Ikaros DNA-binding proteins as integral components of B cell developmental-stage-specific regulatory circuits

Ikaros DNA-binding proteins are critical for the development of lymphocytes and other hematopoietic lineages, but it remains unclear how they cooperate with other regulators of signaling and transcription to achieve ordered gene expression during development. Here, we show that Ikaros proteins regulate the pre-BCR component lambda 5 in a stage-specific manner. In pre-BI cells, Ikaros modulated lambda 5 expression in competition with the transcriptional activator EBF. This required Ikaros binding to the IgII1 (lambda 5) promoter and was abolished either by mutation of the Ikaros DNA-binding domain or by deletion of a single Ikaros site from the IgII1 promoter. At the transition from the pre-BI to pre-BI stage, the expression of the Ikaros family member Aiolos was upregulated and required for the efficient silencing of IgII1. Aiolos expression was controlled by pre-BCR signals via the adaptor protein SLP-65. Thus, pre-BCR signaling regulates Aiolos and the silencing of IgII1 via a developmental-stage-specific feedback loop.