The complement system of rainbow trout (<i>Salmo gairdneri</i>). I. Identification of the serum lytic system homologous to mammalian complement.

Abstract

Abstract In order to definitely demonstrate the presence of the complement system in rainbow trout, we purified IgM to homogeneity from rainbow trout immunized with sheep erythrocytes. We found that normal rainbow trout serum did contain a lytic activity that can lyse sheep erythrocytes at 25°C but only in the presence of rainbow trout IgM (anti-sheep erythrocyte). The identification of this lytic activity with the complement system was further enhanced by the following observations. 1) The lytic activity required both Ca++ and Mg++ ions for the lysis of sensitized sheep erythrocytes. Addition of EDTA or EGTA at a millimolar level completely blocked the lytic activity. 2) Previous incubation with LPS, zymosan, or inulin depleted the lytic activity. Furthermore, rabbit erythrocytes were lysed by rainbow trout serum without rainbow trout antibody. These results suggest that the lytic activity of rainbow trout serum has an alternative pathway. This pathway required only Mg++ ions/as divalent cations, in a manner similar to mammalian complement. 3) The lytic activity was effectively depleted from rainbow trout serum by incubating with IgG fraction of rabbit antiserum directed to 1 of the rainbow trout serum proteins. Addition of minute amounts of rainbow trout serum, not lytic by itself, to depleted serum completely recovered the lytic activity. This antiserum was subsequently identified as anti-rainbow trout C5. 4) Multi-component, macromolecular proteins were isolated from stroma of rabbit erythrocytes lysed by rainbow trout serum or from rainbow trout serum incubated with LPS. These molecules shared many physicochemical properties with membrane attack complexes of the complement system of higher vertebrates.