The absence of VIP production by donor pDCs leads to enhanced in vivo expansion and homing of donor T cells to GVHD target organs. (A-D) B10.BR mice were lethally irradiated and transplanted with 5 × 10³ FACS-purified HSCs and 10⁶ T cells from C57B/6J donors with 5 × 10⁴ FACS-purified pDCs either from wild-type or from VIP-KO donors. (A-B) Comparisons of the absolute numbers of donor T cells in recipient spleens on day +8 and day +15 after transplant. Seven biological replicates from 2 independent experiments were studied. (C-D) Percentage of donor Tregs on CD4 T cells on day +8 and day +15 posttransplant. Seven or 8 biological replicates from 2 independent experiments were analyzed. (E-G) Bioluminescent imaging images of luc⁺ T engraftment in allogeneic recipients. Balb/c (H2K^d) recipients were lethally irradiated and injected with 1 × 10⁶ luc⁺ T cells isolated from C57BL/6J donor mice in combination with 5 × 10³ FACS-purified HSCs and either 5 × 10⁴ wild-type pDC or 5 × 10⁴ VIP KO pDC. Left side radiance scale bar (1000 to 10 000 p/sc/cm²/sr) is for the whole-body image; right side radiance scale bar (10 000 to 100 000 p/sc/cm²/sr) is for the organ images (E). Representative bioluminescent imaging images of luc⁺ donor T cells in Balb/c recipients on day 14. (F) Radiance (p/sc/cm²/sr) of gut, lung, spleen, and paws in Balb/c recipients. N = 6 biological replicates per group, from 2 independent experiments. (G) Total photon flux (p/s) of B10.BR (H2K^k) mice transplanted with 1 × 10⁶ luc⁺ C57BL/6J T cells in combination with 5 × 10³ FACS-purified HSCs and either 5 × 10⁴ wild-type pDC, 5 × 10⁴ VIP KO pDC, or no donor pDC. Serial whole-body images of the same mice were taken at different time points posttransplant. N = 5 nice per group. Statistics: nonparametric Mann-Whitney U test, *P < .05, **P <.01.