Using EAE to better understand principles of immune function and autoimmune pathology

Abstract

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) in which myelin becomes the target of attack by autoreactive T cells. The immune components of the disease are recapitulated in mice using the experimental autoimmune encephalomyelitis (EAE) model. EAE is classically induced by the immunization of mice with encephalitogenic antigens derived from CNS proteins such as proteolipid protein (PLP), myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG). Immunization of susceptible mouse strains with these antigens will induce autoreactive inflammatory T cell infiltration of the CNS. More recently, the advent of clonal T cell receptor transgenic mice has led to the development of adoptive transfer protocols in which myelin-specific T cells may induce disease upon transfer into naïve recipient animals. When used in concert with gene knockout strains, these protocols are powerful tools by which to dissect the molecular pathways that promote inflammatory T cells responses in the central nervous system (CNS). Further, myelin-antigen-specific transgenic T cells may be cultured in vitro under a variety of conditions prior to adoptive transfer, allowing one to study the effects of soluble factors or pharmacologic compounds on T cell pathogenicity. In this review, we describe many of the existing models of EAE, and discuss the contributions that use of these models has made in understanding both T helper cell differentiation and the function of inhibitory T cell receptors. We focus on the step-by-step elucidation of the network of signals required for T helper 17 (Th17) cell differentiation, as well as the molecular dissection of the Tim-3 negative regulatory signaling pathway in Th1 cells.

Keywords: Bat3; CD4+ T lymphocyte, myelin; Experimental autoimmune encephalomyelitis; Interleukin-17; Multiple sclerosis; Tim-3.

Figure 1. Variations in EAE disease course upon active immunization with encephalitogenic peptides

A. The immunization of SJL/J mice with PLP_139-151 can result in an initial paralytic attack, followed by multiple remissions and relapses [24]. B. Immunization of C57BL6/J mice with a high dose of MOG_35-55 causes a chronic disease course in which an initial attack does not resolve (solid line) [17, 33]. By contrast, immunization with a low dose of MOG_35-55 can induce either a multiphasic relapsing-remitting disease course (dotted line) or a single attack followed by a single remission (not shown) [17]. C. Immunization of NOD mice with MOG_35-55 results in a series of mild attacks/relapses followed by remissions. The disease ultimately transitions into a secondary chronic phase that is reminiscent of the secondary progressive phase of MS [56].

Figure 2. Use of MOG_35-55-driven EAE in gene knockout strains to identify a role for Th17 responses in CNS autoimmunity

A. IL-23-driven, and not IL-12-driven, responses are essential for EAE pathology. IFNγ^−/− mice develop highly severe disease (top panel), despite the role of IFNγ as the key Th1 cytokine [–66]. Further, IL-12p35^−/− [67, 69, 70] and IL-12Rβ1^−/− [71] mice, which specifically lack signaling through the Th1-differentiating cytokine IL-12, are susceptible to EAE (second panel), while IL-12p40^−/− mice, which lack IL-12 and IL-23 expression, are resistant [67] (third panel). Importantly, IL-23p19^−/− mice, which lack IL-23 exclusively, are also resistant to EAE [70] (fourth panel). These studies collectively demonstrate a role for the Th17-promoting cytokine IL-23 in EAE pathogenesis. B. Role of TGFβ and IL-6 in promoting EAE. TGFβTg-CD4 Cre mice, which ectopically express TGFβ in CD4 ⁺ T cells, develop highly severe EAE in comparison to control animals [75] (top panel). Further, while IL-6^−/− mice are relatively resistant to EAE (second panel), disease severity increases, and Th17 responses are restored, when CD25+ T_regs are depleted in vivo (bottom panel) [79]. These findings indicate that TGFβ and IL-6 co-operate to promote Th17 responses, and that IL-6 does so in part by suppressing the activity of T_regs.

Figure 3. Role of Tim-3 in normal and pathologic immune responses

Upon encountering its cognate antigen in the context of an IL-12-rich milieu, a T cell will differentiate into a IFNγ⁺ Th1 (CD4⁺) or Tc1 (CD8⁺) cell. A. In the case of an acute infection in which the antigen is ultimately cleared, Th1/Tc1 cells will secrete large quantities of the proinflammatory cytokines IFNγ, TNFα and IL-2, and upon terminal differentiation, will express high levels of Tim-3 [103]. Upon interaction with the Tim-3 ligand galectin-9 [106], these Tim-3⁺ T cells will undergo cell death and will be deleted from the repertoire. B. In instances of chronic viral infections [109, 110] and malignancies [111, 112], Tim-3 expression may become decoupled from the production of inflammatory cytokines. In the presence of persistent antigen, exhausted T cells progressively lose the capacity to produce IFNγ, TNFα and IL-2, and begin to express Tim-3. These exhausted T cells may persist in the periphery but ultimately undergo cell death [113]. C. In the case of autoimmunity, autoreactive Th1 cells produce great quantities of inflammatory cytokines and mediate tissue destruction. In the case of MS, these cells express display defective Tim-3 signaling [107, 108]. One possible mechanism for T cell dysfunction may be increased expression of Bat3, which represses Tim-3 signaling. Strategies that modulate the expression and function of Bat3 may result in augmentation of Tim-3 responses and subsequent amelioration of autoimmunity [16].