A plethora of mechanisms have been proposed to account for the dysfunctional antileukemic lymphocytes in AML, including the production of T-cell- and NK-cell-inhibitory factors by AML blasts,48 a deficient expression of NK-cell receptors on leukemic cells,49 inhibition of antileukemic lymphocytes by mononuclear phagocytes,4 and an impaired stimulatory interaction between the CD28 antigen expressed by T cells and contact antigens on AML blasts