Statin-modified dendritic cells regulate humoral immunity in experimental autoimmune myasthenia gravis

Highlights

• AT-BMDCs regulate humoral immune response in EAMG.

• AT-BMDCs reduce the quantity and the relative affinity of antibodies.

• AT-BMDCs suppress germinal center response.

• AT-BMDCs decrease follicular helper T cells and IL-21.

• AT-BMDCs increase regulatory B cells.

Abstract

We previously demonstrated that atorvastatin induced immature dendritic cells (DCs) derived from spleen in vitro. Administration of these tolerogenic DCs led to amelioration of experimental autoimmune myasthenia gravis (EAMG). The protective effect was mainly mediated by inhibited cellular immune response, including up-regulated regulatory T cells and shifted Th1/Th17 to Th2 cytokines. The present study employed atorvastatin-modified bone marrow-derived DCs (AT-BMDCs) to explore the effect of tolerogenic DCs on humoral immune response of EAMG and further elucidate the underlying mechanisms. Our data showed that AT-BMDCs reduced the quantity and the relative affinity of pathogenic antibodies, suppressed germinal center response, decreased follicular helper T cells and IL-21, and increased regulatory B cells. These results suggest that AT-BMDCs ameliorate EAMG by regulating humoral immune response, thus providing new insights into therapeutic approaches of myasthenia gravis and other autoimmune diseases.

Introduction

Myasthenia gravis (MG) is a classical organ specific autoimmune disease, which is antibody-mediated, T cell-dependent and complement-involved. It impairs the postsynaptic nicotinic acetylcholine receptor (AchR) at the neuromuscular junction (NMJ), leading to neuromuscular transmission failure, thereby inducing muscle weakness and fatigability (Berrih-Aknin et al., 2014). Experimental autoimmune myasthenia gravis (EAMG), induced by immunization of AchR purified from Torpedo californica (Lennon et al., 1975) or synthetic peptide corresponding to region 97–116 of the rat AChRα subunit (R97–116 peptide) (Baggi et al., 2004), serves as a well-established animal model used to explore the immunopathological mechanisms and therapeutic strategies of human MG.

Current immunosuppressive agents for MG treatment, such as corticosteroids and cytotoxic reagents, are more pan-immunosuppressive than specific cell targeted, thus causing numerous side effects. Pathogenic autoantibodies (mostly antibodies against AchR) are detected in a majority of MG patients and damage the neuromuscular transmission via many mechanisms (Meriggioli and Sanders, 2009), indicating that antibody-secreting B cells are essential for the development of MG. Accumulating evidence demonstrates that B cell targeted therapy via elimination of B cells with drugs such as rituximab (anti-CD20 mAb) can relieve symptoms of MG patients (Iorio et al., 2014), which supports the idea that autoreactive B cells may be a prominent candidate for targeted therapies in MG. Moreover, a distinct T cell subset, named follicular helper T (Tfh) cells and characterized by expression of transcription factor B-cell lymphoma 6 (Bcl6), surface molecules including CXC chemokine receptor 5 (CXCR5), inducible T-cell costimulator (ICOS) and programmed death protein-1 (PD-1), and cytokine IL-21, is indicated to supply direct assistance for antibody production of B cells (Crotty, 2011). The contributing role of Tfh cells has been confirmed in many antibody-mediated autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and autoimmune thyroid diseases (Ma et al., 2012, Simpson et al., 2010, Zhu et al., 2012). Recent studies have also reported an aberrant expansion of Tfh population in MG patients (Luo et al., 2013, Zhang et al., 2014a). Taken together, these findings suggest the pathogenic role of humoral immune response mediated by B cells and Tfh cells in MG.

Recently, the discovery of regulatory B cells (Breg) has challenged the conventional roles of B cells as antigen-presenting cells and antibody-secreting cells (Mauri and Ehrenstein, 2008). Adoptive transfer of Breg attenuates severity of some autoimmune diseases in animal models, such as experimental autoimmune encephalomyelitis (EAE) (Matsushita et al., 2010), type 1 diabetes (T1DM) (Hussain and Delovitch, 2007) and collagen-induced arthritis (CIA) (Yang et al., 2012). Sun et al. have reported that IL-10 producing Breg (B10 cells) decrease in MG patients and those who respond well to rituximab treatment exhibit rapid repopulation of B10 cells (Sun et al., 2014). However, data are still scarce about the exact role of Breg in MG.

Previously, our study reported that atorvastatin induced immature dendritic cells (DCs) derived from spleen in vitro and administration of these tolerogenic DCs ameliorated EAMG by inhibiting proliferation of lymphocytes, up-regulating regulatory T cells (Treg), shifting Th1/Th17 to Th2 response and decreasing levels of pathogenic antibodies (Li et al., 2013). The previous study focused on the effect of tolerogenic DCs on cellular immunity, however, the exact mechanisms by which tolerogenic DC treatment triggered reduction of antibodies deserve further investigations. Furthermore, given that bone marrow generates more abundant and immature DCs with higher trafficking property than spleen (Emmanouilidis et al., 2006, Fields et al., 1998), and most studies employing DCs prefer bone marrow-derived dendritic cells (BMDCs) to spleen-derived dendritic cells (SpDCs), we used BMDCs in this study instead of SpDCs in the previous study. Collectively, the aim of the present study was to explore the effect of atorvastatin-modified BMDCs (AT-BMDCs) on humoral immune response in EAMG and we attempted to explore the underlying mechanisms.

In this study, we found that atorvastatin successfully induced immature BMDCs in vitro, of which CD80, CD86 and MHC II molecules were inhibited. AT-BMDC treatment led to amelioration of ongoing EAMG. AT-BMDCs suppressed the quantity and the relative affinity of anti-R97-116 IgG as well as germinal center (GC) response. In addition, Tfh cells and IL-21 were decreased by AT-BMDC treatment. Interestingly, the amount of CD19⁺ B cells was slightly influenced, while Breg were increased by AT-BMDCs. Our data suggest that AT-BMDCs could ameliorate EAMG by regulating humoral immune response and might provide a potential approach and new evidence for MG therapies.