Exocytosis of a subpopulation of specific granules coincides with H2O2 production in adherent human neutrophils.

Abstract

Unlike polymorphonuclear leukocytes (PMN) that are activated in suspension, PMN plated onto extracellular matrix (ECM) proteins exhibited a prolonged delay in the onset of the respiratory burst in response to PMA, FMLP, A23187, or GM-CSF. The present study was focused on examining the events leading to H2O2 release in adherent PMN. The time course of stimulated H2O2 release from PMN plated on fibrinogen (FG) kinetically paralleled cell spreading and lactoferrin release. In contrast, the release of another specific granule component, vitamin B12-binding protein, preceded H2O2 generation by at least 20 min, suggesting the differential mobilization of subpopulations of specific granules. FMLP-stimulated PMN from a patient with leukocyte adhesion deficiency (LAD), lacking CD11/CD18 integrins and unable to adhere to FG, attached and spread on thrombospondin (TSP) and generated substantial amounts of H2O2. However, PMN from a second LAD patient, able to attach but not spread on TSP, failed to generate oxidant. These data indicated that PMN spreading might be a prerequisite for H2O2 generation and that CD11/CD18 integrins are likely not the only surface receptors involved in this response. Exposing PMN to the intracellular Ca2+ chelators MAPTAM or BAPTA significantly reduced H2O2 generation in response to the receptor-mediated agonists, FMLP or GM-CSF, but did not affect PMA-stimulated H2O2 generation. Under conditions that resulted in reduced H2O2 generation (i.e., MAPTAM or BAPTA plus FMLP or GM-CSF), lactoferrin, but not vitamin B12-binding protein, release was also reduced, suggesting a link between the secretion of lactoferrin-containing specific granules and H2O2 production. Since neither MAPTAM or BAPTA blocked PMN spreading, it appeared that cell spreading alone was not sufficient for H2O2 production. Thus, the major requirement for H2O2 generation appeared to be the exocytosis of a distinct population of lactoferrin-containing specific granules. In support of this observation, PMN-like HL-60 cells, which lack specific granules, attached and spread on FG but failed to release H2O2. Thus, we postulate that the delivery of cytochrome b from lactoferrin-containing specific granules to the plasma membrane during activation contributes to oxidant production in PMN adherent to ECM proteins.