Entomopathogenic nematodes (EPNs) have been used in biological control but improvement is needed to realize their full potential for broader application in agriculture. Some improvements have been gained through selective breeding and the isolation of additional species and populations. Having genomic sequences for at least six EPNs opens the possibility of genetic improvement, either by facilitating the selection of candidate genes for hypothesis-driven studies of gene-trait relations or by genomics-assisted breeding for desirable traits. However, the genomic data will be of limited use without a more mechanistic understanding of the genes underlying traits that are important for biological control. Additionally, molecular tools are required to fully translate the genomic resources into further functional studies and better biological control.

Entomopathogenic nematodes are a subgroup of insect-parasitic nematodes that are used in biological control as alternatives or supplements to chemical pesticides. Steinernema scapterisci is an unusual member of the entomopathogenic nematode guild for many reasons including that it is promiscuous in its association with bacteria, it can reproduce in the absence of its described bacterial symbiont, and it is known to have a narrow host range. It is a powerful comparative model within the species and could be used to elucidate parasite specialization. Here we describe a new method of efficiently producing large numbers of S. scapterisci infective juveniles (IJs) in house crickets and for quantifying parasitic activation of the IJs upon exposure to host tissue using morphological features. We found that parasite activation is a temporal process with more IJs activating over time. Furthermore, we found that activated IJs secrete a complex mixture of proteins and that S. scapterisci IJs preferentially activate upon exposure to cricket tissue, reaffirming the description of S. scapterisci as a cricket specialist.

Entomopathogenic nematodes are a subgroup of insect-parasitic nematodes that are used in biological control as alternatives or supplements to chemical pesticides. Steinernema scapterisci is an unusual member of the entomopathogenic nematode guild for many reasons including that it is promiscuous in its association with bacteria, it can reproduce in the absence of its described bacterial symbiont, and it is known to have a narrow host range. It is a powerful comparative model within the species and could be used to elucidate parasite specialization. Here we describe a new method of efficiently producing large numbers of S. scapterisci infective juveniles (IJs) in house crickets and for quantifying parasitic activation of the IJs upon exposure to host tissue using morphological features. We found that parasite activation is a temporal process with more IJs activating over time. Furthermore, we found that activated IJs secrete a complex mixture of proteins and that S. scapterisci IJs preferentially activate upon exposure to cricket tissue, reaffirming the description of S. scapterisci as a cricket specialist.

Parasitic helminths release molecular effectors into their hosts and these effectors can directly damage host tissue and modulate host immunity. Excreted/secreted proteins (ESPs) are one category of parasite molecular effectors that are critical to their success within the host. However, most studies of nematode ESPs rely on in vitro stimulation or culture conditions to collect the ESPs, operating under the assumption that in vitro conditions mimic actual in vivo infection. This assumption is rarely if ever validated. Entomopathogenic nematodes (EPNs) are lethal parasites of insects that produce and release toxins into their insect hosts and are a powerful model parasite system. We compared transcriptional profiles of individual Steinernema feltiae nematodes at different time points of activation under in vitro and in vivo conditions and found that some but not all time points during in vitro parasite activation have similar transcriptional profiles with nematodes from in vivo infections. These findings highlight the importance of experimental validation of ESP collection conditions. Additionally, we found that a suite of genes in the neuropeptide pathway were downregulated as nematodes activated and infection progressed in vivo, suggesting that these genes are involved in host-seeking behavior and are less important during active infection. We then characterized the ESPs of activated S. feltiae infective juveniles (IJs) using mass spectrometry and identified 266 proteins that are released by these nematodes. In comparing these ESPs with those previously identified in activated S. carpocapsae IJs, we identified a core set of 52 proteins that are conserved and present in the ESPs of activated IJs of both species. These core venom proteins include both tissue-damaging and immune-modulating proteins, suggesting that the ESPs of these parasites include both a core set of effectors as well as a specialized set, more adapted to the particular hosts they infect.

Entomopathogenic nematodes (EPNs) are unique parasites due to their symbiosis with entomopathogenic bacteria and their ability to kill insect hosts quickly after infection. It is widely believed that EPNs rely on their bacterial partners for killing hosts. Here we disproved this theory by demonstrating that the in vitro activated infective juveniles (IJs) of Steinernema carpocapsae (a well-studied EPN species) release venom proteins that are lethal to several insects including Drosophila melanogaster. We confirmed that the in vitro activation is a good approximation of the in vivo process by comparing the transcriptomes of individual in vitro and in vivo activated IJs. We further analyzed the transcriptomes of non-activated and activated IJs and revealed a dramatic shift in gene expression during IJ activation. We also analyzed the venom proteome using mass spectrometry. Among the 472 venom proteins, proteases and protease inhibitors are especially abundant, and toxin-related proteins such as Shk domain-containing proteins and fatty acid- and retinol-binding proteins are also detected, which are potential candidates for suppressing the host immune system. Many of the venom proteins have conserved orthologs in vertebrate-parasitic nematodes and are differentially expressed during IJ activation, suggesting conserved functions in nematode parasitism. In summary, our findings strongly support a new model that S. carpocapsae and likely other Steinernema EPNs have a more active role in contributing to the pathogenicity of the nematode-bacterium complex than simply relying on their symbiotic bacteria. Furthermore, we propose that EPNs are a good model system for investigating vertebrate- and human-parasitic nematodes, especially regarding the function of excretory/secretory products.

Most nematodes are small worms that lack enough RNA for regular RNA-seq protocols without pooling hundred to thousand of individuals. We have adapted the Smart-seq2 protocol in order to sequence the transcriptome of an individual worm. While developed for individual Steinernema carpocapsae and Caenorhabditis elegans larvae as well as embryos, the protocol should be adaptable for other nematode species and small invertebrates. In addition, we describe how to analyze the RNA-seq results using the Galaxy online environment. We expect that this method will be useful for the studying gene expression variances of individual nematodes in wild type and mutant backgrounds.

Helminths have coevolved with their hosts, resulting in the development of specialized host immune mechanisms and parasite-specific regulatory products. Identification of new pathways that regulate helminth infection could provide a better understanding of host-helminth interaction and may identify new therapeutic targets for helminth infection. Here we identify the endocannabinoid system as a new mechanism that influences host immunity to helminths. Endocannabinoids are lipid-derived signaling molecules that control important physiologic processes, such as feeding behavior and metabolism. Following murine infection with Nippostrongylus brasiliensis, an intestinal nematode with a life cycle similar to that of hookworms, we observed increased levels of endocannabinoids (2-arachidonoylglycerol [2-AG] or anandamide [AEA]) and the endocannabinoid-like molecule oleoylethanolamine (OEA) in infected lung and intestine. To investigate endocannabinoid function in helminth infection, we employed pharmacological inhibitors of cannabinoid subtype receptors 1 and 2 (CB1R and CB2R). Compared to findings for vehicle-treated mice, inhibition of CB1R but not CB2R resulted in increased N. brasiliensis worm burden and egg output, associated with significantly decreased expression of the T helper type 2 cytokine interleukin 5 (IL-5) in intestinal tissue and splenocyte cultures. Strikingly, bioinformatic analysis of genomic and transcriptome sequencing (RNA-seq) data sets identified putative genes encoding endocannabinoid biosynthetic and degradative enzymes in many parasitic nematodes. To test the novel hypothesis that helminth parasites produce their own endocannabinoids, we measured endocannabinoid levels in N. brasiliensis by mass spectrometry and quantitative PCR and found that N. brasiliensis parasites produced endocannabinoids, especially at the infectious larval stage. To our knowledge, this is the first report of helminth- and host-derived endocannabinoids that promote host immune responses and reduce parasite burden.

Resistin-like molecule α (RELMα) is a highly secreted protein in type 2 (Th2) cytokine-induced inflammation including helminth infection and allergy. In infection with Nippostrongylus brasiliensis (Nb), RELMα dampens Th2 inflammatory responses. RELMα is expressed by immune cells, and by epithelial cells (EC); however, the functional impact of immune versus EC-derived RELMα is unknown. We generated bone marrow (BM) chimeras that were RELMα deficient (RELMα^{-/ -} ) in BM or non BM cells and infected them with Nb. Non BM RELMα^-/- chimeras had comparable inflammatory responses and parasite burdens to RELMα^+/+ mice. In contrast, both RELMα^-/- and BM RELMα^-/- mice exhibited increased Nb-induced lung and intestinal inflammation, correlated with elevated Th2 cytokines and Nb killing. CD11c⁺ lung macrophages were the dominant BM-derived source of RELMα and can mediate Nb killing. Therefore, we employed a macrophage-worm co-culture system to investigate whether RELMα regulates macrophage-mediated Nb killing. Compared to RELMα^{+ /+} macrophages, RELMα^-/- macrophages exhibited increased binding to Nb and functionally impaired Nb development. Supplementation with recombinant RELMα partially reversed this phenotype. Gene expression analysis revealed that RELMα decreased cell adhesion and Fc receptor signaling pathways, which are associated with macrophage-mediated helminth killing. Collectively, these studies demonstrate that BM-derived RELMα is necessary and sufficient to dampen Nb immune responses, and identify that one mechanism of action of RELMα is through inhibiting macrophage recruitment and interaction with Nb. Our findings suggest that RELMα acts as an immune brake that provides mutually beneficial effects for the host and parasite by limiting tissue damage and delaying parasite expulsion.

Parasitic nematodes cause significant morbidity and mortality globally. Excretory/secretory products (ESPs) such as fatty acid- and retinol- binding proteins (FARs) are hypothesized to suppress host immunity during nematode infection, yet little is known about their interactions with host tissues. Leveraging the insect parasitic nematode, Steinernema carpocapsae, we describe here the first in vivo study demonstrating that FARs modulate animal immunity, causing an increase in susceptibility to bacterial co-infection. Moreover, we show that FARs dampen key components of the fly immune response including the phenoloxidase cascade and antimicrobial peptide (AMP) production. Our data also reveal that FARs deplete lipid signaling precursors in vivo as well as bind to these fatty acids in vitro, suggesting that FARs elicit their immunomodulatory effects by altering the availability of lipid signaling molecules necessary for an efficient immune response. Collectively, these data support a complex role for FARs in immunosuppression in animals and provide detailed mechanistic insight into parasitism in phylum Nematoda.