The invention provides methods of enhancing an immune response to a cancer antigen in a mammal comprising inhibiting the activity of the foxo3a gene or gene product in dendritic cells in the mammal. The invention also provides methods of suppressing an immune response to an autoimmune disease antigen in a mammal comprising increasing the activity of the foxo3a gene or gene product in dendritic cells in the mammal. The invention also provides related methods of treating cancer and autoimmune diseases.