Deposition of microparticles by neutrophils onto inflamed epithelium: A new mechanism to disrupt epithelial intercellular adhesions and promote transepithelial migration

Neutrophil [polymorphonuclear leukocyte (PMN)] transepithelial migration (TEM) is a hallmark of inflammatory mucosal disorders. PMN TEM is associated with epithelial injury; however, mechanisms involved in this process are not well defined. The current work describes a new mechanism whereby deposition of PMN membranederived microparticles (PMN-MPs) onto intestinal epithelial cells (IECs) during TEM leads to loss of epithelial cadherins, thus promoting epithelial injury and increased PMN recruitment. PMN-MPs secreted by activated PMNs during TEM displayed a high level of enzymatically active matrix metalloproteinase 9 (MMP-9), and were capable of mediating potent effects on IEC integrity. Isolated PMN-MPs efficiently bound to IEC monolayers and induced cleavage of desmoglein-2 (DSG-2) but not E-cadherin, leading to disruption of IEC intercellular adhesions. Furthermore, PMN-MP binding to intestinal epithelium in vitro in transwell assays and in vivo in ligated intestinal loop preparations facilitated increases in PMN TEM. These effects were MMP-9 dependent and were reversed in the presence of specific pharmacological inhibitors. Finally, we demonstrated that IEC Dsg-2 serves as a barrier for migrating PMNs, and its removal by PMN-MP-associated MMP-9 facilitates PMNtrafficking across epithelial layers. Our findings thus implicate PMN-MPs in PMN-mediated inflammation and epithelial damage, as observed in inflammatory disorders ofmucosal surfaces.