Characterization of human CD25+ CD4+ T cells in thymus, cord and adult blood

Abstract

CD4⁺ CD25⁺ regulatory T cells prevent organ-specific autoimmune diseases in various animal models. We analysed human lymphoid tissues to identify similar CD25⁺ regulatory T cells. Adult peripheral blood contained two populations of CD4⁺ T cells that expressed CD25 at different densities. The larger population (≈ 40%) expressed intermediate levels of CD25 (CD25⁺) and displayed a memory T-cell phenotype (CD45RA⁻/RO⁺, CD45RB^low, CD95⁺, CD62L^low, CD38^low). The smaller population of cells (≈ 2%) expressed very high levels of CD25 (CD25⁺⁺). In addition to the activation/memory T-cell antigens mentioned above they also expressed intracellular CD152 (CTLA-4) as well as enhanced levels of cell-surface CD122, similar to the murine CD4⁺ CD25⁺ regulatory counterpart. To exclude that the CD25⁺⁺ cells had not been recently primed by external antigen we analysed cord blood and thymus. CD25⁺⁺, CD152⁺ and CD122⁺⁺ cells were present in paediatric thymus (10% of CD4⁺ CD8⁻ thymocytes) expressing signs of recent selection (CD69⁺) and in cord blood (5% of CD4⁺ cells) where they showed a naive phenotype. In addition, cord blood contained a small population of CD25⁺ cells (≈ 2% of CD4 T cells) that were CD152⁻ and CD122^low and displayed signs of activation. Together with published data that CD25⁺ CD25⁺⁺ cells from the thymus and peripheral blood are regulatory, our results suggest that regulatory CD25⁺ T cells leave the thymus in a naïve state and become activated in the periphery.