Leukocyte transcellular diapedesis: Rap1b is in control

Abstract

The neutrophil transmigration across the blood endothelial cell barrier represents the prerequisite step of innate inflammation. It is well known that neutrophils cross the endothelial barrier by transmigrating at the endothelial cell junction ('paracellular'). However, in vivo and in vitro evidence have clearly demonstrated occurrence of an alternate mode of migration directly through the endothelial cell body ('transcellular'). Despite our knowledge on mechanisms of transendothelial migration, it remains unclear which factors determine distinct modes of migration. We recently found that the Ras-like Rap1b GTPase limits neutrophil transcellular migration. Rap1b restrains transcellular migration by suppressing Akt-driven invasive protrusions while leaving the paracellular route unaffected. Furthermore, Rap1b limits neutrophil tissue infiltration in mice and prevents hyper susceptibility to endotoxin shock. These findings uncover a novel role for Rap1b in neutrophil migration and inflammation. Importantly, they offer emerging evidences that paracellular and transcellular migration of neutrophils are regulated by separate mechanisms. Here, we discuss the mechanisms of neutrophil transmigration and their clinical importance for vascular integrity and innate inflammation.

Keywords: Rap1b; diapedesis; invadopodia-like protrusions; neutrophil; transcellular migration.

Figure 1.

The leukocyte extravasation cascade is controlled by sequential adhesive interactions between leukocytes and endothelial cells. This schema depicts various steps and the adhesive molecules that are involved at each step. The neutrophil extravasation cascade involves a sequence of tethering and rolling along the endothelium, followed by firm adhesion and arrest onto the endothelium. Subsequently, neutrophils undergo lateral migration or crawling on endothelial cells to find a permissive site for transmigration. It should be noted that subsequent to crossing the endothelial barrier, leukocytes undergo abluminal crawling between endothelial cells and pericytes before crossing the basement membrane and migrating within interstitial tissues [Reproduced from Ref. 51].

Figure 2.

The leukocyte diapedesis. It is now accepted that leukocytes can transmigrate at the junction between 2 endothelial cells (paracellular migration depicted in (A) or directly though endothelial cells (transcellular migration depicted in (B). Paracellular migration is accompanied by the disruption of the endothelial cell junction to form a gap through which the cells migrate. This is accompanied by the reorganization of an adhesive platform and the recycling of adhesive molecules via the LBRC. On the other hand, during transcellular migration, the endothelial cell junctions remain intact. Instead, neutrophil-endothelial cell contacts fuse (represented in blue) and remodel into a transcellular channel forming a path for leukocytes. This necessitates the recruitment of actin-rich membrane, ICAM-enriched caveola and vesicle vesicular vacuolar organelles as well as the recruitment of various adhesive molecules via the LBRC. In addition, the involvement of MMP activity is likely and may help remodeling the leukocyte-endothelial cell interaction to facilitate the formation of the transcellular channel. Several signaling mechanisms important for invasive protrusions and transcellular have been identified.

Figure 3.

Rap1b-deficient neutrophils exhibit enhanced invasive protrusion activity. (A) Electron microscopic analysis of neutrophil invasive protrusions on the surface of endothelial cells, highlighted by black arrows. (B) ECM degradation analysis using Oregon green–labeled gelatin surface. Representative images of gelatin (green) and neutrophils stained for F-actin (red). Previously published in J Exp Med. 2014. doi: 10.1084/jem.20131706