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Background Dysfunction of Tregs results in a breakdown of immunological tolerance and has been implicated in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis (RA) and type 1 diabetes. Treg function is regulated by epigenetic factors and we have previously reported the presence of Tregs expressing an aberrant DNA methylation profile in RA. Objectives The aim of this study was to assess the potential utility of DNA methylation inhibitors for the treatment of RA, using collagen-induced arthritis (CIA) as an animal model. Methods DBA/1 mice were immunised with bovine type II collagen emulsified in complete Freund9s adjuvant. The mice were treated with zebularine (400 mg/kg), decitabine (1 mg/kg) or psammaplin A (10 mg/kg) for 4 days, starting on the day of arthritis onset. Treatment was then stopped and the disease was monitored up to day 10 of arthritis. The expression of Treg genes was measured in lymph nodes on day 10 by qPCR. To assess the effect of DNA methylation inhibitors on generation of Tregs, naive CD4 + CD25 - T cells were cultured with mitomycin C treated APCs plus IL-2, TGFβ and anti-CD3 in the presence or absence of DNA demethylating agents and numbers of CD4 + FoxP3 + Tregs were determined by FACS after 72h. Results Treatment with zebularine resulted in a sustained reduction of arthritis severity, accompanied by an increase in the expression of Treg associated genes, Foxp3, Ctla4 and Tgfb1 , in draining lymph nodes. Treatment with decitabine produced a more profound reduction in disease severity whereas the therapeutic effect of psammaplin A was more transient. All three DNA methylation inhibitors could convert CD4 + CD25 - T cells into CD4 + FoxP3 + Tregs in a dose-dependent manner in vitro . Conclusions This study has shown that pulse treatment with DNA demethylating drugs produces a sustained reduction in the severity of arthritis and promotes the generation of Tregs. The findings raise the possibility that epigenetic drugs can be used on a short-term basis for re-setting tolerance and boosting Treg responses in human autoimmune disease. Acknowledgements This work was supported by grants from Chang Gung Memorial Hospital (Grant Code: CMRPG2D0251) and Idogen. Disclosure of Interest W. Tseng: None declared, I.-S. Huang: None declared, F. Clanchy: None declared, K. McNamee: None declared, D. Perocheau: None declared, P. Ericsson: None declared, H.-O. Sjogren Shareholder of: Hans Olov Sjogren is a co-founder of Idogen and a co-inventor of a patent on the use of zeburaline for the treatment of autoimmune diseases., Z. Xue: None declared, L. Salford Shareholder of: Leif Salford is a co-founder of Idogen and member of the board of Idogen and a co-inventor of a patent on the use of zeburaline for the treatment of autoimmune diseases, A. Sundstedt Employee of: Anette Sundstedt is an employee of Idogen, R. Williams Consultant for: Dr. Richard Williams is on scientific board of Idogen

The major histocompatibility complex (MHC) class I molecules have been shown to be substrates to both of the complement C1 esterases. The effect of a C1 esterase-mediated cleavage of the MHC class I molecules on the activation process of lymphocytes was investigated by including the complement C1 esterase inhibitor (C1-Inh) in the medium during activation of human peripheral lymphocytes by staphylococcal enterotoxin A (SEA). The C1-Inh was shown to inhibit the activation of both CD4+ and CD8+ cells. No effect on activation of B lymphocytes was recorded, although the complement C1 complex was shown to bind to the B lymphocytes. Furthermore, the C1 complex bound to mononuclear cells was shown to be cleaved into molecular weights corresponding to the activated forms of the C1 esterases. The effect of the C1-Inh was much more pronounced at low cell density and the inhibition was not affected by the addition of interleukin-2 (IL-2). However, the inhibition was reduced when the cells were disturbed by addition of new portions of C1-Inh, 24 and 48 hr after the initiation of the activation. This indicates that the C1-Inh interference with the activation of T lymphocytes is mediated through a mechanism that requires some form of cell contact.