Immune defence strategies of generalist and specialist insect herbivores

Ecological immunology examines the adaptive responses of animals to pathogens in relation to other environmental factors and explores the consequences of trade-offs between investment in immune function and other life-history traits. Among species of herbivorous insects, diet breadth may vary greatly, with generalists consuming a wide variety of plant families and specialists restricted to a few species. Generalists may thus be exposed to a wider range of pathogens exerting stronger selection on the innate immune system. To examine whether this produces an increase in the robustness of the immune response, we compared larvae of the generalist herbivore Heliothis virescens and the specialist Heliothis subflexa challenged by entomopathogenic and non-pathogenic bacteria. Heliothis virescens larvae showed lower mortality, a lower number of recoverable bacteria, lower proliferation of haemocytes and higher phagocytic activity. These results indicate a higher tolerance to entomopathogenic bacteria by the generalist, which is associated with a more efficient cell-mediated immune response by mechanisms that differ between these closely related species. Our findings provide novel insights into the consequences of diet breadth and related environmental factors, which may be significant in further studies to understand the ecological forces and investment trade-offs that shape the evolution of innate immunity.     

How parasite interaction strategies alter virulence evolution in multi‐parasite communities

The majority of organisms host multiple parasite species, each of which can interact with hosts and competitors through a diverse range of direct and indirect mechanisms. These within‐host interactions can directly alter the mortality rate of coinfected hosts and alter the evolution of virulence (parasite‐induced host mortality). Yet we still know little about how within‐host interactions affect the evolution of parasite virulence in multi‐parasite communities. Here, we modeled the virulence evolution of two coinfecting parasites in a host population in which parasites interacted through cross immunity, immune suppression, immunopathology, or spite. We show (1) that these within‐host interactions have different effects on virulence evolution when all parasites interact with each other in the same way versus when coinfecting parasites have unique interaction strategies, (2) that these interactions cause the evolution of lower virulence in some hosts, and higher virulence in other hosts, depending on the hosts infection status, and (3) that for cross immunity and spite, whether parasites increase or decrease the evolutionarily stable virulence in coinfected hosts depended on interaction strength. These results improve our understanding of virulence evolution in complex parasite communities, and show that virulence evolution must be understood at the community scale.     

Immune evasion by Babesia bovis and Plasmodium falciparum: cliff-dwellers of the parasite world

Erythrocyte-dwelling parasites, such as Babesia bovis and Plasmodium falciparum, are not accessible to the host immune system during most of their asexual reproductive cycle because they are intracellular. While intracellular, the host immune response must be directed toward the surface of the infected erythrocyte. Immune individuals mount protective antibody and cell-mediated responses which eliminate most of the parasites, yet some survive to establish chronic infections. In this review, David Allred discusses some of the mechanisms used by these parasites to evade individual immune mechanisms targeting the infected erythrocyte to survive in the hostile environment of an effective immune response.     

Ferritins, bacterial virulence and plant defence

The enterobacterial pathogen Erwinia chrysanthemi causes soft rot diseases on a wide range of plants, including the model plant Arabidopsis thaliana. This bacterium proliferates in the host by secreting a set of pectin degrading enzymes responsible for symptom development. In addition, survival of this bacterium in planta requires two high-affinity iron acquisition systems mediated by siderophores and protective systems against oxidative damages, suggesting the implication by both partners of accurate mechanisms controlling their iron homeostasis under conditions of infection. In this review, we address this question and we show that ferritins both from the pathogen and the host are subtly implicated in the control of this interplay.     

Vaccine-driven evolution of parasite virulence and immune evasion in age-structured population: the case of pertussis

Despite enormous success of mass immunization programs in reducing incidence of infectious diseases, vaccine-escape strains have emerged perhaps as a consequence of strong selection pressures exerted on parasites by vaccines. Pertussis presents a well-documented example. As a childhood infection, it exhibits age-specific transmission biased to children. Assuming different transmission rates between children and adults, I study, by means of an age-structured epidemic model, evolutionary dynamics of parasite virulence in a vaccinated population. I find that the age-structure does not affect the evolutionary dynamics of parasite virulence. Also, based on empirical data reporting antigenic divergence with vaccine strains and mutations in virulence-associated genes in pertussis populations, I allow for parallel occurrence of mutations in parasite virulence and associated immune evasion. I conclude that this simultaneous adaptation of both traits may substantially alter the evolutionary course of the parasite. In particular, higher values of virulence are favoured once the parasite is able to evade the transmission-blocking vaccine-induced immunity. On the other hand, lower values of virulence are selected for once the parasite evolves the ability to evade the virulence-blocking vaccine-induced immunity. I emphasize the importance of multi-trait evolution to assess the direction of parasite adaptation more accurately.     

Interference competition and parasite virulence

Within-host competition between parasites, a consequence of infection by multiple strains, is predicted to favour rapid host exploitation and greater damage to hosts (virulence). However, the inclusion of biological variables can drastically change this relationship. For example, if competing parasite strains produce toxins that kill each other (interference competition), their growth rates and virulence may be reduced relative to single-strain infections. Bacteriocins are antimicrobial toxins produced by bacteria that target closely related strains and species, and to which the producing strain is immune. We investigated competition between bacteriocin-producing, insect-killing bacteria (Photorhabdus and Xenorhabdus) and how this competition affected virulence in caterpillars. Where one strain could kill the other, and not vice versa, the non-killing strain was competitively excluded, and insect mortality was the same as that of the killing strain alone. However, when caterpillars were multiply infected by strains that could kill each other, we did not observe competitive exclusion and their virulence was less than single-strain infections. The ubiquity and diversity of bacteriocins among pathogenic bacteria suggest mixed infections will be, on average, less virulent than single infections.     

A crucial role for exopolysaccharide modification in bacterial biofilm formation, immune evasion, and virulence

Biofilms play an important role in many chronic bacterial infections. Production of an extracellular mixture of sugar polymers called exopolysaccharide is characteristic and critical for biofilm formation. However, there is limited information about the mechanisms involved in the biosynthesis and modification of exopolysaccharide components and how these processes influence bacterial pathogenesis. Staphylococcus epidermidis is an important human pathogen that frequently causes persistent infections by biofilm formation on indwelling medical devices. It produces a poly-N-acetylglucosamine molecule that emerges as an exopolysaccharide component of many bacterial pathogens. Using a novel method based on size exclusion chromatography-mass spectrometry, we demonstrate that the surface-attached protein IcaB is responsible for deacetylation of the poly-N-acetylglucosamine molecule. Most likely due to the loss of its cationic character, non-deacetylated poly-acetylglucosamine in an isogenic icaB mutant strain was devoid of the ability to attach to the bacterial cell surface. Importantly, deacetylation of the polymer was essential for key virulence mechanisms of S. epidermidis, namely biofilm formation, colonization, and resistance to neutrophil phagocytosis and human antibacterial peptides. Furthermore, persistence of the icaB mutant strain was significantly impaired in a murine model of device-related infection. This is the first study to describe a mechanism of exopolysaccharide modification that is indispensable for the development of biofilm-associated human disease. Notably, this general virulence mechanism is likely similar for other pathogenic bacteria and constitutes an excellent target for therapeutic maneuvers aimed at combating biofilm-associated infection.     

Immune evasion or avoidance: Fungal skin infection linked to reduced defence peptides in Australian green-eyed treefrogs, Litoria serrata

Many parasites and pathogens suppress host immunity to maintain infection or initiate disease. On the skin of many amphibians, defensive peptides are active against the fungus Batrachochytrium dendrobatidis (Bd), the causative agent of the emerging infectious disease chytridiomycosis. We tested the hypothesis that infection with the fungus may be linked to lower levels of defensive peptides. We sampled both ambient (or constitutive) skin peptides on the ventral surface immediately upon capture, and stored skin peptides induced from granular glands by norepinephrine administration of Australian green-eyed treefrogs, Litoria serrata. Upon capture, uninfected frogs expressed an array of antimicrobial peptides on their ventral surface, whereas infected frogs had reduced skin peptide expression. Expression of ambient skin peptides differed with infection status, and antimicrobial peptides maculatin 1.1 and 2.1 were on average three times lower on infected frogs. However, the repertoire of skin peptides stored in granular glands did not differ with infection status; on average equal quantities were recovered from infected and from uninfected frogs. Our results could have at least two causes: (1) frogs with reduced peptide expression are more likely to become infected; (2) Bd infection interferes with defence peptides by inhibiting release or causing selective degradation of peptides on the skin surface. Immune evasion therefore may contribute to the pathogenesis of chytridiomycosis and a mechanistic understanding of this fungal strategy may lead to improved methods of disease control.     

The evolution of parasite virulence, superinfection, and host resistance

We analyze the evolutionary consequences of host resistance (the ability to decrease the probability of being infected by parasites) for the evolution of parasite virulence (the deleterious effect of a parasite on its host). When only single infections occur, host resistance does not affect the evolution of parasite virulence. However, when superinfections occur, resistance tends to decrease the evolutionarily stable (ES) level of parasite virulence. We first study a simple model in which the host does not coevolve with the parasite (i.e., the frequency of resistant hosts is independent of the parasite). We show that a higher proportion of resistant host decreases the ES level of parasite virulence. Higher levels of the efficiency of host resistance, however, do not always decrease the ES parasite virulence. The implications of these results for virulence management (evolutionary consequences of public health policies) are discussed. Second, we analyze the case where host resistance is allowed to coevolve with parasite virulence using the classical gene-for-gene (GFG) model of host-parasite interaction. It is shown that GFG coevolution leads to lower parasite virulence (in comparison with a fully susceptible host population). The model clarifies and relates the different components of the cost of parasitism: infectivity (ability to infect the host) and virulence (deleterious effect) in an evolutionary perspective.     

Host mortality, predation and the evolution of parasite virulence

One of the most accepted views in the theoretical literature on virulence evolution is that a parasite's virulence will evolve to higher levels when its host's background mortality rate increases. Surprisingly, however, although many sources of background mortality involve predation, there has not yet been any theoretical research that explicitly considers how the dynamics of this important ecological interaction affects virulence evolution. Here, we consider how predation affects virulence evolution by explicitly introducing a predator into a classical susceptible–infected–susceptible epidemiological model. We find that, contrary to previous predictions, different sources of host mortality affect virulence evolution in different ways. Moreover, the way in which virulence evolution is affected depends on how tightly coupled the predator's dynamics are to the host population, and this can result in somewhat counterintuitive results. For example, indirect ecological effects can cause elevated host mortality to result in the evolution of lower parasite virulence, even if this elevated mortality arises from factors unrelated to predation.     

Immune defence,extra-pair paternity,and sexual selection in birds

Secondary sexual characters have been suggested to reliably reflect the ability of individuals to resist debilitating parasites, and females may gain direct or indirect fitness benefits from preferring the most extravagantly ornamented males. Extra-pair paternity provides an estimate of an important component of sexual selection in birds. Species with a high frequency of extra-pair paternity have a variance in realized reproductive success that is greater than the variance in apparent reproductive success, and extra-pair copulations and hence extra-pair paternity by females are often directly associated with the expression of male secondary sexual characters. If sexually dichromatic species have experienced a long period of antagonistic coevolution with their parasites, such species should have evolved larger immune defence organs than sexually monochromatic species. Bird species with sexual dichromatism had larger spleens for their body size than monochromatic species in a comparative analysis. Furthermore, species with a high frequency of extra-pair paternity were sexually dichromatic and had large spleens for their body size. These results are consistent with the hypothesis that females of dichromatic bird species seek extra-pair copulations to obtain indirect fitness benefits in terms of superior resistance of their offspring to virulent parasites.     

Leishmania major parasite stage-dependent host cell invasion and immune evasion

Leishmania pathogenesis is primarily studied using the disease-inducing promastigote stage of Leishmania major. Despite many efforts, all attempts so far have failed to culture the disease-relevant multiplying amastigote stage of L. major. Here, we established a stably growing axenic L. major amastigote culture system that was characterized genetically, morphologically, and by stage-specific DsRed protein expression. We found parasite stage-specific disease development in resistant C57BL/6 mice. Human neutrophils, as first host cells for promastigotes, do not take up amastigotes. In human macrophages, we observed an amastigote-specific complement receptor 3-mediated, endocytotic entry mechanism, whereas promastigotes are taken up by complement receptor 1-mediated phagocytosis. Promastigote infection of macrophages induced the inflammatory mediators TNF, CCL3, and CCL4, whereas amastigote infection was silent and resulted in significantly increased parasite numbers: from 7.1 ± 1.4 (after 3 h) to 20.1 ± 7.9 parasites/cell (after 96 h). Our study identifies Leishmania stage-specific disease development, host cell preference, entry mechanism, and immune evasion. Since the amastigote stage is the disease-propagating form found in the infected mammalian host, the newly developed L. major axenic cultures will serve as an important tool in better understanding the amastigote-driven immune response in leishmaniasis.     

Multiple predator defence strategies in Daphnia pulex and their relation to native habitat

Daphnia may respond with an array of anti-predator defences(behavioural, morphological and life history) to a chemicalcue (kairomone) exuded by its predators: fish and Chaoborus.Given the wide array of potential responses, it is an interestingquestion whether anti-predator defences are coupled or independentof each other. Since anti-predator responses are costly andeven possessing the genetic information to respond to a certainpredator might involve a cost, clones may only react to predatorsthey co-occur with in nature. In this study, we provide evidencefor an uncoupling of responses by Daphnia pulex in several anti-predatordefences against Chaoborus. We were unable to detect a correlationbetween behavioural (migration), morphological (neck-spine induction)and life history [growth rate, neonate size and size at firstreproduction (SFR)] responses. Furthermore, anti-predator responsesdid not always comply with what is commonly believed. We foundthat Daphnia clones can migrate up or down when exposed to fishor Chaoborus kairomone and that population growth rate, neonatesize and SFR can increase or decrease in response to Chaoboruskairomone. We also show patterns in anti-predator defences thatseem to relate to the habitat from which clones were derived.Daphnia clones that were collected in habitats with Chaoborusas the dominant predator tended to react strongly to Chaoboruskairomone by migrating upward and producing neck-spines. Themigration behaviour against fish kairomone in these clones wasoften an unexpected upward migration. The Daphnia clone thatco-existed with fish predators showed a downward migration inthe presence of fish as well as Chaoborus kairomone. Clonesthat had occurred with either both or no predators had mixedresponses. We sometimes found an upward migration in combinationwith smaller body size as a response to Chaoborus kairomone.This may be interpreted as a behavioural defence against Chaoborusand a life-history defence against fish. Daphnia seem not toexhibit defence behaviour against predators they do not co-occurwith. It might be costly for Daphnia to maintain genetic informationto respond to these predators and protect that information fromgenetic drift.     

Empirical support for optimal virulence in a castrating parasite

The trade-off hypothesis for the evolution of virulence predicts that parasite transmission stage production and host exploitation are balanced such that lifetime transmission success (LTS) is maximised. However, the experimental evidence for this prediction is weak, mainly because LTS, which indicates parasite fitness, has been difficult to measure. For castrating parasites, this simple model has been modified to take into account that parasites convert host reproductive resources into transmission stages. Parasites that kill the host too early will hardly benefit from these resources, while postponing the killing of the host results in diminished returns. As predicted from optimality models, a parasite inducing castration should therefore castrate early, but show intermediate levels of virulence, where virulence is measured as time to host killing. We studied virulence in an experimental system where a bacterial parasite castrates its host and produces spores that are not released until after host death. This permits estimating the LTS of the parasite, which can then be related to its virulence. We exposed replicate individual Daphnia magna (Crustacea) of one host clone to the same amount of bacterial spores and followed individuals until their death. We found that the parasite shows strong variation in the time to kill its host and that transmission stage production peaks at an intermediate level of virulence. A further experiment tested for the genetic basis of variation in virulence by comparing survival curves of daphniids infected with parasite spores obtained from early killing versus late killing infections. Hosts infected with early killer spores had a significantly higher death rate as compared to those infected with late killers, indicating that variation in time to death was at least in part caused by genetic differences among parasites. We speculate that the clear peak in lifetime reproductive success at intermediate killing times may be caused by the exceptionally strong physiological trade-off between host and parasite reproduction. This is the first experimental study to demonstrate that the production of propagules is highest at intermediate levels of virulence and that parasite genetic variability is available to drive the evolution of virulence in this system.     

Host resistance and parasite virulence in greenfinch coccidiosis

The question why different host individuals within a population differ with respect to infection resistance is of fundamental importance for understanding the mechanisms of parasite-mediated selection. We addressed this question by infecting wild-caught captive male greenfinches with intestinal coccidian parasites originating either from single or multiple hosts. Birds with naturally low pre-experimental infection retained their low infection status also after reinfection with multiple strains, indicating that natural infection intensities confer information about the phenotypic ability of individuals to resist novel strains. Exposure to novel strains did not result in protective immunity against the subsequent infection with the same strains. Infection with multiple strains resulted in greater virulence than single-strain infection, indicating that parasites originating from different host individuals are genetically diverse. Our experiment thus demonstrates the validity of important but rarely tested assumptions of many models of parasite-mediated selection in a wild bird species and its common parasite.