(A) AT-2 HIV– or CpGB- or resiquimod-activated pDCs (donor A) were incubated with naive CD4⁺ T cells (donor B) for 7 days. The responding T cells were then added to a primary allogeneic MLR (the composition of which is displayed in the inset) using PBMCs (A APC) as stimulatory cells and naive CD4⁺ T cells (B Tn) as responding cells. “CD4” refers to the naive T cells alone; “CD4 + APC” refers to the MLR of the naive CD4⁺ T cells to PBMCs. Proliferation was measured by thymidine incorporation after 5 days. *P < 0.01, Student’s t test, compared with proliferation of naive T cells in the MLR (far-left black bar). Results shown are representative of 5 experiments. (B) pDCs were activated by AT-2 HIV and incubated with naive CD4⁺ T cells. The T cells (T/pDC-HIV) were then added at different ratios to naive CD4⁺ T cells stimulated with anti-CD3 and anti-CD28 antibodies, and proliferation (thymidine incorporation) was measured after 5 days. *P < 0.01, Student’s t test, compared with proliferation of naive T cells alone. Results are representative of 3 experiments. (C) Phenotype of the day 7 T cell population generated by HIV-activated or unactivated pDCs, gated on CD4⁺ T cells. Expression of CD127 and Foxp3 is shown. Percentages of cells in each gate (Foxp3⁺CD127^lo) are indicated. (D) T cells generated as in B were separated into CD25^– and CD25⁺ populations by magnetic bead enrichment and then added to naive T cells (Tn) and stimulated as described in B. Proliferation was measured after 5 days. Results are representative of more than 5 experiments. (E) T cells stimulated as in B were added to naive CD4⁺ T cells in the upper chamber of a Transwell plate or in the same lower compartment as naive CD4⁺ T cells stimulated with anti-CD3 and anti-CD28 antibodies. Proliferation was measured after 5 days by thymidine incorporation. Results are representative of 3 experiments. *P < 0.01, Student’s t test, compared with proliferation of naive T cells alone.