Balancing selection at the tomato RCR3 Guardee gene family maintains variation in strength of pathogen defense

Coevolution between hosts and pathogens is thought to occur between interacting molecules of both species. This results in the maintenance of genetic diversity at pathogen antigens (or so-called effectors) and host resistance genes such as the major histocompatibility complex (MHC) in mammals or resistance (R) genes in plants. In plant-pathogen interactions, the current paradigm posits that a specific defense response is activated upon recognition of pathogen effectors via interaction with their corresponding R proteins. According to the "Guard-Hypothesis," R proteins (the "guards") can sense modification of target molecules in the host (the "guardees") by pathogen effectors and subsequently trigger the defense response. Multiple studies have reported high genetic diversity at R genes maintained by balancing selection. In contrast, little is known about the evolutionary mechanisms shaping the guardee, which may be subject to contrasting evolutionary forces. Here we show that the evolution of the guardee RCR3 is characterized by gene duplication, frequent gene conversion, and balancing selection in the wild tomato species Solanum peruvianum. Investigating the functional characteristics of 54 natural variants through in vitro and in planta assays, we detected differences in recognition of the pathogen effector through interaction with the guardee, as well as substantial variation in the strength of the defense response. This variation is maintained by balancing selection at each copy of the RCR3 gene. Our analyses pinpoint three amino acid polymorphisms with key functional consequences for the coevolution between the guardee (RCR3) and its guard (Cf-2). We conclude that, in addition to coevolution at the "guardee-effector" interface for pathogen recognition, natural selection acts on the "guard-guardee" interface. Guardee evolution may be governed by a counterbalance between improved activation in the presence and prevention of auto-immune responses in the absence of the corresponding pathogen.     

Experimental demonstration of a parasite‐induced immune response in wild birds: Darwin's finches and introduced nest flies

Ecological immunology aims to explain variation among hosts in the strength and efficacy of immunological defenses. However, a shortcoming has been the failure to link host immune responses to actual parasites under natural conditions. Here, we present one of the first experimental demonstrations of a parasite‐induced immune response in a wild bird population. The recently introduced ectoparasitic nest fly Philornis downsi severely impacts the fitness of Darwin's finches and other land birds in the Galápagos Islands. An earlier study showed that female medium ground finches (Geospiza fortis) had P. downsi‐binding antibodies correlating with presumed variation in fly exposure over time. In the current study, we experimentally manipulated fly abundance to test whether the fly does, in fact, cause changes in antibody levels. We manipulated P. downsi abundance in nests and quantified P. downsi‐binding antibody levels of medium ground finch mothers, fathers, and nestlings. We also quantified host behaviors, such as preening, which can integrate with antibody‐mediated defenses against ectoparasites. Philornis downsi‐binding antibody levels were significantly higher among mothers at parasitized nests, compared to mothers at (fumigated) nonparasitized nests. Mothers with higher antibody levels tended to have fewer parasites in their nests, suggesting that antibodies play a role in defense against parasites. Mothers showed no behavioral changes that would enhance the effectiveness of the immune response. Neither adult males, nor nestlings, had P. downsi‐induced immunological or behavioral responses that would enhance defense against flies. None of the parasitized nests fledged any offspring, despite the immune response by mothers. Thus, this study shows that, while the immune response of mothers appeared to be defensive, it was not sufficient to rescue current reproductive fitness. This study further shows the importance of testing the fitness consequences of immune defenses, rather than assuming that such responses increase host fitness.     

Specific Gene Expression Responses to Parasite Genotypes Reveal Redundancy of Innate Immunity in Vertebrates

Vertebrate innate immunity is the first line of defense against an invading pathogen and has long been assumed to be largely unspecific with respect to parasite/pathogen species. However, recent phenotypic evidence suggests that immunogenetic variation, i.e. allelic variability in genes associated with the immune system, results in host-parasite genotype-by-genotype interactions and thus specific innate immune responses. Immunogenetic variation is common in all vertebrate taxa and this reflects an effective immunological function in complex environments. However, the underlying variability in host gene expression patterns as response of innate immunity to within-species genetic diversity of macroparasites in vertebrates is unknown. We hypothesized that intra-specific variation among parasite genotypes must be reflected in host gene expression patterns. Here we used high-throughput RNA-sequencing to examine the effect of parasite genotypes on gene expression patterns of a vertebrate host, the three-spined stickleback (Gasterosteus aculeatus). By infecting naïve fish with distinct trematode genotypes of the species Diplostomum pseudospathaceum we show that gene activity of innate immunity in three-spined sticklebacks depended on the identity of an infecting macroparasite genotype. In addition to a suite of genes indicative for a general response against the trematode we also find parasite-strain specific gene expression, in particular in the complement system genes, despite similar infection rates of single clone treatments. The observed discrepancy between infection rates and gene expression indicates the presence of alternative pathways which execute similar functions. This suggests that the innate immune system can induce redundant responses specific to parasite genotypes.     

Host–parasite coevolution: genetic variation in a virus population and the interaction with a host gene

Host–parasite coevolution is considered to be an important factor in maintaining genetic variation in resistance to pathogens. Drosophila melanogaster is naturally infected by the sigma virus, a vertically transmitted and host‐specific pathogen. In fly populations, there is a large amount of genetic variation in the transmission rate from parent to offspring, much of which is caused by major‐effect resistance polymorphisms. We have found that there are similarly high levels of genetic variation in the rate of paternal transmission among 95 different isolates of the virus as in the host. However, when we examined a transmission‐blocking gene in the host, we found that it was effective across virus isolates. Therefore, the high levels of genetic variation observed in this system do not appear to be maintained because of coevolution resulting from interactions between this host gene and parasite genes.     

Fitness in invasive social wasps: the role of variation in viral load,immune response and paternity in predicting nest size and reproductive output

Within any one habitat, the relative fitness of organisms in a population can vary substantially. Social insects like the common wasp are among the most successful invasive animals, but show enormous variation in nest size and other fitness‐related traits. Some of this variation may be caused by pathogens such as viruses that can have serious consequences in social insects, which range from reduced productivity to colony death. Both individual immune responses and colony‐level traits such as genetic diversity are likely to influence effects of pathogen infections on colony fitness. Here we investigate how infections with Kashmir Bee Virus (KBV), immune response and intracolony genetic diversity (due to queen polyandry) affect nest size in the invasive common wasp Vespula vulgaris. We show that KBV is highly prevalent in wasps and expression of antiviral immune genes is significantly increased with higher viral loads across individuals. Patriline membership within a nest did not influence KBV susceptibility or immune response. A permutational MANCOVA revealed that polyandry, viral load and expression of the immune gene Dicer were significant predictors of variation in nest size. High intracolony genetic diversity due to polyandry has previously been hypothesized to improve colony‐level resistance to parasites and pathogens. Consistent with this hypothesis, we observed genetically diverse colonies to be significantly larger and to produce more queens, although this effect was not driven by the pathogen we investigated. Invasive wasps clearly suffer from pathogens and expend resources, as indicated here by elevated immune gene expression, toward reducing pathogen‐impact on colony fitness.     

Immune responses in normal Indian langur monkeys (Presbytis entellus)--a primate model for visceral leishmaniasis

Abstract: The Indian langur monkey (Presbytis entellus) is an experimental host for a range of human diseases and for the assessment of vaccine candidate antigens to some common parasitic infections. This experimental host is particularly suitable for the follow‐up of immunological responses. To understand some of the mechanism that underlies the defense against experimental pathogens there is a need of the basic knowledge on antibody and cell mediated immune responses. In the present study 25 naïve monkeys were subjected to for assessment of their antibody responses to various human parasitic antigens as well as mitogen induced cellular responses. Only few monkeys were found to have low titer of antiparasitic antibodies. There was compressive dose dependent proliferative response of peripheral blood mononuclear cells. Unlike humans, the blastogenic as well as cytokine responses (IFN‐γ, IL‐2 and IL‐4) to Con A was considerably higher as compared to PHA. These findings are similar to what have been reported in other non‐human primates, confirming the appropriateness of Indian langurs for pre‐clinical trials.     

Pathogen fitness components and genotypes differ in their sensitivity to nutrient and temperature variation in a wild plant-pathogen association

Understanding processes maintaining variation in pathogen life-history stages affecting infectivity and reproduction is a key challenge in evolutionary ecology. Models of host-parasite coevolution are based on the assumption that genetic variation for host-parasite interactions is a significant cause of variation in infection, and that variation in environmental conditions does not overwhelm the genetic basis. However, surprisingly little is known about the stability of genotype-genotype interactions under variable environmental conditions. Here, using a naturally occurring plant-pathogen interaction, I tested whether the two distinct aspects of the infection process - infectivity and transmission potential - vary over realistic nutrient and temperature gradients. I show that the initial pathogen infectivity and host resistance responses are robust over the environmental gradients. However, for compatible responses there were striking differences in how different pathogen life-history stages and host and pathogen genotypes responded to environmental variation. For some pathogen genotypes even slight changes in temperature arrested spore production, rendering the developing infection ineffectual. The response of pathogen genotypes to environmental gradients varied in magnitude and even direction, so that their rankings changed across the abiotic gradients. Hence, the variable environment of spatially structured host-parasite interactions may strongly influence the maintenance of polymorphism in pathogen life-history stages governing transmission, whereas evolutionary trajectories of infectivity may be unaffected by the surrounding environment.     

Signal recognition and transduction mediated by the tomato Pto kinase: a paradigm of innate immunity in plants

Plant disease resistance is the result of an innate host defense mechanism, which relies on the ability of the plant to recognize pathogen invasion and to efficiently mount defense responses. In tomato, resistance to the pathogen Pseudomonas syringae is mediated by the specific interaction between the plant serine/threonine kinase Pto and the bacterial protein AvrPto. This article reviews molecular and biochemical properties that confer to Pto the capability to function as an intracellular receptor and to activate a signaling cascade leading to the induction of defense responses.     

Nucleotide-binding oligomerization domain-like receptors: intracellular pattern recognition molecules for pathogen detection and host defense

The nucleotide binding oligomerization domain-like receptor (NLR) family of pattern recognition molecules is involved in a diverse array of processes required for host immune responses against invading pathogens. Unlike TLRs that mediate extracellular recognition of microbes, several NLRs sense pathogens in the cytosol and upon activation induce host defense signaling pathways. Although TLRs and NLRs differ in their mode of pathogen recognition and function, they share similar domains for microbial sensing and cooperate to elicit immune responses against the pathogen. Genetic variation in several NLR genes is associated with the development of inflammatory disorders or increased susceptibility to microbial infection. Further understanding of NLRs should provide critical insight into the mechanisms of host defense and the pathogenesis of inflammatory diseases.     

Dendritic cells and C-type lectin receptors: coupling innate to adaptive immune responses

Dendritic cells (DCs) have an important function in the initiation and differentiation of immune responses, linking innate information to tailored adaptive responses. Depending on the pathogen invading the body, specific immune responses are built up that are crucial for eliminating the pathogen from the host. Host recognition of invading microorganisms relies on evolutionarily ancient, germline-encoded pattern recognition receptors (PRRs) that are highly expressed on the cell surface of DCs, of which the Toll-like receptors (TLRs) are well characterized and recognize bacterial or viral components. Moreover, they bind a variety of self-proteins released from damaged tissues including several heat-shock proteins. The membrane-associated C-type lectin receptors (CLRs) recognize glycan structures expressed by host cells of the immune system or on specific tissues, which upon recognition allow cellular interactions between DCs and other immune or tissue cells. In addition, CLRs can function as PRRs. In contrast to TLRs, CLRs recognize carbohydrate structures present on the pathogens. Modification of glycan structures on pathogens to mimic host glycans can thereby alter CLR interactions that subsequently modifies DC-induced polarization. In this review, we will discuss in detail how specific glycosylation of antigens can dictate both the innate and adaptive interactions that are mediated by CLRs on DCs and how this balances immune activation and inhibition of DC function.     

Immunological Studies in Ugandan Patients with Hepatocellular Carcinoma

Immunological studies were performed on Ugandan patients with hepatocellular carcinoma to test the hypothesis that the high rate of persistence of hepatitis-associated antigen in these patients is the result of defects in host immune response. The responses to 1-chloro-2,4-dinitrobenzene sensitization and to a battery of recall skin test antigens were normal, as was the humoral antibody response to tularaemia antigen. Neither hypogammaglobulinaemia nor specific immunoglobulin deficiencies were found. Thus it appears unlikely that generalized defects in host immune responses can account for the high incidence of persistent hepatitis B virus infection found in Ugandan patients with hepatocellular carcinoma.     

Genotype specificity among hosts,pathogens, and beneficial microbes influences the strength of symbiont‐mediated protection

The microbial symbionts of eukaryotes influence disease resistance in many host‐parasite systems. Symbionts show substantial variation in both genotype and phenotype, but it is unclear how natural selection maintains this variation. It is also unknown whether variable symbiont genotypes show specificity with the genotypes of hosts or parasites in natural populations. Genotype by genotype interactions are a necessary condition for coevolution between interacting species. Uncovering the patterns of genetic specificity among hosts, symbionts, and parasites is therefore critical for determining the role that symbionts play in host‐parasite coevolution. Here, we show that the strength of protection conferred against a fungal pathogen by a vertically transmitted symbiont of an aphid is influenced by both host‐symbiont and symbiont‐pathogen genotype by genotype interactions. Further, we show that certain symbiont phylogenetic clades have evolved to provide stronger protection against particular pathogen genotypes. However, we found no evidence of reciprocal adaptation of co‐occurring host and symbiont lineages. Our results suggest that genetic variation among symbiont strains may be maintained by antagonistic coevolution with their host and/or their host's parasites.     

The Complex Contributions of Genetics and Nutrition to Immunity in Drosophila melanogaster

Both malnutrition and undernutrition can lead to compromised immune defense in a diversity of animals, and “nutritional immunology” has been suggested as a means of understanding immunity and determining strategies for fighting infection. The genetic basis for the effects of diet on immunity, however, has been largely unknown. In the present study, we have conducted genome-wide association mapping in Drosophila melanogaster to identify the genetic basis for individual variation in resistance, and for variation in immunological sensitivity to diet (genotype-by-environment interaction, or GxE). D. melanogaster were reared for several generations on either high-glucose or low-glucose diets and then infected with Providencia rettgeri, a natural bacterial pathogen of D. melanogaster. Systemic pathogen load was measured at the peak of infection intensity, and several indicators of nutritional status were taken from uninfected flies reared on each diet. We find that dietary glucose level significantly alters the quality of immune defense, with elevated dietary glucose resulting in higher pathogen loads. The quality of immune defense is genetically variable within the sampled population, and we find genetic variation for immunological sensitivity to dietary glucose (genotype-by-diet interaction). Immune defense was genetically correlated with indicators of metabolic status in flies reared on the high-glucose diet, and we identified multiple genes that explain variation in immune defense, including several that have not been previously implicated in immune response but which are confirmed to alter pathogen load after RNAi knockdown. Our findings emphasize the importance of dietary composition to immune defense and reveal genes outside the conventional “immune system” that can be important in determining susceptibility to infection. Functional variation in these genes is segregating in a natural population, providing the substrate for evolutionary response to pathogen pressure in the context of nutritional environment.     

GENETIC VARIATION IN RESISTANCE AND FECUNDITY TOLERANCE IN A NATURAL HOST–PATHOGEN INTERACTION

Individuals vary in their ability to defend against pathogens. Determining how natural selection maintains this variation is often difficult, in part because there are multiple ways that organisms defend themselves against pathogens. One important distinction is between mechanisms of resistance that fight off infection, and mechanisms of tolerance that limit the impact of infection on host fitness without influencing pathogen growth. Theory predicts variation among genotypes in resistance, but not necessarily in tolerance. Here, we study variation among pea aphid (Acyrthosiphon pisum) genotypes in defense against the fungal pathogen Pandora neoaphidis. It has been well established that pea aphids can harbor symbiotic bacteria that protect them from fungal pathogens. However, it is unclear whether aphid genotypes vary in defense against Pandora in the absence of protective symbionts. We therefore measured resistance and tolerance to fungal infection in aphid lines collected without symbionts, and found variation among lines in survival and in the percent of individuals that formed a sporulating cadaver. We also found evidence of variation in tolerance to the effects of pathogen infection on host fecundity, but no variation in tolerance of pathogen‐induced mortality. We discuss these findings in light of theoretical predictions about host‐pathogen coevolution.     

Quantitative association tests of immune responses to antigens of Mycobacterium tuberculosis: a study of twins in West Africa.

There is now considerable evidence that host genetic factors are important in determining the outcome of infection with Mycobacterium tuberculosis (MTB). The aim of this study was to assess the role of several candidate genes in the variation observed in the immune responses to MTB antigens. In-vitro assays of T-cell proliferation, an in-vivo intradermal delayed hypersensitivity response; cytokine and antibody secretions to several mycobacterial peptide antigens were assessed in healthy, but exposed, West African twins. Candidate gene polymorphisms were typed in the NRAMP1, Vitamin D receptor, IL10, IL4, IL4 receptor and CTLA-4 genes. Variants of the loci IL10 (-1082 G/A), CTLA-4 (49 A/G) and the IL4 receptor (128 A/G) showed significant associations with immune responses to several antigens. T-cell proliferative responses and antibody responses were reduced, TNF-alpha responses were increased for subjects with the CTLA-4 G allele. The T-cell proliferative responses of subjects with IL10 GA and GG genotypes differed significantly. IL4 receptor AG and GG genotypes also showed significant differences in their T-cell proliferative responses to MTB antigens. These results yield a greater understanding of the genetic mechanisms that underlie the immune responses in tuberculosis and have implications for the design of therapeutic interventions.     

Proteomic analysis reveals an aflatoxin-triggered immune response in cotyledons of Arachis hypogaea infected with Aspergillus flavus

An immune response is triggered in host cells when host receptors recognize conserved molecular motifs, pathogen-associated molecular patterns (PAMPs), such as β-glucans, and chitin at the cell surface of a pathogen. Effector-triggered immunity occurs when pathogens deliver effectors into the host cell to suppress the first immune signaling. Using a differential proteomic approach, we identified an array of proteins responding to aflatoxins in cotyledons of peanut (Arachis hypogaea) infected with aflatoxin-producing (toxigenic) but not nonaflatoxin-producing (atoxigenic) strains of Aspergillus flavus. These proteins are involved in immune signaling and PAMP perception, DNA and RNA stabilization, induction of defense, innate immunity, hypersensitive response, biosynthesis of phytoalexins, cell wall responses, peptidoglycan assembly, penetration resistance, condensed tannin synthesis, detoxification, and metabolic regulation. Gene expression analysis confirmed the differential abundance of proteins in peanut cotyledons supplemented with aflatoxins, with or without infection with the atoxigenic strain. Similarly, peanut germination and A. flavus growth were altered in response to aflatoxin B1. These findings show an additional immunity initiated by aflatoxins. With the PAMP- and effector-triggered immune responses, this immunity constitutes the third immune response of the immune system in peanut cotyledon cells. The system is also a three-grade coevolution of plant-pathogen interaction.     

植物防御反应与动物免疫应答的比较及其对应性初探

动物经过数亿年的进化直到脊椎动物阶段出现了渐为完整的免疫系统。植物在与各种病原的共进化过程中亦发展了自身的防御系统。随着对植物抗病性的概念及植物防御机制的不断认识,人们发现它与动物的免疫应答有着众多的对应性。这些对应性是否表明植物的防御系统与动物的免疫系统在进化上具有同一性,是否表明它们在防御反应上具有类似的机制值得深思 。     

Cross-fostering reveals an effect of spleen size and nest temperatures on immune responses in nestling European starlings

Immunocompetence may be a good measure of offspring quality, however, factors affecting variation in immune responses are not clear. Research suggests that immune function can vary due to differences in genetics, development conditions and individual quality. Here, I examined factors affecting variation in immune response among nestling European starlings through a split-nest cross-fostering brood manipulation that included two important covariates: spleen size and nest temperatures. Immunocompetence was assessed via a cell-mediated immune response to phytohaemagglutinin (PHA). This paper provides the first direct evidence that individuals with large spleens also mount strong immune responses. Exposure to PHA did not cause splenomegaly, as there was no difference in spleen size between control birds and those injected with PHA. Offspring immune function was affected by common origin and by rearing environment, though rearing environment appeared to exert its influence only through nest temperatures. A comparison of the immune performance of siblings reared in their home nest versus those reared in other nests revealed a strong effect of maternal quality. As the difference in natal clutch size increased, the magnitude of the difference in immune performance between home-reared nestlings versus out-reared nestlings increased. Overall, nestling immune function appears to be determined by the combination of genetic, maternal and environmental effects.     

Toxoplasma gondii: cold stress-induced modulation of antibody responses.

Physical or psychological stressors have been shown to have significant consequences in the immune function and the outcome of disease in human and animal models. Recent work has demonstrated that products released during stress, such as glucocorticoids and catecholamines, can profoundly influence the in vitro growth of pathogens by modulating immune responses. The present study examined the effects of a physical stressor (cold stress) on antigens of Toxoplasma gondii that elicits an antibody-mediated immune response during the acute and chronic phases of infection. Sera obtained from different groups of mice subjected to cold stress during the acute and chronic phases of T. gondii infection were used to measure the levels of antibodies and to localize by Western blot the dominant antigens eliciting IgG and IgM antibody responses. Serum antibodies collected from stressed and infected mice recognized antigens different from those recognized by infected mice without stress. During the acute phase, a stronger IgM antibody response against antigens of 30, 42, 54, and 60 kDa was detected in stressed animals at 3 weeks postinfection. In addition, a 5-kDa antigen was specifically detected in mice subjected to stress during the acute and chronic phases of infection. Levels of specific IgG were increased in infected and in infected and stressed animals that underwent stress in the chronic phase. IgM production did not increase following cold stress in the chronic phase. These results suggest that the strong antibody response in stressed animals is associated with longer parasite persistence in circulation. Stress modulated not only the host immune response but also the ability of parasite antigens to elicit specific antibody responses by the host.