Preparation and properties of monoclonal antibodies against lipopolysaccharide-sensitive serine protease zymogen, factor C, from horseshoe crab (Tachypleus tridentatus) hemocytes.

Seventeen murine monoclonal antibodies (mAbs) against horseshoe crab clotting factor, factor C, were prepared and characterized. When the binding sites of these mAbs were analyzed by immunoblotting, ten mAbs recognized nonreduced factor C, five mAbs were directed against the heavy chain, and two mAbs were directed against the B chain. Three mAbs, 1H4, 2C12, and 2A7, one selected from each group, were used for further study. The mAb 1H4, which recognized only nonreduced factor C molecule, inhibited the factor C activity in a dose-dependent manner. It also inhibited lipopolysaccharide (LPS)- and alpha-chymotrypsin-mediated activations of the zymogen factor C, suggesting that 1H4 binds close to the active site and/or the substrate-binding site located in the serine protease domain (B chain) of factor C. On the other hand, 2C12 and 2A7 recognized, respectively, an epitope located in the heavy and the B chains, and inhibited LPS-mediated activation of factor C, but not alpha-chymotrypsin-mediated activation of factor C or factor C activity. Both F(ab')2 and Fab' fragments derived from 2C12 inhibited LPS-mediated activation in the same manner. These three mAbs did not bind with LPS, although a factor C-mAb complex was able to bind LPS, suggesting that the LPS-mediated activation of the zymogen factor C was induced through intermolecular interaction between the LPS-bound factor C molecules. The dissociation constants (Kd) for 1H4, 2C12, and 2A7 binding to factor C were determined as 1.9 x 10(-9), 0.6 x 10(-10), and 1.8 x 10(-10) M, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)