The present invention relates to genetically engineered cell populations derived from an immortalised/cancerous cell that do not express MHC class I molecules but that are modified to express membrane-bound IL-15, membrane-bound 4-1 BBL ligand, and at least one other membrane bound molecule, such as an interleukin or anti-CD3 antibody. The co-culture of said cells with a population of immune cells results in the activation and expansion of at least one subpopulation of immune cells. Expanded populations of NK cells derived from the co-culture of a mixed cell culture with the stimulatory cell lines may be used in methods of treating cancer or an infectious disease. In a separate embodiment, a plurality of nucleic acids for use in preparing the engineered cell population are provided.