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

Huan Xia,Xiaoqun Wang,Yixiao Zhao,Boyang Ren,Yaoyao Shi,Mingzhao Zhu,Zhongnan Wang,Qian Chai,Suijuan Zhong

doi:10.3389/fimmu.2021.707404

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.

Highlights

T cell development in the thymus starts with hematopoietic progenitor cells (HPCs) transendothelial migration into thymic parenchyma and ends with mature thymocytes reversetransendothelial migration to peripheral blood [1,2,3]
To examine the role of LTbR signaling in thymic egress in more details, we first detected thymocyte populations in Ltbr−/− mice
Consistent with the previous work [8, 9], mature thymocytes, defined as CD62L+CD24− (Figures 1A, B) or CD62L+CD69− (Supplementary Figures 1A, B), were remarkably accumulated in Ltbr−/− mice compared with littermate Ltbr+/− control mice, indicating a deficiency in thymic egress in Ltbr−/− mice

Summary

Introduction

T cell development in the thymus starts with hematopoietic progenitor cells (HPCs) transendothelial migration into thymic parenchyma and ends with mature thymocytes reversetransendothelial migration to peripheral blood [1,2,3]. Zachariah et al used an intravascular procedure to directly label emigrating cells and found that mature thymocytes exit via blood vessels at the CMJ [5]. These previous studies make it obscure how both thymic entry and egress take place at the same place. Our previous work has identified and characterized thymic portal endothelial cells (TPECs) as the cellular basis for thymic homing of HPCs [6] It remains unclear which type of thymic endothelial cells (ECs) is responsible for thymic egress and what is the relationship between that and TPECs

Methods

Results

Conclusion