IDO-Independent Suppression of T Cell Effector Function by IFN-γ–Licensed Human Mesenchymal Stromal Cells

Abstract

Human bone marrow–derived mesenchymal stromal cells (MSCs) inhibit proliferation of activated T cells, and IFN-γ plays an important role in this process. This IFN-γ–licensed veto property is IDO-dependent. To further decipher the mechanistic underpinnings of MSC veto function on T cells, we investigated the effect of MSCs and IFN-γ–licensed MSCs on T cell effector function as assayed by cytokine secretion of T cells. Although MSCs and IFN-γ–licensed MSCs inhibit T cell proliferation, only IFN-γ–licensed MSCs significantly inhibit Th1 cytokine (IFN-γ, TNF-α, and IL-2) production by T cells. Additionally, IFN-γ–licensed MSCs inhibit T cell degranulation as well as single, double, and triple cytokine–producing T cells. Although IFN-γ–licensed MSCs upregulate their IDO activity, we found that MSC IDO catalytic function is dispensable with regard to MSC-driven inhibition of T cell effector function. Novel flow cytometry based functional screening of MSC-expressed, IFN-γ–licensed inhibitory molecules identified B7H1 and B7DC/PD1 pathways as essential effectors in blocking T cell function. Small interfering RNA–mediated blocking of B7H1 and B7DC reverses the inhibitory potential of IFN-γ–licensed MSCs on T cell effector function. Mechanistic analysis revealed that clustering of MHC and coinhibitory molecules are indispensable for the inhibitory effect of IFN-γ MSCs. Although exogenous IL-2 reverses B7H1-Ig–mediated inhibition of T cell proliferation, it does not affect the veto function of IFN-γ MSCs on both T cell proliferation and effector function. Our results reveal a new immunosuppressive property of IFN-γ–licensed MSCs that inhibits T cell effector function independent of IDO but through the ligands for PD1.

Morphological features of IFN-γ–stimulated mesenchymal stromal cells predict overall immunosuppressive capacity

“…In other cell types (HepG2 and HeLa), differences in cell adhesion signaling (drug and matrix-induced) that led to adoption of distinct morphologies were discovered to significantly impact IFN-γ signaling as actin cytoskeletal arrangement was directly linked to STAT1 dephosphorylation (35). This suggests that morphology not only serves as a phenotypic readout for IFN-γ treatment, but could also play a direct role in the magnitude of the MSC immunosuppression response as IFN-γ signaling promotes production of immunomodulatory factors such as indoleamine 2,3-dioxygenase, prostaglandin E 2 , and IL-6 (20,36,37).…”

Defining mesenchymal stromal cells responsiveness to IFN^|^gamma; as a surrogate measure of suppressive potency

“…However MSCs differ from professional APCs such as dendritic cells not only by lacking co-stimulatory molecules B7-1(CD80) and B7-2(CD86) but also IFNγ fails to upregulate them 14) . IFNγ inducible IDO expression plays a major role in MSC's immunosuppressive properties and defines an important component of MSC immune plasticity.…”

“…and, (4) IFNγ licensing is crucial for MSCs to suppress T cell effector functions 14) . All these studies suggest that …”

Adenosine production: a common path for mesenchymal stem-cell and regulatory T-cell-mediated immunosuppression

“…In conditions in which substrate is exhausted and product builds up, T-cell proliferation is inhibited. By expressing this enzyme, MSCs promote consumption of tryptophan and compromise T-cell expansion [168,169]. Another mechanism of MSC immunomodulation is composed of a feed-forward system in which IL-10 and human leukocyte antigen-G5 (HLA-G5) interact.…”