We examined the functional role and mechanisms by which activation of cysteinyl leukotriene-1 receptor (cysLT 1 R) regulates b 2 -integrin adhesion to intercellular adhesion molecule (ICAM)-1 in human polymorphonuclear leukocytes (PMNs) in vitro.Human peripheral blood PMNs and eosinophils were isolated separately from the same mildly atopic donors. Surface expression of cysLT 1 R was identified both in PMNs and in eosinophils by immunofluorescence analysis. Total cysLT 1 R protein was substantially greater in eosinophils than in PMNs as determined by Western blot analysis. However, leukotriene D 4 (LTD 4 ) upregulated b 2 -integrin adhesion of PMNs to ICAM-1 with high efficacy in a time-and concentration-dependent manner. Upregulated b 2 -integrin adhesion of PMNs was related temporally and quantitatively to phosphorylation of 85-kDa cytosolic group IVa phospholipase A2 (gIVaPLA2). Augmented LTD 4 -induced adhesion was blocked significantly by montelukast, a cysLT 1 R antagonist. Trifluoromethylketone (a gIVaPLA2 inhibitor) blocked b 2 -integrin adhesion caused by LTD 4 activation, as did anti-CD18 monoclonal antibody directed against b 2 -integrin on the PMN surface.Our data demonstrate that LTD 4 causes phosphorylation of gIVaPLA2 and upregulation of b 2 -integrin adhesion to ICAM-1 or ICAM-1 surrogate through cysLT 1 R activation. Activation of gIVaPLA2 is a critical step through which b 2 -integrin adhesion is upregulated by the cysLT 1 R expressed on the surface membrane of human PMN.KEYWORDS: Adhesion, cysteinyl leukotriene-1 receptor, group IVa phospholipase A2, polymorphonuclear leukocytes C ysteinyl leukotrienes (cysLTs), prostaglandins and thromboxanes are families of pro-inflammatory mediators arising through metabolism of arachidonic acid. CysLTs are implicated in airway hyperresponsiveness in patients with asthma [1,2]. Leukotriene D 4 (LTD 4 ), a cysLT, is generated extracellularly by gamma-glutamyl transpeptidase from secreted LTC 4 , which is synthesised by intracellular conversion from LTA 4 by LTC 4 synthase [2,3]. LTC 4 and LTD 4 have substantial efficacy in causing airway constriction in asthmatic subjects [1,4,5]. Three subclasses of cysLT receptors have been identified, cysLT 1 R [6-10], cysLT 2 R [6-10] and cysLT 3 R [11,12]. Biological activities such as airway hyperresponsiveness [13], cellular adhesion [14][15][16], cell migration [17,18] and mucus secretion [19] are mediated predominantly through cysLT 1 R activation. Whereas cysLT 2 R has been reported to cause vascular permeability [4,20], blockade of cysLT 3 R appears to prevent hypoxic brain injury [12].The 85-kDa cytosolic group IVa phospholipase A2 (gIVaPLA2) is a critical intracellular messenger protein for cellular adhesion and secretion of bioactive lipid mediators in human eosinophils [21,22] and polymorphonuclear leukocytes (PMNs) [16,23]. Activation of gIVaPLA2 causes production of arachidonate metabolites and lysophospholipids [22][23][24][25][26], which are essential for b 2 -integrin adhesion of granulocytes.Th...