Neutrophil transendothelial migration: updates and new perspectives

Abstract

Neutrophils represent the first line of cellular defense against invading microorganism by rapidly moving across the blood-endothelial cell (EC) barrier and exerting effector cell functions. The neutrophil recruitment cascade to inflamed tissues involves elements of neutrophil rolling, firm adhesion, and crawling onto the EC surface before extravasating by breaching the EC barrier. The interaction between neutrophils and ECs occurs via various adhesive modules and is a critical event determining the mode of neutrophil transmigration, either at the EC junction (paracellular) or directly through the EC body (transcellular). Once thought to be a homogenous entity, new evidence clearly points to the plasticity of neutrophil functions. This review will focus on recent advances in our understanding of the mechanism of the neutrophil transmigration process. It will discuss how neutrophil-EC interactions and the subsequent mode of diapedesis, junctional or nonjunctional, can be context dependent and how this plasticity may be exploited clinically.

Figure 1.

The leukocyte extravasation cascade is controlled by sequential adhesive interactions between leukocytes and ECs. This schema depicts various steps of the extravasation cascade 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 ECs to find a permissive site for transmigration. During this step, the formation of the ICAM-1 adhesome and F-actin–rich filopodia emerging from ECs are critical prior to diapedesis. Diapedesis can occur at EC junctions (paracellular migration) or through the EC body (transcellular migration). Once they have crossed the perivascular basement membrane, neutrophils migrate into the interstitial tissue. They can also return to the blood circulation in a reverse migration process. PMN, polymorphonuclear neutrophil.

Figure 2.

Mechanism of leukocyte polarity and crawling. Upon firm adhesion, neutrophils flatten and adopt a polarized shape with a cell front enriched in polymerized F-actin protrusions (red) and a rear end (or back) enriched in actomyosin contractile filaments (green). This asymmetric shape is controlled by 2 major signaling networks. One is activated at the cell front and involves high levels of PI3K-PIP3 and subsequent Rac1/2-mediated actin polymerization formation. Another is activated at the cell rear and involves phosphatase and tensin homolog (PTEN)–mediated PIP2 and RhoA-driven actomyosin contraction. In addition, Myh9 and MKL1 activity are necessary for proper regulation of F-actin and actomyosin reorganization. Cdc42 signaling, while localized at the cell front, controls the uropod via WASp, which in turn enhances CD11b clustering and RhoA signaling. Lastly, invasive protrusions, which are promoted by Src and Akt signaling but limited by Rap1b, develop at the ventral and lateral part of the leukocyte to scan for a permissive site of transmigration. During polarization, PSGL-1 accumulates at the tip of the uropod and scans for activated platelets in order to maintain neutrophil polarity and crawling properties. In ECs, CD2AP limits the ICAM-1 adhesome. pMLC, phosphorylation of the myosin light chain; ROCK, Rho-associated protein kinase.

Figure 3.

Route of diapedesis. (A) The paracellular route. Paracellular migration is associated with the disruption of the EC junction to form a gap through which the cells migrate, and it involves the sequential engagement of several adhesive receptors, some of which are recruited to the EC plasma membrane via the LBRC. Paracellular migration is favored when the EC content in stress fibers is high, which helps open EC junctions. (B) The transcellular route. During transcellular migration, EC junctions remain intact. Instead, neutrophil-EC contacts fuse (represented in dark red) and remodel into a transcellular channel forming a path for leukocytes. This necessitates the recruitment of an actin-rich membrane, ICAM-enriched caveola, and vesicle vesicular vacuolar organelles (VVO) as well as the recruitment of various adhesive molecules via the LBRC. Several signaling mechanisms important for invasive protrusions and transcellular have been identified. High ICAM density, high integrin signaling, low Rap1b/Tiam1, and subsequent high PI3K/Akt signaling trigger neutrophil invasive protrusions and transcellular migration. CD2AP in ECs destabilizes ICAM clusters and limits transcellular migration. Transcellular migration is favored when the EC content in stress fibers is low but the EC junctions are tight. ESAM, endothelial cell–selective adhesion molecule.