Ruolan Liu

Central nervous system (CNS)–resident natural killer cells suppress Th17 responses and CNS autoimmune pathology

Natural killer (NK) cells of the innate immune system can profoundly impact the development of adaptive immune responses. Inflammatory and autoimmune responses in anatomical locations such as the central nervous system (CNS) differ substantially from those found in peripheral organs. We show in a mouse model of multiple sclerosis that NK cell enrichment results in disease amelioration, whereas selective blockade of NK cell homing to the CNS results in disease exacerbation. Importantly, the effects of NK cells on CNS pathology were dependent on the activity of CNS-resident, but not peripheral, NK cells. This activity of CNS-resident NK cells involved interactions with microglia and suppression of myelin-reactive Th17 cells. Our studies suggest an organ-specific activity of NK cells on the magnitude of CNS inflammation, providing potential new targets for therapeutic intervention.

Differential Effects of IL-21 during Initiation and Progression of Autoimmunity against Neuroantigen

The cytokine IL-21 is closely related to IL-2 and IL-15, a cytokine family that uses the common γ-chain for signaling. IL-21 is expressed by activated CD4+ T cells. We examined the role of IL-21 in the autoimmune disease experimental autoimmune encephalomyelitis (EAE), an animal model for human multiple sclerosis. IL-21 administration before induction of EAE with a neuroantigen, myelin oligodendrocyte glycoprotein peptide 35-55, and adjuvant enhanced the inflammatory influx into the CNS, as well as the severity of EAE. Autoreactive T cells purified from IL-21-treated mice transferred more severe EAE than did the control encephalitogenic T cells. No such effects were observed when IL-21 was administered after EAE progressed. Additional studies demonstrated that IL-21 given before the induction of EAE boosted NK cell function, including secretion of IFN-γ. Depletion of NK cells abrogated the effect of IL-21. Therefore, IL-21, by affecting NK cells, has differential effects during the initiation and progression of autoimmune responses against neuroantigens.

Cooperation of invariant NKT cells and CD4+CD25+ T regulatory cells in the prevention of autoimmune myasthenia.

CD1d-restricted NKT cells and CD4+CD25+ regulatory T (Treg) cells are thymus-derived subsets of regulatory T cells that have an important role in the maintenance of self-tolerance. Whether NKT cells and Treg cells cooperate functionally in the regulation of autoimmunity is not known. We have explored this possibility in experimental autoimmune myasthenia gravis (EAMG), an animal model of human myasthenia gravis, induced by immunization of C57BL/6 mice with the autoantigen acetylcholine receptor. We have demonstrated that activation of NKT cells by a synthetic glycolipid agonist of NKT cells, α-galactosylceramide (α-GalCer), inhibits the development of EAMG. α-GalCer administration in EAMG mice increased the size of the Treg cell compartment, and augmented the expression of foxp3 and the potency of CD4+CD25+ cells to inhibit proliferation of autoreactive T cells. Furthermore, α-GalCer promoted NKT cells to transcribe the IL-2 gene and produce IL-2 protein. Depletion of CD25+ cells or neutralization of IL-2 reduced the therapeutic effect of α-GalCer in this model. Thus, α-GalCer-activated NKT cells can induce expansion of CD4+CD25+ Treg cells, which in turn mediate the therapeutic effects of α-GalCer in EAMG. Induced cooperation of NKT cells and Treg cells may serve as a superior strategy to treat autoimmune disease.

Expansion of regulatory T cells via IL-2/anti-IL-2 mAb complexes suppresses experimental myasthenia.

Human autoimmune diseases are often characterized by a relative deficiency in CD4+CD25+ regulatory T cells (Treg). We therefore hypothesized that expansion of Treg can ameliorate autoimmune pathology. We tested this hypothesis in an experimental model for autoimmune myasthenia gravis (MG), a B‐cell‐mediated disease characterized by auto‐Ab directed against the acetylcholine receptor within neuromuscular junctions. We showed that injection of immune complexes composed of the cytokine IL‐2 and anti‐IL‐2 mAb (JES6‐1A12) induced an effective and sustained expansion of Treg, via peripheral proliferation of CD4+CD25+Foxp3+ cells and peripheral conversion of CD4+CD25−Foxp3− cells. The expanded Treg potently suppressed autoreactive T‐ and B‐cell responses to acetylcholine receptor and attenuated the muscular weakness that is characteristic of MG. Thus, IL‐2/anti‐IL‐2 mAb complexes can expand functional Treg in vivo, providing a potential clinical application of this modality for treatment of MG and other autoimmune disorders.

IL-21 modulates CD4+ CD25+ regulatory T-cell homeostasis in experimental autoimmune encephalomyelitis.

Abstract The homeostasis of CD4 + CD25 +  regulatory T cells (Tregs) depends on the cytokine interleukin (IL)‐2. As IL‐21 shares sequence homology with IL‐2 and the IL‐21 receptors contain a γ‐chain common to IL‐2, we hypothesized that IL‐21 could also affect the homeostasis of Tregs. We tested this hypothesis in experimental autoimmune encephalomyelitis (EAE), an animal model of relapsing‐remitting human multiple sclerosis. We show that blockade of IL‐21 in SJL/J mice before and after the induction of EAE enhances the influx of inflammatory cells into the central nervous system (CNS). The blockade of IL‐21 leads to proliferation of proteolipid peptide (PLP139‐151)‐autoreactive CD4 +  T cells, which are capable to cause severe EAE in adoptively transferred recipient mice. Conversely, Tregs from mice where IL‐21 was blocked, lose their capacity to prevent EAE induced PLP139‐151‐reactive T cells. Notably, direct effects of IL‐21 on Tregs are confirmed by studies of blockade of IL‐21 in mice expressing a green fluorescent protein ‘knocked’ into a Foxp3 allele, in which a reduction of the number of Tregs and a downregulation of their frequency and expression of Foxp3 are observed. These data suggest a role of the IL‐21/IL‐21R axis in the homeostasis of Tregs in CNS autoimmunity.

Autoreactive T Cells Mediate NK Cell Degeneration in Autoimmune Disease

Emerging evidence indicates that NK cells play an important and complex role in autoimmune disease. Humans with autoimmune diseases often have reduced NK cell numbers and compromised NK cell functions. Mechanisms underlying this NK cell degeneration and its biological significance are not known. In this study we show that, in an experimental model of human autoimmune myasthenia gravis induced by a self-Ag, the acetylcholine receptor, NK cells undergo proliferation during the initiation of autoimmunity, followed by significant degeneration associated with the establishment of the autoreactive T cell response. We show that NK cell degeneration was mediated by IL-21 derived from autoreactive CD4+ T cells, and that acetylcholine receptor-immunized IL-21R-deficient mice, with competent NK cells, developed exacerbated autoimmunity. Thus, NK cell degeneration may serve as a means evolved by the immune system to control excessive autoimmunity.

Interleukin-2/interleukin-2 antibody therapy induces target organ natural killer cells that inhibit central nervous system inflammation.

The role of natural killer (NK) cells in regulating multiple sclerosis (MS) is not well understood. Additional studies with NK cells might provide insight into the mechanism of action of MS therapies such as daclizumab, an antibody against the interleukin (IL)‐2R α‐chain, which induces expansion of CD56bright NK cells.

CCL2 recruitment of IL-6-producing CD11b+ monocytes to the draining lymph nodes during the initiation of Th17-dependent B cell-mediated autoimmunity.

The development and function of Th17 cells are influenced in part by the cytokines TGF‐β, IL‐23 and IL‐6, but the mechanisms that govern recruitment and activity of Th17 cells during initiation of autoimmunity remain poorly defined. We show here that the development of autoreactive Th17 cells in secondary lymphoid organs in experimental autoimmune myasthenia gravis – an animal model of human myasthenia gravis – is modulated by IL‐6‐producing CD11b+ cells via the CC chemokine ligand 2 (CCL2). Notably, acetylcholine receptor (AChR)‐reactive Th17 cells provide help for the B cells to produce anti‐AChR antibodies, which are responsible for the impairment of the neuromuscular transmission that contributes to the clinical manifestations of autoimmunity, as indicated by a lack of disease induction in IL‐17‐deficient mice. Thus, Th17 cells can promote humoral autoimmunity via a novel mechanism that involves CCL2.

IL-21 receptor expression determines the temporal phases of experimental autoimmune encephalomyelitis.

The IL-21 receptor (IL-21R) consists of a unique subunit and a common gamma chain (gamma(c)) that is shared with other cytokines including IL-2, IL-4, IL-7, and IL-15. The interaction between IL-21 and IL-21R results in significant effects on both innate and adaptive immune responses. In this study we examined the influence of IL-21R deficiency (IL-21R(-/-)) on the development of experimental autoimmune encephalomyelitis (EAE), an animal model of human multiple sclerosis (MS). IL-21R(-/-) mice developed EAE earlier and more severe neurological impairment than control mice, yet those mice could effectively recover from neurological deficits. The impact on EAE initiation by IL-21R deficiency was associated with a defect of CD4(+)CD25(+) T regulatory (Treg) cells and a down-regulated expression of Foxp3. The recovery from IL-21R(-/-) EAE was correlated with an expansion of Treg cells as well as an organ-specific redistribution of NK cells. These results suggest that a temporal influence of IL-21 on the activity of immunoregulatory circuits can be important in the modulation of the course of the autoimmune disease.

T-bet deficiency decreases susceptibility to experimental myasthenia gravis.

T-bet, a tissue-specific transcription factor, controls T helper 1 (Th1) cell differentiation and IFN-production. Given the reciprocal relationship between Th1 and other types of helper T cells, we hypothesized that T-bet impacts multiple helper and regulatory T (Treg) cells, thereby influencing the magnitude of autoimmune disease. We tested this hypothesis in an experimental model of autoimmune myasthenia gravis (EAMG) of mice. Myasthenia gravis (MG) and EAMG are T cell-driven, IgG autoantibody-mediated disorders that destroy muscles by attacking the target antigen acetylcholine receptor (AChR) or other antigens of skeletal muscle at neuromuscular junctions. We show that, compared to wild-type mice, AChR-primed T-bet(-/-) mice are less susceptible to EAMG. This phenotype is associated with a reduction of autoreactive Th1 cells and augmentation of Th2 and Th17 cells as well as an upregulation of Foxp3 expression by T-bet(-/-)CD4(+)CD25(+) Treg cells. Thus, in our model, T-bet not only specifies the Th1 lineage but also has a broad influence on autoreactive Th2, Th17 and Treg cells. These coordinated effects reduce the genesis of pathogenic antibodies and protect against B cell-mediated EAMG.