Interaction of Lectins with Functionally Pure Complement Components

Abstract As an approach to determine the functional role of the carbohydrate moieties of complement components, the interaction between functionally pure human (Hu) or guinea pig (GP) components and several lectins with differing sugar specificities has been studied. Hu and GP components 1 through 9 were screened for exposed sugar residues by interaction with lectins immobilized by covalent binding to Sepharose 4B. The remaining fluid phase activity of individual components was determined by titration using an appropriate indicator system. The lectins used in this study were castor bean type II (CB II), phytohemagglutinin (PHA), lotus bean (LB), wheat germ agglutinin (WGA) and soy bean (SB). Of the lectins tested, CB II, WGA, PHA, and SB removed fluid phase GP or Hu C1 activity. In addition, CB II bound to Sepharose 4B interacted with Hu or GP C4 and C2. None of the lectins interacted with any component beyond C2 in the classical complement pathway. Unbound PHA or CB II inhibited fluid phase C1 activity, but only CB II affected cell bound C1. Pretreatment of EAC14hu or GP with CB II reduced the number of hemolytically effective EAC142 sites generated by a given concentration of C2 and the rate of decay of the EAC142 formed was decreased. The half life at 37°C of EAC142 pretreated with CB II was > 30 min while for untreated cells it was < 5 min. There was no effect of CB II on the rate of decay of preformed EAC142. In all cases the effects of the lectins were reversed by addition of the sugar specific for lectin binding. This approach suggests that sugar constituents on Hu or GP C1, C4 and C2 may be part of or located near to the binding and/or functional regions of these molecules. Furthermore, treatment of appropriate ceDular intermediates or isolated components with specific sugar degrading enzymes may help to determine whether such sugar residues play a critical role in the recognition/function action and/or control of complement attack.