Regulation of Foxo1 expression is critical for central B cell tolerance and allelic exclusion

Resolving the molecular mechanisms of central B cell tolerance might unveil strategies that prevent autoimmunity. Here, using a mouse model of central B cell tolerance in which Forkhead box protein O1 (Foxo1) is either deleted or over -expressed in B cells, we show that deleting Foxo1 blocks receptor editing, curtails clonal deletion, and decreases CXCR4 expression, allowing high -avidity autoreactive B cells to emigrate to the periphery whereby they mature but remain anergic and short lived. Conversely, expression of degradation -resistant Foxo1 promotes receptor editing in the absence of self -antigen but leads to allelic inclusion. Foxo1 over -expression also restores tolerance in autoreactive B cells harboring active PI3K, revealing opposing roles of Foxo1 and PI3K in B cell selection. Overall, we show that the transcription factor Foxo1 is a major gatekeeper of central B cell tolerance and that PI3K drives positive selection of immature B cells and establishes allelic exclusion by suppressing Foxo1.