Til B lymphocytes preparation [18].Human B-lymphocytes CultureSwitched-memory B cells were seeded at 3 to 46105 cells/mL in 6-well Primaria plates (BD NT 157 Biosciences, Mississauga, Canada) in the presence of 0.56105 cells/cm2 c-irradiated CD154+ L4.5 cells [19,20]. The cells were cultured in IMDM supplemented with 10 ultra low IgG FBS containing 10 mg/mL insulin, 5.5 mg/mL transferrin, 6.7 ng/mL sodium selenite, 100 mg/ml streptomycin and 100 U/ml penicillin G (all from Invitrogen, Burlington, ON, Canada). The culture medium was supplemented with a mix of cytokines, namely 5 ng/mL IL-2 (,50 U/mL), 40 ng/mL IL-10 (,20 U/mL) (both from PeproTech, Rocky Hill, NJ, USA) and 100 U/mL IL-4 (R D Systems, Minneapolis, MN, USA) which sustained expansion and differentiation of human switchedmemory B cells [21,22]. Cultures were fed by replacing at least half of the culture medium every 2? days. Gamma-irradiated adherent L4.5 cells were renewed every 4? days to maintain a constant ratio of about 5 B cells per CD154+ L4.5 cell, which corresponds to about 500 to 2000 CD154 molecules per B cell [23]. Cell counts and viability were evaluated in triplicates by Trypan blue exclusion using a hemocytometer. Expansion factor (EF) was calculated, between time 2 (t2) and time 1 (t1), by using the cellular density (D) and total number of seeded cells (N1) according to the following formula: E = [D2 1]6N1. Generation time (tgen) was calculated within the initiation phase of the growth curve according to the formula: k = 1/ln2 (ln[Nt2] – ln[Nt1])/t2-t1 and tgen = 1/k. EBV DNA was monitored in expanded B cells atIsolation of Peripheral B LymphocytesCD19+ B lymphocytes were isolated from PBMNCs by negative selection using StemSepTM or EasySepTM CD19 cocktail following manufacturer’s instructions (Stem Cell Technologies, Vancouver, BC, Canada). CD19+ B lymphocytes’ purity, as determined by flow cytometry, was higher than 95 in all experiments reported herein. Switched-memory B cells, namely IgG+ or IgA+ cells, 
were further isolated using an EasySepTM custom cocktail containing antibodies directed against IgD and IgM (Stem Cell Technologies). This two-step selection provided untouched B lymphocytes. IgD+IgM+ and IgM+ cells depletion was higher than 95 in all the assays.Large-Scale Expansion of Human B LymphocytesFigure 2. Long-term expansion of switched-memory B lymphocytes. (A) Ten samples of switched-memory B lymphocytes were cultured for 35 to 65 days in the presence of IL-2, IL-4, IL-10 and CD154+ cells (L4.5 cells) at a ratio of five B cells per L4.5 cell. Expansion factors for the ten independent samples were plotted as a function of time (days) in culture. (B) Regression analysis of the ten exponential growth curves presented in (A) resulted in a correlation coefficient of 0.9965 corresponding to the equation y = 10 (0.1344x+0.5415). (C) The proportion of viable cells during long-term culture was monitored on a regular basis for each culture. (D) The mean value and standard deviation of viability ( ) showed a similar evolution for each samples. doi:10.1371/journal.pone.0051946.gthe end of the culture Biotin-NHS cost period by RT-PCR amplification of EBNA1, as previously published [16].Mini-scale CulturesWhen indicated, switched-memory B cell cultures were initiated as described above in 24- or 6-well plates in the presence of cirradiated L4.5 cells up to a total number of 4 to 56106 cells for their subsequent transfer into a Petri dish (100620 mm, BD Biosciences). Cells were.Til B lymphocytes preparation [18].Human B-lymphocytes CultureSwitched-memory B cells were seeded at 3 to 46105 cells/mL in 6-well Primaria plates (BD Biosciences, Mississauga, Canada) in the presence of 0.56105 cells/cm2 c-irradiated CD154+ L4.5 cells [19,20]. The cells were cultured in IMDM supplemented with 10 ultra low IgG FBS containing 10 mg/mL insulin, 5.5 mg/mL transferrin, 6.7 ng/mL sodium selenite, 100 mg/ml streptomycin and 100 U/ml penicillin G (all from Invitrogen, Burlington, ON, Canada). The culture medium was supplemented with a mix of cytokines, namely 5 ng/mL IL-2 (,50 U/mL), 40 ng/mL IL-10 (,20 U/mL) (both from PeproTech, Rocky Hill, NJ, USA) and 100 U/mL IL-4 (R D Systems, Minneapolis, MN, USA) which sustained expansion and differentiation of human switchedmemory B cells [21,22]. Cultures were fed by replacing at least half of the culture medium every 2? days. Gamma-irradiated adherent L4.5 cells were renewed every 4? days to maintain a constant ratio of about 5 B cells per CD154+ L4.5 cell, which corresponds to about 500 to 2000 CD154 molecules per B cell [23]. Cell counts and viability were evaluated in triplicates by Trypan blue exclusion using a hemocytometer. Expansion factor (EF) was calculated, between time 2 (t2) and time 1 (t1), by using the cellular density (D) and total number of seeded cells (N1) according to the following formula: E = [D2 1]6N1. Generation time (tgen) was calculated within the initiation phase of the growth curve according to the formula: k = 1/ln2 (ln[Nt2] – ln[Nt1])/t2-t1 and tgen = 1/k. EBV DNA was monitored in expanded B cells atIsolation of Peripheral B LymphocytesCD19+ B lymphocytes were isolated from PBMNCs by negative selection using StemSepTM or EasySepTM CD19 cocktail following manufacturer’s instructions (Stem Cell Technologies, Vancouver, BC, Canada). CD19+ B lymphocytes’ purity, as determined by flow cytometry, was higher than 95 in all experiments reported herein. Switched-memory B cells, namely IgG+ or IgA+ cells, were further isolated using an EasySepTM custom cocktail containing antibodies 
directed against IgD and IgM (Stem Cell Technologies). This two-step selection provided untouched B lymphocytes. IgD+IgM+ and IgM+ cells depletion was higher than 95 in all the assays.Large-Scale Expansion of Human B LymphocytesFigure 2. Long-term expansion of switched-memory B lymphocytes. (A) Ten samples of switched-memory B lymphocytes were cultured for 35 to 65 days in the presence of IL-2, IL-4, IL-10 and CD154+ cells (L4.5 cells) at a ratio of five B cells per L4.5 cell. Expansion factors for the ten independent samples were plotted as a function of time (days) in culture. (B) Regression analysis of the ten exponential growth curves presented in (A) resulted in a correlation coefficient of 0.9965 corresponding to the equation y = 10 (0.1344x+0.5415). (C) The proportion of viable cells during long-term culture was monitored on a regular basis for each culture. (D) The mean value and standard deviation of viability ( ) showed a similar evolution for each samples. doi:10.1371/journal.pone.0051946.gthe end of the culture period by RT-PCR amplification of EBNA1, as previously published [16].Mini-scale CulturesWhen indicated, switched-memory B cell cultures were initiated as described above in 24- or 6-well plates in the presence of cirradiated L4.5 cells up to a total number of 4 to 56106 cells for their subsequent transfer into a Petri dish (100620 mm, BD Biosciences). Cells were.