Experimental evolution of immunological specificity

The plasticity of immune memory in invertebrates

Whether specific immune protection after initial pathogen exposure (immune memory) occurs in invertebrates has long been uncertain. The absence of antibodies, B-cells and T-cells, and the short lifespans of invertebrates led to the hypothesis that immune memory does not occur in these organisms. However, research in the past two decades has supported the existence of immune memory in several invertebrate groups, including Ctenophora, Cnidaria, Nematoda, Mollusca and Arthropoda. Interestingly, some studies have demonstrated immune memory that is specific to the parasite strain. Nonetheless, other work does not provide support for immune memory in invertebrates or offers only partial support. Moreover, the expected biphasic immune response, a characteristic of adaptive immune memory in vertebrates, varies within and between invertebrate species. This variation may be attributed to the influence of biotic or abiotic factors, particularly parasites, on the outcome of immune memory. Despite its critical importance for survival, the role of phenotypic plasticity in immune memory has not been systematically examined in the past two decades. Additionally, the features of immune responses occurring in diverse environments have yet to be fully characterized.

Immune priming modulates Galleria mellonella and Pseudomonas entomophila interaction. Antimicrobial properties of Kazal peptide Pr13a

Galleria mellonella larvae repeatedly infected with Pseudomonas entomophila bacteria re-induced their immune response. Its parameters, i.e. the defence activities of cell-free hemolymph, the presence and activity of antimicrobial peptides, and the expression of immune-relevant genes were modulated after the re-challenge in comparison to non-primed infected larvae, resulting in better protection. No enhanced resistance was observed when the larvae were initially infected with other microorganisms, and larvae pre-infected with P. entomophila were not more resistant to further infection with other pathogens. Then, the peptide profiles of hemolymph from primed- and non-primed larvae infected with P. entomophila were compared by quantitative RP-HPLC (Reverse Phase - High Performance Liquid Chromatography). The level of carbonic anhydrase, anionic peptide-1, proline peptide-2, and finally, unknown so far, putative Kazal peptide Pr13a was higher in the primed infected animals than in the larvae infected with P. entomophila for the first time. The expression of the Pr13a gene increased two-fold after the infection, but only in the primed animals. To check whether the enhanced level of Pr13a could have physiological significance, the peptide was purified to homogeneity and checked for its defence properties. In fact, it had antibacterial activity: at the concentration of 15 µM and 7.5 µM it reduced the number of P. entomophila and Bacillus thuringiensis CFU, respectively, to about 40%. The antibacterial activity of Pr13a was correlated with changes observed on the surface of the peptide-treated bacteria, e.g. surface roughness and adhesion force. The presented results bring us closer to finding hemolymph constituents responsible for the effect of priming on the immune response in re-infected insects.

Disentangling specific and unspecific components of innate immune memory in a copepod-tapeworm system

Evidence that the innate immune system can respond with forms of memory upon reinfection has been accumulating over the past few years. These phenomena of "immune priming" in invertebrates, and "trained immunity" in vertebrates, are contrary to previous belief that immune memory and specificity are restricted to the adaptive immune system. However, while trained immunity is usually a response with rather low specificity, immune priming has shown highly specific responses in certain species. To date, it is largely unknown how specificity in innate immune memory can be achieved in response to different parasite types. Here, we revisited a system where an exceptionally high degree of innate immune specificity had been demonstrated for the first time, consisting of the copepod Macrocyclops albidus and its natural parasite, the tapeworm Schistocephalus solidus. Using homologous (same family) vs. heterologous (different family) priming-challenge experiments, we first confirm that copepods exposed to the same parasite family benefit from reduced secondary infections. We further focused on exposed-but-not-infected copepods in primary exposure to employ a transcriptomic approach, distinguishing between immunity that was either specific or unspecific regarding the discrimination between tapeworm types. A weighted gene co-expression network (WGCN) revealed differences between specific and unspecific immunity; while both involved histone modification regulation, specific immunity involved gene-splicing factors, whereas unspecific immunity was primarily involved in metabolic shift. We found a functional enrichment in spliceosome in specific immunity, whereas oxidative phosphorylation and carbon metabolism were enriched in unspecific immunity. Our findings allow discrimination of specific and unspecific components of an innate immune memory, based on gene expression networks, and deepen our understanding of basic aspects of immune systems.

Alteration of diet microbiota limits the experimentally evolved immune priming response in flour beetles, but not pathogen resistance

Host-associated microbiota play a fundamental role in the training and induction of different forms of immunity, including inducible as well as constitutive components. However, direct experiments analysing the relative importance of microbiota on diverse forms of evolved immune functions are missing. We addressed this gap by using experimentally evolved lines of Tribolium castaneum that either produced inducible immune memory-like responses (immune priming) or constitutively expressed basal resistance (without priming), as divergent counterstrategies against Bacillus thuringiensis infection. We altered the microbial communities present in the diet (i.e. wheat flour) of these evolved lines using UV irradiation and estimated the impact on the beetle's ability to mount a priming response versus basal resistance. Populations that had evolved immune priming lost the ability to mount a priming response upon alteration of diet microbiota. Microbiota manipulation also caused a drastic reduction in their reproductive output and post-infection longevity. In contrast, in pathogen-resistant beetles, microbiota manipulation did not affect post-infection survival or reproduction. The divergent evolution of immune responses across beetle lines was thus associated with divergent reliance on the microbiome. Whether the latter is a direct outcome of differential pathogen exposure during selection or reflects evolved immune functions remains unclear. We hope that our results will motivate further experiments to understand the mechanistic basis of these complex evolutionary associations between microbiota, host immune strategies and fitness outcomes.

The First Nucleic Acid Strands May Have Grown on Peptides via Primeval Reverse Translation

The central dogma of molecular biology dictates that, with only a few exceptions, information proceeds from DNA to protein through an RNA intermediate. Examining the enigmatic steps from prebiotic to biological chemistry, we take another road suggesting that primordial peptides acted as template for the self-assembly of the first nucleic acids polymers. Arguing in favour of a sort of archaic "reverse translation" from proteins to RNA, our basic premise is a Hadean Earth where key biomolecules such as amino acids, polypeptides, purines, pyrimidines, nucleosides and nucleotides were available under different prebiotically plausible conditions, including meteorites delivery, shallow ponds and hydrothermal vents scenarios. Supporting a protein-first scenario alternative to the RNA world hypothesis, we propose the primeval occurrence of short two-dimensional peptides termed "selective amino acid- and nucleotide-matching oligopeptides" (henceforward SANMAOs) that noncovalently bind at the same time the polymerized amino acids and the single nucleotides dispersed in the prebiotic milieu. In this theoretical paper, we describe the chemical features of this hypothetical oligopeptide, its biological plausibility and its virtues from an evolutionary perspective. We provide a theoretical example of SANMAO's selective pairing between amino acids and nucleosides, simulating a poly-Glycine peptide that acts as a template to build a purinic chain corresponding to the glycine's extant triplet codon GGG. Further, we discuss how SANMAO might have endorsed the formation of low-fidelity RNA's polymerized strains, well before the appearance of the accurate genetic material's transmission ensured by the current translation apparatus.

Differential proteome profiling of bacterial culture supernatants reveals candidates for the induction of oral immune priming in the red flour beetle

Most organisms are host to symbionts and pathogens, which led to the evolution of immune strategies to prevent harm. Whilst the immune defences of vertebrates are classically divided into innate and adaptive, insects lack specialized cells involved in adaptive immunity, but have been shown to exhibit immune priming: the enhanced survival upon infection after a first exposure to the same pathogen or pathogen-derived components. An important piece of the puzzle are the pathogen-associated molecules that induce these immune priming responses. Here, we make use of the model system consisting of the red flour beetle (Tribolium castaneum) and its bacterial pathogen Bacillus thuringiensis, to compare the proteomes of culture supernatants of two closely related B. thuringiensis strains that either induce priming via the oral route, or not. Among the proteins that might be immunostimulatory to T. castaneum, we identify the Cry3Aa toxin, an important plasmid-encoded virulence factor of B. thuringiensis. In further priming-infection assays we test the relevance of Cry-carrying plasmids for immune priming. Our findings provide valuable insights for future studies to perform experiments on the mechanisms and evolution of immune priming.

Pathogen infection induces specific transgenerational modifications to gene expression and fitness in Caenorhabditis elegans

How pathogen infection in a parental generation affects response in future generations to the same pathogen via epigenetic modifications has been the topic of recent studies. These studies focused on changes attributed to transgenerational epigenetic inheritance and how these changes cause an observable difference in behavior or immune response in a population. However, we questioned if pathogen infection causes hidden epigenetic changes to fitness that are not observable at the population level. Using the nematode Caenorhabditis elegans as a model organism, we examined the generation-to-generation differences in survival of both an unexposed and primed lineage of animals against a human opportunistic pathogen Salmonella enterica. We discovered that training a lineage of C. elegans against a specific pathogen does not cause a significant change to overall survival, but rather narrows survival variability between generations. Quantification of gene expression revealed reduced variation of a specific member of the TFEB lipophagic pathway. We also provided the first report of a repeating pattern of survival times over the course of 12 generations in the control lineage of C. elegans. This repeating pattern indicates that the variability in survival between generations of the control lineage is not random but may be regulated by unknown mechanisms. Overall, our study indicates that pathogen infection can cause specific phenotypic changes due to epigenetic modifications, and a possible system of epigenetic regulation between generations.

Re-recognition of innate immune memory as an integrated multidimensional concept

In the past decade, the concept of immunological memory, which has long been considered a phenomenon observed in the adaptive immunity of vertebrates, has been extended to the innate immune system of various organisms. This de novo immunological memory is mainly called "innate immune memory", "immune priming", or "trained immunity" and has received increased attention because of its potential for clinical and agricultural applications. However, research on different species, especially invertebrates and vertebrates, has caused controversy regarding this concept. Here we discuss the current studies focusing on this immunological memory and summarize several mechanisms underlying it. We propose "innate immune memory" as a multidimensional concept as an integration between the seemingly different immunological phenomena.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

Immune Stimulation via Wounding Alters Chemical Profiles of Adult Tribolium castaneum

Group-living individuals experience immense risk of disease transmission and parasite infection. In social and in some non-social insects, disease control with immunomodulation arises not only via individual immune defenses, but also via infochemicals such as contact cues and (defensive) volatiles to mount a group-level immunity. However, little is known about whether activation of the immune system elicits changes in chemical phenotypes, which may mediate these responses. We here asked whether individual immune experience resulting from wounding or injection of heat-killed Bacillus thuringiensis (priming) leads to changes in the chemical profiles of female and male adult red flour beetles, Tribolium castaneum, which are non-social but gregarious. We analyzed insect extracts using GC-FID to study the chemical composition of (1) cuticular hydrocarbons (CHCs) as candidates for the transfer of immunity-related information between individuals via contact, and (2) stink gland secretions, with analysis of benzoquinones as main active compounds regulating 'external immunity'. Despite a pronounced sexual dimorphism in CHC profiles, wounding stimulation led to similar profile changes in males and females with increases in the proportion of methyl-branched alkanes compared to naïve beetles. While changes in the overall secretion profiles were less pronounced, absolute amounts of benzoquinones were transiently elevated in wounded compared to naïve females. Responses to priming were insignificant in CHCs and secretions. We suggest that changes in different infochemicals after wounding may mediate immune status signaling in the context of both internal and external immune responses in groups of this non-social insect, thus showing parallels to social immunity.

Oral Immune Priming Treatment Alters Microbiome Composition in the Red Flour Beetle Tribolium castaneum

It is now well-established that the microbiome is relevant for many of an organism’s properties and that its composition reacts dynamically to various conditions. The microbiome interacts with host immunity and can play important roles in the defenses against pathogens. In invertebrates, immune priming, that is, improved survival upon secondary exposure to a previously encountered pathogen, can be dependent upon the presence of the gut microbiome. However, it is currently unknown whether the microbiome changes upon priming treatment. We here addressed this question in a well-established model for immune priming, the red flour beetle Tribolium castaneum exposed to the entomopathogenic bacterium Bacillus thuringiensis (Bt). After priming treatments, the microbiota composition of beetle larvae was assessed by deep sequencing of the V1-V2 region of the bacterial 16S rRNA gene. We compared the effect of two established routes of priming treatments in this system: injection priming with heat-killed Bt and oral priming via ingestion of filtered sterilized bacterial spore culture supernatants. For oral priming, we used several strains of Bt known to vary in their ability to induce priming. Our study revealed changes in microbiome composition following the oral priming treatment with two different strains of Bt, only one of which (Bt tenebrionis, Btt) is known to lead to improved survival. In contrast, injection priming treatment with the same bacterial strain did not result in microbiome changes. Combined with the previous results indicating that oral priming with Btt depends on the larval microbiome, this suggests that certain members of the microbiome could be involved in forming an oral priming response in the red flour beetle.

Symbiont-mediated immune priming in animals through an evolutionary lens

Protective symbionts can defend hosts from parasites through several mechanisms, from direct interference to modulating host immunity, with subsequent effects on host and parasite fitness. While research on symbiont-mediated immune priming (SMIP) has focused on ecological impacts and agriculturally important organisms, the evolutionary implications of SMIP are less clear. Here, we review recent advances made in elucidating the ecological and molecular mechanisms by which SMIP occurs. We draw on current works to discuss the potential for this phenomenon to drive host, parasite, and symbiont evolution. We also suggest approaches that can be used to address questions regarding the impact of immune priming on host-microbe dynamics and population structures. Finally, due to the transient nature of some symbionts involved in SMIP, we discuss what it means to be a protective symbiont from ecological and evolutionary perspectives and how such interactions can affect long-term persistence of the symbiosis.

The emerging role of epigenetic mechanisms in insect defense against pathogens

Insects resist infection by natural selection that favors the survival and reproduction of the fittest phenotypes. Although the genetic mechanisms mediating the evolution of insect resistance have been investigated, little is known about the contribution of epigenetic mechanisms. Gene expression in response to a pathogen selection pressure is regulated by different mechanisms affecting chromatin plasticity. Whether transgenerational inheritance of genome-wide epigenetic marks contributes to the heritable manifestation of insect resistance is presently debated. Here, we review the latest works on the contributions of chromatin remodeling to insect immunity and adaptation to pathogens. We highlight DNA methylation, histone acetylation, and microRNAs in mediating the transgenerational inherited transcriptional reprogramming of defense-related gene expression and the evolution of insect resistance.

Innate immune memory in invertebrates: Concept and potential mechanisms

Invertebrates are the protagonists of a recent paradigm shift because they now show that vertebrates are not the only group with immune memory. This review discusses the concept of immune priming, its characteristics, and differences with trained immunity and immune enhancement. We include an update of the current status of immune priming within generations in different groups of invertebrates which now include work in 5 Phyla: Ctenophora, Cnidaria, Mollusca, Nematoda, and Arthropoda. Clearly, few Phyla have been studied. We also resume and discuss the effector mechanism related to immune memory, including integrating viral elements into the genome, endoreplication, and epigenetics. The roles of other elements are incorporated, such as hemocytes, immune pathways, and metabolisms. We conclude that taking care of the experimental procedure will discern if results provide or do not support the invertebrates' immune memory and that regarding mechanisms, indeed, there are no studies on the immune memory mechanisms, this is how specificity is reached, and how and where the immune memory is stored and how is recall upon subsequent encounters. Finally, we discuss the possibility of having more than one mechanism working in different groups of invertebrates depending on the environmental conditions.

Current understanding and perspectives on the potential mechanisms of immune priming in beetles

Beetles are the most diverse group of insects in Insecta which can be found in almost every habitat and environment on Earth. The possessing of the rapid and effective immune defenses is one of the important factors for their success. It is generally recognized that beetles only rely on the non-specific innate immune defense, without immunological memory, to fight against pathogens. However, there was cumulative evidence for the innate immune memory in invertebrates, including beetles, over the last decades, implying that insect innate immunity is more complex and has more features than previously thought. In beetles, it has been well documented that the specific or nonspecific enhanced immunocompetence can persist throughout development within generations and can even be transferred to the descendents in the next generation. Although insect immune priming might be shaped by epigenetic modifications and transferring effectors, mRNA and microbial signals, the solid experimental evidence to support the causal relationship between any of them and immune priming is still scarce. The combined usage of 'omics' approaches and CRISPR/Cas9 in the appropriate insect models with well-known genetic background, Tribolium castaneum and Tenebrio molitor, will help us to decipher the molecular mechanisms by which immune priming occurs in beetles in depth.

Current knowledge of immune priming in invertebrates, emphasizing studies on Tenebrio molitor

Vertebrates rely on the most sophisticated adaptive immunity to defend themselves against various pathogens. This includes immunologic memory cells, which mount a stronger and more effective immune response against an antigen after its first encounter. Unlike vertebrates, invertebrates' defense completely depends on the innate immunity mechanisms including humoral and cell-mediated immunity. Furthermore, the invertebrate equivalent of the memory cells was discovered only recently. Since the discovery of transgenerational immune priming (TGIP) in crustaceans, numerous findings have proven the IP in invertebrate classes such as insects. TGIP can be induced through maternal priming pathways such as transcriptional regulation of antimicrobial peptides, and also paternal IP including the induction of proPO system activity. We appraise the diversity and specificity of IP agents to provide sustained immunologic memory in insects, particularly T. molitor in the review. An understanding of IP (more so TGIP) response in T. molitor will deepen our knowledge of invertebrate immunity, and boost the mass-rearing industry by reducing pathogen infection rates.

Immunoglobulin-like receptors and the generation of innate immune memory

Host immunity is classically divided into "innate" and "adaptive." While the former has always been regarded as the first, rapid, and antigen-nonspecific reaction to invading pathogens, the latter represents the more sophisticated and antigen-specific response that has the potential to persist and generate memory. Recent work however has challenged this dogma, where murine studies have successfully demonstrated the ability of innate immune cells (monocytes and macrophages) to acquire antigen-specific memory to allogeneic major histocompatibility complex (MHC) molecules. The immunoreceptors so far identified that mediate innate immune memory are the paired immunoglobulin-like receptors (PIRs) in mice, which are orthologous to human leukocyte immunoglobulin-like receptors (LILRs). These receptor families are mainly expressed by the myelomonocytic cell lineage, suggesting an important role in the innate immune response. In this review, we will discuss the role of immunoglobulin-like receptors in the development of innate immune memory across species.

Serial passage in an insect host indicates genetic stability of the human probiotic Escherichia coli Nissle 1917

Background and objectives

The probiotic Escherichia coli strain Nissle 1917 (EcN) has been shown to effectively prevent and alleviate intestinal diseases. Despite the widespread medical application of EcN, we still lack basic knowledge about persistence and evolution of EcN outside the human body. Such knowledge is important also for public health aspects, as in contrast to abiotic therapeutics, probiotics are living organisms that have the potential to evolve. This study made use of experimental evolution of EcN in an insect host, the red flour beetle Tribolium castaneum, and its flour environment.

Methodology

Using a serial passage approach, we orally introduced EcN to larvae of T.castaneum as a new host, and also propagated it in the flour environment. After eight propagation cycles, we analyzed phenotypic attributes of the passaged replicate EcN lines, their effects on the host in the context of immunity and infection with the entomopathogen Bacillus thuringiensis, and potential genomic changes using WGS of three of the evolved lines.

Results

We observed weak phenotypic differences between the ancestral EcN and both, beetle and flour passaged EcN lines, in motility and growth at 30°C, but neither any genetic changes, nor the expected increased persistence of the beetle-passaged lines. One of these lines displayed distinct morphological and physiological characteristics.

Conclusions and implications

Our findings suggest that EcN remains rather stable during serial passage in an insect. Weak phenotypic changes in growth and motility combined with a lack of genetic changes indicate a certain degree of phenotypic plasticity of EcN.

Lay Summary

For studying adaptation of the human probiotic Escherichia coli strain Nissle 1917, we introduced it to a novel insect host system and its environment using a serial passage approach. After passage, we observed weak phenotypic changes in growth and motility but no mutations or changes in persistence inside the host.

Why do insects evolve immune priming? A search for crossroads

Until recently, it was assumed that insects lack immune memory since they do not have vertebrate-like specialized memory cells. Therefore, their most well studied evolutionary response against pathogens was increased basal immunity. However, growing evidence suggests that many insects also exhibit a form of immune memory (immune priming), where prior exposure to a low dose of infection confers protection against subsequent infection by the same pathogen that acts both within and across generations. Most strikingly, they can rapidly evolve as a highly parallel and mutually exclusive strategy from basal immunity, under different selective conditions and with divergent evolutionary trade-offs. However, the relative importance of priming as an optimal immune strategy also has contradictions, primarily because supporting mechanisms are still unclear. In this review, we adopt a comparative approach to highlight several emerging evolutionary, ecological and mechanistic features of priming vs basal immune responses that warrant immediate attention for future research.

The costs and benefits of basal infection resistance vs immune priming responses in an insect

In insects, basal pathogen resistance and immune priming can evolve as mutually exclusive strategies, with distinct infection outcomes. However, the evolutionary drivers of such diverse immune functions remain poorly understood. Here, we addressed this key issue by systematically analyzing the differential fitness costs and benefits of priming vs resistance evolution in Tribolium beetle populations infected with Bacillus thuringiensis. Surprisingly, resistant beetles had increased post-infection reproduction and a longer lifespan under both starving as well as fed conditions, with no other measurable costs. In contrast, priming reduced offspring early survival, development rate and reproduction. Priming did improve post-infection survival of offspring, but this added trans-generational benefit of immune priming might not compensate for its pervasive costs. Resistance was thus consistently more beneficial. Overall, our work demonstrates the evolutionary change in trans-generational priming response, and provides a detailed comparison of the complex fitness consequences of evolved priming vs resistance.

Mechanisms of transgenerational immune priming in insects

Parents invest in their offspring by preparing them for defense against pathogens and parasites that only the parents have encountered, a phenomenon known as transgenerational immune priming (TGIP). The priming effect can be passed maternally or paternally to the next generation, thus increasing the survival of offspring exposed to the same pathogen. The scope of the resulting immune response can be narrow (primarily targeting the triggering pathogen) or much more general, depending on the underlying mechanism. Maternal TGIP is often narrowly focused because the major mechanism is the transfer of microbes or fragments thereof, encountered by mothers at the larval stage, to the developing eggs along with the uptake of lipophorins and vitellogenins. This induces the expression of zygotic defense genes, including those encoding antimicrobial peptides (AMPs), comparable to the defenses observed in the larvae and adults. Maternal TGIP does not appear to involve the direct vertical transmission of immunity-related effectors such as AMPs (or the corresponding mRNAs) to the eggs. Parental investment in offspring is also mediated by epigenetic mechanisms such as DNA methylation, histone acetylation and microRNA expression, which can be imprinted on the gametes by either parent without changes in the DNA sequence. Epigenetic inheritance is the only known mechanism of paternal TGIP, and results in a more general fortification of the immune response. This review considers the mechanistic basis of TGIP, its role in evolutionary processes such as the establishment of resistance against pathogens, and the impact of pathogens and parasites on the epigenetic machinery of host insects.

Antibiotics accelerate growth at the expense of immunity

Antibiotics have long been used in the raising of animals for agricultural, industrial or laboratory use. The use of subtherapeutic doses in diets of terrestrial and aquatic animals to promote growth is common and highly debated. Despite their vast application in animal husbandry, knowledge about the mechanisms behind growth promotion is minimal, particularly at the molecular level. Evidence from evolutionary research shows that immunocompetence is resource-limited, and hence expected to trade off with other resource-demanding processes, such as growth. Here, we ask if accelerated growth caused by antibiotics can be explained by genome-wide trade-offs between growth and costly immunocompetence. We explored this idea by injecting broad-spectrum antibiotics into wood tiger moth (Arctia plantaginis) larvae during development. We follow several life-history traits and analyse gene expression (RNA-seq) and bacterial (r16S) profiles. Moths treated with antibiotics show a substantial depletion of bacterial taxa, faster growth rate, a significant downregulation of genes involved in immunity and significant upregulation of growth-related genes. These results suggest that the presence of antibiotics may aid in up-keeping the immune system. Hence, by reducing the resource load of this costly process, bodily resources may be reallocated to other key processes such as growth.

Current understanding of immune priming phenomena in insects

It may seem that the most important issues related to insect immunity have already been described. However, novel phenomena observed in recent years shed new light on the understanding of the immune response in insects.The adaptive abilities of insects helped them to populate all ecological land niches.One important adaptive ability of insects that facilitates their success is the plasticity of their immune system. Although they only have innate immune mechanisms, insects can increase their resistance after the first encounter with the pathogen. In recent years, this phenomenon,namedimmunepriming, has become a "hot topic" in immunobiology.Priming can occur within or across generations. In the first case, the resistance of a given individual can increase after surviving a previous infection. Transstadial immune priming occurs when infection takes place at one of the initial developmental stages and increased resistance is observed at the pupal or imago stages. Priming across generations (transgenerationalimmune priming, TGIP) relies on the increased resistance of the offspring when one or both parents are infected during their lifetime.Despite the attention that immune priming has received, basic questions remain to be answered, such as regulation of immune priming at the molecular level. Research indicates that pathogen recognition receptors (PRRs) can be involved in the priming phenomenon. Recent studies have highlighted the special role of microRNAs and epigenetics, which can influence expression of genes that can be transmitted through generations although they are not encoded in the nucleotide sequence. Considerable amounts of research are required to fully understand the mechanisms that regulate priming phenomena. The aim of our work is to analyse thoroughly the most important information on immune priming in insects and help raise pertinent questions such that a greater understanding of this phenomenon can be obtained in the future.

Trained Immunity in Anopheles gambiae: Antibacterial Immunity Is Enhanced by Priming via Sugar Meal Supplemented With a Single Gut Symbiotic Bacterial Strain

Mosquitoes have evolved an effective innate immune system. The mosquito gut accommodates various microbes, which play a crucial role in shaping the mosquito immune system during evolution. The resident bacteria in the gut microbiota play an essential role in priming basal immunity. In this study, we show that antibacterial immunity in Anopheles gambiae can be enhanced by priming via a sugar meal supplemented with bacteria. Serratia fonticola S1 and Enterobacter sp. Ag1 are gut bacteria in mosquitoes. The intrathoracic injection of the two bacteria can result in an acute hemocoelic infection in the naïve mosquitoes with mortality of ∼40% at 24 h post-infection. However, the Enterobacter orSerratia primed mosquitoes showed a better 24 h survival upon the bacterial challenge. The priming confers the protection with a certain degree of specificity, the Enterobacter primed mosquitoes had a better survival upon the Enterobacter but not Serratia challenge, and the Serratia primed mosquitoes had a better survival upon the Serratia but not Enterobacter challenge. To understand the priming-mediated immune enhancement, the transcriptomes were characterized in the mosquitoes of priming as well as priming plus challenges. The RNA-seq was conducted to profile 10 transcriptomes including three samples of priming conditions (native microbiota, Serratia priming, and Enterobacter priming), six samples of priming plus challenges with the two bacteria, and one sample of injury control. The three priming regimes resulted in distinctive transcriptomic profiles with about 60% of genes affected by both bacteria. Upon challenges, different primed mosquitoes displayed different transcriptomic patterns in response to different bacteria. When a primed cohort was challenged with a heterogenous bacterium, more responsive genes were observed than when challenged with a homogenous bacterium. As expected, many canonical immune genes were responsive to the priming and challenge, but much more non-immune genes with various functions were also responsive in the contexts, which implies that the prior priming triggers a delicately coordinated systemic regulation that results in an enhanced immunity against the subsequent challenge. Besides the participation of typical immune pathways, the transcriptome data suggest the involvement of lysosome and metabolism in the context. Overall, this study demonstrated a trained immunity via priming with bacteria in diet.

Insect Behavioral Change and the Potential Contributions of Neuroinflammation-A Call for Future Research

Many organisms are able to elicit behavioral change in other organisms. Examples include different microbes (e.g., viruses and fungi), parasites (e.g., hairworms and trematodes), and parasitoid wasps. In most cases, the mechanisms underlying host behavioral change remain relatively unclear. There is a growing body of literature linking alterations in immune signaling with neuron health, communication, and function; however, there is a paucity of data detailing the effects of altered neuroimmune signaling on insect neuron function and how glial cells may contribute toward neuron dysregulation. It is important to consider the potential impacts of altered neuroimmune communication on host behavior and reflect on its potential role as an important tool in the "neuro-engineer" toolkit. In this review, we examine what is known about the relationships between the insect immune and nervous systems. We highlight organisms that are able to influence insect behavior and discuss possible mechanisms of behavioral manipulation, including potentially dysregulated neuroimmune communication. We close by identifying opportunities for integrating research in insect innate immunity, glial cell physiology, and neurobiology in the investigation of behavioral manipulation.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

Parasite Presence Induces Gene Expression Changes in an Ant Host Related to Immunity and Longevity

Most species are either parasites or exploited by parasites, making parasite-host interactions a driver of evolution. Parasites with complex life cycles often evolve strategies to facilitate transmission to the definitive host by manipulating their intermediate host. Such manipulations could explain phenotypic changes in the ant Temnothorax nylanderi, the intermediate host of the cestode Anomotaenia brevis. In addition to behavioral and morphological alterations, infected workers exhibit prolonged lifespans, comparable to that of queens, which live up to two decades. We used transcriptomic data from cestodes and ants of different castes and infection status to investigate the molecular underpinnings of phenotypic alterations in infected workers and explored whether the extended lifespan of queens and infected workers has a common molecular basis. Infected workers and queens commonly upregulated only six genes, one of them with a known anti-aging function. Both groups overexpressed immune genes, although not the same ones. Our findings suggest that the lifespan extension of infected workers is not achieved via the expression of queen-specific genes. The analysis of the cestodes' transcriptome revealed dominant expression of genes of the mitochondrial respiratory transport chain, which indicates an active metabolism and shedding light on the physiology of the parasite in its cysticercoid stage.

β-Glucan-Induced Trained Immunity in Dogs

Several observations in the world of comparative immunology in plants, insects, fish and eventually mammals lead to the discovery of trained immunity in the early 2010's. The first demonstrations provided evidence that innate immune cells were capable of developing memory after a first encounter with some pathogens. Trained immunity in mammals was initially described in monocytes with the Bacille Calmette-Guerin vaccine (BCG) or prototypical agonists like β-glucans. This phenomenon relies on epigenetic and metabolic modifications leading to an enhanced secretion of inflammatory cytokines when the host encounters homologous or heterologous pathogens. The objective of our research was to investigate the trained immunity, well-described in mouse and human, in other species of veterinary importance. For this purpose, we adapted an in vitro model of trained innate immunity in dogs. Blood enriched monocytes were stimulated with β-glucans and we confirmed that it induced an increased production of pro-inflammatory and anti-microbial compounds in response to bacterial stimuli. These results constitute the first demonstration of trained immunity in dogs and confirm its signatures in other mammalian species, with an implication of cellular mechanisms similar to those described in mice and humans regarding cellular epigenetics and metabolic regulations.

Facilitative priority effects drive parasite assembly under coinfection

Host individuals are often coinfected with diverse parasite assemblages, resulting in complex interactions among parasites within hosts. Within hosts, priority effects occur when the infection sequence alters the outcome of interactions among parasites. Yet, the role of host immunity in this process remains poorly understood. We hypothesized that the host response to the first infection could generate priority effects among parasites, altering the assembly of later-arriving strains during epidemics. We tested this by infecting sentinel host genotypes of Plantago lanceolata with strains of the fungal parasite Podosphaera plantaginis and measuring susceptibility to subsequent infection during experimental and natural epidemics. In these experiments, prior infection by one strain often increased susceptibility to other strains, and these facilitative priority effects altered the structure of parasite assemblages, but this effect depended on host genotype, host population and parasite genotype. Thus, host genotype, spatial structure and priority effects among strains all independently altered parasite assembly. Using a fine-scale survey and sampling of infections on wild hosts in several populations, we then identified a signal of facilitative priority effects, which altered parasite assembly during natural epidemics. Together, these results provide evidence that within-host priority effects of early-arriving strains can drive parasite assembly, with implications for how strain diversity is spatially and temporally distributed during epidemics.

Humic-acid-driven escape from eye parasites revealed by RNA-seq and target-specific metabarcoding

Background

Next generation sequencing (NGS) technologies are extensively used to dissect the molecular mechanisms of host-parasite interactions in human pathogens. However, ecological studies have yet to fully exploit the power of NGS as a rich source for formulating and testing new hypotheses.

Methods

We studied Eurasian perch (Perca fluviatilis) and its eye parasite (Trematoda, Diplostomidae) communities in 14 lakes that differed in humic content in order to explore host-parasite-environment interactions. We hypothesised that high humic content along with low pH would decrease the abundance of the intermediate hosts (gastropods), thus limiting the occurrence of diplostomid parasites in humic lakes. This hypothesis was initially invoked by whole eye RNA-seq data analysis and subsequently tested using PCR-based detection and a novel targeted metabarcoding approach.

Results

Whole eye transcriptome results revealed overexpression of immune-related genes and the presence of eye parasite sequences in RNA-seq data obtained from perch living in clear-water lakes. Both PCR-based and targeted-metabarcoding approach showed that perch from humic lakes were completely free from diplostomid parasites, while the prevalence of eye flukes in clear-water lakes that contain low amounts of humic substances was close to 100%, with the majority of NGS reads assigned to Tylodelphys clavata.

Conclusions

High intraspecific diversity of T. clavata indicates that massively parallel sequencing of naturally pooled samples represents an efficient and powerful strategy for shedding light on cryptic diversity of eye parasites. Our results demonstrate that perch populations in clear-water lakes experience contrasting eye parasite pressure compared to those from humic lakes, which is reflected by prevalent differences in the expression of immune-related genes in the eye. This study highlights the utility of NGS to discover novel host-parasite-environment interactions and provide unprecedented power to characterize the molecular diversity of cryptic parasites.

Evolution of animal immunity in the light of beneficial symbioses

Immune system processes serve as the backbone of animal defences against pathogens and thus have evolved under strong selection and coevolutionary dynamics. Most microorganisms that animals encounter, however, are not harmful, and many are actually beneficial. Selection should act on hosts to maintain these associations while preventing exploitation of within-host resources. Here, we consider how several key aspects of beneficial symbiotic associations may shape host immune system evolution. When host immunity is used to regulate symbiont populations, there should be selection to evolve and maintain targeted immune responses that recognize symbionts and suppress but not eliminate symbiont populations. Associating with protective symbionts could relax selection on the maintenance of redundant host-derived immune responses. Alternatively, symbionts could facilitate the evolution of host immune responses if symbiont-conferred protection allows for persistence of host populations that can then adapt. The trajectory of immune system evolution will likely differ based on the type of immunity involved, the symbiont transmission mode and the costs and benefits of immune system function. Overall, the expected influence of beneficial symbiosis on immunity evolution depends on how the host immune system interacts with symbionts, with some interactions leading to constraints while others possibly relax selection on immune system maintenance. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Immune priming against bacteria in spiders and scorpions?

Empirical evidence of immune priming in arthropods keeps growing, both at the within- and trans-generational level. The evidence comes mostly from work on insects and it remains unclear for some other arthropods whether exposure to a non-lethal dose of a pathogen provides protection during a second exposure with a lethal dose. A poorly investigated group are arachnids, with regard to the benefits of immune priming measured as improved survival. Here, we investigated immune priming in two arachnids: the wolf spider Lycosa cerrofloresiana and the scorpion Centruroides granosus. We injected a third of the individuals with lipopolysaccharides of Escherichia coli (LPS, an immune elicitor), another third were injected with the control solution (PBS) and the other third were kept naive. Four days after the first inoculations, we challenged half of the individuals of each group with an injection of a high dose of E. coli and the other half was treated with the control solution. For scorpions, individuals that were initially injected with PBS or LPS did not differ in their survival rates against the bacterial challenge. Individuals injected with LPS showed higher survival than that of naive individuals as evidence of immune priming. Individuals injected with PBS tended to show higher survival rates than naive individuals, but the difference was not significant—perhaps suggesting a general immune upregulation caused by the wounding done by the needle. For spiders, we did not observe evidence of priming, the bacterial challenge reduced the survival of naive, PBS and LPS individuals at similar rates. Moreover; for scorpions, we performed antibacterial assays of hemolymph samples from the three priming treatments (LPS, PBS and naive) and found that the three treatments reduced bacterial growth but without differences among treatments. As non-model organisms, with some unique differences in their immunological mechanisms as compared to the most studied arthropods (insects), arachnids provide an unexplored field to elucidate the evolution of immune systems.

Adult Mosquitoes Infected with Bacteria Early in Life Have Stronger Antimicrobial Responses and More Hemocytes after Reinfection Later in Life

The immunological strategies employed by insects to overcome infection vary with the type of infection and may change with experience. We investigated how a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, prepares the immune system to face a subsequent bacterial infection. For this, adult female mosquitoes were separated into three groups—unmanipulated, injured, or infected with Escherichia coli—and five days later all the mosquitoes were infected with a different strain of E. coli. We found that an injury or a bacterial infection early in life enhances the ability of mosquitoes to kill bacteria later in life. This protection results in higher mosquito survival and is associated with an increased hemocyte density, altered phagocytic activity by individual hemocytes, and the increased expression of nitric oxide synthase and perhaps prophenoloxidase 6. Protection from a second infection likely occurs because of heightened immune awareness due to an already existing infection instead of memory arising from an earlier, cured infection. This study highlights the dynamic nature of the mosquito immune response and how one infection prepares mosquitoes to survive a subsequent infection.

Social immunity modulates competition between coinfecting pathogens

Coinfections with multiple pathogens can result in complex within-host dynamics affecting virulence and transmission. While multiple infections are intensively studied in solitary hosts, it is so far unresolved how social host interactions interfere with pathogen competition, and if this depends on coinfection diversity. We studied how the collective disease defences of ants - their social immunity - influence pathogen competition in coinfections of same or different fungal pathogen species. Social immunity reduced virulence for all pathogen combinations, but interfered with spore production only in different-species coinfections. Here, it decreased overall pathogen sporulation success while increasing co-sporulation on individual cadavers and maintaining a higher pathogen diversity at the community level. Mathematical modelling revealed that host sanitary care alone can modulate competitive outcomes between pathogens, giving advantage to fast-germinating, thus less grooming-sensitive ones. Host social interactions can hence modulate infection dynamics in coinfected group members, thereby altering pathogen communities at the host level and population level.

Cancer Nano-Immunotherapy from the Injection to the Target: The Role of Protein Corona

Immunotherapy has become a promising cancer therapy, improving the prognosis of patients with many different types of cancer and offering the possibility for long-term cancer remission. Nevertheless, some patients do not respond to these treatments and immunotherapy has shown some limitations, such as immune system resistance or limited bioavailability of the drug. Therefore, new strategies that include the use of nanoparticles (NPs) are emerging to enhance the efficacy of immunotherapies. NPs present very different pharmacokinetic and pharmacodynamic properties compared with free drugs and enable the use of lower doses of immune-stimulating molecules, minimizing their side effects. However, NPs face issues concerning stability in physiological conditions, protein corona (PC) formation, and accumulation in the target tissue. PC formation changes the physicochemical and biological properties of the NPs and in consequence their therapeutic effect. This review summarizes the recent advances in the study of the effects of PC formation in NP-based immunotherapy. PC formation has complex effects on immunotherapy since it can diminish (“immune blinding”) or enhance the immune response in an uncontrolled manner (“immune reactivity”). Here, future perspectives of the field including the latest advances towards the use of personalized protein corona in cancer immunotherapy are also discussed.