Lymphatic endothelial cells as mediators of peripheral tolerance

The ability to specifically recognize foreign antigens while sparing host tissues is a defining characteristic of the immune system. The mechanisms that subdue or eliminate cells that have receptors specific to host-antigens are collectively known as self-tolerance. Until recently the dominant model of tolerance that occurs in lymph nodes (LN) is mediated by immature dendritic cells that present self-antigen to naive self-reactive T cells, resulting in deletion or a state of non-responsiveness, termed anergy. Our lab created a mouse model to study tolerance to tyrosinase, a protein involved in pigment production, whose expression is normally restricted to melanocytes and retinal pigment epithelial cells. Previously, CD8 T cell deletional tolerance to tyrosinase was mediated by a radio-resistant LN cell that directly expressed tyrosinase mRNA. The current work extends previous studies by investigating the identity of this stromal cell, characterizing its tolerogenic properties, and determining the factors that contribute its ability to induce tolerance. We show that LN-resident lymphatic endothelial cells (LEC) express tyrosinase and present Tyr369 to specific CD8 T cells. In addition, we demonstrate that LEC express numerous co-inhibitory molecules, including programmed death ligand-1 (PD-L1), which is required for CD8 deletional tolerance to tyrosinase. We demonstrate that LN-LEC are unique compared to tissue lymphatic LEC in their high expression of PD-L1 and PTA, and their ability to present Tyr 369. We also describe a novel method of discriminating LEC that occupy the LN subcapsule, medulla, and cortex based differential expression of cell surface markers, which will be valuable in investigating specialized functional differences between LEC subpopulations in the LN. In addition, we find that the lymphotoxin pathway and B cells control the representation of LEC in the medullary sinus that express high levels of PD-L1, which have important consequences for deletional tolerance. Overall, this work has identified and characterized LEC as key player in mediating peripheral tolerance.