Abstract
Antigen B (EgAgB) is the most abundant and immunogenic antigen produced by the larval stage (metacestode) of Echinococcus granulosus. It is a lipoprotein, the structure and function of which have not been completely elucidated. EgAgB apolipoprotein components have been well characterised; they share homology with a group of hydrophobic ligand binding proteins (HLBPs) present exclusively in cestode organisms, and consist of different isoforms of 8-kDa proteins encoded by a polymorphic multigene family comprising five subfamilies (EgAgB1 to EgAgB5). In vitro studies have shown that EgAgB apolipoproteins are capable of binding fatty acids. However, the identity of the native lipid components of EgAgB remains unknown. The present work was aimed at characterising the lipid ligands bound to EgAgB in vivo. EgAgB was purified to homogeneity from hydatid cyst fluid and its lipid fraction was extracted using chloroform∶methanol mixtures. This fraction constituted approximately 40–50% of EgAgB total mass. High-performance thin layer chromatography revealed that the native lipid moiety of EgAgB consists of a variety of neutral (mainly triacylglycerides, sterols and sterol esters) and polar (mainly phosphatidylcholine) lipids. Gas-liquid chromatography analysis showed that 16∶0, 18∶0 and 18∶1(n-9) are the most abundant fatty acids in EgAgB. Furthermore, size exclusion chromatography coupled to light scattering demonstrated that EgAgB comprises a population of particles heterogeneous in size, with an average molecular mass of 229 kDa. Our results provide the first direct evidence of the nature of the hydrophobic ligands bound to EgAgB in vivo and indicate that the structure and composition of EgAgB lipoprotein particles are more complex than previously thought, resembling high density plasma lipoproteins. Results are discussed considering what is known on lipid metabolism in cestodes, and taken into account the Echinococcus spp. genomic information regarding both lipid metabolism and the EgAgB gene family.
Highlights
IntroductionThe larval stage of the cestode parasite Echinococcus granulosus is the causative agent of cystic echinococcosis (hydatid disease) in a range of mammalian species (mainly domestic ungulates) as well as in humans
The larval stage of the cestode parasite Echinococcus granulosus is the causative agent of cystic echinococcosis in a range of mammalian species as well as in humans
This work describes the physiological lipids of EgAgB, an important piece of information to complete our knowledge on EgAgB molecular composition
Summary
The larval stage of the cestode parasite Echinococcus granulosus is the causative agent of cystic echinococcosis (hydatid disease) in a range of mammalian species (mainly domestic ungulates) as well as in humans It is a unilocular fluid-filled cyst, which steadily grows inside host visceras (mostly liver and lung). One of the major molecules produced in large amounts by the cyst is a highly immunogenic lipoprotein named antigen B (EgAgB) [1,2], which represents a major diagnostic antigen for human infection [3,4,5] This antigen is present in various larval locations including the parasite cellular layer of the cyst wall (germinal layer), the larval worms or protoscolex (asexually produced towards inside the cyst) and the hydatid cyst fluid (HCF). It is unknown whether it is released throughout the infection or just at a certain time point [10,11]
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