*Purpose: Regulatory T cell (Treg) lymphatic migration is important for immune tolerance. Treg express both programmed death-1 (PD-1) and its ligand PD-L1, which are important immune checkpoints. PD-L1 is also broadly expressed on many immune cells including lymphatic endothelial cells (LEC). The interplay of Treg PD-1 with LEC PD-L1 is not fully understood. We investigated the hypothesis that Treg use surface PD-1 to engage PD-L1 on LEC to regulate lymphatic transendothelial migration (TEM).

*Methods: Human and murine dermal LEC were used in biochemical, phenotypic, and functional analyses of PD-L1 signaling. Human naïve or memory Treg and effector T cells and murine wild type or PD-1^-/- Treg, naïve, and activated CD4 T cells were migrated across LEC in vitro and lymphatic vessels in vivo. Recombinant PD-1 or PD-L1 extracellular domains fused with IgG1 (PD-1 Ig and PD-L1 Ig) were used for induction of PD-L1 and PD-1 signaling respectively. Signaling was blocked with specific anti-PD-1 or anti-PD-L1 Abs.

*Results: Both human and mouse Treg highly express both PD-1 and PD-L1, while human and mouse LEC have high expression of PD-L1, but not PD-1. Ligation of human and mouse Treg PD-1 by immobilized PD-L1 Ig enhanced Treg TEM. Crosslinking PD-L1 on human and mouse LEC with PD-1 Ig increased VCAM-1 and decreased intercellular junction protein VE-cadherin to promote Treg TEM. Blockade of Treg, but not non-Treg, with anti-PD-1 mAb, or blockade of LEC PD-L1 with anti-PD-L1 mAb inhibited Treg, but not non-Treg, TEM . However, blockade of Treg PD-L1, or LEC treatment with PD-1 blocking Ab had no effect on Treg TEM. Transmigration of PD-1^-/- Treg across WT LEC was inhibited compared to WT Treg across WT LEC. Pretreating Treg with anti-PD-1 blocking Ab or pretreating mice with anti-PD-L1 blocking Ab inhibited Treg migration into local draining lymph nodes.

*Conclusions: Treg PD-1 uniquely engages LEC PD-L1 to modulate lymphatic endothelial structure, which then promotes Treg lymphatic TEM. These data demonstrate an important and novel role for Treg PD-1 and LEC PD-L1 in regulation of lymphatic migration and hence Treg suppressive function.

« Back to 2020 American Transplant Congress