Induction of protective immunity to schistosomiasis with immunologically cross-reactive Lumbricus molecules

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.

Purification, characterization, and immunolocalization of paramyosin from the adult stage of Fasciola hepatica

Paramyosin, a vaccine candidate in different helminthiases, was purified from the adult liver fluke Fasciola hepatica using two different procedures. The first started with a crude extraction of paramyosin in high-salt buffer followed by gel filtration chromatography and two precipitation-solubilization cycles; in the second, anion exchange chromatography replaced the gel filtration step. In both cases, the apparent molecular weight of the purified protein determined by sodium dodecyl sulfate gel electrophoresis under reducing and non-reducing conditions was 97 kDa and 200 kDa, respectively. The molecular weights were consistent with the presence of a dimeric protein linked by disulfide bridges. Western blot analysis showed that the dimeric and monomeric forms were both recognized by an antiserum raised against the F. hepatica 97 kDa band (alpha-FhPmy), and by an anti- Schistosoma mansoni paramyosin immune serum. Immunohistochemistry using alpha-FhPmy demonstrated the localization of paramyosin within the subtegumental muscle and in muscle cells surrounding the gut of adult parasites. We also observed labeling of extramuscular structures like testes, surface lamellae of the gut and the tegument of adult flukes.

Characterization and protective potential of the immune response to Taenia solium paramyosin in a murine model of cysticercosis

Paramyosin has been proposed as a vaccine candidate in schistosomiasis and filariasis. However, limited information is available about its protective potential against cysticercosis and the immune response it induces. Immunization of mice with recombinant full-length paramyosin of Taenia solium (TPmy) results in about a 52% reduction in parasite burden after a subsequent challenge by intraperitoneal inoculation of Taenia crassiceps cysticerci. Immunization assays using recombinant fragments of TPmy, corresponding approximately to thirds on the amino, central, or carboxyl regions, suggest that protective epitopes are located mostly in the amino-end third. Proliferation assays using T cells obtained from mice immunized with the full-length recombinant TPmy also showed a preferential response to the amino-terminal fragment. In contrast, antibodies in the sera from these mice predominantly recognize epitopes located in the carboxyl-terminal fragment, being the immunoglobulin G1 subclass, the predominant antibody isotype. Characterization of the cellular immune response induced against the protective amino-terminal fragment reveals production of gamma interferon and interleukin-2, but not interleukin-4, suggesting a Th1-like profile.

Protective mechanisms against the intestinal nematode Strongyloides venezuelensis in Schistosoma japonicum-infected mice

Mice infected with Schistosoma japonicum were resistant to the intestinal nematode, Strongyloides venezuelensis. The numbers of adult S. venezuelensis recovered from mice were significantly decreased when infections were given from 6 weeks after S. japonicum infection. Larval recovery from the lungs showed that significant numbers of subcutaneously inoculated S. venezuelensis larvae were eliminated by 3 days in S. japonicum-infected mice (P < 0.0001), while histology revealed that this was associated with massive eosinophilic infiltration in the lungs. In addition, adult S. venezuelensis worms implanted in the duodenum of S. japonicum-infected mice could not establish in the intestine. This failure was associated with mucosal mastocytosis. Activation of eosinophils and intestinal mast cells was correlated with elevated expression of mRNA for interleukin (IL)-3, IL-4, and IL-5 in S. japonicum-infected mice. Sera from S. japonicum-infected mice recognized S. venezuelensis larva antigens as strongly as those from S. venezuelensis-infected mice, although transfer of sera from S. japonicum-infected mice to normal recipient mice did not protect them from S. venezuelensis challenge infection. It was concluded that the mechanisms for larval killing and adult worm expulsion of S. venezuelensis in S. japonicum-infected mice were identical to those seen in infections with S. venezuelensis only.