Enteric pathogens induce tissue tolerance and prevent neuronal loss from subsequent infections

The enteric nervous system (ENS) controls several intestinal functions including motility and nutrient handling, which can be disrupted by infection-induced neuropathies or neuronal cell death. We investigated possible tolerance mechanisms preventing neuronal loss and disruption in gut motility after pathogen exposure. We found that following enteric infections, muscularis macrophages (MMs) acquire a tissue-protective phenotype that prevents neuronal loss, dysmotility, and maintains energy balance during subsequent challenge with unrelated pathogens. Bacteria-induced neuroprotection relied on activation of gut-projecting sympathetic neurons and signaling via β2-adrenergic receptors (β2AR) on MMs. In contrast, helminth-mediated neuroprotection was dependent on T cells and systemic production of interleukin (IL)-4 and IL-13 by eosinophils, which induced arginase-expressing MMs that prevented neuronal loss from an unrelated infection located in a different intestinal region. Collectively, these data suggest that distinct enteric pathogens trigger a state of disease or tissue tolerance that preserves ENS number and functionality.

A global genotyping survey of Strongyloides stercoralis and Strongyloides fuelleborni using deep amplicon sequencing

Strongyloidiasis is a neglected tropical disease caused by the human infective nematodes Strongyloides stercoralis, Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi. Previous large-scale studies exploring the genetic diversity of this important genus have focused on Southeast Asia, with a small number of isolates from the USA, Switzerland, Australia and several African countries having been genotyped. Consequently, little is known about the global distribution of geographic sub-variants of these nematodes and the genetic diversity that exists within the genus Strongyloides generally. We extracted DNA from human, dog and primate feces containing Strongyloides, collected from several countries representing all inhabited continents. Using a genotyping assay adapted for deep amplicon sequencing on the Illumina MiSeq platform, we sequenced the hyper-variable I and hyper-variable IV regions of the Strongyloides 18S rRNA gene and a fragment of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene from these specimens. We report several novel findings including unique S. stercoralis and S. fuelleborni genotypes, and the first identifications of a previously unknown S. fuelleborni infecting humans within Australia. We expand on an existing Strongyloides genotyping scheme to accommodate S. fuelleborni and these novel genotypes. In doing so, we compare our data to all 18S and cox1 sequences of S. fuelleborni and S. stercoralis available in GenBank (to our knowledge), that overlap with the sequences generated using our approach. As this analysis represents more than 1,000 sequences collected from diverse hosts and locations, representing all inhabited continents, it allows a truly global understanding of the population genetic structure of the Strongyloides species infecting humans, non-human primates, and domestic dogs.

Strongyloidiasis is a neglected tropical disease caused by the human infective worms (nematodes) Strongyloides stercoralis, Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi. Little is known about the genetic diversity of these nematodes and the possibility of geographically isolated genetic types is a particularly interesting research question, given the unique life cycle of these worms which includes both sexual and asexual stages. We extracted DNA from human, dog and primate feces containing Strongyloides, collected from several countries representing all inhabited continents. Using next-generation sequencing, we analyzed three Strongyloides DNA fragments from these specimens; two fragments of the 18S gene and one from the cytochrome c oxidase subunit 1 (cox1) gene. Using this approach, we discovered some unique S. stercoralis and S. fuelleborni genotypes, and identified a previously unknown S. fuelleborni infecting humans within Australia. We compared our data to all 18S and cox1 sequences of S. fuelleborni and S. stercoralis available in public databases and identified several patterns relating to the global distribution of certain genotypes. This knowledge could allow us to infer the origin of human Strongyloides infections in the future, and assess the role certain animals (non-human primates and dogs) might play in the transmission of Strongyloides to humans.

Fatty acid and retinol-binding protein: A novel antigen for immunodiagnosis of human strongyloidiasis

The tenacious human parasitic helminth Strongyloides stercoralis is a significant health problem worldwide. The current lack of a definitive diagnostic laboratory test to rule out this infection necessitates designing more specific diagnostic methods. Fatty acid and retinol-binding protein (FAR) plays a crucial role in the development and reproduction of nematodes. We generated a recombinant form of this protein and determined its applicability for immunodiagnosis of S. stercoralis. The L3 form of S. stercoralis was harvested and used for RNA extraction and cDNA synthesis. The coding sequence of S. stercoralis FAR (SsFAR) was cloned into pET28a(+) vector, expressed in E. coli BL21 and purified. ELISA and immunoblotting were employed to determine the specificity and sensitivity of rSsFAR using a set of defined sera. In addition, we analyzed the phylogenetic relationship of SsFAR with different FAR sequences from other nematodes. The cloned SsFAR had an open reading frame of 447 bp encoding 147 amino acids, with a deduced molecular mass of 19 kD. The SsFAR amino acid sequence was 93% identical to FAR of S. ratti. For differential immunodiagnosis of strongyloidiasis, rSsFAR exhibited 100% sensitivity and 97% specificity. However, cross-reactivity with FAR proteins of other parasites, namely Toxocara canis and Echinococcus granulosus, was noted. Our results provide a novel approach for immunodiagnosis of S. stercoralis infections using rSsFAR with reliable sensitivity and specificity.

Comparative transcriptomics gives insights into the evolution of parasitism in Strongyloides nematodes at the genus, subclade and species level

Strongyloides spp., gastrointestinal nematode parasites of humans and other animals, have genetically identical parasitic and free-living adult life cycle stages. This is an almost unique feature amongst nematodes and comparison of these two stages can provide insights into the genetic basis and evolution of Strongyloides nematode parasitism. Here, we present RNAseq data for S. venezuelensis, a parasite of rodents, and identify genes that are differentially expressed in parasitic and free-living life cycle stages. Comparison of these data with analogous RNAseq data for three other Strongyloides spp., has identified key protein-coding gene families with a putative role in parasitism including WAGO-like Argonautes (at the genus level) and speckle-type POZ-like coding genes (S. venezuelensis-S. papillosus phylogenetic subclade level). Diverse gene families are uniquely upregulated in the parasitic stage of all four Strongyloides species, including a distinct upregulation of genes encoding cytochrome P450 in S. venezuelensis, suggesting some diversification of the molecular tools used in the parasitic life cycle stage among individual species. Together, our results identify key gene families with a putative role in Strongyloides parasitism or features of the parasitic life cycle stage, and deepen our understanding of parasitism evolution among Strongyloides species.

Modulation of CD4+ and CD8+ T Cell Function and Cytokine Responses in Strongyloides stercoralis Infection by Interleukin-27 (IL-27) and IL-37

Strongyloides stercoralis infection is associated with diminished antigen-specific Th1- and Th17-associated responses and enhanced Th2-associated responses. Interleukin-27 (IL-27) and IL-37 are two known anti-inflammatory cytokines that are highly expressed in S. stercoralis infection. We therefore wanted to examine the role of IL-27 and IL-37 in regulating CD4+ and CD8+ T cell responses in S. stercoralis infection. To this end, we examined the frequency of Th1/Tc1, Th2/Tc2, Th9/Tc9, Th17/Tc17, and Th22/Tc22 cells in 15 S. stercoralis-infected individuals and 10 uninfected individuals stimulated with parasite antigen following IL-27 or IL-37 neutralization. We also examined the production of prototypical type 1, type 2, type 9, type 17, and type 22 cytokines in the whole-blood supernatants. Our data reveal that IL-27 or IL-37 neutralization resulted in significantly enhanced frequencies of Th1/Tc1, Th2/Tc2, Th17/Tc17, Th9, and Th22 cells with parasite antigen stimulation. There was no induction of any T cell response in uninfected individuals following parasite antigen stimulation and IL-27 or IL-37 neutralization. Moreover, we also observed increased production of gamma interferon (IFN-γ), IL-5, IL-9, IL-17, and IL-22 and decreased production of IL-10 following IL-27 and IL-37 neutralization and parasite antigen stimulation in whole-blood cultures. Thus, we demonstrate that IL-27 and IL-37 limit the induction of particular T cell subsets along with cytokine responses in S. stercoralis infections, which suggest the importance of IL-27 and IL-37 in immune modulation in a chronic helminth infection.

The genomic basis of parasitism in the Strongyloides clade of nematodes

Soil-transmitted nematodes, including the Strongyloides genus, cause one of the most prevalent neglected tropical diseases. Here we compare the genomes of four Strongyloides species, including the human pathogen Strongyloides stercoralis, and their close relatives that are facultatively parasitic (Parastrongyloides trichosuri) and free-living (Rhabditophanes sp. KR3021). A significant paralogous expansion of key gene families--families encoding astacin-like and SCP/TAPS proteins--is associated with the evolution of parasitism in this clade. Exploiting the unique Strongyloides life cycle, we compare the transcriptomes of the parasitic and free-living stages and find that these same gene families are upregulated in the parasitic stages, underscoring their role in nematode parasitism.

Transposon-mediated chromosomal integration of transgenes in the parasitic nematode Strongyloides ratti and establishment of stable transgenic lines

Genetic transformation is a potential tool for analyzing gene function and thereby identifying new drug and vaccine targets in parasitic nematodes, which adversely affect more than one billion people. We have previously developed a robust system for transgenesis in Strongyloides spp. using gonadal microinjection for gene transfer. In this system, transgenes are expressed in promoter-regulated fashion in the F1 but are silenced in subsequent generations, presumably because of their location in repetitive episomal arrays. To counteract this silencing, we explored transposon-mediated chromosomal integration of transgenes in S. ratti. To this end, we constructed a donor vector encoding green fluorescent protein (GFP) under the control of the Ss-act-2 promoter with flanking inverted tandem repeats specific for the piggyBac transposon. In three experiments, free-living Strongyloides ratti females were transformed with this donor vector and a helper plasmid encoding the piggyBac transposase. A mean of 7.9% of F1 larvae were GFP-positive. We inoculated rats with GFP-positive F1 infective larvae, and 0.5% of 6014 F2 individuals resulting from this host passage were GFP-positive. We cultured GFP-positive F2 individuals to produce GFP-positive F3 L3i for additional rounds of host and culture passage. Mean GFP expression frequencies in subsequent generations were 15.6% in the F3, 99.0% in the F4, 82.4% in the F5 and 98.7% in the F6. The resulting transgenic lines now have virtually uniform GFP expression among all progeny after at least 10 generations of passage. Chromosomal integration of the reporter transgenes was confirmed by Southern blotting and splinkerette PCR, which revealed the transgene flanked by S. ratti genomic sequences corresponding to five discrete integration sites. BLAST searches of flanking sequences against the S. ratti genome revealed integrations in five contigs. This result provides the basis for two powerful functional genomic tools in S. ratti: heritable transgenesis and insertional mutagenesis.

Parasitic roundworms sicken and debilitate over one billion people, most of whom subsist on less than two US dollars per day. There are no vaccines and few drugs available to treat and prevent these infections. Basic research leading to new therapies has been hampered because we lack methods to study gene function in parasitic roundworms. One such method is transgenesis, a process by which gene function is inferred by studying the effects of transferring native or altered copies of genes into subject organisms. Our laboratory has developed a system for transferring synthetic genes into parasitic roundworms of the genus Strongyloides and for obtaining temporary expression of these “transgenes”. Until now, however, we have been unable to propagate these transgenic parasites in the laboratory. This paper describes a new technique that allows us to establish and maintain self-perpetuating lines of transgenic parasites for study. This represents a fundamental advance in the methodology for studying gene function in parasitic roundworms and should greatly facilitate the discovery of new therapies.

Toll-like receptor 4 (TLR4) is required for protective immunity to larval Strongyloides stercoralis in mice

TLR4 is important for immunity to various unicellular organisms and has been implicated in the immune responses to helminth parasites. The immune response against helminths is generally Th2-mediated and studies have shown that TLR4 is required for the development of a Th2 response against allergens and helminth antigens in mice. C3H/HeJ mice, which have a point mutation in the Tlr4 gene, were used in this study to determine the role of TLR4 in protective immunity to the nematode Strongyloides stercoralis. It was demonstrated that TLR4 was not required for killing larval S. stercoralis during the innate immune response, but was required for killing the parasites during the adaptive immune response. No differences were seen in the IL-5 and IFN-gamma responses, antibody responses or cell recruitment between wild type and C3H/HeJ mice after immunization. Protective immunity was restored in immunized C3H/HeJ mice by the addition of wild type peritoneal exudate cells in the environment of the larvae. It was therefore concluded that the inability of TLR4-mutant mice to kill larval S. stercoralis during the adaptive immune response is due to a defect in the effector cells recruited to the microenvironment of the larvae.

No evidence for specificity between host and parasite genotypes in experimental (Nematoda) infections

A key requirement for several theories involving the evolution of sex and sexual selection is a specificity between host and parasite genotypes, i.e. the resistance of particular host genotypes to particular parasite genotypes and the infectivity of particular parasite genotypes for particular host genotypes. Determining the scope and nature of any such specificity is also of applied relevance, since any specificity for different parasite genotypes to infect particular host genotypes may affect the level of protection afforded by vaccination, the efficacy of selective breeding of livestock for parasite resistance and the long-term evolution of parasite populations in response to these control measures. Whereas we have some evidence for the role of specificity between host and pathogen genotypes in viral and bacterial infections, its role in macroparasitic infections is seldom considered. The first empirical test of this specificity for a vertebrate-nematode system is provided here using clonal lines of parasite and inbred and congenic strains of rat that differ either across the genome or only at the major histocompatibility complex. Although significant differences between the resistance of host genotypes to infection and between the fitness of different parasite genotypes are found, there is no evidence for an interaction between host and parasite genotypes. It is concluded that a specificity between host and parasite genotypes is unlikely in this system.

Isolation of a cDNA encoding an IgG immunoreactive antigen (zinc finger protein) of Strongyloides stercoralis

A full length cDNA encoding an IgG immunoreactive antigen of Strongyloides stercoralis is described. A clone containing 1,328 bp insert was selected following screening of S. stercoralis cDNA library with an IgG fraction obtained from a pool of 78 S. stercoralis positive human sera samples. The nucleotide sequence of the 1,328 bp insert was found to be 70.5% A/T, reflecting a characteristic A/T codon bias of S. stercoralis. The nucleotide sequence of this insert identified a cDNA coding for a zinc finger protein. The conceptually translated amino acid sequence of the open reading frame for the IgG immunoreactive antigen of S. stercoralis encodes a 211 amino acid residue protein with an apparent molecular weight of 22.8 kDa and a predicted isoelectric point of 8.71. The diagnostic potential of this IgG immunoreactive antigen of S. stercoralis is also discussed.

Role of IL-5 in innate and adaptive immunity to larval Strongyloides stercoralis in mice

Protective immunity to Strongyloides stercoralis infective larvae in mice has been shown to be dependent on IL-5 based on mAb depletion studies. The goal of this study was to determine the functional role of IL-5 during the innate and adaptive immune response to larval S. stercoralis in mice. In these studies, three strains of mice were used: wild-type C57BL/6J (WT), IL-5 knockout (KO), and IL-5 transgenic (TG). Innate responses to the larvae indicated that there was enhanced survival in the KO animals and decreased survival in the TG animals compared with WT. Furthermore, killing of larvae in TG mice was associated with eosinophil infiltration and degranulation. In studying the adaptive immune response, it was observed that immunization of KO mice did not lead to the development of protective immunity. Experiments were then performed to determine whether KO mice reconstituted with Abs or cells could then develop protective immunity. KO mice displayed protective immunity via a granulocyte-dependent mechanism following injection of purified IgM from immune wild-type animals. Immunity in KO mice could also be reconstituted by the injection of eosinophils at the time of immunization. These eosinophils did not participate in actively killing the challenge infection, but rather were responsible for the induction of a protective Ab response. We conclude that IL-5 is required in the protective immune response for the production of eosinophils, and that eosinophils were involved in larval killing during innate immunity and in the induction of protective Abs in the adaptive immune response.

Influence of rat strain on larval production by the parasitic nematode Strongyloides ratti

The course of infection with Strongyloides ratti in a range of rat strains was assessed by monitoring the production of larvae. To our knowledge, this is the first such study of S. ratti using its natural host Rattus norvegicus. Host strain influenced the pattern of larval production. The results were qualitatively the same for 2 S. ratti lines of North American and Japanese origin.

Host genetic factors do not account for variation in parasite loads in Strongyloides fuelleborni kellyi

Previous work in Papua New Guinea has shown considerable variation in egg counts between different people infected with Strongyloides fuelleborni kellyi, although individual egg loads remained relatively constant over a 14-month period. Possible explanations include genetic predisposition, a surprising longevity of the worms, or external auto-infection. We have now analysed the pedigrees of 177 individuals for whom egg counts were available, and find no evidence for polygenic inheritance of factors related to egg counts. The use of genetic models postulating the segregation of a single unknown susceptibility gene did not enable us, using the data available, to distinguish between this hypothesis and environmental determination of egg counts; nor did we find any association between egg load and the class 1 HLA genotype of the host.