Immune cells involved in demyelinating diseases

Upon the stimulation by specific antigens via the MHC II molecule, naïve CD4⁺ T-cells begin to differentiate into activated T helper (Th)-cells. Different patterns of cytokine production by CD4⁺ T-cells can be observed depending on the state of activation.

These patterns represent a continuum between two phenotypes: the T helper (Th)1 and Th2 phenotype. Th1 cells respond optimally to antigens presented by myeloid dendritic cells and B-cells using the costimulatory molecule CD80. The Th1 phenotype is characterized by the release of pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-1, which are required for the cellular immunity against intracellular pathogens. These cytokines are important for the migration and homing of inflammatory cells to the target site and for the initiation of inflammatory responses. TNF-α promotes the adhesion of inflammatory cells to the vascular endothelium and their subsequent extravasation as well as the activation of macrophages. IL-1 is also important for the activation of macrophages and for the recruitment of further T-lymphocytes. In addition to their physiological function in

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cellular immunity, uncontrolled Th1-cell stimulation promotes delayed-type hypersensitivity reactions in several immune mediated disorders. The Th2 phenotype is characterized by the release of anti-inflammatory cytokines such as transforming growth factor (TGF)-β, IL-4, IL-5 and IL-13 (Tizard, 2008). Th2 cells respond to antigens presented by lymphoid or plasmacytoid dendritic cells and macrophages.

Thus, the balance between the Th1 and Th2 phenotypes determines whether the overall T-cell population has a pro-inflammatory or anti-inflammatory activity (Hartung et al., 2005). Th1-cells dysregulations are observed in different organ-specific autoimmune diseases. For example, CD4⁺ Th1-cells play a crucial role in the development of CNS lesions in MS (Sospedra and Martin, 2005). Th17-cells belong to another T helper-cell lineage, different from Th1- and Th2-cells. These lymphocytes produce cytokines of the IL-17 family, which can be observed during the infection with different bacterial and fungal species. IL-17A and IL-17F are involved in the recruitment, activation and migration of leukocytes. In addition, Th17-cells secrete IL-21 and IL-22. Th17-cells are involved in several infectious CNS diseases (Infante-Duarte et al., 2000) and are supposed to play a role as an early triggering event in autoimmune CNS diseases, such as MS (Graber et al., 2008).

1.3.2 CD8⁺ cytotoxic T-cells

CD8⁺ T-cells have been demonstrated to be important for viral clearance in infectious CNS diseases. T-cell mediated cytotoxicity is triggered by antigens bound to MHC I molecules on the cell surface. IFN-γ increases the MHC I expression on resident cells in the brain in inflammatory diseases (Tizard, 2008). Once activated cytotoxic T-cells kill their target by the induction of apoptosis through the secretion of perforin and granzyme (perforin pathway) or through the stimulation of the death receptor CD95 (CD95 pathway). In addition to antiviral responses, CD8⁺ T-cells also represent a critical cell population in myelin loss disorders (Tsunoda, 2002). Referring to this, CD8⁺ T-cells have been demonstrated to play a divergent role in demyelinating diseases of the CNS. For example, TGF-β-expressing CD8⁺ T-cells function as regulatory cells,

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while interferon (IFN)-γ producing MBP-specific effector CD8⁺ T-cells induce severe CNS autoimmunity (Huseby, 2001; Kelso and Gough, 1988).

1.3.3 Regulatory T-cells

The natural regulatory T-cells (Tregs) with a CD4⁺CD25⁺ phenotype and expression of the transcription factor fork head box P3 (FOXP3) are able to suppress the effector T-cell proliferation via cell to cell contact and cytokine mediated mechanisms (Hori et al., 2003; Sakaguchi et al., 1995; Viglietta et al., 2004). FOXP3 is important for the development and function of these cells. In humans, the dysfunction or lack of FOXP3 expression leads to a rare fatal autoimmune lymphoproliferative disease called immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX;

Wildin et al., 2001). The early onset in males causes severe infections, food allergies, insulin dependent diabetes and enlargement of secondary lymphoid organs (Ziegler, 2006). A FOXP3 mutation is present in the scurfy mice. These mice exhibit a phenotype that is similar to the IPEX disease in humans. The lack of the transcription factor results in an extensive proliferation of CD4⁺ T-cells, subsequent to an elevated cytokine production and inflammation of multiple organs (Ziegler, 2006). In contrast, reduced numbers of T-cells are observed in mice overexpressing the FOXP3 gene.

Further, remaining T-cells in these animals have diminished proliferative and cytolytic capacities as well as an inhibited IL-2 production. The development of a transgenic mice, which enables the selective depletion of FOXP3 (depletion of regulatory T-cell [DEREG]–mice) has provided new insights in the homeostasis of the immune system and in the prevention of autoimmune diseases by Tregs (Lahl et al., 2007).

Tregs play a key role in the control of immune responses in the CNS (Liesz et al., 2009; Sakaguchi, 2006), and are essential for the maintenance of self tolerance (Tang et al., 2008). Tregs modulate the effector function of T-cells and antigen presenting cells by the production of IL-10, TGF-β (Suri-Payer et al., 1998) and cytotoxic T-lymphocyte antigen (CTLA)-4 (Cools et al., 2007). Due to their regulatory properties, an impaired function of Tregs cells predisposes to multifactorial autoimmune diseases,

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such as MS, type I diabetes and Hashimotos’s thyroiditis (Kumar et al., 2006;

Sakaguchi and Powrie, 2007).

1.3.4 B-cells

B-cells regulate immune responses through antibody production and CD4⁺ T-cell activation. Self-reactive B-cells are mostly eliminated in the bone marrow through negative selection. Antigens mediating negative selection are usually membrane-bound self-antigens that deliver strong signals through the B-cell antigen receptor (BCR), leading to apoptosis. However, there are some self-reactive B-cells that escape this selection process, which have the potential to induce an autoimmune disease (Tizard, 2008). Interestingly, in contrast to these autoaggressive cells there are also regulatory B-cells (Bregs), which release anti-inflammatory molecules, such as IL-10.

This B-cell subtype regulates immune responses in mouse autoimmunity and inflammation models, such as the EAE model for MS (Anderton and Fillatreau, 2008;

Matsushita et al., 2008). Autoreactive B-cells and Bregs represent potential targets for new treatments (Yanaba et al., 2008). For example, the drug rituximab is a monoclonal antibody directed against the CD20 protein, which is primarily found on B-cells.

Therefore, rituximab is used in the treatment of hematological neoplasms, such as follicular lymphoma as well as autoimmune diseases, including systemic lupus erythematosus, autoimmune anemia and MS (Leslie, 2009). Overall, these findings highlight the complexity of humoral immune responses in demyelinating diseases (Yanaba et al., 2008).

1.3.5 Antigen presenting cells

Antigen presenting cells (APC) include macrophages, microglia and dendritic cells (DC). Their main function is to present foreign material to lymphocytes to induce an antigen-specific immune response. Antigens are processed in the cytoplasm of APC and subsequently externalized on the cells surface by the MHC class II molecule (Guermonprez et al., 2002). During inflammatory CNS diseases, activated

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macrophages and microglia promote demyelination due to release of soluble factors, such as proteolytic enzymes, pro-inflammatory cytokines and reactive oxygen species (Huseby et al., 2001). These molecules are able to cause myelin damage by the degradation of myelin basic protein (Liuzzi et al., 1995). Macrophages and microglia play a role in viral persistence, as demonstrated in TME (Steurbaut et al., 2008). DC activate the innate immune defense through their continued surveillance and process antigens efficiently to trigger acquired immune responses (Banchereau and Steinman, 1998; Nestle et al., 2001). DC maintain peripheral tolerance to self-antigens and initiate immunity to foreign substances. In addition, DC are able to present processed self-antigens to autoreactive T-cells (Paul, 2007). Furthermore, T-cell differentiation depends upon the contacts with DC. Thus, IL-12 and IL-23 produced by DC, promotes Th1 differentiation and the maintenance of differentiated Th17-cells, while IL-10 produced by DC favors the induction of regulatory T-cells (Steinman, 2007).

1.4 Immune responses in demyelinating diseases