Schistosoma mansoni: circulating and pulmonary leucocyte responses related to the induction of protective immunity in mice by irradiated parasites

Why the radiation-attenuated cercarial immunization studies failed to guide the road for an effective schistosomiasis vaccine: A review

Schistosomiasis is a debilitating parasitic disease caused by platyhelminthes of the genus Schistosoma, notably Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum. Pioneer researchers used radiation-attenuated (RA) schistosome larvae to immunize laboratory rodent and non-human primate hosts. Significant and reproducible reduction in challenge worm burden varying from 30% to 90% was achieved, providing a sound proof that vaccination against this infection is feasible. Extensive histopathological, tissue mincing and incubation, autoradiographic tracking, parasitological, and immunological studies led to defining conditions and settings for achieving optimal protection and delineating the resistance underlying mechanisms. The present review aims to summarize these findings and draw the lessons that should have guided the development of an effective schistosomiasis vaccine.

The irradiated cercariae vaccine model: looking on the bright side of radiation

Schistosomes infect between 200 and 300 million people at any one time. A major strategy to reduce the impact of schistosomiasis on human health is the development of a defined antigen vaccine. Protective immunity induced in mice by irradiated cercariae may serve as a model for the development of a vaccine. In such vaccinated mice, worm burdens resulting from challenge infection can be reduced by more than 90% compared to non-vaccinated mice. During the past three decades, the irradiated-carcariae vaccine model has been dissected in the detail in order to determine factors that may be relevant to vaccination, such as the participating immune compartments, the site and kinetics of the immune response, and the antigens recognized. In this review, Dania Richter, Donald A. Harn and Franz-Rainer Matuschka highlight the research on the vaccine model, focusing on the murine model using gamma-irradiated cercariae of Schistosoma mansoni.

Paramyosin is a major target of the human IgA response against Schistosoma japonicum

Human resistance and susceptibility to schistosomiasis is associated with age and specific antibody isotype responses against worm (SWAP) and egg (SEA) antigens. In a cross-sectional study of 176 individuals infected with Schistosoma japonicum in the Philippines, strikingly similar isotype response patterns against SWAP and SEA was observed when compared to other endemic areas. Interestingly, IgA titres to SWAP correlated with older age among S. japonicum-infected individuals (n = 176, P < 0.01), suggesting a role for this isotype in protective immunity. To identify the molecular targets of human IgA, 17 high-IgA/SWAP responders were identified from the said population. IgA antibodies from the majority (14/17) of these individuals recognized a band of 97 kDa (Sj97), comigrating in immunoblots with the myofibrillar protein paramyosin. The antigen was confirmed as paramyosin by expressed sequence tag (EST)-analysis of four clones obtained by screening an adult S. japonicum cDNA library with pooled IgA antisera and mouse antiparamyosin polyclonal antibodies. The identification of paramyosin as a major target of human IgA raises its potential as a vaccine candidate that targets mucosal immune responses. Since this antigen is exposed on the parasite surface only during the lung stages, we propose that human IgA contributes to parasite attrition during schistosome migration in the lungs.

Nitric oxide produced in the lungs of mice immunized with the radiation-attenuated schistosome vaccine is not the major agent causing challenge parasite elimination

Mice vaccinated with radiation-attenuated cercariae of Schistosoma mansoni exhibit high levels of protection against a challenge with normal larvae. The immune effector mechanism, which operates against schistosomula in the lungs, requires CD4+ T cells capable of producing interferon-gamma (IFN-gamma). This cytokine can stimulate production of nitric oxide (NO), via its ability to up-regulate inducible nitric oxide synthase (iNOS). We have therefore evaluated the potential role of NO in the effector mechanism operating in vaccinated mice. Evidence for the production of NO in the lungs of such animals was obtained from assays on antigen-stimulated airway cell cultures. Enhanced levels of NO, compared with those in cultures from control mice, were detected both after vaccination and after challenge; elevated levels of iNOS mRNA were also present in whole lung after challenge. However, administration of an iNOS inhibitor to vaccinated mice after percutaneous challenge did not significantly increase the worm burden. Furthermore, when mice with a disrupted iNOS gene were vaccinated they showed a highly significant level of protection. Although NO from activated macrophages can mediate cytotoxic killing of newly transformed schistosomula in vitro, we have demonstrated that the addition of erythrocytes to these larvicidal assays abolishes its effects. We interpret this to mean that once migrating schistosomula enter the bloodstream they will be protected against the cytotoxic actions of NO. Our data thus provide little evidence to implicate NO as a major component of the pulmonary effector response to S. mansoni in vaccinated mice.

Recruitment of lymphocytes to the lung through vaccination enhances the immunity of mice exposed to irradiated schistosomes

The effector mechanism, which operates against challenge parasites in the lungs of C57BL/6 mice vaccinated once with irradiated cercariae of Schistosoma mansoni, is mediated by CD4+ T helper lymphocytes. However, adoptive transfer of immunity from vaccinated donors to naive recipients by using sensitized T cells has not proved successful. One explanation may be that the recruitment of sensitized T lymphocytes to the lungs by vaccinating parasites to arm that organ is not reproduced by transfer protocols. We have used the technique of parabiosis, as a means of adoptive transfer, to demonstrate the relevance of pulmonary T cells to protection. Sensitized and naive partners were joined surgically for a 28-day period, coincident with priming of the immune system. A vascular union rapidly developed, and sensitized T cells were detected in the spleens of the naive partners. When parabionts were challenged percutaneously 10 days after separation, the level of immunity transferred to the naive partners was approximately two-thirds that of their vaccinated counterparts. The naive partners, unlike the vaccinated animals, did not recruit lymphocytes to the lungs during the priming period. In contrast, after percutaneous challenge, schistosome-specific lymphocytes were recruited to the lungs of both separated parabionts. The importance of lymphocytes recruited to the lungs during the primary response was revealed by an intravenous challenge with lung schistosomula; this eliminates the opportunity for secondary immune responses prior to parasite arrival in the lungs. In this situation, the vaccinated partners showed 47% immunity while the naive partners were not protected. We conclude that the presence of specific T cells in the lungs at the time of challenge confers a significant advantage, permitting a more effective recall response than in animals lacking such resident cells.

Immune response of mice infected with Schistosoma mansoni is modulated by antifibrotic treatment

Mice infected with Schistosoma mansoni treated with beta-aminopropionitrile (BAPN), an antifibrotic agent, and the antischistosomal drug praziquantel (PZQ) were resistant to challenge for up to 5 weeks post-treatment. The combined treatment resulted in profound changes to the liver granuloma cell matrix and the composition and function of the cellular infiltrate. Although granulocytes always predominated in the infiltrate, the proportion of the cells which were macrophages was higher in mice treated with BAPN alone (39.2%) than in infected mice which were untreated (15.2%) or treated with PZQ alone (12.4%), and much higher than in mice given the combined treatment (1.8%). Two products associated with macrophage activation and cytotoxicity [tumour necrosis factor-alpha (TNF-alpha) and nitric oxide] were only detected in mice harbouring a patent infection and there was a strong positive correlation between the concentrations of each. The relatively low TNF-alpha concentrations in BAPN-treated mice seemed to be associated with the relatively small granulomas observed in these mice. BAPN treatment also led to changes in the proliferative response of the treated mice's macrophages to mitogen and soluble schistosome-egg antigen and in spleen cellularity; these changes are probably associated with the resistance to challenge infection observed in mice given BAPN with PZQ. It is clear that BAPN treatment changes the dynamics of the delayed-type hypersensitivity response within the granuloma and that this impacts on other immunological sites. How this relates to the maintenance of post-treatment resistance to a challenge infection has still to be elucidated.

Immunity to Schistosoma mansoni in mice vaccinated with irradiated cercariae: cytokine interactions in the pulmonary protective response

In C57BL/6 strain mice vaccinated with attenuated cercariae of Schistosoma mansoni, the major site of immune elimination of challenge parasites is the lungs. We have monitored pulmonary events after both vaccination and challenge by bronchoalveolar lavage, and examined the profile of cytokines released by recovered cells upon stimulation with larval antigens in vitro. From 14 days post-vaccination, lavage samples contain infiltrating lymphocytes which produce abundant interferon-gamma (IFN-gamma) and interleukin-3 (IL-3). We suggest that the lymphocytes recruited to the lungs are effector/memory cells of the Th1 subset. Challenge of vaccinated mice results in a second influx of IFN-gamma- and IL-3-secreting cells into the airways, earlier than after vaccination alone, or in appropriate controls. Ablation studies reveal that CD4+ T cells are the source of the IFN-gamma. The timing of cytokine production after both vaccination and challenge coincides with phases of macrophage activation already recorded, and with the presence of parasites in the lungs. Administration of monoclonal antibody directed against IFN-gamma, over the period of challenge elimination, almost completely abrogates protection in vaccinated mice, but does not affect the ratio of Th1:Th2 cells in the lungs. Immunity in this model is not, however, affected by inhibition of nitric oxide production, or neutralization of TNF. We suggest that the effector mechanism may operate by blocking parasite migration, and that loss of protection following neutralization of IFN-gamma may be attributed to changes in composition, density and cohesiveness of pulmonary foci.

Enhancement of eosinophil-mediated cytotoxicity to schistosomula of Schistosoma mansoni by autologous mononuclear cells from patients

Adherent mononuclear cells (monolayer), when co-cultured with autologous peripheral blood eosinophils isolated from patients treated for Schistosoma mansoni infections, enhanced the eosinophil-mediated killing of antibody coated schistosomula. The monolayer increased the activity of the eosinophils by 225%, and was observed in 80% of the patients studied. Heat labile factors other than complement, present in immune serum, further enhanced the ability of eosinophils to kill schistosomula in the presence of the monolayer. On their own the adherent cells did not mediate obvious damage to the parasite. Eosinophils that had been pre-incubated with the monolayer (100 mins) and tested separately, killed equal numbers of schistosomula as in the co-culture assay; this excludes the possibility of concurrent schistosomula cytotoxicity by the two cell populations. The ability of the monolayer to activate eosinophils was not altered by inhibitors of protein synthesis. The monolayer was largely consistent of monocytes as demonstrated by an over 96% positive staining for non-specific esterases.

The role of mononuclear-cell recruitment to the lungs in the development and expression of immunity to Schistosoma mansoni

The role of pulmonary cellular responses in the induction and expression of immunity to Schistosoma mansoni has been evaluated. From experiments in which mice were challenged at intervals after vaccination, we conclude that the resistance mechanism operating in the lungs develops between days 11 and 20. Injection of 51Cr-labelled splenocytes revealed that mononuclear cell recruitment to the lungs, stimulated by the arrival of attenuated schistosomula, intensified progressively between days 10 and 16 post-vaccination. The increased cellularity was reflected in a 19.5% augmentation in the wet-weight index (WW). The period of enhanced cell recruitment therefore coincided with the build-up of resistance. By day 22 post-vaccination infiltration had declined, whilst WW remained elevated. This indicates the persistence of recruited cells, which include schistosome-reactive T lymphocytes, in the lungs. We were unable to demonstrate augmented recruitment of 51Cr-labelled cells after challenge of vaccinated mice, but WW rose slightly, peaking on day 12. Although clearly of a lower order than the primary response, the secondary response was more rapid, implying the existence of immunological memory. These results accord with the concept that schistosome-reactive T lymphocytes recruited after vaccination 'arm' the lungs against the arrival of challenge parasites.

The magnitude and kinetics of delayed-type hypersensitivity responses in mice vaccinated with irradiated cercariae of Schistosoma mansoni

A footpad assay was used to measure the DTH of mice to soluble worm antigens (SWAP), and to living day 7 lung schistosomula, following vaccination and challenge infections with Schistosoma mansoni. DTH to SWAP was first observed on day 10, and reached its maximum on day 17 post-vaccination. Treatment of mice with anti-CD4 antibody on the 3 days prior to footpad challenge completely abrogated this response. Reactivity to living parasites was of a lower order than that to SWAP; it also peaked earlier, on day 10 post-vaccination. By day 35, responsiveness to both sets of antigens had declined almost to control levels. There was no correlation between the level of DTH to living schistosomula, at any time, and the degree of resistance subsequently developed. Percutaneous challenge of vaccinated mice was followed by a resurgence of reactivity to SWAP. This secondary response occurred more rapidly than the primary response, peaking on day 7 post-challenge, and was of a similar magnitude. We were unable to detect a similar recall of DTH to living schistosomula, possibly because the assay was insufficiently sensitive. We conclude that the intensity and kinetics of DTH responsiveness are crucial features of the irradiated vaccine model, and suggest that further investigation of cell-mediated immune reactions, particularly those occurring in the lungs, is vital to a better understanding of events underlying the development and expression of immunity.

Schistosoma mansoni: acquired immunity in mice after the use of oxamniquine at the evolutive skin and pulmonary phases

Mice infected with 350 cercariae of Schistosoma mansoni (LE strain) were treated with oxamniquine, at the dose of 400 mg/kg, 24, 48, 72, and 96 h after infection. Forty days after the treatment, the animals were submitted to a challenge infection with 80 cercariae, through the abdominal and ear skins. The number of immature worms in the animal groups treated 24 and 96 h after the first infection was found to be lower than that in the control group, thus showing that the death of schistosomes by chemotherapy, at the skin and pulmonary phases, causes an acquired resistance state.

Schistosoma mansoni larvicidal activity of murine bronchoalveolar lavage cells

We have investigated the ability of cells obtained from both normal and immune mice by bronchoalveolar lavage (BACs) to kill Schistosoma mansoni larvae in vitro. In cultures with mechanically derived schistosomules, high levels of larvicidal activity were displayed by BACs from both normal and irradiated cercaria-immunized C57BL/6 mice. Based on effector-to-target-cell ratios, BAC-mediated killing was two- to threefold more efficient than killing mediated by macrophage-rich cell populations obtained from the peritoneal cavity. BACs from normal A/J mice were essentially as larvicidal as normal C57BL/6 cells. However, BACs from a strain of mouse (P/J) with a known macrophage defect possessed negligible larvicidal activity. Macrophages made up 85 to 95% of BACs from all three strains tested. In contrast to cells of the IC-21 macrophage cell line, B6 BACs did not show enhanced killing activity when preincubated with lymphokine-containing supernatants. Lung schistosomules harvested 10 days after cercarial penetration were refractory to BAC-mediated killing.

Phenotypic analysis of the cellular responses in regional lymphoid organs of mice vaccinated against Schistosoma mansoni

The cellular responses in regional lymphoid organs of C57B1/6 mice were examined, following protective immunization with 20 krad.-irradiated cercariae of S. mansoni. Marked changes in total cell number were observed, with peak increases of 13.5-fold in the skin-draining (axillary) lymph nodes and 6.9-fold in the lung-draining (mediastinal) lymph nodes. In contrast, cellular responses were small in the spleen and undetectable in the brachial lymph nodes. The temporal pattern of responses was coincident with the kinetics of parasite migration, events in the mediastinal lymph node being apparent only after day 7. Phenotypic analysis of the cellular changes revealed an increase both in T lymphocytes and, to a greater extent, in B lymphocytes. The changes in Thy1+ cells comprised an increase in both L3T4+ and Lyt2+ populations. A comparison of mice exposed to non-immunizing parasites (normal or 80 krad.-irradiated cercariae) with protected animals, revealed smaller and more transient cellular changes in the axillary lymph nodes of the former. We suggest that the successful immunization of mice with attenuated parasites depends upon the prolonged priming of lymphocytes within the lymph nodes draining the skin-exposure site and that the persistence of 20 krad.-irradiated parasites within these nodes provides the requisite stimulus.

Lung-phase immunity to Schistosoma mansoni: definition of alveolar macrophage phenotypes after vaccination and challenge of mice

Phenotypic changes in murine alveolar macrophages have been described in response to vaccination with irradiated cercariae and to a subsequent challenge with normal parasites. Flow cytometric analysis was used to quantify the proportions of cells strongly positive for a number of macrophage surface markers, detected by a panel of monoclonal primary antibodies and fluorescent secondary antibodies. The proportion of Ia+ macrophages sampled by bronchoalveolar lavage increased 5-fold over days 14 to 28 post-vaccination. This upregulation of Ia was accompanied by a sharp decrease in F4/80 expression between days 14 and 21. The low percentage of F4/80+ cells persisted for several weeks after vaccination, and no further change was stimulated by challenge parasites. These altered characteristics are consistent with the 'activation phenotype' induced by other infectious agents. After challenge of immune mice, further changes in macrophage phenotype were slight compared to the responses elicited by vaccination, or to those induced in the challenge control group; Ia expression increased to about three times normal levels. The phenotypic changes correspond both in magnitude and timing with the pattern of alveolar macrophage activation determined in a previous study. The limited changes in phenotype of alveolar macrophages from immunized mice after challenge could indicate that these cells become refractory to reactivation. Overall, the altered macrophage phenotype after vaccination and challenge provides circumstantial evidence for the action of cytokines, particularly interferon-gamma, in lung-phase immunity to schistosomes.