Neutrophils and Emerging Targets for Treatment in Chronic Obstructive Pulmonary Disease

Mariska Meijer; Ger T Rijkers; Frans J van Overveld

Disclosures

Expert Rev Clin Immunol. 2013;9(11):1055-1068. 

In This Article

Infiltration of Inflamed Tissue

Neutrophil infiltration of inflamed tissue occurs in four steps (Figure 2). It begins with a neutrophil rolling past the endothelial surface. Next, the cell needs to be activated, followed by an arrest or adhesion close to the site of infection. Finally, the cell needs to pass through the endothelial layer into the tissue beyond it.[4] This mainly takes place at post-capillary venules, for they have a thin endothelial layer and a diameter that is sufficiently small for neutrophil-endothelial contact yet that is large enough to avoid blockages when neutrophils are in their adhesion phase.[1] At each stage, different adhesion molecules and chemokines are involved.

Figure 2.

Neutrophil infiltration. Sequence of events in the migration of neutrophils into inflamed tissue and the involvement of adhesion molecules, their ligands and proinflammatory cytokines.

The whole process starts with inflammatory signals such as TNF-α, IL-1β, IL-8 and IL-17. Upon encountering an invader, these cytokines originate from activated macrophages,[4] but also from endothelial and epithelial cells.[5] The stimulated endothelial cells start to express the adhesion molecules P-selectin, E-selectin, ICAM-1 and VCAM-1 on their luminal surface.[1,6] These adhesion molecules will bind P-selectin glycoprotein ligand 1 (PSGL-1) and L-selectin, which are both expressed on the surface of the neutrophil. Furthermore, E-selectin binds E-selectin ligand 1 (ESL-1) and CD44, which are also present on the neutrophil surface,[7] slowing down the rolling of the neutrophil. Binding of E-selectin to the CD44 receptor induces an only partially understood p38 signaling pathway which leads to redistribution of the PSGL-1 and L-selectin molecules on the surface of the neutrophil to form clusters. This binding further decreases the speed of neutrophil rolling.[8] The selectin signaling seems to be the most important factor in neutrophil rolling. Nonetheless, it was recently discovered that inhibition of endothelial NO production increases neutrophil rolling and adhesion, via what seems to be an alternative pathway that is independent of selectin signaling.[9]

The next stage, activation and arrest, is mediated by chemokine signaling and the binding of endothelial adhesion molecules ICAM-1 and ICAM-2 to integrins on the neutrophil surface.[10,11] In addition to adhesion molecule expression, the endothelial cells nearby the site of inflammation start secreting chemokines and express lipid chemoattractants on their luminal surface. Some of these chemoattractants were already present, but are now transported from the abluminal to the luminal surface.[12] More chemokines are released by activated macrophages and platelets and transported to the endothelial cells, where they are released.[13] Most chemokine effects can be exerted within milliseconds.[13]

After the neutrophil has been fully arrested, the adhesion of the neutrophil to the endothelial surface is strengthened. Stable binding of ligand to VLA-4 (α4β1-integrin) is rapidly increasing neutrophil infiltration of the inflamed tissue, implicating that internal signals are required for increased adhesion.[13] Several external factors influence the stability of this binding. VLA-4 binding to the endothelial surface is stimulated by the presence of several divalent cations (including Mg2+ and Ca2+), though it is not cation dependent. VLA-4 is unique in this respect, as it appears to be the only β1-integrin that can also be stimulated by Ca2+.[14] Furthermore, an increased curvature of the microvessels enhances VLA-4 binding to the endothelial surface because the curvature increases the sheer stress which enhances neutrophil attachment.[15]

As a result of the outside-in-signaling and the integrin reorientation, the neutrophil changes shape from a radial form to a flattened, polarized form with a distinct front and back. Accordingly, the chemical signaling is translated into cell-body displacements needed for the subsequent extravasation.[16] It has been proposed that this spreading consist of two stages, an initial slow stage when the neutrophil is not yet as tightly connected, and a second, more rapid stage when the neutrophil is tightly bound to the chemoattractants on the endothelial surface. In this process, the microvilli of the neutrophil disappear to make a closer contact possible. It is presumed that this is caused by actin rearrangement.[16]

Now that the neutrophil lies flattened and securely attached against the endothelial wall, it can move on to the final stage: extravasation. To initiate this process, the neutrophil crawls along the endothelial layer in a MAC-1- and ICAM-1-dependent manner in order to find preferred sites for transmigration.[13,17–19] A wide array of other integrins and adhesion molecules are involved in stimulating either transcellular or paracellular migration and each of these molecules can respond differently to inflammatory signals.[13] The duration of transmigration varies between individuals and is mainly influenced by the composition of the basement membrane.[13] Nonetheless, it appears that at some points along the basement membrane there are gaps between pericytes, allowing for easier extravasation.[20] At these sites, there are also higher levels of chemoattractants, which probably is a direct effect of these gaps, as they allow for easier diffusion.[13] The neutrophils are now close to the site of inflammation and will be further directed by chemokines until they have reached this site and can start exerting their functions. During migration, the neutrophils release substantial amounts of proteinases and ROS. This process is known as obligate proteolysis and is an important cause for bystander tissue damage in COPD.[21]

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