Distinct BMI-1 and EZH2 expression patterns in thymocytes and mature T cells suggest a role for polycomb genes in human T cell differentiation

BMI-1 and EZH2 Polycomb-group (PcG) proteins belong to two distinct protein complexes involved in the regulation of hematopoiesis. Using unique PcG-specific antisera and triple immunofluorescence, we found that mature resting peripheral T cells expressed BMI-1, whereas dividing blasts were EZH2⁺. By contrast, subcapsular immature double-negative (DN) (CD4^-/CD8^-) T cells in the thymus coexpressed BMI-1 and EZH2 or were BMI-1 single positive. Their descendants, double-positive (DP; CD4⁺/CD8⁺) cortical thymocytes, expressed EZH2 without BMI-1. Most EZH2⁺ DN and DP thymocytes were dividing, while DN BMI-1⁺/EZH2^- thymocytes were resting and proliferation was occasionally noted in DN BMI-1⁺/EZH2⁺ cells. Maturation of DP cortical thymocytes to single-positive (CD4⁺/CD8^- or CD8⁺/CD4^-) medullar thymocytes correlated with decreased detectability of EZH2 and continued relative absence of BMI-1. Our data show that BMI-1 and EZH2 expression in mature peripheral T cells is mutually exclusive and linked to proliferation status, and that this pattern is not yet established in thymocytes of the cortex and medulla. T cell stage-specific PcG expression profiles suggest that PcG genes contribute to regulation of T cell differentiation. They probably reflect stabilization of cell type-specific gene expression and irreversibility of lineage choice. The difference in PcG expression between medullar thymocytes and mature interfollicular T cells indicates that additional maturation processes occur after thymocyte transportation from the thymus.