39PThe effects of myositis-specific autoantibodies in immune-mediated necrotizing myopathy on muscle fiber contractility

Immune-mediated necrotizing myopathy (IMNM) is the most severe myositis subtype in terms of muscle weakness. Immunosuppressive therapies are still insufficient and there is a need for better and personalized therapies. IMNM is associated with myositis-specific autoantibodies (MSAs) against signal recognition protein-54 (SRP) and 3-hydroxy-3-methylglutaryl CoA reductase (HMGCR), which have been suggested to play a pathogenic role. We have previously shown that the force generating capacity of muscle fibers from IMNM patients is impaired, however we do not know the cause of this impairment. Since these patients have elevated MSAs (up to 1000x fold), this raises the question whether it is caused by the MSAs. Either anti-SRP+, anti-HMGCR+ or seronegative serum was drawn from IMNM patients and healthy controls and stored in our biobank. Subsequently, healthy murine intact muscle fibers from the Flexor Digitorum Brevis (FDB) muscle were isolated for our ex-vivo culture model. After 1 day of culture (to allow the muscle fibers to recover from isolation), the fibers are exposed to the serum for 2 hours. Then the fibers were assessed for contractility in two different set-ups. The first set-up is an innovative high throughput set-up, in which muscle contractility and calcium handling was measured during electrical stimulation. In the second set-up we permeabilized the muscle fibers and mounted them between a force transducer and length motor. Fibers were exposed to solutions with incremental Ca2+-concentrations and the force was measured (without potential confounding effects of Ca2+ cycling by the sarcoplasmic reticulum). Afterwards, the fiber was stored in a buffer for gel electrophoresis to fiber type. Our data suggest that muscle contractility is altered after 2h exposure to serum of IMNM patients compared to healthy controls: during electrical stimulation, sarcomere shortening was higher after exposure to anti-SRP+, anti-HMGCR+ and seronegative serum, indicating altered contractility. Preliminary data suggest that exposure to anti-HMGCR+ serum lowers the maximum force of healthy muscle fibers, but Ca2+-sensitivity of force was not altered. Serum of IMNM patients has a direct impact on muscle contractility. It remains unclear which serum component causes the impact. Therefore, experiments are ongoing with the IgG and IgG-depleted serum of these patients to further investigate the role of the MSAs.