Surface immobilization antigens of Ichthyophthirius multifiliis: Their role in protective immunity

Abstract

In response to infection with the pathogenic ciliate Ichthyophthirius multifiliis, fish produce serum and mucus antibodies that immobilize the parasite in vitro. The antigens responsible for this phenomenon (referred to as immobilization antigens, or i-antigens) are thought to be involved in protective immunity and are being studied in connection with efforts to develop subunit vaccines. Using mammalian antibodies, the i-antigens of Ichthyophthirius have been identified as a family of related surface proteins with M_r's in the 40–60 kDA range. The amino acid sequence deduced from a 1.2 kb cDNA encoding a member of this family predicts a protein with a highly periodic structure characteristic of the i-antigens of the free-living ciliates, Paramecium and Tetrahymena. To date, four distinct immobilization serotypes of I. multifiliis have been identified. Northern hybridization studies indicate that i-antigen genes of Ichthyophthirius are developmentally regulated during the parasite life cycle and are expressed at extraordinarily high levels in the infective stage. Synthesis of i-antigen mRNA transcripts may also be accompanied by novel RNA processing events. A role for the i-antigens in protective immunity is strongly suggested by the results of passive immunization studies with immobilizing monoclonal antibodies (i-mAbs). Following intraperitoneal injection of naïve channel catfish, i-mAbs confer complete protection against an otherwise lethal parasite challenge. In conjunction with ELISA and in vitro immobilization assays, passive immunization experiments indicate that protection requires the presence of antibody at the site of infection (that is, at the surface of fish). The results of these studies are discussed in the light of current knowledge about mechanisms of protection against I. multifiliis, and a model of surface immunity is presented.