Absolute Quantification of Schistosoma mansoni Esophageal Gland Products

Schistosomes are blood-dwelling helminth parasites causing a debilitating disease in the tropics. Major challenges to control the disease persist and vaccines would provide an additional tool, but their development has been problematic. Rhesus macaques can self-cure following schistosome infection, generating antibodies that target schistosome proteins from the tegument, gut and esophagus, the last of which is the least investigated. We developed a dissection technique that permitted comparative proteomics of the schistosome esophagus and gut for detection of secreted antigens. Shotgun proteome analysis of the male schistosomes esophagus identified 13 proteins encoded by microexon genes (MEG), eleven of which were uniquely located in the esophageal glands. Based on this and transcriptome information, a QconCAT was designed for absolute quantification of selected targets. MEGs 12, 4.2, 4.1 and Venom allergen-like protein 7 were the mostly abundant, spanning over 245-6 million copies per cell, while aspartyl protease, palmitoyl thioesterase and a galactosyl transferase were present at <1 million cpc. Antigenic variation by alternative splicing of MEG proteins was confirmed together with a specialised machinery for protein glycosylation in the esophagus. Moreover, some gastrodermal secretions were highly enriched in the gut, while others were more uniformly distributed throughout the parasite, potentially indicating lysosomal activity. Collectively, our findings provide a more rational, better-oriented selection of schistosome vaccine candidates in the context of a proven model of protective immunity.