| Background and objective Hydatid infection caused by the larval stage of Echinococcus granulosus is one of the major world zoonoses, affecting humans as well as domestic and wild animals. Infection of the intermediate hosts constitutes a major public health problem and is a cause of important economic loss in areas of endemicity.Hydatid disease has a worldwide distribution, particularly in countries where pastoral activities are prominent, such as Australia, Argentina, Uruguay and so on(7,8). In our country this disease is also prevalent. The hyperendemic areas are the intensive sheep raising areas of Xinjiang, Qinghai, Tibet, Ningxia and Gansu, as well as some other agricultural areas bordering on the pastoral areas. The endemic areas consist of 40% of the total areas in China (1, 2).Control of this disease mainly depends on public education and dog treatment and has achieved the complete cessation of transmission of Echinococcus granulosus to human in Tasmania and New Zealand (9) However, these have been long and expensive campaigns in highly educated societies in which there exist excellent veterinary and public health infrastructures. Similar campaigns in other areas have been less successful. Our country also uses the public education and dog treatmentapproach to control the infection. But we have not achieved the satisfactory result (10, 11). In China, widespread infection, undeveloped economy in the endemic areas, lack of knowledge of hygiene in residents, the use of dogs to herd sheep in the rural endemic areas, the habit of feeding dogs on home-butchered sheep or other livestocks, all of these factors create vicious cycle. So we must find a more effective strategy to control this disease.An effective vaccine for either the definitive host or the intermediate host should be useful to break the cycle and recombinant DNA methods provided the necessary tools. A vaccine to prevent intermediate hosts from hydatid infection, designated as Eg95, has achieved satisfactory effectiveness (12,13), but it has its own insurmountable problems such as great cost, difficulties in manipulation. These have prevented the commercial release of the vaccine to date. The number of potential definitive hosts is fewer than the number of potential intermediate hosts, so it would be attractive to target the definitive hosts (dogs) for vaccination.Fu screened a cDNA library derived from the adult stage of Echinococcus granulosus with sera from infected dogs and isolated a cDNA clone, designated as EgA31, A recombinant polypeptide encoded by 5'fragment of the EgA31 cDNA can induce strong humoral and cellular immune response. EgA31 should be a potential vaccine against Echinococcus granulosus(5,6).To Express and purify the EgA31 C-terminal peptide for Echinococcusis diagnosis and a vaccine development, wesubcloned the EgA31 into pGEX vector and got the EgA31/GST fusion protein by expression in Escherichia coli. We determined if the fusion protein can elicit the humoral and cellular response in guinea pig. It is a basis for further studying the probability of using the fusion protein in diagnosis and vaccine. And because a substantial degree of immunological cross-reactivity exists among the host-protective antigens of different taeniid species, we also wanted to determine the possibility of the fusion protein against infection with other parasite species.Materials and methods (1) EgA31 3'cDNA was amplified by a pair of primers designed according to the EgA31cDNA sequence. In addition, forward and reverse primers were added with restriction endonuclease Sail and Noti. The PCR products were digested with Sail and Notl for a directional gene cloning to the expression vector pGEX-5X-3.(2) The positive recombinant plasmid(pGEX/EgA31) was transfered into host bacteria Escherichia coli BL21, and the expression of the fusion protein was induced by IPTG and then purified with glutathione-agarose beads(GSTrapFF). SDS-PAGE analysis was performed to detect the expression of fusion protein.(3) Guinea pig...
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