Patterns of Gene Expression in Schistosoma mansoni larvae associated with Infection of the Mammalian Host

Larval schistosomes infect the human host by penetration of unbroken skin, before gaining access to a blood vessel and beginning their intravascular life. The work in this thesis focuses on the mammalian infection process, as it would be the ideal point to interrupt the life cycle. A whole organism approach was taken, and the life cycle stages before, during and after skin penetration were studied, namely the intra-molluscan germ ball (embryonic cercaria), infective cercaria, and the in vitro cultured day 3 schistosomulum (equivalent to skin stage larva). Confocal microscopy was used for a morphological survey of these life cycle stages, establishing the timeline of cercarial embryogenesis and documenting the impressive changes they undergo. The temporal and spatial gene expression patterns underlying the changes were investigated using the first genome-wide microarray for S. mansoni for transcriptional profiling of the three stages. The known repertoire of molecules likely to be secreted during host entry was greatly expanded, particularly the proteases and venom allergen-like proteins. Genes involved in energy production and conservation were up-regulated in the cercaria, as were several genes encoding proteins deployed immediately on arrival in the skin. Additionally, micro exon genes (MEGs) encoding variant secreted proteins were highly upregulated in the schistosomulum, emphasising their likely role after entry into the mammalian host. The transcription of many tegument and gut-associated genes was also increased in the schistosomulum; cathepsins were particularly notable, even in the cercaria, implying that the larval gut becomes active long before blood feeding begins. The first application of whole mount in situ hybridisation to germ balls confirmed localisation of invadolysin and VAL-10 to the nascent acetabular glands. However, SmKK7 and Sm16 were not expressed in these glands, questioning their putative roles in immunomodulation. Finally, three MEGs were revealed to be expressed in tissues at the host-parasite interface.