The cellular and molecular origins of extracellular vesicles released by the helminth pathogen, Fasciola hepatica

Parasitic helminths secrete extracellular vesicles (EVs) which have potent immunomodulatory effects. Whilst the cargo of EVs has been characterised for many species, we know little about the mechanisms that govern their biogenesis and release. Using antibodies raised against a panel of Fasciola hepatica EV (FhEV) marker proteins, we have identified multiple sites of EV production in the parasite. Discrete immunofluorescence patterns were observed within the gastrodermal cells and tegumental syncytium for different marker proteins whilst the protonephridial (excretory) system and parenchymal-type 2 cells were identified as additional sites of production (or transit) of FhEVs. Ligation was used to mechanically block the oral sucker, excretory pore, or both, to determine the effect on FhEV release from live adult flukes in vitro. This revealed that FhEVs are predominately derived from the gut, whilst the tegument releases EVs to a lesser extent. The data also suggest that the protonephridial system contributes to the small (120 K) EV sub-population. Sphingomyelinase (SMase) activity is a key driver of EV biogenesis in mammalian cells and we have previously identified SMases in FhEVs by mass spectrometry. SMase activity associated with isolated FhEVs was susceptible to the chemical inhibitor GW4869 and treatment of adult flukes with GW4869 led to a significant reduction in 120 K EV release in vitro, suggesting that a ceramide-dependent mechanism could drive 120 K EV formation. In contrast, the release of the larger 15 K EVs was only moderately impacted, indicating that they form independently of SMase activity. Ultrastructural observation of GW4869-treated F. hepatica tissue showed severe disruption to the parenchyma and vacuolation of the tegument, gastrodermal cells and epithelial lining of the excretory ducts. This work establishes that targeted disruption of EV biogenesis and release in helminths is possible, and provides proof-of-concept for future studies investigating EV secretion as a target for parasite control.