MHC class I transcription in different mouse cell types. Differential requirement for protein synthesis between B cells and macrophages

Although the MHC class II genes are known to be regulated transcriptionally, the relative rates of transcription of the four classical class II genes in different cell types have not been investigated. Using nuclear transcriptional analysis, we have investigated the transcriptional rates of the class II genes in the macrophage cell line WEHI-3, normal bone marrow-derived macrophages, L-929 cells, and two different B cell lymphoma lines. Kinetic analysis of class II transcription in IFN-γ-treated WEHI-3 cells revealed a 4-h delay, followed by a rapid increase in transcription over the next 20 h. A significant basal level of class II transcription, apparent in bone marrow derived macrophages, was also further enhanced by IFN-γ treatment. None of the class II genes were transcribed in L cells, whereas all class II genes were transcribed constitutively in the B cell lines. In both B cell lines and macrophages, the four class II genes were found to be transcribed at different rates from one another, but the only gene showing a consistent pattern in multiple experiments was A-α, always showing the highest rate. We also investigated the effect of protein synthesis inhibition on class II transcription. Cycloheximide treatment of WEHI-3 cells did not inhibit IFN-γ-induced transcription of the class II genes within 8 h, suggesting that IFN-γ acts on pre-existing trans-acting factors, rather than inducing their synthesis. In contrast, treatment of B cells with cycloheximide for 8 h significantly reduced class II transcription, suggesting that, in B cells, continuous synthesis of a labile-trans-acting factor is required for constitutive expression. These data support the notion that class II expression in B cells is mediated by trans-acting factors distinct from those found in macrophages.