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Background The aetiology of rheumatoid arthritis (RA) is complex. In particular, the vast majority of disease-associated variants implicated by genome-wide association studies are non-coding, leaving genetic mechanisms of adaptive immune dysregulation unresolved. The contribution to this process of epigenetic factors, including the addition of methyl groups to DNA, also remains uncertain. To address these issues and prioritise causal genes for downstream study, genome-wide data incorporating DNA methylation and gene expression measurements from lymphocyte subsets in an early arthritis inception cohort, were available. Methods Whole genome methylation and transcription data from isolated CD4+ T cells and B cells of &amp;gt; 100 well-characterised inflammatory arthritis patients, all of whom were naïve to immunomodulatory treatments and of Northern European ancestry, were obtained (Illumina HumanHT-12 v4 Expression BeadChip and Infinium MethylationEPIC BeadChip arrays, respectively). Genotyping was undertaken using the Illumina Human CoreExome-24 version 1-0 array. After independent pre-processing, normalisation and quality control of paired CD4+ and B lymphocyte data, methylation quantitative trait loci (meQTLs) were first modelled using the MatrixEQTL package in each cell type. Next, at RA risk-associated cis-CpGs, correlations between site-specific methylation and the expression of genes within ±500Kb identified quantitative trait methylations (eQTMs). To infer directionality of SNP-CpG-transcript associations a causal inference test (CIT) was applied. Multiple testing was accounted for, and in vitro assays were used to validate meQTLs at loci of interest and confirm regulatory mechanisms. Further analysis integrated GWAS data from other immune mediated diseases (IMDs) and additional publically available resources. Results We found strong evidence that disease-associated DNA variants regulate cis-CpG methylation of DNA in CD4+ T and/or B cells at 37% RA loci. In general we observed these variants to preferentially modify methylation at sites mapping to lymphocyte enhancers and regions flanking transcription start sites, and at positions bound by the NFκB transcription factor. Using paired, cell-specific transcriptomic data and a statistical approach to infer causality, we then identified examples where site-specific DNA methylation in turn mediates gene expression, including ORMDL3/GSDMB, IL6ST/ANKRD55, FCRL3 and JAZF1 in CD4+ lymphocytes. Leveraging GWAS data we noted that a number of genes regulated in this way highlight mechanisms common to RA, multiple sclerosis and asthma, distinguishing these IMDs from osteoarthritis which is considered a primarily degenerative disease. To validate our findings, cis-meQTL effects at sentinel loci were replicated by pyrosequencing in an independent cohort of genotyped early arthritis patients, and methylation-mediated regulation of FCRL3 expression downstream of the regulatory SNP was confirmed experimentally using a luciferase reporter assay in Jurkat T-cells. Conclusion Our observations highlight important mechanisms of genetic risk in RA and the wider context of autoimmunity. They confirm the utility of DNA methylation profiling as a tool for causal gene prioritisation and, potentially, therapeutic targeting in complex IMD. Disclosures A.D. Clark None. N. Nair None. A.E. Anderson None. N. Thalayasingam None. N. Naamane None. A.J. Skelton None. J. Diboll None. A. Barton None. S. Eyre None. J.D. Isaacs None. L.N. Reynard None. A.G. Pratt Grants/research support; I am a recipient of an unrestricted, investigator initiated research grant from Pfizer, paid to Newcastle University.

O11 Lymphocyte DNA methylation mediates genetic risk at RA risk loci that are shared with other immune mediated diseases