DN thymocytes later develop into CD4+CD8+(double positive [DP]) cells, then pass through a CD8 intermediate single positive phase followed by differentiation into either CD4+or CD8+(single positive [SP]) cells. [5]. In younger patients, T cell recovery can take up to 12 months, while in older patients, T cells may never reach pretransplant levels [6]. The Desmopressin Acetate significant delays in T cell reconstitution in older patients can be attributed to age-related thymic involution and the coincident decrease in export of nave T cells from the thymus [6]. In general, the thymi of young recipients are capable of supporting the production of a new repertoire of selected, functional T cells. This does, however , depend on several factors: thymic damage due to radio/chemotherapy, engraftment of donor-derived hematopoietic precursors, and graft-versus-host disease (GVHD) occurrence and treatment. In this review, we will discuss the use of adoptive precursor cell therapies as a means to enhance T cell reconstitution and decrease morbidity and mortality following HSCT. == Mouse T cell development == T cells develop in the thymus, unlike other lymphoid lineages, which develop in the bone marrow from resident hematopoietic stem cells [7, 8]. Although the thymus is essential for T cell differentiation, it does not contain self-renewing T cell progenitors. Instead, bone marrow-derived precursors enter the thymus via the bloodstream [912]. Although T cell precursors in the early stages of T cell development maintain their potential to develop into non-T lineages [13, 14], under normal circumstances, the majority will follow a well-characterized differentiation pathway. Thymic entry is a gated phenomenon in which progenitors are able to engraft only when space is available in thymic niches and microvascular entry sites or gates are open [1518]. Engraftment in the thymus is further regulated by a variety of adhesion interactions and chemotactic events [19]. P-selectin glycoprotein ligand 1 (PSGL-1) and CD44 [20], as well as the CC chemokine receptor 9 (CCR9) [2123] facilitate progenitor entry into the thymus. The identity of the thymus-seeding progenitor remains unknown; however , the phenotype of the early T lineage progenitor (ETP), the earliest thymocyte subset, is defined as lineage marker-negative (lin)CD44+CD25c-kit+[24]. As shown inFig. 1, ETPs are, therefore , contained within the Desmopressin Acetate most immature thymocyte subset, CD4CD8(double negative [DN]) cells. DN thymocytes later develop into CD4+CD8+(double positive [DP]) cells, then pass through a CD8 intermediate single positive phase followed by differentiation into either CD4+or CD8+(single positive [SP]) cells. The DN compartment is further divided on the basis of CD44 and CD25 expression [2527]. In order to mature, thymocytes traverse the thymus and receive differentiation signals from thymic stromal (especially epithelial) and dendritic cells. DN1 (CD44+CD25) cells are found in the cortex near Desmopressin Acetate the corticomedullary junction, the site at Rabbit Polyclonal to CIDEB which bloodborne precursors enter the thymus [2830]. Accordingly, the subsequent developmental stages of thymocytes are observed in distinct regions of the thymus. DN2 (CD44+CD25+) cells are located in the cortex, DN3 (CD44CD25+) near the subcapsular region, and DN4 (CD44CD25) and DP cells begin to travel from the subcapsular region back toward the medulla [2830]. Subsequent thymocyte subsets are primarily located in the medulla where they finish the process of T cell receptor (TCR) rearrangement and undergo positive and negative selection, all of which require interaction with the thymic microenvironment. == Fig. 1 . == Comparison of known stages of mouse and human T cell development Desmopressin Acetate in the thymus.