Do individual branches of immune defence correlate? A comparative case study of scavenging and non‐scavenging birds

Costs of immunity are widely believed to play an important role in life history evolution, but most studies of ecological immunology have considered only single aspects of immune function. It is unclear whether we should expect correlated responses in other aspects of immune function not measured, because individual branches of immune defence may differ in their running costs and thus may compete unequally for limiting resources, resulting in negatively correlated evolution. In theory such selection pressure may be most intense where species are hosts to more virulent parasites, thus facing a higher potential cost of parasitism. These issues are relatively unstudied, but could influence the efficacy of attempting to estimate the scale and cost of host investment in immune defence. Here, in a comparative study of birds we found that species that scavenge at carcasses, that were hypothesised to be hosts to virulent parasites, had larger spleens for their body size and higher blood total leukocyte concentrations (general measures of immune function) than non-scavengers. These results support the hypothesis that scavengers are subject to strong parasite-mediated selection on immune defences. However, measures of specific branches of immune function revealed that scavengers had a relatively lower proportion of lymphocytes than phagocytic types of leukocytes, suggesting robust front line immune defences that could potentially reduce the need for mounting relatively energetically costly lymphocyte-dependent immune responses. Following experimental inoculation, scavengers produced significantly larger humoral immune responses, but not cell-mediated immune responses, than non-scavengers. However, the sizes of cell-mediated and humoral immune responses were not correlated across species. These results suggest that single measures of immune defence may not characterise the overall immune strategy, or reveal the likely costs involved.     

Incubation period and immune function: a comparative field study among coexisting birds

Developmental periods are integral components of life history strategies that can have important fitness consequences and vary enormously among organisms. However, the selection pressures and mechanisms causing variation in length of developmental periods are poorly understood. Particularly puzzling are prolonged developmental periods, because their selective advantage is unclear. Here we tested the hypotheses that immune function is stronger in species that are attacked at a higher rate by parasites and that prolonged embryonic development allows the development of this stronger immune system. Through a comparative field study among 12 coexisting passerine bird species, we show that species with higher blood parasite prevalence mounted stronger cellular immune responses than species with lower prevalence. These results provide support for the hypothesis that species facing greater selection pressure from parasites invest more in immune function. However, species with longer incubation periods mounted weaker cellular immune responses than species with shorter periods. Therefore, cellular immune responses do not support the hypothesis that longer development time enhances immunocompentence. Future studies should assess other components of the immune system and test alternative causes of variation in incubation periods among bird species.     

Exposure to natural pathogens reveals costly aphid response to fungi but not bacteria

Immune responses are costly, causing trade‐offs between defense and other host life history traits. Aphids present a special system to explore the costs associated with immune activation since they are missing several humoral and cellular mechanisms thought important for microbial resistance, and it is unknown whether they have alternative, novel immune responses to deal with microbial threat. Here we expose pea aphids to an array of heat‐killed natural pathogens, which should stimulate immune responses without pathogen virulence, and measure changes in life‐history traits. We find significant reduction in lifetime fecundity upon exposure to two fungal pathogens, but not to two bacterial pathogens. This finding complements recent genomic and immunological studies indicating that pea aphids are missing mechanisms important for bacterial resistance, which may have important implications for how aphids interact with their beneficial bacterial symbionts. In general, recent exploration of the immune systems of non‐model invertebrates has called into question the generality of our current picture of insect immunity. Our data highlight that taking an ecological approach and measuring life‐history traits to a broad array of pathogens provides valuable information that can complement traditional approaches.     

Wild skylarks seasonally modulate energy budgets but maintain energetically costly inflammatory immune responses throughout the annual cycle

A central hypothesis of ecological immunology is that immune defences are traded off against competing physiological and behavioural processes. During energetically demanding periods, birds are predicted to switch from expensive inflammatory responses to less costly immune responses. Acute phase responses (APRs) are a particularly costly form of immune defence, and, hence, seasonal modulations in APRs are expected. Yet, hypotheses about APR modulation remain untested in free-living organisms throughout a complete annual cycle. We studied seasonal modulations in the APRs and in the energy budgets of skylarks Alauda arvensis, a partial migrant bird from temperate zones that experiences substantial ecological changes during its annual cycle. We characterized throughout the annual cycle changes in their energy budgets by measuring basal metabolic rate (BMR) and body mass. We quantified APRs by measuring the effects of a lipopolysaccharide injection on metabolic rate, body mass, body temperature, and concentrations of glucose and ketone. Body mass and BMR were lowest during breeding, highest during winter and intermediate during spring migration, moult and autumn migration. Despite this variation in energy budgets, the magnitude of the APR, as measured by all variables, was similar in all annual cycle stages. Thus, while we find evidence that some annual cycle stages are relatively more energetically constrained, we find no support for the hypothesis that during these annual cycle stages birds compromise an immune defence that is itself energetically costly. We suggest that the ability to mount an APR may be so essential to survival in every annual cycle stage that skylarks do not trade off this costly form of defence with other annual cycle demands.     

Explaining variable costs of the immune response: selection for specific versus non-specific immunity and facultative life history change

The cost of the immune response is variable and may provide a sufficient selective pressure to produce adaptations that minimise those costs under high infection prevalence. Here, using invertebrates as a model, I suggest two possible mechanisms that maintain variation in responses that covary with costs. First I propose that infection prevalence should balance costs through the selection of optimal patterns of "specific" and "non-specific" immune pathways concomitantly expressed in the immune response. Second, I propose that life history adjustments (e.g. earlier reproduction in response to infection) could have been selected to minimise the cost of successful immune responses and consequently may result in the maintenance of costly immunity.     

The evolutionary economics of immunity

How much of its resources should an individual invest in a costly immune system? In this article, we apply an evolutionarily stable strategy analysis to an epidemic model to answer this question. On the one hand, an investment in immune function confers protection to infectious agents by reducing host susceptibility, pathogen virulence, or the length of the infectious period. On the other hand, an immune system is costly since it absorbs resources that otherwise might be invested in increasing the host's fertility or longevity. In addition, an active immune system may be able to clear pathogens efficiently but at the same time may result in immunopathology. By means of a reproductive value approach, we show how to compare the costs and benefits of an immune system systematically and how to derive the evolutionarily stable level of immune function. We then apply these methods to various plausible scenarios. The analysis reveals that the relationship between the life span of an organism and the optimal level of investment in immune function is less straightforward than one might expect. First, the prevalence of infection is reduced to the lowest possible level only under special circumstances. Second, members of a long-lived species do not necessarily have to invest more in immune function than those of a short-lived species. In fact, the opposite may be true. Third, the outcome of evolution can be contingent on the initial conditions. Depending on its initial investment strategy, a population may evolve to a state where very much or almost nothing is invested in a costly immune system.     

Optimal immune responses: immunocompetence revisited

The function of the immune system of an animal is to provide defence against infection, in order to maximize fitness. Understanding this and, particularly, how limiting resources are traded off between costly immune responses and other physiological demands, is central to properly understanding life-history traits and their evolution. Here, we propose that functional (rather than immunological) measures of immune responses should be used when investigating this. We further suggest that optimal immune responses are context specific, rather than generic; that is, a maximum immune response is not necessarily optimal. The nature of an optimal immune response will depend on the specific circumstances and infection status of the animal. Identifying and understanding such optimality requires that the effects of different immune strategies on fitness be considered.     

Two arms are better than one: parasite variation leads to combined inducible and constitutive innate immune responses

Parasites represent a major threat to all organisms which has led to the evolution of an array of complex and effective defence mechanisms. Common to both vertebrates and invertebrates are innate immune mechanisms that can be either constitutively expressed or induced on exposure to infection. In nature, we find that a combination of both induced and constitutive responses are employed by vertebrates, invertebrates and, to an extent, plants when they are exposed to a parasite. Here we use a simple within-host model motivated by the insect immune system, consisting of both constitutive and induced responses, to address the question of why both types of response are maintained so ubiquitously. Generally, induced responses are thought to be advantageous because they are only used when required but are too costly to maintain constantly, while constitutive responses are advantageous because they are always ready to act. However, using a simple cost function but with no a priori assumptions about relative costs, we show that variability in parasite growth rates selects for a strategy that combines both constitutive and induced defences. Differential costs are therefore not necessary to explain the adoption of both forms of defence. Clearly, hosts are likely to be challenged by variable parasites in nature and this is sufficient to explain why it is optimal to deploy both arms of the innate immune system.     

Low cost antiviral activity of Plodia interpunctella haemolymph in vivo demonstrated by dose dependent infection

Given the ubiquity of infectious disease it is important to understand the way in which hosts defend themselves and any costs that they may pay for this defence. Despite this, we know relatively little about insect immune responses to viruses when compared to their well-characterized responses to other pathogens. In particular it is unclear whether there is significant haemocoelic response to viral infection. Here we directly examine this question by examining whether there is a dose-dependency in infection risk when a DNA virus is injected directly into the haemocoel. Infection from direct injection into the haemocoel showed a clear dose dependency that is indicative of an active intrahaemocoelic immune response to DNA viruses in insects. In contrast to the natural oral infection route, we found no measurable sublethal effects in the survivours from direct injection. This suggests that the immune responses in the haemocoel are less costly than those that occur earlier.     

Insularity effects on bird immune parameters: A comparison between island and mainland populations in West Africa

Oceanic islands share several environmental characteristics that have been shown to drive convergent evolutionary changes in island organisms. One change that is often assumed but has seldom been examined is the evolution of weaker immune systems in island species. The reduction in species richness on islands is expected to lead to a reduced parasite pressure and, given that immune function is costly, island animals should show a reduced immune response. However, alternative hypotheses exist; for example, the slower pace of life on islands could favor the reorganization of the immune system components (innate vs. acquired immunity) on islands. Thus far, few island species have been studied and no general patterns have emerged. Here, we compared two immune parameters of birds from São Tomé and Príncipe islands to those of their close relatives at similar latitudes on the mainland (Gabon, West Africa). On islands, the acquired humoral component (total immunoglobulins) was lower for most species, whereas no clear pattern was detected for the innate component (haptoglobin levels). These different responses did not seem to arise from a reorganization of the two immune components, as both total immunoglobulins and haptoglobin levels were positively associated. This work adds to the few empirical studies conducted so far which suggest that changes in immune parameters in response to insularity are not as straightforward as initially thought.     

Divergent immune priming responses across flour beetle life stages and populations

Growing evidence shows that low doses of pathogens may prime the immune response in many insects, conferring subsequent protection against infection in the same developmental stage (within‐life stage priming), across life stages (ontogenic priming), or to offspring (transgenerational priming). Recent work also suggests that immune priming is a costly response. Thus, depending on host and pathogen ecology and evolutionary history, tradeoffs with other fitness components may constrain the evolution of priming. However, the relative impacts of priming at different life stages and across natural populations remain unknown. We quantified immune priming responses of 10 natural populations of the red flour beetle Tribolium castaneum, primed and infected with the natural insect pathogen Bacillus thuringiensis. We found that priming responses were highly variable both across life stages and populations, ranging from no detectable response to a 13‐fold survival benefit. Comparing across stages, we found that ontogenic immune priming at the larval stage conferred maximum protection against infection. Finally, we found that various forms of priming showed sex‐specific associations that may represent tradeoffs or shared mechanisms. These results indicate the importance of sex‐, life stage‐, and population‐specific selective pressures that can cause substantial divergence in priming responses even within a species. Our work highlights the necessity of further work to understand the mechanistic basis of this variability.     

Immune Investment Is Explained by Sexual Selection and Pace-of-Life,but Not Longevity in Parrots (Psittaciformes)

Investment in current reproduction should come at the expense of traits promoting future reproduction, such as immunity and longevity. To date, comparative studies of pace-of-life traits have provided some support for this, with slower paced species having greater immune function. Another means of investment in current reproduction is through secondary sexual characters (SSC). Investment in SSC''s is considered costly, both in terms of immunity and longevity, with greater costs being borne by species with more elaborate traits. Yet within species, females prefer more ornate males and those males are typically immunologically superior. Because of this, predictions about the relationship between immunity and SSC''s across species are not clear. If traits are costly, brighter species should have reduced immune function, but the opposite is true if SSC''s arise from selection for more immunocompetent individuals. My approach was to investigate immune investment in relation to SSC''s, pace-of-life and longevity while considering potentially confounding ecological factors. To do so I assessed leukocyte counts from in a novel group, the Psittaciformes. Investment in SSC''s best explained investment in immunity: species with brighter plumage had higher leukocyte counts and those with a greater degree of sexual dichromatism had fewer. Ecological variables and pace-of-life models tended to be poor predictors of immune investment. However, shorter incubation periods were associated with lower leukocyte counts supporting the notion that species with a fast pace-of-life invest less in immunity. These results suggest that investment in reproduction in terms of fast pace-of-life and sexual dichromatism results in reduced immunity; however, investment in plumage colour per se does not impose a cost on immunity across species.     

Protein-poor diet reduces host-specific immune gene expression in Bombus terrestris

Parasites infect hosts non-randomly as genotypes of hosts vary in susceptibility to the same genotypes of parasites, but this specificity may be modulated by environmental factors such as nutrition. Nutrition plays an important role for any physiological investment. As immune responses are costly, resource limitation should negatively affect immunity through trade-offs with other physiological requirements. Consequently, nutritional limitation should diminish immune capacity in general, but does it also dampen differences among hosts? We investigated the effect of short-term pollen deprivation on the immune responses of our model host Bombus terrestris when infected with the highly prevalent natural parasite Crithidia bombi. Bumblebees deprived of pollen, their protein source, show reduced immune responses to infection. They failed to upregulate a number of genes, including antimicrobial peptides, in response to infection. In particular, they also showed less specific immune expression patterns across individuals and colonies. These findings provide evidence for how immune responses on the individual-level vary with important elements of the environment and illustrate how nutrition can functionally alter not only general resistance, but also alter the pattern of specific host–parasite interactions.     

Trade‐offs in evolutionary immunology: just what is the cost of immunity?

It has become increasingly clear that life-history patterns among the vertebrates have been shaped by the plethora and variety of immunological risks associated with parasitic faunas in their environments. Immunological competence could very well be the most important determinant of life-time reproductive success and fitness for many species. It is generally assumed by evolutionary ecologists that providing immunological defences to minimise such risks to the host is costly in terms of necessitating trade-offs with other nutrient-demanding processes such as growth, reproduction, and thermoregulation. Studies devoted to providing assessments of such costs and how they may force evolutionary trade-offs among life-history characters are few, especially for wild vertebrate species, and their results are widely scattered throughout the literature. In this paper we attempt to review this literature to obtain a better understanding of energetic and nutritional costs for maintaining a normal immune system and examine how costly it might be for a host who is forced to up-regulate its immunological defence mechanisms. The significance of these various costs to ecology and life history trade-offs among the vertebrates is explored. It is concluded that sufficient evidence exists to support the primary assumption that immunological defences are costly to the vertebrate host.     

Senescence in immune priming and attractiveness in a beetle

Age‐related decline in immune activity is referred to as immunosenescence and has been observed for both the adaptive immune response of vertebrates and the innate immune system of invertebrates. Because maintaining a basic level of immune defence and mounting an immune response is costly, optimal investment in immune function should vary over a wide range of individual states such as the individual’s age. In this study, we tested whether the immune response and immunological priming within individuals become less efficient with age using mealworm beetles, Tenebrio molitor, as a model organism. We also tested whether ageing and immunological priming affected the odours produced by males. We found that young males of T. molitor were capable of mounting an immune response a sterile nylon monofilament implant with the potential to exhibit a simple form of immune memory through mechanisms of immune priming. Older males did not increase their immune response to a second immune challenge, which negatively affected their sexual attractiveness and remaining life span. Our results indicate that the immune system of older males in T. molitor is less effective, suggesting complex evolutionary trade‐offs between ageing, immune response and sexual attractiveness.     

Brood size and immunity costs in zebra finches Taeniopygia guttata

Birds rearing experimentally enlarged broods have lower antibody responses to a novel antigen, and we tested three hypotheses that could explain this result. We used zebra finches Taeniopygia guttata inoculated with sheep red blood cells (SRBC) as a study system, for which this trade-off was previously demonstrated. 1. Compensatory cellular immunity: The humoral immune response is slow, and removal of SRBC through up-regulated cellular immunity could pre-empt an antibody response. However, cellular immune response to PHA decreased with increasing brood size, allowing rejection of this hypothesis. 2. Costs of antibody-production: Chicks in large broods grow less well, and birds with large broods may allocate resources to chicks instead of antibodies when these are costly. Compared to saline controls, SRBC suppressed metabolic rate in the hours following immunisation, but there was no effect in the following night, or at any time 4 and 8 days later. Fitness costs were measured by repeatedly immunising parents with SRBC while rearing young. Chick growth, parental condition, and subsequent reproduction of the parents were not affected by SRBC. We conclude that the costs of antibody formation cannot explain the trade-off between brood size and antibody responsiveness. 3. Costs of immune system maintenance: Maintaining a system enabling antibody-formation may be very costly, and birds rearing large broods may have down-regulated this system. Based on this hypothesis we predicted that antibody formation would still be reduced in parents rearing large broods when immunised after rearing the chicks. Our results confirmed this prediction, and we suggest that birds rearing large broods have lower antibody responses because they economised on the maintenance costs of the immune system.     

Contrasting adaptive immune defenses and blood parasite prevalence in closely related Passer sparrows

Immune system components differ in their functions and costs, and immune defense profiles are likely to vary among species with differing ecologies. We compared adaptive immune defenses in two closely related species that have contrasting inflammatory immune responses, the widespread and abundant house sparrow (Passer domesticus) and the less abundant tree sparrow (Passer montanus). We found that the house sparrow, which we have previously shown mounts weaker inflammatory responses, exhibits stronger adaptive immune defenses, including antibody responses, natural antibody titers, and specific T-cell memory, than the tree sparrow. Conversely, tree sparrows, which mount strong inflammatory responses, also mount stronger nonspecific inflammatory T-cell responses but weaker specific adaptive responses. Prevalence of avian malaria parasite infections, which are controlled by adaptive immune defenses, was higher in the geographically restricted tree sparrow than in the ubiquitous house sparrow. Together these data describe distinct immune defense profiles between two closely related species that differ greatly in numbers and distributions. We suggest that these immunological differences could affect fitness in ways that contribute to the contrasting abundances of the two species in North American and Western Europe.     

NK-like and oxidative burst activities are the main early cellular innate immune responses activated after virus inoculation in reservoir fish

Viral diseases are a major problem in fish farming and a deeper knowledge of the immunological mechanisms playing a part in the antiviral defence is still important. Moreover, fish farming practices (high densities, new areas of culture and egg/larvae/adult transport) are significantly increasing the spread of viruses and the number of susceptible or reservoir fish species. In this last point, no studies have focused on the immunological mechanisms playing a part in the antiviral responses in reservoir and non-susceptible fish species. Thus, we have evaluated the very early innate immune responses of gilthead seabream (Sparus aurata) to the virus causing viral haemorrhagic septicaemia (VHSV) in salmonids since this virus has been found in seabream and neighbouring farmed marine fish species acting as a viral reservoir. The virus was detected in liver, head-kidney, spleen and peritoneal cavity suggesting that the virus reached these tissues but did not replicate as viral expression was almost absent by 72h post-inoculation. Interestingly, VHSV provoked an influx of leucocytes to the peritoneal cavity and a redistribution of peritoneal exudate (PELs) and head-kidney (HKLs) leucocytes and their innate immune responses (non-specific cytotoxic (NCC or NK-like) activity, phagocytosis, reactive oxygen intermediate (ROI) production and myeloperoxidase (MPO) activity) were generally increased demonstrating that the immune system is activated and involved in the clearance of the virus. Strikingly, NK-like, ROI and MPO were the most enhanced by the presence of VHSV in both PELs and HKLs suggesting that these early innate immune events are crucial during early viral infection stages in non-susceptible or reservoir species. Differences in the immunological mechanisms between susceptible and reservoir species and with other particulate antigens are discussed.     

Examination of the immune responses of males and workers of the leaf-cutting ant Acromyrmex echinatior and the effect of infection

Summary. Parasites represent significant challenges to social insects. The high density, interaction rate and relatedness of individuals within colonies are all predicted to make social insect colonies particularly vulnerable to parasites. To cope with this pressure, social insects have evolved a number of defence mechanisms. These include the immune response, which, aside from in bumblebees, has been relatively little studied in social insects. Here we compare the immune responses of males and workers of the leaf-cutting ant Acromyrmex echinatior and examine the effect upon immunocompetence of prior exposure to a virulent parasite. Males have a far lower immune response than workers, suggesting either haploid susceptibility or reduced investment in immunity by the short-lived males. There was also significantly less variation in the immune response of males than of workers, which may be due to leaf-cutting ant workers being more variable in age or more genetically diverse within colonies. When exposed to the entomopathogenic fungus Metarhizium, workers expressed a substantially reduced immune response 96 h after infection, suggesting that the immune system was either depleted by having to respond to the Metarhizium infection or was depressed by the parasite. The results suggest that the immune response is a costly and limited process, but further experiments are needed to distinguish between the alternative explanations for the effects observed.Received 3 August 2004; revised 3 February 2005; accepted 2 March 2005.