The role of B-cells in immunity against adult Strongyloides venezuelensis

Generation of IgG antibodies against Strongyloides stercoralis in mice via immunization with recombinant antigens A133 and Ss-IR

Strongyloidiasis, caused by the nematode Strongyloides stercoralis, remains a threat to global public health, and a vaccine would be useful to control the disease, especially in developing countries. This study aimed to evaluate the efficacy of recombinant proteins, A133 and Ss-IR, as potential vaccine candidates against strongyloidiasis by investigating the humoral and cellular immune responses in immunized mice. Respective antigens were adjuvanted with Complete Freund's Adjuvant (prime) and Incomplete Freund's Adjuvant (boost) and administered intraperitoneally (prime) and subcutaneously (boost) to female BALB/c mice. For antigen-only doses, only antigens were injected without adjuvants. Altogether, 1 prime dose, 4 booster doses, and 2 antigen-only doses were administered successively. ELISAs were conducted to assess the antibody responses, along with flow cytometry and cytokine ELISA to elucidate the cellular immune responses. Results showed that A133 and Ss-IR induced the production of IgG1 and IgG2a, with A133 generating more robust IgG2a responses than Ss-IR. Flow cytometry findings indicated that effector CD8+T-cells and memory B-cells activity were upregulated significantly for A133 only, whereas cytokine ELISA demonstrated that a Th1/Th2/Th17 mixed cell responses were triggered upon vaccination with either antigen. This preliminary study illustrated the good potential of recombinant A133 and Ss-IR as vaccine candidates against S. stercoralis. It provided information on the probable immune mechanism involved in host defence and the elicitation of protection against S. stercoralis.

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'.

The Strongyloides bioassay toolbox: A unique opportunity to accelerate functional biology for nematode parasites

Caenorhabditis elegans is a uniquely powerful tool to aid understanding of fundamental nematode biology. While C. elegans boasts an unrivalled array of functional genomics tools and phenotype bioassays the inherent differences between free-living and parasitic nematodes underscores the need to develop these approaches in tractable parasite models. Advances in functional genomics approaches, including RNA interference and CRISPR/Cas9 gene editing, in the parasitic nematodes Strongyloides ratti and Strongyloides stercoralis provide a unique and timely opportunity to probe basic parasite biology and reveal novel anthelmintic targets in species that are both experimentally and therapeutically relevant pathogens. While Strongyloides functional genomics tools have progressed rapidly, the complementary range of bioassays required to elucidate phenotypic outcomes post-functional genomics remain more limited in scope. To adequately support the exploitation of functional genomic pipelines for studies of gene function in Strongyloides a comprehensive set of species- and parasite-specific quantitative bioassays are required to assess nematode behaviours post-genetic manipulation. Here we review the scope of the current Strongyloides bioassay toolbox, how established Strongyloides bioassays have advanced knowledge of parasite biology, opportunities for Strongyloides bioassay development and, the need for investment in tractable model parasite platforms such as Strongyloides to drive the discovery of novel targets for parasite control.

Microscopic and molecular evaluation of Strongyloides venezuelensis in an experimental life cycle using Wistar rats

INTRODUCTION

Strongyloides venezuelensis is a nematode whose natural host is rats. It is used as a model for the investigation of human strongyloidiasis caused by S. stercoralis. The latter is a neglected tropical disease in Ecuador where there are no specific plans to mitigate this parasitic illness.

OBJECTIVE

To evaluate the stages of S. venezuelensis in an experimental life cycle using Wistar rats.

MATERIALS AND METHODS

Male Wistar rats were used to replicate the natural biological cycle of S. venezuelensis and describe its morphometric characteristics, as well as its parasitic development. Furthermore, the production of eggs per gram of feces was quantified using two diagnostic techniques and assessment of parasite load: Kato-Katz and qPCR.

RESULTS

Viable larval stages (L1, L2, L3) could be obtained up to 96 hours through fecal culture. Parthenogenetic females were established in the duodenum on the fifth day postinfection. Fertile eggs were observed in the intestinal tissue and fresh feces where the production peak occurred on the 8th. day post-infection. Unlike Kato-Katz, qPCR detected parasitic DNA on days not typically reported.

CONCLUSIONS

The larval migration of S. venezuelensis within the murine host in an experimental environment was equivalent to that described in its natural biological cycle. The Kato-Katz quantitative technique showed to be quick and low-cost, but the qPCR had greater diagnostic precision. This experimental life cycle can be used as a tool for the study of strongyloidiasis or other similar nematodiasis.

Context-dependent roles of B cells during intestinal helminth infection

The current approaches to reduce the burden of chronic helminth infections in endemic areas are adequate sanitation and periodic administration of deworming drugs. Yet, resistance against some deworming drugs and reinfection can still rapidly occur even after treatment. A vaccine against helminths would be an effective solution at preventing reinfection. However, vaccines against helminth parasites have yet to be successfully developed. While T helper cells and innate lymphoid cells have been established as important components of the protective type 2 response, the roles of B cells and antibodies remain the most controversial. Here, we review the roles of B cells during intestinal helminth infection. We discuss the potential factors that contribute to the context-specific roles for B cells in protection against diverse intestinal helminth parasite species, using evidence from well-defined murine model systems. Understanding the precise roles of B cells during resistance and susceptibility to helminth infection may offer a new perspective of type 2 protective immunity.

Age range implications of rats over Strongyloides venezuelensis infection

OBJECTIVE

To evaluate the dynamics of S. venezuelensis infection in Wistar rats of different age ranges.

DESIGN

Thirty-five (n = 35, 7 per group) male Wistar rats were distributed according to age into five groups: 2, 3, 6, 12 and 18 months old (mo). The rats were infected by S. venezuelensis and eggs per gram of feces (EPG) were measured at 3, 9, 15 and 21 days post-infection (dpi). All animals were killed at 21 dpi, thymus, lungs and small intestines were removed, and relative weight calculated. The adult worms recovered from the small intestines and blood cells were counted.

RESULTS

Rats in advanced age presented higher parasite oviposition at 9 dpi and posterior reduction of EPG, while young rats still showed higher oviposition at 15 dpi and 21 dpi. At 12 and 18 mo, the rats had greater number of adult worms, which with low fecundity, eosinophilia and least concentration of monocytes. The fecundity of worms was more expressive in young rats. A strong correlation was observed between age and EPG at 9 dpi (R = 0.72, p < 0.0001), at 15 (R = -0.66, p < 0.0001) and at 21 dpi (R = -0.65, p < 0.0001), as well as age and numbers of worms at 21 dpi (R = 0.74, p < 0.0001). The relative weight of the thymus, lungs and small intestines were higher in rats at 2 and 3 mo in comparison to the older groups of rats.

CONCLUSIONS

Aging process interfered on host-parasite relationship and changed the dynamics of infection of S. venezuelensis in Wistar rats.

Secretome analysis of Strongyloides venezuelensis parasitic stages reveals that soluble and insoluble proteins are involved in its parasitism

Background

Parasites excrete and secrete a wide range of molecules that act as the primary interface with their hosts and play critical roles in establishing parasitism during different stages of infection. Strongyloides venezuelensis is a gastrointestinal parasite of rats that is widely used as a laboratory model and is known to produce both soluble and insoluble (adhesive) secretions during its parasitic stages. However, little is known about the constituents of these secretions.

Results

Using mass spectrometry, we identified 436 proteins from the infective third-stage larvae (iL3s) and 196 proteins from the parasitic females of S. venezuelensis. The proteins that were secreted by the iL3s were enriched with peptidase activity, embryo development and the oxidation-reduction process, while those of the parasitic females were associated with glycolysis, DNA binding (histones) and other unknown functions. Trypsin inhibitor-like domain-containing proteins were identified as the main component of the adhesive secretion from parasitic females. An absence of secretion signals in many of the proteins indicated that they are secreted via non-classical secretion pathways.

Conclusions

We found that S. venezuelensis secretes a wide range of proteins to establish parasitism. This includes proteins that have previously been identified as being involved in parasitism in other helminths as well as proteins that are unique to this species. These findings provide insights into the molecular mechanisms underlying Strongyloides parasitism.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3266-x) contains supplementary material, which is available to authorized users.

Strongyloides infection in rodents: immune response and immune regulation

The human pathogenic nematode Strongyloides stercoralis infects approximately 30-100 million people worldwide. Analysis of the adaptive immune response to S. stercoralis beyond descriptive studies is challenging, as no murine model for the complete infection cycle is available. However, the combined employment of different models each capable of modelling some features of S. stercoralis life cycle and pathology has advanced our understanding of the immunological mechanisms involved in host defence. Here we review: (i) studies using S. stercoralis third stage larvae implanted in diffusion chambers in the subcutaneous tissue of mice that allow analysis of the immune response to the human pathogenic Strongyloides species; (ii) studies using Strongyloides ratti and Strongyloides venezuelensis that infect mice and rats to extend the analysis to the parasites intestinal life stage and (iii) studies using S. stercoralis infected gerbils to analyse the hyperinfection syndrome, a severe complication of human strongyloidiasis that is not induced by rodent specific Strongyloides spp. We provide an overview of the information accumulated so far showing that Strongyloides spp. elicits a classical Th2 response that culminates in different, site specific, effector functions leading to either entrapment and killing of larvae in the tissues or expulsion of parasitic adults from the intestine.

Karyotype and reproduction mode of the rodent parasite Strongyloides venezuelensis

SUMMARY Strongyloides venezuelensis is a parasitic nematode that infects rodents. Although Strongyloides species described to date are known to exhibit parthenogenetic reproduction in the parasitic stage of their life cycle and sexual reproduction in the free-living stage, we did not observe any free-living males in S. venezuelensis in our strain, suggesting that the nematode is likely to depend on parthenogenetic reproduction. We confirmed by cytological analysis that S. venezuelensis produces eggs by parthenogenesis during the parasitic stage of its life cycle. Phylogenetic analysis using nearly the full length of 18S and D3 region of 28S ribosomal RNA gene suggested that S. venezuelensis is distantly related to another rodent parasite, namely Strongyloides ratti, but more closely related to a ruminant parasite, Strongyloides papillosus. Karyotype analysis revealed S. venezuelensis reproduces with mitotic parthenogenesis, and has the same number of chromosomes as S. papillosus (2n = 4), but differs from S. ratti (2n = 6) in this regard. These results, taken together, suggest that S. venezuelensis evolved its parasitism for rodents independently from S. ratti and, therefore, is likely to have a different reproductive strategy.

Importance of Both Innate Immunity and Acquired Immunity for Rapid Expulsion of S. venezuelensis

In the first part of this review, we described the relevant roles of endogenous IL-33 for accumulation of ILC2 and eosinophils even in the lungs of Rag2^−/− mice. Type II alveolar epithelial (ATII) cells express IL-33 in their nucleus and infection with Strongyloides venezuelensis induces IL-33 production by increasing the number of ATII cells possibly by the action of chitin. IL-33 from ATII cells induces ILC2 proliferation and at the same time activates them to produce IL-5 and IL-13, which in combination induce lung eosinophilic inflammation, aiding to expel infected worms in the lungs. In the second part, we showed that, although AID^−/− mice normally develop Th2 cells and intestinal mastocytosis after infection with S. venezuelensis, they need adoptive transfers of immune sera from S. venezuelensis infected mice to obtain the capacity to promptly expel S. venezuelensis. Thus, intestinal nematode infection induces various Th2 immune responses (e.g., Th2 cell, ILC2, goblet cell hyperplasia, intestinal mastocytosis, smooth muscle cell contraction, local and systemic eosinophilia, and high serum level of IgE and IgG1). However, all of them are not necessary for rapid expulsion of intestinal nematodes. Instead, some combinations of Th2 immune responses are essentially required.