The origins of cancer immunotherapy date back more than 2 decades to when researchers explored ways to amplify the immune system to attack tumor cells. At the time, scientists hypothesized that tumors were able to evade immune surveillance because tumor cells lacked costimulatory molecules required to signal T cells to the presence of the tumor.

However, in the mid-1990s, James P. Allison, PhD, and his colleagues at the University of California, Berkeley, Cancer Research Laboratory discovered that rather than lack costimulatory molecules, tumor cells and other immune cells expressed inhibitory molecules that disabled T cells from recognizing and eliminating tumors.