TWENTY-THIRD ANNUAL EASTERN FISH HEALTH WORKSHOP JOHN CARVER INN, PLYMOUTH, MA 30 MARCH - 2 APRIL, 1998 Mechanisms Of Immunity Against The Parasitic Ciliate, Ichthyopthirius multifiliis. Theodore G. Clark1 and Harry W. Dickerson2 1Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853; 2Department of Medical Microbiology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602 Icthyopthirius multifiliis (Ich) invades the skin and gill epithelia of a wide variety of freshwater fish. While it is highly pathogenic, animals that survive controlled infections become resistant to subsequent challenge. Icthyopthirius is therefore a suitable candidate for vaccine development. At the same time, it becomes an extremely useful model for studying host-parasite interactions. Based on recent work, it appears that resistance to I. multifiliis involves a novel mechanism of humoral immunity affecting parasite behavior. Rather than being killed on the host, a majority of parasites exit fish prematurely in response to antibody binding. The target antigens involved in this process are a class of highly abundant surface membrane proteins referred to as immobilization antigens, or i-antigens. Direct observation of parasites as they leave fish suggest several alternative hypotheses that explain forced exit. Regardless of the precise mechanism, however, it is clear that the phenomenon requires antigen cross-linking (most probably eliciting a signal transduction cascade within the cell). We have recently shown that the i-antigens of Ichthyopthirius are anchored to the plasma membrane through a glycosylphosphatidyl inositol (GPI) moiety, and a number of potential transduction pathways are currently being tested for activation by specific antibodies. Parallel efforts are also underway to characterize the i-antigens in more detail. In this regard, we have cloned and sequenced a gene encoding the 48 kDa i-antigen of a parasite isolate designated Gl. Based on its deduced amino acid sequence, the protein contains a series of tandem repeats (about 80 amino acids each) that predict zinc-binding domains. Homologies between i-antigens and the variant specific proteins (VSPs) of the mammalian gut parasite, Giardia lamblia, will be discussed. Finally, progress toward the development of a practical vaccine will be addressed.   Return to 23rd Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page