Studies on immunity to Schistosoma mansoni in vivo: whole-body irradiation has no effect on vaccine-induced resistance in mice

Do schistosome vaccine trials in mice have an intrinsic flaw that generates spurious protection data?

The laboratory mouse has been widely used to test the efficacy of schistosome vaccines and a long list of candidates has emerged from this work, many of them abundant internal proteins. These antigens do not have an additive effect when co-administered, or delivered as SWAP homogenate, a quarter of which comprises multiple candidates; the observed protection has an apparent ceiling of 40-50%. We contend that the low level of maturation of penetrating cercariae (~32% for Schistosoma mansoni) is a major limitation of the model since 68/100 parasites fail to mature in naïve mice due to natural causes. The pulmonary capillary bed is the obstacle encountered by schistosomula en route to the portal system. The fragility of pulmonary capillaries and their susceptibility to a cytokine-induced vascular leak syndrome have been documented. During lung transit schistosomula burst into the alveolar spaces, and possess only a limited capacity to re-enter tissues. The acquired immunity elicited by the radiation-attenuated (RA) cercarial vaccine relies on a pulmonary inflammatory response, involving cytokines such as IFNγ and TNFα, to deflect additional parasites into the alveoli. A principal difference between antigen vaccine protocols and the RA vaccine is the short interval between the last antigen boost and cercarial challenge of mice (often two weeks). Thus, after antigen vaccination, challenge parasites will reach the lungs when both activated T cells and cytokine levels are maximal in the circulation. We propose that "protection" in this situation is the result of physiological effects on the pulmonary blood vessels, increasing the proportion of parasites that enter the alveoli. This hypothesis will explain why internal antigens, which are unlikely to interact with the immune response in a living schistosomulum, plus a variety of heterologous proteins, can reduce the level of maturation in a non-antigen-specific way. These proteins are "successful" precisely because they have not been selected for immunological silence. The same arguments apply to vaccine experiments with S. japonicum in the mouse model; this schistosome species seems a more robust parasite, even harder to eliminate by acquired immune responses. We propose a number of ways in which our conclusions may be tested.

Radiation-attenuated schistosome vaccination – a brief historical perspective

The high level of protection which can be induced by vaccination of a range of hosts, from rodents to primates, with live radiation-attenuated schistosome larvae offers great promise for development of a human schistosome vaccine. Studies of the irradiated vaccine models benefitted from significant funding during the 1970–90s and much was learned concerning the inducers, targets and mechanisms of immunity. Less progress was made in definition of the protective antigens involved. The application of new techniques for identifying membrane and secreted antigens has recently provided new vaccine candidates and a new impetus for schistosome vaccine development. This article is intended as an overview of some of the main lessons learned from the studies of the irradiated vaccines as a backdrop to renewed interest in schistosome vaccine development.

Schistosoma mansoni: inflammatory foci around larvae in the peritoneal cavity of naive mice is radiosensitive

Innate attack to Schistosoma mansoni cercariae was evaluated in irradiated mice. It was observed that 70% of the larvae from mice sacrificed one day after whole body irradiation with 400 or 800 rads were surrounded by cluster reactivities, without difference from controls. Differences were apparent on day 5 after irradiation with sub lethal (400 rads) or lethal doses (800 rads) suggesting that innate defence to infection take at least 5 days to be affected by low dose whole-body radiation.

Th1 cytokine mRNA expression dominates in the skin-draining lymph nodes of C57BL/6 mice following vaccination with irradiated Schistosoma mansoni cercariae, but is down-regulated upon challenge infection

Vaccination of C57BL/6 mice with irradiated cercariae of Schistosoma mansoni results in the induction of high levels of immunity to subsequent infection. The events occurring in the lymph nodes draining the exposure site have been analysed ex vivo by reverse transcription-polymerase chain reaction (RT-PCR) and the timing of cytokine gene expression following exposure has been established. After vaccination, spatial separation of the T-helper type 1 (Th1) and Th2 responses was evident, with interferon-gamma (IFN-gamma), interleukin-2 (IL-2) and IL-12 mRNA peaking earlier than mRNA for IL-4, IL-5 and IL-10. In contrast to the profiles observed post-vaccination, following challenge the IL-4 mRNA was predominant in the draining lymph nodes, with IFN-gamma message levels barely detectable above the naive level. These observations are confirmed by the analysis of IL-4 and IFN-gamma mRNA using competitive PCR. From these studies it is clear that irradiated cercariae are more able to promote a protective Th1 response, with normal parasites eliciting higher IL-4 and IL-5 expression upon both primary and secondary stimulation.

Effect of cholera toxin on vaccine-induced immunity and infection in murine schistosomiasis mansoni

Intradermal vaccination of mice with soluble adult worm antigen (SWAP) in combination with Mycobacterium bovis BCG (Swedish strain) induced significant protection against subsequent infection with Schistosoma mansoni cercariae. When cholera toxin (CT) was used as an adjuvant in combination with SWAP or fraction A, no significant protection was observed. However, intradermal vaccination in combination with CT triggered a strong anti-SWAP antibody response and induced a strong delayed-type hypersensitivity response to schistosome antigens (SWAP or fraction A), one significantly higher than that in the SWAP-BCG group. In addition, vaccinating mice intranasally with SWAP or cercarial antigen together with CT as adjuvant failed to induce any significant protection. Surprisingly, mice given CT alone intranasally revealed a significantly enhanced worm burden. These findings suggest that mucosal application of CT may modulate the host-parasite relationship in favor of parasite survival.

Mechanisms involved in the loss of antibody-mediated adherence of macrophages to lung-stage schistosomula of Schistosoma mansoni in vitro

Sera from rabbits vaccinated with irradiated cercariae mediated cell (P388D1 or mouse peritoneal macrophage) adherence to lung-stage schistosomula (LS) but such antibody-mediated cell adherence was short-lived in contrast to cell adherence to mechanically transformed schistosomula (MS). Thus LS lost 50% of their adherent cells within 3-6 h in culture and up to 90% by 24 h, whereas adherence to MS was undiminished during this time. Rapid loss of adherent cells was unique to schistosomula that had developed to the lung stage because schistosomula recovered from the skin up to 3 days post-infection did not exhibit the rapid cell loss shown by 3-day LS. To determine whether cell loss was caused by loss of surface antigenicity during culture LS were cultured on their own for up to 24 h and at various intervals samples of schistosomula were tested for antigenicity by addition of immune serum and cells. Levels of adherence to both MS and LS were maintained throughout the incubation period. When antibody-opsonized schistosomula were washed and indicator cells added at progressive intervals, persistence of adherence was again demonstrated, showing that antibody binding to LS had not promoted surface antigen loss or degradation of bound antibody. It was then shown, by adding fresh macrophages to cultures up to 24 h old that LS which had lost their adherent cells nevertheless retained bound antibody, and comparison of adherence of 'used' and 'fresh' cells to MS and LS showed that the cytoadherence properties of macrophages were not significantly reduced during their culture with LS from which cells had been lost.(ABSTRACT TRUNCATED AT 250 WORDS)

Experimental models of immunization against schistosomes: lessons for vaccine development

Schistosomiasis is a debilitating, and sometimes deadly, parasitic infection that afflicts hundreds of millions of people living in developing countries. One of the best hopes for control of this disease is vaccine development. Studies on experimental models of attenuated vaccines have proven that high levels of protective immunity can be achieved. In these systems, resistance has been shown to be directed against the migrating larval stages of the parasite and to have both cellular and humoral components. Several candidate vaccine immunogens have been identified on the basis of antibody reactivity. However, the level of protection induced by immunization with nonliving vaccines has at yet not approached the level observed with attenuated infection. Current challenges to vaccine development include identification of protective T cell immunogens, determination of ways to strengthen the immunogenicity of isolated parasite antigens, and development of methodologies to selectively stimulate protective, as opposed to ineffective or even detrimental, immune responses.

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.

Schistosoma mansoni: larval damage and role of effector cell(s) in the synergy between vaccine immunity and praziquantel treatment

A number of authors have demonstrated that the schistosomicidal compound, Praziquantel (Pzq), depends for its action upon the immune status of the host (Sabah et al. 1985; Brindley & Sher, 1987; Doenhoff et al. 1987). We have attempted to define the synergistic interaction between immuno- and chemotherapy further, using the murine irradiated vaccine model of schistosomiasis mansoni. In vaccinated mice, resistance operates in the skin but not the lungs; drug targeted towards lung-stage worms exacerbates lung-phase immunity, however, as depicted by the increased number and size of inflammatory reactions in the pulmonary tissues. Parasites are often found trapped within such foci. In the present investigation, light and ultrastructural studies have been utilized to examine the nature and extent of damage inflicted upon lung-stage larvae recovered from day 6 Pzq-treated vaccinated mice. Such studies have revealed that damage involves muscle disorganization, internal disruption and occasionally, loss of the tegument; in the latter case, cells are often seen attached to the denuded lung worms. To identify the crucial cellular effector cell(s) involved in the synergy between immuno- and chemotherapy, cell depletion studies have been performed in vivo. It would appear from these experiments that eosinophils or lymphocytes rather than neutrophils or macrophages are important effector cells in this synergy. Histological studies argue in favour of eosinophils being the key effector cells.

Vaccination in murine schistosomiasis with adult worm-derived antigens: variables influencing protection in outbred mice

Previous studies have shown that both permissive (mouse) and partially permissive (rabbit) hosts develop high levels of resistance against Schistosoma mansoni infection after vaccination with a multiple antigen extract (SE) obtained by incubation of living adult worms in saline, plus bacterial adjuvant. To investigate variables influencing SE-induced protection in murine schistosomiasis, a series of distinct vaccination protocols were performed focussing on the immunization dose, carrier systems, route, site and amplitude of challenge infection, and time between immunization and challenge. In addition, a new approach was adopted to evaluate SE protective activity, by means of population analysis of worm burden frequency distributions in a large scale study of vaccination in outbred Swiss mice. Distinct curves of frequency and a drastic difference in worm burden distribution of frequencies from SE-vaccinated x non-vaccinated mice were found. It was shown that SE could generate 75% mean protection in outbred mice even in the absence of adjuvant. In addition SE immunization was also able to induce full protection against lethal infection. SE-induced protection could be modulated by such parameters as dose of SE immunization/challenge interval, and route of cercariae injection. These data show that SE yields very high protective activity in outbred mice, and may provide a further insight for rational design of a vaccine in experimental schistosomiasis.

Protective immunity to Schistosoma mansoni in mice is dependent on antibody and complement but not on radiosensitive leukocytes

The role of complement in the control of the Schistosoma mansoni infection in mice was investigated in vivo. The number of schistosomula recovered from the lung 5 days post-infection was used as a parasitological criterion of immunity. A significant difference in worm burden was observed between normal and immune mice. In contrast, when cobra venom factor (CVF) was injected into normal or immune mice 3 h before challenge, a significant increase in worm burden was noticed compared to untreated mice. We also investigated the protective mechanisms in mice that had been exposed to 650 rads of 60Co gamma radiation before challenge infection. Our results show that gamma-irradiated immune mice, depleted of more than 90% of their circulating or tissue leukocytes, are still able to destroy most of the parasites of a challenge infection with cercariae, suggesting that the radiosensitive leukocytes are not essential in the effector mechanisms of this protective immunity to S. mansoni. These results provide evidence of a role for the complement system, in association with radioresistant effector cells, in protective immunity occurring in the first hours after infection with S. mansoni.

Schistosoma mansoni: histological analysis of the synergistic interaction between vaccine immunity and praziquantel therapy in the lungs of mice

Naive CBA mice and mice vaccinated 4 weeks previously with gamma-irradiated cercariae of S. mansoni were challenged percutaneously with normal cercariae and then treated with 500 mg/kg body weight of Praziquantel (Pzq). The drug was administered intradermally on day 1 or intramuscularly on day 6, thus targeting against skin stage or lung stage challenge larvae respectively. The skin site of challenge and/or the lungs were removed at various time points to provide samples for histological examination. As reported elsewhere (Flisser, Delgado & McLaren 1989) the efficacy of Pzq was significantly enhanced in vaccinated mice and was influenced by the treatment regime. Histological analysis revealed that when Pzq was administered I/D on day 1 to vaccinated mice, the inflammatory response to challenge differed in extent but not nature from that seen in vaccinated but untreated cohorts. This correlates with worm recovery data showing no (this study), or only marginal synergy between drug treatment and immunity using this regimen of drug treatment (Flisser et al. 1989). Following the day 6 protocol of drug delivery, however, lungs from treated vaccinated mice exhibited many large inflammatory reactions containing trapped challenge larvae. In contrast, lungs from untreated vaccinated mice had only few foci which were small and rarely contained trapped larvae. These data again correlate well with worm recovery data showing that there is a highly significant synergy between vaccination and drug treatment administered at this time (Flisser et al. 1989; this study). It would seem, therefore, that Pzq exacerbates lung phase immunity in the NIMR vaccine mouse model where skin phase immunity predominates and pulmonary attrition is normally minimal. The results are discussed in the light of published data concerning the effector mechanisms thought to characterize skin and lung phase vaccine resistance in the murine model.

Schistosoma mansoni: evidence that site-dependent host responses determine when and where vaccine immunity is expressed in different rodent species

Laboratory rodents vaccinated with highly irradiated cercariae of Schistosoma mansoni develop significant levels of specific acquired resistance yet effect challenge elimination in different organs. Mice and guinea-pigs are at opposite ends of the spectrum in this respect since, in our hands, vaccinated mice kill challenge parasites in the skin whereas vaccinated guinea-pigs kill challenge parasites predominantly in the liver. To determine whether this phenomenon is host-dependent (site) or parasite-dependent (stage), we have transferred worms harvested from mice or guinea-pigs into vaccinated recipient guinea-pigs. The results show that mouse-derived 5-day lung worms and 9-day liver worms that are essentially refractory to vaccine resistance in mice are indeed susceptible to vaccine resistance in guinea-pigs. Identical levels of susceptibility were recorded for lung-stage larvae introduced via the foot vein so as to experience lung and liver mechanisms, or via the mesenteric vein to bypass the lung, thereby confirming that vaccine resistance in guinea-pigs operates in the liver. Mouse worms and guinea-pig worms exhibited equivalent levels of susceptibility at all stages of development. Thirteen-day-old larvae from either donor species were on the border-line of vulnerability, while 20-day-old worms were totally refractory to vaccine immunity in guinea-pigs. These data show that vaccine immunity in different rodent species is a site-dependent, rather than a stage-dependent phenomenon. There is, however, an upper age limit of schistosome vulnerability which is common to worms harvested from different donor species.

Evidence that radio-sensitive cells are central to skin-phase protective immunity in CBA/Ca mice vaccinated with radiation-attenuated cercariae of Schistosoma mansoni as well as in naive mice protected with vaccine serum

Naive CBA/Ca mice and CBA/Ca mice vaccinated 4 weeks previously with radiation-attenuated cercariae of Schistosoma mansoni were subjected to 550 rad of whole body (gamma) irradiation and then challenged 3 days later with normal cercariae. The perfusion recovery data showed that this procedure reduced the primary worm burden in naive mice by 22% and the challenge worm burden in vaccinated mice by 82%. Irradiation also ablated the peripheral blood leucocytes of both mouse groups by 90-100% at the time of challenge. Histological data revealed that such treatment caused a dramatic change in number, size and leucocyte composition of cutaneous inflammatory skin reactions that characterize challenged vaccinated mice and are known to entrap invading larvae; cutaneous eosinophils were preferentially abolished by this treatment. Polyvaccine mouse serum that conferred protection passively upon naive recipient mice, failed to protect naive/irradiated mice when administered by the same protocol. Distraction of macrophages by treatment of mice with silica did not affect the establishment of a primary worm burden and reduced the protection exhibited by vaccinated mice by only 16%. These data indicate that radio-sensitive cells are important to both innate and specific acquired resistance in this mouse model and that macrophages contribute only marginally to the expression of vaccine immunity.

Schistosoma mansoni: analysis of the humoral and cellular basis of resistance in guinea-pigs vaccinated with radiation-attenuated cercariae

This study addresses the humoral and cellular basis of specific acquired immunity in the guinea-pig irradiated vaccine model of schistosomiasis mansoni. Rodents vaccinated with 500 gamma-irradiated cercariae and then splenectomized 4.5 weeks later showed a 33% reduction in resistance to challenge as compared to vaccinated animals or vaccinated/sham splenectomized controls. Serum harvested from once vaccinated individuals conferred modest levels of resistance upon naive recipients in some experiments, but transfer was not achieved consistently. Serum from vaccinated and thrice boosted rodents (Vbbb) routinely transferred around 45% immunity, however, provided it was given in 4 ml aliquots on day 9 post-challenge; Vbbb serum thus transferred 50% of donor immunity. Interestingly, multiple doses of this protective serum given on and either side of day 9 did not enhance the protection achieved with a single 4 ml aliquot. Neither peripheral lymph node cells nor splenocytes from the polyvaccinated serum donors were able to transfer resistance to recipient guinea-pigs and they failed to augment the protection achieved with Vbbb serum. Foot-pad testing revealed no correlation between delayed hypersensitivity responses and immunity to challenge in vaccinated guinea-pigs. Although polyvaccine guinea-pig serum successfully protected homologous recipients, it failed to protect mice when administered either at the time of challenge (the optimal schedule for transfer of polyvaccine mouse serum), or around day 9 (the optimal schedule for guinea-pigs). Similarly, guinea-pigs could not be protected with polyvaccine rat serum that conferred 75% resistance upon naive recipient rats.

Evidence for enhancement of IgG1 subclass expression in mice polyvaccinated with radiation-attenuated cercariae of Schistosoma mansoni and the role of this isotype in serum-transferred immunity

Serum or immunoglobulin fractions of serum from CBA/Ca mice vaccinated three or four times with radiation-attenuated cercariae of Schistosoma mansoni have been investigated for their capacity to confer protection upon naive mice. The data confirm that around 35% protection can be transferred with polyvaccine mouse serum administered in 0.5-ml aliquots 1 h before challenge (intravenously) and 24 h post-challenge (intraperitoneally). We show in addition, however, that polyvaccine serum is also protective when injected into the skin site of challenge as a single 0.05-ml aliquot. In contrast, lymphocytes obtained from the donors of protective serum conferred only 13% protection upon recipient mice. The passive cutaneous anaphylaxis assay showed that IgG1 is incremented by polyvaccination, while passive transfer experiments revealed that of the different isotypes fractionated from whole protective serum, only IgG1 has the capacity to protect naive recipients against challenge. The resistance transferred by IgG1 represents more than 60% of that obtained with whole serum and can be achieved using either the intravenous/intraperitoneal or the subcutaneous administration regimen. Recipients of serum given via the subcutaneous route exhibit cutaneous inflammatory focal reactions which comprise 20% eosinophils and 80% mononuclear cells; these foci entrap challenge larvae. The importance of IgG1 subclass expression to the success of serum-transferred resistance is discussed.

Immunity to Schistosoma mansoni in vivo: contradiction or clarification?

In recent years controversy and contradiction have hindered the elucidation of the immune effector mechanisms that are most effective against Schistosoma mansoni - an essential goal for the development of an effective vaccine. However, recent in-vivo studies have clarified the relative contributions of such mechanisms to protection. Here, Dario Vignali and colleagues summarize current evidence that suggests that both antibody and CD4+ T cells, in cooperation with macrophages, are crucial for the development of an effective response. In addition, a model is presented that may account for some of the discrepancies observed and which could be used as a basis for future research.

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

The leucocyte responses in peripheral blood and pulmonary airways are described following vaccination of mice with radiation-attenuated parasites, and subsequent challenge with normal parasites. Percutaneous vaccination stimulated a large and sustained expansion of the circulating lymphocyte pool, more marked than after intradermal vaccination with lung schistosomula which induced comparable levels of resistance. Macrophages and lymphocytes infiltrated the pulmonary airways in response to vaccination by both routes, the lymphocytes being particularly abundant after intradermal vaccination. Exposure of mice to an equivalent number of normal cercariae induced an earlier lymphocytosis of short duration; far fewer macrophages and lymphocytes infiltrated the lungs than after vaccination. An intense but transient pulmonary eosinophilia peaked at 3 weeks after primary exposure to either normal or attenuated parasites. Percutaneous challenge of vaccinated mice elicited higher levels of circulating lymphocytes than challenge of unsensitized controls. However, whilst leucocyte numbers of all cell types were still elevated in the airways at challenge as a consequence of vaccination, no further cellular recruitment was observed coincident with parasite elimination. Our data are compatible with the hypothesis that the mechanism of immunity in once-vaccinated mice involves a T lymphocyte-macrophage interaction triggered by antigen release from lung schistosomula.

Histological examination of the cellular reactions around schistosomula of Schistosoma mansoni in the lungs of sublethally irradiated and unirradiated, immune and control rats

Histopathological data on the cellular reactions (foci) around Schistosoma mansoni schistosomula in the lungs of both irradiated (750 rad) and unirradiated, passively immunized and normal rats were consistent with the idea that a significant proportion of immune-mediated attrition in passively immunized rats occurs in the lungs. In unirradiated rats, immune serum elicited an enhanced (i.e. larger) and accelerated (i.e. more rapidly developing) inflammatory cellular infiltration around lung-stage parasites when administered 5 days post-infection, when the parasites were already in the lungs. This demonstrated the antigenicity of lung-stage schistosomula and their potential as targets for immune attack. In irradiated rats, innate immunity was decreased as judged by an increase in the number of worms recovered by portal perfusion, and was accompanied by an overall decreased percentage of trapped parasites compared with unirradiated controls, suggesting that trapping in the lungs is involved in innate, as well as acquired immunity. In contrast to the results in unirradiated rats, passive transfer of immune serum into irradiated recipients did not result in larger lung foci than in the NRS-recipients. However, there was evidence of an accelerated response resulting in an essentially similar ratio of trapped parasites (VRS- compared with NRS-recipients) in irradiated rats, as compared with unirradiated rats, reflecting the similar levels of resistance manifested in both groups of rats. This also lent credence to the notion that it was the speed of immune recognition of the migrating schistosomula and the establishment of trapping foci that were of greater importance rather than the size of the enveloping granulomata.(ABSTRACT TRUNCATED AT 250 WORDS)