Review article Role of the innate immune system in the pathogenesis of multiple sclerosis Roopali Gandhi 1, Alice Laroni 1, Howard L. Weiner ��� Center for Neurologic Diseases, Brigham and Women's Hospital, 77 Avenue Louis Pasteur, NRB641, Boston, MA 02115, United States a b s t r a c t a r t i c l e i n f o Article history: Received 6 July 2009 Received in revised form 5 October 2009 Accepted 6 October 2009 Keywords: Multiple sclerosis Dendritic cells Mast cells NK cells NK T cells Microglial cells Gamma Delta T cells Innate immune system Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease with heterogeneous clinical presentations and course. MS is considered to be a T cell mediated disease but in recent years contribution of innate immune cells in mediating MS pathogenesis is being appreciated. In this review, we have discussed the role of various innate immune cells in mediating MS. In particular, we have provided an overview of potential anti-inflammatory or pro-inflammatory function of DCs, microglial Cells, NK cells, NK-T cells and gamma delta T cells along with their interaction among themselves and with myelin. Given the understanding of the role of the innate immune cells in MS, it is possible that immunotherapeutic intervention targeting these cells may provide a better and effective treatment. �� 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Dendritic cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Microglial cells/macrophages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Natural killer cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Mast cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Invariant NK-T cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Gamma-delta T cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction Being the earliest defense against pathogens, the innate immune system fights against infections and protects against self or innocuous antigens. Various cell types that compose the innate immune system share antigen recognition ability through their invariant receptors which do not undergo rearrangement and have no immunological memory. We might compare the components of the innate immune system not only to first line soldiers, but also to sentinels, who encounter the enemy and report to the ���comman- ders���, the T and B lymphocytes, thus activating the adaptive immune response. Multiple sclerosis (MS) is a chronic inflammatory demyelinating autoimmune disease of the central nervous system (CNS) of unknown etiology and heterogeneous clinical symptoms and course (Weiner, 2004). Depending upon clinical presentation and course, MS is classified either as relapsing remitting (RR), primary progressive (PP) or secondary progressive (SP). About 87% of MS patients exhibit a RR course of disease (Weiner, 2008), characterized by acute attack (relapse) followed by partial or full recovery (remission) occurring at variable intervals (Debouverie et al., 2008). Of these RR���MS patients, about two-thirds transition to the secondary progressive phase where Journal of Neuroimmunology 221 (2010) 7���14 ��� Corresponding author. E-mail address: hweiner@rics.bwh.harvard.edu (H.L. Weiner). 1 These authors contributed equally to this review. 0165-5728/$ ��� see front matter �� 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jneuroim.2009.10.015 Contents lists available at ScienceDirect Journal of Neuroimmunology journal homepage: www.elsevier.com/locate/jneuroim

neurologic disability progresses in the absence of attacks (Runmarker and Andersen, 1993 Weiner, 2008). About 10% of MS patients have a primary progressive course manifested by progressive worsening from onset (Weiner, 2009). Much has been done to understand the etiology of MS, with a major focus on the role of the adaptive immune system. It has been suggested that myelin-specific auto-reactive lymphocytes, mainly IFN-�� secreting T helper 1 (���Th1���) cells (Baker et al., 1991 Bettelli et al., 2004) and/or IL-17 producing ���Th17��� cells (Bettelli et al., 2008 Korn et al., 2007) are primed in periphery by unknown factors, after which they migrate to CNS, leading to demyelination and axonal loss and subsequent neurological disability (Sospedra and Martin, 2005). Recent studies have suggested that the innate immune system also plays an important role both in the initiation and progression of MS by influencing the effector function of T and B cells (Weiner, 2008). The effector cells, in turn, express cytokines and activation markers that further activate innate immune cells (Monney et al., 2002). In this review, we will discuss the potential role of the innate immune system in the pathogenesis of MS and EAE (the murine model of MS) specifically, dendritic cells, microglial cells, natural killer cells, natural-killer T cells, mast cells and gamma-delta T cells. 2. Dendritic cells Dendritic cells (DCs) are ���professional antigen presenting cells��� that play an important role in promoting activation and differentiation of na��ve T cells. DCs are classified into different categories based on their surface markers. A widely accepted classification distinguishes human DCs into two categories: myeloid (Lin-CD11c+) and lymphoid/ plasmacytoid (Lin-CD11cdimCD123+) (Lipscomb and Masten, 2002 MacDonald et al., 2002). The interaction of DCs with T cells is crucial in determining T cell differentiation into either effector T cells (Th1, Th2 and Th17 cells) or regulatory T cells (natural Tregs and induced Tr1 cells) (Gilliet and Liu, 2002 Shortman and Heath, 2001). DCs can also affect NK cells function where they can either stimulate NK cell- mediated cytotoxicity (Fernandez et al., 1999) or ���prime��� NK responses toward viral and bacterial pathogens (Lucas et al., 2007). Myeloid dendritic cells (mDCs) can activate NK cells and selectively trigger the proliferation of the CD56bright NK cell subset (Vitale et al., 2004). Similarly, plasmacytoid dendritic cells (pDCs) can also interact with NK cells tostimulate their effector function and induce selectiveCD56bright NK cell expansion (Romagnani et al., 2005). In EAE pathogenesis, several studies have suggested the involvement of DCs particularly, showing accumulation of these cells in CNS during inflammation (Bailey et al., 2007 Serafini et al., 2000), and in studies utilizing in vitro transfer models of activated antigen pulsed DCs (Dittel et al., 1999 Weir et al., 2002). These DCs activate encephalitogenic T cells and result in either induction of disease (Bailey et al., 2007 Dittel et al., 1999 Weiretal.,2002)ortolerance(Khouryetal.,1995 Xiaoetal.,2004), depending upon the activation state of DCs and mechanism of antigen uptake (El Behi et al., 2005). DCs isolated from the CNS of EAE mice, induced by injection of PLP178���191, are the most potent stimulators of na��ve T cells or helper T cells in the presence or absence of endogenous peptide, suggesting the possible contribution of DCs in epitope spreading (spreading T cell reactivity to antigens in addition to initial disease inducing epitope), in the CNS during the disease (McMahon et al., 2005 Miller et al., 2007). In humans, there are studies demonstrating altered DC phenotype/ function in peripheral blood. In MS patients, DCs have an activated phenotype with an increased expression of activation markers and an aberrant secretion of proinflammatory cytokines. mDC-mediated inflammation is more pronounced in SP���MS patients than in RR���MS patients (Karni et al., 2006). These DCs show increased expression of the activation markers CD40 and CD80 in SP and RR patients whereas a decreased expression of programmed death ligand-1 (PDL1), an immunoregulatory molecule,wasobservedonly inprogressivepatients. In the same study, mDCs isolated from SP���MS patients showed an enhanced production of IL-12 in response to IFN-�� and LPS (Karni et al., 2006). This activated phenotype of DCs in both RR��� and SP���MS patients is accompanied by an enhanced pro-inflammatory T cell response as defined by increased secretion of TNF-�� and IFN-��. Similarly, it has been shown that monocyte-derived DCs differentiated from MS patients, secrete more pro-inflammatory cytokines such as IFN-��, TNF-�� Huang et al., 1999), IL-6 (Huang et al., 1999) and IL-23 (Th-17 bias cytokine) (Vaknin-Dembinsky et al., 2006 Vaknin-Dembinsky et al., 2008). In addition to promoting MS pathology by secretion of pro- inflammatory cytokines, DCs also secrete the glycoprotein osteopon- tin, which is involved in chemotaxis, activation and differentiation of immune cells. Increased expression of osteopontin has been reported in brain lesions during EAE (Hur et al., 2007), as well as in DCs isolated from mice with EAE and from MS patients, which in turn may be related to skewed differentiation of Th1 and Th17 cells during the disease (Hur et al., 2007 Murugaiyan et al., 2008). As discussed previously, plasmacytoid DCs (pDCs) represent another subset of DCs involved in both innate and adaptive immunity including protection from microbial infections and the generation of immunoregulatory immune responses (Siegal et al., 1999). pDCs isolated from peripheral blood of MS patients exhibit an altered phenotype with decreased or delayed expression of the activation markers CD86, CD83, CD40 and 4-IBBL, in addition to their altered functionality in terms of T cell proliferation and generation of regulatory T cells (Stasiolek et al., 2006). Taken together, these studies suggest that DCs are important in promoting pro-inflammatory T cell responses in MS and are also linked to determination of the RR and SP disease phases. Based on these observations, a number of DC-based therapies, both antigen- specific and non-specific, have been tested in EAE and other animal models with varied success. Multiple approaches have been used to modify DCs to treat EAE including the following: a) injection of either mature DCs (Zhang et al., 2002) or of IFN-�� treated DCs (Xiao et al., 2004) or of DCs treated with an autoantigenic peptide and matured in the presence of TNF-�� Menges et al., 2002), suppressed clinical severity of disease and inflammation in CNS b) transfer of DCs transduced with SOCS-3 (suppressor of cytokine signaling inhibitor), could inhibit EAE by promotion of IL-10 and through inhibition of IL- 12, IFN-�� and IL-23 secretion (Li et al., 2006) c) treatment of mice with in vitro stem cell derived DCs loaded with auto-antigens and expressing either death receptors like TRAIL or regulatory molecules like PDL-1 could prevent EAE (Hirata et al., 2005) d) Mitomycin C- treated DCs loaded with MBP (Terness et al., 2008) could inhibit the induction of disease or reduce severity of EAE. 3. Microglial cells/macrophages Microglial cells comprise 10���20% of glial cells and are the most common immune cells in the CNS. Microglial cells are considered resident macrophages of the nervous system, being involved in phagocytosis, antigen presentation and production of cytokines (Benveniste, 1997). Microglial cells are rapidly activated in response to injury, neuro-degeneration, infection, tumors and inflammation. Until now, there are no unique markers distinguishing microglial cells from blood-derived macrophages in the CNS. Microglial/macrophage cell activation contributes to MS and EAE pathology through antigen presentation and secretion of pro-inflam- matory cytokines (Benveniste, 1997). Persistent activation of microglial cells has also been observed in the chronic phase of relapsing-remitting EAE and a correlation has been observed between activated microglial cells and loss of neuronal synapses (Rasmussen et al., 2007). Similarly, profound activation of microglial cells has been reported in MS, more frequently in progressive than in RR patients and specifically in association with inflammation of white matter (Kutzelnigg et al., 2005). The role of microglial cells in antigen presentation is based on their expression of various molecules involved in antigen presentation such 8 R. Gandhi et al. / Journal of Neuroimmunology 221 (2010) 7���14