(A) Transrepression assays in Lrf^–/–p19Arf^–/– immortalized MEFs transfected with empty vector, WT Lrf, or dimerization-deficient (I20A) Lrf expression vector along with mouse p19Arf luciferase-based reporter. Data represent mean with SD. (B) Growth curves of Lrf^+/+ and Lrf^–/– MEFs after infection with empty, WT, or I20A Lrf retroviral vector. y axis shows relative cell numbers measured by attenuance (D) at 595 nm. Error bars indicate SD. (C) Schematic representation of the CSEμmarCD19-IRES-GFP vector. (D) BM HSCs were harvested from CD45.2⁺ Lrf^Flox/Flox mb-1 Cre⁺ mice after 5-FU injection, infected with B cell–specific lentivirus, and transferred to CD45.1⁺ recipient mice. The recipient mice were immunized 2 months after transplantation and GC formation analyzed 7 days after immunization. (E) Representative FACS profiles of GCB cell development in recipient mice. GFP was predominantly expressed in B cells (B220⁺CD19⁺), confirming B cell–specific transgene expression. Only WT-Lrf–transduced CD45.2⁺ donor B cells (from Lrf^Flox/Flox mb-1 Cre⁺ mice) could give rise to GCB cells upon immunization. (F) Bar graph showing ratios between GFP-negative and -positive GCB cells within total CD45.2⁺ GCB cells for each group (4 mice per group). Error bars indicate SD. (G) Key roles of LRF in mature B cell lineage fate and GC reaction. LRF may regulate FOB versus MZB lineage fate determination by counteracting the Notch2/Dll1 pathway, while it regulates GCB cell proliferation and survival by repressing ARF.