Thymic Egress Is Regulated by T Cell-Derived LTβR Signal and via Distinct Thymic Portal Endothelial Cells

Abstract

Thymic blood vessels at the perivascular space (PVS) are the critical site for both homing of hematopoietic progenitor cells (HPCs) and egress of mature thymocytes. It has been intriguing how different opposite migrations can happen in the same place. A subset of specialized thymic portal endothelial cells (TPECs) associated with PVS has been identified to function as the entry site for HPCs. However, the cellular basis and mechanism underlying egress of mature thymocytes has not been well defined. In this study, using various conventional and conditional gene-deficient mouse models, we first confirmed the role of endothelial lymphotoxin beta receptor (LTβR) for thymic egress and ruled out the role of LTβR from epithelial cells or dendritic cells. In addition, we found that T cell-derived ligands lymphotoxin (LT) and LIGHT are required for thymic egress, suggesting a crosstalk between T cells and endothelial cells (ECs) for thymic egress control. Furthermore, immunofluorescence staining analysis interestingly showed that TPECs are also the exit site for mature thymocytes. Single-cell transcriptomic analysis of thymic endothelial cells suggested that TPECs are heterogeneous and can be further divided into two subsets depending on BST-1 expression level. Importantly, BST-1hi population is associated with thymic egressing thymocytes while BST-1lo/− population is associated with HPC settling. Thus, we have defined a LT/LIGHT-LTβR signaling–mediated cellular crosstalk regulating thymic egress and uncovered distinct subsets of TPECs controlling thymic homing and egress, respectively.

The Lineage Differentiation and Dynamic Heterogeneity of Thymic Epithelial Cells During Thymus Organogenesis

“…Cluster EC 3 was characterized by high expression of P-selectin ( Selp ) and Lrg1 ( Figure 7C ). The gene expression specificity identified these cells as thymic portal ECs (TPECs) that mediate thymic progenitor cell entry ( 75 , 76 ). Cluster EC 4 specifically expressed Bmx and Fbln5 ( Figure 7C ).…”

“…Ly6C − Selp + thymic portal ECs (TPECs), of which the differentiation is controlled by Ltβr signaling, mediate thymic progenitor cell entry ( 75 , 76 ). We therefore analyzed the dynamic expression of these genes during development.…”

Mesenchymal stromal cells in the thymus

“…The functional diversity of mural cells is likely related to the diversity of endothelial cells in the thymus. Distinct subsets of thymic endothelial cells associated with the perivascular space have been shown to serve as the entry site for ETPs or exit site for mature T cells [84][85][86]. Endothelial cells of the capillaries and arterioles in the thymus express claudin-5, tight-junction-forming protein that helps form the blood-thymus-barrier, which insulates the thymic microenvironment from blood-borne molecules, including antigens [87].…”