Signal integration and crosstalk during thymocyte migration and emigration

Abstract

The thymus produces self-tolerant functionally competent T cells. This process involves the import of multipotent haematopoietic progenitors that are then signalled to adopt the T cell fate. Expression of T cell-specific genes, including those encoding the T cell receptor (TCR), is followed by positive and negative selection and the eventual export of mature T cells. Significant progress has been made in elucidating the signals that direct progenitor cell trafficking to, within and out of the thymus. These advances are the subject of this Review, with a particular focus on the role of reciprocal cooperative and regulatory interactions between TCR- and chemokine receptor-mediated signalling.

Figure 1

Overview of regulated migration events during T cell development. Within the bone marrow, hematopoietic stem cells (HSCs) differentiate into multipotential progenitors (MPPs). A subset of MPPs that are FLT3^hi initiates transcription of the genes encoding RAG1 and RAG2 and are termed lymphoid-primed multipotential progenitors (LMPPs; also known as ELPs). Subsequent lymphoid-primed bone marrow progenitors include common lymphoid progenitor (CLPs). All these progenitors will make T cells if placed within the thymus. However, the ability of hematopoietic progenitors to migrate to the thymus is regulated, requiring among other molecules the CC-chemokine receptor 9 (CCR9), which is expressed by subsets of LMPPs and CLPs, and CCR7, which is expressed by some LMPPs and highly expressed on CLPs. The identity of in vivo thymus settling progenitors (TSPs) has not been precisely determined, but they likely include LMPPs and CLPs. TSPs enter the thymus near the cortico-medullary junction, where they may spend a prolonged period of time as KIT⁺ DN1 thymocytes (also known as ETPs) before migrating to the subcapsular zone as double-negative 2 (DN2) and double-negative 3 (DN3) cells. DN3 cells require CXCR4 to efficiently develop into double-positive (DP) thymocytes. These DP thymocytes express high levels of CCR9; however, the function of CCR9 in DP thymocytes remains unclear. DP thymocytes that undertake appropriate interactions with self peptide–MHC complexes on cortical thymic epithelial cells (positive selection), upregulate expression of CCR7 and mature into single-positive (SP) mature T cells that migrate into the thymic medulla. Residence in the medulla (where negative selection occurs) is followed by emigration regulated by S1P₁.

Figure 2

Model for regulation of CD4 SP and CD8 SP thymocyte export by TCR signalling. A. “Semi-mature” SP thymocytes receive activating signals through their TCRs by interaction with self-peptide–MHC complexes expressed by medullary TECs (mTECs) and DCs. TCR signalling results in the AKT-mediated phosphorylation of FOXO1 resulting in the transcriptional inactivation and nuclear export of FOXO1. Retention of FOXO1 in the cytoplasm leads to down-regulation of Klf2, which is required for transcription of Ccr7, Sell (encoding CD62L) and S1pr1 (encoding S1P₁). In addition, TCR signalling induces expression of CD69, which downregulates the surface expression of S1P₁. Loss and/or prevention of S1P₁ surface expression prevents premature S1P-mediated thymocyte export from the thymus. B. SP thymocytes that do not undergo or survive negative selection become functionally mature and TCR signalling is terminated. Cessation of TCR signalling terminates AKT-mediated FOXO1 phosphorylation and results in nuclear translocation of transcriptionally active FOXO1. FOXO1 promotes transcription of Klf2, which induces the expression of Ccr7, Sell and S1pr1. Termination of TCR signalling also results in eventual downregulation of CD69, alleviating CD69-directed downregulation of S1P₁ surface expression. Increased surface expression of S1P₁ renders SP thymocytes responsive to S1P chemotaxis and promotes their migration towards the cortio-medullary junction and their export from the thymus.