Phenotypic Comparison of Extrathymic Human Bone-Marrow-Derived T Cells with Thymic-Selected T Cells Recovered from Different Tissues

We have previously described extrathymic generation of human T cells from purified stem cells in the bone marrow of athymic immune deficient mice. This system provides a pure population of extrathymic human T cells that is devoid of contamination by peripheral expansion of thymic-selected T cells. In the current studies, we phenotypically compared the extrathymic human T cells (Ex-T) to T cells from human peripheral blood leukocytes (PBL), umbilical cord blood (CB), bone marrow (BM), and postnatal thymus. There were few CD4+/CD8+ double positive (DP) cells in PBL, CB, BM, and Ex-T, in comparison with over 85% DP cells in thymus. More CD8+ and CD4dim cells were observed in Ex-T than in the thymic-selected cells. Ex-T and T cells in thymus and peripheral tissues differed in their CD8 isoforms. There were more TCRγ/δ T cells in PBL, CB, BM, and Ex-T than in thymus. Similar to the bright CD3+ T cells in thymus, T cells in PBL, CB, and BM were CD3 bright and expressed the adhesion molecules CD44 and L-selectin (CD62L), while intermediate CD3 T cells in thymus lacked CD44 and L-selectin. However, the majority of Ex-T only expressed CD44 but not L-selectin. In summary, thymic- and extrathymic-derived T cells are phenotypically different. The identification of extrathymically derived T cells in humans will allow us to begin to understand their role in the early contribution to immune recovery posttransplantation and their possible involvement in autoimmunity and other disease states.