Response of the tegument of Fasciola hepatica to infection and immunization sera in vitro.

The migratory phase is a critical time for Fasciola hepatica as it must locate, penetrate and migrate through the alimentary tract to the liver parenchyma whilst under attack from the host immune response. Here, scanning and transmission electron microscopy were used to monitor the in vitro effects of sera (with, and without, complement depletion) on F. hepatica newly excysted juveniles (NEJs) and flukes recovered at 7, 35, 70 and 98 days post infection (dpi) from the liver and bile ducts of rats. Test sera were from these F. hepatica-infected rats. A F. hepatica NEJ-specific rabbit antiserum was also used. All fluke stages demonstrated release of the tegumental glycocalyx and microvesicles and intense activity within the tegumental syncytium characterized by eccrine secretion of T-0/T-1/T-2 secretory bodies with subsequent microvillar formation and shedding of microvesicles from the apical plasma membrane. Exposure of both NEJs and 35 dpi flukes to 35 and 70 dpi rat sera produced significant amounts of eccrine-derived secretory material and putative attached immunocomplex. Rabbit anti-F. hepatica NEJ-specific antiserum produced similar responses at the NEJ tegument, including binding of putative immunocomplex to the surface, but with additional blistering of some regions of the apical plasma membrane. Our data suggest that immune sera stimulates multiple interrelated secretory mechanisms to maintain the integrity of the tegumental barrier in response to immune attack. Concurrent release of microvesicles may also serve to both divert the immune response away from the fluke itself and permit delivery of immunomodulatory cargo to immune effector cells.