LAUSR

We aimed to evaluate in vivo effects of abatacept on phenotypes of T and B cells in the circulation of myositis patients in a sub‐study of the ARTEMIS trial. Twelve patients with paired frozen PBMCs before and after 6‐month abatacept treatment were included in this sub‐study where mass cytometry (CyTOF) was chosen as a technology to be tested for its utility in a real‐life clinical immune monitoring setting. Using CyTOF, the peripheral T cell phenotypes demonstrated considerable variation over time and between individuals precluding the identification of treatment‐specific changes. We therefore conclude that studies of patient cohorts displaying wide clinical heterogeneity using mass cytometry must be relatively large in order to be suited for discovery research and immune monitoring. Still, we did find some correlations with functional muscle outcome, namely positive correlations between the ratio of CD4+ T cells and CD8+ T cells (CD4/CD8) in peripheral blood samples both at baseline and after treatment with muscle endurance improvement as assessed by the functional index‐2 (FI‐2) test. Our data suggest that the CD4/CD8 ratio in circulation at time of active disease may be a predictor of treatment efficacy in myositis patients.