Mesenchymal stem cells transmigrate between and directly through tumor necrosis factor-α-activated endothelial cells via both leukocyte-like and novel mechanisms.

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.

Cellular and extracellular programming of cell fate through engineered intracrine-, paracrine-, and endocrine-like mechanisms.

A cell's fate is tightly controlled by its microenvironment. Key factors contributing to this microenvironment include physical contacts with the extracellular matrix and neighboring cells, in addition to soluble factors produced locally or distally. Alterations to these cues can drive homeostatic processes, such as tissue regeneration/wound healing, or may lead to pathologic tissue dysfunction. In vitro models of cell and tissue microenvironments are desirable for enhanced understanding of the biology and ultimately for improved treatment. However, mechanisms to exert specific control over cellular microenvironments remains a significant challenge. Genetic modification has been used but is limited to products that can be manufactured by cells and release kinetics of therapeutics cannot easily be controlled. Herein we describe a non-genetic approach to engineer cells with an intracellular depot of phenotype altering agent/s that can be used for altering cell fate via intracrine-, paracrine-, and endocrine-like mechanisms. Specifically, we show that human mesenchymal stem cells (MSCs) can be engineered with poly lactide-co-glycolic acid (PLGA) particles containing dexamethasone, which acts on cytoplasmic receptors. The controlled release properties of these particles allowed for sustained intracellular and extracellular delivery of agent to promote differentiation of particle-carrying cells, as well as neighboring cells and distant cells that do not contain particles.

Chemical engineering of mesenchymal stem cells to induce a cell rolling response.

Covalently conjugated sialyl Lewis X (SLeX) on the mesenchymal stem cell (MSC) surface through a biotin-streptavidin bridge imparts leukocyte-like rolling characteristics without altering the cell phenotype and the multilineage differentiation potential. We demonstrate that the conjugation of SLeX on the MSC surface is stable, versatile, and induces a robust rolling response on P-selectin coated substrates. These results indicate the potential to increase the targeting efficiency of any cell type to specific tissue.