Abstract Thymic homing of hematopoietic progenitor cells (HPCs) is an essential step for the subsequent T cell development. Previously we have identified a subset of specialized thymic portal endothelial cells (TPECs), which is important for thymic HPC homing. However, the underlying molecular mechanism remains still unknown. Here we found that signal regulatory protein alpha (SIRPα) is preferentially expressed on TPECs. Disruption of CD47-SIRPα signaling in mice resulted in reduced number of thymic early T cell progenitors (ETPs) and impaired thymic HPC homing. Mechanistically, SIRPα-deficient ECs and CD47-deficient lymphocytes demonstrated impaired transendothelial migration (TEM). Specifically, SIRPα intracellular ITIM motif-initiated downstream signaling in ECs was found to be required for TEM in a SHP2- and Src-dependent manner. Furthermore, CD47-signaling from migrating cells and SIRPα intracellular signaling were found to be required for VE-cadherin endocytosis in ECs. Functionally, SIRPα signaling is required for T cell regeneration upon sub-lethal total body irradiation (SL-TBI); CD47-SIRPα signaling blockade post SL-TBI diminishes antitumor immunity. Thus, our study reveals a novel role of endothelial SIRPα signaling for thymic HPC homing for T cell regeneration and antitumor immunity. Graphic abstract Thymic homing of hematopoietic progenitor cells is fundamental to the T cell-based adaptive immunity, yet the molecular basis of this process is less clear. We discovered that endothelial SIRPα signaling, engaged by migrating cell derived CD47 ligand, regulates thymic homing of hematopoietic progenitor cells for T cell regeneration and antitumor immunity. SIRPα is preferentially expressed on thymic portal endothelial cells. Endothelial SIRPα regulates thymic homing of hematopoietic progenitor cells. CD47-SIRPα downstream signaling induces VE-cadherin endocytosis. CD47-SIRPα signaling blockade impairs thymic T cell regeneration and antitumor immunity.
