A schistosome infection is initiated when the parasite penetrates the skin of a susceptible host. Relatively large quantities of protein are released by transforming cercariae compared to later larval stages. This represents the first parasite material to which the host's immune system is exposed, yet little is known about the proteins which are released during the first few hours post-transformation. We have shown that antiserum raised against such molecules was capable of imparting protection against a schistosome challenge infection upon passive transfer to naïve mice. By screening a cercarial cDNA library with this serum, 38 positive clones were identified. Sequence analysis showed these to represent 8 different molecules which included Schistosoma mansoni 21·7 kDa antigen, calcium-binding-protein and the vaccine candidate glutathione S-transferase (Sm28GST). In addition, 5 clones were isolated, 1 of which had significant homology to many cytochrome C proteins, another with leukocyte elastase inhibitors and 3 which represented novel molecules. Four clones were expressed in a prokaryotic high-level expression vector, sera produced against each purified recombinant protein and used subsequently to probe Western blots and parasite sections. The leukocyte elastase inhibitor homologue and 2 unknowns induced significant proliferation by lymph node cells recovered from mice vaccinated with irradiated cercariae. More strikingly, the 2 novel proteins stimulated very high levels of interferon γ (IFNγ) secretion both by lymph node cells and those recovered by broncho-alveolar lavage from the lungs of vaccinated mice. Such results will be discussed in the context of vaccine development.

Lung-stage schistosomula are the target of protective immunity in mice vaccinated with attenuated cercariae of Schistosoma mansoni. Therefore, proteins present at this developmental stage, and in particular those which are secreted, are a potential source of novel vaccine candidates. However, little information is available about such molecules. Here we describe the cDNA clones identified by screening expression libraries with serum raised against proteins released by lung-stage schistosomula. In total, 11 different cDNA species were identified, 6 of which have been described previously in S. mansoni; these included fructose 1,6-bisphosphate aldolase and Sm21.7 which together accounted for two-thirds of all positive clones. Of the 5 newly described schistosome genes, 1 cDNA had a high degree of homology to the s5a subunit of 26S proteasomes, most significant being with the human protein. The remaining 4 clones showed no significant homologies to any genes sequenced previously. Fructose 1,6-bisphosphate aldolase, Sm21.7, the proteasome homologue and 1 unknown clone (A26) have been expressed in a bacterial expression system and serum produced against each recombinant protein. Immunolocalization showed fructose 1,6-bisphosphate aldolase, Sm21.7 and the proteasome homologue to be most abundant in muscle cells whilst clone A26 was distributed throughout many tissues, but was most abundant in the tegument. Analysis of the cellular immune responses of vaccinated mice showed 3 of the 4 expressed clones to be highly immunogenic, inducing the secretion of large quantities of the Th1-type cytokine interferon gamma.