PMPs selectively target CCR6⁺HLA-DR⁺ Tregs prior to and during activation. Tregs were isolated from healthy donors and cultured for 7 days with anti-CD3/anti-CD28 mAb–coated beads, recombinant human IL-2, IL-15, IL-1β, and 1.5 × 10⁶ allogeneic PMPs isolated from platelet apheresis units of three healthy donors. Tregs were stained for surface markers and analyzed by flow cytometry at days 0, 1, 4, and 7. (A) CD41⁺ PMP-bound Tregs (n = 5 Treg donors) and the expression level of PSGL-1 on the Tregs (n = 3 PMP donors). (B) CCR6⁺HLA-DR⁺ Tregs present in the CD41⁻ and CD41⁺ subpopulations. (C) Example CCR6/HLA-DR dot plots of PMP-negative (CD41⁻) and PMP-positive (CD41⁺) Treg. Mean and SD for 3 PMP donors are shown. *P < .05. Results representative of 5 Treg donors are shown. (D) Day 7 cell proliferation of Pacific Blue succinimidyl ester–labeled Tregs cocultured with and without PMPs was assessed. The division (Div.) index, proliferation (Prol.) index, and percentage of divided cells were determined for total Tregs and CCR6⁺HLA-DR⁺ Tregs using FlowJo 7.6. Results are shown for Tregs cultured alone and Tregs cocultured with PMPs of 3 different PMP donors.