Continuous thymic homing of haematopoietic progenitor cells (HPCs) via the blood is critical for normal T-cell development. However, the nature and the differentiation programme of specialized thymic endothelial cells (ECs) controlling this process remain poorly understood. Here using conditional gene-deficient mice, we find that lymphotoxin beta receptor (LTβR) directly controls thymic ECs to guide HPC homing. Interestingly, T-cell deficiency or conditional ablation of T-cell-engaged LTβR signalling results in a defect in thymic HPC homing, suggesting the feedback regulation of thymic progenitor homing by thymic products. Furthermore, we identify and characterize a special thymic portal EC population with features that guide HPC homing. LTβR is essential for the differentiation and homeostasis of these thymic portal ECs. Finally, we show that LTβR is required for T-cell regeneration on irradiation-induced thymic injury. Together, these results uncover a cellular and molecular pathway that governs thymic EC differentiation for HPC homing.

The establishment of T cell central tolerance critically relies on the development and maintenance of the medullary thymic epithelial cells (mTECs). Disrupted signaling of lymphotoxin beta receptor (LTβR) results in dramatically reduced mTEC population. However, whether LTβR directly or indirectly control mTECs remains undetermined; how LTβR controls this process also remain unclear. In this study, by utilizing K14-Cre × Ltbrfl/fl conditional knockout (cKO) mice, we show that epithelial intrinsic LTβR was essential for the mTEC development postnatally. Mechanistically, LTβR did not directly impact the proliferation or survival of mTECs; the maturation of mTECs from MHC-IIlo to MHC-IIhi stage was also unaltered in the absence of LTβR; interestingly, the number of mTEC progenitors (Cld3,4hiSSEA-1+) was found significantly reduced in LTβR cKO mice at the neonatal stage, but not at E18.5. Consequently, epithelial deficiency of LTβR resulted in significant defect of thymic negative selection as demonstrated using OT-I and RIP-OVA transgenic mouse system. In summary, our study clarifies the epithelial intrinsic role of LTβR on mTEC development and function; more importantly, it reveals a previously unrecognized function of LTβR on the control of the size of mTEC progenitor population.