Getting leukocytes to the site of inflammation

Abstract

There is no "response" in either the innate or adaptive immune response unless leukocytes cross blood vessels. They do this through the process of diapedesis, in which the leukocyte moves in ameboid fashion through tightly apposed endothelial borders (paracellular transmigration) and in some cases through the endothelial cell itself (transcellular migration). This review summarizes the steps leading up to diapedesis, then focuses on the molecules and mechanisms responsible for transendothelial migration. Surprisingly, many of the same molecules and mechanisms that regulate paracellular migration also control transcellular migration, including a major role for membrane from the recently described lateral border recycling compartment. A hypothesis that integrates the various known mechanisms of transmigration is proposed.

Figure 1

Sequential steps in leukocyte emigration are controlled by interactions between specific molecules on leukocytes and their counter-receptors on endothelial cells. The steps in leukocyte emigration described in the text are depicted schematically here. For each step, the molecules that interact between leukocyte and endothelial cell are printed in the same color. This diagram is not all-inclusive, and other molecules may mediate each of these events for distinct leukocyte types under different inflammatory conditions. Endothelial cells are depicted in blue and the leukocytes in pink with blue nuclei. Protrusions on the leukocyte surface in the capture/tethering step represent microvilli that bear L-selectin and very late antigen 4 (VLA-4). The lightning bolt at the activation step represents the triggering of inside-out activation of leukocyte integrins by signals from the endothelium and endothelial surface via G protein–coupled receptors. Parentheses around LFA-1 and ICAM-2 in the locomotion step indicate that these molecules have been shown to play a role in this step for monocytes in vitro but have not been verified in vivo. The basal lamina is depicted as the orange strip separating the underside of the endothelium from the remainder of the extracellular matrix (ECM). Many members of the β1 family of integrins are involved in migration through the ECM and are represented by “β1.” Most of the β1 ligands are not listed on the ECM side of this cartoon. ESL-1, E-selectin ligand 1; s-Le^x sialyl-Lewis x antigen; VCAM-1, vascular cell adhesion molecule 1; PSGL-1, P-selectin glycoprotein ligand 1; LFA-1, lymphocyte function-associated antigen 1; Mac-1, macrophage-1 antigen; PAF, platelet activating factor; PAF-R, PAF receptor; ICAM, intercellular adhesion molecule; PECAM-1, platelet/endothelial cell adhesion molecule 1; PECAM-1 (d1/2), PECAM-1 domains 1 and 2 are responsible for this step; PECAM-1 (d6), PECAM-1 domain 6 is responsible for this step; α6β1, VLA-6, the integrin responsible for binding laminin; HSPG, heparan sulfate proteoglycan.

Figure 2

Schematic view of the movement of the lateral border recycling compartment (LBRC) during paracellular transmigration. Note: For Figures 2–4, the drawing is not to scale. Endothelial cell thickness is exaggerated to allow depiction of the LBRC and various signaling molecules. In reality the endothelial cell is ≤ 0.5 μm thick, while the leukocyte is ~7 – 10 μm in diameter. (a) Constitutive recycling of the LBRC is ongoing as the leukocyte locomotes toward the intercellular border. (b) Upon reaching the apical side of the endothelial border, leukocyte platelet/endothelial cell adhesion molecule (PECAM) engages endothelial cell PECAM, which is enriched in plasma membrane at the cell borders (and in the LBRC). (c) In cases of PECAM-dependent transmigration, leukocyte PECAM-endothelial cell PECAM interaction triggers a signal(s) (lightning bolt) that redirect recycling of the LBRC to the site of leukocyte engagement. In cases of PECAM-independent transmigration, some other interaction triggers this signal. (d) Membrane from the interconnected vesicles of the LBRC move to surround the leukocyte. (e) Recruitment of the LBRC continues as the leukocyte passes across the endothelial cell border. Modified from Wedlich D, ed. Cell Migration in Development and Disease. Fig. 13.2, page 246. Published 2005. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Figure 3

A unified schematic view of paracellular transendothelial migration. (See text for details and Figure 2 legend for note on the scale.) (a) Clustering of interecellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) through engagement of their leukocyte integrin counterreceptors (αβ in diagram) initiates activation of Src, Rho A, and Rac-1, as well as increased cytosolic free calcium ion (↑[Ca⁺²]_i). Phosphorylation of cortactin by Src stimulates f-actin rearrangements in the cortical cytoplasm, which facilitates more ICAM-1 clustering. (b) These signals lead to activation of myosin light chain kinase (MLCK), inactivation of protein phosphatase 1c (PP1c) through Rho kinase (ROCK), and phosphorylation of vascular endothelial specific cadherin (VE-cadherin), inducing release of the associated catenins (β-catenin, plakoglobin, p120). (c) Leukocyte PECAM engagement of endothelial cell PECAM and/or other leukocyte–endothelial cell interactions at the apical surface of the endothelial border activate kinesin molecular motors in the endothelial cell and stimulate targeted trafficking of lateral border recycling compartment (LBRC) membrane to the vicinity of the leukocyte. (d) Targeted trafficking of LBRC membrane continues as the leukocyte moves into the border between endothelial cells, now enlarged by the contribution of membrane from the LBRC. This process continues until transmigration is complete. CaM, calmodulin; ROS, reactive oxygen species; circled P, phosphorylated state. Reprinted by permission.

Figure 4

Schematic view of the movement of the lateral border recycling compartment (LBRC) during transcellular transmigration. (See Figure 2 legend for note on the scale.) (a) The signal that recruits targeted recycling to the vicinity of the leukocyte (lightning bolt) is given while the leukocyte is on the apical surface of the endothelial cell. (b) This causes targeting of the LBRC membrane along cortical microtubules toward the leukocyte. (c) A single fusion event may be sufficient to allow the entire interconnected LBRC to come into contact with the leukocyte. (d–f) As the leukocyte passes through the endothelial cell, additional LBRC membrane is recruited to surround the leukocyte. The transmigration pore is thus essentially a para-junctional junction lined by molecules the leukocyte needs to interact with (eg, PECAM, CD99, JAM-A) and lacking those it needs to bypass (eg, VE-cadherin and associated proteins). (g) At the basal surface a second fusion event may be necessary to allow the leukocyte to complete its migration (h) across the endothelial cell. Reprinted by permission.