Mysteries of a Therapy Unveiled

By Jason Socrates Bardi

"Nature loves to hide."

—Heraclitus of Ephesus, Fragments, circa 500 B.C.

Several years ago, a blood protein called activated protein C was found to lower the mortality in patients who acquire severe sepsis. Six months ago, activated protein C was approved in recombinant form by the Food and Drug Administration for use in severe sepsis after the protein proved to be effective at lowering mortality. Today the drug is sold under the brand name Xigris and is manufactured by Eli Lilly.

Despite its demonstrated efficacy, and despite the fact that scientists had pondered its beneficial therapeutic effect for a decade, exactly how activated protein C improved the prognosis for sepsis had remained a mystery.

Now a group of researchers at The Scripps Research Institute (TSRI) have described how activated protein C works. The group, led by TSRI Associate Professor Wolfram Ruf, has elucidated the signaling pathway through which activated protein C works—the receptors on the surface of cells it binds to and activates—and have published these results in one of the latest issues of the journal Science.

"For the first time," says Ruf, "we know which players are involved."

The Bacterial Death Knell

Septic shock, also known as sepsis and systemic inflammatory response syndrome, is a fast-moving, dramatic, and often fatal disease and is a major problem in U.S. hospitals and hospitals worldwide.

The prognosis for sepsis is dire. According to the National Institutes of Health, two percent of all hospital admissions suffer from sepsis, which typically has a 30 percent mortality rate and can be as high as 60 percent. Sepsis is one of the ten leading causes of both infant and adult mortality in the United States, and directly caused over 30,000 deaths in 1999 alone, according to the Centers for Disease Control and Prevention (CDC). And the prognosis is especially dire for children.

In a widespread infection, the response of the immune system is triggered by chemical components of microorganisms, such as endotoxin in certain bacteria. Endotoxin activates innate immune cells known as monocytes that induce inflammation at the site of infection. Monocyte/macrophages release pro-inflammatory cytokines like TNF-a and Interleukin-6 (IL-6), which makes a person feverish. This inflammation is necessary because without it, the body cannot fight off the bacterial infection.

"This is your first line of defense," says Ruf.

The inflammation that fights the infection can spiral out of control and lead to septic shock syndrome. One of the signs of severe sepsis is the activation of coagulation within the vasculature. Platelets disappear and fibrinogen is consumed. Many different parts of the body can be affected by this consumptive coagulopathy. Widespread coagulation in the blood vessels of vital organs leads to blockade of the microcirculation and whole organs can shut down. Frequently, the vital function of kidneys and lungs are affected. In patients with sepsis, the levels of inflammatory cytokines like IL-6 stay high.

"The organ failure is the major problem that results from the inflammation within the vasculature," says Ruf.

Therapeutic approaches that reduce inflammation proved to make the patients worse off than they were without treatment because the therapies compromised their immune response to the bacteria. For many years, the best treatment has been to administer broad antibiotics to try to quell the infection, and the rise of antibiotic-resistant bacteria in the last few decades has promised to exacerbate the problem.

The Protective Protein Pathway

Several years ago, clinical observations led to the idea of using protein C as a treatment for sepsis. In certain patients, particularly in children with severe meningococcal sepsis, there was a dramatic decrease in the level of activated protein C in the blood. Researchers thought that if this protein was disappearing during severe sepsis, perhaps administering it to patients would help.

And, indeed, that proved to be the case. Activated protein C fights inflammation without compromising the body's ability to fight the bacteria and lowers the mortality due to sepsis. But nobody knew how activated protein C was mediating anti-inflammatory reactions.

The Ruf laboratory, drawing on several years of work on related areas of research, figured out the pathway through which activated protein C works.

The related area that Ruf studies concerns the interaction of proteins that circulate in the bloodstream and are involved in the blood clotting cascade with "receptor" proteins, which are displayed on cells on the inner surface of blood vessels.

The blood clotting cascade is a tightly controlled mechanism designed primarily to prevent blood loss due to injury, but is also linked to diseases like cancer and sepsis. During a bacterial infection, when the monocytes are drawn to a tissue by the presence of endotoxins, they upregulate a cell surface receptor, tissue factor, which drives blood clotting. Tissue factor was shown to be an important contributor to the inflammation in sepsis by animal experiments that were carried out when Ruf was a postdoctoral fellow in Thomas Edgington's laboratory.


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TSRI scientists Matthias Riewald (left) and Wolfram Ruf led the effort that determined how activated protein C works. Photo by Kevin Fung.