The long-term goal of this project is to identify the biological roles of the LT/TNF family. These cytokines contribute importantly to inflammation and lymphoid organ development, including regulation of high endothelial venules (HEVs) in lymph nodes (LNs). In the next years, regulation of lymphatic vessels (LVs) will be studied. Though the ontogenic process of lymphangiogenesis has been described, the initiators of the process are unknown. Lymphangiogenesis in inflammation is even less well understood. Several cell types, including B cells and macrophages have been invoked, either as sources of VEGFs or as precursors of lymphatic endothelial cells. Lymphangiogenesis at different sites and under different conditions may be regulated differently. Aim 1. To identify the mechanism of inflammation-induced lymphangiogenesis in LNs. Remodeling of the LN vasculature that occurs during inflammation includes changes in expression of HEV genes, lymphangiogenesis, and the appearance of vessels positive for both HEV and LV markers. To test the hypothesis that LVs arise from HEVs in lymphangiogenesis mice that express green fluorescent protein under the regulation of a HEV unique gene, HEC-6ST, and mice to be developed that express red fluorescent protein under the regulation of a LV gene, Prox-1, will be evaluated by confocal and multiphoton intravital microscopy. Experiments will be carried out to determine if information is exchanged between the vessels. The roles of cells, signals, and cytokines in this process will be identified. Aim 2. To identify the mechanism of inflammation-induced lymphangiogenesis in the skin. Lymphangiogenesis occurs in the skin after immunization with ovalbumin and complete Freund[unreadable]s adjuvant. This is enhanced in LT&#946;-/- mice. A remarkable mononuclear infiltrate is present in the region of the immunization site. The hypothesis, that both LT&#945;and TNF&#945;derived from cells in that site, contribute to skin lymphangiogenesis, will be tested by identifying the cells, signals, and cytokines, and other lymphangiogenic factors required for the process. Testing the hypothesis that LT/TNF family members, key molecules in lymphoid organ development and inflammation, can induce lymphatic vessels will provide an understanding of the factors that can induce lymphangiogenesis and thus provide therapeutic insight into the pathology of lymphedema. The data will suggest modalities for treatment of lymphatic vessel insufficiency by controlled local administration of cytokines. The analysis of interactions between lymphatic vessels and HEVs will provide insight into how these two vascular systems are regulated, maintained, and cooperate during an immune response will provide insight into lymphangiogenesis of inflammation in transplantation, autoimmunity, infectious diseases, and cancer.