The present invention relates to a method for preparing accessory cells, said accessory cells may themselves be used for preparing activated NK cells that may be used in various therapeutic protocols (e.g. cancer treatment). More particularly, the present invention relates to a method for preparing an accessory cell comprising the steps consisting of i) providing a cell and ii) inhibiting in said cell the expression of a gene encoding for a Killer-Cell Immunoglobulin-like Receptor(s) (KIR) ligand.