Anti–galectin-1 mAb can block the in vitro suppressive activity of naturally occurring human CD4⁺CD25⁺ Treg cells. Freshly isolated and CFSE labeled CD4⁺CD25⁻ and CD4⁺CD25⁺ T cells or a combination of both (1:1 ratio) were stimulated with anti-CD3/anti-CD28–coated beads for 5 days. (A) Number of viable (Annexin V⁻/7-AAD⁻) CFSE-labeled CD4⁺CD25⁻ T cells and CD4⁺CD25⁺ T cells cultured alone and the number of viable CFSE-CD4⁺CD25⁻ T cells in the cocultures. Data are mean ± SD, representative of 1 representative experiment of 10 experiments (*P < .015). (B) The number of viable CFSE-labeled CD4⁺CD25⁻ T cells was determined in each cell division. The CFSE-labeled CD4⁺CD25⁻ T cells were cultured alone or in combination with CD4⁺CD25⁺ T cells (1:1 ratio) with or without 0.1 μg/mL of anti–galectin-1 mAb. Data represent mean + SD of 4 experiments. Proliferation of anti-CD3/anti-CD28–activated (C) and allogeneic PBMNC-stimulated (D) CD4⁺CD25⁻ T cells and CD4⁺CD25⁺ T cells cultured alone and at decreasing ratios from 1:1 to 1:0.125, respectively. Cells were cultured for 5 days, 16 hours before harvesting cultures were pulsed with ³[H]TdR. Data are means ± SD from 1 experiment representative of 3 experiments that were performed (*P < .015).