TNFR1 Signalling Is a Critical Checkpoint in the Generation of Myeloid-Derived Suppressor Cells

Abstract

Purpose: : To investigate the role of myeloid regulation of T cell responses in experimental autoimmune uveoretinitis, specifically the role of TNF receptor 1 (TNFR1) signalling in the development of a subset of macrophages termed myeloid-derived suppressor cells (MDSC). Macrophages (Mϕ) and T cells are central to retinal damage during non-infectious ocular inflammation, and TNF- is critical to this destruction. We have previously demonstrated that IFNγ-mediated activation of Mϕ to produce nitric oxide (NO), which suppresses T cell proliferation, requires autocrine TNF- signaling and intact TNFR1. Furthermore, in vivo studies demonstrated CD11b⁺ Gr-1⁺ myeloid cell infiltrate during ocular inflammation, a phenotype that is characteristic of MDSC, and which suppress T cell proliferation.

Methods: : To investigate the role of TNFR1 signalling in MDSC generation further, we cultured either wild type (WT) or TNFR1^-/- bone-marrow derived macrophages with OVA_323-339-specific TcR transgenic OT-II CD4+ T cells in the presence or absence of cognate OVA peptide. Experiments assessed cell surface marker expression, and activation of macrophages and T cells.

Results: : Activation of WT Mϕ by co-culture with T cells and cognate peptide resulted in up-regulation of Gr-1 in WT Mϕ but not TNFR1-/- Mϕ, indicating a requirement for both T cells and TNFR1 signals to generate MDSC. The suppressive mechanism by which Mϕ regulated T cells was reversible, as T cells removed from co-culture still proliferated, demonstrating that antigen presentation by Mϕ to T cells generates T cells that are poised to divide but held in check by factors within the local microenvironment. High levels of PGE₂ are detected in co-cultures of WT Mϕ and T cells, but not in TNFR1^-/- Mϕ co-cultures, and selective blockade of PGE₂ production leads to WT Mϕ-induced T cell proliferation. Addition of exogenous PGE₂ removes the requirement for TNF- signalling in the maturation of these monocytes and allows TNFR1^-/- Mϕ to function as MDSC.

Conclusions: : TNFR1 signalling is critical for the development of Mϕ that inhibit T cell proliferation. TNFR1^-/- Mϕ are unable to differentiate into inhibitory MDSC because they do not produce PGE₂ that provides a maturation signal, or NO, which is an essential part of the effector mechanism that inhibits T cell proliferation.

Keywords: immunomodulation/immunoregulation • inflammation • uveitis-clinical/animal model