DOWN-REGULATION OF THE MHC CLASS-I ANTIGEN-PROCESSING MACHINERY AFTERONCOGENIC TRANSFORMATION OF MURINE FIBROBLASTS

Malignant transformation is often associated with genetic alterations providing tumor cells with mechanisms for escape from immune surveilla nce. Human and murine tumors of various origin as well as in vitro mod els of viral and oncogenic transformation express reduced levels of ma jor histocompatibility complex (MHC) class I antigens resulting in dec reased sensitivity to MHC class I-restricted cytotoxic T lymphocyte (C TL)-mediated lysis, We here investigate whether the suppressed MHC cla ss I surface expression of ras-transformed fibroblasts is due to dysre gulation of the genes of the antigen-processing machinery, the peptide transporters TAP-1 and TAP-2 and the proteasome subunits LMP-2 and LM P-7, and whether it can be restored by gene transfer. In comparison to parental NIH3T3 cells, the ras oncogenic transformants revealed reduc ed TAP and LMP mRNA expression and impaired function of these genes, l eading to deficient peptide transport and peptide loading of MHC class I molecules resulting in instable expression of the MHC class I compl ex on the cell surface. Enhanced H-2 surface expression due to stabili zation of the MHC class I complex could be achieved by culturing ras t ransformants at low, unphysiological temperature (26 degrees C) or by loading these cells with either exogenous human beta 2-microglobulin o r MHC class I-binding peptide alone or in combination. Furthermore, in terferon-gamma treatment was capable to enhance the expression of TAP, LMP and MHC class I molecules in both parental as well as ras-transfo rmed fibroblasts. Stable transfection of the human TAP-I cDNA into ras transformants caused a partial reconstitution of the peptide transpor t and an enhancement of the MHC class I surface expression, whereas th e level of MHC class I biosynthesis was not affected by TAP-1 overexpr ession in parental cells. Together these results point to the existenc e of an association between oncogenic transformation and deficiencies in the MHC class I antigen-restricted immunosurveillance, suggesting i ntervention strategies involving specific MHC class I-binding peptides or transfection of the LMP and/or TAP genes to overcome the expressio n of the immune escape phenotype.