J Leukoc Biol. 2015 Jun;97(6):1101-9.
Recovery from experimental autoimmune uveitis promotes induction of antiuveitic inducible Tregs.
- 1Department of Ophthalmology, Boston University School of Medicine, Boston, Massachusetts, USA.
- 2Department of Ophthalmology, Boston University School of Medicine, Boston, Massachusetts, USA email@example.com.
The recovery of EAU, a mouse model of endogenous human autoimmune uveitis, is marked with the emergence of autoantigen-specific regulatory immunity in the spleen that protects the mice from recurrence of EAU. This regulatory immunity is mediated by a melanocortin-driven suppressor APC that presents autoantigen and uses adenosine to activate an antigen-specific CD4(+) Tregs through the A2Ar. These cells are highly effective in suppressing uveitis, and they appear to be inducible Tregs. In this study, we determined whether they are inducible or natural Tregs and identified the dependent mechanism for the function of these post-EAU Tregs. The post-EAU spleen CD25(+)CD4(+) T cells were sorted for NRP-1 expression and transferred to recipient mice immunized for EAU. The sorted NRP-1(-), but not the NRP-1(+), Tregs suppressed EAU. These NRP-1(-) Tregs coexpress PD-1 and PD-L1. Treatment of naive APCs with α-MSH promoted a regulatory APC that induced CD25(+) CD4(+) Tregs in a CD73-dependent manner. These Tregs were PD-L1(+) PD-1(+) NRP-1(-) FOXP3(+) HELIOS(-) and suppressed EAU when transferred to recipient mice. In contrast, PD-1(-) T cells did not suppress EAU, indicating that PD-1 is necessary for the suppressive activity of iTregs. Moreover, these Tregs did not suppress effector T cells when the PD/-1/PD-L1 pathway was blocked. These results demonstrate that post-EAU Tregs are inducible Tregs, which use a PD-1/PD-L1 mechanism to suppress disease.
KEYWORDS: adenosine; melanocortins; neuroimmunomodulation; ocular immunobiology; suppressor cells
The eye is an immune privileged tissue that expresses within its microenvironment anti-inflammatory molecules to prevent induction inflammation. While it is rare, inflammation, uveitis, resulting from autoimmune disease does occur. To understand the cause, and to evaluate potential treatments for autoimmune uveitis experimental autoimmune uveoretinitis (EAU) mouse models are studied (1). The use of this model has led to understanding uveitis induction needs an innate immune component to breaks central tolerance, and make changes within the ocular microenvironment to allow inflammation. Susceptibility to uveitis may be a combination of genetic predisposition along with a contribution by the gut microbiome. The EAU model has demonstrated that the autoimmune disease is mediated by both Th1 and Th17 cells with antigen-specificity to retinal autoantigens. However, the most curious feature of the EAU model is that mice recover from uveitis without any intervention (2). We asked what is the mechanism that clears the inflammation, and can it be used to suppress uveitis when it appears?
One of the anti-inflammatory molecules within the healthy immune privileged eye is the neuropeptide alpha-melanocyte stimulating hormone (a-MSH) (3). It has an important role both in the eye, and systemically in suppressing and regulating the extent of inflammation. One of the actions of a-MSH on T cells is that it converts pro-inflammatory effector T cells into regulatory T cells (4). This was done in an antigen-specific manner. It is understood that Treg cells increase at the resolution of EAU, and may be the mediators of resolution. There are at resolution retinal antigen-specific Treg cells in the regional lymph nodes and the spleen. The retinal autoantigen-specific Treg cells in the spleen of the post-EAU mice provide the mice with resistance to reactivation of EAU (5, 6). Mice with the melanocortin 5 receptor (MC5r) (one of 4 receptors for a-MSH) knocked out still resolve EAU, but they lack this protective regulatory immunity in the spleen. When we examined the spleen immune cells of post-EAU wild type mice it was found that it is the antigen presenting cells (APC) not the T cells that have to express MC5r. The melanocortin pathway mediated by a-MSH through MC5r expands a population of suppressor APC that express the markers of F4/80, CD11b, Ly6G, and Ly6Clow. In addition, their induction of antigen-specific Treg cells requires generation of adenosine that binds to the adenosine 2A receptor on the T cells (7). Injecting EAU mice with a-MSH expands both populations of cells in the spleen and rapidly suppresses uveitis. What is interesting is that suppression of uveitis by a-MSH is not dependent on a-MSH generated regulatory immunity. This has suggested that the suppression of uveitis is in part by a-MSH’s nonspecific anti-inflammatory activity in immune cells. The added benefit is that induction of regulatory immunity provides long-term tolerance. Also, it is possible to treat APC with a-MSH and provide antigen in culture. These treated APC activate antigen-specific Treg cells that suppress autoimmune disease when adoptively transferred. This as suggested that at the resolution of EAU there is through MC5r the expansion of suppressor APC in the spleen that by the adenosinergic pathway mediate the counter-conversion of retinal autoantigen-specific effector T cell into Treg cells.
In this work, the type of Treg cell that is activated in the post-EAU mouse spleen and by a-MSH-treated APC was studied (8). The previous publications showed that these Treg cells are CD25+ CD4+ FoxP3+. This manuscript demonstrated that the Treg cells do not express Neuropilin-1 indicating that they are inducible Treg cells. This further supports that idea that the melanocortin-stimulated APC are promoting the counter-conversion of effector T cells. Also, the manuscript addressed the mechanistic question of how these Treg cells function. Stimulation of programmed death-1 (PD-1) receptor inhibits T cell activation and promotes Treg cell activity to maintain tolerance (9). In this work we identify that the Treg cells from EAU-recovered mice express PD-1 and the ligand for PD-1, PD-L1. Moreover, blocking PD-L1 on these Treg cells prevents suppression of retinal autoantigen specific effector T cells. Therefore, as uveitis resolves, a retinal autoantigen specific Treg cell that functions through the PD-1/PD-L1 pathway is induced systemically with the capacity to provide resistance to reactivation of autoimmune uveitis and suppression of memory immunity to the autoantigen. It is as if stimulating the melanocortin/adenosinergic-pathway resets immune tolerance through the expansion of autoantigen-specific inducible Treg cells
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