Researchers have long tried to understand how natural killer (NK) cells, the body's first line of defense against infection, recognize sick cells, such as those virally infected or transformed. In 1999, investigators found the ligands for an activating NK receptor, NKG2D. These ligands, called MICA and MICB, are expressed endogenously when cells are under stress, infected, or turning into tumors. In a form of assisted cellular suicide, the ligands migrate to the cell's surface where they send for the executioners: the NKG2D-carrying immune cells.
Since this discovery, research activity has erupted around NKG2D and its newly described ligands. A group from Seattle's Fred Hutchinson Cancer Research Center confirmed the role of this NK-cell receptor in stimulating an immune response through CD8+ T cells in one of this issue's Hot Papers.1 Prior to the finding, says Lewis Lanier, University of California, San Francisco, most thought NKG2D was involved only in NK-cell ...
