Figure 1.

A low level of CD154 stimulation allowed maintenance of memory B lymphocytes. CD19-purified human peripheral blood B lymphocytes (3.75 × 10⁵ cells/ml) containing 40% CD27⁺ cells were stimulated in the presence of variable numbers of gamma-irradiated CD154⁺ L4.5 cells in complete medium containing 100 U/ml interleukin (IL)-4 for 7 days. Ratios corresponding to 1–100 B lymphocytes per L4.5 cell were obtained by varying the number of L4.5 cells while keeping constant the seeding number of B lymphocytes. (a) Expansion factors were evaluated using viable cell counts performed in triplicate at the indicated times. Error bars may be smaller than the symbols. (b) On day 5, the proportions of CD19⁺ CD27⁺ and CD19⁺ CD27^– cells were determined by flow cytometry, using phycoerythrin (PE)-conjugated anti-CD27 and peridinin chlorophyll protein (PerCP)-cyanin 5.5-conjugated anti-CD19 antibodies. PB, phenotype of peripheral blood B cells. Viable cells were ≥ 95% CD19⁺ throughout the 7 days of stimulation. These results are representative of two independent experiments.