Samantha L. Bailey

Epitope spreading initiates in the CNS in two mouse models of multiple sclerosis

Chronic progression of two T cell–mediated central nervous system (CNS) demyelinating models of multiple sclerosis, relapsing EAE (R-EAE) and Theilers murine encephalomyelitis virus–induced demyelinating disease (TMEV-IDD) is dependent on the activation of T cells to endogenous myelin epitopes (epitope spreading). Using transfer of carboxyfluorescein succinyl ester (CFSE)-labeled T-cell receptor (TCR)-transgenic T cells and mixed bone marrow chimeras, we show that activation of naive proteolipid protein (PLP)139–151-specific T cells in SJL mice undergoing PLP178–191-induced R-EAE or TMEV-IDD occurs directly in the CNS and not in the cervical lymph nodes or other peripheral lymphoid organs. Examination of the antigen-presentation capacity of antigen-presenting cell (APC) populations purified from the CNS of mice with PLP178–191-induced R-EAE shows that only F4/80−CD11c+CD45hi dendritic cells (DCs) efficiently present endogenous antigen to activate naive PLP139–151-specific T cells in vitro. In contrast, DCs as well as F4/80+CD45hi macrophages and F4/80+CD45lo microglia activate a PLP139–151-specific helper T cell line. The data suggest that naive T cells enter the inflamed CNS and are activated by local APCs, possibly DCs, to initiate epitope spreading.

CNS myeloid DCs presenting endogenous myelin peptides 'preferentially' polarize CD4+ TH-17 cells in relapsing EAE

Peripherally derived CD11b+ myeloid dendritic cells (mDCs), plasmacytoid DCs, CD8α+ DCs and macrophages accumulate in the central nervous system during relapsing experimental autoimmune encephalomyelitis (EAE). During acute relapsing EAE induced by a proteolipid protein peptide of amino acids 178–191, transgenic T cells (139TCR cells) specific for the relapse epitope consisting of proteolipid protein peptide amino acids 139–151 clustered with mDCs in the central nervous system, were activated and differentiated into T helper cells producing interleukin 17 (TH-17 cells). CNS mDCs presented endogenously acquired peptide, driving the proliferation of and production of interleukin 17 by naive 139TCR cells in vitro and in vivo. The mDCs uniquely biased TH-17 and not TH1 differentiation, correlating with their enhanced expression of transforming growth factor-β1 and interleukins 6 and 23. Plasmacytoid DCs and CD8α+ DCs were superior to macrophages but were much less efficient than mDCs in presenting endogenous peptide to induce TH-17 cells. Our findings indicate a critical function for CNS mDCs in driving relapses in relapsing EAE.

Dendritic cells with TGF-β1 differentiate naïve CD4+CD25− T cells into islet-protective Foxp3+ regulatory T cells

CD4+CD25+Foxp3+ regulatory T cells (T regs) are important for preventing autoimmune diabetes and are either thymic-derived (natural) or differentiated in the periphery outside the thymus (induced). Here we show that β-cell peptide-pulsed dendritic cells (DCs) from nonobese diabetic (NOD) mice can effectively induce CD4+CD25+Foxp3+ T cells from naïve islet-specific CD4+CD25− T cells in the presence of TGF-β1. These induced, antigen-specific T regs maintain high levels of clonotype-specific T cell receptor expression and exert islet-specific suppression in vitro. When cotransferred with diabetogenic cells into NOD scid recipients, T regs induced with DCs and TGF-β1 prevent the development of diabetes. Furthermore, in overtly NOD mice, these cells are able to significantly protect syngeneic islet grafts from established destructive autoimmunity. These results indicate a role for DCs in the induction of antigen-specific CD4+CD25+Foxp3+ T cells that can inhibit fully developed autoimmunity in a nonlymphopoenic host, providing an important potential strategy for immunotherapy in patients with autoimmune diabetes.

The integrated stress response prevents demyelination by protecting oligodendrocytes against immune-mediated damage

In response to ER stress, the pancreatic endoplasmic reticulum kinase (PERK) coordinates an adaptive program known as the integrated stress response (ISR) by phosphorylating the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha). IFN-gamma, which activates the ER stress response in oligodendrocytes, is believed to play a critical role in the immune-mediated CNS disorder multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE). Here we report that CNS delivery of IFN-gamma before EAE onset ameliorated the disease course and prevented demyelination, axonal damage, and oligodendrocyte loss. The beneficial effects of IFN-gamma were accompanied by PERK activation in oligodendrocytes and were abrogated in PERK-deficient animals. Our results indicate that IFN-gamma activation of PERK in mature oligodendrocytes attenuates EAE severity and suggest that therapeutic approaches to activate the ISR could prove beneficial in MS.

Antigen Presentation in the CNS by Myeloid Dendritic Cells Drives Progression of Relapsing Experimental Autoimmune Encephalomyelitis

Abstract:  Chronic progression of relapsing experimental autoimmune encephalomyelitis (R‐EAE), a mouse model of multiple sclerosis (MS), is dependent on the activation of T cells to endogenous myelin epitopes, that is, epitope spreading. This review focuses on the cellular and molecular mechanisms underlying the process of epitope spreading. Surprisingly, activation of naïve T cells to endogenous myelin epitopes in SJL mice undergoing R‐EAE occurs directly in the central nervous system (CNS), a site generally perceived to be immunologically privileged. Determination of the antigen presentation capacity of antigen‐presenting cell (APC) populations purified from the CNS of mice with established R‐EAE shows that peripherally derived CD11b+CD11c+CD45hi myeloid dendritic cells (mDCs) most efficiently present endogenous myelin antigens to activate both preactivated effector myelin‐specific T cells and naïve T cells. The mDCs, which drive epitope spreading, preferentially polarize pathogenic Th17 responses correlating with their enhanced expression of TGF‐β1, IL‐6, and IL‐23. Both B220+CD11c+ plasmacytoid (pDCs) and CD8α+CD11c+ (CD8 DCs) were superior to CD11b+CD11c–CD45hi macrophages, but less efficient than mDCs at presenting endogenous peptide to induce Th17 cells. In contrast, CNS‐resident CD11b+CD11c–CD45low microglia purified from the inflamed CNS were found to be largely incapable of activating either naïve or effector T cells.

PD-1 ligands expressed on myeloid-derived APC in the CNS regulate T-cell responses in EAE

Disease progression in experimental autoimmune encephalomyelitis (EAE) is regulated by programmed death receptor 1 (PD‐1) and its ligands, B7‐H1 (programmed death ligand 1 (PD‐L1)) and B7‐DC (PD‐L2). B7‐H1 and B7‐DC have negative regulatory effects upon binding PD‐1 on activated T cells and B7‐H1 deficiency increases severity of both diabetes and EAE. However, the role of PD‐L expression on different APC in the CNS in regulating local T‐cell function during relapsing EAE has not been examined. Our data show that the majority of CNS CD4+ T cells isolated during acute EAE are PD‐1+, and T cells specific for relapse‐associated epitopes express PD‐1 upon antigen stimulation in the CNS. B7‐H1 and B7‐DC are differentially expressed on discrete APC populations in the inflamed CNS. B7‐H1 and PD‐1 have mainly inhibitory functions on CNS T cells. B7‐H1 negatively regulates the stimulation of activated PD‐1+ TH cells, in co‐cultures with microglia and different CNS‐infiltrating APC presenting endogenously processed peptides. The preponderance of IFN‐γ+ versus IL‐17+ T cells in the CNS of B7‐H1−/− mice suggests that B7‐H1 more selectively suppresses TH‐1 than TH‐17 responses in vivo. In contrast, blockade of B7‐DC has less pronounced regulatory effects. Overall, the results demonstrate that B7‐H1 expressed by CNS myeloid APC negatively regulates T‐cell activation during acute relapsing EAE.

T-cell response dynamics in animal models of multiple sclerosis: implications for immunotherapies.

Multiple sclerosis (MS) is a multifactorial autoimmune disease of the central nervous system with a complex immune nature and varied clinical presentation. Current therapies for MS are limited by toxicity and efficacy, so interest has now turned to specifically modulating autoreactive T-cell responses. Murine MS models, such as experimental autoimmune encephalomyelitis (EAE), have proved invaluable for understanding the immune components of MS and for designing and testing potential immunotherapies. Here, we review the current knowledge of the mechanisms of induction and progression of EAE and MS and the immunotherapies that have resulted from studies of the EAE model.

CNS dendritic cells in inflammation and disease

Multiple sclerosis (MS) is a multi-factorial disease associated with chronic autoimmune inflammation of the central nervous system (CNS). In MS, and the relevant animals models experimental autoimmune encephalomyelitis (EAE), and Theiler’s murine encephalitis virus-induced demyelinating disease (TMEV-IDD), myelin destruction is mediated by neuroantigen-specific CD4 T cells (Gonatas et al., 1986; Miller et al., 1997; Wekerle, 1991). MS and EAE share clinical and histopathological similarities. Mononuclear cells (MNCs) accumulate in demyelinated lesions in the white and grey matter of the brain and spinal cord. Infiltrates are composed of CD4 and CD8 T cells, B cells, macrophages and dendritic cells (DCs). CD4 T cells and DCs are critical for the initiation and progression of EAE as CD4 T cell depletion renders mice resistant to EAE (Jameson et al., 1994; McDevitt et al., 1987; Sedgwick and Mason, 1986; Waldor et al., 1985), and DCs from the CNS uniquely activate naïve myelin-specific T cells by acquiring and processing endogenous myelin peptides (Bailey et al., 2007). The initiating events in MS are unknown, but studies in the EAE and TMEV-IDD animal models indicate that CNS damage is caused by direct and indirect effects of inflammatory cytokines and chemokines (e.g. TNF, IFN-γ, IL-17, CCL2, etc.) (Begolka et al., 1998; Chen et al., 2006; Karpus et al., 1995; Powell et al., 1990), that induce the activation and recruitment of monocyte/macrophages and resident microglia, that cause axon damage and demyelination by bystander mechanisms (Cammer et al., 1978; Rivers and Schwentker, 1935). EAE can be induced in a variety of mouse strains by immunizing with mouse spinal cord homogenate or myelin proteins and peptides in complete Freund’s adjuvant (CFA), or by the transfer of activated CD4 T cells expanded in the lymph nodes of myelin/CFA immunized donor mice. For initiation of EAE, activated CD4 T cells must encounter myelin peptides presented by MHC class II-expressing cells in the CNS (Hickey and Kimura, 1988; Tompkins et al., 2002). T cell activation/re-activation in the CNS is quickly followed by the recruitment of other peripheral immune populations, and subsequently clinical symptoms ranging from tail tone loss and hind weakness to hind and fore-limb paralysis.