Schistosoma mansoni: peripheral and tissue eosinophilia in infected rats

Schistosomiasis Mansoni-Recruited Eosinophils: An Overview in the Granuloma Context

Eosinophils are remarkably recruited during schistosomiasis mansoni, one of the most common parasitic diseases worldwide. These cells actively migrate and accumulate at sites of granulomatous inflammation termed granulomas, the main pathological feature of this disease. Eosinophils colonize granulomas as a robust cell population and establish complex interactions with other immune cells and with the granuloma microenvironment. Eosinophils are the most abundant cells in granulomas induced by Schistosoma mansoni infection, but their functions during this disease remain unclear and even controversial. Here, we explore the current information on eosinophils as components of Schistosoma mansoni granulomas in both humans and natural and experimental models and their potential significance as central cells triggered by this infection.

Development of Schistosoma mansoni in the laboratory rat analyzed by light and confocal laser scanning microscopy

The kinetic of maturation (schistogram) of Schistosoma mansoni worms grown in laboratory rats was studied by light and confocal laser scanning microscopy. Infected rats with the BH strain were weekly euthanized 3-9weeks pi. Recovered flukes stained with hydrochloric carmine were preserved as whole-mounts and analyzed by confocal and brightfield microscopy. Worms displayed varying degrees of maturation of the reproductive system at weeks 3-6. Male worms showed complete maturation of the reproductive system at week 6, while female worms completed their maturation at week 7. Males presented few tubercles in tegument in all weeks. Despite the presence of a developing embryo within the ootype, no uterine egg was found. The schistogram in rats follows a pattern similar to that observed in mice hosts.

Profiles of Th1 and Th2 cytokines after primary and secondary infection by Schistosoma mansoni in the semipermissive rat host

In contrast to most mouse strains, rats eliminate the primary schistosome burden around 4 weeks postinfection and subsequently develop protective immunity to reinfection. In rat schistosomiasis, we have shown predominant expression of a Th2-type cytokine response at the mRNA level after primary infection. In the present study, we showed a significant increase in interleukin-4 (IL-4) mRNA expression in inguinal lymph nodes early after a secondary infection. IL-5 mRNA expression showed a significant increase at days 2 and 4 postreinfection in the spleen and lymph nodes, respectively. We did not detect any gamma interferon (IFN-gamma) mRNA after a challenge infection. Analysis of cytokine secretion by stimulated spleen cells after a primary infection showed predominant expression of IL-4 with maximum production on day 21, accompanied by production of IL-5 from day 11 to day 67. A significant increase in IFN-gamma secretion was detected at day 21. Analysis of immunoglobulin G2b (IgG2b) and IgG2c (Th1-related isotypes) showed undetectable levels of IgG2b, but detectable levels of specific IgG2c antibodies were observed from day 42. The analysis of Th2-related isotypes showed high specific IgG1 and IgG2a antibody titers from day 29. After a secondary infection, only IL-4 and IL-5 secretion was sustained. This is supported by the increased production of Th2-related isotypes. These findings showed that S. mansoni infection can drive Th2 responses in rats in the absence of egg production which is required to induce a Th2 response in mice and are in favor of the role of Th2-type cytokines in protective immunity against reinfection.

Interleukin-12 as an adjuvant for an antischistosome vaccine consisting of adult worm antigens: protection of rats from cercarial challenge

Our group previously demonstrated that a detergent extract (fraction S3) prepared from immature (4-week) Schistosoma mansoni parasites can induce partial, serum-transferable immunity to challenge infection in rats when administered as an alum precipitate. In the present study, we examined whether S3 prepared from adult (7-week) worms could similarly induce protection and whether immunity could be positively influenced by treatment with interleukin-12 (IL-12). IL-12 coadministered to Fischer rats and C57BL/6 mice at the time of S3 vaccination altered the prechallenge kinetics of S3-specific antibody titers in both species, ultimately leading to a stable enhancement of titers (relative to those in animals vaccinated without IL-12) in mice but not rats. Immunoblot analysis of prechallenge immune sera demonstrated that IL-12 treatment was associated with changes in the S3 antigen recognition profile in each species. Isotyping of specific antibodies in S3- plus IL-12-vaccinated mice prior to challenge infection revealed a moderate elevation in immunoglobulin G1 (IgG1) responses, strongly enhanced IgG2a and IgG2b responses, as well as diminished total serum IgE responses compared to those in mice given S3 only. In vaccinated rats, IL-12 profoundly suppressed specific IgG1 and enhanced IgG2b responses but did not affect IgG2a responses. S3- plus IL-12-vaccinated rats also produced less total IgE upon challenge infection. Enumeration of worm burdens revealed that vaccination with S3 plus IL-12 conferred 50% protection from cercarial challenge to rats, whereas rats given S3 only were not protected; mice were not protected by S3 vaccination regardless of IL-12 coadministration. The protection observed in S3- plus IL-12-vaccinated rats could not be transferred with serum, suggesting participation of an activated cellular component in the expression of immunity.

Immunity to schistosomes: progress toward vaccine

Among the major parasitic infections, schistosomiasis may be the most promising candidate for human vaccination. Information about mechanisms of immunity, gained mainly from experimental models but likely to be relevant to human infection, indicates a dynamic balance between protective and regulatory (blocking) mechanisms. Besides cell-mediated responses leading to macrophage activation, antibody-dependent cell-mediated cytotoxicity systems involving precise antibody isotypes and nonlymphoid cells (mononuclear phagocytes, eosinophils, and platelets) appear to be essential effectors of immune attack. The slow development of immunity in humans seems related to the production of antibodies that cross-react with schistosomulum surface antigen and block the binding of antibodies of the effector isotype. Schistosomes that survive in the bloodstream and produce chronic infections may evade the immune system as a result of intrinsic changes in membrane susceptibility and of transient expression of target antigens; at other stages of the parasite life cycle, cross-reactive molecules may be secreted that play an essential role in the induction of immunity. Several schistosome proteins have been characterized as candidates for vaccination. Among these, an antigen of 28 kilodaltons has been cloned and shown to be immunogenic in humans and protective in mice, rats, and baboons.

Limitations to schistosome growth and maturation in nonpermissive hosts

The life cycle of schistosomes is reviewed in its various steps, both in permissive hosts (in which the cycle is completed) and in nonpermissive hosts (which excrete no viable eggs as a result of the infection). A large worm loss occurs at (or after) the lung stage in both types of hosts ("normal attrition") and some nonpermissive hosts (like the rat) have an additional elimination of worms from the portal circulation. Worm growth and reproductive maturation are also impaired in several nonpermissive hosts and the possible host-parasite interactions leading to such limitations are discussed, with special reference to hormonal influences. Attention is also given to peculiar phenomena occurring in some hosts, like the late portal worm elimination in rhesus monkeys, the migration from mesenteric veins to lungs in rats, and the block to egg excretion in guinea pigs. The steps of the schistosome life cycle which appear vulnerable in several hosts are contrasted with the steps which are carried out successfully in the majority of hosts studied.

The eosinophilic leukocyte: structure and function

The evidence reviewed here indicates that the eosinophil has the ability to kill many species of helminths and likely does so during worm infection. This toxic ability appears to be regulated by several other cells including mast cells, monocytes, and T lymphocytes. Eosinophils kill helminths through their ability to generate potent oxidants and through their content of cationic proteins, which likely achieve high concentrations at points of granule deposition. Eosinophils also participate in inflammation in human disease especially asthma, skin diseases, and heart disease. Though present concepts hold that the mast cell is the cornerstone of the allergic inflammatory response (450), the findings that eosinophils bind IgE and are activated by antigen-IgE complexes and that the eosinophil can elaborate many inflammatory mediators raise the possibility that the eosinophil might also be involved in the initiation of inflammatory responses. Finally, an eosinophil-related protein appears to play an undefined role in human reproduction.

Induction of a protective immune response in rats by injection of live or killed S. mansoni worms

Resistance to S. mansoni cercarial challenge, induced by prior infection of rats with cercariae, is analysed by substitution of alternatives to the primary cercarial exposure. Intravenous injections of either live or killed 4-week stage worms have been examined, utilizing two different routes of injection, one or two injected doses, and two different methods to kill worms. In addition to assessing peripheral blood eosinophil levels, anti-schistosome antibody titres, and challenge worm burdens in the injected and control recipients, sera from these rat groups collected 3 weeks after challenge were used for passive immunization studies in rats. Recipients of intravenous live worm injections were resistant to a cercarial challenge and yielded sera capable of passively immunizing rats. Recipients of intravenous killed worm injections were not significantly resistant to a challenge infection, yet paradoxically, yielded sera capable of passively immunizing rats. Serum from challenge control rats was not significantly protective in the passive immunization assay. There were no correlations between peripheral blood eosinophil levels or anti-schistosome antibody titres and resistance (either active or passive). We conclude that killed worms induce an immune response which, although insufficient to provide protection by itself, can be recalled or augmented by a challenge infection to result in significant levels of serum protective activity.

Interactions between eosinophils and antibodies: in vivo protective role against rat schistosomiasis

An original protocol of cell transfer from Schistosoma mansoni-infected rats to normal recipient rats is used to investigate the protective role of phagocytic cell populations, described as effector cells in vitro, against a challenge infection with S. mansoni. Nonadherent, eosinophil-enriched and -adherent, macrophage-rich cell preparations, injected via intradermal and subcutaneous routes at the precise site of exposure to cercariae, were able to significantly protect the recipient rats. The time-course study of this protective effect according to the time after infection of donor rats revealed that eosinophils were the major cell population involved in the early phase of infection (4 to 5 weeks), whereas macrophages could also be incriminated thereafter. A rosette assay using anti-immunoglobulin-coated erythrocytes indicated a sequence of the various antibody isotypes under study (IgG1, IgG2a, IgE) on the eosinophil surface, during the course of infection. As previously shown in vitro, cytophilic antibodies seemed to participate in the protective effect of eosinophils, since eosinophil-enriched cells from normal rats, sensitized in vitro with immune complexes present in infected rat serum, could also confer significant protection. These observations establish therefore the relevance between our previous in vitro studies and rat resistance to a challenge infection with S. mansoni, underlining the major role played by the interaction between antibodies and phagocytic cells (eosinophils and macrophages).

Recovery of parasites at different stages of migration following infection of rats with Schistosoma mansoni

The technique of tail amputation is utilized as a method for interrupting the migration process of Schistosoma mansoni schistosomula from the skin of Fischer rats infected by exposure of the tail to cercariae. The yields of schistosomula recovered from the lungs at different times post-infection are compared, using rats with or without tail amputation. Residence times of schistosomula in skin and lungs, as well as their transit time and efficiency of migration between these sites, are estimated. At least one-third of the infecting cercariae migrate from skin to lung in rats. Amputation of the tail on days 4 or 5 post-infection isolates a definable number of schistosomula in the lung and their migration to the portal circulation can be followed. The kinetics of this migration in rats and mice is compared and a significant difference is revealed.