Bystander T cells help provides activation signals for peripheral and tonsillar B cells. (A) Purified CD19⁺ peripheral blood B cells were CFSE-labeled and cultured for 7 days in the presence of CpG, anti–human IgM, and IL-2 (left) or irradiated autologous peripheral CD4⁺ T cells preactivated (right) or not (middle) with ChAGlyCD3 antibody. The CFSE dilution profiles of B cells (CD19⁺ gate) stained on day 7 for CD38 (top) or CD25 (bottom) are shown. (B) Tonsillar B-cell subsets in 2 representative tonsils as assessed by staining with CD19, CD20, CD38, and IgD antibodies. The quadrant corresponding to memory B cells is highlighted as the IgD-negative, CD38-negative/low subset (CD19⁺CD20⁺ gate). (C) Purified CFSE-labeled CD19⁺ tonsillar B cells were cultured for 7 days in the presence of CpG, anti–human IgM, and IL-2 (left) or irradiated autologous peripheral CD4⁺ T cells preactivated (right) or not (middle) with ChAGlyCD3 antibody. The CFSE dilution profiles of B cells (CD19⁺ gate) stained on day 7 for CD38 (top) and CD20 (bottom) are shown.