Researchers have found a way to selectively block the ability of white blood cells to "crawl" toward the sites of injury and infection when such mobility drives disease, according to a study published today in The Journal of Experimental Medicine.
The results suggest a new treatment approach for autoimmune diseases like rheumatoid arthritis, lupus and multiple sclerosis, and for conditions made worse by misplaced inflammation, like atherosclerosis, stroke and transplant rejection, researchers said.
Where a single-celled amoeba moves to find food, human cells migrate as part of complex bodily functions like immunity. Disease-fighting cells for instance move toward bacteria and cells infected with viruses, which they target for destruction. Unfortunately, the same cells can mistakenly attack the body's own cells or drive inflammation too far, worsening the problem they rushed in to solve.
A team of researchers at the University of Rochester Medical Center has been studying proteins called integrins that enable T cells, a major subset of immune cells, to migrate. The integrin-related mechanisms described for the first time in the current paper suggest a way to shut down only those T cells currently in the act of disease-related migration, while leaving in place reserves needed in the likely event that another infection occurs during treatment. Making the mechanistic discoveries possible was a successful effort by the team to capture on video the first detailed images of fast-migrating T cells and the behavior of key proteins related to migration, which had been tagged with fluorescence. Twelve videos of T cells, and their key migration proteins, in action are part of the publication and are available online.
“There are many cases where it would be incredibly useful to precisely block integrin activation, and thus T cell migration,” said Minsoo Kim, Ph.D., assistant professor of Microbiology and Immunology within the David H. Smith Center for Vaccine Biology and Immunology at the Medical Center, and lead author of the article. “Good examples include when our immune system attacks our own cells, or rejects a lifesaving transplant or clogs our blood vessels by mistake. The problem is that past, system-wide attempts that block all integrin activation, like the multiple sclerosis drug Tysabri, shut down not only unwanted inflammation in one locale, but also vital immune defenses elsewhere, leaving patients vulnerable to infection.”
Two mechanisms make cell migration, or programmed directional movement, possible. The first, called chemotaxis, tells the cell which direction to move in. Cell surface proteins sense and follow chemicals and molecules they are attracted to toward wherever those attractants are most concentrated. T cells, named after the thymus (T) where they mature, move toward the byproducts of bacteria and viruses.
The second migratory mechanism is propulsion. In between infections and injuries, inactive T cells ride along with the bloodstream. T cells “realize” when they pass by part of a blood vessel wall close to the site of an injury or infection. Integrins on their surfaces unfold and grab onto key proteins on the surface of blood vessel wall cells (e.g. ICAM), resisting the surrounding blood flow. The T cells then pass through the vessel wall, and once outside the bloodstream, crawl along the tissue scaffolding toward the site of injury.
In a T cell at rest, integrins are distributed evenly over the entire surface of the T cell. When the cell gets ready to move, however, activated integrins cluster on the leading edge of the cell in the direction the cell wants to move in. They bind to their counterpart adhesion proteins like ICAM on the surface that the T cell is moving across. The T cell then contracts using its cell skeleton to pull itself over the leading edge integrins. Finally, the integrins on the trailing edge of the cell let go. Without precise changes that enable the front end to gain traction, and the tail to let go, the cell cannot migrate.