Ratti Infection Research Articles

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.

An immunoassay system to investigate epidemiology of Rocahepevirus ratti (rat hepatitis E virus) infection in humans

Background & AimsRat hepatitis E virus (Rocahepevirus ratti; HEV-C1) is an emerging cause of hepatitis E that is divergent from conventional human-infecting HEV variants (Paslahepevirus balayani; HEV-A). Validated serological assays for HEV-C1 are lacking. We aimed to develop a parallel enzymatic immunoassay (EIA) system that identifies individuals with HEV-C1 exposure. We also aimed to conduct the first HEV-C1 seroprevalence study in humans using this validated EIA system. MethodsExpressed HEV-A (HEV-A4 p239) and HEV-C1 (HEV-C1 p241) peptides were characterised. Blood samples were simultaneously tested in HEV-A4 p239 and HEV-C1 p241 IgG EIAs. An optical density (OD) cut-off-based interpretation algorithm for identifying samples seropositive for HEV-A or HEV-C1 was validated using RT-PCR-positive infection sera. This algorithm was used to measure HEV-C1 seroprevalence in 599 solid organ transplant recipients and 599 age-matched immunocompetent individuals. ResultsBoth peptides formed virus-like particles. When run in HEV-A4 p239 and HEV-C1 p241 EIAs, HEV-A and HEV-C1 RT-PCR-positive samples formed distinct clusters with minimal overlap in a two-dimensional plot of optical density values. The final EIA interpretation algorithm showed high agreement with RT-PCR results (Cohen’s κ = 0.959) and was able to differentiate HEV-A and HEV-C1 infection sera with an accuracy of 94.2% (95% CI: 85.8–98.4%). HEV-C1 IgG seroprevalence was 7/599 (1.2%) among solid organ transplant recipients and 4/599 (0.7%) among immunocompetent individuals. Five of 11 (45.5%) of these patients had history of transient hepatitis of unknown cause. ConclusionsHEV-C1 exposure was identified in 11/1198 (0.92%) individuals in Hong Kong indicating endemic exposure. This is the first estimate of HEV-C1 seroprevalence in humans. The parallel IgG EIA algorithm is a valuable tool for investigating epidemiology and risk factors for HEV-C1 infection. Impact and ImplicationsRat hepatitis E virus has recently been discovered to infect humans, but antibody tests for this infection are lacking, making it difficult to gauge how common this infection is. We developed an antibody test algorithm that can identify individuals with past rat hepatitis E virus exposure. We used this algorithm to estimate rat hepatitis E exposure rates in humans in Hong Kong and found that approximately 1% of all tested people had been exposed to this virus previously.

Limited role of mast cells during infection with the parasitic nematode Litomosoides sigmodontis.

Mast cells are innate effector cells that due to their localization in the tissue form the first line of defense against parasites. We have previously shown that specifically mucosal mast cells were essential for the termination of the intestinal Strongyloides ratti infection. Here, we analyze the impact of mast cells on the immune response and defense against the tissue-dwelling filarial nematode Litomosoides sigmodontis using mast cell-deficient Cpa3cre mice. Despite an increase and an activation of mast cells at the site of infection in wildtype BALB/c mice the outcome of L. sigmodontis infection was not changed in mast cell-deficient BALB/c Cpa3cre mice. In Cpa3cre mice neither vascular permeability induced by blood-sucking mites nor the migration of L3 was altered compared to Cpa3 wildtype littermates. Worm burden in the thoracic cavity was alike in the presence and absence of mast cells during the entire course of infection. Although microfilaremiae in the peripheral blood increased in mast cell-deficient mice at some time points, the infection was cleared with comparable kinetics in the presence and absence of mast cells. Moreover, mast cell deficiency had no impact on the cytokine and antibody response to L. sigmodontis. In summary, our findings suggest that mast cells are not mandatory for the initiation of an appropriate immune response and host defense during L. sigmodontis infection in mice.

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.

Parasitological and transcriptomic comparison of Strongyloides ratti infections in natural and in suboptimal permissive hosts

The nematode genus Strongyloides consists of fairly species-specific small intestinal parasites of various vertebrates, among them the human pathogen S. stercoralis. Between the parthenogenetic parasitic generations these worms can also form single facultative sexual free-living generations. In addition to their primary hosts, several species can also live more or less well in other permissive hosts, which are sometimes not very closely related with the normal host. For example, S. stercoralis can also infect dogs and non-human primates.Here we compare the infection and reproductive success over time and the gene expression profiles as determined by quantitative sequencing of S. ratti parasitizing in its natural host rat and in the permissive host gerbil. We show that in gerbils fewer infective larvae successfully establish in the host, but those that do accomplish this survive and reproduce for longer and produced a higher proportion of males during the first two month of infection. Globally, the gene expression profiles in the two hosts are very similar. Among the relatively few differentially expressed genes, astacin-like and acetylcholinesterase genes are prominently represented. In the future it will be interesting to see if these changes in the suboptimal host are indeed ecologically sensible responses to the different host.

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: Cpa3Cre mice, which lack mucosal and connective tissue mast cells, remained infected for more than 150 days. Mcpt5Cre R-DTA mice, which lack connective tissue mast cells only, and basophil-deficient Mcpt8Cre mice terminated the infection after 1 month with wild-type kinetics despite their initial increase in intestinal parasite burden. Because Cpa3Cre 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.

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.

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

It is estimated that 30–100million 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.

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.

Strongyloides ratti infection modulates B and T cell responses to third party antigens

It is estimated that over one third of the world population is infected with helminths, Strongyloides ssp. accounting for approximately 30–100 million cases. As helminth infections often result in a modulation of the host’s immune system, infected people may display impaired responses to concurrent infections and to third party antigens. Here, we employ the experimental system of murine Strongyloides ratti infection to investigate the impact of helminth infections on experimental vaccinations. We demonstrate that concurrent infection with S. ratti strongly affected the humoral response to a thymus dependent model antigen, whereby predominantly Th1 associated IgG2b production was suppressed. We provide evidence that this suppression was due to modulation of T helper cell and not B cell function as the responses to a thymus independent model antigen remained unchanged in S. ratti infected mice. Moreover, using an adoptive transfer system, we show that infection with S. ratti directly interfered with antigen-specific proliferation of T cell receptor transgenic CD4+ T helper cells in vivo. Finally, using IL-10 deficient mice and mice that selectively lack T helper cell derived IL-10 we rule out a role for host-derived IL-10 in mediating the suppression of thymus dependent model antigen response in S. ratti infected mice.

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.

Strongyloides rattiinfection induces transient nematode-specific Th2 response and reciprocal suppression of IFN-γ production in mice

Over one-third of the world population is infected with parasitic helminths, Strongyloides ssp. accounting for approximately 30-100 million infected people. In this study, we employ the experimental system of murine Strongyloides ratti infection to investigate the interaction of this pathogenic nematode with its mammalian host. We provide a comprehensive kinetic description of the immune response to S. ratti infection that was reflected by induction of antigen-specific IgM and IgG1, mast cell activation and a Th2-like cytokine response. T cells derived from infected mice displayed an increased IL-3, IL-4, IL-5, IL-13 and IL-10 response to CD3-engagement in comparison with T cells derived from naïve mice. The IFN-gamma response to CD3-engagement that was well detectable in T cells derived from naïve mice, however, was suppressed in T cells derived from infected mice. Both, the induction of the S. ratti-specific Th2 response and the suppression of pro-inflammatory cytokines were transient and observed in strict correlation to the course of infection and the number of infective larvae used. Finally, comparing artificial infections induced by subcutaneous injection of larvae to natural infections, we observed similar antigen-specific T cell responses although the natural infection led to a significantly lower worm burden.

Strongyloides ratti: implication of mast cell-mediated expulsion through FcεRI-independent mechanisms

In order to examine whether FcepsilonRI-dependent degranulation of intestinal mast cells is required for expulsion of intestinal nematode Strongyloides ratti, CD45 exon6-deficient (CD45-/-) mice were inoculated with S. ratti. In CD45-/- mice, egg excretion in feces persisted for more than 30 days following S. ratti larvae inoculation, whereas in wild-type (CD45+/+) mice, the eggs completely disappeared by day 20 post-infection. The number of intestinal mucosal mast cells, which are known effector cells for the expulsion of S. ratti, was 75% lower in CD45-/- mice compared with that in CD45+/+ mice. Adoptive transfer of wild-type T cells from CD45+/+ mice into CD45-/- mice reduced the duration of S. ratti infection to comparable levels observed in CD45+/+ mice, with concomitant increases in intestinal mucosal mast cells. These results showed that CD45 is not involved in the effector function of intestinal mucosal mast cells against S. ratti infection. Since FcepsilonRI-dependent degranulation of mast cells is completely impaired in these CD45 knockout mice, we conclude that FcepsilonRI-dependent degranulation is not required in the protective function of intestinal mucosal mast cells against primary infection of S. ratti.

Genes important in the parasitic life of the nematode Strongyloides ratti

Parasitic nematodes are important pathogens of humans and other animals. The genus Strongyloides has both a parasitic and a free-living adult generation. S. ratti infections of its rat host are negatively affected by the host immune response, such that a month after infection, worms are lost from the hosts. Here we have investigated the changes in parasite gene expression that occur as the anti- S. ratti immune pressure increases. Existing S. ratti expressed sequence tags were used to construct a microarray consisting of 2227 putative genes. This was probed with cDNA prepared from parasites subject to low or high immune pressures. There are significant changes in the gene expression of S. ratti when subject to different immune pressures. Most of the genes whose expression changes have no significant alignment to known genes. These data together with previous S. ratti EST data were then used to identify genes that we hypothesise are central to the parasitic life of S. ratti and, perhaps, other parasitic nematodes. These analyses have identified genes likely to play a key role in the parasitic life of S. ratti; these genes should be the priority for further investigation.

The immune response during a Strongyloides ratti infection of rats

A range of immune parameters was measured during a primary infection of Strongyloides ratti in its natural rat host. The immune parameters measured were interleukin-4 (IL-4) and interferon-gamma from both the spleen and mesenteric lymph node (MLN) cells; parasite-specific immunoglobulin G(1)(IgG(1)), IgG(2a) and IgG(2b) in serum and in intestinal tissue; parasite-specific IgG and total IgE in serum; parasite-specific and total IgA in intestinal tissue and rat mast cell protease II in intestinal tissue. Parasite-specific IgG(1), IgG(2a) and total IgE in serum and parasite-specific IgA and rat mast cell protease II in intestinal tissue all occurred at significantly greater concentrations in infected animals, compared with non-infected animals. Similarly, the production of IL-4 by MLN cells stimulated with parasitic female antigen or concanavalin A occurred at significantly greater concentrations in infected animals, compared with non-infected animals. In all, this suggests that there is a T-helper 2-type immune response during a primary S. ratti infection. These data also show the temporal changes in these components of the host immune response during a primary S. ratti infection.

Density-dependent immune responses against the gastrointestinal nematode Strongyloides ratti

Negative density-dependent effects on the fitness of parasite populations are an important force in their population dynamics. For the parasitic nematode Strongyloides ratti, density-dependent fitness effects require the rat host immune response. By analysis of both measurements of components of parasite fitness and of the host immune response to different doses of S. ratti infection, we have identified specific parts of the host immune response underlying the negative density-dependent effects on the fitness of S. ratti. The host immune response changes both qualitatively from an inflammatory Th1- to a Th2-type immune profile and the Th2-type response increases quantitatively, as the density of S. ratti infection increases. Parasite survivorship was significantly negatively related to the concentration of parasite-specific IgG1 and IgA, whereas parasite fecundity was significantly negatively related to the concentration of IgA only.

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

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

Strongyloides ratti: the role of interleukin-5 in protection against tissue migrating larvae and intestinal adult worms.

To determine the role of interleukin-5 (IL-5) and eosinophils in protection against Strongyloides ratti, mice treated with anti-IL-5 monoclonal antibody (mAb) were infected with S. ratti larvae. Strongyloides ratti egg numbers in faeces (EPG) in mAb treated mice were higher than those in control mice on days 6 and 7 after inoculation. The numbers of migrating worms in mAb treated mice 36 h after inoculation were higher than those observed in control mice. Intestinal worm numbers in mAb treated mice 5 days after inoculation were higher than those in control mice. These results show that eosinophils effectively protected the host against S. ratti infection by mainly the larval stage in primary infections. The involvement of eosinophils in protection against secondary infection was also examined. Before secondary infection, mice were treated with anti-IL-5 mAb and infected with S. ratti. Patent infections were not observed in either mAb treated or control Ab treated mice. The numbers of migrating worms in the head and lungs of mAb treated mice increased to 60% of that in primary infected mice. Intestinal worms were not found in mAb treated mice or in control mice after oral implantation of adult worms. Eosinophils were therefore mainly involved in protection against tissue migrating worms in secondary infections.