A cell-based bioluminescence assay reveals dose-dependent and contextual repression of AP-1-driven gene expression by BACH2

Whereas effector CD4 + and CD8 + T cells promote immune activation and can drive clearance of infections and cancer, CD4 + regulatory T (T reg ) cells suppress their function, contributing to both immune homeostasis and cancer immunosuppression. The transcription factor BACH2 functions as a pervasive regulator of T cell differentiation, promoting development of CD4 + T reg cells and suppressing the effector functions of multiple effector T cell (T eff ) lineages. Here, we report the development of a stable cell-based bioluminescence assay of the transcription factor activity of BACH2. Tetracycline-inducible BACH2 expression resulted in suppression of phorbol 12-myristate 13-acetate (PMA)/ionomycin-driven activation of a luciferase reporter containing BACH2/AP-1 target sequences from the mouse Ifng + 18k enhancer. BACH2 expression repressed the luciferase signal in a dose-dependent manner but this activity was abolished at high levels of AP-1 signalling, suggesting contextual regulation of AP-1 driven gene expression by BACH2. Finally, using the reporter assay developed, we find that the histone deacetylase 3 (HDAC3)-selective inhibitor, RGFP966, inhibits BACH2-mediated repression of signal-driven luciferase expression. In addition to enabling mechanistic studies, this cell-based reporter may enable identification of small molecule agonists or antagonists of BACH2 function for drug development.