Noninvasive imaging in ACE was not complicated by the putative immunoprivilege and enabled visualization of the interaction between CD4⁺ T_eff cells and their target cells. (A–D) The efficacies of β cell antigen–specific CD4⁺ T_eff and T_reg cells in the endogenous pancreatic islet were compared with that of islet grafts in the anterior chamber of the eye (ACE). (A) For immune responses in native pancreata, NOD.SCID mice were injected with either BDC2.5 T_eff cells or T_reg–T_eff mixture. Damage to pancreatic islets was monitored by reading blood glucose (BG) levels. Animals with consecutive readings of BG > 250 mg/deciliter were considered diabetic, or failed T_reg cell protection if T_reg cells were co-transferred with T_eff cells. For immune responses in ACE grafts, NOD.SCID mice were rendered diabetic through streptozotocin destruction of pancreatic β cells, and then transplanted with pancreatic islets from allogeneic (B6) or syngeneic (NOD.SCID) donors into ACE. After stable engraftment (>2 wk after transplantation), CD4⁺ T_eff cells alone (B) or T_reg–T_eff mixture (C) was injected into the graft-bearing mice. Graft damage was monitored by BG readings. (D) Summary of results in A–C. The cumulative incidence of diabetes was calculated for the groups of animals (n = 4–26; 2–3 experiments) presented individually in A–C. (E) 3D rendering of in vivo fluorescence micrographs for apoptotic signal Annexin V in areas of CD4⁺ BDC2.5 T_eff cells engaging in direct contact with β cell targets. The results represent two experiments with a total of 4 mice analyzed with snapshot imaging of 1–3 islets per animal. Bar, 100 µm. See Video 1.