S protein binds to serum-treated agarose beads independently of complement activation and the formation of the terminal complement complex on the beads.

Comparison of initial (early-phase) and terminal (late-phase) sequence activation of complement by agarose beads and endotoxin was evaluated in an enzyme immunoassay (EIA) of serum levels of C3c and C9 neoepitopes, respectively. EIA and Western blotting with anti-S protein monoclonal antibody revealed lower S protein values and weaker S protein bands in serum activated by agarose beads than by endotoxin, implying that S protein was removed from serum by binding to agarose. The binding of S protein to the beads was confirmed by radioimmunoassay and was found to be equal in normal and heat-inactivated serum. In contrast, the terminal complement complex was formed only on agarose beads incubated with normal serum and not with inactivated serum.