Leukocyte Extravasation Steps

Leukocyte extravasation is the process where leukocytes migrate to the infected or damaged tissue site from the circulatory system. Formerly, leuckocyte extravasation process was described with 3 main steps: rolling, activation and arrest (1). As basic research advanced, these 3 steps were refined and expanded. In the current literature, steps such as slow rolling and adhesion strengthening are accepted as additional steps.

The leukocyte extravasation starts with tethering (capture). In this step, P-selectin glycoprotein ligand 1 (PSGL1) binds to L-selectin. This event triggers leukocyte-leukocyte interactions and these interactions mediate tethering. Tethering also assists leukocytes which cannot produce ligands that bound to P-selectin or E-selectin to reached the damaged site. As selectins interact with their ligands, leukocytes are able to bind to the damaged endothelium while in blood flow. Endothelium is key for leukocyte rolling as the cells which form endothelium express E-selectin and P-selectin (1).

Integrins are also critical for the rolling and firm leukocyte adhesion steps. For instance, very late antigen 4 (VLA4 / a4b1 — integrin) supports the rolling step of the lymphocytes in the venules of central nervous system. Moreover, slow rolling[1] requires involvement b2 integrins together with E-selectin to occur in vivo (1).

Leukocyte arrest starts with the engagement of chemokines/chemoattractants. Furthermore, binding of integrins and immunoglobulin superfamily members[2] mediate this process. As inflammation occurs, endothelial cells get trigerred by cytokines and start to adhesion molecule expression and chemoattractant/chemokine synthesis. These synthesized chemokines trigger monocyte arrest. Integrins from the subfamilies of b1 and b2 can also play a key role in leukocyte arrest as differential expression of these integrins have a huge role in specificity in leukocyte arrest (1).

After leukocyte arrest, adhesion strengthening is mediated via integrin signalling. In 2006, Giagulli et al. demonstrated that outside-in signalling mechanism of integrins support the attached neutrophils which are under flow. Absence of this signalling mechanism led by b2 — integrin chain led to a serial detachment of neutrophils. Thus, there is a strong evidence of a step after arrest that stabilizes the leukocyte adhesion (2). Moreover, very late antigen-4 integrin’s (VLA4) interaction with CD44 antigen aids T-cell adhesion while under flow (3).

Crawling and transmigration are the last steps of leukocyte extravasation. Monocytes and neutrophils go inside of blood vessels by making a crawling motion with the regulation of Macrophage-1 antigen (MAC1) and CD54. If crawling is suppressed, transmigration happens through transcellular route instead of the paracellular route. After transmigration is done either through the paracellular or transcellular pathway, leukocytes arrive to site of infection from the circulatory system (4).

Figure 1. Leukocyte extravasation steps. Illustrated by Cengizhan Büyükdağ, derived from Ley et al. (1)

[1]a rolling type mostly seen in neutrophils with a velocity under 5 μm/sec.

[2] e.g. intercellular adhesion molecule-1 (ICAM1) or vascular cell adhesion protein-1 (VCAM1)

References

1- Ley, K., Laudanna, C., Cybulsky, M. I., & Nourshargh, S. (2007). Getting to the site of inflammation: The leukocyte adhesion cascade updated. Nature Reviews Immunology, 7(9), 678–689. https://doi.org/10.1038/nri2156

2- Giagulli, C., Ottoboni, L., Caveggion, E., Rossi, B., Lowell, C., Constantin, G., … & Berton, G. (2006). The Src family kinases Hck and Fgr are dispensable for inside-out, chemoattractant-induced signaling regulating β2 integrin affinity and valency in neutrophils, but are required for β2 integrin-mediated outside-in signaling involved in sustained adhesion. The Journal of Immunology, 177(1), 604–611.

3- Nandi, A., Estess, P., & Siegelman, M. (2004). Bimolecular complex between rolling and firm adhesion receptors required for cell arrest: CD44 association with VLA-4 in T cell extravasation. Immunity, 20(4), 455–465.

4- Phillipson, M., Heit, B., Colarusso, P., Liu, L., Ballantyne, C. M., & Kubes, P. (2006). Intraluminal crawling of neutrophils to emigration sites: a molecularly distinct process from adhesion in the recruitment cascade. The Journal of experimental medicine, 203(12), 2569–2575.