Bactericidal/Permeability-Increasing Protein (BPI) - 0.2mg

The existence of autoantibodies to bactericidal/permeability-increasing protein (BPI) was first described when it was noticed that 11 of 51 sera that were cANCA positive in immunofluorescence, recognized a 57 kDa antimicrobial cationic protein known as CAP57 in both ELISA and Western blot.  It is now known that CAP57 and BPI are identical.

More recently BPI purified from neutrophil granula was used to directly demonstrate the existence of anti-BPI autoantibodies in a range of vasculitic disorders by ELISA and Western blot. BPI-ANCA have since been demonstrated to be present in a range of clinical conditions including ulcerative colitis, Crohn’s disease, cystic fibrosis, rheumatoid arthritis, systemic lupus erythematosus, mixed connective tissue disease, Behcet’s disease, bronchieactasis, primary sclerosing cholangitis and chronic liver diseases.

BPI is a highly cationic protein that is stored in the azurophil granula of neutrophils. BPI is a potent antibacterial agent with selective toxicity towards Gram-negative bacteria. It binds to the lipopolysaccharides present in the outer envelope of these bacteria to exert immediate bacteriostatic effects and later, bactericidal effects. Analysis of the activity of proteolytic fragments and of the crystal structure of BPI reveal that the protein consists of two functionally distinct domains: a potent antibacterial and anti-endotoxin 20 kDa domain at the amino-terminal and a carboxyl terminal domain that is responsible for BPI’s opsonic activity.

Epitope studies using a panel of recombinant truncated BPI proteins indicate that anti-neutrophil cytoplasmic antibodies to BPI recognise the carboxyl terminal domain. It has also been suggested elsewhere that BPI-ANCA binding sites are likely to be conformational epitopes, as is the case with autoantibodies to proteinase 3 and myeloperoxidase.