Acquisition of donor HLA−DR by recipient APCs. Naïve WT B6 mice were used as recipients of cardiac grafts from HLA−DR15 transgenic mice. HLA−DR expression in the recipients was analyzed using a pan DR antibody. A, Acquisition of donor HLA−DR on graft−infiltrating dendritic cells at day 14 post-transplant: B6 recipient of a DR15 graft was sacrificed at day 14 post-transplant. Live CD45+ graft−infiltrating cells were analyzed for donor HLA−DR expression. HLA−DR expression was noted on the graft−infiltrating dendritic cells, as seen by the small shift in the histogram (red) as compared with the negative control (black). Red line, Test DR15 to B6; Blue, WT DR15+ cells; Black, WT B6 cells. B, Acquisition of donor HLA−DR on lymph node DC at day 35 post-transplant: B6 recipient rejected the DR Tg heart at day 35 post-transplant. Lymph node cells were analyzed for HLA−DR expression. HLA−DR was expressed on both mDCs and pDCs, but not on monocytes (data not shown). Red line, Test DR15 to B6; blue, WT B6 cells; black, HLA−DR PE FMO. DC, dendritic cells.