Cercarial Challenge Research Articles

A protective monoclonal antibody specifically recognizes and alters the catalytic activity of schistosome triose-phosphate isomerase.

mAb M.1 was previously shown to recognize a 28-kDa Ag in all stages of the human helminth parasite, Schistosoma mansoni, and to bind to the surface membranes of newly transformed schistosomula in a transient manner. Here we demonstrate that M.1 passively transfers partial resistance (41-49%) to cercarial challenge in naive mice. Thus, the 28-kDa Ag recognized by M.1 is a putative vaccine candidate. After immunoaffinity purification, tryptic digests of the 28-kDa Ag were prepared and individual peptides were sequenced. Amino terminus sequences of tryptic peptides of the 28-kDa Ag had high (79-87%) sequence homology with the mammalian glycolytic/gluconeogenic enzyme triosephosphate isomerase (TPI). Purified, native 28-kDa Ag from adult parasites was shown to function enzymatically in an analogous manner to yeast and mammalian TPI in the reverse reaction. Addition of M.1 antibody to the enzyme reaction altered the catalytic activity of schistosome TPI. To determine the immunologic cross-reactivity of this vaccine candidate with mammalian TPI, Western blot analysis was performed and demonstrated that M.1 was immunologically specific for the schistosome enzyme.

Experimental life history of Echinostoma hortense

The complete life cycle of Echinostoma hortense has been maintained in the laboratory, using Lymnaea pervia snails and Rana nigromaculata tadpoles as the first and second intermediate hosts. ICR mice was used as the definitive host. Within the egg of E. hortense, the miracidium was fully matured in 13 days of incubation at 29-30 degrees C. The miracidium was 93.8 x 53.6 microns in average size, covered with numerous cilia of 7-11 microns length. The epidermal plates were arranged in 6-8-4-2 formula. The first generation rediae (1.19 x 0.27 mm in average size) were observed in 14 days after miracidial challenge to the snails, and the second generation rediae (1.40 x 0.26 mm in average size) in 30 days. The average size of the cercaria was 295.5 x 145.0 microns. Their head crown was poorly developed, and collar spines were not yet observed. After a cercarial challenge to the tadpoles, all of the tadpoles became infected and the average worm recovery rate was 88.5%. The majority of the metacercariae (75.5%) were recovered from the muscle of the tadpole's posterior body and the rest (24.3%) from their gills. The metacercariae from the tadpoles were elliptical, and 167.7 x 129.9 microns in average size. The recovery rate of adults from the mice was different by the age of the metacercariae grown in the tadpoles. The metacercariae younger than 5 hrs could not infect mice whereas those older than 6 hrs could infect mice. The recovery rate became higher as the metacercaria matured, with the peak recovery rate of 90.0% at the metacercarial age of 9 days. Thereafter the recovery rate decreased to 55.0% at the age of 50 days. As shown by the above results, the whole life cycle of E. hortense has been completed in the laboratory. At least 55-58 days were required to maintain one egg-to-egg cycle of E. hortense.

Monoclonal anti-idiotypic and anti-anti-idiotypic antibodies from mice immunized with a protective monoclonal antibody against Schistosoma mansoni.

To study the role of idiotypic anti-idiotypic interactions in schistosomiasis, mice were immunized with a mAb, E.1, which bound to soluble egg and larval stage Ag and also passively transferred resistance to cercarial challenge in mice. Subsequently, hybridomas were produced from E.1 immunized mice and tested for the ability to inhibit E.1 binding to soluble egg Ag. The results showed that anti-idiotypic mAb (Ab2) were produced. The range of inhibitory activity was from 33 to 100%. By using a direct Ab2 binding assay, the Ab2 were shown to be idiotypic specific, not isotype specific. It was also found that six of the hybridomas bound to soluble egg Ag and were therefore anti-anti-idiotypic antibodies (Ab3). Ab3 were shown to be inhibited from binding to soluble egg Ag by Ab2. To the authors' knowledge, this is the first time that an in vivo network relevant to an infectious organism has been reproduced in vitro such that both Ab2 and Ab3 were produced from the same animals independent of exposure to parasite Ag.

Purification and immunochemical characterization of a 22 kilodalton surface antigen from Schistosoma mansoni

A 22 kDa antigen (Sm22) was purified from schistosomula membrane extracts by immunoaffinity chromatography with monoclonal antibody M.2. Western blotting suggested that the epitope bound by M.2 required a specific conformational folding of the molecule, which was sensitive to reducing agents. Two-dimensional electrophoresis of purified Sm22 demonstrated that the single 22 kDa protein recognized by M.2 on one-dimensional gel analysis was composed of at least two isomorphs. Additional Western blotting showed that Sm2 was one of the major antigens recognized by mouse anti-irradiated cercariac serum, and that this same serum recognized at least one epitope which was not sensitive to reducing agents. The mice vaccinated with irradiated cercariae were shown to be 75% protected from cercarial challenge. Sera from a rabbit immunized with Sm22 contained antibodies which bound to the surface of schistosomula and detected a single protein at 22 kDa by immunoprecipitation or Western blot. The rabbit anti-Sm22 sera also immunoprecipitated a 22 kDa in vitro translation product, indicating that at least one epitope on Sm22 is not dependent on glycosylation.

Induction of protective immunity against Schistosoma mansoni by a non-living vaccine. V. Effects of varying the immunization and infection schedule and site.

Intradermal (i.d.) injection with non-living schistosome antigens plus bacterial adjuvant protects mice against subsequent infection with Schistosoma mansoni. This protection became apparent within 2 weeks after a single inoculation and persisted for at least 8 weeks. Administration of one or more booster immunizations enhanced the level of protection but never produced complete resistance to cercarial challenge under the conditions tested. All immunized mice recognized the Sm 97 antigen in adult worms, as measured by antibody reactivity in ELISA, although the level of reactivity did not correlate with the level of resistance. Significant protection was observed when mice were immunized in the skin of the chest, the footpad or at the base of the tail if challenge infection was administered percutaneously either on the abdomen or back. However, when mice were infected by cercarial exposure of the tail skin, the level of resistance was consistently lower regardless of the immunization site. Vaccinated mice were not resistant to infection with lung stage parasites. These results demonstrate that the i.d. vaccine induces significant and persistent resistance in mice, the level of which is strengthened by booster immunization. The resistance is unrelated to inflammation at the site of immunization. However, immune response at the challenge site may play a critical role in the effector mechanism of resistance in this model.

Vaccine-induced immunity in mice against Schistosoma mansoni trickle cercarial infections.

Experiments were performed to determine the level of protection in immunized mice to multiple small (trickle) Schistosoma mansoni cercarial challenges. C57BL/6 mice were immunized with either 50 Krad-irradiated cercariae or a soluble worm antigenic extract injected intradermally in conjunction with the adjuvant BCG. A series of cercarial challenges (40 cercariae per exposure), beginning approximately one month after immunization, was given at 4-week intervals. To prevent any contribution of egg-related pathology, the mice were exposed to male cercariae only. The results showed that highly significant levels of protection developed at all times after cercarial challenge. Regardless of the type of immunization or the number of cercarial challenges, the levels of protection which appeared after one, two, three, or four challenge exposures were nearly identical, and equalled those in mice exposed at a single time to a cumulative total of the several exposures. As expected, titers of anti-schistosome antibodies were higher in the immunized and challenged groups than in those challenged alone, and titers increased anamnestically after challenge infection. However, increased antibody titers were not associated with increased resistance in trickle challenged mice. Overall, we showed that the level of vaccine-induced immunity does not change, under these conditions, in the face of repeated cercarial challenges. Also, these results show that single mass cercarial challenges may legitimately be used in experimental situations to assess the effectiveness of vaccines which might be used in subjects repeatedly exposed to cercariae in field situations.

Macrophage activation as an immune correlate to protective immunity against schistosomiasis in mice immunized with an irradiated, cryopreserved live vaccine.

Immune responses against Schistosoma mansoni were evaluated in C57BL/6 mice injected with one of two populations of irradiated schistosomules, the larval preparations differing only in the degree of freezing-induced damage sustained upon cryopreservation. Mice injected with larvae which successfully withstood cryopreservation showed a significant reduction in worm burden following cercarial challenge. No protection was achieved in mice which received larvae damaged by a suboptimal thawing rate. Parallel comparison of several humoral and cellular responses in mice which received either inoculum revealed that induction of activated macrophages and production of macrophage-activating lymphokine activity were the strongest correlates to development of protective immunity. Protected mice also showed marginal 30-min skin test reactivity and weak but transient 24-h delayed-type hypersensitivity to a soluble adult worm preparation. In contrast, indistinguishable levels of circulating antibodies to soluble and tegumental antigens developed in the two immunization groups, and antigen-stimulated lymphocyte blastogenic responses were strong and essentially equivalent in magnitude. These studies strongly suggested that in this new model for investigating anti-schistosome effector mechanisms, responses contributing to the development of activated macrophages may be essential for induction of protective immunity.

Reevaluation of the potential of Fasciola hepatica antigens for immunization against Schistosoma mansoni infection.

Previous workers have reported that significant resistance to Schistosoma mansoni cercarial challenge can be induced in mice by immunization with various Fasciola hepatica antigen preparations. We have attempted to reproduce these observations and have also carried out similar experiments in rats. In our hands, however, CBA mice did not develop either antibodies against the schistosomular surface, or significant resistance to challenge after immunization with heterologous antigens. Similar results were obtained in Fischer rats, but rats did develop significant resistance after immunization with homologous (schistosomular) antigens, even without adjuvant, and the homologous 'protective' antigens could be separated by antibody-affinity chromatography using serum from rats vaccinated with irradiated cercariae. Thus, we found no evidence for the existence of shared 'protective' antigens between S. mansoni and F. hepatica, and suggest that this is further evidence that where heterologous resistance can be demonstrated between these two parasites, it is probably due to immunologically non-specific mechanisms.

Evidence that a protective membrane epitope is involved in early but not late phase immunity in Schistosoma mansoni.

An anti-egg monoclonal antibody E.1, which is partially protective in passive transfer experiments, is shown in this study to recognize a membrane epitope on cercariae, schistosomula, and the ciliary plates of miracidia. E.1 did not bind to the surface membranes of lung or adult worms, or recognize secreted egg antigen in infected liver tissue. The E.1 epitope was present in the glycocalyx of cercariae, as well as on the syncytial membrane as determined by electron microscopy. Immunoprecipitation of iodinated surfaces of cercariae and schistosomula demonstrated E.1 binding to a high m.w. moiety in cercariae, which corresponds to the glycocalyx because it was not immunoprecipitated from schistosomula. In addition, a band at 38,000 daltons was immunoprecipitated from both cercariae and schistosomula. When compared with in vitro cultured parasites, schistosomula that were obtained from mice 1 to 24 hr after tail vein injection showed significant loss of E.1 binding. Consistent with the rapid loss of antigen in vivo, E.1 antibody was unable to passively transfer protection to naive mice when administered 5 days after cercarial challenge.

Schistosoma mansoni: The cutaneous response to cercarial challenge in naive guinea pigs and guinea pigs vaccinated with highly irradiated cercariae

Schistosoma mansoni: the cutaneous response to cercarial challenge in naive guinea pigs and guinea pigs vaccinated with highly irradiated cercariae. International Journal for Parasitology 16: 491–510. Naive guinea pigs and guinea pigs vaccinated 4 weeks previously with highly irradiated cercariae were challenged percutaneously with normal cercariae. Skin samples from the challenge site were then harvested at varying times to provide histological, quantitative and ultrastructural data on the respective cellular responses to cercarial invasion. The primary cutaneous reaction was characterised by neutrophils; these cells reached peak numbers (16% of total cells) by 18 h. Eosinophils and basophils made only a small contribution to the infiltrate (2.9 and 5.7% respectively). Some basophils showed evidence of anaphylactic degranulation, others seemed to be damaged, but most appeared normal. Mononuclear cells of varied morphology were present at all times, but activated fibroblasts were prominent, and collagen deposition increased with time. Degranulating mast cells were recognised at 24 and 48 h. Dead schistosomula were never seen in naive-challenged skin, although one or two of the observed larvae showed minor tegumental abnormalities. In vaccinated guinea pigs, the cutaneous cellular response to challenge was significantly enhanced, with basophils dominating the reaction (33% of total cells at 24 h). Many basophils were already degranulating by the anaphylactic mechanism at 3 h post challenge, and free basophil granules were seen frequently. Both intact cells and free granules congregated in close proximity to invading larvae. Eosinophils were also present in greater numbers at secondary reaction sites, but they never exceeded 6% of the total infiltrate. Mononuclear cells believed to be immature eosinophils were prominent from 3 h. For the first time, the mechanism of eosinophil attachment and degranulation onto a multicellular target, described previously only from in vitro investigations, was recognized in vivo. Neutrophil numbers matched those recorded in naive-challenged skin at 3 and 6 h, but declined thereafter, while mast cells were seen degranulating at these early times. Mononuclear cells again presented a variety of morphological appearances; of particular note were large cells that had phagocytosed debris and were presumed to be macrophages, and small rounded cells with scant cytoplasm and few organelles, that may have been lymphocytes. By 12 h, large areas of the dermis had become severely disorganised and numerous, free basophil granules were distributed amongst the other cellular constituents. Dead schistosomula, denuded of tegument, were clearly recognised at 6 h, and such individuals invariably had neutrophils attached to their exposed muscle layers. Since dead schistosomula were not identified in naive-challenged guinea pig skin, it is concluded that a percentage of the challenge larvae, however small, is preferentially killed in the skin of the vaccinated animals.

Cutaneous sensitivity induced by immunization with irradiated Schistosoma mansoni cercariae : II. Autoimmunoregulation of cell-mediated cutaneous sensitivity and its reversal

C57BL/6 mice sensitized by abdominal, dermal exposure to irradiated (50 kR) Schistosoma mansoni cercariae develop partial protection against subsequent exposure with unattenuated cercariae and express cell-mediated cutaneous sensitivity upon challenge with irradiated cercariae. Autoimmunoregulation occurs as a part of this sensitization, and can be demonstrated by augmentation of cutaneous sensitivity upon use of appropriate regimens of cyclophosphamide. Mice exposed to irradiated cercariae by either intraperitoneal or ear pinna routes developed a transient hyporesponsiveness to cercarial challenge. This unresponsiveness was also reversed by pretreatment with cyclophosphamide. Timed removal of the site of effective sensitization (abdominal skin) 7 days after exposure consistently led to reduced cutaneous responsiveness. This artificially induced hyporesponsiveness was reversed by either cyclophosphamide treatment or systemic administration of anti-I-J b, but not anti-I-J k sera. The data indicate the involvement of cyclophosphamide-sensitive, I-J-bearing T-suppressor cells or factors in the autoimmunoregulation that controls this cutaneous sensitivity. Parallel challenge infection studies in immunized mice treated with cyclophosphamide demonstrated that the resultant augmentation of cutaneous sensitivity did not lead to concomitantly elevated levels of resistance. Furthermore, successful adoptive cell transfer of cutaneous responsiveness also did not ensure protection against cercarial challenge. These observations indicate that dermal cell-mediated anti-cercarial responsiveness is not a sufficient mechanism to explain resistance in mice immunized with irradiated cercariae.

Induction of protective immunity against Schistosoma mansoni by a nonliving vaccine. IV. Fractionation and antigenic properties of a soluble adult worm immunoprophylactic activity.

An aqueous buffer-soluble, nonparticulate fraction of adult Schistosoma mansoni worms (SWAP) was separated by gel filtration on Ultragel AcA-34, and portions of the eluate were tested for their capacity to induce protective immunity against cercarial challenge when administered intradermally to mice in combination with the adjuvant BCG. All of the immunogenic activity was found in a single peak of protein excluded in the void volume of the column. This same fraction was determined by SDS-PAGE and Western immunoblotting to be unique in that it contained a component of Mr (X 10(-3) 97 (97,000) recognized monospecifically by antibodies from mice vaccinated with unseparated SWAP plus BCG. Similarly, the protective fraction was unique in possessing the capacity to elicit 24 hr delayed footpad swelling responses, as well as lymphokine production, in SWAP-BCG-immunized mice. These results suggest that the immunogenic activity of SWAP resides in a restricted population of molecules, and possibly in the 97,000 antigen detected with antibodies from vaccinated animals. Because both the protective capacity of unfractionated SWAP and the serologic reactivity of the 97,000 antigen are sensitive to digestion with protease, it is likely that the immunologic activity of these molecules is dependent on peptide-bonded structural elements.

IgE antibody and resistance to infection. II. Effect of IgE suppression on the early and late skin reaction and resistance of rats to Schistosoma mansoni infection.

Most helminth parasites induce a strong IgE antibody and elevated eosinophil response in their mammalian hosts and a number of in vitro studies have suggested that IgE, possibly in association with eosinophils, may be an essential element of the host protective immunity against helminth infections. To assess the role of IgE in protective immunity, we examined the effect of suppressing the IgE antibody response on rat immunity to Schistosoma mansoni. Suppression was achieved in neonates by injections of rabbit anti-epsilon chain gamma-globulins, control rats received injections of unspecific gamma-globulins. IgE suppression caused a marked reduction of the inflammatory reaction that developed in the skin of immune rats at the site of a cercarial challenge: the early (30 to 60 min) wheal and flare reaction was abolished, and the late cutaneous reaction (6 to 18 h) associated with intense pruritus, edema and local eosinophilia was greatly reduced. This shows that IgE was critical to the recruitment of effector cells and molecules in the skin during the first 24 h following parasite invasion. Worms were recovered 18 to 30 days after a primary infection and 18 days after a challenge infection from IgE-suppressed and control rats. IgE-suppressed rats cured a first infection as rapidly as the control rats; however, they were two to three times less efficient than the controls at eliminating a second or a third challenge. These observations demonstrate that IgE antibodies are essential for the full development of rat acquired protective immunity against Schistosoma mansoni.

Schistosoma mansoni: Challenge attrition during the lung phase of migration in vaccinated and serum-protected rats

Serum harvested from Sprague-Dawley rats twice vaccinated with gamma-irradiated cercariae of Schistosoma mansoni is able to protect naive recipients against a challenge infection, but the challenge parasites are susceptible to immune elimination over a very short period of time. Thus, vaccinated rat serum protects recipients against a percutaneous cercarial challenge when transferred on Day +5 but not Day 0 and protects recipients against a tail vein challenge with 5-day-old lung worms when transferred on Days 0 or +1, but not Days +4 or +5. Rats challenged with lung worms via the tail vein and given serum on Day +3 exhibit approximately half the protection expressed by counterparts that received serum on Day 0. However, vaccinated rat serum does not protect naive recipients against a lung worm challenge introduced directly into the liver. These data indicate that immune elimination of challenge parasites in the vaccinated rat model is site-dependent rather than stage-dependent, and most probably occurs during the lung phase of parasite migration.

Induction of protective immunity against Schistosoma mansoni by a non living vaccine. I. Partial characterization of antigens recognized by antibodies from mice immunized with soluble schistosome extracts.

A single intradermal injection of frozen and thawed schistosomula in conjunction with the bacterial adjuvant Mycobacterium bovis strain Bacille Calmette Guerin, Phipps substrain (BCG) induced significant levels of resistance to challenge Schistosoma mansoni infection in C57BL/6 mice. Immunization with the aqueous fraction remaining after 100,000 X G centrifugation of the larval lysate was also protective under these conditions, suggesting that some immunogenic determinants may not be membrane associated. Frozen-thawed cercariae and soluble components of adult worms also protected against challenge infection in these experiments. These observations indicate that soluble immunogens are present in both early and late developmental stages of the parasite, and therefore may be good candidate antigens for an immunochemically defined vaccine against schistosomiasis. Induction of humoral reactivity against soluble or membrane antigens was examined in mice protected against cercarial challenge by prior exposure to frozen-thawed larvae, soluble larval, or soluble adult antigens plus BCG. Animals that were immunized with frozen-thawed larvae produced low but significant levels of antibodies against larval surface antigens when examined by indirect immunofluorescence or by immunoprecipitation of surface-labeled schistosomula. Mice immunized with soluble antigens, however, showed negligible antibody reactivity against surface membrane antigens. Because mice immunized with soluble antigens were resistant to challenge infection, these results strongly suggest that anti-surface membrane reactivity is not required in the mechanism of protective immunity in this model. Sera from mice immunized with either total freeze-thaw larval lysate or soluble schistosome extracts all showed strong reactivity against soluble antigens, as detected by ELISA. Western blot analysis showed these antisera to react with a restricted number of high m.w. antigens that were present both in schistosomula and in adult worms. These antigens are therefore likely to play a major role in the development of resistance in this model as immunogens and/or as targets of protective immune response.

Evidence against the existence of specific Schistosoma mansoni subpopulations which are resistant to irradiated vaccine-induced immunity.

When mice are immunized with irradiated Schistosoma mansoni cercariae a proportion of the subsequent cercarial challenge always escapes killing and matures to egg-laying adults. This report investigates the possibility that incomplete immunity in this system is governed by a genetically-determined insusceptibility of a particular schistosome subpopulation. To do this we tested whether more immunoresistant schistosomes would develop following successive passages of progeny of the resistant worms through immunized mice. Mice were immunized with 500 50 Krad-irradiated cercariae, and challenged with normal cercariae when immunity was at its peak. After five successive passages through snails and immune mice, progeny of those parasites which escaped immune killing were no more refractory to vaccine-induced resistance than the original stock maintained in nonimmune mice. Additionally, the "passaged" isolates did not differ from the original stock in their ability to induce protection following irradiation. Our results indicate that with this model of acquired resistance incomplete immunity is unlikely to be due to a subpopulation of the parasites possessing a genetically-determined insusceptibility to killing.

The Fate of Challenge Schistosomula in the Murine Anti-Schistosome Vaccine Model

Mice exposed to irradiated cercariae of Schistosoma mansoni develop a partial resistance to subsequent parasite challenge. In this study we utilized histopathologic methods to investigate the fate of both the immunizing and challenge cercariae in C57BL/6J mice. After immunization by percutaneous infection, a large number of the 50 Kr irradiated organisms could be detected in tissue sections of lung. However, as early as 2 weeks after immunization, the majority of these schistosomula apparently had died, leaving residual inflammatory foci. The numbers of these foci then gradually declined during the next 4 weeks of examination. Cercarial challenge of mice vaccinated 4 weeks previously provoked an intense eosinophil-enriched inflammatory response in percutaneously exposed ear pinnae. Despite these pronounced tissue reactions, no evidence of significant parasite damage or attrition was detected in this migration site. In contrast, schistosomula arriving in the lungs of vaccinated mice produced a greater number of residual inflammatory foci than did larvae appearing in the lungs of normal mice. In addition, challenge schistosomula were cleared from the lungs of vaccinated mice at a slower rate than they were from the lungs of control mice. These observations suggest that the lung is a major site of parasite attrition for both immunizing and challenge infections in the mouse irradiated vaccine model.

Passive transfer of resistance and the site of immune-dependent elimination of the challenge infection in rats vaccinated with highly irradiated cercariae of Schistosoma mansoni.

The immune-dependent elimination of a challenge infection in rats vaccinated with highly-irradiated cercariae of Schistosoma mansoni was analysed by passive transfer of serum, recovery of the challenge from the lungs and liver and by transferring lung-stage schistosomula. Recipients of serum from rats immunized with either unirradiated, 20 or 40 krad.-irradiated cercariae, most of which die in the liver, lungs and skin, respectively, were equally resistant if the serum was injected on the day of infection or 5-7 days after infection. In addition, vaccinated rat serum transferred to mice and vaccinated rabbit serum transferred to rats conferred comparable protection when injected on day 0 or 5 days after infection of the recipients. This apparent susceptibility of the lung schistosomula to immune attack was confirmed by challenging 20 or 40 krad.-irradiated cercariae vaccinated rats with lung-stage schistosomula derived either from mice or rats. All the detectable attrition of a cercarial challenge in vaccinated rats occurred between 7 and 10 days post-challenge, before the parasites reached the liver. Since there was no evidence of damage or attrition in the skin or lungs before day 7 it was concluded that immune-dependent elimination occurred rapidly following a 'window of sensitivity' coinciding with the migration of the parasites from the lungs to the liver.

Schistosoma mansoni: Vaccination of mice with 10-krad-irradiated, cryopreserved schistosomules

Protection against a Schistosoma mansoni cercarial challenge was evaluated in mice immunized with a vaccine composed of 10-krad-irradiated, cryopreserved schistosomules. The level of resistance induced in C57 B1 6 or NMRI (CV) mice increased with the number of schistosomules injected. Up to 83% reduction in challenge worm burden was achieved when 5000 schistosomules were injected per mouse. Intramuscular injection of the vaccine was superior to subcutaneous. Multiple immunizations, up to 3 at 4-week intervals, did not increase the resistance induced by a single immunization. A high level of protection developed in as little as 2 weeks and was maintained through at least 12 weeks postimmunization. The vaccine irradiated with 10 krad from either a 60-cobalt or 137-cesium source induced equivalent levels of resistance, and no differences were found in the immunogenicity of vaccines comprised of organisms irradiated as cercariae or as 1- to 3-hr-old schistosomules. These findings are basic to the development of a cryopreserved, live vaccine against schistosomiasis of humans or domestic animals.

Schistosoma mansoni: radiation dose and morphologic integrity of schistosomules as factors for an effective cryopreserved live vaccine.

The effectiveness of a cryopreserved, irradiated schistosomule vaccine against an homologous Schistosoma mansoni cercarial challenge was tested in C57B1/6 mice. Highly significant levels of protection developed consistently when mice were immunized with the vaccine irradiated at 10-20 Krad, i.e., doses below that considered optimal for irradiated cercariae (50 Krad). Cryopreserved schistosomules irradiated at 10 or 20 Krad induced greater levels of protection than did schistosomules irradiated at 2, 5, 30, or 50 Krad. Protective immunity developed as early as 3 weeks post-immunization. When immunizing inocula were injected at various times post-thaw, or when schistosomule subpopulations of normal-appearing, damaged or dead organisms were injected, those populations which had appeared to sustain the least degree of damage were those most capable of stimulating protective immunity. These findings highlight the hazards of extrapolating conditions considered standard for an irradiated cercarial vaccine to one in which cryopreservation, for storage of the schistosomules, is an added stress.