The Shrinking Schistosomal Egg Granuloma: How Accessory Cells Control T Cell-Mediated Pathology

Schistosomes are blood-dwelling trematode helminths which cause disease and death in millions of people. The immunopathologic basis of the disease is a granulomatous reaction against the parasite eggs, which in the case of infection with Schistosoma mansoni takes place in the liver and gastrointestinal tract. A granuloma is a well-circumscribed aggregate of inflammatory cells (mostly macrophages, eosinophils, and lymphocytes) embedded in a mixture of extracellular matrix components. Based on observations made in a suitable murine model, it is now known that the granuloma is a cell-mediated delayed-type hypersensitivity (DTH) reaction carried out by egg antigen-specific CD4 super(+) T helper (Th) lymphocytes expressing heterodimeric alpha beta chain receptors.