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Secretory phospholipase A 2 (sPLA 2 ) type IIa, elevated in inflammation, breaks down membrane phospholipids and generates arachidonic acid. We hypothesized that sPLA 2 will hydrolyze red blood cells that expose phosphatidylserine (PS) and generate lysophosphatidic acid (LPA) from phosphatidic acid that is elevated in PS-exposing red blood cells. In turn, LPA, a powerful lipid mediator, could affect vascular endothelial cell function. Although normal red blood cells were not affected by sPLA 2 , at levels of sPLA 2 observed under inflammatory conditions (100 ng/ml) PS-exposing red blood cells hemolyzed and generated LPA (1.2 nM/10 8 RBC). When endothelial cell monolayers were incubated in vitro with LPA, a loss of confluence was noted. Moreover, a dose-dependent increase in hydraulic conductivity was identified in rat mesenteric venules in vivo with 5 M LPA, and the combination of PS-exposing red blood cells with PLA 2 caused a similar increase in permeability. In the presence of N-palmitoyl L-serine phosphoric acid, a competitive inhibitor for the endothelial LPA receptor, loss of confluence in vitro and the hydraulic permeability caused by 5 M LPA in vivo were abolished. The present study demonstrates that increased sPLA 2 activity in inflammation in the presence of cells that have lost their membrane phospholipid asymmetry can lead to LPA-mediated endothelial dysfunction and loss of vascular integrity.Secretory phospholipase A 2 (sPLA 2 ) 2 type IIa is a low molecular weight, ubiquitous enzyme that is elevated in inflammation. The enzyme generates arachidonic acid from phospholipids for the generation of thromboxanes and leukotrienes and, as such, acts as an essential mediator in the inflammatory pathways (1). The specific membranes targeted by the enzyme for the generation of arachidonic acid during inflammation have not been clearly defined. Secretory PLA 2 has a strong preference for phospholipids that are negatively charged at physiologic pH: phosphatidylserine (PS) and phosphatidylethanolamine (PE) (2). In normal mammalian cells, these phospholipids are mainly (PE) or exclusively (PS) confined to the inner layer of the plasma membrane (1-3). Loss of this asymmetric distribution and exposure of PS on the external surface generates a thrombogenic surface and signals macrophages to remove cells by phagocytosis (1-5). Although normal mammalian cells do not seem to act as targets for sPLA 2 , loss of phospholipid asymmetry in plasma membranes and the exposure of PS have been shown to render them vulnerable to phospholipid hydrolysis (1). In addition, it has been well established that bacterial membranes are excellent substrates for this enzyme (6). Together, membranes with altered phospholipid packing are potential targets for sPLA 2 type IIa.We have previously shown that sPLA 2 is elevated after injury, predicts hypoxemia, and is related to multiorgan failure (7,8). In addition, elevated levels of sPLA 2 predict the onset of the acute chest syndrome in sickle cell disease (9 -11), the severe lung damage that...