Inhibition of COX-2 activity in DCs prevents IVIg-mediated Treg expansion. (A) Amount of secretion of PGE₂ by DCs. DCs were cultured in GM-CSF and IL-4 alone (Ctr) or with IVIg for 24 hours. In parallel, DCs were treated with COX-2 inhibitor NS-398 followed by treatment with IVIg (NS-398+IVIg). PGE₂ in cell-free culture supernatants was measured by ELISA (mean ± SEM, n = 7). (B) Percentage (mean ± SEM, n = 7) of CD4⁺CD25⁺FoxP3⁺ cells in the DC-CD4⁺ T cell cocultures. DCs were treated as explained above. These DCs were washed extensively and cocultured with CD4⁺ T cells for 4 days. Tregs (CD4⁺CD25⁺FoxP3⁺) were analyzed by flow cytometry. (C) DCs were cultured in GM-CSF and IL-4 alone (Ctr) or with DMSO (solvent control) followed by IVIg or with COX-2 inhibitor celecoxib followed by IVIg for 24 hours. The amount of secretion of PGE₂ by DCs was measured (mean ± SEM, n = 7). (D) Percentage (mean ± SEM, n = 4) of CD4⁺CD25⁺FoxP3⁺ cells in the DC-CD4⁺ T cell cocultures. DCs were pretreated with DMSO, NS-398, or celecoxib followed by IVIg and then cocultured with CD4⁺ T cells, *P < .05; **P < .01; ***P < .001.