What's in SWAP? Abundance of the principal constituents in a soluble extract of Schistosoma mansoni revealed by shotgun proteomics.

backgroundThe soluble antigen preparation of adult schistosomes (SWAP) has often been used to probe host responses against these blood-dwelling parasites. Despite its long-established use there is limited knowledge about its composition. The information we provide here on the molecular composition of SWAP may contribute as a guide for a rational selection of antigenic targets.

methodsLabel-free quantitative shotgun proteomics was employed to determine the composition and abundance of SWAP constituents. Briefly, paired adult Schistosoma mansoni worms were sonicated in PBS pH 7.2 and ultracentrifuged for recovery of the soluble supernatant. An aliquot was subjected to trypsin digestion. Resulting peptides were separated under ultra-high performance liquid chromatography and analysed using an orbitrap mass spectrometer. Spectral data were interrogated using SequestHT against an in-house S. mansoni database. Proteins were quantified by recording the mean area under curve obtained for up to three most intense detected peptides. Proteins within the 90(th) percentile of the total SWAP mass were categorized according to their sub-cellular/tissue location.

resultsIn this work we expanded significantly the list of known SWAP constituents. Through application of stringent criteria, a total of 633 proteins were quantitatively identified. Only 18 proteins account to 50% of the total SWAP mass as revealed by their cumulative abundance. Among them, none is predicted as a secreted molecule reinforcing the point that SWAP is dominated by cytosolic and cytoskeletal proteins. In contrast, only 3% of the mass comprised proteins proposed to function at the host-parasite interfaces (tegument and gut), which could conceivably represent vulnerable targets of a protective immune response. Paradoxically, at least 11 SWAP proteins, comprising ~25% of its mass, have been tested as vaccine candidates.

conclusionsOur data suggest that use of SWAP to probe host responses has greatest value for diagnostic purposes or assessing intensity of infection. However, the preparation is of limited utility as an antigen source for investigating host responses to proteins expressed at or secreted from worm-host interfaces. The data also pose the question as to why vaccination with SWAP, containing so many proposed vaccine candidates, has no additive or even synergistic effects on the induction of protection.