Binding and lethal effect of complement from Oncorhynchus mykiss on Gyrodactylus derjavini (Platyhelminthes: Monogenea)

A lethal effect of rainbow trout Oncorhynchus mykiss plasma containing intact complement factors on Gyrodactylus derjavini was demonstrated. It was associated with binding of complement factor C3 to certain carbohydrate-rich parasite structures. Parasites were exposed in vitro to plasma from rainbow trout. Untreated plasma from infected and uninfected fish showed lethal effects on the parasites within 1 h, whereas prior heat inactivation of complement prevented any parasite killing. Inhibition of the classical pathway by EGTA (ethyleneglycol-tetraacetic acid) treatment of plasma did not prevent the killing. It was shown by immuno-cytochemical assays that C3 bound directly to certain parasite structures (cephalic gland openings, parasite body, hamulus sheath). In contrast, no immunoglobulin binding was detected on the parasites. Lectin-binding assays indicated mannose-rich regions in the cephalic gland openings and lactose derivatives in the hamulus sheath. Galactose derivatives showed a general distribution in the glycocalyx. Thus, the antibody-independent plasma effect on the gyrodactylids are ascribed to the alternative complement pathway, and it is suggested that some carbohydrate epitopes on the parasites are involved in the C3 activation.