An Introduction to Ecoimmunology
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Ecoimmunology is the study of the causes and consequences of variation in immunity. This integrative field builds on and complements comparative immunology by evaluating immunity across levels of biological organization using an evolutionary and ecological framework. One major accomplishment of ecoimmunology has been to demonstrate that immune defenses have costs, and these costs can be context-dependent. As a result, researchers assess immunity not only in terms of specific immune pathways but also as functional, emergent traits, such as resistance against and tolerance of parasites. Ecoimmunology also highlights the importance of integration across physiological systems, including the immune, endocrine, and nervous systems, in the production of immune defenses. The field has demonstrated the importance of genetics, developmental environment, and current conditions to individual variation in immune function and emphasized the traits of species, such as body size, that could be integral to immune phenotypes. Furthermore, ecoimmunological studies have shown that heterogeneity in immunity at the individual and species levels can have important consequences for disease dynamics within populations and communities as well as population cycling. Herein we discuss these key research areas in ecoimmunology as they relate to three central themes: (1) immunity in the context of the whole organism, (2) heterogeneity in immunity, and (3) the broad consequences of individual variation in immunity. We conclude by reviewing future directions for ecoimmunology, focusing on those that could have important implications for public health, conservation, and wildlife management.
KeywordsEcoimmunology Ecological immunology Immunocompetence Costs of immunity Tolerance Resistance Trade-off Sickness behavior Psychoneuroimmunology Endocrinology Glucocorticoid Testosterone Life history Pace-of-life Body mass Scaling Competence Host-parasite interactions Disease dynamics Population dynamics
Immunological research has made astounding progress in describing the cellular and molecular mechanisms underlying the defenses to parasites. Researchers have developed life-saving medical procedures based on experiments conducted on genetically identical animals that have been raised in sterile, carefully controlled laboratory housing (National Research Council 2004). Yet, the reality remains that animals, including humans, are not genetically homogeneous, and both develop and live in variable environments. As a result, the ability to defend against parasites (i.e., immunity) varies among individuals, populations, and species (Schmid-Hempel 2003). Why should we care about variation in immunity? The sources and consequences of immune variation have implications for public health, wildlife management, agriculture, and conservation. Understanding the sources of variation in immunity, such as which genes influence the likelihood of developing diseases or how nutrition influences susceptibility to infection, should help identify management strategies to minimize the effects of disease (Jones et al. 2014; Bulik-Sullivan et al. 2015; Farh et al. 2015). Furthermore, integrative approaches to immunology will enhance our ability to understand the equally intriguing consequences of that variation, such as disease dynamics and the evolution of life history traits.
Immunity in the Context of the Whole Organism
Ecoimmunologists investigate immunity as a characteristic of the whole organism rather than a series of isolated mechanisms. Although this holistic strategy has long been the goal, the approaches used to characterize immunity have evolved over time. Early efforts were relatively simplistic and focused on measuring the strength of responses to immune challenges or the magnitude of available defenses (Martin et al. 2006b). New information about the mechanisms underlying immunity and the costs of parasite defenses have led to fundamental changes in how immunity is studied (Brock et al. 2014). The complexities of immune function and the interdependent nature of physiological systems is facilitating a transition to more integrative approaches today.
Immunocompetence: The Search for a Simple Immune Measure
The quest for a measure of organismal-level immunity began with the concept of immunocompetence, or an individual’s ability to prevent or eliminate a parasite infection (Demas et al. 2011). Because immunocompetence is so broad, it is logistically difficult (or even impossible) to assess. Moreover, faith in the validity of the concept led early ecoimmunologists to underestimate the complexities of parasite defenses. Early ecoimmunology studies tended to use one or two immune metrics and interpreted measured variation as an indication of overall antiparasite defense (e.g., Saino and Calza 1999; Johnsen et al. 2000; Møller and Erritzoe 2000). One such immune metric was the phytohemagglutinin (PHA) skin test, which was thought to signal the strength of T-cell-mediated immunity (Smits et al. 1999; Tella et al. 2008). For example, a study of barn swallows (Hirundo rustica) demonstrated that adults producing offspring with strong responses to the PHA skin test were less likely to survive until the next breeding season (Saino and Calza 1999). The authors interpreted this result as indicating that producing highly immunocompetent offspring was associated with survival costs. Although this approach pointed to interesting relationships between immunity and other traits, it began to fall out of favor after researchers raised concerns about the general concept of immunocompetence and the interpretation of this and other specific immune assays (Adamo 2004; Viney et al. 2005; Martin et al. 2006b). Whereas immunologists know well that the immune system is complex and different immune pathways are important for responding to different parasites, early ecoimmunologists hoped to be able to overcome this problem and capture something meaningful with simple assays. Now, though, the concept of immunocompetence is understood as too broad to be useful (Demas et al. 2011). This recognition arose when ecoimmunologists began interpreting results of their assays in terms of the specific immune components measured, not immunocompetence in a broad sense (Martin et al. 2006b). This epiphany was reinforced when researchers found that variation in immune measures , such as the PHA skin test or white blood cell counts, only sometimes predicted responses to an actual infection, and usually they did so weakly (Adamo 2004; Adelman et al. 2014). What the concept of immunocompetence sought to capture, though, that there is an optimal immune response for different species or individuals, still resonates in the field today (Adamo 2004; Demas et al. 2011). But just what is an optimal immune response?
Optimal Immunity : More Isn’t Always Better
The Costs of Immunity Influence Evolutionary Trade-Offs
Evolutionary costs of immunity can arise because of genetic architecture. The genes that influence immune phenotypes can regulate other traits as well or can be inherited with other genes. Similarly, transcription factors that regulate immunity may also regulate other traits (Downs et al. 2014). As a result, there are negative genetic correlations between traits that promote immunity and traits associated with other characteristics that enhance fitness (Rolff and Siva-Jothy 2003; Ardia et al. 2011). Therefore, the evolution of immunity is influenced by genetically correlated traits.
Antagonistic pleiotropy occurs when a single gene causes negative correlations between an immune trait and another trait that can enhance fitness (Rolff and Siva-Jothy 2003; Roff and Fairbairn 2007). Antagonist pleiotropy can force trade-offs that are not easily altered by selection because breaking these trade-offs requires a gene duplication event (Roff and Fairbairn 2007). Fruit fly (Drosophila melanogaster) defenses against parasitoid wasps present one of the most extreme examples of antagonistic pleiotropy influencing immunity. A wasp oviposits eggs inside the host fly, and if allowed to develop, the wasp’s offspring kill the fly larva by consuming it from the inside out. Some Drosophila genotypes can resist the wasp’s attack by producing hemocytes (blood cells) that encapsulate the eggs, destroying them (Poirie et al. 2000; Kraaijeveld et al. 2001). However, this and other defenses have costs: more resistant Drosophila genotypes (i.e., those that can better control infections) also have slower feeding rates than more susceptible Drosophila. Subsequently, resistant individuals are less likely to survive in high-competition environments (Kraaijeveld and Godfray 1997; Kraaijeveld et al. 2001). Evidence suggests that a single gene might regulate the trade-off between competition and parasitoid defenses (Hodges et al. 2012). Antagonistic pleiotropy also is suspected to play a role in human aging (Franceschi et al. 2000; Goto 2008). Inflammation, an important part of the innate immune response, also contributes to cellular senescence, tissue damage, and several diseases associated with aging such as arthritis and Alzheimer’s (Lambeth 2007; Freund et al. 2010). The protective advantages conferred by inflammation in younger individuals becomes costly as those individuals age, when inflammation can become chronic and more likely to cause disease (Lambeth 2007; Freund et al. 2010).
Often it is unclear whether immune trade-offs with a genetic basis are caused by pleiotropy or linkage disequilibrium, when genes are inherited together because of their proximity on a chromosome (Saltz et al. 2017). In these scenarios, correlations between traits can be broken through genetic recombination. A meta-analysis revealed that artificial selection for rapid growth in chickens led to attenuated immune responses (van der Most et al. 2011). However, selection for various immune traits was not always associated with reduced growth rates, and the strengths of relationships varied among lines of chickens and forms of immunity, suggesting a mechanism other than pleiotropy (van der Most et al. 2011).
The degree to which immunity is traded off with other traits depends on the relative strength of the evolutionary pressures (e.g., parasite risk, predation, conspecific competition) (Lazzaro and Little 2009). The outcome of evolutionary immune trade-offs thus vary across environments. Defenses of Drosophila against the parasitoid wasps vary geographically, indicating that variable selection pressures across environments lead to local optima for immunity (Kraaijeveld and Godfray 1999). For example, humidity influences the biological pathways enabling resistance to parasitoids and, thus, can alter the costs of immunity. As a result, humidity could shape the local evolution of defenses.
The Costs of Immunity Cause Trade-Offs Within Individuals
The aforementioned evolutionary costs of immunity have physiological bases. For instance, the development, activation, and even mitigation of parasite defenses can impose substantial costs on organisms (Lochmiller and Deerenberg 2000). Experimental activation of immune responses can cause increases in metabolic rate and body mass loss, indicating that immunity has energetic costs (Demas et al. 2011; Ots et al. 2001; Freitak et al. 2003; Martin et al. 2003; Eraud et al. 2005; Amat et al. 2007). Nutrient availability is also critical. For example, carotenoid availability limits the expression of immune traits in juvenile (Saino et al. 2003; Klasing et al. 2006; Tyndale et al. 2008) and adult (Blount et al. 2003; Amar et al. 2004) birds and fish. Similarly, the amino acid lysine is integral to leukocyte function and the biosynthesis of proteins associated with innate immunity (Iseri and Klasing 2014). Indeed, in chickens, the lysine required during the acute-phase response to a bacterial infection is equivalent to that in 355 feathers or 17% of an egg (Iseri and Klasing 2014). Because any resources allocated to immunity are unavailable for other activities, the resource costs of immunity can drive allocation trade-offs (Norris and Evans 2000; Lochmiller and Deerenberg 2000; Ardia et al. 2011; Downs et al. 2014). For example, energy limitation underlies a trade-off between wound healing and reproduction in the ornate tree lizard (Uta ornatus) (French et al. 2009). Injuries occur frequently in nature, and wound healing involves multiple components of the immune system. Experimentally wounding a lizard’s skin reduced egg follicle size if the studied female was food restricted. However, when food was unlimited, there was no trade-off between wound healing and reproductive investment (French et al. 2007a, b). In addition, food restriction had no effect on wound healing in females before reproduction (French et al. 2007a). Similar allocation trade-offs between reproduction and immunity are relatively common and can be found across taxa, including insects, mammals, reptiles, and birds (Schwenke et al. 2016; Norris and Evans 2000; Hayes and Shonkwiler 2001; French et al. 2009).
Allocation trade-offs with immunity are not limited to reproduction. Other energetically costly functions, such as tissue growth or dispersal to new areas (e.g., migration), are also subject to allocation trade-offs (Soler et al. 2003; Moreno-Rueda 2010; Altizer et al. 2011). For example, some forms of immune function in thrushes (Catharus spp. and Hylocichla mustelina) are reduced during migration when compared to breeding conspecifics (Owen and Moore 2006), and resource availability is likely to underlie these reductions (Owen and Moore 2008). At a migratory stopover site, thrushes in poor energetic condition (lower fat deposits, muscle mass, body-size-controlled condition index) had lower leukocyte and lymphocyte counts; among birds captured and brought into captivity, mass gain was associated with greater responsiveness to an immune challenge (Owen and Moore 2008). Importantly, the resource costs of immunity will only lead to trade-offs under conditions of resource limitations. If individuals can obtain sufficient resources, they can support immunity in addition to other costly activities, and there will be no allocation trade-off (van Noordwijk and de Jong 1986; Downs et al. 2014).
Immunity not only imposes energetic costs on individuals; activating immune defenses can harm hosts as well. Autoimmune disease is one form of such a cost. For example, Guillain-Barré syndrome is the result of mounting an immune response against neurons (Yuki and Hartung 2012). Lupus occurs when antibodies target multiple host organ systems (Lipsky 2001). Immune responses targeting an infection can also cause damage to hosts. During malaria infections, the loss of red blood cells is often much higher than could be achieved by the malarial parasites alone (Price et al. 2001; Evans et al. 2006). Evidence suggests that the immune response, specifically T-cell activity, damages uninfected red blood cells, increasing the severity of malarial anemia (Evans et al. 2006). In addition, immune responses to malaria and other parasites can increase concentrations of reactive oxygen metabolites (ROMs), often called free radicals (Costantini and Møller 2009). High concentrations of ROMs can damage tissue and even accelerate rates of senescence (Finkel and Holbrook 2000; Monaghan and Haussmann 2006; Haussmann and Marchetto 2010). Because individuals can harm their own tissues by activating parasite defenses, mounting overly strong immune responses could result in a loss of fitness (Råberg et al. 1998; Graham et al. 2010, 2011).
Behaviors associated with immune responses, sickness behaviors, can also be costly. When individuals mount particular types of immune responses, they exhibit specific behaviors, including lethargy, anorexia, and low libido and alertness (Adelman and Martin 2009; Adelman and Hawley 2016). Many of these behavioral changes are regulated by proinflammatory cytokine activity in the nervous system (Adelman and Martin 2009). Sickness behaviors are thought to be adaptive; they can promote energy conservation for defending against parasites (Hart 1988) or possibly support tissue repair (Medzhitov et al. 2012). However, elicitation of sickness behaviors can also reduce survival or reproductive success. For example, immune-challenged field crickets, Gryllus campestris, were at increased risk of predation because they slowed responses to predators and spent more time in exposed areas, possibly because of differing thermoregulation needs during infection (Otti et al. 2011). Sickness behaviors can also carry opportunity costs because they directly interfere with activities such as foraging or breeding effort. Immune-challenged house sparrows (Passer domesticus) reduced the rate at which they fed their offspring, had higher rates of nest abandonment, and, ultimately, fledged fewer chicks (Bonneaud et al. 2003). However, individuals appear to have evolved to minimize the opportunity costs associated with sickness behavior. Male song sparrows (Melospiza melodia morphna) exhibit greater sickness behaviors in response to an immune challenge during the nonbreeding season than during the breeding season, when sickness behaviors could impair reproduction (Owen-Ashley and Wingfield 2006). This seasonal variation in sickness behaviors might be hormonally mediated; experimental increases in testosterone suppressed sickness behaviors in male Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii) (Ashley et al. 2009).
Complex Immune Phenotypes: Using Resistance and Tolerance to Characterize Whole-Organism Responses to Parasites
At both individual and evolutionary scales, the optimal parasite defenses will be those that maximize fitness (Schmid-Hempel 2003; Ardia et al. 2011). Studies of the costs versus benefits of immunity initiated new directions in ecoimmunology. In particular, researchers have started characterizing immunity as a complex, integrated response rather than the activity of a single physiological system operating without constraints of other life processes (Graham et al. 2011; Brock et al. 2014). Central to this new framework are resistance and tolerance (Råberg et al. 2009).
Resistance and tolerance are distinct, but not mutually exclusive, strategies for defending against parasites. Broadly speaking, parasite resistance is fighting infection and involves limiting the number of parasites infecting a host. Before a parasite successfully infects a host, resistance can include avoidance or physically blocking a parasite invasion (Best et al. 2014; Kutzer and Armitage 2016). Infected hosts can resist parasites by removing parasites or limiting their reproduction, often through actions of the host’s immune system (Råberg et al. 2009). More resistant individuals will have fewer parasites or a faster rate of parasite clearance, and as a result, increased host resistance has a negative impact on parasite fitness (Råberg et al. 2007). In observational studies of free-living animals, parasite load is often used as an indicator of resistance (e.g., Coltman et al. 2001; Schoenle et al. 2017b). In experiments, resistance is often measured as the change in parasite load over time (e.g., Råberg et al. 2007, 2009). Resistance, perhaps obviously, has been the focus of traditional immunology.
In contrast, tolerance emphasizes the costs of infection rather than control of parasite burden (Råberg 2014; Kutzer and Armitage 2016). The concept of tolerance comes from the plant sciences, where researchers realized that plants could gain fitness by minimizing damage caused by herbivores, fungi, and other threats rather than avoiding, clearing, or otherwise walling off threats entirely (Caldwell et al. 1958; Schafer 1971; Strauss and Agrawal 1999). Tolerance is defined as the minimization of the costs of infection per parasite (Råberg et al. 2009). In other words, tolerance involves the regulation of damage accrued during an infection, independent of parasite load. Tolerance can be achieved by reducing the damage caused by the host’s own defense mechanisms (Schneider and Ayres 2008). Consider the example of the Soay sheep mentioned earlier (Fig. 2) in which some of the antibodies that protected from parasites and promoted overwinter survival were also associated with low fecundity (Graham et al. 2010; Nussey et al. 2014). The immune system of the Soay sheep can cause immunopathology, so preventing antibody levels from becoming too high could increase tolerance. Individuals can also increase tolerance by mitigating damage caused by parasites (Schneider and Ayres 2008). For example, the human intestinal helminth, Schistosoma mansoni, causes extensive tissue damage in multiple organ systems as it progresses through its life cycle (Allen and Wynn 2011). The production of Th2 cytokines promotes the repair of parasite-damaged tissues and thereby increases tolerance to the helminths (Allen and Wynn 2011). Although a relatively new area of study, ongoing research indicates that tolerance is an important trait for understanding host–parasite interactions. Tolerance is often heritable (Mazé-Guilmo et al. 2014; Parker et al. 2014), associated with host fitness (Hayward et al. 2014b), and impactful on parasite evolution (Cousineau and Alizon 2014; Cressler et al. 2015).
As forms of immunity, resistance and tolerance have great advantages relative to the concept of immunocompetence and immune variation as measured by specific assays. Because resistance and tolerance assess how individuals manage parasite loads and costs of infection for specific parasites, they are biologically relevant performance metrics. Furthermore, resistance and tolerance are integrative measures of immunity at the level of the whole organism; they incorporate the effects of multiple branches of the immune system, as well as behaviors and other physiological systems (e.g., endocrine and nervous system), on organisms’ parasite loads and the costs of infection.
Integrative Ecoimmunology: Immunity as Part of a Complex, Dynamic System
Immunity is the product of interconnected physiological systems. Most people experienced this first hand through unfortunate and uncomfortable encounters with the flu. Although flu viruses do not usually enter the central nervous system, the response to the infection affects the brain, leading to fatigue, reduced alertness, reduced appetite, and poor mood (Kelley and McCusker 2014). Indeed, an infection anywhere in the body, such as the lungs or sinuses, can induce a proinflammatory cytokine response in the brain that results in many unpleasant flu symptoms (Jurgens et al. 2012). This common experience is just one example of how the immune system and parasite defenses are inextricably linked to other physiological systems. Addressing immunity in the context of the whole organism requires understanding interactions between immune defenses and other components of physiology.
An entire field, psychoneuroimmunology , is dedicated to understanding how the immune, endocrine, and nervous systems are related to behavior (Ader et al. 1995). Interest in these connections grew following the discovery of evidence suggesting that stress and depression could influence immune function (Irwin 2008). For example, early work in the field demonstrated that the death of a spouse was associated with weak responses to the PHA skin test (Bartrop et al. 1977; Schleifer et al. 1983). Evidence also suggests that psychological stress can increase susceptibility to infectious disease (Cohen et al. 1991) and suppress the ability to heal wounds (Kiecolt-Glaser et al. 1996). Physiological links between the nervous and immune systems underlie the relationship between stress and disease. For example, the neuropeptide corticotropin-releasing hormone (CRH) is released from the hypothalamus in response to stress and is elevated in depressed humans (Irwin 2008). When high levels of CRH bind to receptors in the brain, CRH suppresses both innate and cell-mediated immune responses (Irwin et al. 1987; Strausbaugh and Irwin 1992; Irwin 2008).
Ecoimmunologists have also devoted substantial efforts toward understanding the mechanistic links between stress and immune function. Researchers working on nonhuman vertebrates have primarily focused on glucocorticoids. Glucocorticoids increase in circulation when organisms are faced with psychological or physical challenges (Bonier et al. 2009; Boonstra 2012). They promote a suite of physiological and behavioral changes, including alterations in immune function, metabolism, and reproductive capability (Sapolsky et al. 2000). Interestingly, glucocorticoids can enhance or suppress immune function, and in some cases do both simultaneously depending on the threat or context (Sapolsky et al. 2000; Martin 2009). The relationship between glucocorticoids and immune function is highly dependent on past and current environmental conditions (French et al. 2009), host life priorities (Martin et al. 2005), and the amplitude, duration, and frequency of glucocorticoid secretion (Martin 2009; McCormick et al. 2015).
A substantial proportion of ecoimmunology studies investigating connections among physiological systems have focused on endocrine–immune interactions. Ecoimmunologists might emphasize endocrinology because many hormones can act as physiological integrators (Cohen et al. 2012), factors for which many cell types have receptors. Because so many cells carry receptors for the same hormones, the activities of disparate systems can be coordinated for the same tasks (Martin et al. 2011b). Moreover, many hormone concentrations rapidly respond to changes in the physical or social environment, further facilitating a match of the whole-organism phenotypes to current conditions (e.g., Maher et al. 2013). For example, melatonin transduces information about day length into a hormonal signal that mediates seasonal changes in immunity (Guerrero and Reiter 2002; Hardeland et al. 2011). In general, melatonin tends to enhance immune function (Martin et al. 2008; Weil et al. 2015), and by supporting a stronger immune response during winter, when days are short and risk of some infections is relatively high, melatonin might bolster immunity (Nelson 2004). Other hormonal integrators can also create links between immunity and other physiological traits. For example, in male red grouse, Lagopus lagopus scoticus, testosterone increased both nematode burden and comb size (Mougeot et al. 2005a, b). Grouse combs are colorful ornaments, and the form of these sexually selected traits influences female mate choice and male–male interactions (Moss et al. 1979; Rintamaki et al. 1993). Thus, the link between testosterone, comb size, and resistance to nematodes might ensure that the comb is an honest signal in this system; only high-quality males, who will have high levels of testosterone and a large comb, can cope with high parasite burdens (Folstad and Karter 1992; Mougeot et al. 2004).
Heterogeneity in Immunity
Sources of Individual Heterogeneity in Immunity
Spatiotemporal variation in infection risk as well as the value of particular sickness behaviors or other costly immune responses influence investments in immune traits. Moreover, forms of optimal immunity are apt to vary over the course of an organism’s lifespan. Both the rate and extent of immune investments as well as subsequent immune development are contingent on the life history strategy of a population. In all hosts, there can be permanent, semipermanent, and reversible effects of genes and environment, ultimately altering immunity across environments (Fig. 3a).
Genetic Sources of Immune Heterogeneity
Hosts can express a range of immune phenotypes, and the breadth of that range is determined by genotype, developmental history, and maternal and other epigenetic effects. Genotypes vary in many immune traits, including cytokine production (de Craen et al. 2005) as well as organismal traits such as resistance and tolerance (Hayward et al. 2014a; Mazé-Guilmo et al. 2014; Parker et al. 2014). For example, nucleotide polymorphisms were associated with substantial variation in the ability to subdue bacterial infection in wild-sourced Drosophila melanogaster (Lazzaro et al. 2004). Furthermore, immune phenotypes are also influenced by the environment and gene-by-environment interactions. For example, Drosophila resistance to the ectoparasitic mite, Macrocheles subbadius, is also negatively genetically correlated with larval competitive ability (Luong and Polak 2008). However, the relative success of more resistant versus more competitive Drosophila varies with density and temperature, resulting in an environment-dependent evolutionary trade-off.
Developmental Environment and Immune Heterogeneity
Early developmental experiences can also affect the expression of immune traits, sometimes throughout life. For example, parents can transmit antibodies to their offspring that shape immunity both immediately and in the long term (Hasselquist and Nilsson 2009). In mammals, some antibody isotypes pass across the placenta and via lactation shortly after birth (Grindstaff et al. 2003; Boulinier and Staszewski 2008). Maternal antibodies can also be transferred to offspring in the egg yolks of birds, fish, and reptiles (Grindstaff et al. 2003; Boulinier and Staszewski 2008). Neonates tend to be particularly vulnerable to parasites in early life. Short-term bolstering of resistance by maternal antibodies can enhance offspring growth rates and maturation (Hasselquist and Nilsson 2009). The effects of maternal antibodies can extend beyond the neonatal phase, shaping immune function into adulthood. In laboratory animals, exposure to antibodies during development altered the immune response to antigens later in life (Wikler et al. 1980; Elliott and Kearney 1992; Lundin et al. 1999). In one study of Wistar Furth rats, the effects of neonatal antibody exposure increased the immune response to a bacterial antigen in the next generation (Lundin et al. 1999). Other components of the developmental environment , such as food availability and parental care, can also influence immune phenotype. In Gambian villages supported by subsistence farming, adult body condition and the mass of newborns were tightly linked to the growing season; body condition was significantly lower during the seasonal postharvest period when little food is available (Moore et al. 1999). People born during the postharvest period were at higher risk of death from infectious disease, suggesting long-term impacts of early-life food limitation. Other types of parental care can also affect immune traits. For example, wood ducks (Aix sponsa) incubated at low temperatures were less responsive to immune challenges than those incubated at average temperatures measured in the wild (Durant et al. 2012).
Environmental Conditions Influence Immune Trait Expression
Although genes and the developmental environment constrain the expression of immune traits , immunity remains responsive to fluctuations in the environment throughout organisms’ lives. For example, monarch butterflies’ (Danaus plexippus) ability to resist and tolerate the protozoan parasite Ophryocystic elektroscirrha depend on which species of milkweed the butterflies ate (Sternberg et al. 2012). Relationships between resource availability and immunity can be complex, however. Cuban tree frogs (Osteopilus septentrionalis) fed a high resource diet (i.e., more crickets) were more resistant to penetration of nematodes through the skin; if an infection was already established, though, they were also more tolerant to infection (Knutie et al. 2017). Environmental effects on immunity are not limited to resource availability. Perceived predation risk induce system-wide changes in physiology and behavior (Clinchy et al. 2013; Newman et al. 2013), including immune functions (Navarro et al. 2003; Stoks et al. 2006; Groner et al. 2013; Adamo et al. 2017). For example, house sparrows (Passer domesticus) exposed to predators mounted lower responses to an immune challenge (Navarro et al. 2003).
Life History Stage and Strategy
As discussed previously, parasite defenses are inextricably tied to life history strategy (Sheldon and Verhulst 1996; Ricklefs and Wikelski 2002). Life history can be defined as the patterns of investment an organism makes in growth, reproduction, and survival over its lifetime. Life history stages are periods characterized by specific patterns of investment in some processes over others (e.g., reproduction, immunity, growth, migration) (Ricklefs 1977; Stearns 1992; Roff 2002). The ability to protect against parasites varies across life history stages , including from juvenile to adult as well breeding to nonbreeding. For example, juvenile field voles (Microtus agrestis) were more resistant to than tolerant of macroparasites (e.g., intestinal worms, arthropods), whereas the opposite was true of adults, who tended to favor tolerance over resistance (Jackson et al. 2014). This difference could be attributed to the maturation of the immune system with age or to an adaptive strategy mediated by different risk of exposure to parasites or the greater value of growth and maturation versus finding a mate and breeding, depending on individual age. For instance, infections could be more harmful during development, favoring investment in resistance over tolerance in younger animals. Immunity also often changes from the breeding to nonbreeding season, most likely because individuals face different challenges (Zuk and Stoehr 2002; Love et al. 2008). Similarly, we observe sex differences in immunity because males and females engage in different activities during these periods (Zuk and McKean 1996; Zuk and Stoehr 2002). For example, responses to an immune challenge vary across breeding and nonbreeding life history stages in resource-limited female zebra finches (Taeniopygia guttata), but not in males (Love et al. 2008). Female zebra finches tend to invest more energy and effort in breeding than males, so differences in immune phenotype might reflect differences in the resources available to the immune system.
Physiology and Body Condition Influence Immune Heterogeneity
All of the sources of individual heterogeneity in immunity discussed here are also likely to influence other components of physiology, such as metabolism or the endocrine system (Ricklefs and Wikelski 2002). As a result, factors that influence immunity might not be acting directly but instead through another mechanism. Body condition is a potential mediator of immunity. For example, short day length (i.e., simulated winter photoperiod) causes a reduction in both body fat and humoral immunity in Siberian hamsters (Drazen et al. 2001). Experimental removal of fat from hamsters also causes a reduction in humoral immunity, suggesting that the presence of fat mediates the effect of day length on immunity (Demas et al. 2003). In this system, the effect of fat on immunity is driven by leptin, a hormone produced by adipose cells (Carlton and Demas 2014, 2015; Carlton et al. 2014). Poor body condition is not always associated with decreasing investment in immunity. North American elk (Cervus elaphus) in worse condition invested more in constitutive immune defenses than elk in better condition (Downs et al. 2015). Similarly, mallards in worse condition were less susceptible to infection with an influenza virus and had shorter periods of viral shedding (Arsnoe et al. 2011). Individuals in poor body condition might invest more in less energetically costly components of immunity, or constitutive defenses, to prevent and remove infections quickly because allowing parasites to become established could require more energetically costly specific immune responses (Downs et al. 2015).
Heterogeneity in Immunity Across Species
The fields of ecoimmunology and comparative immunity converge at the study of immune heterogeneity across species. Comparative immunology emphasizes the evolution of immunity and the extent of species-level variation in immune traits (Cooper 2003). Thus, comparative immunology enhances our understanding of how species can defend against parasites. Furthermore, ecoimmunology complements comparative immunology by evaluating why heterogeneity in immunity persists through the study of the selective forces and constraints that influence the evolution of immunity (Martin et al. 2011a; Downs et al. 2014). In this section, we discuss two potential drivers of variation in immunity across species: life history and body size.
Variation in Species’ Life History Underlies Heterogeneity in Immunity
Species-level immune variation is the product of evolutionary history, including historic selective pressures and inherited genetic and physiological constraints (Fig. 3b). One way to understand this variation is to consider parasite defenses in the context of life history traits. Life history traits covary along a continuum called the pace-of-life axis (Hille and Cooper 2015). Species with a fast pace of life tend to grow quickly, reproduce early and in high numbers, and have short lifespans. Species with a slow pace of life express the opposite traits: slow maturation, late reproduction, few offspring per reproductive bout, long lifespans, and large body sizes. Thus, evaluating how immunity maps onto the pace-of-life axis will provide insights into species-level immunity variation (Sheldon and Verhulst 1996; Zuk and Stoehr 2002). The most basic prediction is that individuals with a slower pace of life should exhibit greater immunity because investing in parasite defenses should enhance survival because long-lived organisms should encounter more parasites during their lifetimes (Williams 1966; Ricklefs 1998).
Multiple researchers have hypothesized that fast paces of life are associated with constitutive innate immunity and inflammatory responses, whereas slow paces of life are linked to acquired immunity and anti-inflammatory responses (Lee 2006; Sears et al. 2011). Evidence supporting this hypothesis exists both within and across species (Tieleman et al. 2005; Martin et al. 2006a, 2007; Previtali et al. 2012; Downs et al. 2013; Pap et al. 2015). For example, house sparrows with a slower pace of life exhibit stronger acquired immune responses and lower inflammatory response than fast-living house sparrows (Martin et al. 2006a). Similarly, rodent species with slower paces of life had the lowest innate and strongest acquired immune responses (Martin et al. 2007; Previtali et al. 2012). Multiple cross-species analyses of birds have found that higher mass-adjusted basal metabolic rate, which is associated with a fast pace of life, correlates with lower innate immune responses (Tieleman et al. 2005; Pap et al. 2015; but see Versteegh et al. 2012). There is also evidence that selection for maximal metabolic rate, a trait associated with a fast pace of life , enhanced innate immunity and decreased adaptive immunity in mice (Downs et al. 2013).
Studies have also evaluated the relationship of resistance and tolerance with life history strategy. Because inflammation can cause substantial damage to host tissue as well as to parasites, tolerance is predicted to be associated with anti-inflammatory responses, acquired immunity, and a slower pace of life (Sears et al. 2011). Greater resistance to parasites is predicted to be associated with fast-paced living and stronger innate and proinflammatory immune responses. A comparative study of 13 amphibian species found support for these predictions (Johnson et al. 2012). Fast-living species suffered more severe limb malformations and more pathology during trematode infection than slow-living ones. Counter to predictions for resistance, though, faster-pace-of-life species had greater parasite burdens than slower-pace-of-life species. In another study (Sears et al. 2015), tadpoles with a faster pace of life were more resistant, but slower-pace-of-life species were more tolerant of the same type of trematode infection.
Species-Level Variation in Immunity Can Scale with Body Mass
Body size, as a trait in itself, might affect inter- and intraspecific variation in immunity (Langman and Cohn 1987; Wiegel and Perelson 2004; Perelson et al. 2006; Han et al. 2015). Numerous traits of animals change in a predictable manner, or scale, with body size (Schmidt-Nielson 1984; Calder 1996). We can predict with remarkable accuracy the lifespan, heart rate, sleeping patterns, metabolism, and many other characteristics of organisms from body size alone. Several theoretical models of the immune system suggest that the relationship between body size and immunity is the result of physiological structure (e.g., vasculature, tissue organization) or metabolic rate, which could drive immune cell generation and transport throughout the body (Langman and Cohn 1987; Wiegel and Perelson 2004; Perelson et al. 2006). Scaling of immune function might also be related to the role that immune function plays in regulating life history traits. Immune function supports survival, so longer-lived individuals are predicted to invest more in immunity. In addition, lifespan increases with body mass and immunity might scale in the same way because the immune system is required to function for a longer time (Wiegel and Perelson 2004).
Some empirical evidence suggests that some aspects of immunity , such as white blood cell numbers (Nunn 2002; Nunn et al. 2003; Tian et al. 2015) and the costs of immune responses (Brace et al. 2017), are related to body size. Presently, all conclusions about body mass and immunity are premature. We do not yet know how size affects defenses against parasites. However, it is important to generate such information because it has important implications for public health. For instance, the effectiveness and toxicity of most pharmaceuticals are assessed initially in small mammals such as rodents. Drug effects do not scale directly with size, though, but we rarely take this into account when developing human treatment plans (Blanchard and Smoliga 2008; Mahmood et al. 2016; Smoliga and Blanchard 2017). Similarly, the most dangerous novel infectious diseases (e.g., Ebola, Nipah, SARS, Hendra viruses, among others) have spread recently from animals to people. Information on immune scaling could help us predict which species are most likely to dilute and amplify infection risk for other species, including humans (Han et al. 2015).
Broad Consequences of Individual Variation in Immunity
Competence is therefore an organismal trait comprised of processes that operate at multiple levels of organization (VanderWaal and Ezenwa 2016). Moreover, it seems so plastic that in some cases individuals within species could be as distinct as groups of species (Gervasi et al. 2015). Depending on where an organism (genotype) finds itself, its role in a disease epidemic can be quite different. For instance, differences in diet quality among laboratory mice (Mus musculus) influence eosinophil levels, and variation in eosinophils predicts the amount of helminth eggs shed by mice (Budischak et al. 2015). In Grant’s gazelles (Nanger granti), nematode shedding rates are higher in territorial males than in nonterritorial males and females (Ezenwa 2004). This particular difference in competence among classes of gazelle could be attributed to differences in testosterone, which is associated with immune phenotype as well as the likelihood of defending a territory and parasitism (Ezenwa et al. 2012). Species differences in immunity and competence also underlie spatial variation in Lyme disease risk (LoGiudice et al. 2003; Keesing et al. 2010). Highly disturbed forest sites in the Northeastern United States tend to have less species diversity overall and higher densities of the white-footed mouse (Peromyscus leucopus). The white-footed mouse is a highly competent host for Lyme, at least in part due to its immune profile (Donahue et al. 1987; Martin et al. 2007; Previtali et al. 2012; Ostfeld et al. 2014). When there are both a low diversity of hosts available for the vector of Lyme disease, the tick Ixodes scapularis, and high densities of the white-footed mouse, then a higher proportion of ticks carry the Lyme-causing bacterium, increasing the risk of disease for all hosts in the community (LoGiudice et al. 2003).
Variation in immunity has the potential to influence population dynamics independent of disease (Lochmiller 1996). Individuals require resources to both develop their immune system and maintain its ability to respond to threats (Demas et al. 2011; Klasing 1988; Ots et al. 2001; Klasing et al. 2006). These costs of immunity can influence individuals’ ability to invest in reproduction or other traits important to survival, thereby influencing birth and death rates of populations (Downs and Stewart 2014). For example, environmental conditions, such as precipitation or human disturbance, can shift trade-offs between investment in immunity and reproduction in multiple species of reptiles (Smith and French 2017). Immunity also contributes to survival and population dynamics through influences on recruitment and longevity, which in turn affect lifetime reproductive success. For example, cell-mediated immunity was a better predictor of recruitment nesting into a breeding population of pied flycatchers (Ficedula hypoleuca) after 2 years than mass or hatch date, more traditionally used predictors of recruitment (Moreno et al. 2005).
Few empirical data sets link variation in host immunity to host population dynamics directly, an application of comparative immunology that remains relatively unexplored. However, immunity is linked to both disease transmission and infection outcomes, and parasite effects on host fitness can influence population dynamics. Parasites influence host fitness both by directly killing hosts and through sublethal effects that indirectly reduce survival rates or alter reproductive output (Hatcher et al. 2006). For example, avian malaria (Plasmodium spp.) can cause rapid mortality, particularly in areas where the parasites were recently introduced (Atkinson and Van Riper III 1991; Atkinson et al. 1995; LaPointe et al. 2012). Avian malaria is particularly deadly for many Hawaiian birds, and mathematical models indicate that the parasites are a major factor causing population declines and blocking the recovery of at-risk species (Samuel et al. 2011). In addition, climate warming is extending the reach of the parasites and their vectors in Hawaii, further restricting the host-population distributions. Over most of its range, avian malaria infections tend to be chronic, but infection has sublethal effects that can reduce fitness (Bennett et al. 1993; Merino et al. 2000; Valkiūnas 2005; Marzal et al. 2005; Asghar et al. 2015; Schoenle et al. 2017a). Even sublethal effects of parasites can alter population dynamics, though. For example, intestinal nematodes reduced reproductive success by 2–13% in the Svalbard reindeer (Rangifer tarandus) (Albon et. al 2002). This reduction in fecundity was sufficient to limit reindeer population growth, even without any measurable effects of nematodes on reindeer survival. In some cases, it is the interaction between the effects of parasites and other environmental challenges that underlie population-level patterns. A study of white-footed and deer mice (Peromyscus maniculatus) demonstrated that population crashes were caused by the combination of nematode infections and decreases in resource availability (Pedersen and Grieves 2008). Experiments manipulating both food availability and parasite infection indicated that while each stressor was involved in population regulation, both were required to replicate population crashes.
Conclusions and Future Directions
The field of ecoimmunology has begun to explain why we observe substantial variation in immune defenses and provide some understanding of the implications of that variation. Perhaps the greatest contribution of the field has been to establish that immunity has costs, and those costs can lead to trade-offs within individuals and at an evolutionary scale (Lochmiller and Deerenberg 2000; Schmid-Hempel 2003; Brock et al. 2014). Identifying the costs of parasite defenses led the field in new directions. One of the most influential developments has been the concept of tolerance, which incorporates the costs of infection into measures of immunity. The study of tolerance has expanded beyond ecoimmunology, reaching into human biomedicine, wildlife ecology, and public health. For example, public concern has been growing around an “antibiotic crisis” caused by the rise of drug-resistant microbes and a decline in the discovery of effective antibiotics (Martens and Demain 2017). Shifting the focus of treatments from resistance to tolerance presents a potential solution: the development of drugs that reduce the virulence of infections rather than parasite elimination (Vale et al. 2014, 2017; Totsika 2017). Furthermore, ecoimmunology has begun to evaluate the constraints that shape immunity and, by combining forces with disease ecology, to use information about immunity to predict the spread of infectious disease (Hawley and Altizer 2011; VanderWaal and Ezenwa 2016).
An important goal of future research is to understand the fitness consequences of different immune phenotypes and the resulting consequences for infectious disease dynamics. Individuals’ ability to resist or tolerate infection can change with environmental conditions and life history stage or strategy (Blanchet et al. 2010; Jackson et al. 2014; Sears et al. 2015; Knutie et al. 2017). However, very few studies compare the fitness benefits of a more tolerant to a more resistant phenotype. Studies of Soay sheep defenses against intestinal nematodes find that resistance to nematodes is associated with lower reproductive success (Hayward et al. 2014a), whereas tolerance is associated with greater reproductive success (Hayward et al. 2014b). Furthermore, resistance is independent of tolerance in this system, indicating that individuals are not forced to trade off investment in these strategies (Hayward et al. 2014a). Thus, to understand how defense strategies affect disease dynamics, it might be important to address how resistance and tolerance influence fitness independently. To our knowledge, no studies have assessed the fitness benefits of resistance and tolerance across contexts. Like other components of immunity, resistance and tolerance could vary adaptively across environments or life history stages. Because resistance and tolerance influence hosts’ parasite load and the duration of infection, these strategies can have important implications for infectious disease transmission, parasite prevalence, and even the evolution of virulence (Restif and Koella 2004; Miller et al. 2006; Adelman 2014; Gopinath et al. 2014). Determining how investment in resistance and tolerance within populations scales to disease dynamics within populations is an important future direction.
Another important frontier in ecoimmunology is to understand how environmental change influences disease dynamics. Chemical pollutants from industry and agriculture can alter immune function and have been associated with multiple disease outbreaks in wildlife (Grasman 2002). For example, the industrial contaminants polychlorinated biphenyls have been linked to infectious disease deaths in numerous marine mammal species, including harbor porpoises (Phocoena phocoena) and striped dolphins (Stenella coeruleolba) (Ross 2002). Urbanization is linked to changes in disease dynamics (Bradley and Altizer 2006), and traits of the urban environment, such as light pollution, alter immune function (Bedrosian et al. 2011). Climate change can also alter immunity (Hernroth et al. 2012), which could amplify the risks of infectious disease outbreaks as the ranges of parasites and vectors change in response to a warming climate (Greer et al. 2008). The frameworks are already in place to understand the relationships between environmental factors and immunity as well as immunity and disease dynamics. By improving our understanding of the causes and consequences of immune variation in the context of anthropogenic change, ecoimmunology could provide insights that have implications for public health and conservation.
Finally, building on the connections between the fields of ecoimmunology and comparative immunology will continue to advance our understanding of diversity in immune function. Both fields have made substantial progress through research on both model and nonmodel organisms and by addressing how multiple physiological systems interact to produce immune defenses (Cooper 1984, 2002, 2006; Martin et al. 2006b). In this chapter we discussed two areas of research that link ecoimmunology and comparative physiology: (1) the selective forces and constraints that influence heterogeneity in immunity and (2) how variation in immunity can influence disease dynamics. Characteristics inherent in species , such as body size or life history, can constrain or influence selection on immune traits and, thus, help explain patterns of immunity and disease spread across species that are not explained by phylogenetic relationships (Calder 1996; Han et al. 2015; Brace et al. 2017). Investigating comparative immunology in the context of community ecology is changing how we evaluate infectious disease risk. Because multiple species within a community can be at risk for the same parasitic infections, evaluating heterogeneity in host immunity can help parameterize models predicting infectious disease spread (Adelman 2014; VanderWaal and Ezenwa 2016). Incorporation of ecological factors and organismal traits into comparative studies of immunity can provide insights into both the evolution of immune defenses and community-level consequences of cross-species variation in immunity.
Box 1 The Origins and Central Tenets of Ecoimmunology: A Brief History
Ecoimmunology emerged when researchers began formally considering immunology in an ecological and evolutionary context. One of the first papers to link immunology to ecology was published in 1973 and addressed age-related variation in the prevalence and intensity of schistosome infections in humans (Martin et al. 2011a). It suggested that the lower infection prevalence and intensity observed in older humans could be driven by ecological factors (e.g., parasite exposure, death of hosts with high-intensity infection) in addition to or instead of acquired immunity (Warren 1973). More recent work has confirmed these claims: patterns of human contact with water, management of waterways, and antibody levels are all important in predicting the risk of schistosome infection (Brooker 2007; Moira et al. 2010). Although it was not established as a field until the 1990s, other early studies addressed ideas that remain at the core of ecoimmunology, including trade-offs between physiological functions (Williams 1966), mechanistic links between the immune system and other aspects of physiology (Grossman 1985), the costs and benefits of controlling parasite loads (Behnke et al. 1992), and the effects of resource availability on immunity (Klasing 1988).
Advances in behavioral ecology and endocrinology associated with sexual selection theory were key to the formalization of the field of ecoimmunology. The Hamilton and Zuk hypothesis proposed a role for parasite defenses in influencing the evolution of sexually selected traits, which enable the opposite sex to assess the quality of potential mates (e.g., colorful feathers in birds, “push-up” display in some lizards) (Hamilton and Zuk 1982). Hamilton and Zuk postulated that sexually selected traits could serve as honest signals of resistance to parasites. The later publication of the immunocompetence handicap hypothesis (ICHH) in 1992 promoted interest in immune mechanisms that could mediate honesty in sexual traits (Folstad and Karter 1992). The ICHH posited that androgens (e.g., testosterone) support both the expression of male secondary sexual traits and suppress immune function, and as a result, sexual trait expression depends on parasite exposure. A meta-analysis revealed weak support for the hypothesis, probably because the ICHH is too simple to capture the complexities of how endocrine–immune interactions influence phenotype (Roberts et al. 2004). Nevertheless, the ICHH was significant to the development of ecoimmunology because it was one of the first widely tested theories linking immunological mechanisms to larger-scale biological processes.
In 1996, Sheldon and Verhulst published an influential paper coining the phrase “ecological immunology,” which is arguably the beginning of ecoimmunology as a field. In this paper, Sheldon and Verhulst discussed the costs of immune responses and suggested that immunity is subject to trade-offs. Furthermore, they highlighted that trade-offs can take both physiological and evolutionary forms (Sheldon and Verhulst 1996). Early in the development of ecoimmunology, research on physiological trade-offs primarily involved allocation trade-offs: how organisms distribute limited resources across various functions (Sheldon and Verhulst 1996; Lochmiller and Deerenberg 2000; Schmid-Hempel 2003). As the field grew, research expanded to include other costs individuals pay to develop, maintain, and activate parasite defenses (Adelman and Martin 2009; Graham et al. 2010). Around the same time, evolutionary trade-offs attracted attention and were found to occur when selection favored the investment in one trait to the detriment of another (Schmid-Hempel 2003). This form of trade-off is represented by negative genetic correlations often seen among traits (Schmid-Hempel 2003; Ardia et al. 2011). Today, research addresses the consequences of costs and trade-offs for host–parasite coevolution as well as host population cycling and disease dynamics (e.g., Cousineau and Alizon 2014; VanderWaal and Ezenwa 2016).
Ecoimmunology has continued to grow since its inception in 1996, and the integrative nature of the field endures. The number of ecoimmunology publications produced annually (identified using the search terms ecol* and immunol* in the ISI Web of Science database) has increased more than tenfold since the establishment of the field in 1996 (Fig. 4). Ecoimmunology research incorporates techniques and theories from diverse fields, including ecology, evolutionary biology, epidemiology, comparative physiology, animal behavior, neurobiology, genomics, and proteomics. In 2009, the National Science Foundation funded a Research Coordination Network in Ecological Immunology, and in 2014, the Society for Integrative and Comparative Biology introduced a Division of Ecoimmunology and Disease Ecology (Martin et al. 2014). Going forward, we anticipate that ecoimmunology will continue to make valuable contributions to our understanding of host–parasite interactions.
LAS, CJD, and LBM were supported by NSF grants IOS-1656618 (to LBM) and IOS-1656551 (to CJD).
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