SURFACE-ANTIGEN CROSS-LINKING TRIGGERS FORCED EXIT OF A PROTOZOAN PARASITE FROM ITS HOST

We used the common fish pathogen Ichthyophthirius multifiliis as a mod el for studying interactions between parasitic ciliates and their vert ebrate hosts, Although highly pathogenic, Ichthyophthirius can elicit a strong protective immune response in fish after exposure to controll ed infections, To investigate the mechanisms underlying host resistanc e, a series of passive immunization experiments were carried out using mouse monoclonal antibodies against a class of surface membrane prote ins, known as immobilization antigens (or i-antigens), thought to play a role in the protective response, Such antibodies bind to cilia and immobilize I. multifiliis in vitro, Surprisingly, we found that passiv e antibody transfer in vivo caused rapid exit of parasites from the ho st, The effect was highly specific for a given I. multifiliis serotype , F(ab)(2) subfragments had the same effect as intact antibody, wherea s monovalent Fab fragments failed to protect, The activity of Fab coul d, nevertheless, be restored after subsequent i.p. injection of bivale nt goat anti-mouse IgG, Parasites that exit the host had detectable an tibody on their surface and appeared viable in all respects, These fin dings represent a novel instance among protists in which protective im munity (and evasion of the host response) result from an effect of ant ibody on parasite behavior.