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Khandoga, Alexander (2009): Mechanisms of leukocyte transendothelial and interstitial migration during inflammation in vivo. Dissertation, LMU München: Faculty of Medicine
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Abstract

The emigration of leukocytes from the circulation is a critical step during immune surveillance and inflammatory reactions that is governed by a coordinated interplay involving a spectrum of adhesion and signal molecules. While a great deal has been learned about the early steps of leukocyte recruitment, i.e. rolling and adhesion, little is known about the subsequent steps, transendothelial and interstitial migration when leukocytes migrate across the endothelial layer lining the blood vessel and move to the sites of inflammation. In particular, it is not fully understood which endothelial receptors are responsible for extravasation of leukocytes into the perivascular space. Moreover, the mechanisms of interstitial migration of leukocytes during inflammation remain to be clarified in vivo. In the first part of the study, we analyzed the role of ESAM for leukocyte migration in vivo. ESAM is a novel adhesion receptor which is specifically expressed at endothelial tight junctions and on platelets. Using RLOT intravital microscopy of the murine cremaster muscle, we have shown that IL-1β-induced leukocyte transmigration was reduced by about 50% in ESAM-deficient mice without affecting leukocyte rolling and adhesion. In summary, ESAM at endothelial tight junctions participates in the migration of neutrophils through the vessel wall. In the second part of the study, we investigated the role of two other recently discovered receptors, CD99 and CD99L2, for leukocyte migration. Similar to ESAM, these receptors are expressed at endothelial cell contacts but did not belong to any of the known protein families. We demonstrate that CD99 and CD99L2 mediate transendothelial migration of neutrophils in vivo without any effect on leukocyte rolling and adhesion. Finally, we show that the inhibitory effect of anti-CD99 and CD99L2 antibodies on cytokine-induced leukocyte transmigration in cremasteric venules is amplified in PECAM-1-/- mice. This fact suggests that a functional relationship between PECAM-1 and CD99/ CD99L2 might exist in mediating leukocyte transmigration. Taken together, our study provides the first evidence for a role of CD99 and CD99L2 in the process of leukocyte transendothelial migration in vivo. In the third part of the study, we established a novel approach allowing the visualization and analysis of directional leukocyte interstitial migration in vivo. Our technique combines RLOT and multicolor fluorescence microscopy with microinjection for local application of chemoattractants. In the mouse cremaster muscle, we show that microinjection of chemoattractants (MIP-1α and PAF) induced directional leukocyte polarization and migration. Combination of RLOT microscopy with fluorescence microscopy allowed simultaneous visualization and analysis of migratory behavior of different leukocyte subsets upon chemotactic stimulation. Moreover, this approach enabled an imaging of subcellular events such as mitochondria redistribution in single polarized interstitially migrating leukocytes in vivo. This technique opens new avenues for investigations of the mechanisms of interstitial migration of leukocytes as well as the observation of morphological changes and subcellular events in different leukocyte subsets during their interstitial migration in vivo.