PASSIVE-IMMUNIZATION OF CHANNEL CATFISH (ICTALURUS-PUNCTATUS) AGAINSTTHE CILIATED PROTOZOAN PARASITE ICHTHYOPHTHIRIUS-MULTIFILIIS BY USE OF MURINE MONOCLONAL-ANTIBODIES

Fish acquire immunity against the ciliated protozoan parasite Ichthyop hthirius multifiliis following sublethal infection, The immune respons e includes the elaboration of humoral antibodies against a class of ab undant surface membrane proteins referred to as immobilization antigen s (i-antigens), Antibodies against these proteins immobilize the paras ite in vitro, suggesting a potential role for the i-antigens in protec tive immunity, To test this hypothesis, passive immunization experimen ts were carried out with naive channel catfish, Ictalurus punctatus, u sing immobilizing murine monoclonal antibodies (MAbs), Fish were compl etely protected against lethal challenge following intraperitoneal inj ection of 20 to 200 mu g of MAb, Although fish succumbed to infection at lower doses, palliative effects were observed with as little as 2 m u g of antibody, In experiments in which animals were challenged at va rious times following inoculation, an inverse relationship between par asite load and serum immobilizing activity was seen, Of seven MAbs whi ch conferred protection, all were immunoglobulin G class antibodies. T he only immobilizing MAb that failed to protect was an immunoglobulin M antibody that was absent from surface mucosa as determined by enzyme -linked immunosorbent assay, The implications of these findings for th e development of a vaccine against I. multifiliis and immunity against surface pathogens of fish are discussed.