Abstract: The invention provides a method of preventing or reducing the severity of a cancer in a subject by stimulating the subject's immune response against the cancer. The invention provides, for example, a method of stimulating an immune response in a subject by administering to the subject tumor cells that are substantially similar to the subject's cancer cells and that are genetically modified to reduce or inhibit the expression of one or more immunosuppressive agents. The invention also provides a method of preventing or reducing the severity of cancer in a subject by stimulating the subject's immune response against the cancer by administering to the subject tumor cells that are substantially similar to the subject's cancer cells and that are genetically modified to prevent the expression of an immunosuppressive agents and, in combination with the genetically modified tumor cells, an immunostimulatory agent. The invention further provides compositions useful for practicing the claimed methods.