Andrew W. Taylor

Review of the activation of TGF-β in immunity

The evolutionarily conserved TGF‐β proteins are distributed ubiquitously throughout the body and have a role in almost every biological process. In immunity, TGF‐β has an important role in modulating immunity. Much is understood about the process of TGF‐β production as a latent molecule and of the consequences and the intercellular signaling of active TGF‐β binding to its receptors; however, there is little discussed between the production and activation of TGF‐β. This review focuses on what is understood about the biochemical and physiological processes of TGF‐β activation and identifies the gaps in understanding immune cell activation of TGF‐β. A mechanistic understanding of the process activating TGF‐β can lead to regulating multiple biological systems by enhancing or inhibiting TGF‐β activation.

Ocular immune privilege.

It has been over 60 years since the phrase immune privilege was used by Sir Peter Medawar to describe the lack of an immune response against allografts placed into the ocular microenvironment. Since then, we have come to understand that the mechanisms of ocular immune privilege include unique anatomical features of a blood barrier and a lack of direct lymphatic drainage. Also, we know that the ocular microenvironment is rich with immunosuppressive molecules that influence the activity of immune cells. Moreover, the placement of foreign antigen into the ocular microenvironment can induce a systemic form of tolerance to the foreign antigen called anterior chamber-associated immune deviation (ACAID). Many soluble immunomodulators are found in aqueous humour, and are a mixture of growth factors, cytokines, neuropeptides, and soluble receptors. This is a continuously growing list. The mechanisms of ocular immune privilege induce apoptosis, promote the production of anti-inflammatory cytokines, and mediate the activation of antigen-specific regulatory immunity. These mechanisms of immune privilege also attempt to impose themselves upon immunity within the uveitic eye. The adaptation of several anatomical and biochemical mechanisms to establish an immune privileged microenvironment within the eye makes the eye immunologically unique. It is a tissue site where we may learn how immunity is regulated in inflammation and at rest. Success in translating the lessons of ocular immune privilege to other tissues has the potential to drastically change the therapy and clinical outcomes of autoimmune diseases and allograft survival.

Ocular Immune Privilege in the Year 2010: Ocular Immune Privilege and Uveitis

The phrase “immune privilege” was coined by Peter Medawar to describe the absence of an immune response to allografts placed into the anterior chamber of the eye or brain. We now understand that immune privilege is more than a passive microenvironment with a distinctive anatomical structure that holds back immunity. The ocular microenvironment actively engages the immune system with immunosuppressive biochemical mechanisms. The unique characteristics of ocular immune privilege appear designed to protect the eye from damage while preserving foveal vision, thus providing the host with a definite survival advantage. However, the protection is not always sufficient and the eye becomes susceptible to uveitis. Uveitis is an intraocular inflammatory disorder that encompasses a wide range of underlying etiologies. It may be idiopathic or associated with systemic disease or infection. Understanding the biochemistry of immune privilege has the potential to identify its weaknesses that allow for immunity to break through.

Both MC5r and A2Ar Are Required for Protective Regulatory Immunity in the Spleen of Post–Experimental Autoimmune Uveitis in Mice

The ocular microenvironment uses a poorly defined melanocortin 5 receptor (MC5r)-dependent pathway to recover immune tolerance following intraocular inflammation. This dependency is seen in experimental autoimmune uveoretinitis (EAU), a mouse model of endogenous human autoimmune uveitis, with the emergence of autoantigen-specific regulatory immunity in the spleen that protects the mice from recurrence of EAU. In this study, we found that the MC5r-dependent regulatory immunity increased CD11b+F4/80+Ly-6ClowLy-6G+CD39+CD73+ APCs in the spleen of post-EAU mice. These MC5r-dependent APCs require adenosine 2A receptor expression on T cells to activate EAU-suppressing CD25+CD4+Foxp3+ regulatory T cells. Therefore, in the recovery from autoimmune disease, the ocular microenvironment induces tolerance through a melanocortin-mediated expansion of Ly-6G+ regulatory APCs in the spleen that use the adenosinergic pathway to promote activation of autoantigen-specific regulatory T cells.

Following EAU Recovery There Is an Associated MC5r-Dependent APC Induction of Regulatory Immunity in the Spleen

PURPOSE IRBPp-specific regulatory immunity is found in the spleens of mice recovered from experimental autoimmune uveoretinitis (EAU). Induction of this regulatory immunity is dependent on the expression of the melanocortin 5 receptor (MC5r). Therefore, the authors investigated whether dependence on the expression of MC5r was with the T cells or with the APCs mediating protective regulatory immunity in the EAU-recovered mouse spleen. METHODS Wild-type and MC5r-/- mice were immunized to induce EAU. The IRBPp-stimulated T-cell response in spleens of wild-type and MC5r-/- mice were compared for surface markers and cytokine production. Spleen APC were isolated and used to stimulate cytokine production and regulatory activity in IRBP-specific T cells from wild-type or MC5r-/- mice assayed in culture by ELISA, by flow cytometry, and in vivo by adoptive transfer into EAU mice. RESULTS IRBPp-specific CD25+CD4+ T cells from spleens of EAU-recovered wild-type mice express a Treg cell phenotype of FoxP3 and TGF-β compared with the effector T-cell phenotype of IFN-γ and IL-17 production in EAU-recovered MC5r-/- mice. APCs from the spleens of wild-type mice recovering from EAU promoted regulatory T-cell activation in IRBP-specific effector T cells from the spleens of EAU-recovering MC5r-/- mice. Spleen APCs from EAU-recovering wild-type, but not MC5r-/-, mice induced TGF-β expression by primed IRBP-specific effector T cells. CONCLUSIONS Dependence on MC5r expression is with an APC that promotes or selectively activates IRBP-specific FoxP3+ TGF-β+ CD25+CD4+ Treg cells in the spleens of EAU-recovered mice.

The neuropeptides α-MSH and NPY modulate phagocytosis and phagolysosome activation in RAW 264.7 cells

Within the immunosuppressive ocular microenvironment, there are constitutively present the immunomodulating neuropeptides alpha-melanocyte stimulating hormone (α-MSH) and neuropeptide Y (NPY) that promote suppressor functionality in macrophages. In this study, we examined the possibility that α-MSH and NPY modulate phagocytic activity in macrophages. The macrophages treated with α-MSH and NPY were significantly suppressed in their capacity to phagocytize unopsonized Escherichia coli and Staphylococcus aureus bioparticles, but not antibody-opsonized bioparticles. The neuropeptides significantly suppressed phagolysosome activation, and the FcR-associated generation of reactive oxidative species as well. This suppression corresponds to neuropeptide modulation of macrophage functionality within the ocular microenvironment to suppress the activation of immunogenic inflammation.

Re-evaluating the treatment of acute optic neuritis

Clinical case reports and prospective trials have demonstrated a reproducible benefit of hypothalamic-pituitary-adrenal (HPA) axis modulation on the rate of recovery from acute inflammatory central nervous system (CNS) demyelination. As a result, corticosteroid preparations and adrenocorticotrophic hormones are the current mainstays of therapy for the treatment of acute optic neuritis (AON) and acute demyelination in multiple sclerosis. Despite facilitating the pace of recovery, HPA axis modulation and corticosteroids have failed to demonstrate long-term benefit on functional recovery. After AON, patients frequently report visual problems, motion perception difficulties and abnormal depth perception despite ‘normal’ (20/20) vision. In light of this disparity, the efficacy of these and other therapies for acute demyelination require re-evaluation using modern, high-precision paraclinical tools capable of monitoring tissue injury. In no arena is this more amenable than AON, where a new array of tools in retinal imaging and electrophysiology has advanced our ability to measure the anatomic and functional consequences of optic nerve injury. As a result, AON provides a unique clinical model for evaluating the treatment response of the derivative elements of acute inflammatory CNS injury: demyelination, axonal injury and neuronal degeneration. In this article, we examine current thinking on the mechanisms of immune injury in AON, discuss novel technologies for the assessment of optic nerve structure and function, and assess current and future treatment modalities. The primary aim is to develop a framework for rigorously evaluating interventions in AON and to assess their ability to preserve tissue architecture, re-establish normal physiology and restore optimal neurological function.

Ex-vivo tolerogenic F4/80+ antigen-presenting cells (APC) induce efferent CD8+ regulatory T cell-dependent suppression of experimental autoimmune uveitis

It is known that inoculation of antigen into the anterior chamber (a.c.) of a mouse eye induces a.c.‐associated immune deviation (ACAID), which is mediated in part by antigen‐specific local and peripheral tolerance to the inciting antigen. ACAID can also be induced in vivo by intravenous (i.v.) inoculation of ex‐vivo‐generated tolerogenic antigen‐presenting cells (TolAPC). The purpose of this study was to test if in‐vitro‐generated retinal antigen‐pulsed TolAPC suppressed established experimental autoimmune uveitis (EAU). Retinal antigen‐pulsed TolAPC were injected i.v. into mice 7 days post‐induction of EAU. We observed that retinal antigen‐pulsed TolAPC suppressed the incidence and severity of the clinical expression of EAU and reduced the expression of associated inflammatory cytokines. Moreover, extract of whole retina efficiently replaced interphotoreceptor retinoid‐binding protein (IRBP) in the preparation of TolAPC used to induce tolerance in EAU mice. Finally, the suppression of EAU could be transferred to a new set of EAU mice with CD8+ but not with CD4+regulatory T cells (Treg). Retinal antigen‐pulsed TolAPC suppressed ongoing EAU by inducing CD8+ Treg cells that, in turn, suppressed the effector activity of the IRBP‐specific T cells and altered the clinical symptoms of autoimmune inflammation in the eye. The ability to use retinal extract for the antigen raises the possibility that retinal extract could be used to produce autologous TolAPC and then used as therapy in human uveitis.

Recovery from experimental autoimmune uveitis promotes induction of antiuveitic inducible Tregs

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.

The Alpha-Melanocyte Stimulating Hormone Induces Conversion of Effector T Cells into Treg Cells

The neuropeptide alpha-melanocyte stimulating hormone (α-MSH) has an important role in modulating immunity and homeostasis. The production of IFN-γ by effector T cells is suppressed by α-MSH, while TGF-β production is promoted in the same cells. Such α-MSH-treated T cells have immune regulatory activity and suppress hypersensitivity, autoimmune diseases, and graft rejection. Previous characterizations of the α-MSH-induced Treg cells showed that the cells are CD4+ T cells expressing the same levels of CD25 as effector T cells. Therefore, we further analyzed the α-MSH-induced Treg cells for expression of effector and regulatory T-cell markers. Also, we examined the potential for α-MSH-induced Treg cells to be from the effector T-cell population. We found that the α-MSH-induced Treg cells are CD25+  CD4+ T cells that share similar surface markers as effector T cells, except that they express on their surface LAP. Also, the α-MSH treatment augments FoxP3 message in the effector T cells, and α-MSH induction of regulatory activity was limited to the effector CD25+ T-cell population. Therefore, α-MSH converts effector T cells into Treg cells, which suppress immunity targeting specific antigens and tissues.