Background: Autoantibodies can be identified in approximately 60% of UK children with JDM. With the development of quantitative techniques such as ELISA to detect these autoantibodies there is a growing interest in the clinical utility of autoantibodies in predicting disease activity and the risk of associated complications. To date, the titre of anti-MDA5 has been shown to be useful in predicting response to treatment in Japanese children with JDM and in adults small studies have also shown a relationship between the titres of anti-Jo-1, anti-MDA5, anti-HMGCR and anti-SRP with disease activity measures. Methods: Serum samples and matched clinical data were obtained from 285 patients with JDM recruited to the UK Juvenile Dermatomyositis Cohort and Biomarker Study. After initial screening for the presence of autoantibodies by immunoprecipitation, the antibody titre of those with anti-TIF1g, anti-NXP2 and anti-MDA5 was assessed by ELISA using recombinant antigen. First and last available samples were analysed for each autoantibody subgroup (32 patients with anti-TIF1g, 30 with anti-NXP2 and 15 with anti-MDA5) along with multiple serial samples in those where an initial sample was available within 6 months of diagnosis (12 patients with anti-TIF1g, 8 with anti-NXP2 and with 4 anti-MDA5). Between three and eight samples were available per patient. Results: Autoantibody titre was seen to change over the follow-up period and reduced in the majority of patients in all autoantibody subgroups. In the last available sample (taken 1-19 years post diagnosis) autoantibody titre had reduced below the ELISA positive threshold in eight (25%) patients with anti-TIF1g, seven (23%) with anti-NXP2 and eight (53%) with anti-MDA5. Analysis of serial samples showed a relationship between autoantibody titre and change in disease activity as measured by the physician global assessment score (PGAS). While numbers were too small to demonstrate statistical significance within individual patients, trends were striking and a strong positive correlation was confirmed. Conclusion: Autoantibody titre in JDM changes over time and reduces in the majority of patients. Within individual patients autoantibody titre correlates with disease activity as measured by the Physician Global Assessment Score. A corresponding rise in autoantibody titre to mirror Physician Global Assessment Score in those patients who relapsed, suggests that this does not simply represent regression to the mean. Limitations of this study include few patients with serial samples available, variably timed blood sampling and non-standardized treatment. Further work is needed to confirm these results and to assess whether alterations in autoantibody titre pre-date changes in disease activity; if so, and given the clinical sub-phenotypes associated with these antibodies, the routine testing of autoantibodies early in diagnostic workup is to be recommended. Disclosure statement: S.L.T. has received research support in the form of a BMA Doris Hillier Grant 2012. All other authors have...

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