Stepwise reprogramming of B cells into macrophages.

Area of Science:

• Hematopoiesis and cell fate determination
• Molecular mechanisms of cell differentiation
• Transcription factor networks in immunity

Background:

• Hematopoietic lineages arise from multipotent progenitors, guided by lineage-restricted transcription factors.
• The specific transcription factors governing lymphoid versus myeloid cell fate decisions are not fully understood.
• Understanding these factors is crucial for controlling immune cell development and function.

Purpose of the Study:

• To identify transcription factors capable of reprogramming differentiated B cells into other hematopoietic lineages.
• To elucidate the molecular mechanisms by which cell fate decisions are reversed or altered.
• To investigate the role of C/EBPalpha, C/EBPbeta, and PU.1 in B cell to macrophage reprogramming.

Main Methods:

• Enforced expression of C/EBPalpha and C/EBPbeta in differentiated B cells.
• Analysis of key lineage-specific gene expression, including Pax5, CD19, PU.1, and Mac-1.
• Experiments utilizing PU.1-deficient pre-B cells to dissect the roles of individual transcription factors.

Main Results:

• Enforced C/EBPalpha and C/EBPbeta expression rapidly reprogrammed B cells into macrophages.
• C/EBPs inhibited the B cell factor Pax5, downregulating its target CD19.
• C/EBPs synergized with endogenous PU.1 to upregulate myeloid markers like Mac-1.
• Reprogramming required endogenous PU.1 for myeloid marker activation, though CD19 downregulation occurred independently.

Conclusions:

• C/EBPalpha and C/EBPbeta are potent drivers of B cell to macrophage reprogramming.
• The reprogramming process involves remodeling the transcription factor network by inhibiting B cell factors and activating myeloid factors.
• Endogenous PU.1 is essential for the full myeloid reprogramming of B cells, highlighting a critical cooperation between transcription factor families.