Mixed lymphocyte reaction.

DCs were generated as described previously (83). Briefly, BM cells isolated from BALB/c mice were cultured with recombinant mouse GM-CSF and recombinant mouse IL-4 for 7 days. DCs were then irradiated (1,000 cGy) and used as stimulators. T cells purified from WT B6 or Pdl1^–/– mice were labeled with CFSE and used as responder cells. Mixed lymphocyte reaction experiments were performed by coculturing of responder and stimulator cells at different ratios in 96-well plates. Cells were harvested on day 5 and day 6, and T cell proliferation was determined by CFSE dilution.