The endotoxin-binding bactericidal/permeability-increasing protein (BPI): a target antigen of autoantibodies.

The bactericidal/permeability-increasing protein (BPI) is an endotoxin-binding neutrophil leukocyte-granule protein with antibacterial and anti-endotoxin properties. A recombinant form of BPI (rBPI21) has been developed and is being tested as a therapeutic agent to treat gram-negative bacterial infections and exposure to gram-negative bacterial endotoxin. BPI is also a target antigen of anti-neutrophil cytoplasmic autoantibodies (ANCA). BPI-ANCA are present in cystic fibrosis, inflammatory bowel disease, vasculitis, and primary sclerosing cholangitis; presence of BPI-ANCA appears associated with a higher inflammatory disease activity and greater organ damage. BPI-ANCA as well as ANCA directed at other neutrophil-granule proteins may exacerbate inflammation by nonspecific effects of extracellular and cell-associated immune complexes. BPI-ANCA may further worsen inflammation by reducing the ability of BPI to promote clearance of gram-negative bacteria and bacterial-associated endotoxin.