The present invention generally relates to methods of preparing leukocytes, particularly T cells, ex vivo for use in immunotherapy, particularly cancer immunotherapy. More specifically, the invention relates to methods for the preparation of leukocytes exhibiting cytotoxic properties for use in adoptive cell transfer. The invention also relates to cells and compositions including them for cancer immunotherapy. The invention also relates to methods of immunotherapy, particularly cancer immunotherapy. The present invention relates to a method for producing a leukocyte that has an enhanced capacity for killing a target cell, the method including contacting the leukocyte with a PTPN2 inhibitor in conditions for enabling the inhibitor to inactivate PTPN2 in the leukocyte, thereby producing a leukocyte that has an enhanced capacity for killing a target cell. Preferably, the leukocyte is contacted with the PTPN2 inhibitor in the absence of a T helper cell.
