Cancer immunotherapy by antisense suppression of Ii protein in MHC-class-II-positive tumor cells

This study was aimed at creating a more effective tumor cell vaccine by sup pressing Ii protein in the presence of MHC class II molecules within a canc er cell. Absence of the Ii protein, which normally blocks the antigenic-pep tide-binding site of MHC class II molecules at synthesis in the endoplasmic reticulum, presumably increases the range of cancer-related epitopes prese nted to CD4(+) helper T cells. Effective suppression of Ii protein was achi eved with an antisense, phosphorothioate oligonucleotide, which was selecte d on the basis of (1) the RNase H activation assay, (2) an assay for Ii pro tein suppression, and (3) a test for potency with respect to the extent of base sequence ("sequence walking"). The SaI murine sarcoma, which is MHC-cl ass-I+ and MHC-class-II-, Ii-protein, upon transfection With genes for eith er interferon gamma or the MHC class II transactivator, came to express MHC class II molecules and Ii protein. In each line of transfected tumor cells , the antisense oligonucleotide profoundly suppressed Ii protein in 35%-55% cells, without affecting expression of MHC class II molecules. Inoculation of mice with such Ii-protein-suppressed tumor vaccine cells,after either f ormaldehyde fixation or X-irradiation, led to much greater protection again st challenge with the parental SaI sarcoma than did inoculation with untrea ted cells. This approach to cancer cell vaccination can be applied in a wid e range of human tumors.