STAT5, but not STAT3, induces proliferation in human B cells. (A) Total tonsil CD19⁺ B cells were transduced with LZRS-control-IRES-GFP (■) or LZRS-STAT3ER-IRES-GFP (●) and cultured on CD40L-L cells with IL-2 and IL-4 in the presence of tamoxifen (1 μM). The percentage of GFP-positive cells was determined continuously throughout the culture period. Data represent means (± SD) of 2 independent experiments. (B) Absolute numbers in time of CA-STAT5b⁺ tonsil B cells cultured in the presence or absence of CD40L-expressing L cells with or without IL-2 and IL-4. ■ represent plus CD40L plus cytokines; □, plus CD40L minus cytokines; ●, minus CD40L plus cytokines; and ○, minus CD40L minus cytokines. Data represent means (± SD) of 3 independent experiments. Identical results were obtained with 8 different donors. (C) Absolute numbers in time of CA-STAT5b⁺ PB B cells cultured in the presence or absence of CD40L-expressing L cells and with or without IL-2 and IL-4 as indicated in panel B. Data represent means (± SD) of 4 independent experiments. Genescan analysis for VH FR3-JH region. (D) L428: positive control for monoclonality. (E) Tonsil B cells: positive control for polyclonality. (F) CA-STAT5b⁺ tonsil B cells cultured on CD40L-expressing L cells. (G) CA-STAT5b⁺ PB B cells cultured on CD40L-expressing L cells. (H) CA-STAT5b⁺ tonsil B cells cultured without CD40L-expressing L cells. Analysis of the VH FR1-JH and VH FR2-JH region gave identical results. Similar results were obtained with 4 other tonsil CA-STAT5b⁺ B-cell lines and 2 other PB CA-STAT5b⁺ B-cell lines.