The invention relates to the field of immunotherapy, more in particular to a composition for use in the treatment of cancer. The invention also relates to a composition obtainable by such a method, such as a pharmaceutical composition. More in particular, the invention relates to an ex vivo method for obtaining a composition suitable for the treatment of cancer in a subject, comprising the steps of providing primary tumor cells derived from the subject, and ex vivo contacting the tumor cells with an inhibitor of a bromodomain and extra-terminal domain family member (BET inhibitor).