Researchers spur the immune system to rout malignancies
Abnormal cells often grow silently in the body. In most cases, though, they never get a chance to develop into cancer. The immune system homes in on these overly prolific cells and annihilates them first.
The cancers that people become aware of occur when aberrant cells somehow evade the immune system and continue to divide aggressively. If left unchecked, such cells form a malignant tumor.
Now, medical researchers have recruited a specialized cell from the immune system into the battle against cancer. A Swiss and German team has published results of a study in which this strategy successfully targeted the deadly skin cancer melanoma. A U.S. team used the same immune therapy to treat prostate cancer, the second leading cause of death in men in the United States.
"These studies are demonstrating that it may well be possible to immunize against cancer," says Steven A. Rosenberg of the National Cancer Institute in Bethesda, Md. Rosenberg has also tested an immune therapy on melanoma patients, with promising results.
Such immune strategies are sometimes called cancer vaccines. Traditionally, vaccines prime the body to fight off an infection before it gets started. Cancer vaccines, in contrast, rev up the immune system to rout an existing disease.
Researchers suggest that the vaccines will provide an alternative to chemotherapy, in which doctors give patients round after round of cell-killing drugs in the hope of destroying all the malignant cells. Chemotherapy may control a cancer for a time, but in many cases, cancerous cells break off from the main tumor, travel through the bloodstream, and eventually spread the disease.
Since the 1980s, researchers looking for a different approach have been laboring to find ways of boosting the immune system's response to cancer. Their hard work is finally starting to pay off. "It's not just a pipe dream," says Harmon Eyre, chief medical officer of the American Cancer Society (ACS) in Atlanta. "There's evidence now that this is working."
The element of the immune system that shows the most promise for cancer therapy is the dendritic cell, a type of white blood cell.
Cancer cells make proteins not usually found in healthy cells. Fragments of those abnormal proteins are called peptides and appear on the surface of the malignant cell like a red flag. Dendritic cells typically prowl the bloodstream in search of dead or dying cells. When it finds such a cell, the dendritic cell gobbles up the cellular debris and sprints to the lymph nodes, the command center of the immune system. This action has led Michael T. Lotze of the University of Pittsburgh to call dendritic cells the "track stars" of the immune system.
Upon reaching the lymph nodes, the dendritic cell, which has a number of arms, holds out the cancer cell peptide--much as a runner would pass a baton in a relay race. The other white cells scan the peptide, and those best able to kill the cancer cell proliferate rapidly, forming an army of killer T cells. Released into the bloodstream, the defenders seek out the flagged cancer cells and trigger a self-destruct mechanism in them. In effect, the T lymphocytes force the cancer cells to kill themselves.
Many cancers, unfortunately, evade recognition by dendritic cells and thus avoid an immune-mediated death. Recent experiments with cancer vaccines have attempted to encourage dendritic cells to take notice of a masked cancer threat and sound the alarm.
Melanoma, a cancer that is on the rise in many countries, including the United States, has been a target of potential immune therapies for almost a decade. Frank O. Nestle of the University of Zurich Medical School and Dirk Schadendorf of the University of Heidelberg in Germany recently used dendritic cells to attack melanoma, a cancer of the pigment-producing skin cells. …