Immunopathology of intestinal helminth infection

Immune-mediated regulation of chronic intestinal nematode infection

Gastrointestinal nematode infection is extremely prevalent worldwide in humans and animals. Infection levels vary between individuals in infected populations and exhibit a negative binomial distribution, and some individuals appear to be predisposed to certain infection levels. Moreover, infection tends to be chronic, despite evidence for the acquisition of some degree of acquired immunity. The host is subject to constant and repeated antigenic challenge, and individuals vary in the response they make. While a considerable amount of information is emerging on the immunoregulatory mechanisms operating during acute nematode infection from a variety of laboratory model systems, relatively little work has been carried out on the immune mechanisms underlying chronic infection. This review details some of the work that has addressed this important facet of gut nematode infection, highlighting studies from model systems that give insight into the induction of nonprotective immunity, while at the same time avoiding the induction of host-damaging pathology.

Interleukin (IL) 5 levels and eosinophilia in patients with intestinal parasitic diseases

AIM

Intestinal parasitic diseases are commonly accompanied with diarrhoeal symptoms and allergic reactions. Eosinophilia occurs as a result of IL-5 synthesized from Th2 cells during allergic reactions. IL-5 acts as a factor activating eosinophils. The aim of this study was to compare the IL-5 cytokine measurements in serum samples and cell cultures. And also to compare eosinophilia observed in helminth infections and protozoon infections accompanied with allergy.

METHODS

Twenty-three patients who presented with diarrhoeal symptoms and allergic complaints were tested positive for intestinal parasites, as well as 21 controls with allergic complaints who did not have any intestinal parasites were included in this study. IL-5 production in in vitro cell cultures prepared by using phytohemaglutinin (PHA) to stimulate peripheral blood mononuclear cells (PBMC) obtained from the blood samples taken from these patients were compared with the IL-5 level in serum. Furthermore, the IL-5 production in protozoon and helminth infections was also compared. Absolute eosinophil values in 1 mm(3) of blood were calculated by means of peripheral smear in both groups within the scope of the study.

RESULTS

Parasites such as helminth detected in 15 (65.2%) and protozoon in 8 (34.8%) of the patients were included in this study. As regards the values of the sera in both patients with parasite infection and controls, the IL-5 production was found to be higher in the cell culture supernatant (P<0.001 and P<0.05). When the IL-5 level of the patients with helminth parasites was compared with that of those with protozoon, it was determined that the IL-5 level in serum was more significant in the patients with protozoon than in those with helminth (P<0.05). In the study group, the patients were found to have parasites, the percentage of eosinophil was 7.0% compared to 6.5% in the control group. Thus, there was no significant difference between the eosinophil values (P>0.05).

CONCLUSION

It was found that IL-5 cytokine levels in serum samples from the patients with helminth and protozoon displayed more measurable values as compared to the IL-5 levels after stimulation with mitogen. It is concluded that IL-5 acts as a triggering factor in the toxiallergic complaints commonly seen in helminth and protozoon infections.

Trichinella spiralis: Stimulation of mast cells by TSL-1 antigens trigger cytokine mRNA expression and release of IL-4 and TNF through an Ig-independent pathway

Previous in vitro studies have demonstrated that mast cells (MC) can be directly activated by Trichinella spiralis larvae 1 (TSL-1) antigens. To characterize even more this activation of MC and their possible role on induction and regulation of the Type 2 response generated against T. spiralis infection, we studied the interaction between a hybrid rat MC line (HRMC), murine bone marrow MC (mBMMC), and TSL-1 antigens. Immunofluorescence staining and flow cytometry analysis showed that TSL-1 antigens bound to the surface of HRMC cells, resulting in the transcriptional induction and in the release of TNF and IL-4. Besides, an increase of IL-4 intracellular expression was also observed in mBMMC. This suggests that MC may play an important role in the early immune response against T. spiralis and may be a source of cytokines, that regulate the final onset of the immune mechanisms which determine the course of the infection.

Expression profiling reveals novel innate and inflammatory responses in the jejunal epithelial compartment during infection with Trichinella spiralis

Infection with intestinal nematodes induces profound pathological changes to the gut that are associated with eventual parasite expulsion. We have applied expression profiling as an initial screening process with oligonucleotide microarrays (Affymetrix MG-U74AV2 gene chips) and time course kinetics to investigate gene transcription triggered by the intraepithelial nematode Trichinella spiralis in jejunal epithelium from BALB/c mice. Of the 4,114 genes detected, 2,617 were present in all uninfected and T. spiralis-infected replicates, 8% of which were notably upregulated, whereas 12% were downregulated at the time of worm expulsion (day 14 postinfection). Upregulation of goblet cell mucin gene transcripts intestinal mucin gene 3 (MUC3), calcium chloride channel 5 (CLCA5), and goblet cell gene 4 (GOB4) is consistent with enhanced production and alteration of mucus, whereas a 60- to 70-fold upregulation of transcripts for mast cell proteases 1 and 2 (MCPT-1 and -2) is consistent with intraepithelial mucosal mast cell recruitment. Importantly, there was novel expression of sialyltransferase 4C (SIAT4C), small proline-rich protein 2A (SPRR2A), and resistin-like molecule beta (RELMbeta) on day 14 postinfection. In contrast, DNase I and regenerating protein 3 (REG3) transcripts were substantially downregulated. Time course analyses revealed early (within 48 h of infection) induction of Siat4c, Sprr2A, and Relmbeta and later (within 120 h) induction of Mcpt-1 and -2. The findings demonstrate early innate responses and later inflammatory changes within the epithelium. The early epithelial responses may be associated both with repair (Sprr2A) and with the development of innate immunity (Siat4c and Relmbeta).

Expression of integrin-alphaE by mucosal mast cells in the intestinal epithelium and its absence in nematode-infected mice lacking the transforming growth factor-beta1-activating integrin alphavbeta6

Peak intestinal mucosal mast cell (MMC) recruitment coincides with expulsion of Trichinella spiralis, at a time when the majority of the MMCs are located within the epithelium in BALB/c mice. Although expression of integrin-alpha(E)beta(7) by MMCs has not been formally demonstrated, it has been proposed as a potential mechanism to account for the predominantly intraepithelial location of MMCs during nematode infection. Co-expression of integrin-alpha(E)beta(7) and the MMC chymase mouse mast cell protease-1, by mouse bone marrow-derived mast cells, is strictly regulated by transforming growth factor (TGF)-beta(1). However, TGF-beta(1) is secreted as part of a latent complex in vivo and subsequent extracellular modification is required to render it biologically active. We now show, for the first time, that intraepithelial MMCs express integrin-alpha(E)beta(7) in Trichinella-infected BALB/c and S129 mice. In S129 mice that lack the gene for the integrin-beta(6) subunit and, as consequence, do not express the epithelial integrin-alpha(v)beta(6), integrin-alpha(E) expression is virtually abolished and recruitment of MMCs into the intestinal epithelium is dramatically reduced despite significant overall augmentation of the MMC population. Because a major function of integrin-alpha(v)beta(6) is to activate latent TGF-beta(1,) these findings strongly support a role for TGF-beta(1) in both the recruitment and differentiation of murine MMCs during nematode infection.

Detection of inducible nitric oxide synthase in Schistosoma japonicum and S. mansoni

Schistosoma japonicum and S. mansoni were tested for reactivity with an anti-inducible nitric oxide (iNOS) antibody and the distribution of iNOS was studied by immunofluorescent tests in different stages of the parasites. Reactivity was associated with the tegument in both larval schistosomes (sporocysts and cercariae) and eggs. With adult worms, the majority of the immunofluorescence was predominantly subtegumental in S. japonicum and parenchymal in S. mansoni. Fluorescence was also observed in host tissues (snails and mouse liver). In Western blots, the enzyme of S. japonicum had an apparent molecular weight of about 210 kDa. The possible role of worm and host iNOS in the parasite-host interrelation remains to be clarified.

Granulomatous inflammation during Heligmosomoides polygyrus primary infections in FVB mice

Host responses to primary infections with Heligmosomoides polygyrus were studied in fast responding FVB mice (H-2(q)). Pathological changes in the intestinal mucosa, mesenteric lymph nodes and spleen were examined. Features of the fast response were typical: low effectiveness of infection and limiting of parasite survival and egg production, with worm expulsion occurring about 60 days post-infection. The intestinal inflammatory response involved infiltration by different cells into the intestinal mucosa and granulomata formation. As is typical for intestinal nematode infection enteropathy, decreased villus:crypt ratio and hyperplasia of goblet and Paneth cells were also present. Reactions of the intestinal mucosa, mesenteric lymph nodes and spleen increased over time post-infection and after worm expulsion. Enteropathy may help worm expulsion by creating an unfavourable environment for H. polygyrus. The implications of these findings and the potential role of intestinal intraepithelial lymphocytes in the pathogenesis of generated lesions are discussed.

Mice intranasally immunized with a recombinant 16-kilodalton antigen from roundworm Ascaris parasites are protected against larval migration of Ascaris suum

Protective immunity to the pig roundworm, Ascaris suum, has been demonstrated by immunization of pigs with antigens derived from the parasite's larval stages. We identified a protective antigen commonly expressed in the human and pig Ascaris infections as a 16-kDa protein (As16), which has no similarity at the amino acid level to mammalian proteins but has some similarity to those of the filarial parasites and Caenorhabditis elegans gene product. Localization analysis revealed that the native As16 was highly expressed in the adult worm intestine, hypodermis, and cuticles. In addition, As16 was detected in the parasite excretory and secretory products. Mice intranasally vaccinated with Escherichia coli-expressed recombinant As16 (rAs16), coupled with cholera toxin B subunit, generated a significant increase in the level of rAs16-specific immunoglobulin G (IgG) and IgE in serum. Mucosal IgA levels were also increased. The recombinant protein evoked a mixed (both Th1 and Th2) type of immune response characterized by elevated levels of gamma interferon and interleukin-10 in the culture supernatants of activated spleen cells. An increased level of IgG1 and IgG2a in serum was also observed. The vaccinated mice showed a reduction by 58% in the recovery of challenged larvae compared to a nonvaccinated control. These results suggest the possibility of developing a mucosal vaccine for human and pig ascariasis.

The relative involvement of Th1 and Th2 associated immune responses in the expulsion of a primary infection of Heligmosomoides polygyrus in mice of differing response phenotype

T helper cell (Th1 and Th2) associated responses were examined following a primary infection with the gastrointestinal nematode Heligmosomoides polygyrus in five inbred strains of mice with different resistance phenotypes. Levels of (i) mast cell protease, (ii) specific IgE, (iii) nitric oxide and (iv) specific IgG2a, as markers of Th2 and Th1 associated responses, respectively, were determined in sera and intestinal fluids and correlated with worm burdens. The "fast" responder (resistant) strains SWR and SJL produced strong Th2 and Th1 associated responses respectively in a mutually exclusive fashion. The F1 hybrid (SWRxSJL) F1, showed rapid expulsion of the parasite and expressed both intense Th1 and Th2 responses, suggesting synergism between Th1 and Th2 activity in these mice. The results indicate that both Th2 and Th1 responses operate in mice following a primary infection with H. polygyrus and that each Th response may be involved to a greater or lesser degree within certain strains. Resistance to H. polygyrus was found to correlate only to the intensity of either the gut-associated mastocytosis or nitric oxide production in these strains but not to either specific IgE or IgG2a titres. Chronic infections in the "slow" response phenotype mouse strains CBA and C57BL/10, were associated with both poor Th2 and poor Th1-associated responses attributed to a general parasite-mediated immunosuppression of the host immune response to infection.

Biological variation in Trichinella species and genotypes

At present, the genus Trichinella comprises seven species of which five have encapsulated muscle larvae (T. spiralis, T. nativa, T. britovi, T. nelsoni and T. murrelli) and two do not (T. pseudospiralis and T. papuae) plus three genotypes of non-specific status (T6, T8 and T9). The diagnostic characteristics of these species are based on biological, biochemical and genetic criteria. Of biological significance is variation observed among species and isolates in parameters such as infectivity and immunogenicity. Infectivity of Trichinella species or isolates is determined, among other considerations, by the immune status of the host in response to species- or isolate-specific antigens. Common and particular antigens determine the extent of protective responses against homologous or heterologous challenge. The kinetics of isotype, cytokine and inflammatory responses against T. spiralis infections are isolate-dependent. Trichinella spiralis and T. pseudospiralis induce different dose-dependent T-cell polarizations in the early host response, with T. spiralis initially preferentially promoting Th1-type responses before switching to Th2 and T. pseudospiralis driving Th2-type responses from the outset.

Is there a common mechanism of gastrointestinal nematode expulsion?

Parasitic gastrointestinal (GI) nematodes are one of the most commonly acquired infections in the world. Although they cause relatively little mortality, infections result in high levels of morbidity that can result in developmental consequences in infected children and cause significant economic loss in infected animals. Over the last 30 years there has been extensive research into the mechanisms controlling the expulsion of gastrointestinal nematodes. Although many of the effector mechanisms that contribute to the loss of the parasite have been defined, we still appear to be some way from understanding the actual cause of parasite loss. Part of this stems from the different responses induced by different gastrointestinal parasites. It is clear that a Th2 response is essential for the expulsion of GI helminths; however, each of the characteristic immunological effector mechanisms induced following infection with these parasites may not be required or may be insufficient in isolation, but together they operate to expel GI helminths. These responses then succeed more efficiently in some cases than in others to induce parasite expulsion. The contribution made by various effector mechanisms to the expulsion of these parasites may therefore be a reflection of both the niche which the parasite inhabits as well as possible evasive/suppressive mechanisms employed by the parasites. In this review the various aspects of parasite expulsion will be described and the controversial issues in the field will be discussed.

The epidemiological consequences of optimisation of the individual host immune response

We present a simple unscaled, quantitative framework that addresses the optimum use of resources throughout a host's lifetime based on continuous exposure to parasites (rather than evolutionary, genetically explicit trade-offs). The principal assumptions are that a host's investment of resources in growth increases its survival and reproduction, and that increasing parasite burden reduces survival. The host reproductive value is maximised for a given combination of rates of parasite exposure, host resource acquisition and pathogenicity, which results in an optimum parasite burden (for the host). Generally, results indicate that the optimum resource allocation is to tolerate some parasite infection. The lower the resource acquisition, the lower the proportion of resources that should be devoted to immunity, i.e. the higher the optimum parasite burden. Increases in pathogenicity result in reduced optimum parasite burdens, whereas increases in exposure result in increasing optimum parasite burdens. Simultaneous variation in resource acquisition, pathogenicity and exposure within a community of hosts results in overdispersed parasite burdens, with the degree of heterogeneity decreasing as mean burden increases. The relationships between host condition and parasite burden are complicated, and could potentially confound data analysis. Finally, the value of this approach for explaining epidemiological patterns, immunological processes and the possibilities for further work are discussed.

Oral transfer of adult Ancylostoma ceylanicum hookworms into permissive and nonpermissive host species

Syrian hamsters become anemic and exhibit delayed growth following oral infection with third-stage Ancylostoma ceylanicum hookworm larvae. Here we describe experiments designed to determine the feasibility of adult worm transfer (AWT) between hosts, a technique that would facilitate the specific study of bloodfeeding hookworms in vivo without prior exposure of the host to larva-specific antigens, permit the ex vivo manipulation of adult parasites prior to reimplantation, and also allow for cross-species transfer of worms. Weanling hamsters given an oral AWT of 40 or 60 mixed-sex A. ceylanicum worms rapidly developed anemia; in the higher-dose group, hemoglobin levels declined from prechallenge levels by 44% within 4 days following AWT. Long-term survival of transferred worms was demonstrated by recovery of parasites from the intestines 42 days after AWT. AWT hamsters acquired humoral immune responses against soluble adult hookworm extracts and excretory-secretory products that were comparable in magnitude to those of animals given a typical infection with larvae. In AWT experiments employing the nonpermissive murine model, C57BL/6 mice given adult worms rapidly became anemic and lost weight in a manner similar to AWT hamsters. Infection of additional mouse strains demonstrated that while C57BL/10 and CD-1 mice also developed anemia following AWT, BALB/c mice were resistant. The technique of AWT to mice may further our understanding of hookworm pathogenesis by allowing the study of adult hookworm infections in a species with well-characterized genetics and an abundance of available reagents.

Parameters of intestinal inflammation in mice given graded infections of the nematode Trichinella spiralis

Four parameters of the intestinal inflammatory response (numbers of mucosal mast cells (MMC) and Paneth cells, villus:crypt ratios and mitotic figures) were measured in mice exposed to varying doses of infective larvae of Trichinella spiralis. The aim of the experiments was to determine whether generation of these components of inflammation required a threshold level of infection and whether, once triggered, inflammation became pan-mucosal. Near maximal MMC and Paneth cell responses were elicited even with infections as low as 35 larvae; changes in villus:crypt ratios and in mitotic indices also occurred at this level of infection, but were progressively greater with increasing levels of infection. In all infected mice, including those infected with 35 larvae, MMC and Paneth cell responses extended over most of the small intestine. These data are interpreted as showing: (i) that the intestinal mucosa is highly responsive to T. spiralis infection; (ii) that once triggered, components of the inflammatory response are amplified by T cell-dependent mechanisms, becoming pan-mucosal; and (iii) that MMC and Paneth cell responses, which require cell division and differentiation, become maximal at a lower infection threshold than changes in the villus:crypt ratio or in mitotic indices, which directly reflect increased rates of division in crypt cells.

Tyvelose and protective responses to the intestinal stages of Trichinella spiralis

The unusual sugar tyvelose is the immunodominant portion of the major larval glycoprotein antigens of Trichinella spiralis, which play an important role in generating immunity against the intestinal stages of infection. The possibility that the tyvelose component itself may have a host- or parasite-protective role in the intestine was tested by following the outcome of challenge infections in mice primed and boosted with tyvelose-BSA, or in mice primed with tyvelose-BSA before boosting with larval antigen. Although antibody responses were raised against tyvelose there was no evidence of protective immunity against the intestinal stages, as assessed by total adult worm recovery or by size and fecundity of female worms in immunized mice. Equally, priming with tyvelose-BSA before boosting with larval antigen had no effect on the expression of immunity against a challenge infection. The predominant antibody isotype recorded in all immunized mice was IgG1, suggesting the induction of type 2 T cell responses, and this was confirmed by cytokine analysis, mesenteric node lymphocytes of all mice showing production of IL-5 but not IFN-gamma. Clearly immunization with tyvelose had no significant effect on T cell polarization. The data show that, with the experimental design employed, there was no evidence for a functional role of tyvelose in either host- or parasite-protection during the intestinal phase of infection.

Intranasal immunization with recombinant Ascaris suum 14-kilodalton antigen coupled with cholera toxin B subunit induces protective immunity to A. suum infection in mice

Animals can be rendered immune to Ascaris parasites by immunization with infectious-stage larvae. The specific parasite gene products that mediate protective responses in ascariasis are unknown. We have identified a cDNA encoding Ascaris suum 14-kDa antigen (As14) and evaluated the vaccinal effect of the Escherichia coli-expressed recombinant protein (rAs14). GenBank analysis showed that As14 has low similarity at the amino acid level to a Caenorhabditis elegans gene product and to antigens of the filarial nematodes but not to other known proteins. In addition, As14 homologues were found to be expressed in human and dog roundworms. In mice that received intranasal administration of rAs14 coupled with cholera toxin B subunit (rAs14-CTB), there was a 64% reduction of recovery of larvae compared with that in the nontreated group. The vaccinated mice showed a significant increase in the total serum immunoglobulin G (IgG) levels and the mucosal IgA responses. Elevation of the rAs14-specific IgE response was also seen. Measurement of the IgG subclasses showed a higher level of IgG1 and a lower level of IgG2a antibody response in the sera of the immunized mice, suggesting that protection was associated with a type II immune response. As14 is the first protective antigen against A. suum infection to be identified. Our immunization trial results in laboratory animals suggest the possibility of developing a mucosal vaccine for parasitic diseases caused by ascarid nematodes.

Oral tolerance

The ability of the mucosal immune system to distinguish between harmful and harmless antigens is essential for mounting protective immune responses and preventing the induction of mucosal pathology yet the basis for this remains unclear. As fed antigen can also exert systemic effects understanding oral tolerance and priming will also have important consequences for therapy and vaccination. Here we will not only review the increasing amount of information about the potential mechanisms of oral tolerance and priming but also attempt to shed some light on how differences in the uptake and handling i.e. 'the journey' of orally administered antigen may promote these mechanisms.