The invention provides a method of radiosensitizing a tumor in a subject by contacting the tumor with a cytokine or a nucleic acid molecule encoding a cytokine. The invention also provides a method of radiosensitizing a tumor in a subject by administering, at a site other than the tumor, a cell genetically modified to express a cytokine. The invention further provides a method of reducing the severity of a cancer in a subject by administering a cytokine at the site of the tumor or by immunizing the subject at a site other than the tumor with tumor cells genetically modified to express a cytokine, and treating the tumor with radiotherapy.