Time Course of Intercellular Adhesion Molecule-1 Upregulation Following Endotoxin Administration

Abstract

Bacterial cell wall derived lipopolysaccharide (LPS) is the primary endotoxin contributing to localized in_ammation and systemic toxicity as a result of gram-negative infection. Complexes of LPS are major components of the outer membrane of gram negative bacteria and are released as a result of cell lysis as well as during active growth [1]. Circulating LPS can bind with several blood components including LPS binding protein (LBP). The LPS-LBP complex is capable of interacting with leukocyte boundor soluble-CD14, which stimulates leukocytes and endothelial cells, respectively [2]. In addition, circulating LPS can activate complement which eventually leads to the release of in_ammatory mediators from several cell types including platelets, mast cells, and endothelial cells. Adherence of polymorphonuclear leukocytes (PMN) to the endothelium is one of the initial steps in an acute in_ammatory response. Following activation by in_ammatory mediators, interactions between leukocytes and endothelial cells begin with transient adhesive forces which result in the rolling of leukocytes along the endothelium. This rolling is primarily mediated by the selectin family of adhesion molecules (i.e., P-, E-, and L-selectin) [3–5]. Early shedding of L-selectin from the leukocyte leads to a rapid upregulation of the b2 integrin CD11/CD18 on the leukocyte cell surface. Interactions between CD11/CD18 and ICAM-1 are responsible in large part for the ~rm adhesion of leukocytes to the endothelial surface and their subsequent extravasation [6,7]. This has been demonstrated in several studies in which the administration of a monoclonal antibody directed against ICAM-1 inhibited PMN adherence to endothelial cells both in vivo and in vitro [8–10]. The progression of endotoxin-induced in_ammation relies to a large extent on the surface expression of cellular adhesion molecules on both leukocytes and endothelial cells. Although changes in the af~nity of these molecules may be involved, the regulation of leukocyte-endothelial cell interactions is primarily the result in quantitative changes in cell surface expression. Of the endothelial-derived adhesion molecules, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1, and E-selectin are upregulated by a number of stimuli including interleukin (IL)-1, tumor necrosis factor-a (TNF-a), and LPS. Marked upregulation of cellular adhesion molecules in endotoxemia may be due to LPS directly or to the release of humoral factors that are initiated by endotoxin [11,12]. Nevertheless, the time course of upregulation appears to differ with each stimulus and adhesion molecule, possibly relating to the fact that these endothelial adhesion molecules are under transcriptional regulation. In order to investigate the effects of endotoxin administration on the upregulation of cell adhesion molecules, we utilized an ex vivo model of leukocyte adherence to rat superior mesenteric artery endothelium.