IVIg induces a mild proliferation of Treg rather than their de novo generation. TCR-HA transgenic T cells were fractioned by cell sorting into a CD4⁺CD25⁻ Tconv population and a CD4⁺CD25⁺ population (Treg) and labeled with CFSE. Three groups of BALB/c mice received intravenous injections of Tconv cells, Treg cells, or a mixture of both (90% Tconv + 10%Treg). Mice were injected with 2 μg HA peptide in footpad the next day and received daily infusion of IVIg (open symbols) or PBS (solid symbols). They were killed 10 days later, and cells of the draining LN were analyzed by flow cytometry. (A) Cells were gated on CD4⁺CFSE⁺ population, and the mean percentages of (B) Foxp3⁺ and (C) Foxp3⁻ cells were compared (*P ≤ .05). (D) IVIg induces in vitro proliferation of Treg. 5 × 10⁴ cells/well of either Treg or Tconv were stimulated with 1 μg/mL of coated anti-CD3 Ab and 10 ng/mL of IL-2 and in the presence (open bars) or absence (solid bars) of IVIg. Proliferation was assessed by [³H] thymidine incorporation (**P < .001). Data are representative of results from 3 experiments. Error bars represent SD.