Lack of CD8⁺ cell activation in naive recipients of Tie2-LacZ vascularized organ grafts. (A) Spontaneous β-gal–specific CD8⁺ T-cell reactivity measured by tetramer analysis. C57BL/6 recipients received either C57BL/6 (B6 → B6) or Tie2-LacZ (T2 → B6) hearts, and the presence of β-gal_497-504–specific CD8⁺ T cells in blood was assessed by flow cytometry on day 20 after transplantation. (B) Activation of CD8⁺ Bg1 T cells after adoptive transfer of 1.5 × 10⁷ CFSE-labeled Bg1 splenocytes in C57BL/6 recipients on day 10 after transplantation. (i,iii) Heterotopic heart transplantation with donor organs from C57BL/6 (n = 7, i) and Tie2-LacZ (n = 8, iii) mice. Orthotopic liver transplantation with donor organs from C57BL/6 (n = 5, i) and Tie2-LacZ (n = 4, iii) mice. Mice were killed on day 4 following adoptive transfer, and cells from blood were analyzed by flow cytometry. Values in the histograms represent mean percentages (± SEM) of proliferating CD8⁺Thy1.1⁺ cells.