Fish cells involved in mucosal immunity against enteric helminths.

In recent decades, the mucosal immunology of higher vertebrates has been an intensively explored area of research and yet little is known regarding this in fish. The intestinal mucosa is continuously exposed to a broad spectrum of antigens derived from ingested food, water, commensal flora and pathogens. Recent comparative histological and ultrastructural studies on the uninfected intestines of chub Squalius cephalus, European eels Anguilla anguilla, and catfish Silurus glanis and in conspecifics naturally infected with acanthocephalans and platyhelminths, however, have begun to explore this. Endoparasitic helminths frequently cause intestinal inflammation inducing the recruitment of various immune cells to the site of infection. In each of the fish-helminth systems that were studied, a massive hyperplastic granulocyte response involving mast cells (MCs) and neutrophils in the epithelia, lamina propria and submucosa was seen in close proximity to the point of parasite attachment. An increase in the number of rodlet cells (RCs) in the intestinal epithelium of parasite-infected specimens was also seen. Hyperplasia and hypertrophy of mucous cells in infected intestines was evident. In transmission electron microscopy sections of all three infected fish hosts, MCs and mucous cells were frequently seen in contact with one another with evident degranulation of those MCs in close proximity to mucous cells. In fish the mechanism by which mucosal MCs might induce the secretion of excess mucus is unknown and is the main focus of this study. The role of MCs, neutrophils, RCs and mucous cells in the mucosal immune system of fish will be discussed.