Regular Article VASCULAR BIOLOGY CLEC-2 is required for development and maintenance of lymph nodes

Lymph nodes (LNs) are organized anatomical structures distributed at strategic sites alongside the lymphatic vasculature that provide the hub of the acquired immune system. Their organization is supported by stromal cell populations, allowing maximal interaction between antigen-loaded dendritic cells (DCs) migrating through the afferent lymphatic vasculature and recirculating lymphocytes entering through the blood vasculature. The combination of the markers CD31 (PECAM-1), Gp38/Podoplanin, CD35, and RANKL has allowed the identification of at least 5 stromal cell populations: blood endothelial cells (BECs; Gp38CD31CD35RANKL), lymphatic endothelial cells (LECs; Gp38CD31CD35RANKL), fibroblastic reticular cells (FRCs; Gp38CD31CD35RANKL), follicular DCs (FDCs; Gp38CD31CD35RANKL) and marginal reticular cells (MRCs; Gp38CD31CD35RANKL). These populations are responsible for immune cell interaction and antigen presentation, ultimately providing the anatomical base for the organization of the acquired immune response. LN development begins during embryogenesis, following precise timing according to anatomical location: the mesenteric LNs developfirst, followed by the others along the anterior-posterior axis. A key event is the recruitment and clustering of the hematopoietic lymphoid tissue inducer (LTi) cells expressing lymphotoxin a1b2. Engagement by LTi cells of the lymphotoxin b receptor (LTbR) on immature stromal cells will direct their maturation to lymphoid tissue organizer (LTo) cells that regulate further LTi cell clustering and development of the anlage. The lymphatic vasculature is integral to LN structure; however, its importance in the development of the anlage remains unclear. Despite the development of the LN anlagen is synchronized with the formation of the lymph sacs, which characterize the beginning of the establishment of the lymphatic vasculature, LN formation is initiated normally in Prox-1 embryos which are devoid of lymph sacs and lymphatic vessels. Due to the early lethality of Prox-1 embryos, the definitive outcome of the LN anlagen, which is disorganized in the absence of the lymphatic vasculature, remains unknown. In adult LNs, the lymphatic vasculature is embedded in the LN structure and interruption of the afferent lymphatic vessels has been shown to cause LN regression and loss of high endothelial venules (HEVs). CLEC-2 (encoded by Clec1b), a C-type lectin-like receptor is an endogenous ligand for Gp38/Podoplanin expressed at high levels on megakaryocyte-derived platelets, inflammatory DCs, and possibly other hematopoietic cells. CLEC-2 contains a half immunoreceptor tyrosine-based activationmotif (ITAM), also known as a hemITAM, and signals via the tyrosine kinase Syk, mediating platelet activation and thrombus formation.Deficiency inCLEC-2 or its ligand Gp38/Podoplanin results in systemic edema and abnormal lymphatic