TRANSCRIPTIONAL AND TRANSLATIONAL CONTROL OF TNF-ALPHA GENE-EXPRESSION IN HUMAN MONOCYTES BY MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II LIGANDS

While non-stimulated primary human monocytes exhibit very low levels o f tumor necrosis factor (TNF)-alpha mRNA, direct binding of the staphy lococcal exotoxin toxic shock syndrome toxin-1 (TSST-1) to major histo compatibility complex (MHC) class II molecules results in a fast (peak 1 h after stimulation), transient induction (sevenfold) of TNF-alpha mRNA. This induction correlates with a fourfold increase in transcript ion rates of the TNF-alpha gene, as detected by run-on assays, and doe s not require de novo protein synthesis. Mapping of DNase-I hypersensi tive sites (DHS) discloses two constitutive DHS, one located far upstr eam (within the TNF-beta promoter) and the other centered at -39 +/- 4 0 bp relative to the major TNF-alpha transcription start site, suggest ing that the TNF-alpha gene was transcriptionally competent even prior to MHC class II engagement. Furthermore, stimulation of human monocyt es with either TSST-1 or lipopolysaccharide increases the translationa l efficiency of TNF-alpha mRNA, as shown by a shift in the distributio n of this mRNA species in polysome gradients and the translation rates of TNF-alpha measured by immunoprecipitation from cells pulsed with [ S-35] methionine. The increase in translation efficiency of TNF-alpha mRNA is independent of the half-life of TNF-alpha transcripts, which u nder the conditions used is unchanged. Taken together, our data indica te that TNF-alpha expression is tightly regulated by MHC class II liga nds, both al the transcriptional and translational levels.