KLRK1 and tropical spastic paraparesis: The decreased frequency of NK-like cells in patients with HAM/TSP, the documentation of an inverse correlation between the frequencies of these cells with proviral load in HC but not in HAM/TSP, and the documentation that in vitro blockage of NKG2D increases proviral load indicate that cytotoxicity mediated by NK cells may be an important mechanism in the control of proviral load and in preventing progression from HC status to HAM/TSP.