Mechanism of Diapedesis: Importance of the Transcellular Route

Abstract

The neutrophil transmigration across the blood endothelial cell barrier represents the prerequisite step of innate inflammation. Neutrophil recruitment to inflamed tissues occurs in a well-defined stepwise manner, which includes elements of neutrophil rolling, firm adhesion, and crawling onto the endothelial cell surface before transmigrating across the endothelial barrier. This latter step known as diapedesis can occur at the endothelial cell junction (paracellular) or directly through the endothelial cell body (transcellular). The extravasation cascade is controlled by series of engagement of various adhesive modules, which result in activation of bidirectional signals to neutrophils and endothelial cells for adequate cellular response. This review will focus on recent advances in our understanding of mechanism of leukocyte crawling and diapedesis, with an emphasis on leukocyte-endothelial interactions and the signaling pathways they transduce to determine the mode of diapedesis, junctional or nonjunctional. I will also discuss emerging evidence highlighting key differences in the two modes of diapedesis and why it is clinically important to understand specificity in the regulation of diapedesis.

Keywords: Diapedesis; Leukocyte extravasation cascade; Locomotion; PI3K; Rap1b; Rho GTPases; 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 moving across the endothelial barrier, leukocytes undergo abluminal crawling between endothelial cells and pericytes before crossing the basement membrane and migrating within interstitial tissues (Nourshargh, Hordijk, & Sixt, 2010).

Figure 2

The leukocyte diapedesis. (A) Representation of the transcellular cup made of clusters of leukocyte integrins and endothelial ICAM. Some studies have observed the formation of actin-microvilli embracing the transmigrating leukocyte. (B) It is now accepted that leukocytes can transmigrate at the junction between two endothelial cells (paracellular migration depicted in left panel) or directly though endothelial cells (transcellular migration depicted in right panel). 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 (gray in the print version)) 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. High ICAM density, high integrin signaling, low Rap1b, and subsequent high PI3K/Akt signaling trigger neutrophil invasive protrusions and transcellular migration.

Figure 3

Signaling mechanism of leukocyte locomotion. Upon firm adhesion, leukocyte flattens and adopts a polarized shape with a cell front enriched in polymerized F-actin protrusions and a rear enriched in actomyosin contractile filaments. This asymmetric shape is controlled by two major signaling networks. One is activated at the front leading to high levels of PI3K–PIP3 and subsequent Rac1-mediated actin polymerization formation. One at the cell rear leads to PTEN-mediated PIP2 and RhoA-driven actomyosin contraction. In addition, Cdc42 signaling at the front communicates with the rear of the cells by sending WASp signals to the uropod to activate integrins and RhoA signaling. Lastly, invasive protrusions mediated by Src signaling develop at the ventral and lateral part of the leukocyte to scan for permissive site of transmigration.