Interferon-γ Converts Human Microvascular Pericytes into Negative Regulators of Alloimmunity through Induction of Indoleamine 2,3-Dioxygenase 1

Early acute rejection of human allografts is mediated by circulating alloreactive host effector memory T cells (TEM). TEM infiltration typically occurs across graft post-capillary venules and involves sequential interactions with graft-derived endothelial cells (ECs) and pericytes (PCs). While the role of ECs in allograft rejection has been extensively studied, contributions of PCs to this process are largely unknown. This study aimed to characterize the effects and mechanisms of interactions between human PCs and allogeneic TEM. We report that unstimulated PCs, like ECs, can directly present alloantigen to TEM but while IFN-γ-activated ECs (γ-ECs) show increased ability to stimulate alloreactive T cells, IFN-γ-activated PCs (γ-PCs) instead suppress TEM proliferation but not cytokine production or signaling. RNA sequencing analysis of PCs, γ-PCs, ECs, and γ-ECs reveal induction of indoleamine 2,3-dioxygenase 1 (IDO1) in γ-PCs to significantly higher levels than in γ-ECs that correlates with tryptophan depletion in vitro. Consistently, shRNA knockdown of IDO1 markedly reduces -PC-mediated immunoregulatory effects. Adoptively transferred T cells induced PC expression of IDO1 in allogeneic human skin grafts on immunodeficient mice. We conclude that immunosuppressive properties of human PCs are not intrinsic but instead result from IFN-γ-induced IDO1-mediated tryptophan depletion. Examination of gene expression by unstimluated versus IFN-γ-stimulated endothelial cells or pericytes