Interleukin-12 and the host response to parasitic helminths; the paradoxical effect on protective immunity and immunopathology

Influence of levamisole and Freund's adjuvant on mouse immunisation with antigens of adults of the liver fluke Fasciola hepatica Linnaeus, 1758

We have studied the influence of both levamisole (AL) and Freund's adjuvant (AF) on the immunisation of mice with the secretory antigens of adults of the liver fluke Fasciola hepatica Linnaeus, 1758. Total IgG antibodies were detected in all groups where the F. hepatica antigen was administered, been levels of IgG1 increased respect to IgG2a antibodies. During immunisation, IL-4 and IFN-γ were only detected in AL and AF groups, but after infection, IL-4 boosted in all groups. IFN-γ increased two fold in AF and AL groups compared to the saline solution (AS) group. Worm recovering was of 32-35% in groups administered without antigen whereas in AS, AL and AF groups recovering was of 25%, 12% and 8%, respectively. Macroscopical lesions in the liver were scarce in AL and AF groups. Our data suggest that immunisation of mice with antigens of F. hepatica enhances the immune response avoiding both liver damage and worm establishment after challenge infection. The murine model of fasciolosis has appeared to be useful to elucidate the mechanism by which the parasite modulates immune responses toward a Th2 type but also the development of Th1 type-inducing vaccines.

Acquired immune heterogeneity and its sources in human helminth infection

Similarities in the immunobiology of different parasitic worm infections indicate that co-evolution of humans and helminths has shaped a common anti-helminth immune response. However, recent in vitro and immuno-epidemiological studies highlight fundamental differences and plasticity within host-helminth interactions. The 'trade-off' between immunity and immunopathology inherent in host immune responses occurs on a background of genetic polymorphism, variable exposure patterns and infection history. For the parasite, variation in life-cycle and antigen expression can influence the effector responses directed against them. This is particularly apparent when comparing gastrointestinal and tissue-dwelling helminths. Furthermore, insights into the impact of anti-helminthic treatment and co-infection on acquired immunity suggest that immune heterogeneity arises not from hosts and parasites in isolation, but also from the environment in which immune responses develop. Large-scale differences observed in the epidemiology of human helminthiases are a product of complex host-parasite-environment interactions which, given potential for exposure to parasite antigens in utero, can arise even before a parasite interacts with its human host. This review summarizes key differences identified in human acquired immune responses to nematode and trematode infections of public health importance and explores the factors contributing to these variations.

Gastrointestinal parasites: potential therapy for refractory inflammatory bowel diseases

Crohn's disease and ulcerative colitis are chronic relapsing inflammatory bowel diseases (IBDs). Different pharmacological agents are currently used in several combinations to control the inflammatory process. Recently, antibodies against the proinflammatory cytokine tumor necrosis factor-alpha appeared to be very effective in treating patients with Crohn's disease. However, due to the fact that the pathogen causing IBD is still unknown, no causative treatment is currently available that is able to make the disease disappear. Recently, the hygiene hypothesis of the development of immunological diseases was proposed, stating that raising children in extremely hygienic environments with less exposure to parasite infections may negatively affect the development of the immune system, predisposing them to immunologic diseases such as IBD. This hypothesis is supported by experimental data showing that helminthic parasites protect against T helper (TH) type 1 cell-mediated gastrointestinal inflammations like Crohn's disease. Both TH-2 cells and regulatory T cells may be involved in this immunomodulatory mechanism. Here, we review the experimental and clinical studies in favor of the hygiene hypothesis, opening perspectives on new therapies for IBD.

A STAT4-dependent Th1 response is required for resistance to the helminth parasite Taenia crassiceps

To determine the role of STAT4-dependent Th1 responses in the regulation of immunity to the helminth parasite Taenia crassiceps, we monitored infections with this parasite in resistant mice lacking the STAT4 gene. While T. crassiceps-infected STAT4(+/+) mice rapidly resolved the infection, STAT4(-/-) mice were highly susceptible to infection and displayed large parasite loads. Moreover, the inability of STAT4(-/-) mice to control the infection was associated with the induction of an antigen-specific Th2-type response characterized by significantly higher levels of Th2-associated immunoglobulin G1 (IgG1) and total IgE as well as interleukin-4 (IL-4), IL-10, and IL-13 than those in STAT4(+/+) mice, who produced significantly more gamma interferon. Furthermore, early after infection, macrophages from STAT4(-/-) mice produced lower levels of the pro-inflammatory cytokines IL-12, tumor necrosis factor alpha, IL-1 beta, and nitric oxide (NO) than those from STAT4(+/+) mice, suggesting a pivotal role for macrophages in mediating protection against cysticercosis. These findings demonstrate a critical role for the STAT4 signaling pathway in the development of a Th1-type immune response that is essential for mediating protection against the larval stage of T. crassiceps infection.

Brugia malayi microfilariae induce cell death in human dendritic cells, inhibit their ability to make IL-12 and IL-10, and reduce their capacity to activate CD4+ T cells

Parasite Ag-specific T cell unresponsiveness and diminished IFN-gamma production are immunologic hallmarks of patent infection with lymph-dwelling filarial nematodes. Although this diminished responsiveness is directed primarily against the intravascular microfilarial (MF) parasite stage and mediated in part by reduced APC function, the mechanisms involved are not fully understood. In this report, we demonstrate that human dendritic cells (DC) exposed to live MF up-regulate both the cell surface and gene expression of CD54 (ICAM-1). Moreover, live MF result in a 3-fold increase in DC death compared with MF-unexposed DC, primarily due to apoptosis. Notably, microarray and real-time RT-PCR data indicate that live MF concurrently up-regulate mRNA expression of proinflammatory molecules such as IL-8, RANTES, IL-1alpha, TNF-alpha, and IL-beta in DC, the presence of which is also detected at the protein level, while inhibiting the production of IL-12 (p40 and p70) and IL-10. Soluble excretory-secretory products from live MF diminished IL-12 and IL-10 production and induced DC death, although to a lesser degree. Moreover, exposure of DC to live MF resulted in a decrease in the ability of DC to promote CD4(+) T cell production of IFN-gamma and IL-5. Our findings clearly suggest that the interaction between live MF and DC is complex but contributes to the hyporesponsiveness and parasite persistence associated with the MF(+) state in the infected human. These data further suggest that MF induce an orchestrated response in APC that leads to a diminished capacity to function appropriately, which in turn has significant consequences for CD4(+) T cells.

When T-helper cells don't help: immunopathology during concomitant infection

Disease directly caused by immune system action is known as immunopathology. Many factors may lead the immune system to cause rather than cure disease, and autoimmune, allergic, and infection-related immunopathological diseases affect millions of people worldwide. This review presents an analysis of T-helper cell mediated, infection-related immunopathology within the framework of evolutionary ecology. A proximate cause of infection-related immunopathology is an error in the type of T-helper response induced. Distinct subsets of T-helper cells enable different effector mechanisms and therefore work optimally against different types of parasites (e.g., extracellular versus intracellular parasites). Immune responses that cure rather than cause disease require that the T-helper subset be tailored to the parasite. It is thus critical for the immunophenotype to match the "environment" of the parasitic infection. As in other cases of adaptive plasticity, a mismatch between an organism's phenotype and the selective environment can decrease fitness. T-helper response induction may be confounded by coinfection of a single host by multiple parasite species. Because of normally adaptive feedback loops that lend to polarize T-helper responses, it can become impossible for the immune system to mount effective, conflicting responses concurrently. Immunophenotype-environment mismatches may thus be inevitable when simultaneous, conflicting immune responses are required. An ultimate cause of infection-related immunopathology in a multiparasite selection regime is the T-helper response polarization that can propagate response errors and constrain the ability of the immune system to resolve conflicting response requirements. A case study is used to illustrate how coinfection can exacerbate immunopathology and to frame testable predictions about optimal responses to coinfection (e.g., is the observed joint response to coinfection accurately predicted by the average of the component single-infection optimal responses, where the single-infection optima are weighted by the contribution of each to fitness). The case study includes immunological and pathological data from mice infected by Schistosoma mansoni alone and by S. mansoni in combination with Toxoplasma gondii. Such data can inform hypothesis tests of evolutionary ecological principles, and ecological analysis can in turn clarify assumptions about responses to coinfection for a greater understanding of the immune system. The synthesis of evolutionary ecology and immunology could therefore be of mutual benefit to the two disciplines.

Cathepsin L proteinases as vaccines against infection with Fasciola hepatica (liver fluke) in ruminants

Fasciolosis, caused by trematodes of the genus Fasciola, is one of the most important diseases of farmed ruminants in temperate and tropical zones. The appearance of Fasciola hepatica populations that are resistant to common flukicidal drugs means that new methods of treatment will soon be required. The future prospect for the development of anti-liver fluke vaccines is optimistic and given their consumer acceptability and environmental friendliness, offer the best way forward. Cathepsin L proteases (Fhe CL 1 and Fhe CL 2), secreted by liver flukes at all stages of their development in the mammalian host, are believed to play important roles in facilitating parasite migration (tissue degradation), feeding and immuno-evasion. The authors consider them prime targets for which new vaccines can be developed. Vaccine studies in cattle and sheep have shown that protection levels of up to 72 and 79 per cent, respectively, can be obtained with immunisations of cathepsin Ls in Freunds' adjuvant. The vaccine also exhibited high anti-embryonation/anti-fecundity effects on parasites that survived in vaccinated animals and thus could have a major impact on the transmission of disease to the intermediate host. While natural infections in sheep and cattle appear to elicit non-protective Th2 immune responses, the authors' studies indicate that the protection induced by vaccination involves elements of a Th1 response.

Signaling via interleukin-4 receptor alpha chain is required for successful vaccination against schistosomiasis in BALB/c mice

Although protective immunity in C57BL/6 mice induced by a single dose of the radiation-attenuated schistosome vaccine is believed to be mediated by Th1-type immune responses, we here report that in BALB/c mice protection can also depend upon signaling via the interleukin-4 (IL-4) receptor which conventionally governs the development of Th2-type immune responses. We show that in BALB/c mice deficient for the IL-4 receptor alpha chain (IL-4Ralpha(-/-)), which are unresponsive to IL-4 and IL-13, vaccine-induced protection is abrogated compared with that in wild-type (WT) mice. In vaccinated IL-4Ralpha(-/-) mice, IL-12p40 production by cells from the skin exposure site was elevated, although gamma interferon (IFN-gamma) production in draining lymphoid tissues was similar in WT and IL-4Ralpha(-/-) mice. Nevertheless, the effector response in IL-4Ralpha(-/-) mice was Th1 biased with elevated IFN-gamma in the lungs and higher immunoglobulin G2a (IgG2a) and IgG2b titers but negligible quantities of Th2-associated IgG1 and IgE. Interestingly, levels of IL-4 were equivalent in WT and IL-4Ralpha(-/-) mice, indicating that Th2 responses were not dependent upon signaling by IL-4 or IL-13. No differences in the phenotype and composition of the pulmonary effector mechanism that might explain the failure to induce protection in IL-4Ralpha(-/-) mice were detected. However, passive transfer of partial protection to naive IL-4Ralpha(-/-) mice, using serum from vaccinated WT mice, indicates that Th2-associated antibodies such as IgG1 have a role in parasite elimination in BALB/c strain mice and that signaling via IL-4R can be an important factor in the generation of protection.

Prenatal immune priming with helminth infections: parasite-specific cellular reactivity and Th1 and Th2 cytokine responses in neonates

The present investigation aimed to determine to what extent maternal helminth infection primes parasite-specific cellular responsiveness in neonates. Umbilical cord mononuclear blood cells (UCBC) and peripheral blood mononuclear cells (PBMC) from mothers proliferated in response to mitogenic stimulation with concanavalin A, as well as to bacterial Streptococcus pyogenes-derived (streptolysin O) and helminth-specific antigens of Necator americanus and Onchocerca volvulus. Cellular responses to Echinococcus multilocularis (Em) and Oesophagostomum bifurcum (Oes), helminth parasites not endemic in the study area, were absent (for Em) or very low (for Oes due to antigenic cross-reactivity). Cellular responsiveness to mitogen and antigens was higher in mothers than in their neonates. Several Th1-type (IL-2, IL-12, and IFN-gamma) and Th2-type (IL-5 and IL-10) cytokines were produced by UCBC from neonates and PBMC from mothers. Low levels of IFN-gamma were elicited by UCBC in response to helminth and bacterial antigens, while secretion of IL-2 was pronounced and similarly high in neonates and their mothers. Amounts of IL-5 produced by UCBC in response to bacterial SL-O and mitogenic stimulation (PHA) were low, but equivalent levels of IL-5 were induced by intestinal helminth and filaria-derived antigens in neonates and mothers. A pronounced production of IL-10 and IL-12 by UCBC was observed--spontaneous IL-10 and IL-12 secretion by UCBC was higher in neonates than by PBMC from mothers. Net amounts of IL-10 elicited by helminth antigens were similar, while net IL-12 in response to mitogen, and bacterial and helminth antigens was significantly higher in mothers than their offspring. Our results indicate that human maternal helminth infection does sensitize in utero for parasite-specific cellular responsiveness in offspring, and also activates specific production of several cytokines, and such children do not present a dominant expression of immunity of either Th1 or Th2.

Vaccination against helminth parasites--the ultimate challenge for vaccinologists?

Helminths are multicellular pathogens which infect vast numbers of human and animal hosts, causing widespread chronic disease and morbidity. Vaccination against these parasites requires more than identification of effective target antigens, because without understanding the immunology of the host-parasite relationship, ineffective immune mechanisms may be invoked, and there is a danger of amplifying immunopathogenic responses. The fundamental features of the immune response to helminths are therefore summarised in the context of vaccines to helminth parasites. The contention between type-1 and type-2 responses is a central issue in helminth infections, which bias the immune system strongly to the type-2 pathway. Evidence from both human and experimental animal infections indicates that both lineages contribute to immunity in differing circumstances, and that a balanced response leads to the most favourable outcome. A diversity of immune mechanisms can be brought to bear on various helminth species, ranging from antibody-independent macrophages, antibody-dependent granulocyte killing, and nonlymphoid actions, particularly in the gut. This diversity is highlighted by analysis of rodent infections, particularly in comparisons of cytokine-depleted and gene-targeted animals. This knowledge of protective mechanisms needs to be combined with a careful choice of parasite antigens for vaccines. Many existing candidates have been selected with host antibodies, rather than T-cell responses, and include a preponderance of highly conserved proteins with similarities to mammalian or invertebrate antigens. Advantage has yet to be taken of parasite genome projects, or of directed searches for novel, parasite-specific antigens and targets expressed only by infective stages and not mature forms which may generate immunopathology. With advances under way in parasite genomics and new vaccine delivery systems offering more rapid assessment and development, there are now excellent opportunities for new antihelminth vaccines.

Immune responses to the radiation-attenuated schistosome vaccine: what can we learn from knock-out mice?

The goal of an effective schistosomiasis vaccine has proved elusive but the protective immunity induced in mice by radiation-attenuated cercaria larvae provides an appropriate model from which such a vaccine might be developed. Using gene-disrupted mice, we have analysed the process of immune priming by attenuated larvae of Schistosoma mansoni and the nature of the pulmonary effector response directed against a challenge infection. Vaccination stimulates expansion of IFNgamma-producing T-helper cells in the skin-draining lymph nodes. IL-12 is crucial in determining the Thl direction of this initial response but the cells of origin and the parasite components which stimulate its production are unknown. In the effector response, focal aggregates comprising mainly mononuclear cells accumulate around challenge larvae in the lungs, a process orchestrated by IFNgamma. This cytokine up-regulates nitric oxide synthase activity but we were unable to implicate nitric oxide as a cytotoxic agent causing challenge parasite elimination. An alternative action for IFNgamma may be to up-regulate adhesion molecule expression, increasing the cohesiveness of effector foci the better to block parasite migration, but the adhesive interactions so far examined do not appear relevant. In contrast, TNF induction is essential to protection, and we are currently testing the hypothesis that it determines the speed of the effector response following arrival of challenge larvae in the lungs.