Transplantation of allogeneic TCD STAT1^−/− BM expands CD9⁻Siglec H^hi pDCs in recipient spleens and BM that show poor IFNα and IL-12 production, but enhanced IL-10 production. (A) CpG stimulation of pDCs isolated from the BM or spleen of nontransplanted (naïve) STAT1^+/+ and STAT1^−/− mice were compared with pDCs isolated 14 days after lethally irradiated C3H.SW recipients were transplanted with TCD STAT1^+/+ and STAT1^−/− BM. All pDCs were analyzed by flow cytometry, by first gating on CD11b⁻CD11c^int cells, and then by gating on B220⁺ and intracellular IFNα. A representative mouse is shown for each group. (B) The percentage of IFNα⁺ pDCs were multiplied by the splenic or BM count to calculate absolute number of cells. (C) pDCs isolated from each organ from nontransplanted (naïve) and alloHSCT recipients were also incubated with CpG and were analyzed for fold change compared with unstimulated controls for IFNα gene expression by quantitative RT-PCR, and (D) IFNα protein secretion by ELISA of cell supernatants from ex vivo cultures. In addition, pDCs isolated 14 days after alloHSCT were activated overnight ex vivo with CpG and were analyzed for (E) IL-12 gene expression and (F) IL-10 production by ELISA. (G) Lethally irradiated recipients received TCD miHA-mismatched BM from STAT1^+/+ or STAT1^−/− donors (129 → C3H.SW), and on day +14 pDCs from BM and spleen were analyzed for expression of CD9 and Siglec H, then the ratio of CD9⁻Siglec H^hi (tolerogenic) to CD9⁺Siglec H^lo (stimulatory) pDCs was calculated in each organ. N = 5 to 6 mice/group, data are representative from 1 of 2 similar experiments. *P < .05.