DiscussionSchistosome miracidia transform to primary sporocysts soon after entry into the snail intermediate host. This event is characterized by the shedding of ciliated epidermal plates concurrently with the formation of the sporocyst tegumental syncytium. During this transformation process a diverse array of larval proteins (larval transformation proteins or LTP) are released into surrounding snail tissues, thereby representing the first major source of host-interactive molecule. Previous proteomic analyses of whole LTP released in vitro by transforming S. mansoni miracidia included several marker proteins typically found in exosome-like extracellular vesicles (ELVs). Because ELVs have not been investigated in early developing schistosome larvae, we proceeded to isolate ELVs from LTP (24-hr cultures) and characterize their proteomic profile. Isolated nanoparticles exhibited mean and mode diameters of 130 and 66 nm (NanoSight LM10), respectively. Protein comparisons between ELV-enriched samples and LTP-depleted or ELV-wash fractions revealed an enrichment of known exosome-associated proteins. Among the 25 most common proteins associated with exosomes (exoCarta), 10 were present in ELVs isolated from 24h S. mansoni LTP, including tetraspanin/CD63 receptor, annexin, enolase, GAPDH and others. Additional proteins involved in cell signaling, RNA metabolism, transcriptional regulation, cell cycle regulation, and stress response were also identified. Fluorescent-labeled ELVs bound to, and were internalized by, a subset of circulating Biomphalaria glabrata snail hemocytes, as well as to cells of the B. glabrata embryonic (Bge) cell line. Studies are currently in progress to further characterize ELVs released during early larval development and their potential roles in regulating snail cell/parasite interactions.