Differential Roles of LTβR in Endothelial Cell Subsets for Lymph Node Organogenesis and Maturation.

Cellular cross-talk mediated by lymphotoxin αβ-lymphotoxin β receptor (LTβR) signaling plays a critical role in lymph node (LN) development. Although the major role of LTβR signaling has long been considered to occur in mesenchymal lymphoid tissue organizer cells, a recent study using a VE-cadherincre Ltbrfl/fl mouse model suggested that endothelial LTβR signaling contributes to the formation of LNs. However, the detailed roles of LTβR in different endothelial cells (ECs) in LN development remain unknown. Using various cre transgenic mouse models ( Tekcre , a strain targeting ECs, and Lyve1cre , mainly targeting lymphatic ECs), we observed that specific LTβR ablation in Tekcre+ or Lyve1cre+ cells is not required for LN formation. Moreover, double- cre -mediated LTβR depletion does not interrupt LN formation. Nevertheless, Tekcre Ltbrfl/fl mice exhibit reduced lymphoid tissue inducer cell accumulation at the LN anlagen and impaired LN maturation. Interestingly, a subset of ECs ( VE-cadherin + Tekcre-low/neg ECs) was found to be enriched in transcripts related to hematopoietic cell recruitment and transendothelial migration, resembling LN high ECs in adult animals. Furthermore, endothelial Tek was observed to negatively regulate hematopoietic cell transmigration. Taken together, our data suggest that although Tekcre+ endothelial LTβR is required for the accumulation of hematopoietic cells and full LN maturation, LTβR in VE-cadherin+ Tekcre-low/neg ECs in embryos might represent a critical portal-determining factor for LN formation.