Tracey Sciuto

In vivo, leukocyte transendothelial migration (TEM) occurs at endothelial cell junctions (paracellular) and nonjunctional (transcellular) locations, whereas in vitro models report that TEM is mostly paracellular. The mechanisms that control the route of leukocyte TEM remain unknown. Here we tested the hypothesis that elevated intercellular adhesion molecule-1 (ICAM-1) expression regulates the location of polymorphonuclear leukocyte (PMN) TEM. We used an in vitro flow model of tumor necrosis factor-alpha (TNF-alpha)-activated human umbilical vein endothelium cells (HUVECs) or an HUVEC cell line transfected with ICAM-1GFP (green fluorescent protein) and live-cell fluorescence microscopy to quantify the location of PMN adhesion and TEM. We observed robust transcellular TEM with TNF-alpha-activated HUVECs and ICAM-1GFP immortalized HUVECS (iHUVECs). In contrast, primary CD3+ T lymphocytes exclusively used a paracellular route. Endothelial ICAM-1 was identified as essential for both paracellular and transcellular PMN transmigration, and interfering with ICAM-1 cytoplasmic tail function preferentially reduced transcellular TEM. We also found that ICAM-1 surface density and distribution as well as endothelial cell shape contributed to transcellular TEM. In summary, ICAM-1 promotes junctional and nonjunctional TEM across inflamed vascular endothelium via distinct cytoplasmic tail associations.

Systemically administered adult mesenchymal stem cells (MSCs), which are being explored in clinical trials to treat inflammatory disease, exhibit the critical ability to extravasate at sites of inflammation. We aimed to characterize the basic cellular processes mediating this extravasation and compare them to those involved in leukocyte transmigration. Using high-resolution confocal and dynamic microscopy, we show that, like leukocytes, human bone marrow-derived MSC preferentially adhere to and migrate across tumor necrosis factor-α-activated endothelium in a vascular cell adhesion molecule-1 (VCAM-1) and G-protein-coupled receptor signaling-dependent manner. As several studies have suggested, we observed that a fraction of MSC was integrated into endothelium. In addition, we observed two modes of transmigration not previously observed for MSC: Paracellular (between endothelial cells) and transcellular (directly through individual endothelial cells) diapedesis through discrete gaps and pores in the endothelial monolayer, in association with VCAM-1-enriched "transmigratory cups". Contrasting leukocytes, MSC transmigration was not preceded by significant lateral migration and occurred on the time scale of hours rather than minutes. Interestingly, rather than lamellipodia and invadosomes, MSC exhibited nonapoptotic membrane blebbing activity that was similar to activities previously described for metastatic tumor and embryonic germ cells. Our studies suggest that low avidity binding between endothelium and MSC may grant a permissive environment for MSC blebbing. MSC blebbing was associated with early stages of transmigration, in which blebs could exert forces on underlying endothelial cells indicating potential functioning in breaching the endothelium. Collectively, our data suggest that MSC transmigrate actively into inflamed tissues via both leukocyte-like and novel mechanisms.