The Release of Platelet-Activating Factor during Phagocytosis by Polymorphonuclear Neutrophils and Monocytes

Abstract

Platelet-activating factor (PAF) is a soluble factor that induces platelet aggregation and release reaction (1–4), independently from ADP- and Arachidonic Acid/Thromboxane A₂ -mediated pathways (2, 4). PAF is alow molecular weight phospholipid (1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine)(5,6). PAF was initially obtained from antigen-stimulated, IgE-sensitized rabbit basophils (2). In man PAF release from basophils still awaits confirmatory evidence, mainly because in man basophils constitute a much smaller fraction of the total leukocytes than in the rabbit. Lewis recovered a PAF-like activity from purified leukemic basophils in the presence of Calcium Ionophore (7). Later, we have shown that immune complexes (IC), the specific antigen, anaphylatoxin and neutrophil cationic proteins stimulate PAF release from human leukocytes (8). Although basophil degranulation was coincident with PAF release, the cell populations used in those experiments were relatively impure. More recently, ultrastructural evidence of platelet interaction only with degranulated basophils but not with either neutrophils or monocytes was obtained when basophils were degranulated by the specific antigen, anti-IgE or Synachten (9). However, PAF release occurred independently from basophil degranulation during phagocytosis by polymorphonuclear neutrophils (PMN) and monocytes (9,10). Lotner et al. demonstrated that purified human PMN are capable of releasing PAF (11). The purpose of the present report was to comparatively study the mechanism of PAF release from PMN and monocytes under phagocytic stimuli.