Protection to Fasciola hepatica in the gut mucosa of immune rats is associated with infiltrates of eosinophils, IgG1 and IgG2a antibodies around the parasites

We investigated the immune effector mechanisms that underlie protection aga inst F. hepatica in the gut wall of immune rats, using (immuno)histochemist ry. In the lamina propria of immune Wistar rats, four weeks after oral infe ction, frequencies of IgE-positive cells, eosinophils and mucosal mast cell s were significantly increased, compared with naive rats. These factors rep resent the traditional effector mechanisms against helminths. No significan t differences were detected between the two groups in frequencies of IgM-, IgG2a-, IgG1- and IgA- positive cells, CD4- and CD8-positive cells, NK cell s, macrophages, neutrophils or goblet cells. Upon challenge of immune rats with F. hepatica in an ex vivo gut segment, NEJs that migrated through the (sub)mucosa were coated with IgG1 and IgG2a antibodies and surrounded by eo sinophils. No IgE or IgA antibodies were detected on the parasites. The ons et of these immune effector responses, two h after challenge, was related t o the expression of protection. These results suggest that NEJs are killed by an eosinophil-mediated cytotoxic response involving IgG antibodies. Thes e antibodies were not produced in the intestine, but infiltrated the gut up on challenge. The observed immune effector responses were not restricted to the site where the primary infection is located, namely the small intestin e, but were also detected in the large intestine. The presence of the prote ctive immune mechanisms in two other rat strains demonstrates the pivotal i mportance of these responses, irrespective the genetic,background of the ho st.