No abstract

Bactericidal/permeability-increasing protein (BPI), a cationic protein present in the azurophilic granule and on the surface of polymorphonuclear leukocytes, specifically interacts with lipopolysaccharide (LPS). This study demonstrates for the first time, using flow cytometry with specific anti-BPI monoclonal antibody (MAb), that human peripheral blood monocytes express BPI on their cell surface. The monocyte cell surface BPI was shown to bind to LPS, because binding of anti-BPI MAb 4E3 (which is known not to react with BPI to which LPS is bound) to cell surface BPI was strongly reduced after preincubation of cells with LPS. However, cell surface BPI did not quantitatively contribute to the interaction of LPS with the monocyte cell membrane, since preincubation of cells with 4E3 did not block binding of LPS-fluorescein isothiocyanate to monocytes. The origin of the monocyte cell surface BPI remains to be further elucidated.

A sandwich ELISA was developed specific for human bactericidal/permeability-increasing protein (BPI), using Mg++ ions to abrogate disturbance by lipopolysaccharide of BPI measurement and to prevent aspecific adherence of BPI to solid phase. In fresh EDTA or heparinized plasma of healthy volunteers BPI was not detectable, whereas in serum BPI was present, indicating that coagulation activates polymorphonuclear leukocytes to release BPI. Furthermore, BPI was present in plasma of critically ill intensive care unit (ICU) patients, in bronchoalveolar lavage fluid of patients suspected of having pneumonia, in wound fluid, and in pleural fluid. In sub-groups of samples with culture-proven bacteria, mean BPI levels were increased compared with subgroups without bacteria, although the differences were only significant in EDTA plasma of ICU patients. These findings indicate the presence of BPI during pathologic conditions. The physiologic role of the released BPI has to be further elucidated.

In this study, the release of bactericidal/permeability-increasing protein (BPI), which is stored in polymorphonuclear leukocytes (PMNL), was analyzed in a whole blood ex vivo system. Of the microbial products tested, lipopolysaccharide (LPS) most potently induced BPI release; FMLP, serum-treated zymosan (STZ), and lipoteichoic acid (LTA) also induced BPI release. In addition, the inflammatory mediator tumor necrosis factor (TNF)-alpha potently activated PMNL in whole blood, via TNF receptor p55, to release BPI, whereas interleukin (IL)-1, IL-8, platelet activating factor, and C5a were poor inducers of BPI release. STZ and phorbol myristate acetate, but not LPS, FMLP, or LTA, stimulated isolated PMNL to release BPI. BPI was released in comparable magnitude with the azurophilic granule protein elastase. Furthermore, both proteins were released with similar kinetics, which started within 30 min after onset of stimulation and lasted 1-4 h.