Richard A. O'Connor

Cutting Edge: Th1 Cells Facilitate the Entry of Th17 Cells to the Central Nervous System during Experimental Autoimmune Encephalomyelitis

It has recently been proposed that experimental autoimmune encephalomyelitis, once considered the classical Th1 disease, is predominantly Th17 driven. In this study we show that myelin-reactive Th1 preparations devoid of contaminating IL-17+ cells are highly pathogenic. In contrast, Th17 preparations lacking IFN-γ+ cells do not cause disease. Our key observation is that only Th1 cells can access the noninflamed CNS. Once Th1 cells establish the experimental autoimmune encephalomyelitis lesion, Th17 cells appear in the CNS. These data shed important new light on the ability of Th1 vs Th17 cells to access inflamed vs normal tissue. Because the IL-17-triggered release of chemokines by stromal cells could attract many other immune cells, allowing Th17 cells to access the tissues only under conditions of inflammation may be a key process limiting (auto)immune pathology. This has major implications for the design of therapeutic interventions, many of which are now aiming at Th17 rather than Th1 cells.

The Inflamed Central Nervous System Drives the Activation and Rapid Proliferation of Foxp3+ Regulatory T Cells

Resolution of experimental autoimmune encephalomyelitis requires a large cohort of Foxp3+ regulatory T cells (Tregs) within the CNS. In this study, we have used the passive transfer of murine experimental autoimmune encephalomyelitis using myelin-reactive T cells to study the development of this Treg response. Rapid proliferation of Tregs within the CNS (which is not seen in lymphoid organs) drives a switch in the balance of CNS proliferation from T effectors to Tregs, correlating with recovery. This proliferative burst drives a local over-representation of Vβ8+ Tregs in the CNS, indicative of an oligoclonal expansion. There is also evidence for a small, but detectable, myelin oligodendrocyte glycoprotein-reactive Treg component expanded without prior immunization. Furthermore, CNS-derived Tregs, taken during recovery, suppressed the proliferation of CNS-derived effectors in response to myelin oligodendrocyte glycoprotein. Under these conditions, Tregs could also limit the level of IFN-γ production, but not IL-17 production, by CNS-derived effectors. These data establish the CNS as an environment that permits extensive Treg proliferation and are the first to demonstrate Treg expansion specifically within the tissues during the natural resolution of autoimmune inflammation.

A Pivotal Role for CD40-Mediated IL-6 Production by Dendritic Cells during IL-17 Induction In Vivo

The costimulatory requirements for Th17 development remain to be defined. In this study, we show that CD40-deficient animals immunized with the Gram-positive bacterium Propionibacterium acnes were specifically impaired in their ability to mount an IL-17 response, but not that of IFN-γ. The same cytokine imbalance resulted from in vivo priming with pathogen-pulsed, CD40-deficient dendritic cells (DC). Engagement of CD40 on P. acnes-conditioned DC stimulated the release of IL-12, IL-23, and IL-6, of which IL-6 alone proved essential for Th17 differentiation. Compared with wild-type DC, priming with those lacking expression of CD40 resulted in reduced disease severity during experimental autoimmune encephalomyelitis, coincident with reduced IL-17 production. Our data delineate sequential requirements for DC expression of CD40 and production of IL-6 during Th17 polarization in vitro and in vivo, and reveal distinct costimulatory requirements for Th1 vs Th17 generation.

Prostaglandin E2 constrains systemic inflammation through an innate lymphoid cell–IL-22 axis

A prostaglandin barrier to inflammation Blood-borne bacterial infections and severe trauma can send the immune system into overdrive, causing it to pump out inflammatory mediators, sometimes at lethal doses. Duffin et al. now report on a role for prostaglandins in keeping systemic inflammation in check. Systemic inflammation correlates with decreased production of the prostaglandin E2 (PGE2). Blocking PGE2 signaling in mice led to severe inflammation associated with the translocation of gut bacteria. PGE2 acts on innate lymphoid cells, which produce interleukin-22, a secreted protein that helps promote intestinal integrity. Science, this issue p. 1333 Prostaglandin E2 prevents systemic inflammation by maintaining gut barrier integrity. Systemic inflammation, which results from the massive release of proinflammatory molecules into the circulatory system, is a major risk factor for severe illness, but the precise mechanisms underlying its control are not fully understood. We observed that prostaglandin E2 (PGE2), through its receptor EP4, is down-regulated in human systemic inflammatory disease. Mice with reduced PGE2 synthesis develop systemic inflammation, associated with translocation of gut bacteria, which can be prevented by treatment with EP4 agonists. Mechanistically, we demonstrate that PGE2-EP4 signaling acts directly on type 3 innate lymphoid cells (ILCs), promoting their homeostasis and driving them to produce interleukin-22 (IL-22). Disruption of the ILC–IL-22 axis impairs PGE2-mediated inhibition of systemic inflammation. Hence, the ILC–IL-22 axis is essential in protecting against gut barrier dysfunction, enabling PGE2-EP4 signaling to impede systemic inflammation.

Myelin-reactive, TGF-β-induced regulatory T cells can be programmed to develop Th1-like effector function but remain less proinflammatory than myelin-reactive Th1 effectors and can suppress pathogenic T cell clonal expansion in vivo.

Interest in the use of regulatory T cells (Tregs) as cellular therapeutics has been tempered by reports of naturally occurring Tregs losing Foxp3 expression and producing IL-17, raising concerns over a switch to pathogenic function under inflammatory conditions in vivo. TGF-β–induced Tregs (inducible Tregs [iTregs]), generated in large numbers in response to disease-relevant Ags, represent the most amenable source of therapeutic Tregs. Using Foxp3-reporter T cells recognizing myelin basic protein (MBP), we investigated the capacity of iTregs to produce effector-associated cytokines under proinflammatory cytokine conditions in vitro and whether this translated into proinflammatory function in vivo. In contrast with naturally occurring Tregs, iTregs resisted conversion to an IL-17–producing phenotype but were able to express T-bet and to produce IFN-γ. iTregs initiated their T-bet expression during their in vitro induction, and this was dependent on exposure to IFN-γ. IL-12 reignited iTreg expression of T-bet and further promoted iTreg production of IFN-γ upon secondary stimulation. Despite losing Foxp3 expression and expressing both T-bet and IFN-γ, MBP-responsive IL-12–conditioned iTregs induced only mild CNS inflammation and only when given in high numbers. Furthermore, iTregs retained an ability to suppress naive T cell clonal expansion in vivo and protected against the development of experimental autoimmune encephalomyelitis. Therefore, despite bearing predictive hallmarks of pathogenic effector function, previously Foxp3+ iTregs have much lower proinflammatory potential than that of MBP-responsive Th1 cells. Our results demonstrate that autoprotective versus autoaggressive functions in iTregs are not simply a binary relationship to be determined by their relative expression of Foxp3 versus T-bet and IFN-γ.

Translational Mini‐Review Series on Th17 Cells: CD4+ T helper cells: functional plasticity and differential sensitivity to regulatory T cell‐mediated regulation

OTHER ARTICLES PUBLISHED IN THIS MINI‐REVIEW SERIES ON Th17 CELLS
Function and regulation of human T helper 17 cells in health and disease. Clin Exp Immunol 2009; doi:10.1111/j.1365‐2249.2009.04037.x
Induction of interleukin‐17 production by regulatory T cells. Clin Exp Immunol 2009; doi:10.1111/j.1365‐2249.2009.04038.x
Are T helper 17 cells really pathogenic in autoimmunity? Clin Exp Immunol 2009; doi:10.1111/j.1365‐2249.2009.04039.x
Development of mouse and human T helper 17 cells. Clin Exp Immunol 2009; doi:10.1111/j.1365‐2249.2009.04041.x

Activation of CD4⁺ Foxp3⁺ regulatory T cells proceeds normally in the absence of B cells during EAE.

B cells and regulatory T (Treg) cells can both facilitate remission from experimental auto immune encephalomyelitis (EAE), a disease of the central nervous system (CNS) used as a model for multiple sclerosis (MS). Considering that B‐cell‐depletion therapy (BCDT) is used to treat MS patients, we asked whether Treg‐cell activation depended on B cells during EAE. Treg‐cell proliferation, accumulation in CNS, and augmentation of suppressive activity in the CNS were normal in B‐cell‐deficient mice, indicating that B cells are not essential for activation of the protective Treg‐cell response and thus provide an independent layer of regulation. This function of B cells involved early suppression of the encephalitogenic CD4+ T‐cell response, which was enhanced in B‐cell‐deficient mice. CD4+ T‐cell depletion was sufficient to intercept the transition from acute‐to‐chronic EAE when applied to B‐cell‐deficient animals that just reached the peak of disease severity. Intriguingly, this treatment did not improve disease when applied later, implying that chronic disability was ultimately maintained independently of pathogenic CD4+ T cells. Collectively, our data indicate that BCDT is unlikely to impair Treg‐cell function, yet it might produce undesirable effects on T‐cell‐mediated autoimmune pathogenesis.

Multi-faceted control of autoaggression: Foxp3+regulatory T cells in murine models of organ-specific autoimmune disease

The discovery of forkhead box p3 (Foxp3) as the critical transcriptional controller of suppressive function in murine CD4(+) T regulatory (Treg) cells has allowed precise analyses of these cells in a range of immunopathological models. Recent data have revealed key roles for Foxp3+ Tregs in murine models of human organ-specific autoimmune conditions. Do these Tregs target the same autoantigens recognized by the autoaggressive T cells that need to be controlled? Under steady state conditions there may not be a need for such a shared recognition to dampen spontaneous anti-self priming in the lymphoid organs. However, when they are needed to control ongoing inflammation, Tregs recognizing autoantigens found in the diseased organ appear to have significantly stronger suppressive powers. We reflect on these observations that clearly have relevance for the translation of Treg-targeting immune therapies to human disease.

Adjuvant Immunotherapy of Experimental Autoimmune Encephalomyelitis: Immature Myeloid Cells Expressing CXCL10 and CXCL16 Attract CXCR3+CXCR6+ and Myelin-Specific T Cells to the Draining Lymph Nodes Rather Than the Central Nervous System

CFA is a strong adjuvant capable of stimulating cellular immune responses. Paradoxically, adjuvant immunotherapy by prior exposure to CFA or live mycobacteria suppresses the severity of experimental autoimmune encephalomyelitis (EAE) and spontaneous diabetes in rodents. In this study, we investigated immune responses during adjuvant immunotherapy of EAE. Induction of EAE in CFA-pretreated mice resulted in a rapid influx into the draining lymph nodes (dLNs) of large numbers of CD11b+Gr-1+ myeloid cells, consisting of immature cells with ring-shaped nuclei, macrophages, and neutrophils. Concurrently, a population of mycobacteria-specific IFN-γ–producing T cells appeared in the dLNs. Immature myeloid cells in dLNs expressed the chemokines CXCL10 and CXCL16 in an IFN-γ–dependent manner. Subsequently, CD4+ T cells coexpressing the cognate chemokine receptors CXCR3 and CXCR6 and myelin oligodendrocyte glycoprotein (MOG)-specific CD4+ T cells accumulated within the chemokine-expressing dLNs, rather than within the CNS. Migration of CD4+ T cells toward dLN cells was abolished by depleting the CD11b+ cells and was also mediated by the CD11b+ cells alone. In addition to altering the distribution of MOG-specific T cells, adjuvant treatment suppressed development of MOG-specific IL-17. Thus, adjuvant immunotherapy of EAE requires IFN-γ, which suppresses development of the Th17 response, and diverts autoreactive T cells away from the CNS toward immature myeloid cells expressing CXCL10 and CXCL16 in the lymph nodes.

Inflammation-associated genes: risks and benefits to Foxp3+ regulatory T-cell function.

Foxp3+ regulatory T (Treg) cells prevent the development of autoimmunity and immunopathology, as well as maintaining homeostasis and tolerance to commensal microorganisms. The suppressive activity of Treg cells is their defining characteristic, generating great interest in their therapeutic potential. However, suppressive and effector functions are not entirely exclusive. Considerable evidence points to the ability of supposedly anti‐inflammatory Foxp3‐expressing Treg cells to also express transcription factors that have been characterized as cardinal drivers of T effector cell function. We will consider the mounting evidence that Treg cells can function in non‐suppressive capacities and review the impetus for this functional change, its relevance to developing immune and autoimmune responses and its significance to the development of Treg‐based therapies.