Involvement of CCR9 at multiple stages of adult T lymphopoiesis

The chemokine CCL25 is constitutively expressed in the thymus, and its receptor CCR9 is expressed on subsets of developing thymocytes. Nevertheless, the function of CCL25/CCR9 in adult thymopoiesis remains unclear. Here, we demonstrate that purified CCR9 −/− hematopoietic stem cells are deficient in their ability to generate all major thymocyte subsets including double‐negative 1 (DN1) cells in competitive transfers. CCR9 −/− bone marrow contained normal numbers of lineage − Sca‐1 + c‐kit + , common lymphoid progenitors, and lymphoid‐primed multipotent progenitors (LMPP), and CCR9 −/− LMPP showed similar T cell potential as their wild‐type (WT) counterparts when cultured on OP9–δ‐like 1 stromal cells. In contrast, early thymic progenitor and DN2 thymocyte numbers were reduced in the thymus of adult CCR9 −/− mice. In fetal thymic organ cultures (FTOC), CCR9 −/− DN1 cells were as efficient as WT DN1 cells in generating double‐positive (DP) thymocytes; however, under competitive FTOC, CCR9 −/− DP cell numbers were reduced significantly. Similarly, following intrathymic injection into sublethally irradiated recipients, CCR9 −/− DN cells were out‐competed by WT DN cells in generating DP thymocytes. Finally, in competitive reaggregation thymic organ cultures, CCR9 −/− preselection DP thymocytes were disadvantaged significantly in their ability to generate CD4 single‐positive (SP) thymocytes, a finding that correlated with a reduced ability to form TCR‐MHC‐dependent conjugates with thymic epithelial cells. Together, these results highlight a role for CCR9 at several stages of adult thymopoiesis: in hematopoietic progenitor seeding of the thymus, in the DN‐DP thymocyte transition, and in the generation of CD4 SP thymocytes.