Tolerance and chimerism is dependent on Tregs, their expression of IL-10, and NKT cells. (A) Experimental scheme: WT or Jα18−/− BALB/c host mice were given C57BL/6 neonatal heart transplants on day 0. ATS was injected intraperitoneally on days 0, 2, 6, 8, and 10. TLI was given over 14 days at 10 doses of 240 cGy each. Some hosts were given a single dose of anti-CD25 mAb intraperitoneally on day −7 to deplete Treg cells. On day 15, 50 × 106 C57BL/6 bone marrow cells were injected intravenously without, or with CD4+CD25+ T cells sorted from WT, or IL-10−/− BALB/c mice. (B) Heart graft survival in nondepleted TLI/ATS WT, or nondepleted Jα18−/− BALB/c mice given C57BL/6 heart transplants and bone marrow cell infusion (nondepl WT, n = 9 mice), (nondepl Jα18−/−, n = 10 mice), or nondepleted BALB/c hosts given DBA/2 heart transplants without bone marrow cell infusion (DBA/2 without BM, n = 10 mice). Nondepl WT versus nondepl Jα18−/−, (P < .01) by log-rank Mantel-Cox test. (C) Heart graft survival in Treg depleted and TLI/ATS conditioned WT hosts given heart transplants and bone marrow cell infusion (CD25 depl WT, n = 12 mice), Treg depleted and TLI/ATS conditioned WT hosts given heart transplants and bone marrow cell infusion with add-back of sorted 1 × 106 CD4+CD25+ T cells from WT BALB/c mice (CD25 depl WT + WT Treg, n = 11 mice), Treg depleted TLI/ATS Ja18−/− BALB/c mice given heart transplants and bone marrow cell infusion (CD25 depl Ja18−/− + WT Treg, n = 8) with add-back of sorted 1 × 106 CD4+CD25+ T cells from WT BALB/c mice, and Treg depleted and TLI/ATS conditioned WT hosts given heart transplants and bone marrow cell infusion with add-back of sorted 1 × 106 CD4+CD25+ T cells from IL-10−/− BALB/c mice (CD25 depl WT + IL-10−/− Treg, n = 9 mice). CD25 depl WT versus CD25 depl + WT Treg (P < .001); CD25 depl WT + WT Treg versus CD25 depl Jα18−/− + WT Treg (P < .001), CD25 depl WT + WT Treg versus + IL-10−/− Treg (P < .001) by log-rank Mantel-Cox test. (D) Mean percentages of donor type cells on day 28 after marrow transplantation in nondepleted WT BALB/c mice given heart and marrow transplants (nondepl WT, n = 8 mice), CD25 depleted WT hosts given transplants (CD25 depl WT, n = 6 mice), CD25 depleted WT hosts given transplants and sorted 1 × 106 CD4+CD25+ T cells from WT BALB/c mice (CD25 depl WT + WT Treg, n = 9 mice), nondepleted TLI/ATS Ja18−/− BALB/c mice given transplants (nondepl Ja18−/−, n = 10 mice), CD25 depleted TLI/ATS Ja18−/− BALB/c mice given transplants with add-back of sorted 1 × 106 CD4+CD25+ T cells from WT BALB/c mice (CD25 Ja18−/− + WT Treg, n = 7 mice), and CD25 depleted WT hosts given transplants and sorted 1 × 106 CD4+CD25+ T cells from IL-10−/− BALB/c mice (CD25 depl WT + IL-10−/− Treg, n = 9 mice). Comparison of means (Student 2-tailed t test) gave P values shown on the panels; NS > .05, *P < .05, **P < .01, ***P < .001. T cells-TCR+; B cells-B220+; Macrophages- Mac1+; Granulocytes-Gr1+ cells.