*_I. CELLULAR AND MOLECULAR IMMUNOLOGY OF MIGRATION AND LYMPH NODE STRUCTURE_* *_Regulatory T Cells and Lymphatic Endothelial Cells: Regulatory Interactions for Migration and Function_* Our investigations unraveled the key signaling pathways controlling Treg migration. Thymus derived (nTreg) and peripherally induced Treg (iTreg) express high cell surface levels of lymphotoxinab(LTab). Treg, but not Tconv, use LTabto migrate from grafts across afferent lymphatic endothelial cells (LEC) and into dLN. Treg LTabbinds the LEC LTb-receptor (LTbR) to stimulate non-canonical NFkB-inducing kinase (NIK). NIK signaling facilitates Treg transendothelial migration (TEM) by inducing LEC responses, including increased CCL21 expression, VCAM-1 rearrangement, and increases in basal lamellipodia that engage Treg. Our new preliminary studies now show that Treg-LEC interactions: 1.) use LTab-LTbR to condition LEC for permissiveness for TEM; 2.) determine the transition of Treg to become exTreg; and 3.) re...