PI3 Kinase and FOXO1 activity differentially control B cells in the germinal center light and dark zone

PI3K signaling and FOXO transcription factors play opposing roles at several B cell developmental stages. We show here abundant nuclear FOXO1 expression in the proliferative compartment of the germinal center (GC), its dark zone (DZ), and PI3K activity, downregulating FOXO1, in the GC light zone (LZ), where cells are selected for further differentiation. However, here FOXO1 is expressed in c-Myc+ cells destined for DZ reentry. Upon FOXO1 ablation by genetic means or induction of PI3K activity GCs become devoid of their DZ, due at least partly to the downregulation of the chemokine receptor CXCR4. While this is known to prevent proper cyclic selection of cells expressing high-affinity antibodies, the initiation of immunoglobulin switching is essentially dependent on FOXO1 activity. All samples were obtained from mouse germinal center (GC) B cells (Cgamma1-cre; YFP, P110*, FOXO1 f/f or PTEN f/f animals). Cells used for microarray analysis were FACS sorted cells.