Progenitor Cells for High Endothelium in the Immune Response

PROJECT SUMMARY SUMMARY: High endothelial venules (HEV) are specialized portals for lymphocyte entry into lymphoid tissues and sites of chronic inflammation from the blood. Along with flat walled postcapillary venules, they regulate immune cell trafficking in physiologic and pathologic settings including autoimmune diseases, inflammation and cancer. HEV in lymph nodes draining sites of immune challenge expand dramatically to support enhanced lymphocyte recruitment, with new high endothelial cells (HEC) arising by proliferation and by neogenesis from capillary resident progenitors (CRP). The molecular pathways that control HEV expansion and differentiation from capillary precursors are as yet unclear; but generation of a comprehensive atlas of lymph node blood endothelial cell subsets, molecular phenotypes and responses to immune challenge now allows us to identify candidate pathways involved. Our fundamental focus in this renewal application is therefore to identify and characterize novel mechanisms of HEV specialization and generation in the immune response. In this context we will mechanistically define pathways that i) regulate progenitor cell homeostasis and transitional EC expansion in immune angiogenesis; ii) induce the unique "high" endothelial morphology and metabolism of HEV, regulating "HEVness" and function; and iii) control the cell-fate decision that directs transitional EC towards lymphocyte-recruiting HEV (vs non-HEV PCV) differentiation. We will apply innovative approaches to address these fundamental gaps in knowledge including scRNAseq and mass proteomic EC profiling, novel pan-EC and CRP-specific gene-targeted mice, and nanoparticle methods to manipulate EC-subset specific gene expression. Elucidation of the mechanisms of endothelial cell specialization and homeostasis in lymphoid tissues, including mechanisms regulating endothelial cells that control lymphocyte homing, will lead to novel targets and approaches for the control of autoimmune inflammation and for therapeutic regulation of immune cell traffic for vaccination and cancer immunity.