Caveolin-1 Expression Determines the Route of Neutrophil Extravasation through Skin Microvasculature

Interleukin-8 plays a key role in the acute inflammatory response by mediating recruitment of neutrophils through vessel walls into affected tissues. During this process, molecular signals guide circulating blood neutrophils to target specific vessels for extravasation and to migrate through such vessels via particular routes. Our results show that levels of endothelial caveolin-1, the protein responsible for the induction of the membrane domains known as caveolae, are critical to each of these processes. We demonstrate that, in response to the intradermal injection of interleukin-8, neutrophils are preferentially recruited to a unique subset of venules that express high levels of intercellular adhesion molecule-1 and low levels of caveolin-1. Our results show that neutrophils traverse human dermal microvascular endothelial cells using one of two pathways: a transcellular route directly through the cell or a paracellular route through cellular junctions. Caveolin-1 expression appears to favor the transcellular path while down-regulation of caveolin-1 promotes the paracellular route.Wounding of the epithelium and entry of a foreign body elicit a series of responses from the innate immune system. One of the main hallmarks of acute inflammation is neutrophil infiltration at the affected site.^1,2 In response to injury or infection, resident phagocytic cells become activated and release inflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-8. TNF-α activates the vascular endothelium causing vasodilation and cellular infiltration.³ IL-8 functions as a critical chemotactic factor attracting neutrophils from the blood to the affected area.^1,4It is currently thought that leukocyte recruitment and migration through the vasculature is an active process not only for migrating blood cells but also for endothelial cells lining the vessels. Initially, inflammatory cytokines or bacterial endotoxins induce expression of P- and E-selectin on the surface of microvascular endothelial cells.^5,6 These molecules recognize carbohydrate counterligands on the surface of circulating leukocytes and mediate the tethering and rolling of these cells along vessel walls.^5,6,7 Firm adhesion is then initiated through the upregulation of endothelial adhesion molecules such as intercellular adhesion molecule (ICAM)-1 and vascular adhesion molecule (VCAM)-1, which bind to integrins expressed on the leukocyte surface.^5,7,8 Finally, the leukocyte is induced to migrate through the vessel in a process known as diapedesis.^5,6,7Among the many proteins implicated in the process of diapedesis, the adhesion molecule ICAM-1, which is up-regulated on activated endothelium, and caveolin-1, which is expressed on most terminally differentiated cell types but is largely undetectable in white blood cells, have been most closely associated with the route of transendothelial migration in in vitro systems.^7,9,10 A recent study by Millan et al clearly demonstrates that ICAM-1 and caveolin-1 are involved in directing the path of T lymphoblast migration through human umbilical vein endothelial cells (HUVECs).⁷Although both caveolin-1 and ICAM-1 have been associated with leukocyte transendothelial migration in vitro, the distribution of these proteins in vessels used by migrating leukocytes in vivo remain unclear. While all endothelial cells (ECs) share common features, the vascular tree is known to be extremely heterogeneous. As a result, the precise molecular profile of selectins and adhesion molecules defining vessels targeted for extravasation by circulating leukocytes is unknown. Furthermore, since the phenotype of vessel ECs is determined in large part by their unique in vivo microenvironment, site specific and regional differences in the expression of molecules contributing to the regulation of leukocyte transmigration have yet to be thoroughly characterized.^11,12 In this study, we have examined the in vivo molecular profile of vessels targeted by circulating neutrophils in response to IL-8 in the skin and have determined the effect of the expression of these factors on the route of neutrophil transmigration in vitro.