The HLA class II expression is controlled by the transcriptional activator CIITA. The transcription of CIITA is controlled by different promoters, among which promoter-IV is inducible by IFN-gamma. We analysed the regulation of HLA class II molecules by IFN-gamma in a large series of human
neuroblastoma cell lines. No induction of surface or intracellular HLA class II molecules and of specific
mRNA was observed, in all
neuroblastomas, with the exception of a nonprototypic cell line,
ACN. In a large subset of
neuroblastomas IFN-gamma induced expression of CIITA
mRNA, derived from promoter-IV, which was not methylated. In contrast, in another subset of
neuroblastomas, CIITA was not inducible by IFN-gamma and CIITA promoter-IV was either completely or partially methylated. Interestingly, the use of
DNA demethylating agents restored CIITA gene transcriptional activation by IFN-gamma, but not HLA class II expression. The defect of HLA class II was not related to alterations in RFX or NF-Y
transcription factors, as suggested by EMSA or RFX gene transfection experiments. In addition, the transfection of a functional CIITA
cDNA failed to induce HLA class II expression in typical
neuroblastoma cells. Confocal microscopy and Western blot analysis suggested a defective nuclear translocation and/or reduced
protein synthesis in CIITA-transfected NB cells. Altogether, these data point to multiple mechanisms preventing HLA class II expression in the
neuroblastoma, either involving CIITA promoter-IV silencing, or acting at the CIITA post-transcriptional level.