M A Gijon

The role of platelet‐activating factor and peptidoleukotrienes in the vascular changes of rat passive anaphylaxis

1 The role of platelet‐activating factor (PAF) and peptidoleukotrienes as putative mediators of some of the vascular changes triggered by antigen was investigated in rats passively sensitized with monoclonal anti‐DNP (2,4‐dinitrophenyl) IgE. 2 Lethal anaphylaxis with respiratory distress, systemic hypotension, detachment of the intestinal mucosa, leukopenia and extravasation of protein‐rich plasma was observed after antigen challenge of rats sensitized with partially purified monoclonal IgE at concentrations of 15 mg protein kg−1. 3 Analysis of the peritoneal fluid obtained after i.v. challenge with DNP‐BSA (bovine serum albumin) showed the presence of significant amounts of PAF (101 ± 8 pg/rat), whereas this mediator was undetectable in control animals. Leukotriene D4 was the predominant peptidoleukotriene that could be recovered after antigen challenge, and showed an extremely high concentration (92 ± 15 ng/rat) as compared to PAF levels. 4 Extravasation of protein‐rich plasma was observed shortly after challenge and reached a maximum at 30 min. Treatment of animals with i.v. PCA 4248 (1–2 mg kg−1) and WEB 2086 (1 mg kg−1), two chemically unrelated compounds which are antagonists of the PAF‐receptor, produced a significant reduction of the extravasation of protein‐rich plasma. 5 The same degree of protection could be afforded by MK‐886, an inhibitor of leukotriene biosynthesis. Combined treatment with WEB 2086 and MK‐886 provided greater inhibition of protein‐rich plasma extravasation than either compound alone. PCA 4248 was also found to inhibit in a dose‐dependent manner the systemic hypotension observed upon DNP‐BSA challenge. 6 These data indicate that the lipid mediators PAF and peptidoleukotrienes are major effectors of the vascular disturbances observed in rat passive IgE‐mediated anaphylaxis.

Effect of immunological stimulation on the production of platelet-activating factor by rat peritoneal cells : its relevance to anaphylactic reactions

The production of platelet-activating factor (PAF) by rat peritoneal cells was studied using as stimuli either monoclonal IgE, IgG1 or IgG2b anti-DNP (2,4-dinitrophenyl), and DNP-BSA. Peritoneal cells sensitized in vitro with any of these antibodies at concentrations higher than 10 nM and challenged with 1 microM DNP-BSA produced PAF. PAF production was also elicited by preformed IgE/ and IgG2b/DNP-BSA immune complexes, preferentially at a large antigen/antibody ratio. The production of PAF was unrelated to the activation of mast cells, since it occurred in populations depleted of mast cells by adherence to plastic dishes. Moreover, the release of [3H]serotonin from IgE-sensitized mast cells showed a time-course more rapid than PAF production and occurred in cells sensitized with IgE at concentrations lower than those required for PAF formation. In contrast, peritoneal cells sensitized with IgG1 and IgG2b failed to release [3H]serotonin. Rat peritoneal cells showed a significant ability to catabolize PAF by intracellular PAF-acetylhydrolase in view of both the amounts of enzyme activity assayed in cellular homogenates, and the 15-fold increase on controls of PAF quantities detected in peritoneal cells treated with phenylmethylsulfonyl fluoride (PMSF), a known inhibitor of PAF-acetylhydrolase. The PAF activity produced upon PMSF addition showed a retention time on reverse-phase HPLC which suggests structural identity to PAF produced by either immunological challenge or ionophore A23187. These data suggest that PAF formed during rat passive anaphylaxis reactions depends on the activation of mononuclear phagocytes. This production may be triggered by two types of low affinity receptors: Fc epsilon RII/CD23 and Fc gamma R. The ability of peritoneal cells to catabolize PAF by intracellular acetylhydrolase seems unaffected by immunological stimulation.

Study of Some Mechanisms that may Contribute to the Presence of High Levels of Phospholipase A2 Activity in Plasma of Patients with Septicemia

Phospholipase A2 (EC 3.1.1.4) is a family of phosphatide 2-acylhydrolases that play an important role in the metabolism of phospholipids and membrane homeostasis [1–4]. These functions of phospholipase A2 seem relevant to the pathogenesis of various clinical conditions, since the presence of high concentrations of phospholipase A2 activity in plasma and inflammatory exudates of patients suffering from inflammatory arthritis, peritonitis and septic shock has been reported. Administration of exogenous extracellular phospholipase into experimental animals causes inflammatory hyperemia [5], and a correlation of serum levels of phospholipase A2 with the magnitude of hypotension has been shown in endotoxin shock [6,7]. The possible pathogenetic role of soluble phospholipase A2 in endotoxin shock in connection to the cytokine network has been emphasized by several reports showing the secretion of type II phospholipase A2 by rat mesangial cells [8,9], liver cells [1011] and human synovial cells [12,13] in response to tumor necrosis factor and interleukin ls. A recent report from this laboratory has shown the presence of high amounts of PAF associated with the platelets of patients with septicemia [14] and this raises the question as to whether this finding might be related to the secretion of phospholipase A2 activity and an ensuing formation of lipid mediators.