When T-helper cells don't help: immunopathology during concomitant infection

Disease directly caused by immune system action is known as immunopathology. Many factors may lead the immune system to cause rather than cure disease, and autoimmune, allergic, and infection-related immunopathological diseases affect millions of people worldwide. This review presents an analysis of T-helper cell mediated, infection-related immunopathology within the framework of evolutionary ecology. A proximate cause of infection-related immunopathology is an error in the type of T-helper response induced. Distinct subsets of T-helper cells enable different effector mechanisms and therefore work optimally against different types of parasites (e.g., extracellular versus intracellular parasites). Immune responses that cure rather than cause disease require that the T-helper subset be tailored to the parasite. It is thus critical for the immunophenotype to match the "environment" of the parasitic infection. As in other cases of adaptive plasticity, a mismatch between an organism's phenotype and the selective environment can decrease fitness. T-helper response induction may be confounded by coinfection of a single host by multiple parasite species. Because of normally adaptive feedback loops that lend to polarize T-helper responses, it can become impossible for the immune system to mount effective, conflicting responses concurrently. Immunophenotype-environment mismatches may thus be inevitable when simultaneous, conflicting immune responses are required. An ultimate cause of infection-related immunopathology in a multiparasite selection regime is the T-helper response polarization that can propagate response errors and constrain the ability of the immune system to resolve conflicting response requirements. A case study is used to illustrate how coinfection can exacerbate immunopathology and to frame testable predictions about optimal responses to coinfection (e.g., is the observed joint response to coinfection accurately predicted by the average of the component single-infection optimal responses, where the single-infection optima are weighted by the contribution of each to fitness). The case study includes immunological and pathological data from mice infected by Schistosoma mansoni alone and by S. mansoni in combination with Toxoplasma gondii. Such data can inform hypothesis tests of evolutionary ecological principles, and ecological analysis can in turn clarify assumptions about responses to coinfection for a greater understanding of the immune system. The synthesis of evolutionary ecology and immunology could therefore be of mutual benefit to the two disciplines.     

Diurnal variation around an optimum and near‐critically high temperature does not alter the performance of an ectothermic aquatic grazer

The growing threat of global climate change has led to a profusion of studies examining the effects of warming on biota. Despite the potential importance of natural variability such as diurnal temperature fluctuations, most experimental studies on warming are conducted under stable temperatures. Here, we investigated whether the responses of an aquatic invertebrate grazer (Lymnaea stagnalis) to an increased average temperature differ when the thermal regime is either constant or fluctuates diurnally. Using thermal response curves for several life‐history and immune defense traits, we first identified the optimum and near‐critically high temperatures that Lymnaea potentially experience during summer heat waves. We then exposed individuals that originated from three different populations to these two temperatures under constant or fluctuating thermal conditions. After 7 days, we assessed growth, reproduction, and two immune parameters (phenoloxidase‐like activity and antibacterial activity of hemolymph) from each individual. Exposure to the near‐critically high temperature led to increased growth rates and decreased antibacterial activity of hemolymph compared to the optimum temperature, whilst temperature fluctuations had no effect on these traits. The results indicate that the temperature level per se, rather than the variability in temperature was the main driver altering trait responses in our study species. Forecasting responses in temperature‐related responses remains challenging, due to system‐specific properties that can include intraspecific variation. However, our study indicates that experiments examining the effects of warming using constant temperatures can give similar predictions as studies with fluctuating thermal dynamics, and may thus be useful indicators of responses in nature.     

Sexual comparisons in immune ability, survival and parasite intensity in two damselfly species

Recent evolutionary studies have suggested that females have a more robust immune system than males. Using two damselfly species (Hetaerina americana and Argia tezpi), we tested if females produced higher immune responses (as phenoloxidase and hydrolytic enzymes), had a higher survival (using a nylon implant inserted in the abdomen and measuring survival after 24h) and fewer parasites (gregarines and water mites) than males. We also tested whether immune differences should emerge in different body areas (thorax vs. abdomen) within each sex with the prediction that only females will differ with the abdomen having a higher immune response than their thorax since the former area, for ecological and physiological reasons, may be a target zone for increased immune investment. Animals were adults of approximately the same age. In both species, females were more immunocompetent than males, but only in H. americana females were immune responses greater in the abdomen than in the thorax. However, there were no differences in survival and parasite intensity or the probability of being parasitised between the sexes in either of the two species. Thus, this study lends partial support to the principle that females are better at defending than males despite the null difference in parasitism and survival.     

Measuring the immune system of the three‐spined stickleback – investigating natural variation by quantifying immune expression in the laboratory and the wild

Current understanding of the immune system comes primarily from laboratory‐based studies. There has been substantial interest in examining how it functions in the wild, but studies have been limited by a lack of appropriate assays and study species. The three‐spined stickleback (Gasterosteus aculeatus L.) provides an ideal system in which to advance the study of wild immunology, but requires the development of suitable immune assays. We demonstrate that meaningful variation in the immune response of stickleback can be measured using real‐time PCR to quantify the expression of eight genes, representing the innate response and Th1‐, Th2‐ and Treg‐type adaptive responses. Assays are validated by comparing the immune expression profiles of wild and laboratory‐raised stickleback, and by examining variation across populations on North Uist, Scotland. We also compare the immune response potential of laboratory‐raised individuals from two Icelandic populations by stimulating cells in culture. Immune profiles of wild fish differed from laboratory‐raised fish from the same parental population, with immune expression patterns in the wild converging relative to those in the laboratory. Innate measures differed between wild populations, whilst the adaptive response was associated with variation in age, relative size of fish, reproductive status and S. solidus infection levels. Laboratory‐raised individuals from different populations showed markedly different innate immune response potential. The ability to combine studies in the laboratory and in the wild underlines the potential of this toolkit to advance our understanding of the ecological and evolutionary relevance of immune system variation in a natural setting.     

Loss/retention and evolution of NBS-encoding genes upon whole genome triplication of Brassica rapa

A genome triplication took place in the ancestor of Brassiceae species after the split of the Arabidopsis lineage. The postfragmentation and shuffling of the genome turned the ancestral hexaploid back to diploids and caused the radiation of Brassiceae species. The course of speciation was accompanied by the loss of duplicate genes and also influenced the evolution of retained genes. Of all the genes, those encoding NBS domains are typical R genes that confer resistance to invading pathogens. In this study, using the genome of Arabidopsis thaliana as a reference, we examined the loss/retention of orthologous NBS-encoding loci in the tripled Brassica rapa genome and discovered differential loss/retention frequencies. Further analysis indicated that loci of different retention ratios showed different evolutionary patterns. The loci of classesII and III (maintaining two and three syntenic loci, respectively, multi-loci) show sharper expansions by tandem duplications, have faster evolutionary rates and have more potential to be associated with novel gene functions. On the other hand, the loci that are retained at the minimal rate (keeping only one locus, class I, single locus) showed opposite patterns. Phylogenetic analysis indicated that recombination and translocation events were common among multi-loci in B. rapa, and differential evolutionary patterns between multi- and single-loci are likely the consequence of recombination. Investigations towards other gene families demonstrated different evolutionary characteristics between different gene families. The evolution of genes is more likely determined by the property of each gene family, and the whole genome triplication provided only a specific condition.     

Innate versus adaptive immunity in sticklebacks: evidence for trade-offs from a selection experiment

In vertebrates, the immune system consists of two arms of different characteristics: the innate and the acquired immune response. Parasites that are only shortly exposed to the immune system are most efficiently attacked by fast, constitutive innate immune mechanisms. Here, we experimentally selected within four fish families for high innate resistance versus susceptibility of three-spined sticklebacks (Gasterosteus aculeatus) against infection with the eye-fluke (Diplostomum pseudospathacaeum), a parasite whose metacercariae are protected from the immune system within the eye lens. We predicted that in families with high susceptibility, the adaptive immune system would be upregulated when challenged with infection. In accordance, we found that MHC class IIB expression is increased by approximately 50% in those lines selected for higher parasite load (i.e. low innate response). This suggests extensive genetic correlations between innate and adaptive immune system and/or crosstalk between both lines of defense. An efficient, specific innate immune response might reduce overall activation of the immune system and potentially alleviate associated effects of immunopathology.     

Conflicting selection alters the trajectory of molecular evolution in a tripartite bacteria–plasmid–phage interaction

Bacteria engage in a complex network of ecological interactions, which includes mobile genetic elements (MGEs) such as phages and plasmids. These elements play a key role in microbial communities as vectors of horizontal gene transfer but can also be important sources of selection for their bacterial hosts. In natural communities, bacteria are likely to encounter multiple MGEs simultaneously and conflicting selection among MGEs could alter the bacterial evolutionary response to each MGE. Here, we test the effect of interactions with multiple MGEs on bacterial molecular evolution in the tripartite interaction between the bacterium, Pseudomonas fluorescens, the lytic bacteriophage, SBW25φ2, and conjugative plasmid, pQBR103, using genome sequencing of experimentally evolved bacteria. We show that individually, both plasmids and phages impose selection leading to bacterial evolutionary responses that are distinct from bacterial populations evolving without MGEs, but that together, plasmids and phages impose conflicting selection on bacteria, constraining the evolutionary responses observed in pairwise interactions. Our findings highlight the likely difficulties of predicting evolutionary responses to multiple selective pressures from the observed evolutionary responses to each selective pressure alone. Understanding evolution in complex microbial communities comprising many species and MGEs will require that we go beyond studies of pairwise interactions.     

TNF superfamily member, TL1A, is a potential mucosal vaccine adjuvant

The identification of cytokine adjuvants capable of inducing an efficient mucosal immune response against viral pathogens has been long anticipated. Here, we attempted to identify the potential of tumor necrosis factor superfamily (TNFS) cytokines to function as mucosal vaccine adjuvants. Sixteen different TNFS cytokines were used to screen mucosal vaccine adjuvants, after which their immune responses were compared. Among the TNFS cytokines, intranasal immunization with OVA plus APRIL, TL1A, and TNF-α exhibited stronger immune response than those immunized with OVA alone. TL1A induced the strongest immune response and augmented OVA-specific IgG and IgA responses in serum and mucosal compartments, respectively. The OVA-specific immune response of TL1A was characterized by high levels of serum IgG1 and increased production of IL-4 and IL-5 from splenocytes of immunized mice, suggesting that TL1A might induce Th2-type responses. These findings indicate that TL1A has the most potential as a mucosal adjuvant among the TNFS cytokines.     

Immune-challenged house wren broods differ in the relative strengths of their responses among different axes of the immune system

Single components of the immune system are widely used to assess immune function in free-living vertebrates. However, as different immunological components are triggered by different types of threats and may be regulated independently, there is little reason to assume that they should respond similarly if challenged. We investigated whether three commonly assayed immune responses, cutaneous immune activity (phytohaemagglutinin assay), antibody response (tetanus toxoid immunization), and plasma bactericidal activity (Escherichia coli killing) are positively related in nestling house wrens (Troglodytes aedon). Multivariate analysis revealed significant differences in overall immune responsiveness among broods (i.e. nests), primarily attributable to differences in plasma bactericidal activity. Among broods, humoral immune response was negatively related to cutaneous immune activity and positively related to plasma bactericidal activity. We found no significant relationships among these measures of immunity among individual nestlings within broods. Our results suggest that different broods (i.e. families) invest differentially in the various branches of the immune system. Further study is needed to characterize the roles of maternal, genetic and environmental effects on the expression of this physiological bias.     

Divergence in Olfactory Host Plant Preference in D. mojavensis in Response to Cactus Host Use

Divergence in host adaptive traits has been well studied from an ecological and evolutionary perspective, but identification of the proximate mechanisms underlying such divergence is less well understood. Behavioral preferences for host plants are often mediated by olfaction and shifts in preference may be accompanied by changes in the olfactory system. In this study, we examine the evolution of host plant preferences in cactophilic Drosophila mojavensis that feeds and breeds on different cacti throughout its range. We show divergence in electrophysiological responses and olfactory behavior among populations with host plant shifts. Specifically, significant divergence was observed in the Mojave Desert population that specializes on barrel cactus. Differences were observed in electrophysiological responses of the olfactory organs and in behavioral responses to barrel cactus volatiles. Together our results suggest that the peripheral nervous system has changed in response to different ecological environments and that these changes likely contribute to divergence among D. mojavensis populations.     

The Evolution of the Major Hepatitis C Genotypes Correlates with Clinical Response to Interferon Therapy

Background

Patients chronically infected with hepatitis C virus (HCV) require significantly different durations of therapy and achieve substantially different sustained virologic response rates to interferon-based therapies, depending on the HCV genotype with which they are infected. There currently exists no systematic framework that explains these genotype-specific response rates. Since humans are the only known natural hosts for HCV–a virus that is at least hundreds of years old–one possibility is that over the time frame of this relationship, HCV accumulated adaptive mutations that confer increasing resistance to the human immune system. Given that interferon therapy functions by triggering an immune response, we hypothesized that clinical response rates are a reflection of viral evolutionary adaptations to the immune system.

Methods and Findings

We have performed the first phylogenetic analysis to include all available full-length HCV genomic sequences (n = 345). This resulted in a new cladogram of HCV. This tree establishes for the first time the relative evolutionary ages of the major HCV genotypes. The outcome data from prospective clinical trials that studied interferon and ribavirin therapy was then mapped onto this new tree. This mapping revealed a correlation between genotype-specific responses to therapy and respective genotype age. This correlation allows us to predict that genotypes 5 and 6, for which there currently are no published prospective trials, will likely have intermediate response rates, similar to genotype 3. Ancestral protein sequence reconstruction was also performed, which identified the HCV proteins E2 and NS5A as potential determinants of genotype-specific clinical outcome. Biochemical studies have independently identified these same two proteins as having genotype-specific abilities to inhibit the innate immune factor double-stranded RNA-dependent protein kinase (PKR).

Conclusion

An evolutionary analysis of all available HCV genomes supports the hypothesis that immune selection was a significant driving force in the divergence of the major HCV genotypes and that viral factors that acquired the ability to inhibit the immune response may play a role in determining genotype-specific response rates to interferon therapy.     

Identification of immune-based prostate cancer subtypes using mRNA expression

Immune infiltration in Prostate Cancer (PCa) was reported to be strongly associated with clinical outcomes. However, previous research could not elucidate the diversity of different immune cell types that contribute to the functioning of the immune response system. In the present study, the CIBERSORT method was employed to evaluate the relative proportions of immune cell profiling in PCa samples, adjacent tumor samples and normal samples. Three types of molecular classification were identified in tumor samples using the ‘CancerSubtypes’ package of the R software. Each subtype had specific molecular and clinical characteristics. In addition, functional enrichment was analyzed in each subtype. The submap and Tumor Immune Dysfunction and Exclusion (TIDE) algorithms were also used to predict clinical response to the immune checkpoint blockade. Moreover, the Genomics of Drug Sensitivity in Cancer (GDSC) database was employed to screen for potential chemotherapeutic targets for the treatment of PCa. The results showed that Cluster I was associated with advanced PCa and was more likely to respond to immunotherapy. The findings demonstrated that differences in immune responses may be important drivers of PCa progression and response to treatment. Therefore, this comprehensive assessment of the 22 immune cell types in the PCa Tumor Environment (TEM) provides insights on the mechanisms of tumor response to immunotherapy and may help clinicians explore the development of new drugs.     

Evolutionary histories of expanded peptidase families in Schistosoma mansoni

Schistosoma mansoni is one of the three main causative agents of human schistosomiasis, a major health problem with a vast socio-economic impact. Recent advances in the proteomic analysis of schistosomes have revealed that peptidases are the main virulence factors involved in the pathogenesis of this disease. In this context, evolutionary studies can be applied to identify peptidase families that have been expanded in genomes over time in response to different selection pressures. Using a phylogenomic approach, we searched for expanded endopeptidase families in the S. mansoni predicted proteome with the aim of contributing to the knowledge of such enzymes as potential therapeutic targets. We found three endopeptidase families that comprise leishmanolysins (metallopeptidase M8 family), cercarial elastases (serine peptidase S1 family) and cathepsin D proteins (aspartic peptidase A1 family). Our results suggest that the Schistosoma members of these families originated from successive gene duplication events in the parasite lineage after its diversification from other metazoans. Overall, critical residues are conserved among the duplicated genes/proteins. Furthermore, each protein family displays a distinct evolutionary history. Altogether, this work provides an evolutionary view of three S. mansoni peptidase families, which allows for a deeper understanding of the genomic complexity and lineage-specific adaptations potentially related to the parasitic lifestyle.     

Back-and-forth hermaphroditism: phylogenetic context of reproductive system evolution in subdioecious Daphne laureola

Recent phylogenetic analyses of sexual reproductive systems supported the evolutionary pathway from hermaphroditism to dioecy via gynodioecy in different groups of angiosperms. In this study, we explore the evolution of sexual reproductive systems in Daphne laureola L. (Thymelaeaceae), a species with variation in reproductive system among population. Sequences from the ITS region of the nuclear ribosomal cistron and two plastid markers (psbA-trnH and ndhF) were analyzed and used to map the population reproductive system along the molecular phylogeny. Our results support D. laureola as a monophyletic lineage with three different clades within the Iberian Peninsula. The hermaphroditic populations belong to two different clades, whereas gynodioecy is ubiquitous but characteristic of the third clade, which grouped together all the North-Western Iberian populations sampled, including the apparently oldest haplotype sampled. Gynodioecy appears as the most likely basal condition of the 13 analyzed populations, but different evolutionary transitions in reproductive sexual system were traced within each D. laureola clade. Both ecological conditions and (meta)population dynamics may help explain plant reproductive system evolution at the microevolutionary scale. Phylogenetic studies in which the historical relationships between populations differing in reproductive system can be ascertained will help to clarify the process.     

Microevolutionary selection dynamics acting on immune genes of the green‐veined white butterfly,Pieris napi

Insects rely on their innate immune system to successfully mediate complex interactions with their microbiota, as well as the microbes present in the environment. Previous work has shown that components of the canonical immune gene repertoire evolve rapidly and have evolutionary characteristics originating from interactions with fast‐evolving microorganisms. Although these interactions are likely to vary among populations, there is a poor understanding of the microevolutionary dynamics of immune genes, especially in non‐Dipteran insects. Here, we use the full set of canonical insect immune genes to investigate microevolutionary dynamics acting on these genes between and among populations by comparing three allopatric populations of the green‐veined white butterfly, Pieris napi (Linné; Lepidoptera, Pieridae). Immune genes showed increased genetic diversity compared to genes from the rest of the genome and various functional categories exhibited different types of signatures of selection, at different evolutionary scales, presenting a complex pattern of selection dynamics. Signatures of balancing selection were identified in 10 genes, and 17 genes appear to be under positive selection. Genes involved with the cellular arm of the immune response as well as the Toll pathway appear to be enriched among our outlier loci, regardless of functional category. This suggests that the targets of selection might focus upon an entire pathway, rather than functional subsets across pathways. Our microevolutionary results are similar to previously observed macroevolutionary patterns from diverse taxa, suggesting that either the immune system is robust to dramatic differences in life history and microbial communities, or that diverse microbes exert similar selection pressures.     

Diversification of MIF immune regulators in aphids: link with agonistic and antagonistic interactions

Background

The widespread use of genome sequencing provided evidences for the high degree of conservation in innate immunity signalling pathways across animal phyla. However, the functioning and evolutionary history of immune-related genes remains unknown for most invertebrate species. A striking observation coming from the analysis of the pea aphid Acyrthosiphon pisum genome is the absence of important conserved genes known to be involved in the antimicrobial responses of other insects. This reduction in antibacterial immune defences is thought to be related to their long-term association with beneficial symbiotic bacteria and to facilitate symbiont maintenance. An additional possibility to avoid elimination of mutualistic symbionts is a fine-tuning of the host immune response. To explore this hypothesis we investigated the existence and potential involvement of immune regulators in aphid agonistic and antagonistic interactions.

Results

In contrast to the limited antibacterial arsenal, we showed that the pea aphid Acyrthosiphon pisum expresses 5 members of Macrophage Migration Inhibitory Factors (ApMIF), known to be key regulators of the innate immune response. In silico searches for MIF members in insect genomes followed by phylogenetic reconstruction suggest that evolution of MIF genes in hemipteran species has been shaped both by differential losses and serial duplications, raising the question of the functional importance of these genes in aphid immune responses. Expression analyses of ApMIFs revealed reduced expression levels in the presence, or during the establishment of secondary symbionts. By contrast, ApMIFs expression levels significantly increased upon challenge with a parasitoid or a Gram-negative bacteria. This increased expression in the presence of a pathogen/parasitoid was reduced or missing, in the presence of facultative symbiotic bacteria.

Conclusions

This work provides evidence that while aphid’s antibacterial arsenal is reduced, other immune genes widely absent from insect genomes are present, diversified and differentially regulated during antagonistic or agonistic interactions.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-762) contains supplementary material, which is available to authorized users.     

Selection for brain size impairs innate,but not adaptive immune responses

Both the brain and the immune system are energetically demanding organs, and when natural selection favours increased investment into one, then the size or performance of the other should be reduced. While comparative analyses have attempted to test this potential evolutionary trade-off, the results remain inconclusive. To test this hypothesis, we compared the tissue graft rejection (an assay for measuring innate and acquired immune responses) in guppies (Poecilia reticulata) artificially selected for large and small relative brain size. Individual scales were transplanted between pairs of fish, creating reciprocal allografts, and the rejection reaction was scored over 8 days (before acquired immunity develops). Acquired immune responses were tested two weeks later, when the same pairs of fish received a second set of allografts and were scored again. Compared with large-brained animals, small-brained animals of both sexes mounted a significantly stronger rejection response to the first allograft. The rejection response to the second set of allografts did not differ between large- and small-brained fish. Our results show that selection for large brain size reduced innate immune responses to an allograft, which supports the hypothesis that there is a selective trade-off between investing into brain size and innate immunity.     

Variation and covariation among life-history traits in Rumex acetosella from a successional old-field gradient

In this study morphological variation and the potential for competition to affect biomass and seedling selection of the families of five populations of Rumex acetosella L. sampled along a successional old-field gradient have been investigated. Seeds from 25 families were submitted to four competitive regimes: no competition (one plant per pot), medium competition (two plants/ pot taking plants from the same population), high within-population competition (four individuals from the same population in a pot) and high between-population competition (four individuals from two different populations in a pot). Eight traits were analysed after 3 months of growth for variation among families within populations. A significant difference among families within the two older populations was recorded for sexual biomass and related components. High sensitivity of these traits to density was observed in all populations except the youngest, suggesting specialization to particular environmental conditions in late successional populations, and a good adaptive capacity to buffer environmental variation in the pioneer population. Little significant interaction between competitive regimes and families within populations was found, i.e. genotypes within each population showed little variation in their response to environmental variation. Genotypic variance decreased with increasing competitive conditions for the majority of the traits. However, the percentage of variance in sexual reproduction explained by family was stable among treatments. Tradeoffs between vegetative reproduction and sexual reproduction were recorded at the population level along the successional gradient, with increasing competitive conditions. As succession proceeds, we observed a decrease in sexual reproduction and an increase in vegetative reproduction. At the family level, correlation among traits were similar when plants were grown in the absence of competition and at high density, with a significant negative correlation between sexual reproduction and vegetative reproduction. For both sprout number and sexual biomass, the performance of families grown under all the treatments was positively correlated. Together these results indicate allocational constraints on the reproductive biology of R. acetosella that may be favoured by natural selection and have influenced population differentiation along the successional gradient. However, they also revealed that the potential exists for evolutionary specialization through plasticity, in response to variation in environmental conditions.