on emerging methods for the separation, detection, mapping, and/or quantification of trace elements and metalloproteins in biological systems at organism, organ, cell, and sub-cellular levels required to understand the linkage of trace metal cycles to the pathogenesis of infectious diseases. Metal-coded affinity tagging (MeCAT) technology, laser ablation (LA) ICPMS alone or in combination with polyacrylamide gel electrophoresis (PAGE) for separation and screening of metalloproteins, LC/ICP-MS methods for the separation of metal species, Metal Isotope native RadioAutography in Gel Electrophoresis (MIRAGE) in metalloproteomics, HPLCDF- ICP-MS, inductively coupled plasma and electrospray ionization linear trap-Orbitrap mass spectrometry (ICP-ESI-MS), sensor-specific imaging of metal proteomes in vivo optical imaging, and nanoparticle-based strategies are examples of new and emerging technologies that hold promise in the study of heavy metals and infectious diseases. This group will discuss how these techniques can be used alone and in combination with various in silico methods to study metals and the metalloproteome.
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