BLIMP1 controls GC B cell expansion and exit through regulating cell cycle progression and key transcription factors BCL6 and IRF4.

In B cells BLIMP1 is required for plasma cell differentiation. BLIMP1 is also expressed in some dark zone (DZ) germinal center (GC) B cells (GCBCs), yet the role of BLIMP1 in GCBCs is not understood. Here we generated mixed bone marrow (BM) chimeric mice using Prdm1+/+ CD19Cre and Prdm1fl/fl CD19Cre BM, allowing us to examine the cell-intrinsic functions of BLIMP1 in GCBC, independent of antibody or antigen levels. Strikingly, BLIMP1-deficient B cells quickly dominate GCs and persist for a much longer time compared to the wild type cells. BLIMP1 deficiency promotes positive selection of GCBC and enhances cell-cycle progression. In addition, BLIMP1 deficiency enhanced class-switching. Mechanistically, BLIMP1-deficient GCBC fail to downregulate BCL6 and to upregulate IRF4, indicating that BLIMP1 controls the expression of these transcription factors that mediate exit from the GC. These studies revealed unique functions of BLIMP1 in regulating GCBC responses that impact long-lived immune compartments. Overall design: Single Cell RNAseq expression profiling of Blimp1 KO and WT GCBC