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Under normal conditions white blood cells, or leukocytes, circulate in the blood stream waiting to be called by damaged tissue to the site of injury or infection. Movement of the leukocytes into the damaged tissue from the blood requires the cells to squeeze between the endothelial cells that line the blood vessel walls. This rapid process is called Trans-Endothelial cell Migration (TEM), or diapedesis, and leads to the normal inflammation of the tissue. However, the white blood cell must migrate through the endothelial cells of the vessel wall without jeopardizing the integrity of the blood vessel. Just how this is accomplished has long been a source of debate among biologists and immunologists.

Now, a paper in Nature by scientists at Weill Cornell Medical College, led by Dr. William A. Muller, Professor of Pathology and Laboratory Medicine, makes great strides towards understanding how the white blood cell moves through the endothelial cells into the damaged tissue. In their process of exploration, the Muller team has made three significant discoveries. First, they have discovered a new internal membrane structure in the endothelial cells, called the endothelial surface-connected compartment, which lies just below the cell surface (the regular outer cell membrane). Second, they have observed that in normal endothelial cells, parts of this membrane shuttle in a wave-like manner between this compartment and the cell surface. Lastly, they have found a new role and function for the adhesion molecule PECAM (Platelet/Endothelial Cell Adhesion Molecule), which Dr. Muller's laboratory previously discovered. PECAM, which is essential for TEM, is found in abundance within this intracellular membrane and could serve as a traffic warden during diapedesis. These findings are crucial because they may provide new targets for anti-inflammatory therapy and lead to more focused treatments.

Inflammation is the body's normal response to an injury or infection, says Dr. Muller. However, inflammation that is misdirected or out of control is at the root of many human diseases, such as autoimmune diseases, arteriosclerosis, and asthma. Through understanding how the movement of white blood cells into the tissue is regulated, we hope to find ways to inhibit harmful inflammatory responses.

Dr. Muller explains that a score of adhesion proteins, molecules with the specific function of attaching one cell to another, help the moving white blood cell (or leukocyte) attach to the endothelial cells during migration, thereby keeping the blood vessel intact. One such molecule, the membrane protein PECAM, is apparently critical to this process of moving the leukocytes through the vessel wall acting as the major traffic warden during migration.

Using anti-PECAM antibodies with electron and confocal microscopes, Dr. Muller and colleagues discovered the novel membrane structure just below the cell surface membrane. Under normal conditions, the PECAM-containing membrane recycles evenly around the cell border. However, when white blood cells begin to migrate across the endothelial cells, the recycling PECAM becomes directed to the specific part of the endothelial cell border across which the leukocyte is migrating.

The newly observed internal membrane, or compartment of membrane just underneath the intercellular junction of the two endothelial cells, contains one third of the cell's total PECAM, notes Dr. Muller, and is constitutively recycling it to the cell surface and back.

White blood cells also express PECAM, leading Dr. Muller's team to hypothesize that the interaction between the PECAM on the endothelial cell and the PECAM on the white blood cell is essential for migration. In in vitro experiments, they prove this by using antibodies to block the PECAM on the white blood cells. They demonstrate that while the white blood cells can still attach to the endothelial cells, the recycling of PECAM is never redirected, and the leukocytes are unable to migrate into the tissue.

In summary, these observations reveal a new mechanism, membrane trafficking within the endothelial cell, with PECAM as the traffic warden, involved in white blood cell transmigration. The membrane recycling provides a fresh source of PECAM to adhere to and guide the migrating white blood cell while possibly expanding the endothelial cell surface to accommodate the extra cell volume.

Early data suggest that the recycling of PECAM may also serve to dilute the amount of VE-cadherin, another adhesion molecule at the junction between endothelial cells that, unlike PECAM, is antagonistic to the migration of leukocytes (in that it serves to maintain vessel-wall integrity). Further experiments are also being conducted to determine how this mechanism of PECAM and membrane recycling may influence other adhesion proteins involved in migration, such as JAM and CD99.

These results provide a new kind of target for anti-inflammatory therapy, observes Dr. Muller. Instead of altering levels of molecules in the blood that can affect even healthy cells, the therapy could specifically target only those cells that are misbehaving, effectively controlling the traffic and ticketing the 'traffic warden.'

Dr. William Muller, both an M.D. and Ph.D., is also an Attending Pathologist at NewYork-Presbyterian Hospital Weill Cornell Medical Center. First author Dr. Zahra Mamdouh and contributing author Dr. Xia Chen are members of the Department of Pathology and Laboratory Medicine at Weill Cornell Medical College. Dr. Frederick R. Maxfield, Chairman of Biochemistry, and Dr. Lynda M. Pierini, Assistant Professor of Microbiology & Immunology, at Weill Cornell, also contributed significantly to this work.

This research was supported by the National Institutes of Health, the Charles H. Revson Foundation, and an Atorvastatin Research Award from Pfizer/Parke Davis.