Chimerism and up-regulation of PD-1 on CD4⁺ Tcon and CD8⁺ T cells are enhanced by NKT cells in an IL-4–dependent manner. (A-B) Percentage of PD-1^hi cells among gated CD8⁺ (A) or CD4⁺CD25⁻ Tcon cells (B) T cells from spleen in Ja18^−/− BALB/c mice given transplants (Ja18^−/− TX, n = 6 mice); Ja18^−/− BALB/c mice given transplants with add-back of sorted 0.4 × 10⁶ NKT cells from WT BALB/c mice (Ja18^−/− TX + WT NKT, n = 7 mice), and Ja18^−/− BALB/c mice given transplants with add-back of sorted 0.4 × 10⁶ NKT cells from IL-4^−/− BALB/c mice (Ja18^−/− TX + IL-4^−/− NKT, n = 5 mice). (C) Mean percentages of donor type cells in splenic mononuclear cell gate from Ja18^−/− BALB/c mice given transplants (Ja18^−/− TX, n = 9 mice); Ja18^−/− BALB/c mice given transplants with add-back of sorted 0.4 × 10⁶ NKT cells from WT BALB/c mice (Ja18^−/− TX + WT NKT, n = 7 mice); and Ja18^−/− BALB/c mice given transplants with add-back of sorted 0.4 × 10⁶ NKT cells from IL-4^−/− BALB/c mice (Ja18^−/− TX + IL-4^−/− NKT, n = 5 mice). Comparison of means (Student 2-tailed t test) gave P values shown on the panels; NS > .05, *P < .05, **P < .01, ***P < .001.