Comparative characterization of two galectins excreted-secreted from intestine-dwelling parasitic versus free-living females of the soil-transmitted nematode Strongyloides

Strongyloides ratti infection in mice: immune response and immune modulation

Strongyloides ratti is a natural parasite of wild rats and most laboratory mouse strains are also fully permissive. The infection can be divided into three distinct phases: the tissue migration of the infective third stage larvae during the first two days, the early intestinal establishment of S. ratti parasites molting to adults on days three to six and the later intestinal parasitic phase until the end of infection. Immunocompetent mice terminate the S. ratti infection after one month and are semi-resistant to a second infection. Employing the powerful tools of mouse immunology has facilitated a detailed analysis of the initiation, execution and regulation of the immune response to S. ratti. Here we review the information collected to date on the protective immune response to migrating S. ratti larvae in tissues and to adult parasites in the intestine. We show that depending on the phase of infection, a site-specific portfolio of immune effector mechanisms is required for infection control. In addition, we summarize the strategies employed by S. ratti to evade the immune system and survive long enough in its host to replicate despite an effective immune response. Selected murine studies using the closely related Strongyloides venezuelensis will be discussed. This article is part of the Theo Murphy meeting issue 'Strongyloides: omics to worm-free populations'.

Novel insights into the somatic proteome of Strongyloides stercoralis infective third-stage larvae

BACKGROUND

Strongyloidiasis is a neglected tropical disease affecting an estimated 600 million people, particularly in resource-limited settings. The infection can persist lifelong due to unusual auto-infective cycle of Strongyloides stercoralis. The lack of a diagnostic gold standard and limited knowledge of the mechanisms underpinning this chronic infection are key issues in disease management. To date, only a few proteomics studies have been conducted to elucidate the molecular mechanisms associated with Strongyloides parasitism or to highlight novel immunological markers, with the result that our knowledge of S. stercoralis proteome remains limited. This study aims at expanding the characterization of S. stercoralis infective larvae (iL3) in order to further explore the mechanisms of parasitism and to highlight possible novel targets for serodiagnosis.

METHODS

iL3 obtained from an infected subject were analysed by high-throughput tandem mass spectrometry. To achieve a more comprehensive characterization of the iL3 proteome we analysed the experimental dataset using an automatic search strategy combined with manual annotation, which included gene ontology (GO) analysis, InterPro annotation, assessment of the homology with Homo sapiens and other pathogens of clinical importance and B-cell epitope prediction.

RESULTS

Our pipeline identified 430 S. stercoralis proteins, 187 (43%) of which were uncharacterized. Oxidoreductases and peptidases were amongst the most represented protein categories, as highlighted by molecular function GO analyses, while membrane and mitochondrial proteins were the most represented cellular component GO categories. A high proportion of proteins bearing the CAP, SCP or thioredoxin domain or belonging to cysteine-rich secretory, transthyretin-like or peptidase protein families were also identified. Additionally, we highlighted nine proteins displaying low homology with H. sapiens or other related pathogens and bearing amino acid sequences with immunogenic properties.

CONCLUSIONS

Our comprehensive description and annotation of the S. stercoralis iL3 proteome contribute to expanding the 'omics characterization of this parasite and provide experimental evidence on the most represented proteins associated with S. stercoralis parasitism, as inferred from genomic and transcriptomic data. Moreover, novel candidate immunogenic proteins to be evaluated as novel serological diagnostic markers are highlighted.

The interaction of host and nematode galectins influences the outcome of gastrointestinal nematode infections

Galectins are a family of proteins that bind β-galactosides and play key roles in a variety of cellular processes including host defence. They have been well studied in hosts but less so in gastrointestinal nematodes. Both host and parasite galectins are present in the gastrointestinal tract following infection. Parasite galectins can both bind antibody, especially highly glycosylated IgE and be bound by antibody. Parasite galectins may act as molecular sponges that soak up antibody. Host galectins promote mast cell degranulation while parasite galectins inhibit degranulation. Host and parasite galectins can also bind mucins and influence mucus viscosity. As the protective response against gastrointestinal nematode infection is partly dependent on IgE mediated mast cell degranulation and mucus, the interactions between host and parasite galectins play key roles in determining the outcome of infection.

Galectins from Onchocerca ochengi and O. volvulus and their immune recognition by Wistar rats, Gudali zebu cattle and human hosts

Background

During the last two decades research on animal filarial parasites, especially Onchocerca ochengi, infecting cattle in savanna areas of Africa revealed that O. ochengi as an animal model has biological features that are similar to those of O. volvulus, the aetiological agent of human onchocerciasis. There is, however, a paucity of biochemical, immunological and pathological data for O. ochengi. Galectins can be generated by parasites and their hosts. They are multifunctional molecules affecting the interaction between filarial parasites and their mammalian hosts including immune responses. This study characterized O. ochengi galectin, verified its immunologenicity and established its immune reactivity and that of Onchocerca volvulus galectin.

Results

The phylogenetic analysis showed the high degree of identity between the identified O. ochengi and the O. volvulus galectin-1 (ß-galactoside-binding protein-1) consisting only in one exchange of alanine for serine. O. ochengi galectin induced IgG antibodies during 28 days after immunization of Wistar rats. IgG from O. ochengi-infected cattle and O. volvulus-infected humans cross-reacted with the corresponding galectins. Under the applied experimental conditions in a cell proliferation test, O. ochengi galectin failed to significantly stimulate peripheral blood mononuclear cells (PBMCs) from O. ochengi-infected cattle, regardless of their parasite load.

Conclusion

An O. ochengi galectin gene was identified and the recombinantly expressed protein was immunogenic. IgG from Onchocerca-infected humans and cattle showed similar cross-reaction with both respective galectins. The present findings reflect the phylogenetic relationship between the two parasites and endorse the appropriateness of the cattle O. ochengi model for O. volvulus infection research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02064-3.

Brugia malayi galectin 2 is a tandem-repeat type galectin capable of binding mammalian polysaccharides

Galectins are among the most abundant excretory/secretory (ES) products of filarial worms, but their role in filarial biology is poorly understood. Galectin-2 (Lec-2), a major component of Brugia malayi extracellular vesicles, is released by filarial worms, and was recently identified in the serum of persons with loiasis. We therefore sought to clone and characterize Lec-2, and to develop reagents to examine its potential as a biomarker and its role in parasite biology. We cloned and expressed recombinant B. malayi Lec-2 (rBmLec-2), generated a Lec-2-specific monoclonal antibody (4B4), and used it to confirm the presence of Lec-2 in B. malayi ES products and whole worm lysate. We show that Lec-2 is absent in B. malayi oocytes, and increases in concentration as embryos mature. Recombinant BmLec-2 hemagglutinates rabbit red blood cells at concentrations less than 1 μg/mL, and this is abrogated by single amino acid substitutions in the predicted carbohydrate recognition domains. rBmLec-2 binds multiple LacNAc oligosaccharides on a mammalian carbohydrate array. Sera from 17/23 (78 %) persons with microfilaremic loiasis and 4/10 (40 %) persons with bancroftian filariasis had detectable antibody to Lec-2 by western blot. Our studies confirm the functionality of BmLec-2 and indicate anti-Lec-2 antibody responses are common in persons with filariasis. These studies set the stage for further examination of the role of Lec-2 in filarial biology and in filarial-host interactions.

Advances in the Molecular and Cellular Biology of Strongyloides spp

Purpose of Review

This paper constitutes an update of recent studies on the general biology, molecular genetics, and cellular biology of Strongyloides spp. and related parasitic nematodes.

Recent Findings

Increasingly, human strongyloidiasis is considered the most neglected of neglected tropical diseases. Despite this, the last 5 years has seen remarkable advances in the molecular biology of Strongyloides spp. Genome sequences for S. stercoralis, S. ratti, S. venezuelensis, S. papillosus, and the related parasite Parastrongyloides trichosuri were created, annotated, and analyzed. These genomic resources, along with a practical transgenesis platform for Strongyloides spp., aided a major achievement, the advent of targeted mutagenesis via CRISPR/Cas9 in S. stercoralis and S. ratti. The genome sequences have also enabled significant molecular epidemiologic and phylogenetic findings on human strongyloidiasis, including the first genetic evidence of zoonotic transmission of S. stercoralis between dogs and humans. Studies of molecular signaling pathways identified the nuclear receptor Ss-DAF-12 as one that can be manipulated in the parasite by exogenous application of its steroid ligands. The chemotherapeutic implications of this were unscored by a study in which a Ss-DAF-12 ligand suppressed autoinfection by S. stercoralis in a new murine model of human strongyloidiasis.

Summary

Seminal advances in genomics of Strongyloides spp. have transformed research into strongyloidiasis, facilitating fundamental phylogenetic and epidemiologic studies and aiding the deployment of CRISPR/Cas9 gene disruption and editing as functional genomic tools in Strongyloides spp. Studies of Ss-DAF-12 signaling in S. stercoralis demonstrated the potential of this pathway as a novel chemotherapeutic target in parasitic nematodes.

IgG reactivity with 40-35 kDa soluble and membrane antigen of Strongyloides venezuelensis in immunocompromised patients

Immunocompromised patients constitute a risk group for the development of severe clinical forms of human strongyloidiasis. The diagnosis of this infection is primarily performed by parasitological techniques, but with low sensitivity. Serological techniques appear as an alternative, especially with heterologous antigens use. The aim of this study was to perform the Western blot technique by using S. venezuelensis infective third stage larva (iL3) soluble (TS) and membrane (TM) saline antigens to reveal immunoreactive bands in immunocompromised patients with strongyloidiasis. Serum samples from 117 parasitologically well-characterized patients were divided into four groups: S. stercoralis positive and immunocompetent (S + IC); S. stercoralis positive and immunocompromised (S + IP); negative and immunocompetent (S-IC); negative and immunocompromised (S-IP). A 40-35 kDa band was recognized by 100% of patients in the S + IC group in both antigenic fractions, and by 62.5% and 50% in the S + IP group using the TS and TM fractions, respectively. A 29 kDa band was recognized by 86.3% and 72.7% (for TS and TM, respectively) of patients in the S + IC group, and only by 12.5% of patients in the S + IP group on the TM antigen. Regardless of the patients' immunological condition, the 40-35 kDa band from S. venezuelensis was detected more frequently and can be used as an important marker to the immunodiagnosis of human strongyloidiasis.