How Stem Cells Exit The Bloodstream

New research from a laboratory at North Carolina State University has changed our understanding of how therapeutic stem cells exit the bloodstream.  Understanding this new process, which has been given the name “angiopellosis” may not only increase our understanding of how intravenous stem cells home to their target tissues, but also how metastatic cancer cells invade new sites.

When white blood cells are summoned to a site of infection, they exit the bloodstream by means of a rather well understood process called “Leukocyte extravasation” or “diapedesis.”.

Leukocyte extravasation mostly occurs in post-capillary venules, where hemodynamic shear force are low.  This process is characterized by 4 steps: 1)  “chemoattraction;” 2)  “rolling adhesion;” 3) “tight adhesion;” and 4)  “(endothelial) transmigration.”. If any of these steps are inhibited, diapedesis does not occur.

White blood cells or leukocytes phagocytose or gobble up foreign particles, produce antibodies, secrete inflammatory response triggers (histamine and heparin), and neutralize histamine.  In general, leukocytes defend an organism and protect it from disease by promoting or inhibiting inflammatory responses. Leukocytes do most of their specific functions in tissues and they use the blood as a transport medium to reach the tissues of the body.


Below is a brief summary of each of the four steps involved in leukocyte extravasation:

1) Chemoattraction

Upon recognition of and activation by pathogenic organisms, resident macrophages in the affected tissue release small signaling proteins called “cytokines” such as IL-1, TNFα and chemokines (small molecules that induce cell migration). IL-1, TNFα and other blood-based  molecules induce the endothelial cells that line blood vessels near the site of infection to express cellular adhesion molecules, including selectins.  Circulating leukocytes are localized to the site of injury or infection as a result of secreted chemokines.

2) Rolling adhesion

Sugar residues on they surfaces of circulating leukocytes bind to these selectin molecules on the inner wall of the blood vessels.  This interaction, however, is relatively modest in its binding strength.  The sugar-selectin interaction causes the leukocytes to roll along the inner surface of the vessel wall as transient bounds are constantly broken and reformed between selectins and cell-bound sugars.

The carbohydrate binding partner for P-selectin, P-selectin glycoprotein ligand-1 (PSGL-1), is an expressed by different types of leukocytes. The binding of PSGL-1 on the leukocyte to P-selectin on the endothelial cell allows for the leukocyte to roll along the endothelial surface. This interaction can be fine-tuned by the different ways that sugars are attached to PSGL-1.   These different forms of PSGL-1 that have distinct patterns of sugar attachment have unique affinities for different selectins.  This gives different leukocytes varying abilities to migrate to distinct specific sites within the body.

3) Tight adhesion

The chemokines released by macrophages activate the rolling leukocytes and induce them to synthesize surface integrin molecules.   Integrin molecules create high-affinity associations between cells and bind tightly to complementary receptors expressed on endothelial cells. This immobilized the leukocytes, despite the shear forces of the ongoing blood flow.

4) Transmigration

The internal cytoskeleton of the leukocytes are reorganizes that the leukocytes spread out over the endothelial cells. In this form, leukocytes extend pseudopodia and pass through gaps between endothelial cells.  This migratory step requires the expression of PECAM proteins on both the surface  of the leukocytes and the endothelial cells.  PECAM interaction effectively pulls the cell through the endothelium. Once through the endothelium, the leukocyte must penetrate the underlying basement membrane.  The mechanism by which the leukocytes does this remains a source of some dispute.  Once in the interstitial fluid, leukocytes migrate along a gradient of attractant molecules towards the site of injury or infection.

When stem cells are administered intravenously, they too Havre a similar ability to leave the bloodstream, but the means by which they do so was poorly understood.

Ke Cheng and colleagues examined zebrafish and used genetically engineered fish whose blood vessels glowed a fluorescent color.  Next, these fish were injected with leukocytes, and stem cells from rats, humans, and dogs that had been labeled with a red fluorescent protein.  These cells were followed by means of time-lapsed, three-dimensional light sheet microscopic imaging.  This technology allowed Cheng and others to view the stem cells as they left the blood vessels.

As predicted, the leukocytes exited the bloodstream by means of leukocytes extravasation.  The stem cells, however, were actively expelled from the blood vessels by the endothelial cells.  The endothelial cells membranes moved around the stem cells, surrounded them, moved them through the endothelial cells and then extruded them on the opposite side of the blood vessel.  This is a very different process than diapedesis in which the leukocyte is the active participant.  In the case of the stem cells, the endothelial cells are the active participants and the stem cells passively exit the bloodstream.  Cheng and company called this process angiopellosis.

Other differences between angiopellosis and diapedesis involved the time of the process.  Diapedesis can occur rather quickly whereas angiopellosis takes hours.  During diapedesis, one cell moves at a time, but during angiopellosis, several cells are moved at a time.

How effective of a method is this to leave the bloodstream?  If cancer cells used angiopellosis to facilitate metastasis, cent we inhibit it?

Further work should answer these important questions.  This work was published in the journal Stem Cells, 2016; DOI:10.1002/stem.2451.