T cell accumulation in the bone marrow of NOD mice is due to homing. A. Comparison of steady-state level of CD4 T cell proliferation in the bone marrow of 15–16 week-old Balb/c and prediabetic NOD mice. Bone marrow cells were stained for TCR, CD4, CD44 and Ki67. Representative Ki67 versus CD44 expression profiles are shown for TCR+CD4+ cells. The numbers are percentage of Ki67+ cells. B. Comparison of T cell homing to the bone marrow and lymph nodes in Balb/c and NOD mice. CFSE-labeled T cells from Balb/c or NOD mice were injected intravenously into both Balb/c and NOD recipients (12–16 week-old) and analyzed 2 hours following the transfer. Homing index is calculated by dividing percentage of CFSE-positive donor CD4 T cells in the bone marrow or lymph nodes by that in the spleen of the same recipient. Mean ± SD of homing index of CD4 T cells in at least four mice per group is shown. *p < 0.01.