The bactericidal/permeability-increasing protein (BPI) is membrane-associated in azurophil granules of human neutrophils, and relocation occurs upon cellular activation

Neutrophilic granulocytes contain the 55 kDa bactericidal/permeability-incr easing protein (BPI). BPI binds to lipopolysaccharides (LPS), and exerts ba cteriostatic and bactericidal effects against a wide variety of Gram-negati ve bacterial species.;We have investigated the subcellular location of BPI in immature and mature neutrophils using cryotechnique for immunoelectron m icroscopy. BPI was found to colocate with myeloperoxidase (MPO), a marker f or azurophil granules, and it also showed the same pattern of distribution as CD63, a transmembrane-anchored protein. This suggests that BPI is membra ne-associated in the azurophil granules in neutrophils. Its presence in azu rophil granules was further confirmed by the finding of BPI in the azurophi l granules of neutrophil promyelocytes of the bone marrow. Induction of sel ective release of azurophilic granules by the Na-ionophore monensin resulte d in fusion of endosomes with azurophil granules, leading to the formation of large vacuoles containing MPO, CD63, and BPI. After phagocytosis of seru m-treated zymosan (STZ), BPI was detected in phagosomes, both in associatio n with membranes as well as in the lumen, suggesting the release of BPI int o activated compartments. The results show that BPI is present in azurophil granules, is probably primarily membrane-associated, and is relocated afte r activation, following the same route as MPO and CD63.