From transcription to cell surface expression, the induction of MHC class II I-Aα by interferon-γ in macrophages is regulated at different levels

Abstract.

Using mouse bone marrow-derived macrophages we determined the role of interferon (IFN)-γ at the different steps in expression of the I-Aα chain of MHC class II molecules, from transcription to the cell surface. Levels of transcription, RNA, and protein were low in cells not stimulated with IFN-γ. Treatment with IFN-γ for 24 or 48 h induced an increase in mRNA levels (7- and 12-fold) that did not correlate with the increase in transcription (2.5- and 2.7-fold). The half-life of mRNA was not modified by IFN-γ. These data suggest a block at the level of translation. In fact, IFN-γ increased ribosome loading, which confirms regulation at the translational level. Treatment with IFN-γ increased protein synthesis (6-fold after 48 h) and level of expression at the cell surface (3- and 9-fold after 24 and 48 h, respectively). Interestingly, treatment with IFN-γ also increased the I-Aα protein half-life from 2 to 6–7 h. This is the first attempt to determine qualitatively and quantitatively the regulation of an inducible gene at all the putative levels of control. The data indicate that IFN-γ plays a critical role in MHC class II protein expression in macrophages through the regulation of different steps, from transcription to surface expression.