Unraveling the dual role of Pax5: Activation and repression in B cell lineage maintenance 9054

Abstract Description The transcription factor Pax5 is essential for B cell lineage maintenance. Conditional inactivation of Pax5 leads to loss of B cell identity, allowing differentiation into T cells, natural killer cells, and myeloid cells. While it is known that Pax5 activates B cell-specific genes and represses non-B cell programs, the precise gene network that needs to be controlled by Pax5 for stable B cell commitment remains unknown. To address this, we performed time-course single-cell transcriptomics and ATAC-sequencing on B cells after conditional Pax5 deletion. This analysis provided a comprehensive list of transcription factors (TFs) and transcriptional regulators controlled by Pax5. We then conducted multiplexed loss- and gain-of-function experiments to respectively test the functional roles of Pax5-activated and Pax5-repressed TFs and transcriptional regulators. Our findings suggest that Pax5 collaborates with the TF Bcl11a to repress a myeloid program in B cells, ensuring a stable B cell identity. Additionally, we discovered that continuous repression of the TF Zbtb7b is vital for maintaining the B cell fate. Overexpression of Zbtb7b disrupts the B cell transcriptional program, reverting it to an early hematopoietic stem-like state. Our results provide a comprehensive picture of the gene interaction network through which Pax5 maintains stable B cell commitment and may have broad implications for understanding immune cell development. Funding Sources Supported by: General Atlantic Pre-doctoral Fellowship Topic Categories Hematopoiesis and Immune System Development (HEM)