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BackgroundThe trematodeFasciola hepaticais the most widespread causative agent of fasciolosis, a parasitic disease that mainly affects humans and ruminants worldwide. DuringF. hepaticainfection, newly excysted juveniles (FhNEJ) emerge in the duodenum of the mammalian host and migrate towards the definitive location of the parasite, the intra-hepatic biliary ducts. Understanding howF. hepaticatraverses the intestinal wall and migrates towards the liver is pivotal for the development of more successful strategies against fasciolosis. The central enzyme of the mammalian fibrinolytic system is plasmin, a serine protease whose functions are exploited by a number of parasite species owing to its broad spectrum of substrates, including components of tissue extracellular matrices. The aim of the present work is to understand whether FhNEJ co-opt the functions of their host fibrinolytic system as a mechanism to facilitate trans-intestinal migration.Methodology/Principal FindingsAn FhNEJ tegument protein extract (FhNEJ-Teg) was obtainedin vitro, and its capability to bind the zymogen plasminogen (PLG) and enhance its conversion to the active protease, plasmin, were analyzed by a combination of enzyme-linked immunosorbent, chromogenic and immunofluorescence assays. Additionally, PLG-binding proteins in FhNEJ-Teg were identified by 2D electrophoresis coupled to mass-spectrometry analysis, and the interactions were validated using FhNEJ recombinant proteins.Conclusions/SignificanceOur results show that FhNEJ-Teg contains proteins that bind PLG and stimulate its activation to plasmin, which could facilitate the traversal of the intestinal wall by FhNEJ and contribute to the successful establishment of the parasite within its mammalian host. Altogether, our findings contribute to a better understanding of host-parasite relationships during early fasciolosis and may be exploited from a pharmacological and/or immunological perspective for the development of treatment and control strategies against this global disease.Author SummaryFasciolosis is a disease caused by parasites of the genusFasciola, of whichF. hepaticastands out as it has successfully spread all over the world and infects humans and animals throughout the entire global geography. Definitive hosts become infected by ingestion of aquatic plants or water contaminated with metacercariae, which excyst in the duodenum and release the so-called newly excysted juvenile flukes (FhNEJ). FhNEJ traverse the intestinal wall and evolve into immature parasites that migrate through the peritoneum and liver parenchyma until they reach their definitive location inside the major biliary ducts, where adult worms develop and egg shedding starts. In order to cross the intestinal wall, FhNEJ are endowed with a repertoire of proteases that degrade components of the intestinal extracellular matrix, and we hypothesized that they may also co-opt the proteolytic functions of plasmin, the central enzyme of the mammalian fibrinolytic system, to migrate more efficiently across host tissues. In this study, we demonstrate that FhNEJ express proteins on their tegument surface that interact with plasminogen, the zymogen of plasmin, and stimulate its conversion into its active form, which could potentially be used for trans-intestinal migration and contribute to the successful establishment of the parasite within its mammalian host.