ANALYSIS OF THE SOLUBLE AND MEMBRANE-BOUND IMMOBILIZATION ANTIGENS OFICHTHYOPHTHIRIUS-MULTIFILIIS

The immobilization antigens (i-antigens) are a class of highly abundan t surface membrane proteins found on a number of holotrich ciliates. I n Ichthyophthirius multifiliis (an obligate parasite of fish) these an tigens appear to be targets of the host immune response. While the i-a ntigens of Ichthyophthirius are predominantly membrane-associated prot eins, we now find that they are released into the water surrounding th e parasite in a highly enriched form. The membrane-associated and wate r soluble proteins appear indistinguishable by antigenic means, as wel l as by several biochemical criteria including peptide mapping, mobili ty in reducing and non-reducing SDS-polyacrylamide gels, and relative glycosylation. Antibodies raised against the membrane-associated antig ens react with the water soluble proteins on Western blots. Not surpri singly, immuoocytochemical localization studies show binding of these antibodies to surface membranes of the cell. In addition, however, ant ibody binding is also detectible on the membranes of a secretory organ elle (that is, mucocysts) present in the cortical cytoplasm. The signi ficance of these findings with regard to the potential role of the i-a ntigens in infection and immunity is discussed.