Mucosal IgM Antibody with d-Mannose Affinity in Fugu Takifugu rubripes Is Utilized by a Monogenean Parasite Heterobothrium okamotoi for Host Recognition.

How parasites recognize their definitive hosts is a mystery; however, parasitism is reportedly initiated by recognition of certain molecules on host surfaces. Fish ectoparasites make initial contact with their hosts at body surfaces, such as skin and gills, which are covered with mucosa that are similar to those of mammalian guts. Fish are among the most primitive vertebrates with immune systems that are equivalent to those in mammals, and they produce and secrete IgM into mucus. In this study, we showed that the monogenean parasite Heterobothrium okamotoi utilizes IgM to recognize its host, fugu Takifugu rubripes Oncomiracidia are infective larvae of H. okamotoi that shed their cilia and metamorphose into juveniles when exposed to purified d-mannose-binding fractions from fugu mucus. Using liquid chromatography-tandem mass spectrometry analysis, proteins contained in the fraction were identified as d-mannose-specific IgM with two d-mannose-binding lectins. However, although deciliation was significantly induced by IgM and was inhibited by d-mannose or a specific Ab against fugu IgM, other lectins had no effect, and IgM without d-mannose affinity induced deciliation to a limited degree. Subsequent immunofluorescent staining experiments showed that fugu d-mannose-specific IgM binds ciliated epidermal cells of oncomiracidium. These observations suggest that deciliation is triggered by binding of fugu IgM to cell surface Ags via Ag binding sites. Moreover, concentrations of d-mannose-binding IgM in gill mucus were sufficient to induce deciliation in vitro, indicating that H. okamotoi parasites initially use host Abs to colonize host gills.