Thymic stromal lymphopoietin-activated plasmacytoid dendritic cells induce the generation of FOXP3⁺ regulatory T cells in human thymus

Human thymus contains major dendritic cell (DC) subsets, myeloid DCs (mDCs), and plasmacytoid DCs (pDCs). We previously showed that mDCs, educated by thymic stromal lymphopoietin (TSLP) produced by the epithelial cells of the Hassall's corpuscles, induced differentiation of CD4⁺CD25^- thymocytes into Forkhead Box P3⁺ (FOXP3⁺) regulatory T cells (T_R) within the medulla of human thymus. In this study, we show that pDCs expressed the TSLP receptor and IL-7 receptor α complexes upon activation and became responsive to TSLP. TSLP-activated human pDCs secrete macrophage-derived chemokine CCL-22 and thymus- and activation-regulated chemokine CCL-17 but not Th1- or Th2-polarizing cytokines. TSLP-activated pDCs induced the generation of FOXP3⁺ T_R from CD4 ⁺CD8^-CD25^- thymocytes, which could be strongly inhibited by Th1-polarizing cytokine IL-12 or Th2-polarizing cytokine IL-4. Interestingly, the FOXP3⁺ T_R induced by the TSLP-pDCs expressed more IL-10 but less TGF-β than that induced by the TSLP-mDCs. These data suggest that TSLP expressed by thymic epithelial cells can activate mDCs and pDCs to positively select the FOXP3⁺ T_R with different cytokine production potential in human thymus. The inability of TSLP to induce DC maturation without producing Th1- or Th2-polarizing cytokines may provide a thymic niche for T_R development.