Cell therapies often induce harmful reactions in patients, so researchers have devised a 'safety switch' and tested it in a common procedure — the infusion of T cells in patients with blood cancers. Malcolm Brenner at Baylor College of Medicine in Houston, Texas, and his colleagues worked with T cells that are frequently given to reduce the risk of relapse and boost immune recovery after stem-cell transplantation. They modified the cells to include a gene that encodes an inducible protein crucial to cell death.

When the T cells provoked the dangerous graft-versus-host disease (GVHD) in four children with acute leukaemia, an otherwise inert drug was infused to activate the protein. Within 30 minutes, more than 90% of the T cells had been eliminated. The GVHD resolved within 24–48 hours and had not returned one year later.

N. Engl. J. Med. 365, 1673–1683 (2011)