Interferon-gamma-induced MHC class I expression and defects in Jak/Stat signalling in methylcholanthrene-induced sarcomas

Seventy-eight uncloned tumour cell lines, each established from a primary sarcoma induced with methylcholanthrene in immunocompetent nu/+ BALB/c and C.B.-17 mice or in immunodeficient nu/nu BALB/c and severe combined immunodeficient (SCID) mice, were examined for sensitivity to interferon-gamma (IFN-gamma) as measured by tumour cell augmentation of major histocompatibility complex (MHC) class I expression. The tumour cells were cultured with IFN-gamma and their expression of K-d, D-d and L-d was measured by fluorescence-activated cell sorter analysis. All but three of the 78 tumour Lines up-regulated K-d, D-d and L-d to a variable degree in response to IFN-gamma, indicating that IFN-gamma resistance is not a common property of these sarcomas. The tumour cell lines varied greatly in their MHC class I expression before as well as after IFN-gamma stimulation. There was a tendency towards a higher MHC expression after IFN-gamma stimulation in tumour lines from immunocompetent mice compared to immunodeficient mice, but no common maximum MHC class I expression level was found for the 78 tumour cell lines. Three of the tumour lines, all from immunodeficient mice, completely failed to respond to IFN-gamma by up-regulating MHC class I expression. The same three also displayed absence of IFN-gamma-induced Stat1 beta tyrosine phosphorylation and low Stat1 alpha tyrosine phosphorylation, indicating a defect in the signal transduction pathway affecting phosphorylation of Stat1. These findings strongly suggest a link between defects in Stat1 phosphorylation and the failure to up-regulate MHC class I. In all tumour lines tested, the Stat1 Western blotting revealed a 78 kDa protein (p78) not previously described.