Trypsin-binding immunoglobulin G and associated antigen in cystic fibrosis

Trypsin-binding immunoglobulin G (TBIgG) is found in the sera of a high proportion of patients with cystic fibrosis. We previously reported that TBIgG preferentially binds human cationic trypsin rather than trypsin from other animal species. Binding affinity is enhanced by complex formation with bovine pancreatic trypsin inhibitor, which is known to induce characteristic conformational modifications in the active site region of the trypsin molecule. To identify the human trypsin-like antigen associated with TBIgG, we have studied the effects of conformational changes of cationic trypsin induced by limited proteolysis based on competitive binding studies. It is shown that the most likely TBIgG-related self-antigen is an 11,000-dalton fragment that is a cleavage product of the complex formed by trypsin and alpha 1-protease inhibitor. This result emphasizes the occurrence of circulating trypsinogen activation and is interpreted to be a consequence of the protease-antiprotease imbalance, which has been well documented by previous investigators in cystic fibrosis and also in other lung diseases associated with an inflammatory state.