Cellular responses during liver fluke infection in sheep and its evasion by the parasite.

The cellular immune response in sheep to an acute and chronic primary and an acute secondary liver fluke infection were examined by immunohistology of liver tissue and flowcytometry of lymphocytes from the draining hepatic lymph nodes. Ten days after primary infection, portal tract areas surrounding migratory tunnels were infiltrated with CD4+ and CD8+ lymphocytes with fewer B cells and T19+ T cells. Micro abscesses were distributed sporadically in the liver parenchyma and young flukes could be easily observed in the liver tissue free from inflammatory cells. More intensive infiltration of the portal tract areas was observed during a secondary liver fluke infection characterized by a pronounced increase in eosinophils, B cells and CD4+ T cells. In addition, there was an increase in MHC class II+ fibroblastic-like cells surrounding the migratory tracts. In contrast to the primary infection, no young flukes were observed in the same tissue areas during the secondary infection. Chronic primary infections were characterized by perilobular fibrosis and a predominance of CD8+ and gamma delta-TCR+T19- T cells distributed within fibrotic strands. Distinct B cell follicles were observed in the fibrotic strands and near major bile ducts and necrotic patches. Pronounced lymphocyte infiltration could occasionally be observed surrounding liver fluke eggs lodged in liver tissue. A progressive increase in lymph node weight, cell number and CD4/CD8 ratio was observed in the acute and chronic primary infections. The role of the infiltrating cell populations and possible mechanism of immune evasion by the parasite are discussed.