"It does address one of the problems with T cell therapies, which is survival and persistence of these cells," says Darrel Irvine, a bioengineer at MIT who was not involved in the research. Irvine points out that one concern in enhancing T cells' growth is the possibility that growth could spin out of control. "But they have an elegant solution," he says. "Through this drug, they can turn it off."

Because the switch is made from RNA rather than proteins, it may also avoid another issue with existing modified T cells--triggering an attack from the immune system. "An important feature of their system is that it should not trigger a T cell response against the cells themselves," says Carl June, a pathologist at the University of Pennsylvania.

In addition to its implications for cancer treatment, the research is a significant step forward for synthetic biology. Most synthetic biology parts are created and tested in microbes, such as the research workhorses yeast and E. coli. But the new study showed this particular part can work in human cells as well.

Another of the switch's benefits is its modular design. Scientists can swap in different sensor and actuator components, so that different drugs can be used to trigger production of other immune molecules."We want to make them respond to inert molecules like vitamins, which are inexpensive and easy to use and nontoxic for the patient," says Jensen.

Smolke's team is now working on altering the system to work with other FDA-approved drugs, and engineering the switch into T cells with tumor recognition systems. Smolke says clinical testing of the technology is still a ways off--they need to show that the system is safe in animals, and that it can slow tumor growth in animal models of cancer.