The general principle for tumor cells to escape from immune surveillance is to prevent tumor antigens from being recognized by the immune system. Many methods have been developed to increase the immunogenecity of the tumor cells. The most efficient methods are able to force tumor cells to present their own tumor antigens to the immune system. Stimulating Th cells by converting tumor cells into MHC class Ⅱ+/Ii- antigen presenting cells is one of the most efficient technologies. Using antisense methods, we suppress the expression of the Ii protein that normally co-expresses with MHC class Ⅱ molecules and blocks the antigenic peptide binding site of MHC class Ⅱ molecules during synthesis in the endoplasmic reticulum. In such tumor cells, the unprotected MHC class Ⅱ molecules pick up endogenous tumor antigenic peptides, which have been transported into the ER for binding to MHC class Ⅰ molecules. Simultaneous presentation of tumor antigens by both MHC class Ⅰ and Ⅱ molecules generates a robust and long-lasting anti-tumor immune response. MHC class Ⅱ+/Ii- tumor cells are potent tumor cell vaccines and also cure a significant number of animals with renal and prostate tumors. We have developed analogous human gene vectors that are suitable for most patients and cancers.