Vaccination with Strongyloides ratti heat shock protein 60 increases susceptibility to challenge infection by induction of Th1 response

Soil-transmitted helminthic vaccines: Where are we now?

It has been tested and proven that vaccination is still the best strategy to combat infectious diseases. However, to date, there are still no vaccines against human soil-transmitted helminthic diseases, despite their high prevalence globally, particularly in developing countries and rural areas with tropical climates and poor sanitation. The development of vaccines against helminths is riddled with obstacles. Helminths have a complex life cycle, multiple stages within the same host with stage-specific antigen expression, and the ability to regulate host immune reactions to evade the immune response. These elements contribute to the main challenge of helminthic vaccines: the identification of effective vaccine candidates. Therefore, this article reviews the current progress and potential future direction of soil-transmitted helminthic vaccines, particularly against Trichuris trichiura, Ascaris lumbricoides, Strongyloides stercoralis, Necator americanus and Ancylostoma duodenale. The study design employed was a systematic review, using qualitative meta-summary synthesis. Preclinical studies and clinical trials on the development of protein subunit vaccines against the five soil-transmitted helminths were searched on PubMed and Scopus. Effectiveness was indicated by a reduction in worm burden or larval output, an increase in specific IgG levels, or an increase in cytokine production. Our findings show that only the hookworm vaccine against N. americanus is in the clinical trial phase, while the rest is still in exploratory research and pre-clinical development phase.

Heat shock protein 60 in parasitic helminths: A role in immune responses and therapeutic applications

Heat shock protein 60 (HSP60) is an unique member of the heat shock protein family, being involved in parasite infections. To cope with harsh environments where parasites live, HSP60s are indispensable and involved in a variety of biological processes. HSP60s have relative low similarity among parasites, but their ATPase /Mg²⁺ active sites are highly conserved. The interactions of HSP60s with signaling pathway regulators in immune cells suggest a crucial role in immune responses, rendering them a potential therapeutic target. This paper reviews the current understandings of HSP60s in parasitic helminths in aspects of molecular characteristics, immunoregulatory responses and HSP60-based therapeutics.

IL-33 facilitates rapid expulsion of the parasitic nematode Strongyloides ratti from the intestine via ILC2- and IL-9-driven mast cell activation

Parasitic helminths are sensed by the immune system via tissue-derived alarmins that promote the initiation of the appropriate type 2 immune responses. Here we establish the nuclear alarmin cytokine IL-33 as a non-redundant trigger of specifically IL-9-driven and mast cell-mediated immunity to the intestinal parasite Strongyloides ratti. Blockade of endogenous IL-33 using a helminth-derived IL-33 inhibitor elevated intestinal parasite burdens in the context of reduced mast cell activation while stabilization of endogenous IL-33 or application of recombinant IL-33 reciprocally reduced intestinal parasite burdens and increased mast cell activation. Using gene-deficient mice, we show that application of IL-33 triggered rapid mast cell-mediated expulsion of parasites directly in the intestine, independent of the adaptive immune system, basophils, eosinophils or Gr-1+ cells but dependent on functional IL-9 receptor and innate lymphoid cells (ILC). Thereby we connect the described axis of IL-33-mediated ILC2 expansion to the rapid initiation of IL-9-mediated and mast cell-driven intestinal anti-helminth immunity.

Beneficial effects of Strongyloides venezuelensis antigen extract in acute experimental toxoplasmosis

BACKGROUND

Toxoplasma gondii is a protozoan with worldwide distribution and triggers a strong Th1 immune response in infected susceptible hosts. On the contrary, most helminth infections are characterized by Th2 immune response and the use of helminth-derived antigens to regulate immune response in inflammatory disorders has been broadly investigated.

OBJECTIVES

The aim of this study was to investigate whether treatment with Strongyloides venezuelensis antigen extract (SvAg) would alter immune response against T gondii.

METHODS

C57BL/6 mice were orally infected with T gondii and treated with SvAg, and parasitological, histological and immunological parameters were investigated.

RESULTS

It was observed that SvAg treatment improved survival rates of T gondii-infected mice. At day 7 post-infection, the parasite load was lower in the lung and small intestine of infected SvAg-treated mice than untreated infected mice. Remarkably, SvAg-treated mice infected with T gondii presented reduced inflammatory lesions in the small intestine than infected untreated mice and decreased intestinal and systemic levels of IFN-γ, TNF-α and IL-6. In contrast, SvAg treatment increased T gondii-specific IgA serum levels in infected mice.

CONCLUSIONS

S venezuelensis antigen extract has anti-parasitic and anti-inflammatory properties during T gondii infection suggesting as a possible alternative to parasite and inflammation control.

Rbpj expression in regulatory T cells is critical for restraining TH2 responses

The transcriptional regulator Rbpj is involved in T-helper (T_H) subset polarization, but its function in T_reg cells remains unclear. Here we show that T_reg-specific Rbpj deletion leads to splenomegaly and lymphadenopathy despite increased numbers of T_reg cells with a polyclonal TCR repertoire. A specific defect of Rbpj-deficient T_reg cells in controlling T_H2 polarization and B cell responses is observed, leading to the spontaneous formation of germinal centers and a T_H2-associated immunoglobulin class switch. The observed phenotype is environment-dependent and can be induced by infection with parasitic nematodes. Rbpj-deficient T_reg cells adopt open chromatin landscapes and gene expression profiles reminiscent of tissue-derived T_H2-polarized T_reg cells, with a prevailing signature of the transcription factor Gata-3. Taken together, our study suggests that T_reg cells require Rbpj to specifically restrain T_H2 responses, including their own excessive T_H2-like differentiation potential.

Transcriptional regulator Rbpj is involved in T-helper subset differentiation. Here the authors show that expression of Rbpj in regulatory T cells is required to both regulate T_H2 responses and regulate T_reg T_H2 differentiation potential.

Basophils are dispensable for the establishment of protective adaptive immunity against primary and challenge infection with the intestinal helminth parasite Strongyloides ratti

Infections with helminth parasites are controlled by a concerted action of innate and adaptive effector cells in the frame of a type 2 immune response. Basophils are innate effector cells that may also contribute to the initiation and amplification of adaptive immune responses. Here, we use constitutively basophil-deficient Mcpt8-Cre mice to analyze the impact of basophils during initiation and execution of the protective type 2 responses to both, a primary infection and a challenge infection of immune mice with the helminth parasite Strongyloides ratti. Basophil numbers expanded during parasite infection in blood and mesenteric lymph nodes. Basophil deficiency significantly elevated intestinal parasite numbers and fecal release of eggs and larvae during a primary infection. However, basophils were neither required for the initiation of a S. ratti-specific cellular and humoral type 2 immune response nor for the efficient protection against a challenge infection. Production of Th2 cytokines, IgG1 and IgE as well as mast cell activation were not reduced in basophil-deficient Mcpt8-Cre mice compared to basophil-competent Mcpt8-WT littermates. In addition, a challenge infection of immune basophil-deficient and WT mice resulted in a comparable reduction of tissue migrating larvae, parasites in the intestine and fecal release of eggs and L1 compared to mice infected for the first time. We have shown previously that S. ratti infection induced expansion of Foxp3+ regulatory T cells that interfered with efficient parasite expulsion. Here we show that depletion of regulatory T cells reduced intestinal parasite burden also in absence of basophils. Thus basophils were not targeted specifically by S. ratti-mediated immune evasive mechanisms. Our collective data rather suggests that basophils are non-redundant innate effector cells during murine Strongyloides infections that contribute to the early control of intestinal parasite burden.

Recombinant subunit vaccines for soil-transmitted helminths

Soil-transmitted helminths (STHs) collectively infect one fourth of all human beings, and the majority of livestock in the developing world. These gastrointestinal nematodes are the most important parasites on earth with regard to their prevalence in humans and livestock. Current anthelmintic drugs are losing their efficacies due to increasing drug resistance, particularly in STHs of livestock and drug treatment is often followed by rapid reinfection due to failure of the immune system to develop a protective response. Vaccines against STHs offer what drugs cannot accomplish alone. Because such vaccines would have to be produced on such a large scale, and be cost effective, recombinant subunit vaccines that include a minimum number of proteins produced in relatively simple and inexpensive expression systems are required. Here, we summarize all of the previous studies pertaining to recombinant subunit vaccines for STHs of humans and livestock with the goal of both informing the public of just how critical these parasites are, and to help guide future developments. We also discuss several key areas of vaccine development, which we believe to be critical for developing more potent recombinant subunit vaccines with broad-spectrum protection.

Th2/1 Hybrid Cells Occurring in Murine and Human Strongyloidiasis Share Effector Functions of Th1 Cells

Infections by the soil-transmitted threadworm Strongyloides stercoralis affect 30–100 million people worldwide, predominantly in tropic and sub-tropic regions. Here we assessed the T helper cell phenotypes in threadworm-infected patients and experimental murine infections with focus on CD4⁺ T cells co-expressing markers of Th2 and Th1 differentiation. We show that mice infected with the close relative S. ratti generate strong Th2 responses characterized by the expansion of CD4⁺ GATA-3⁺ cells expressing IL-4/-5/-13 in blood, spleen, gut-draining lymph nodes, lung and gut tissue. In addition to conventional Th2 cells, significantly increased frequencies of GATA-3⁺T-bet⁺ Th2/1-hybrid cells were detected in all organs and co-expressed Th2- and Th1-cytokines at intermediate levels. Assessing the phenotype of blood-derived CD4⁺ T cells from South Indian patients infected with S. stercoralis and local uninfected control donors we found that GATA-3 expressing Th2 cells were significantly increased in the patient cohort, coinciding with elevated eosinophil and IgE/IgG4 levels. A fraction of IL-4⁺CD4⁺ T cells simultaneously expressed IFN-γ hence displaying a Th2/1 hybrid phenotype. In accordance with murine Th2/1 cells, human Th2/1 cells expressed intermediate levels of Th2 cytokines. Contrasting their murine counterparts, human Th2/1 hybrids were marked by high levels of IFN-γ and rather low GATA-3 expression. Assessing the effector function of murine Th2/1 cells in vitro we found that Th2/1 cells were qualified for driving the classical activation of macrophages. Furthermore, Th2/1 cells shared innate, cytokine-driven effector functions with Th1 cells. Hence, the key findings of our study are that T helper cells with combined characteristics of Th2 and Th1 cells are integral to immune responses of helminth-infected mice, but also occur in helminth-infected humans and we suggest that Th2/1 cells are poised for the instruction of balanced immune responses during nematode infections.

Mucosal mast cells are indispensable for the timely termination of Strongyloides ratti infection

Mast cells and basophils are innate immune cells with overlapping functions that contribute to anti-helminth immunity. Mast cell function during helminth infection was previously studied using mast cell-deficient Kit-mutant mice that display additional mast cell-unrelated immune deficiencies. Here, we use mice that lack basophils or mucosal and connective tissue mast cells in a Kit-independent manner to re-evaluate the impact of each cell type during helminth infection. Neither mast cells nor basophils participated in the immune response to tissue-migrating Strongyloides ratti third-stage larvae, but both cell types contributed to the early expulsion of parasitic adults from the intestine. The termination of S. ratti infection required the presence of mucosal mast cells: Cpa3^Cre mice, which lack mucosal and connective tissue mast cells, remained infected for more than 150 days. Mcpt5^Cre R-DTA mice, which lack connective tissue mast cells only, and basophil-deficient Mcpt8^Cre mice terminated the infection after 1 month with wild-type kinetics despite their initial increase in intestinal parasite burden. Because Cpa3^Cre mice showed intact Th2 polarization and efficiently developed protective immunity after vaccination, we hypothesize that mucosal mast cells are non-redundant terminal effector cells in the intestinal epithelium that execute anti-helminth immunity but do not orchestrate it.

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.

Structural and functional characterization of a novel scFv anti-HSP60 of Strongyloides sp

Phage display is a powerful technology that selects specific proteins or peptides to a target. We have used Phage Display to select scFv (single-chain variable fragment) clones from a combinatorial library against total proteins of Strongyloides venezuelensis. After scFv characterization, further analysis demonstrated that this recombinant fragment of antibody was able to bind to an S. venezuelensis antigenic fraction of ~65 kDa, present in the body periphery and digestive system of infective larvae (L3), as demonstrated by immunofluorescence. Mass spectrometry results followed by bioinformatics analysis showed that this antigenic fraction was a heat shock protein 60 (HSP60) of Strongyloides sp. The selected scFv was applied in serodiagnosis by immune complexes detection in serum samples from individuals with strongyloidiasis using a sandwich enzyme-linked immunosorbent assay (ELISA), showing sensitivity of 97.5% (86.84–99.94), specificity of 98.81 (93.54–99.97), positive likelihood ratio of 81.60 and an area under the curve of 0.9993 (0.9973–1.000). Our study provided a novel monoclonal scFv antibody fragment which specifically bound to HSP60 of Strongyloides sp. and was applied in the development of an innovative serodiagnosis method for the human strongyloidiasis.

Cutting Edge: the BTLA-HVEM regulatory pathway interferes with protective immunity to intestinal Helminth infection

Helminths exploit intrinsic regulatory pathways of the mammalian immune system to dampen the immune response directed against them. In this article, we show that infection with the parasitic nematode Strongyloides ratti induced upregulation of the coinhibitory receptor B and T lymphocyte attenuator (BTLA) predominantly on CD4(+) T cells but also on a small fraction of innate leukocytes. Deficiency of either BTLA or its ligand herpes virus entry mediator (HVEM) resulted in reduced numbers of parasitic adults in the small intestine and reduced larval output throughout infection. Reduced parasite burden in BTLA- and HVEM-deficient mice was accompanied by accelerated degranulation of mucosal mast cells and increased Ag-specific production of the mast cell-activating cytokine IL-9. Our combined results support a model whereby BTLA on CD4(+) T cells and additional innate leukocytes is triggered by HVEM and delivers negative signals into BTLA(+) cells, thereby interfering with the protective immune response to this intestinal parasite.

Vaccination against helminth parasite infections

Helminth parasites infect over one fourth of the human population and are highly prevalent in livestock worldwide. In model systems, parasites are strongly immunomodulatory, but the immune system can be driven to expel them by prior vaccination. However, no vaccines are currently available for human use. Recent advances in vaccination with recombinant helminth antigens have been successful against cestode infections of livestock and new vaccines are being tested against nematode parasites of animals. Numerous vaccine antigens are being defined for a wide range of helminth parasite species, but greater understanding is needed to define the mechanisms of vaccine-induced immunity, to lay a rational platform for new vaccines and their optimal design. With human trials underway for hookworm and schistosomiasis vaccines, a greater integration between veterinary and human studies will highlight the common molecular and mechanistic pathways, and accelerate progress towards reducing the global health burden of helminth infection.

Foxp3⁺ regulatory T cells delay expulsion of intestinal nematodes by suppression of IL-9-driven mast cell activation in BALB/c but not in C57BL/6 mice

Accumulating evidence suggests that IL-9-mediated immunity plays a fundamental role in control of intestinal nematode infection. Here we report a different impact of Foxp3⁺ regulatory T cells (Treg) in nematode-induced evasion of IL-9-mediated immunity in BALB/c and C57BL/6 mice. Infection with Strongyloides ratti induced Treg expansion with similar kinetics and phenotype in both strains. Strikingly, Treg depletion reduced parasite burden selectively in BALB/c but not in C57BL/6 mice. Treg function was apparent in both strains as Treg depletion increased nematode-specific humoral and cellular Th2 response in BALB/c and C57BL/6 mice to the same extent. Improved resistance in Treg-depleted BALB/c mice was accompanied by increased production of IL-9 and accelerated degranulation of mast cells. In contrast, IL-9 production was not significantly elevated and kinetics of mast cell degranulation were unaffected by Treg depletion in C57BL/6 mice. By in vivo neutralization, we demonstrate that increased IL-9 production during the first days of infection caused accelerated mast cell degranulation and rapid expulsion of S. ratti adults from the small intestine of Treg-depleted BALB/c mice. In genetically mast cell-deficient (Cpa3-Cre) BALB/c mice, Treg depletion still resulted in increased IL-9 production but resistance to S. ratti infection was lost, suggesting that IL-9-driven mast cell activation mediated accelerated expulsion of S. ratti in Treg-depleted BALB/c mice. This IL-9-driven mast cell degranulation is a central mechanism of S. ratti expulsion in both, BALB/c and C57BL/6 mice, because IL-9 injection reduced and IL-9 neutralization increased parasite burden in the presence of Treg in both strains. Therefore our results suggest that Foxp3⁺ Treg suppress sufficient IL-9 production for subsequent mast cell degranulation during S. ratti infection in a non-redundant manner in BALB/c mice, whereas additional regulatory pathways are functional in Treg-depleted C57BL/6 mice.

Potential of recombinant inorganic pyrophosphatase antigen as a new vaccine candidate against Baylisascaris schroederi in mice

The intestinal nematode Baylisascaris schroederi is an important cause of death for wild and captive giant pandas. Inorganic pyrophosphatases (PPases) are critical for development and molting in nematode parasites and represent potential targets for vaccination. Here, a new PPase homologue, Bsc-PYP-1, from B. schroederi was identified and characterized, and its potential as a vaccine candidate was evaluated in a mouse challenge model. Sequence alignment of PPases from nematode parasites and other organisms show that Bsc-PYP-1 is a nematode-specific member of the family I soluble PPases. Immunohistochemistry revealed strong localization of native Bsc-PYP-1 to the body wall, gut epithelium, ovary and uterus of adult female worms. Additionally, Bsc-PYP-1 homologues were found in roundworms infecting humans (Ascaris lumbricoides), swine (Ascaris suum) and dogs (Toxocara canis). In two vaccine trials, recombinant Bsc-PYP-1 (rBsc-PYP-1) formulated with Freund complete adjuvant induced significantly high antigen-specific immunoglobulin (Ig)G but no IgE or IgM responses. Analysis of IgG-subclass profiles revealed a greater increase of IgG1 than IgG2a. Splenocytes from rBsc-PYP-1/FCA-immunized mice secreted low levels of T helper (Th)1-type cytokines, interferon-γ and interleukin (IL)-2, while producing significantly high levels of IL-10 and significantly elevated levels of IL-4 (Th2 cytokines) after stimulation with rBsc-PYP-1 in vitro. Finally, vaccinated mice had 69.02–71.15% reductions (in 2 experiments) in larval recovery 7 days post-challenge (dpc) and 80% survival at 80 dpc. These results suggest that Th2-mediated immunity elicited by rBsc-PYP-1 provides protection against B. schroederi, and the findings should contribute to further development of Bsc-PYP-1 as a candidate vaccine against baylisascariasis.

Passive immunization with a monoclonal IgM antibody specific for Strongyloides ratti HSP60 protects mice against challenge infection

It is estimated that 30-100 million people are infected with the pathogenic nematode Strongyloides stercoralis worldwide but parasite control is still based on anti-helminthic treatment. To develop protective vaccination strategies, we use the murine model of Strongyloides ratti infection. We have shown recently that vaccination with alum-precipitated, but not with native or CFA-emulsified S. ratti heat shock protein 60 (srHSP60) conferred protection to challenge infection. Here we describe the generation of a monoclonal IgM specific for srHSP60. Anti-srHSP60 detected human and srHSP60 and stained S. ratti infective larvae in vitro. Passive immunization of mice with monoclonal anti-srHSP60 IgM led to reduced numbers of migrating larvae in lung and head, reduced numbers of parasitic adults in the small intestine and reduced larval output upon S. ratti challenge infection. Taken together, our findings highlight the relevance of srHSP60 as vaccine candidate for the induction of antibody-mediated protection against Strongyloides infection.