Strongyloides ratti infection induces transient nematode-specific Th2 response and reciprocal suppression of IFN-gamma production in mice

Strongyloidiasis

Strongyloidiasis is a neglected tropical disease caused primarily by the roundworm Strongyloides stercoralis. Strongyloidiasis is most prevalent in Southeast Asia and the Western Pacific. Although cases have been documented worldwide, global prevalence is largely unknown due to limited surveillance. Infection of the definitive human host occurs via direct skin penetration of the infective filariform larvae. Parasitic females reside in the small intestine and reproduce via parthenogenesis, where eggs hatch inside the host before rhabditiform larvae are excreted in faeces to begin the single generation free-living life cycle. Rhabditiform larvae can also develop directly into infectious filariform larvae in the gut and cause autoinfection. Although many are asymptomatic, infected individuals may report a range of non-specific gastrointestinal, respiratory or skin symptoms. Autoinfection may cause hyperinfection and disseminated strongyloidiasis in immunocompromised individuals, which is often fatal. Diagnosis requires direct examination of larvae in clinical specimens, positive serology or nucleic acid detection. However, there is a lack of standardization of techniques for all diagnostic types. Ivermectin is the treatment of choice. Control and elimination of strongyloidiasis will require a multifaceted, integrated approach, including highly sensitive and standardized diagnostics, active surveillance, health information, education and communication strategies, improved water, sanitation and hygiene, access to efficacious treatment, vaccine development and better integration and acknowledgement in current helminth control programmes.

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'.

Retinoic acid drives intestine-specific adaptation of effector ILC2s originating from distant sites

Adaptation of immune cells to tissue-specific microenvironments is a crucial process in homeostasis and inflammation. Here, we show that murine effector type 2 innate lymphoid cells (ILC2s) from various organs are equally effective in repopulating ILC2 niches in other anatomical locations where they adapt tissue-specific phenotypes of target organs. Single-cell transcriptomics of ILC2 populations revealed upregulation of retinoic acid (RA) signaling in ILC2s during adaptation to the small intestinal microenvironment, and RA signaling mediated reprogramming of kidney effector ILC2s toward the small intestinal phenotype in vitro and in vivo. Inhibition of intestinal ILC2 adaptation by blocking RA signaling impaired worm expulsion during Strongyloides ratti infection, indicating functional importance of ILC2 tissue imprinting. In conclusion, this study highlights that effector ILC2s retain the ability to adapt to changing tissue-specific microenvironments, enabling them to exert tissue-specific functions, such as promoting control of intestinal helminth infections.

IRF4 is required for migration of CD4+ T cells to the intestine but not for Th2 and Th17 cell maintenance

The transcription factor Interferon Regulatory Factor 4 (IRF4) is central in control of T cell activation and differentiation. Deficiency of IRF4 results in severe immune deficiency and affects maturation and function of most if not all T cell subsets. Here we use mouse infection models for Citrobacter rodentium and Strongyloides ratti to analyze the function of IRF4 in T helper (Th) 17 and Th2 cell responses, respectively. IRF4 deficient mice were impaired in the control of both pathogens, failed to mount Th17 and Th2 cell responses and showed impaired recruitment of T helper cells to the intestine, the infection site of both pathogens. Compromised intestinal migration was associated with reduced expression of the intestinal homing receptors α4β7 integrin, CCR9 and GPR15. Identification of IRF4 binding sites in the gene loci of these receptors suggests a direct control of their expression by IRF4. Competitive T cell transfer assays further demonstrated that loss of one functional Irf4 allele already affected intestinal accumulation and Th2 and Th17 cell generation, indicating that lower IRF4 levels are of disadvantage for Th2 and Th17 cell differentiation as well as their migration to the intestine. Conversion of peripheral CD4⁺ T cells from an Irf4 wildtype to an Irf4 heterozygous or from an Irf4 heterozygous to a homozygous mutant genotype after C. rodentium or S. ratti infection did not reduce their capacity to produce Th17 or Th2 cytokines and only partially affected their persistence in the intestine, revealing that IRF4 is not essential for maintenance of the Th2 and Th17 phenotype and for survival of these T helper cells in the intestine. In conclusion, we demonstrate that the expression levels of IRF4 determine Th2 and Th17 cell differentiation and their intestinal accumulation but that IRF4 expression is not crucial for Th2 and Th17 cell survival.

Eosinophils and Neutrophils Eliminate Migrating Strongyloides ratti Larvae at the Site of Infection in the Context of Extracellular DNA Trap Formation

Parasitic nematodes such as hookworms actively penetrate the skin of their hosts, encountering skin-resident innate immune cells that represent the host´s first line of defense. Here we use Strongyloides ratti as a model for an intestinal helminth parasite with tissue migrating stages. We show that interception and killing of migrating larvae in mice during a 1^st infection occurred predominantly in skin and muscle tissue before larvae migrated via lung and head tissue to the intestine. Inhibition of larval migration was even more efficient in immune mice during a 2^nd infection where larvae barely left the site of entry i.e. the foot. Using cell-deficient mice we show that interception in the tissue was predominantly mediated by neutrophils and eosinophils while basophils and mast cells were dispensable in vivo. Likewise, neutrophils and eosinophils inhibited S. ratti L3 motility in vitro in the context of ETosis. Thereby eosinophils were strictly dependent on the presence of anti-S. ratti antibodies while neutrophils inhibited L3 motility as such. Also, MPO and MMP-9 were released by neutrophils in response to L3 alone, but immune plasma further stimulated MPO release in an antibody-dependent manner. In summary, our findings highlight the central role of the skin as first line of defense against helminth parasites in both, innate and adaptive immunity.

Transgenic expression of a T cell epitope in Strongyloides ratti reveals that helminth-specific CD4+ T cells constitute both Th2 and Treg populations

Helminths are distinct from microbial pathogens in both size and complexity, and are the likely evolutionary driving force for type 2 immunity. CD4+ helper T cells can both coordinate worm clearance and prevent immunopathology, but issues of T cell antigen specificity in the context of helminth-induced Th2 and T regulatory cell (Treg) responses have not been addressed. Herein, we generated a novel transgenic line of the gastrointestinal nematode Strongyloides ratti expressing the immunodominant CD4+ T cell epitope 2W1S as a fusion protein with green fluorescent protein (GFP) and FLAG peptide in order to track and study helminth-specific CD4+ T cells. C57BL/6 mice infected with this stable transgenic line (termed Hulk) underwent a dose-dependent expansion of activated CD44hiCD11ahi 2W1S-specific CD4+ T cells, preferentially in the lung parenchyma. Transcriptional profiling of 2W1S-specific CD4+ T cells isolated from mice infected with either Hulk or the enteric bacterial pathogen Salmonella expressing 2W1S revealed that pathogen context exerted a dominant influence over CD4+ T cell phenotype. Interestingly, Hulk-elicited 2W1S-specific CD4+ T cells exhibited both Th2 and Treg phenotypes and expressed high levels of the EGFR ligand amphiregulin, which differed greatly from the phenotype of 2W1S-specific CD4+ T cells elicited by 2W1S-expressing Salmonella. While immunization with 2W1S peptide did not enhance clearance of Hulk infection, immunization did increase total amphiregulin production as well as the number of amphiregulin-expressing CD3+ cells in the lung following Hulk infection. Altogether, this new model system elucidates effector as well as immunosuppressive and wound reparative roles of helminth-specific CD4+ T cells. This report establishes a new resource for studying the nature and function of helminth-specific T cells.

Elucidating different pattern of immunoregulation in BALB/c and C57BL/6 mice and their F1 progeny

Helminths are large multicellular parasites that infect one quarter of the human population. To prolong their survival, helminths suppress the immune responses of their hosts. Strongyloides ratti delays its expulsion from the gut by induction of regulatory circuits in a mouse strain-specific manner: depletion of Foxp3⁺ regulatory T cells (Treg) improves the anti-S. ratti immunity in BALB/c but not in C57BL/6 mice. In the current study we compare the hierarchy of immunoregulatory pathways in BALB/c, C57BL/6 mice and their F1 progeny (BALB/c × C57BL/6). Using multicolor flow cytometry, we show that S. ratti induces a distinct pattern of inhibitory checkpoint receptors by Foxp3⁺ Treg and Foxp3^- T cells. Intensity of expression was highest in C57BL/6 and lowest in BALB/c mice, while the F1 cross had an intermediate phenotype or resembled BALB/c mice. Treg subsets expanded during infection in all three mouse strains. Similar to BALB/c mice, depletion of Treg reduced intestinal parasite burden and increased mucosal mast cell activation in S. ratti-infected F1 mice. Our data indicate that Treg dominate the regulation of immune responses in BALB/c and F1 mice, while multiple regulatory layers exist in C57BL/6 mice that may compensate for the absence of Treg.

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.

Advances in the Development of Anti-Haemonchus contortus Vaccines: Challenges, Opportunities, and Perspectives

The gastrointestinal nematode parasite Haemonchus contortus (H. contortus) is a resident of tropical and subtropical regions worldwide that imposes significant production losses, economic losses, and animal health issues in the small ruminant industry, particularly sheep and goats. Considerable efforts have been made to understand how immunity is elicited against H. contortus infection. Various potential vaccine antigens have been tested by different methods and strategies applied in animal models, and significant progress has been made in the development of vaccines against H. contortus. This review highlighted and shared the knowledge about the current understanding of host immune responses to H. contortus and ongoing challenges in the development of a protective, effective, and long-lasting vaccine against H. contortus infection. We have also pinpointed some achievements and failures in the development and testing of vaccines, which will establish a road map for future research directions to explore new effective vaccine candidates for controlling and preventing H. contortus infection.

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.

Interleukin-9 promotes early mast cell-mediated expulsion of Strongyloides ratti but is dispensable for generation of protective memory

IL-9 is a cytokine with pleiotropic function that mediates allergic inflammation and immunity to intestinal helminth parasites. Accumulating evidence suggests that IL-9 acts via both, initiation and regulation of adaptive immune responses and direct activation of intestinal effector pathways. Here we use IL-9 receptor deficient mice on BALB/c and C57BL/6 genetic background to dissect effector and regulatory functions of IL-9 during infection with the parasitic nematode Strongyloides ratti. IL-9 receptor-deficient mice displayed increased intestinal parasite burden and prolonged infection irrespective of the genetic background of the mice. Increased parasite burden was correlated to a reciprocally reduced early degranulation of mucosal mast cells, reduced intestinal IL-13 expression and caused by IL-9 receptor deficiency on hematopoietic cells. We observed additional significant changes in the adaptive immune response to S. ratti infection in the absence of the IL-9 receptor that depended on the mouse strain. However, the generation of protective memory to a second infection was intact in IL-9 receptor-deficient mice, irrespective of the genetic background. In summary, our results support a central role for IL-9 as an early mast cell activating effector cytokine during intestinal helminth infection while non-redundant functions in the initiation and amplification of adaptive immune responses were not apparent.

High prevalence of S. Stercoralis infection among patients with Chagas disease: A retrospective case-control study

BACKGROUND

We evaluate the association between Trypanosoma cruzi infection and strongyloidiasis in a cohort of Latin American (LA) migrants screened for both infections in a non-endemic setting.

METHODOLOGY

Case-control study including LA individuals who were systematically screened for T. cruzi infection and strongyloidiasis between January 2013 and April 2015. Individuals were included as cases if they had a positive serological result for Strongyloides stercoralis. Controls were randomly selected from the cohort of individuals screened for T. cruzi infection that tested negative for S. stercoralis serology. The association between T. cruzi infection and strongyloidiasis was evaluated by logistic regression models.

PRINCIPAL FINDINGS

During the study period, 361 individuals were screened for both infections. 52 (14.4%) individuals had a positive serological result for strongyloidiasis (cases) and 104 participants with negative results were randomly selected as controls. 76 (48.7%) indiviuals had a positive serological result for T. cruzi. Factors associated with a positive T. cruzi serology were Bolivian origin (94.7% vs 78.7%; p = 0.003), coming from a rural area (90.8% vs 68.7%; p = 0.001), having lived in an adobe house (88.2% vs 70%; p = 0.006) and a referred contact with triatomine bugs (86.7% vs 63.3%; p = 0.001). There were more patients with a positive S. stercoralis serology among those who were infected with T. cruzi (42.1% vs 25%; p = 0.023). Epidemiological variables were not associated with a positive strongyloidiasis serology. T. cruzi infection was more frequent among those with strongyloidiasis (61.5% vs 42.3%; p = 0.023). In multivariate analysis, T. cruzi infection was associated with a two-fold increase in the odds of strongyloidiasis (OR 2.23; 95% CI 1.07-4.64; p = 0.030).

CONCLUSIONS

T. cruzi infection was associated with strongyloidiasis in LA migrants attending a tropical diseases unit even after adjusting for epidemiological variables. These findings should encourage physicians in non-endemic settings to implement a systematic screening for both infections in LA individuals.

Strongyloides stercoralis infection increases the likelihood to detect Trypanosoma cruzi DNA in peripheral blood in Chagas disease patients

OBJECTIVES

In a previous study performed by our group, Strongyloides stercoralis infection in patients with Chagas disease was associated with higher proportion of Trypanosoma cruzi DNA detection in peripheral blood. The aim of the study was to confirm this association in a larger cohort of patients.

METHODS

Cross-sectional study of all patients with Chagas disease diagnosed from 2005 to 2015 during blood donation at the Catalan Blood Bank. Demographic data and T. cruzi RT-PCR were collected. S. stercoralis infection diagnosis was based on a serological test.

RESULTS

Two hundred and two blood donors were included. T. cruzi RT-PCR was positive in 72 (35.6%) patients, and S. stercoralis serology was positive in 22 (10.9%) patients. Patients with positive S. stercoralis serology had higher proportion of positive T. cruzi RT-PCR than those with negative serology (54.5% vs. 33.3%, P = 0.050), and the difference increased when taking a serological index cut-off of 2.5, which increases the specificity of the test to detect a confirmed strongyloidiasis (60% vs. 33%, P = 0.017).

CONCLUSIONS

Patients with Chagas disease with positive S. stercoralis serology had higher proportion of positive T. cruzi RT-PCR in peripheral blood than those with negative serology, which reflects the potential immunomodulatory effects of S. stercoralis in T. cruzi co-infected patients.

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.

Impact of Helminth Infection on the Clinical and Microbiological Presentation of Chagas Diseases in Chronically Infected Patients

BACKGROUND

Helminth infections are highly prevalent in tropical and subtropical countries, coexisting in Chagas disease endemic areas. Helminth infections in humans may modulate the host immune system, changing the Th1/Th2 polarization. This immunological disturbance could modify the immune response to other infections. The aim of this study is to evaluate the relationship between clinical, microbiological and epidemiological characteristics of Chagas disease patients, with the presence of helminth infection.

METHODS

A prospective observational study was conducted at Vall d'Hebron University Hospital (Barcelona, Spain). Inclusion criteria were: age over 18 years, diagnosis of Chagas disease, and not having received specific treatment for Chagas disease previously to the inclusion. The study protocol included Chagas disease assessment (cardiac and digestive evaluation, detection of T. cruzi DNA measured by PCR in peripheral blood), and helminth infection diagnosis (detection of IgG anti-Strongyloides stercoralis by ELISA, microscopic examination of stool samples from three different days, and specific faecal culture for S. stercoralis larvae).

RESULTS

Overall, 65 patients were included, median age was 38 years, 75.4% were women and most of them came from Bolivia. Cardiac and digestive involvement was present in 18.5% and 27.7% of patients respectively. T. cruzi PCR was positive in 28 (43.1%) patients. Helminth infection was diagnosed in 12 (18.5%) patients. No differences were observed in clinical and epidemiological characteristics between patients with and without helminth infection. Nevertheless, the proportion of patients with positive T. cruzi PCR was higher among patients with helminth infection compared with patients without helminth infection (75% vs 35.8%, p = 0.021).

CONCLUSIONS

We observed a high prevalence of S. stercoralis infection among chronic Chagas disease patients attended in our tropical medicine unit. Strongyloidiasis was associated with significantly higher proportion of positive T. cruzi RT-PCR determined in peripheral blood.

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.

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.

Strongyloides stercoralis infection in marmosets: replication of complicated and uncomplicated human disease and parasite biology

BACKGROUND

Strongyloides stercoralis can undergo an alternative autoinfective life cycle in the host, which, in some individuals can lead to a lethal infection. However, due to a number of factors, such as, the majority of those infected are from low-income backgrounds and the limitation in experimental models for studying human S. stercoralis, strongyloidiasis remains neglected. Improved knowledge of animal models that are susceptible to this parasite is needed in order to investigate the immunological mechanisms involved during infection and in particular to further understand the natural history of the autoinfective cycle.

METHODS

Callithrix penicillata were inoculated subcutaneously with 100 (n = 2), 300 (n = 4) or 500 (n = 9) third-stage infective larvae (L3i) of S. stercoralis of human origin. Three marmosets received smaller inocula (i.e., one received 100 and two received 300 L3i) to ensure a greater capacity to withstand the infection after immunosuppression, which was triggered by administration of dexamethasone during early patency. Qualitative faecal analyses began at 7 days post-infection (DPI), and semi-quantitative tests were also performed for the dexamethasone-treated primates and the three matched controls. During the necropsies, specimens of S. stercoralis were recovered and tissue fragments were processed for histopathology.

RESULTS

The mean prepatency and patency periods were 16.1 ± 3.0 and 161.1 ± 72.2 DPI, respectively. The marmosets typically tolerated the infection well, but immunosuppressed individuals exhibited higher numbers of larvae in the faeces and progressive clinical deterioration with late disseminated infection. In these cases, the number of females recovered was significantly higher than the number of inoculated L3i. Large quantities of larvae were observed migrating through the host tissues, and histopathology revealed pulmonary and intestinal injuries consistent with those observed in human strongyloidiasis.

CONCLUSIONS

Both complicated and uncomplicated strongyloidiasis occur in C. penicillata that is described as a susceptible small non-human primate model for S. stercoralis. This host permits the maintenance of a human strain of the parasite in the laboratory and can be useful for experimental investigations of strongyloidiasis. In parallel, we discuss data related to the autoinfective cycle that provides new insights into the biology of S. stercoralis.

Helminth parasites alter protection against Plasmodium infection

More than one-third of the world's population is infected with one or more helminthic parasites. Helminth infections are prevalent throughout tropical and subtropical regions where malaria pathogens are transmitted. Malaria is the most widespread and deadliest parasitic disease. The severity of the disease is strongly related to parasite density and the host's immune responses. Furthermore, coinfections between both parasites occur frequently. However, little is known regarding how concomitant infection with helminths and Plasmodium affects the host's immune response. Helminthic infections are frequently massive, chronic, and strong inductors of a Th2-type response. This implies that infection by such parasites could alter the host's susceptibility to subsequent infections by Plasmodium. There are a number of reports on the interactions between helminths and Plasmodium; in some, the burden of Plasmodium parasites increased, but others reported a reduction in the parasite. This review focuses on explaining many of these discrepancies regarding helminth-Plasmodium coinfections in terms of the effects that helminths have on the immune system. In particular, it focuses on helminth-induced immunosuppression and the effects of cytokines controlling polarization toward the Th1 or Th2 arms of the immune response.

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.

Increased susceptibility to Strongyloides venezuelensis infection is related to the parasite load and absence of major histocompatibility complex (MHC) class II molecules

In human and murine models strongyloidiasis induce a Th2 type response. In the current study we investigated the role of different loads of Strongyloides venezuelensis in the immune response raised against the parasite and the participation of the major histocompatibility complex (MHC) class II molecule in the disease outcome in face of the different parasite burden. The C57BL/6 wild type (WT) and MHC II(-/-) mice were individually inoculated by subcutaneous injection with 500 or 3000 S. venezuelensis L3. The MHC II(-/-) mice infected with 3000L3 were more susceptible to S. venezuelensis infection when compared with WT groups, in which the parasite was completely eliminated. The production of Th2 cytokines and specific IgG1 or IgE antibodies against parasite were significantly lowered in MHC II(-/-) infected mice with different larvae inoculums. The infection of MHC II(-/-) mice with S. venezuelensis induced slight inflammatory alterations in the small intestine, and these lesions were lower when compared with WT mice, irrespective of the parasite load utilized to infect animals. Finally, we concluded that MHC class II molecules are essential in the immune response against S. venezuelensis mainly when infection occurs with high parasite inoculum.

Suppression of ovine lymphocyte activation by Teladorsagia circumcincta larval excretory-secretory products

Teladorsagia circumcincta is an important pathogenic nematode of sheep. It has been demonstrated previously that stimulation of murine T lymphocytes with excretory-secretory (ES) products derived from fourth stage larvae of T. circumcincta (Tci-L4-ES) results in de novo expression of Foxp3, a transcription factor intimately involved in regulatory T cell function. In the current study, Foxp3⁺ T cell responses in the abomasum and the effects of Tci-L4-ES on ovine peripheral blood mononuclear cells (PBMC) following T. circumcincta infection were investigated. T. circumcincta infection resulted in a significant increase in numbers of abomasal Foxp3⁺ T cells, but not an increase in the proportion of T cells expressing Foxp3. Unlike in mice, Tci-L4-ES was incapable of inducing T cell Foxp3 expression but instead suppressed mitogen-induced and antigen-specific activation and proliferation of ovine PBMC in vitro. This effect was heat labile, suggesting that it is mediated by protein(s). Suppression was associated with up-regulation of interleukin-10 (IL-10) mRNA, and specific monoclonal antibody neutralisation of IL-10 resulted in a 50% reduction in suppression, indicating involvement of the IL-10 signaling pathway. Suppression was significantly reduced in PBMC isolated from T. circumcincta infected vs. helminth-naïve lambs, and this reduction in suppression was associated with an increase in Tci-L4-ES antigen-specific T cells within the PBMC. In conclusion, we have identified a mechanism by which T. circumcincta may modulate the host adaptive immune response, potentially assisting survival of the parasite within the host. However, the impact of Tci-L4-ES-mediated lymphocyte suppression during T. circumcincta infection remains to be determined.

Intestinal mast cells and eosinophils in relation to Strongyloides ratti adult expulsion from the small and large intestines of rats

Mucosal mast cells (MMC) play a crucial role in the expulsion of Strongyloides ratti adults from the small intestine of mice. We reported the large intestinal parasitism of S. ratti in rats, and there has been no report on MMC in the large intestine of the natural host. We studied kinetics of MMC, together with eosinophils, in the upper and lower small intestines, caecum and colon of infected rats. Two distinct phases of mastocytosis were revealed: one in the upper small intestine triggered by stimulation of ‘ordinary’ adults, and the other in the colon stimulated by ‘immune-resistant’ adults that started parasitizing the colon around 19 days post-infection. In all 4 intestinal sites, the MMC peaks were observed 5–7 days after the number of adult worms became the maximum and the height of MMC peaks appeared to be dependent on the number of parasitic adults, suggesting an important role played by worms themselves in the MMC buildup.

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.

Nematode-induced interference with the anti-Plasmodium CD8+ T-cell response can be overcome by optimizing antigen administration

Malaria is still responsible for up to 1 million deaths per year worldwide, highlighting the need for protective malaria vaccines. Helminth infections that are prevalent in malaria endemic areas can modulate immune responses of the host. Here we show that Strongy-Ioides ratti, a gut-dwelling nematode that causes transient infections, did not change the efficacy of vaccination against Plasmodium berghei. An ongoing infection with Litomosoides sigmodontis, a tissue-dwelling filaria that induces chronic infections in BALB/c mice, significantly interfered with vaccination efficacy. The induction of P. berghei circumspor-ozoite protein (CSP)-specific CD8(+) T cells, achieved by a single immunization with a CSP fusion protein, was diminished in L. sigmodontis-infected mice. This modulation was reflected by reduced frequencies of CSP-specific CD8(+) T cells, reduced CSP-specific IFN-y and TNF-a production, reduced CSP-specific cytotoxicity, and reduced protection against P. berghei challenge infection. Implementation of a more potent vaccine regime, by first priming with CSP-expressing recombinant live Salmonella prior to CSP fusion protein immunization, restored induction of CSP-specific CD8(+) T cells and conferred almost sterile immunity to P. berghei challenge infection also in L. sigmodontis-infected mice. In summary, we show that appropriate vaccination regimes can overcome helminth-induced interference with vaccination efficacy.

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

The control of strongyloidiasis affecting approximately 100 million people - caused by the gastrointestinal nematode Strongyloides stercoralis - is still based on anti-helminthic treatment. In the current study we analysed the immune response to Strongyloides ratti heat shock protein 60 (srHSP60) as a possible vaccine candidate in the murine system. We show that srHSP60 is a target of both, humoral and cellular response in S. ratti-infected mice. Strikingly, vaccination with srHSP60 without adjuvant or with CFA induced a S. ratti-specific Th1 response in vivo that did not confer protection but slightly increased larval output during challenge infection. Using in vitro T cell stimulation assays we provide further evidence that srHSP60 skewed activated T cells towards a Th1 response that interfered with efficient clearance of S. ratti infection. Vaccination with alum-precipitated srHSP60, in contrast, overruled the Th1-inducing activity intrinsic to srHSP60, induced a Th2 response, and conferred partial protection against a challenge infection. As srHSP60 is actively secreted by S. ratti during all life stages, our findings strongly suggest that srHSP60 induced polarization towards a Th1 response reflects a mechanism of immune evasion by this pathogenic nematode.

Innate and adaptive immunity to the nematode Strongyloides stercoralis in a mouse model

Mice have been used to the study the mechanisms of protective innate and adaptive immunity to larval Strongyloides stercoralis. During primary infection, neutrophils and eosinophils are attracted by parasite components and kill the larvae by release of granule products. Eosinophils also function as antigen-presenting cells for the induction of a Th2 response. B cells produce both IgM and IgG that collaborate with neutrophils to kill worms in the adaptive immune response. Vaccine studies have identified a recombinant diagnostic antigen that induced high levels of immunity to infection with S. stercoralis in mice. These studies demonstrate that there are redundancies in the mechanisms used by the immune response to kill the parasite and that a vaccine with a single antigen may be suitable as a prophylactic vaccine to prevent human strongyloidiasis.

Life cycle stage-resolved proteomic analysis of the excretome/secretome from Strongyloides ratti--identification of stage-specific proteases

A wide range of biomolecules, including proteins, are excreted and secreted from helminths and contribute to the parasite's successful establishment, survival, and reproduction in an adverse habitat. Excretory and secretory proteins (ESP) are active at the interface between parasite and host and comprise potential targets for intervention. The intestinal nematode Strongyloides spp. exhibits an exceptional developmental plasticity in its life cycle characterized by parasitic and free-living generations. We investigated ESP from infective larvae, parasitic females, and free-living stages of the rat parasite Strongyloides ratti, which is genetically very similar to the human pathogen, Strongyloides stercoralis. Proteomic analysis of ESP revealed 586 proteins, with the largest number of stage-specific ESP found in infective larvae (196), followed by parasitic females (79) and free-living stages (35). One hundred and forty proteins were identified in all studied stages, including anti-oxidative enzymes, heat shock proteins, and carbohydrate-binding proteins. The stage-selective ESP of (1) infective larvae included an astacin metalloproteinase, the L3 Nie antigen, and a fatty acid retinoid-binding protein; (2) parasitic females included a prolyl oligopeptidase (prolyl serine carboxypeptidase), small heat shock proteins, and a secreted acidic protein; (3) free-living stages included a lysozyme family member, a carbohydrate-hydrolyzing enzyme, and saponin-like protein. We verified the differential expression of selected genes encoding ESP by qRT-PCR. ELISA analysis revealed the recognition of ESP by antibodies of S. ratti-infected rats. A prolyl oligopeptidase was identified as abundant parasitic female-specific ESP, and the effect of pyrrolidine-based prolyl oligopeptidase inhibitors showed concentration- and time-dependent inhibitory effects on female motility. The characterization of stage-related ESP from Strongyloides will help to further understand the interaction of this unique intestinal nematode with its host.

Stage-specific excretory-secretory small heat shock proteins from the parasitic nematode Strongyloides ratti--putative links to host's intestinal mucosal defense system

In a search for molecules involved in the interaction between intestinal nematodes and mammalian mucosal host cells, we performed MS to identify excretory-secretory proteins from Strongyloides ratti. In the excretory-secretory proteins of the parasitic female stage, we detected, in addition to other peptides, peptides homologous with the Caenorhabditis elegans heat shock protein (HSP)-17, named Sra-HSP-17.1 (∼ 19 kDa) and Sra-HSP-17.2 (∼ 18 kDa), with 49% amino acid identity. The full-length cDNAs (483 bp and 474 bp, respectively) were identified, and the genomic organization was analyzed. To allow further characterization, the proteins were recombinantly expressed and purified. Profiling of transcription by quantitative real-time-PCR and of protein by ELISA in various developmental stages revealed parasitic female-specific expression. Sequence analyses of both the DNA and amino acid sequences showed that the two proteins share a conserved α-crystallin domain and variable N-terminals. The Sra-HSP-17s showed the highest homology with the deduced small HSP sequence of the human pathogen Strongyloides stercoralis. We observed strong immunogenicity of both proteins, leading to strong IgG responses following infection of rats. Flow cytometric analysis indicated the binding of Sra-HSP-17s to the monocyte-macrophage lineage but not to peripheral lymphocytes or neutrophils. A rat intestinal epithelial cell line showed dose-dependent binding to Sra-HSP-17.1, but not to Sra-HSP-17.2. Exposed monocytes released interleukin-10 but not tumor necrosis factor-α in response to Sra-HSP-17s, suggesting the possible involvement of secreted female proteins in host immune responses.

Strongyloides ratti infection induces expansion of Foxp3+ regulatory T cells that interfere with immune response and parasite clearance in BALB/c mice

To escape expulsion by their host's immune system, pathogenic nematodes exploit regulatory pathways that are intrinsic parts of the mammalian immune system, such as regulatory T cells (Tregs). Using depletion of Treg mice, we showed that Foxp3(+) Treg numbers increased rapidly during infection with the nematode Strongyloides ratti. Transient depletion of Tregs during the first days of infection led to dramatically reduced worm burden and larval output, without aggravation of immune pathology. The transient absence of Tregs during primary infection did not interfere with the generation of protective memory. Depletion of Tregs at later time points of infection (i.e., day 4) did not improve resistance, suggesting that Tregs exert their counterregulatory function during the priming of S. ratti-specific immune responses. Improved resistance upon early Treg depletion was accompanied by accelerated and prolonged mast cell activation and increased production of types 1 and 2 cytokines. In contrast, the blockade of the regulatory receptor CTLA-4 specifically increased nematode-specific type 2 cytokine production. Despite this improved immune response, resistance to the infection was only marginally improved. Taken together, we provide evidence that Treg expansion during S. ratti infection suppresses the protective immune response to this pathogenic nematode and, thus, represents a mechanism of immune evasion.