"We don't think the immune system has evolved to handle cancers," Schreiber notes. "Cancer is typically a disease of the elderly, who have moved beyond their reproductive years, so there probably was no evolutionary pressure for the immune system to find a way to fight cancer."
Schreiber, Smyth and Old speculate that from the immune system's point-of-view, a cancerous cell may look like a cell infected by an invading microorganism. To overcome the safeguards that prevent the immune system from attacking the body's own tissues, the tumor has to have a high level of immunogenicity, or ability to provoke an immune reaction. Cancer cells can reduce their immunogenicity by changing the materials they present to the immune system to more closely resemble those presented by normal tissue. This enables the third outcome of the immunoediting theory: escape.
Equilibrium sometimes may be a more common outcome of tumor-immune encounters than elimination. According to the researchers' theory, some of us may harbor dormant tumors that either developed spontaneously or from exposure to carcinogens. They propose that these quiescent tumors are unleashed only as we age or are exposed to environmental, infectious or physical stresses that cause a breakdown of the immune system.
To follow up, researchers plan a molecular-level investigation of what happens in tumors and the immune system during equilibrium. They also want to test their results' applicability both in humans and in different types of cancers.
"For example, we need to look at which tissues are regularly edited by the immune system and at how closely the immune system watches over these tissues," Schreiber says. "If you completely knock out the immune system in mice, you'll see
|Contact: Michael C. Purdy|
Washington University School of Medicine