Suppression of B-cell proliferation by activated CD4⁺CD25⁺ T cells is not mediated by death receptors. (A) Fas:Fc has no effect on CD4⁺CD25⁺ T-cell–mediated suppression of B cells. LPS-primed B cells (5 × 10⁴) were cultured with activated CD4⁺CD25⁺ T cells in the presence (▴, ▪) or absence (▵, □) of anti-CD3 (2 μg/mL) and in the presence (▴, ▵) or absence of Fas:Fc (20 μg/mL; ▪, ▪) for 72 hours and the proliferation of B cells was measured as in Figure 1A. (B) Suppression of the proliferation of Fas-deficient B cells by CD4⁺CD25⁺ T cells. Fas-deficient B cells (▪, □) from MRL/lpr or WT (▴, ▵) B cells (5 × 10⁴) were stimulated with LPS and cultured with activated CD4⁺CD25⁺ T cells in the presence (▴, ▪) or absence (▵, □) of anti-CD3. (C-D) FasL-deficient CD4⁺CD25⁺ T cells suppressed the proliferation of both WT and Fas-deficient B cells. LPS-primed WT (C) or Fas-deficient (D) B cells (5 × 10⁴) were cultured with activated CD4⁺CD25⁺ T cells from WT (▪, ▴) or FasL-deficient (gld/gld) mice (□, ▵) in the presence (□, ▪) or absence (▴, ▵) of anti-CD3. (E) The induction of B-cell death is not dependent on Fas/FasL and TRIALl/TRAILR pathways. Preactivated CD4⁺CD25⁺ T cells were cultured with LPS-primed B cells at a 5:1 ratio for 8 hours in the presence (▪) or absence (□) of anti-CD3. Where indicated, Fas:Fc (20 μg/mL), or TRAILR(1-4):Fc (a mixture of 10 μg/mL R1:Fc, 0.5 μg/mL R2:Fc, 50 ng/mL R3:Fc, and 10 μg/mL R4:Fc) was included in the coculture. B-cell apoptosis was analyzed as described in Figure 2A. Results are expressed as means ± SD (n = 3). *P < .01.