Persistence of recipient MHC class II–expressing cells after allogeneic HSCT. Lethally irradiated C57BL/6 recipients received 10⁷ TCD-BM + 10⁶ LC from either allogeneic BALB/c donors or syngeneic C57BL/6 donors. Fourteen days after transplant, recipient mice were injected with 2 × 10⁶ CFSE-labeled lymphocytes from 4C TCR-tg mice (direct allospecificity toward the BALB/c MHC class II molecule I-A^d) that were on the Ly5.1 congenic background. Recipient intrahepatic lymphocytes were harvested 3 days later and FACS-analyzed. (A) Schematic of experiment. (B) FACS gates for detection of 4C TCR-tg T cells and CFSE analysis. Results shown are representative of 4 mice in each group.