Immune regulation by helminth parasites: cellular and molecular mechanisms

Gastrointestinal nematode infection is associated with variation in innate immune responsiveness

Ex vivo monocyte cytokine responses (IL-1beta, TNF-alpha, IL-12p70, IL-10, TGF-beta) to bacterial TLR2 and TLR4 ligands were quantified in 47 gastrointestinal (GI) nematode-exposed children in Pemba Island, Tanzania. Worminess (estimated by faecal egg counts (FEC)) had a positive relationship with pro-inflammatory TNF-alpha and IL-1beta responsiveness to the TLR ligands. In particular, there was a strong significant relationship with TNF-alpha response to TLR4 ligand (LPS). There were no significant associations between regulatory responses (IL-10, TGF-beta) and worminess. These results are consistent with the possibility that GI nematodes modulate innate responses and may indicate a potential mechanism for interactions between GI nematodiasis and important bystander pathogens.

Infections and allergy - helminths, hygiene and host immune regulation

There is increasing evidence that helminth infections can protect the host against Th2-mediated allergic pathologies, even though helminths themselves are strong Th2 inducers. In murine model systems, alleviation of allergy is not achieved through immune deviation to Th1, but is linked to expansion of regulatory T cell activity. Parasite infection does not prevent allergen sensitisation, but restricts the Th2 effector phase responsible for inflammation. Suppression of allergic inflammation can be transferred by Treg phenotype cells from an infected, allergen-naïve animal to an uninfected, sensitized recipient. Patent allergy in humans is also known to be modulated by helminth infections, suggesting that a similar regulatory network may be controlling immunopathologic disease in man.

The beneficial helminth parasite?

There is unequivocal evidence that parasites influence the immune activity of their hosts, and many of the classical examples of this are drawn from assessment of helminth infections of their mammalian hosts. Thus, helminth infections can impact on the induction or course of other diseases that the host might be subjected to. Epidemiological studies demonstrate that world regions with high rates of helminth infections consistently have reduced incidences of autoimmune and other allergic/inflammatory-type conditions. Here I review and assess the possible ways by which helminth infections can block or modulate concomitant disease processes. There is much to be learned from careful analysis of immuno-regulation in helminth-infected rodents and from an understanding of the immune status of acutely and chronically infected humans. The ultimate reward from this type of investigation will likely be a more comprehensive knowledge of immunity, novel ways to intervene in the immune response to alleviate autoimmune and allergic diseases (growing concerns in economically developed areas), and perhaps the development of helminth therapy for patients suffering from specific inflammatory, autoimmune or allergic disorders.

rOv-ASP-1, a recombinant secreted protein of the helminth Onchocercavolvulus, is a potent adjuvant for inducing antibodies to ovalbumin, HIV-1 polypeptide and SARS-CoV peptide antigens

We studied the adjuvanticity of recombinant Onchocerca volvulus activation associated protein-1 (rOv-ASP-1) for ovalbumin (OVA) in mice. After a single immunization and one boost, rOv-ASP-1 exceeded the efficacy of alum or MPL + TDM adjuvants in terms of end-point total IgG or IgG1 and IgG2a anti-OVA titres. Using the helminth-derived adjuvant, IgG isotype responses to OVA were of a mixed Th1/Th2 profile and spleen cell cytokines exclusively Th1-type. The potent adjuvanticity of rOv-ASP-1 was confirmed in mice vaccinated with a 37-mer peptide from the S protein of SARS-CoV and an HIV-1 gp120-CD4 chimeric polypeptide antigen. Unusually for a helminth product, the rOv-ASP-1 adjuvant augmented not only Th2 but also Th1 responses, the latter property being of potential utility in stimulating anti-viral immune responses.

Intragastric administration of Mycobacterium vaccae inhibits severe pulmonary allergic inflammation in a mouse model

BACKGROUND

Coexistence with harmless microorganisms such as lactobacilli, saprophytic mycobacteria and some helminths, throughout evolution, may have shaped the host immune system. Exposure to such organisms may have therapeutic benefits by triggering immunoregulatory mechanisms that control inappropriate immune responses to self, gut contents or allergens.

OBJECTIVE

We determined whether treatment with Mycobacterium vaccae by gavage influences the host immune response both locally and systemically. We also investigated whether delivery by this route prevents severe symptoms of disease in a murine model of pulmonary allergic inflammation.

RESULTS

A single intragastric administration of M. vaccae induced a transient increase in the production of IL-10 and IFN-gamma by mesenteric lymph nodes cells and splenocytes. In addition, in a mouse model of pulmonary allergic inflammation, a single treatment with M. vaccae by gavage not only diminished the total cellular infiltrate and the eosinophilic component induced by subsequent intratracheal allergen challenge, but also biased local and systemic cytokine production towards IL-10. Delivery of M. vaccae by gavage was as effective as subcutaneous treatment.

CONCLUSION

This is the first report to suggest that heat-killed mycobacteria can down-regulate symptoms of allergic inflammation by the intragastric route. These data suggest an alternative route of treatment with M. vaccae for patients with allergic conditions.

Can an allograft become harmonious with its recipient by analogous mechanisms to the long-term survival of a parasite in its host? Transplantation is similar to parasitism and chronic parasite infection can prolong allograft survival

Organ transplantation is very similar to the parasitism, the immune responses to the allograft and parasite share some general characters in acute stage. But host-parasite interplay seems harmonious and clinically asymptomatic carriers act as long-term reservoirs for transmission even several decades. It is proposed that the allograft also can survive long-term like the parasite harmonious with the host if we exploit the mechanism of the parasitism. How can the parasite escape the immunologic cleaning? Recent research found that: (1) Almost all the parasites infection can break the balance of Th1/Th2 and induce the Th2 bias. The level of cytokine excreted by Th2 is increased. (2) Parasite infection can interfere in antigen presentation processing through cystatins and "dendritic cell paralysis". (3) Regulatory T cells were found surrounding the local site of parasite worms. (4) Parasite infections can switch of eicosanoid metabolism, and the anti-inflammatory mediator of the plasma is increasingly manifest in the chronic infection stage. And in animal model, chronic infection can prolong the allograft survival. If the hypotheses are correct, it will give another novel therapeutic option for patients with organ transplantation.

Co-infected C57BL/6 mice mount appropriately polarized and compartmentalized cytokine responses to Litomosoides sigmodontis and Leishmania major but disease progression is altered

This study examines the capacity of the mammalian host to fully compartmentalize the response to infection with type 1 vs. type 2 inducing organisms that infect different sites in the body. For this purpose, C57BL/6 mice were infected with the rodent filarial nematode Litomosoides sigmodontis followed by footpad infection with the protozoan parasite Leishmania major. In this host, nematode infection is established in the thoracic cavity but no microfilariae circulate in the bloodstream. We utilized quantitative ELISPOT analysis of IL-4 and IFN-gamma producing cells to assess cytokine bias and response magnitude in the lymph nodes draining the sites of infection as well as more systemic responses in the spleen and serum. Contrary to other systems where co-infection has a major impact on bias, cytokine ratios were unaltered in either local lymph node. The most notable effect of co-infection was an unexpected increase in the magnitude of the IFN-gamma response to L. major in mice previously infected with L. sigmodontis. Further, lesion development was significantly delayed in these mice. Thus, despite the ability of the immune system to appropriately compartmentalize the immune response, interactions between responses at distinct infection sites can alter disease progression.

Echinostomes as experimental models for interactions between adult parasites and vertebrate hosts

Echinostomes are intestinal trematodes that, for years, have served as experimental models in different areas of parasitology. However, the usefulness of these trematodes in experimental parasitology has been underappreciated. In this article, we examine the characteristics that make echinostomes useful models for analysis of the interactions between adult parasites and vertebrate hosts, particularly in relation to the host-related factors that determine the establishment of the parasites.

Intestinal helminthiasis in Colombian children promotes a Th2 response to Helicobacter pylori: possible implications for gastric carcinogenesis

BACKGROUND

Colombians living in coastal Tumaco have a lower incidence of Helicobacter pylori-associated gastric cancer compared with residents of Pasto in the high Andes. Considering the risk for H. pylori disease seems affected by features of bacterial virulence and host polymorphisms, other poorly understood influences, such as concurrent helminthiasis, may also be important.

METHODS

Fecal samples from 211 children were tested for parasites and sera from another cohort of 159 children and 92 adults were tested for IgE and H. pylori-specific IgG.

RESULTS

Most individuals (95%) from both areas were H. pylori seropositive, with a predominant response of IgG1 followed by IgG2 and low IgG3 and IgG4 antibodies. Compared with Pasto children, Tumaco children were more commonly infected with helminths (P = 0.000), had higher serum IgE levels (P < 0.03), and had higher Th2-associated IgG1 responses to H. pylori (P < 0.0002). Other IgG isotype responses all increased with age but were not significantly different between children and adults from either area.

CONCLUSIONS

These results suggest that intestinal helminthiasis in children promotes Th2-polarizing responses to H. pylori and may decrease gastric cancer risk in these individuals later in life. Concurrent helminthiasis may alter inflammatory responses to H. pylori and thus affect the progression of gastritis to gastric atrophy, dysplasia, and cancer.

Comparative dynamics and phenotype of the murine immune response to Trichinella spiralis and Trichinella pseudospiralis

Infection of NIH mice with Trichinella spiralis and Trichinella pseudospiralis results in qualitatively comparable immune responses. Antigen-specific proliferation by mesenteric lymph node cells was transient and temporally associated with intestinal infection, but in contrast was sustained throughout infection by splenocytes. Early cytokine production by mesenteric lymph node cells was dominated by interleukin 10, but also IL-5 and IL-4, with rapid resolution following parasite expulsion from the gut. Splenocytes showed a mixed profile of cytokine production, although again dominated by IL-10 and sustained over 60 days of infection. All antibody classes were evident, with early production of IgA and IgG1, and subsequent secretion of other subclasses including IgG2a. Granulocytic infiltration of the spleen was significantly greater in T. spiralis infection. The concentration of serum corticosterone generally remained within normal boundaries, although was raised by day 60 in T. spiralis-infected mice. We conclude that the systemic suppression of inflammation reported for T. pseudospiralis does not result from selective induction of regulatory cytokines, or a major difference in the immune response to infection with T. spiralis.

Mucosal immunity and allergic responses: lack of regulation and/or lack of microbial stimulation?

Allergic hyperreactivity is defined as an exaggerated immune response [typically immunoglobulin E (IgE) but also non-IgE mediated] toward harmless antigenic stimuli. The prevalence of allergic disease has increased dramatically during the last 20 years, especially in developed countries. Both genetic and environmental factors contribute to susceptibility to allergy. Evidence has emerged supporting the hypothesis that a reduction in antigenic stimulation brought about by widespread vaccination, improvements in standards of hygiene, and extensive use of antibiotics has contributed to the dysregulation of T-helper 2 cell (Th2) type responsiveness that typifies allergy. Regulation of the inherently Th2-biased mucosal immune response is crucial both to the maintenance of homeostasis at this strategic defensive barrier and to the prevention of allergic disease. The ability of Th1 responses to counter-regulate Th2 reactivity is well characterized. More recently, interest has centered on regulatory T cells, which can suppress both Th1 and Th2 cells through the secretion of immunosuppressive cytokines such as interleukin-10 and transforming growth factor-beta. In this review, we discuss the basic cellular mechanisms of allergic diseases at mucosal surfaces, focusing on allergic responses to food, before examining newer work that suggests the induction of allergic hyperreactivity is due to a deficient immunoregulatory network, a lack of microbial stimulation, or both.

Depletion of immature B cells during Trypanosoma cruzi infection: involvement of myeloid cells and the cyclooxygenase pathway

The ability of a microorganism to elicit or evade B cell responses represents a determinant factor for the final outcome of an infection. Although pathogens may subvert humoral responses at different stages of B cell development, most studies addressing the impact of an infection on the B cell compartment have focused on mature B cells within peripheral lymphoid organs. Herein, we report that a protozoan infection, i.e. a Trypanosoma cruzi infection, induces a marked loss of immature B cells in the BM, which also compromises recently emigrated B cells in the periphery. The depletion of BM immature B cells is associated with an increased rate of apoptosis mediated by a parasite-indirect mechanism in a Fas/FasL-independent fashion. Finally, we demonstrated that myeloid cells play an important role in B cell depletion, since CD11b(+) BM cells from infected mice secrete a product of the cyclooxygenase pathway that eliminates immature B cells. These results highlight a previously unrecognized maneuver used by a protozoan parasite to disable B cell generation, limiting host defense and favoring its chronic establishment.

Helicobacter pylori and Schistosoma japonicum co-infection in a Chinese population: helminth infection alters humoral responses to H. pylori and serum pepsinogen I/II ratio

The effects of helminth infection on humoral IgG responses and clinical outcome of gastric Helicobacter pylori infection are unknown. IgG and IgG subclass responses to H. pylori and serum pepsinogen I/II ratio, a marker of gastric atrophy, were investigated in a Schistosoma japonicum prevalent Chinese population. H. pylori, CagA and IgG subclass responses were assayed by ELISA. Serum pepsinogen I and pepsinogen II were assayed by ELISA and the pepsinogen I/II ratio determined. In 150 subjects, infection with S. japonicum and H. pylori was 55.3% and 51.3%, respectively. H. pylori IgG titres and CagA seropositivity were significantly lower (P<0.05) in co-infected subjects, and differences in H. pylori IgG isotype responses were evident. In H. pylori positives, a significantly higher (P<0.05) pepsinogen I/II ratio was observed in co-infected subjects. The difference between S. japonicum positive and negative subjects was only evident in H. pylori CagA seronegative subjects. In conclusion, S. japonicum co-infection with H. pylori is associated with alterations in IgG responses to H. pylori and less gastric atrophy.

Regulatory T cells modulate Th2 responses induced by Brugia pahangi third-stage larvae

Infection of BALB/c mice with Brugia pahangi third-stage larvae (L3) results in the production of interleukin-4 (IL-4), IL-5, and IL-10 with a resultant down-regulation in Th1 responses. Previously, this was thought to reflect a skewing of immune responses towards a Th2 phenotype by the infective stage of the parasite. In this study, we show that exposure to the L3 of Brugia also induces the expansion of a population of CD4 cells that express CD25 and cytotoxic-T-lymphocyte-associated antigen 4 in an IL-4-independent fashion. By quantitative reverse transcription-PCR, we show that the CD25+ population is highly enriched in mRNA for the Foxp3 transcription factor and that these cells express significantly more IL-10 mRNA than the CD25- population, suggesting a likely regulatory phenotype. The functional capacity of these cells was demonstrated using a neutralizing CD25 monoclonal antibody (MAb). Mice treated with this MAb demonstrated elevated levels of antigen (Ag)-specific proliferation in vitro, and levels of Ag-specific Th2 cytokines were significantly increased. These results suggest a complex network of regulation in L3-infected mice with Th2 cells limiting the Th1 response, while T-regulatory cells modulate Th2 responses.

Schistosomes: the road from host-parasite interactions to vaccines in clinical trials

Insights over recent years into the interactions between helminths, including schistosomes, and the immune system have generated new concepts in immunology and significant advances in vaccine strategies. Here, we report recent advances that substantially increase our understanding of the nature of the host innate and adaptive responses to schistosomes and on strategies elaborated by the parasite to manipulate such responses. We also describe the long road that has allowed us to move from the identification of an anti-schistosome vaccine candidate, a 28kDa glutathione-S-transferase, to its recent evaluation in human clinical trials.

Diminished Expression and Function of TLR in Lymphatic Filariasis: A Novel Mechanism of Immune Dysregulation

Lymphatic filariasis is a disease characterized by immune dysregulation involving APC and T cell populations. To assess the contribution of TLR in mediating this dysregulation, we examined the expression of TLR1, TLR2, TLR4, and TLR9 on B cells and monocytes of filaria-infected and uninfected individuals. Baseline expression of TLR was significantly lower in B cells but not in monocytes of the filaria-infected group compared with the uninfected group. Upon stimulation with filarial Ag, a diminished up-regulation of TLR was observed in both B cells and monocytes of infected individuals. Finally, stimulation of B cells and monocytes with TLR ligands resulted in decreased B cell and monocyte activation/cytokine production, indicating a state of immune tolerance. This dysregulation is associated with diminished CD4(+) T cell production of IFN-gamma and IL-5. The diminished expression and function of TLR is thus a likely consequence of chronic Ag stimulation and could serve as a novel mechanism underlying the dysfunctional immune response in filariasis.

Regulation of allergy and autoimmunity in helminth infection

Parasitic infections are a major theme in the "hygiene hypothesis", as allergies and autoimmune diseases are less prevalent in countries with higher burdens of helminths and other parasitic organisms. Helminths"-the grouping of multicellular worm parasites including nematodes, cestodes and trematodes-tend to establish long-lived, chronic infections indicating successful down-modulation of the host immune system. In this review, we describe the intricate immunology of host-helminth interactions and how parasites manipulate immune responses to enhance their survival. In so doing, they often minimise immunopathology and, it is suggested, reduce host susceptibility to, and severity of allergic and autoimmune diseases. Studies on helminth-infected communities and individuals support the hypothesis that an immuno-regulatory network promoted by parasites extends its influence to limiting allergies. Experimental models are now probing more deeply into the area of immune modulation by helminths, and we discuss the likely mechanisms by which helminths could be establishing a strongly regulatory environment. Understanding and harnessing the modulatory capacity of helminths may uncover novel therapeutic interventions, mimicking and exploiting their evolution for our benefit. Parasitic infections are a major theme in the "hygiene hypothesis", as allergies and autoimmune diseases are less prevalent in countries with higher burdens of helminths and other parasitic organisms. Helminths"-the grouping of multicellular worm parasites including nematodes, cestodes and trematodes-tend to establish long-lived, chronic infections indicating successful down-modulation of the host immune system. In this review, we describe the intricate immunology of host-helminth interactions and how parasites manipulate immune responses to enhance their survival. In so doing, they often minimise immunopathology and, it is suggested, reduce host susceptibility to, and severity of allergic and autoimmune diseases. Studies on helminth-infected communities and individuals support the hypothesis that an immuno-regulatory network promoted by parasites extends its influence to limiting allergies. Experimental models are now probing more deeply into the area of immune modulation by helminths, and we discuss the likely mechanisms by which helminths could be establishing a strongly regulatory environment. Understanding and harnessing the modulatory capacity of helminths may uncover novel therapeutic interventions, mimicking and exploiting their evolution for our benefit.

Inducible costimulator is required for type 2 antibody isotype switching but not T helper cell type 2 responses in chronic nematode infection

Inducible costimulator (ICOS) has been suggested to perform an important role in T helper cell type 2 (Th2) responses, germinal center formation, and isotype switching. The role of ICOS in chronic Th2 responses was studied in a nematode model with the filarial parasite, Brugia malayi. Contrary to expectations, we did not observe a significant defect in IL-4-producing Th2 cells in ICOS-/- mice or in eosinophil recruitment. We also found that ICOS was not required for the differentiation of alternatively activated macrophages (AAMPhi) that express Ym1 and Fizz1. Although the production of IgE was slightly reduced in ICOS-/- mice, this was not as significant as in CD28-/- mice. In contrast to live infection, the primary response of ICOS-/- mice immunized with soluble B. malayi antigen and complete Freund's adjuvant resulted in significantly fewer IL-4-producing cells in the lymph nodes. As previously reported, we observed a defect in antibody isotype switching toward the IgG1 isotype in ICOS-/- mice during live infection. Interestingly, there was a significant enhancement of parasite-specific IgG3 isotype antibodies. CD28-/- and MHC class II-/- mice also had enhanced parasite-specific IgG3 isotype antibodies. Our results suggest that ICOS is not required to maintain a chronic cellular Th2 response. The primary role of ICOS in a chronic helminth infection could be to drive antibodies toward type 2 isotypes. T-independent antibody response to the parasite could be enhanced in the absence of costimulation and T cell help.

Allergen-specific IgE and IgG4 are markers of resistance and susceptibility in a human intestinal nematode infection

IgG4 has been proposed to act as a 'blocking antibody' due to its ability to compete for the same epitopes as IgE thus preventing IgE-dependent allergic responses. IgG4 and IgE are both elevated in helminth infections and strong anti-parasite IgE responses are associated with resistance to infection. We wished to determine the relationship between anti-parasite IgG4 and IgE and Ascaris lumbricoides infection status. We examined anti-parasite responses, including antibody levels to recombinant Ascaris allergen-1A (rABA-1A), a target of serum IgE in endemic populations. Worm burden was indirectly estimated by measuring parasite egg output in a cross-sectional human population (N = 105). Levels of anti-parasite IgG4 and IgE in patients' plasma were quantified by immunoassay. Global anti-parasite antibody responses did not bear any significant relationships with intensity of Ascaris infection. Individuals who had detectable levels of IgE but not IgG4 to rABA-1A (11%) had lower average levels of infection compared with individuals who produced anti-rABA-1A IgG4 (40%) and sero-negative individuals (49%) (P = 0.008). The ratio of IgG4/IgE in rABA-1A responders positively correlated with intensity of infection (P < 0.025). IgG4 levels positively correlated with infection level in younger children (age 4-11) where average levels of infection were increasing (P = 0.038), whereas allergen specific IgE emerged as a correlate of immunity in older children and adults (age 12-36) where infection levels were decreasing (P = 0.048). Therefore, in a gastrointestinal helminth infection, differential regulation of anti-allergen antibody isotypes relate to infection level. Our results are consistent with the concept that IgG4 antibody can block IgE-mediated immunity and therefore allergic processes in humans.

Parasite role reversal: worms on trial

Asthma has reached epidemic proportions globally. This has been attributed by many to improved hygiene. The frequent failure of conventional pharmaceuticals to manage the disease has led to the introduction of parasites as a potential alternative therapy for asthma and other immunological diseases. In this article, we briefly review the immunological rationale underpinning therapeutic parasitic infection, describe recently initiated trials, and highlight potential risks and benefits of introducing parasites into patient cohorts.

Immunity, immunoregulation and the ecology of trichuriasis and ascariasis

Immune responses to human roundworm (Ascaris lumbricoides) and whipworm (Trichuris trichiura) and their role in controlling worm populations are reviewed. Recent immunoepidemiological data implicate T(H)2-mediated responses in limiting A. lumbricoides and T. trichiura populations. Reinfection studies further suggest that IL-5 cytokine responses are negatively associated with adult recruitment in T. trichiura but not A. lumbricoides and may therefore be involved in negative intraspecific and interspecific interactions mediated through the host immune system. The importance of inducible immunoregulatory networks in the ecology of the host-parasite relationship is considered, with particular regard to possible manipulative strategies by the parasites. This aspect of the worms' interaction with the host immune system is both poorly known and potentially central to an understanding of parasite population dynamics and the evolutionary pressures that have shaped present-day host-parasite associations. Some possible implications of worm-mediated immunomodulation for the occurrence of bystander infectious diseases in human populations and the management of de-worming programmes are also discussed.

Intestinal worms and human allergy

The immunoepidemiological interactions between intestinal worm (or geohelminth) infections and allergy are of great interest to parasitologists, immunologists, and allergists because of the close similarities between the human immune response to geohelminth parasites and environmental allergens. Allergic diseases appear to be most rare in populations living in the rural tropics with high rates of infection with geohelminth parasites, and this has led to suggestions that the relationship between geohelminth infections and allergy may be causal. Allergic sensitization and disease results from a complex interaction between environmental exposures and genetic background, and the numerous epidemiological studies that have investigated the relationship between allergy and geohelminth infections have provided conflicting findings. The strongest epidemiological evidence for a causal association is provided by intervention studies that demonstrate evidence for an effect of anthelmintic treatment on atopy or asthma risk. There is evidence also for an inverse relationship between geohelminth infection and either atopy or asthma symptoms from cross-sectional studies that have been conducted in areas of high infection prevalence. Chronic geohelminth infections could affect allergy risk by modulation of the immune response to environmental allergens, and an area of great research activity at present is the investigation of the role of regulatory T cells in modulating host inflammatory responses. However, a causal association between geohelminth infections and allergy remains to be proven, and prospective and intervention studies are required that investigate the development of allergy in early life at a time when humans are first exposed to geohelminth parasites and their antigens.

A worm's eye view of the immune system: consequences for evolution of human autoimmune disease

Humans and the many parasites that we can host have co-evolved over millions of years. This has been compared to an arms race in which the immune armoury of the human has evolved to deal with potential pathogens and the pathogen has evolved strategies to evade, and in some cases use, the immune system of the human host. Recently, there have been marked changes in the exposure of individuals in the developed world to both microorganisms and metazoan parasites, so the immune stimuli such organisms provide no longer have a role in our lives. As we discuss here, this is a marked perturbation, and the absence of the associated immunomodulation might have led to the increased emergence of autoimmune diseases.

Removal of Regulatory T Cell Activity Reverses Hyporesponsiveness and Leads to Filarial Parasite Clearance In Vivo

Human filarial parasites cause chronic infection associated with long-term down-regulation of the host's immune response. We show here that CD4+ T cell regulation is the main determinant of parasite survival. In a laboratory model of infection, using Litomosoides sigmodontis in BALB/c mice, parasites establish for >60 days in the thoracic cavity. During infection, CD4+ T cells at this site express increasing levels of CD25, CTLA-4, and glucocorticoid-induced TNF receptor family-related gene (GITR), and by day 60, up to 70% are CTLA-4(+)GITR(high), with a lesser fraction coexpressing CD25. Upon Ag stimulation, CD4(+)CTLA-4(+)GITR(high) cells are hyporesponsive for proliferation and cytokine production. To test the hypothesis that regulatory T cell activity maintains hyporesponsiveness and prolongs infection, we treated mice with Abs to CD25 and GITR. Combined Ab treatment was able to overcome an established infection, resulting in a 73% reduction in parasite numbers (p < 0.01). Parasite killing was accompanied by increased Ag-specific immune responses and markedly reduced levels of CTLA-4 expression. The action of the CD25(+)GITR+ cells was IL-10 independent as in vivo neutralization of IL-10R did not restore the ability of the immune system to kill parasites. These data suggest that regulatory T cells act, in an IL-10-independent manner, to suppress host immunity to filariasis.

Signaling pathways in Th2 development

In order for an immune response to be successful, it must be of the appropriate type and magnitude. Intracellular residing pathogens require a cell-mediated immune response, whereas extracellular pathogens evoke a humoral immune response. T-helper (Th) cells orchestrate the immune response and are divided into two subsets, Th1 and Th2 cells. Here, we discuss the mechanisms of Th2 development with a focus on signal transduction pathways that influence Th2 differentiation.

Heterologous expression of the filarial nematode alt gene products reveals their potential to inhibit immune function

Background

Parasites exploit sophisticated strategies to evade host immunity that require both adaptation of existing genes and evolution of new gene families. We have addressed this question by testing the immunological function of novel genes from helminth parasites, in which conventional transgenesis is not yet possible. We investigated two such novel genes from Brugia malayi termed abundant larval transcript (alt), expression of which reaches ~5% of total transcript at the time parasites enter the human host.

Results

To test the hypothesis that ALT proteins modulate host immunity, we adopted an alternative transfection strategy to express these products in the protozoan parasite Leishmania mexicana. We then followed the course of infection in vitro in macrophages and in vivo in mice. Expression of ALT proteins, but not a truncated mutant, conferred greater infectivity of macrophages in vitro, reaching 3-fold higher parasite densities. alt-transfected parasites also caused accelerated disease in vivo, and fewer mice were able to clear infection of organisms expressing ALT. alt-transfected parasites were more resistant to IFN-γ-induced killing by macrophages. Expression profiling of macrophages infected with transgenic L. mexicana revealed consistently higher levels of GATA-3 and SOCS-1 transcripts, both associated with the Th2-type response observed in in vivo filarial infection.

Conclusion

Leishmania transfection is a tractable and informative approach to determining immunological functions of single genes from heterologous organisms. In the case of the filarial ALT proteins, our data suggest that they may participate in the Th2 bias observed in the response to parasite infection by modulating cytokine-induced signalling within immune system cells.

New lessons from old pathogens: what parasitic infections have taught us about the role of nuclear factor-kappaB in the regulation of immunity

The nuclear factor-kappaB (NF-kappaB) family of transcription factors is activated by many infectious and inflammatory stimuli. This family regulates the expression of multiple genes, whose products include cytokines, chemokines, adhesion molecules, and antiapoptotic factors that are important components of the innate and adaptive immune response. A prominent role of NF-kappaB transcription factors in resistance to a variety of infectious diseases was revealed by studies with mice that lack individual family members. However, little is known about the basis for these effects or about the role of individual family members during a coordinated immune response to infection. Diverse parasites such as Toxoplasma gondii, Leishmania major, and Trichuris muris provide a unique opportunity to understand the role of the NF-kappaB system in the development of innate and adaptive immunity to these infections. The basis for resistance and susceptibility to these parasites is well understood, and studies using these experimental systems have provided unique insights into the role of NF-kappaB in the regulation of T-helper 1 cell (Th1) and Th2 type responses. It has become clear that NF-kappaB family members have cell lineage-specific functions and that their relative importance varies with type of infection as well as route of pathogen entry. Thus, studies with models of parasitic infection have revealed that individual NF-kappaB family members perform distinct, nonoverlapping, and biologically significant functions in the regulation of immunity and inflammation.

Helminth parasites--masters of regulation

Immune regulation by parasites is a global concept that includes suppression, diversion, and conversion of the host immune response to the benefit of the pathogen. While many microparasites escape immune attack by antigenic variation or sequestration in specialized niches, helminths appear to thrive in exposed extracellular locations, such as the lymphatics, bloodstream, or gastrointestinal tract. We review here the multiple layers of immunoregulation that have now been discovered in helminth infection and discuss both the cellular and the molecular interactions involved. Key events among the host cell population are dominance of the T-helper 2 cell (Th2) phenotype and the selective loss of effector activity, against a background of regulatory T cells, alternatively activated macrophages, and Th2-inducing dendritic cells. Increasingly, there is evidence of important effects on other innate cell types, particularly mast cells and eosinophils. The sum effect of these changes to host reactivity is to create an anti-inflammatory environment, which is most favorable to parasite survival. We hypothesize therefore that parasites have evolved specific molecular strategies to induce this conducive landscape, and we review the foremost candidate immunomodulators released by helminths, including cytokine homologs, protease inhibitors, and an intriguing set of novel products implicated in immune suppression.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

Helminth antigens modulate TLR-initiated dendritic cell activation

There is increasing awareness that helminth infections can ameliorate proinflammatory conditions. In part, this is due to their inherent ability to induce Th2 and, perhaps, regulatory T cell responses. However, recent evidence indicates that helminths also have direct anti-inflammatory effects on innate immune responses. In this study, we address this issue and show that soluble molecules from the eggs of the helminth parasite Schistosoma mansoni (SEA) suppress LPS-induced activation of immature murine dendritic cells, including MHC class II, costimulatory molecule expression, and IL-12 production. SEA-augmented LPS-induced production of IL-10 is in part responsible for the observed reduction in LPS-induced IL-12 production. However, analyses of IL-10(-/-) DC revealed distinct IL-10-independent suppressive effects of SEA. IL-10-independent mechanisms are evident in the suppression of TLR ligand-induced MAPK and NF-kappaB signaling pathways. Microarray analyses demonstrate that SEA alone uniquely alters the expression of a small subset of genes that are not up-regulated during conventional TLR-induced DC maturation. In contrast, the effects of SEA on TLR ligand-induced DC activation were striking: when mixed with LPS, SEA significantly affects the expression of >100 LPS-regulated genes. These findings indicate that SEA exerts potent anti-inflammatory effects by directly regulating the ability of DC to respond to TLR ligands.

Dendritic cells activated with products released by schistosome larvae drive Th2-type immune responses, which can be inhibited by manipulation of CD40 costimulation

The early immune events in response to infective larvae of the parasitic helminth Schistosoma mansoni are poorly understood, but here for the first time we report on the potential of products released by schistosome larvae (material released in the first 3 h after transformation [0-3hRP]) to stimulate the maturation of dendritic cells (DC) and alter their T-cell-polarizing function. This was performed in comparison with lipopolysaccharide (LPS) and zymosan A, which classically activate DC to prime for Th1- and Th2-type responses, respectively. In our study, immature bone marrow-derived DC stimulated in vitro with 0-3hRP exhibited up-regulated expression of major histocompatibility complex class II, CD40, and CD86 and increased production of interleukin 12p40 (IL-12p40) and IL-6, albeit at lower levels than in response to LPS or zymosan A. Using an in vitro ovalbumin peptide-restricted priming assay, DC matured with 0-3hRP exhibited a potent capacity to drive Th2 polarization of CD4(+) cells from DO11.10 transgenic mice. This was characterized by increased IL-4 production (but not gamma interferon) of a magnitude similar to that primed by DC matured with zymosan A. Inoculation of DO11.10 mice with 0-3hRP-activated DC pulsed with ovalbumin peptide also led to the development of a Th2-type polarized response in the skin-draining lymph nodes and spleen. However, ligation of CD40 on DC by anti-CD40 antibody treatment reversed the ability of 0-3hRP-activated DC to prime for Th2-type responses and instead caused the induction of a more Th1-type response.

Thioredoxin peroxidase secreted by Fasciola hepatica induces the alternative activation of macrophages

Alternatively activated macrophages (AAMphi) are primarily associated with the chronic stages of parasitic infections and the development of a polarized Th2 response. We have shown that Fasciola hepatica infection of BALB/c mice induces a polarized Th2 response during both the latent and chronic stage of disease. The activation status of macrophages was analyzed in this model of helminth infection by evaluating the expression of genetic markers of alternative activation, namely, Fizz1, Ym1, and Arg1. AAMphi were recruited to the peritoneum of mice within 24 h of F. hepatica infection and after intraperitoneal injection of parasite excretory-secretory (ES) products. Administration of a recombinant antioxidant thioredoxin peroxidase (TPx), which is contained within the ES products, also induced the recruitment of AAMphi to the peritoneum. In vitro studies showed that this recombinant TPx directly converts RAW 264.7 macrophages to an alternatively activated phenotype characterized by the production of high levels of interleukin-10 (IL-10), prostaglandin E(2), corresponding with low levels of IL-12. Our data suggest that the Th2 responses induced by the helminth F. hepatica are mediated through the secretion of molecules, one of which is TPx, that induce the recruitment and alternative activation of macrophages.

Th2 induction by Nippostrongylus secreted antigens in mice deficient in B cells, eosinophils or MHC Class I-related receptors

The populations of immune system cells necessary for induction of the Th2 response have yet to be fully defined. Among the most consistent natural stimuli for Th2 responses are helminth parasites, such as the gastrointestinal nematode Nippostrongylus brasiliensis. Strong Th2 bias in immune responsiveness to live parasite infection can be reproduced by injection of a soluble Nippostrongylus excretory/secretory product (NES) collected from adult worm parasites in vitro. Injection of soluble NES induces a residual type-2 response (IL-13) even in IL-4-deficient mice, and drives a fully polar Th2 response in IL-5-deficient animals. A potent IL-10 response is observed irrespective of IL-4 or IL-5 gene status. While MHC Class II knockout animals fail to mount any IL-4 or IL-10 response, the Th2 bias is intact in Class I knockouts, indicating that CD8+ or NK-like T cells restricted to classical or non-classical Class I molecules do not play an essential role in Th2 induction. B cell-deficient microMT animals also show responses, which are strongly skewed to IL-4 production. Thus, Th2 induction by Nippostrongylus antigens is independent of B cells, or MHC class I presentation, and does not require a sufficiency of eosinophils in vivo.

Cluster analysis of schistosome-specific antibody responses partitions the population into distinct epidemiological groups

Immuno-epidemiological studies in schistosomiasis continue to generate large amounts of immunology data, whose analysis requires sophisticated statistical approaches. Here cluster analysis, is used to explore the relationship between immune responses and observed epidemiological patterns of schistosome infection in two Zimbabwean communities. Analysis of cross-sectional antibody data (IgA, IgE, IgG1, IgG2, IgG3, IgG4 and IgM directed against Schistosoma haematobium soluble egg antigen (SEA)) showed that cluster analysis partitioned the data into distinct epidemiological groups based on all seven antibody isotypes (defined by age, infection intensity, treatment status and history of infection) confirming an already known partitioning based on IgA/IgG1 production. All treated participants (children) changed cluster membership following treatment from clusters where IgA was the predominant antibody to clusters where IgG1 predominated. There was a differential distribution of IgE and IgG4 between clusters consistent with the recently proposed balance between T-helper cells (Th) 1, Th2 and regulatory T cells. The analysis suggested that naturally acquired anti-schistosome responses associated with resistance to infection were different from drug-induced responses associated with resistance to re-infection. Furthermore, the analysis suggested that parasite-specific immune responses were dynamic. The analysis conducted on data from participants resident in the S. mansoni endemic area who were all children partitioned the data into two clusters, one with predominately pre-treatment data (cluster 1) and the other with post-treatment data (cluster 2). The antibody profiles of both clusters were most similar to the profile of people with a modified Th2 response. Following treatment 43% of the children in cluster 1 moved to cluster 2, which generally had higher levels of antibodies. A detailed study of factors determining which children moved between the clusters showed that it was mostly the older, infected children who moved to cluster 2. The results of the analysis are discussed in terms of current theories of the development of acquired immunity to schistosomiasis. The relative merits of cluster analysis as a statistical tool for analysing these data are also discussed.

Helminth infection protects mice from anaphylaxis via IL-10-producing B cells

Modulation of the immune system by infection with helminth parasites, including schistosomes, is proposed to reduce the levels of allergic responses in infected individuals. In this study we investigated whether experimental infection with Schistosoma mansoni could alter the susceptibility of mice to an extreme allergic response, anaphylaxis. We formally demonstrate that S. mansoni infection protects mice from an experimental model of systemic fatal anaphylaxis. The worm stage of infection is shown to mediate this protective effect. In vivo depletion studies demonstrated an imperative role for B cells and IL-10 in worm-mediated protection. Furthermore, worm infection of mice increases the frequency of IL-10-producing B cells compared with that in uninfected mice. However, transfer of B cells from worm-infected mice or in vitro worm-modulated B cells to sensitized recipients exacerbated anaphylaxis, which was attributed to the presence of elevated levels of IL-4-producing B cells. Worm-modulated, IL-10-producing B cells from IL-4-deficient, but not IL-5-, IL-9- or IL-13-deficient, mice conferred complete resistance to anaphylaxis when transferred to naive mice. Therefore, we have dissected a novel immunomodulatory mechanism induced by S. mansoni worms that is dependent on an IL-10-producing B cell population that can protect against allergic hypersensitivity. These data support a role for helminth immune modulation in the hygiene hypothesis and further illustrate the delicate balance between parasite induction of protective regulatory (IL-10) responses and detrimental (IL-4) allergic responses.

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.

Impact of Nef-mediated downregulation of major histocompatibility complex class I on immune response to simian immunodeficiency virus

Functional activities that have been ascribed to the nef gene product of simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) include CD4 downregulation, major histocompatibility complex (MHC) class I downregulation, downregulation of other plasma membrane proteins, and lymphocyte activation. Monkeys were infected experimentally with SIV containing difficult-to-revert mutations in nef that selectively eliminated MHC downregulation but not these other activities. Monkeys infected with these mutant forms of SIV exhibited higher levels of CD8(+) T-cell responses 4 to 16 weeks postinfection than seen in monkeys infected with the parental wild-type virus. Furthermore, unusual compensatory mutations appeared by 16 to 32 weeks postinfection which restored some or all of the MHC-downregulating activity. These results indicate that nef does serve to limit the virus-specific CD8 cellular response of the host and that the ability to downregulate MHC class I contributes importantly to the totality of nef function.

The immunoepidemiology of human hookworm infection

Advances in hookworm immunoepidemiology are reviewed. Recent studies demonstrate a mixed Th1/Th2 response in human hookworm infection, with immunosuppression of specific and nonspecific IFN-gamma responses. There is increasing evidence for protective immunity in human hookworm infection, including anti-larval IL-5- and IgE-dependent mechanisms, and for immunological interactions between hookworm infection and other diseases.

Selective maturation of dendritic cells byNippostrongylus brasiliensis-secreted proteins drives Th2 immune responses

Helminth infections at mucosal and tissue sites strongly polarize towards Th2 immune responses, following pathways which have yet to be elucidated. We investigated whether dendritic cells (DC) exposed to gastrointestinal nematodes induce Th2 differentiation and, if so, whether this outcome reflects the absence of DC activation (the default hypothesis). We studied secreted proteins from the parasite Nippostrongylus brasiliensis, which induce Th2 development in vivo without live infection. Murine bone marrow-derived DC pulsed with N. brasiliensis excretory/secretory antigen (NES) can, on transfer to naive recipients, prime mice for Th2 responsiveness. Heat inactivation of NES abolishes both its ability to drive Th2 responses in vivo and its capacity to stimulate DC for Th2 induction. NES, but not heat-inactivated NES, up-regulates DC maturation markers associated with Th2 promotion (CD86 and OX40L), with little change to CD80 and MHC class II. Moreover, DC exposed to NES readily produce IL-6 and IL-12p40, but not IL-12p70. LPS induced high IL-12p70 levels, except in DC that had been pre-incubated with NES. These data contradict the default hypothesis, demonstrating that a helminth product (NES) actively matures DC, selectively up-regulating CD86 and OX40L together with IL-6 production, while blocking IL-12p70 responsiveness in a manner consistent with Th2 generation in vivo.

Basophils produce IL-4 and accumulate in tissues after infection with a Th2-inducing parasite

Using mice in which the eGfp gene replaced the first exon of the Il4 gene (G4 mice), we examined production of interleukin (IL)-4 during infection by the intestinal nematode Nippostrongylus brasiliensis (Nb). Nb infection induced green fluorescent protein (GFP)pos cells that were FcepsilonRIpos, CD49bbright, c-kitneg, and Gr1neg. These cells had lobulated nuclei and granules characteristic of basophils. They were found mainly in the liver and lung, to a lesser degree in the spleen, but not in the lymph nodes. Although some liver basophils from naive mice express GFP, Nb infection enhanced GFP expression and increased the number of tissue basophils. Similar basophil GFP expression was found in infected Stat6-/- mice. Basophils did not increase in number in infected Rag2-/- mice; Rag2-/- mice reconstituted with CD4 T cells allowed significant basophil accumulation, indicating that CD4 T cells can direct both tissue migration of basophils and enhanced IL-4 production. IL-4 production was immunoglobulin independent and only partially dependent on IL-3. Thus, infection with a parasite that induces a "Th2-type response" resulted in accumulation of tissue basophils, and these cells, stimulated by a non-FcR cross-linking mechanism, are a principal source of in vivo IL-4 production.

Cytokine response profiles predict species-specific infection patterns in human GI nematodes

This study investigated associations between pre-treatment cytokine expression and infection patterns, before and after de-worming, in humans exposed to two gastrointestinal nematode species. Quantitative measures of Ascaris lumbricoides and Trichuris trichiura infection (based on faecal egg counts) were estimated immediately before and 8-9 months after treatment in a Cameroonian population. Whole blood cytokine responses to parasite-derived antigens were assayed immediately pre-treatment. An overall measure of the tendency towards species-specific infection (increasing with A. lumbricoides faecal egg counts and decreasing with T. trichiura faecal egg counts) was significantly positively related to IL-10 levels in older (14-57 year) hosts. There was a significant negative influence of IL-5 on reinfection probability in T. trichiura but not A. lumbricoides. This effect coincided with reduced reinfection success in T. trichiura compared to A. lumbricoides. T(H)2 cytokine expression by younger hosts (4-13 year) was negatively associated with contemporary A. lumbricoides faecal egg counts before treatment. Following treatment, the pre-treatment T(H)2 cytokine expression data for younger hosts (now reflecting responsiveness 8-9 months in the past) were negatively associated with T. trichiura faecal egg counts. Taken together, these observations suggest a successional interaction between T(H)2-driven immune responses and species infection over time. However, any differential effects of the measured immune responses on species-specific recruitment, maturation and mortality were superimposed upon (and outweighed by) the effects of other factors favouring coinfection.

RELMbeta/FIZZ2 is a goblet cell-specific immune-effector molecule in the gastrointestinal tract

Gastrointestinal (GI) nematode infections are an important public health and economic concern. Experimental studies have shown that resistance to infection requires CD4(+) T helper type 2 (Th2) cytokine responses characterized by the production of IL-4 and IL-13. However, despite >30 years of research, it is unclear how the immune system mediates the expulsion of worms from the GI tract. Here, we demonstrate that a recently described intestinal goblet cell-specific protein, RELMbeta/FIZZ2, is induced after exposure to three phylogenetically distinct GI nematode pathogens. Maximal expression of RELMbeta was coincident with the production of Th2 cytokines and host protective immunity, whereas production of the Th1 cytokine, IFN-gamma, inhibited RELMbeta expression and led to chronic infection. Furthermore, whereas induction of RELMbeta was equivalent in nematode-infected wild-type and IL-4-deficient mice, IL-4 receptor-deficient mice showed minimal RELMbeta induction and developed persistent infections, demonstrating a direct role for IL-13 in optimal expression of RELMbeta. Finally, we show that RELMbeta binds to components of the nematode chemosensory apparatus and inhibits chemotaxic function of a parasitic nematode in vitro. Together, these results suggest that intestinal goblet cell-derived RELMbeta may be a novel Th2 cytokine-induced immune-effector molecule in resistance to GI nematode infection.

Schistosoma mansoni worms induce anergy of T cells via selective up-regulation of programmed death ligand 1 on macrophages

Infectious pathogens can selectively stimulate activation or suppression of T cells to facilitate their survival within humans. In this study we demonstrate that the trematode parasite Schistosoma mansoni has evolved with two distinct mechanisms to suppress T cell activation. During the initial 4- to 12-wk acute stages of a worm infection both CD4(+) and CD8(+) T cells are anergized. In contrast, infection with male and female worms induced T cell anergy at 4 wk, which was replaced after egg laying by T cell suppression via a known NO-dependent mechanism, that was detected for up to 40 wk after infection. Worm-induced anergy was mediated by splenic F4/80(+) macrophages (Mphi) via an IL-4-, IL-13-, IL-10-, TGF-beta-, and NO-independent, but cell contact-dependent, mechanism. F4/80(+) Mphi isolated from worm-infected mice were shown to induce anergy of naive T cells in vitro. Furthermore, naive Mphi exposed to live worms in vitro also induced anergy in naive T cells. Flow cytometry on in vivo and in vitro worm-modulated Mphi revealed that of the family of B7 costimulatory molecules, only programmed death ligand 1 (PD-L1) was selectively up-regulated. The addition of inhibitory mAb against PD-L1, but not PD-L2, to worm-modulated Mphi completely blocked the ability of these cells to anergize T cells. These data highlight a novel mechanism through which S. mansoni worms have usurped the natural function of PD-L1 to reduce T cell activation during early acute stages of infection before the subsequent emergence of egg-induced T cell suppression in the chronic stages of infection.

KINETICS OF ECHINOSTOMA CAPRONI (TREMATODA: ECHINOSTOMATIDAE) ANTIGENS IN FECES AND SERUM OF EXPERIMENTALLY INFECTED HAMSTERS AND RATS

This study reports on the kinetics of antibody production to Echinostoma caproni and the dynamics of antigens in feces and sera in 2 experimental hosts (hamsters and rats) that display different degrees of susceptibility with this echinostome. Echinostoma caproni produced chronic infections in hamsters, whereas rats lost the infection at 49-56 days postinfection (DPI). Hamsters developed higher antibody responses than rats, probably in relation to different intestinal absorptions of worm antigens in each host species. The levels of coproantigens were indicative of the course of infection in each host. Positive coproantigen levels were detected at 1-2 DPI in both hosts, and the values remained positive until the end of the experiment in hamsters; in rats, the coproantigen levels reverted to negative values, coinciding with the loss of infection. High levels of circulating antigens were detected in hamsters from 21 DPI to the end of the study. In contrast, low levels of E. caproni seroantigens were detected in rats only. These observations may reflect the differences in local inflammatory responses induced by E. caproni in each host species.

Signal sequence analysis of expressed sequence tags from the nematode Nippostrongylus brasiliensis and the evolution of secreted proteins in parasites

BACKGROUND

Parasitism is a highly successful mode of life and one that requires suites of gene adaptations to permit survival within a potentially hostile host. Among such adaptations is the secretion of proteins capable of modifying or manipulating the host environment. Nippostrongylus brasiliensis is a well-studied model nematode parasite of rodents, which secretes products known to modulate host immunity.

RESULTS

Taking a genomic approach to characterize potential secreted products, we analyzed expressed sequence tag (EST) sequences for putative amino-terminal secretory signals. We sequenced ESTs from a cDNA library constructed by oligo-capping to select full-length cDNAs, as well as from conventional cDNA libraries. SignalP analysis was applied to predicted open reading frames, to identify potential signal peptides and anchors. Among 1,234 ESTs, 197 (~16%) contain predicted 5' signal sequences, with 176 classified as conventional signal peptides and 21 as signal anchors. ESTs cluster into 742 distinct genes, of which 135 (18%) bear predicted signal-sequence coding regions. Comparisons of clusters with homologs from Caenorhabditis elegans and more distantly related organisms reveal that the majority (65% at P < e-10) of signal peptide-bearing sequences from N. brasiliensis show no similarity to previously reported genes, and less than 10% align to conserved genes recorded outside the phylum Nematoda. Of all novel sequences identified, 32% contained predicted signal peptides, whereas this was the case for only 3.4% of conserved genes with sequence homologies beyond the Nematoda.

CONCLUSIONS

These results indicate that secreted proteins may be undergoing accelerated evolution, either because of relaxed functional constraints, or in response to stronger selective pressure from host immunity.

Fimbriated Porphyromonas gingivalis is more efficient than fimbria-deficient P. gingivalis in entering human dendritic cells in vitro and induces an inflammatory Th1 effector response

Porphyromonas gingivalis is a fimbriated mucosal pathogen implicated in chronic periodontitis (CP). The fimbriae are required for invasion of the gingival mucosa and for induction of CP in animal models of periodontitis. CP is associated with infection of immature dendritic cells (DCs) by P. gingivalis in situ and with increased numbers of dermal DCs (DDCs) and mature DCs in the lamina propria. The role of fimbriae in gaining entry into human DCs and how this modulates the inflammatory and effector immune responses, however, have not been explored. To address this, we generated monocyte-derived DCs (MDDCs) in vitro which phenotypically and functionally resemble DDCs. We show here that virulent fimbriated P. gingivalis 381, in contrast to its fimbria-deficient mutant, P. gingivalis DPG3, efficiently gains entry to MDDCs in a manner dependent on active cell metabolism and cytoskeletal rearrangement. In addition, uptake of 381, unlike DPG3, induces DCs to undergo maturation, upregulate costimulatory molecules, and secrete inflammation cytokines interleukin-1beta (IL-1beta), IL-6, tumor necrosis factor alpha, IL-10, and IL-12. Moreover, MDDCs pulsed with 381 also stimulated a higher autologous mixed lymphocyte reaction and induced a Th1-type response, with gamma interferon (IFN-gamma) being the main cytokine. Monocytes used as controls demonstrated fimbria-dependent uptake of 381 as well but produced low levels of inflammatory cytokines compared to MDDCs. When MDDCs were pulsed with recombinant fimbrillin of P. gingivalis (10 micro g/ml), maturation of MDDCs was also induced; moreover, matured MDDCs induced proliferation of autologous CD4(+) T cells and release of IFN-gamma. Thus, these results establish the significance of P. gingivalis fimbriae in the uptake of P. gingivalis by MDDCs and in induction of immunostimulatory Th1 responses.