Cancer cells hide in plain sight. The healthy immune system is precisely tuned to kill diseased cells, but it often falters when it comes to cancer cells. Researchers have tried many ways of bolstering the immune system’s response to cancer – with limited success. But two new gene therapy approaches show promise.
One, now in human testing, uses gene therapy to help the immune system better recognize specific kinds of cancer cells. Another, already used to eradicate tumors in mice, uses gene therapy to alter stem cells, which in turn make immune cells that combat the specific cancer – a treatment that would last a lifetime.
The immune system is a complex network of many kinds of cells, some killers, some regulators. Researchers have tried intervening in several different places in this network to rally it against cancer. But “nobody has figured out a way to make cancer vaccines work,” says Steven Rosenberg, head of tumor immunology in the National Cancer Institute’s Center for Cancer Research.
Rosenberg’s group and another in southern California are trying a new approach by going after one of the most important cells in the immune system, the T cell. T cells have receptors that can recognize specific antigens – protein markers that can signal disease or infection – on other cells. Cancer cells bear antigens, but somehow, they shut down T cells’ activity. Cancer patients have T cells specific to their disease. These “look like perfectly good T cells but they don’t respond to cancer. We’re not sure how it happens,” says David Baltimore, president of the California Institute of Technology, who heads the California group.
Baltimore and Rosenberg are both using gene therapy to mobilize T cells against cancer. First, they remove T cells from a patient who has recovered from, for example, a melanoma tumor. From these cells, they clone a gene whose protein product, a T cell receptor, has a strong affinity for a melanoma antigen. Then they construct a virus that can deliver this gene to other T cells.
Rosenberg is currently conducting a clinical trial using a T cell receptor gene for melanoma in patients with advanced disease. Patients’ blood is drawn, their T cells are removed, incubated with the virus, then replaced. Rosenberg declined to discuss his results because it would jeopardize upcoming publication in a scientific journal. But he hopes to start clinical trials on other cancers soon: his group has isolated the receptor for an antigen made in large quantities in half of all common cancers, including those of the breast, lung, and prostate.
Baltimore’s lab research has gone beyond T cells to the immune cells’ precursor, stem cells. Throughout life, stem cells in the bone marrow replenish blood cells, including those involved in the immune system like T cells. Because stem cells are continuously replenishing the immune system, giving them a gene that combats cancer would mean “a life-long supply of tumor-specific T cells,” says Baltimore. Using the stem cell technique in mice with existing tumors (while providing mice with supplements of another type of immune cell) leads mice to completely destroy their tumors, says Lili Yang, a research in Baltimore’s lab.
Baltimore and Yang have started a research group to translate their mouse success into a human therapy. Like Rosenberg, they will perform their first clinical trial with patients with advanced melanoma, and will work first with T cells rather than stem cells. “We have to do T cells first to look at the safety and efficacy profile, and [more work in] animal models is needed to find out what the requirements are” before moving to stem cell therapies, says James Economou, chief of surgical oncology at the University of California at Los Angeles Medical School. (Other institutions involved in the planned clinical trials include Children’s Hospital Los Angeles and the University of Southern California.) Baltimore hopes the group will treat its first patient next year.
The group will address safety concerns by delivering a single gene called TK along with the receptor gene. The added gene will make the T cells visible on PET scans; the group will work on imaging the therapy with the inventor of PET, Michael Phelps, professor molecular and medical pharmacology at UCLA. The TK gene can also act as a suicide gene: if patients’ T cells turn cancerous, the researchers will give them a drug toxic to cells making TK.
Baltimore has high hopes for his gene therapy work. Rather than having patients undergo the painful process of a bone marrow transplant, which is necessary to remove and replace the stem cells, “the holy grail is just to put the virus in humans and have it home to the stem cells.” Baltimore won the Nobel prize in medicine in 1975 for his work on the kind of viruses used in gene therapy, and is now working on developing stem cell-targeted viruses.
If the current and planned clinical trials are a success, it would have a tremendous impact on cancer patients. “Although we can cure half of the people with cancer, half will die. That’s almost 600,000 deaths per year in the U.S.,” says Rosenberg. “So we’re looking for new treatments.”