The human BCL6 [19, 20] and BCL-XL [21] encoding cDNAs were further cloned in a LZRS retroviral expression vector, around a T2A cleavage-promoting peptide sequence and upstream a cassette containing an internal ribosome entry site (IRES) and the gene encoding GFP. We therefore obtained a LZRS vector in the following configuration: BCL6-T2A-BCLXL-IRES-GFP [9] . Transfection of Phoenix-GALV packaging cells and virus production were performed as previously described [22] . Before retroviral transduction, memory B cells were activated on c-irradiated (50 Gy) mouse L cell fibroblasts stably expressing CD40L (CD40L-L cells) in the presence of 25-50 ng/ml recombinant mouse interleukin-21 (rmIL-21, R&D systems) for 36 h [19] . The B cells were washed, mixed with retroviral supernatants in Retronectin-coated plates (Takara), centrifuged at room temperature for 60 min at 360 g, and subsequently incubated with the retroviruses at 37uC, 5% CO 2 for 6-8 h. Transduced B cells were maintained in co-cultures using CD40L-L cells (10 5 cells/ml) and in standard IMDM (Gibco) culture medium supplemented with 8% fetal bovine serum (FBS; HyClone), penicillin/streptomycin (Roche) and 25 ng/ml rmIL-21.