A MONOCLONAL-ANTIBODY AGAINST A PEPTIDE SEQUENCE OF FIBRINOGEN GAMMA-CHAIN ACTS AS AN INHIBITOR OF FACTOR XIII-MEDIATED CROSS-LINKING OF HUMAN FIBRIN

Recurrent hemorrhage has been reported in humans as a result of acquir ed antibody inhibitors which interfere with the crosslinking of fibrin by factor XIII. One type of these inhibitors (Type III) prevents acti vated factor XIII from acting on fibrin. We have generated an ant:fibr in monoclonal antibody, called mAb 4A5, which binds to a peptide seque nce at the carboxyl-terminus of human fibrinogen gamma-chains. MAb 4A5 acts like a Type III inhibitor and prevents proper factor XIII-mediat ed crosslinking. Pre-incubation of fibrinogen or pooled human plasma w ith mAb 4A5, but not mAb D2 (specific for the carboxyl terminus of fib rin alpha-chains), resulted in clots which are soluble in either 5 M u rea or 1% monochloroacetic acid. DS-PAGE and immunoblotting analysis o f these clots confirmed that mAb 4A5 inhibited gamma-chain crosslinkin g in plasma clots and fibrin clots. Results from a factor XIII activit y assay demonstrated that biotinylcadaverine crosslinking into fibrin by factor XIII could be inhibited by mAb 4A5 but not mAb D2, arguing t hat mAb 4A5 acted by binding the crosslinking site of factor XIII. Stu dies of the immunoreactivity of these mAbs with 12 different animal sp ecies showed that the gamma-chain epitope recognized by mAb 4A5 was mo re conserved than the alpha-chain epitope recognized by mAb D2. The sp ecies fibrinogens, recognized by mAb 4A5 in binding assays, also showe d impaired crosslinking when mAb 4A5 was present during the clotting r eaction.