Disclosed herein are novel compositions and methods for stimulation of and the production or expansion of natural killer (NK) cells. Numbers of NK cells can be increased following contact with exosomes modified with one or more stimulatory peptides. Methods and compositions for the production of exosomes, wherein the exosomes comprises stimulatory peptides are also described. Also described are methods of treating cancer using the disclosed NK-stimulating exosomes or NK cells stimulated by the disclosed methods.