In the version of this caption initially published, the cover artwork was credited to Erin Dewalt, based on imagery from the author, rather than stating that it was created by Michael B. Battles and the design was by Erin Dewalt. The error has been corrected in the HTML and PDF versions of the caption. ERRATUM In the version of this article initially published, the genus name 'Mycoplasma' was incorrectly used in place of the correct 'Mycobacterium'. The error has been corrected in the HTML and PDF versions of the article. ERRATUM npg

Objective: To study activin signaling and its blockade in sporadic inclusion body myositis (sIBM) through translational studies and a randomized controlled trial. Methods:We measured transforming growth factor b signaling by SMAD2/3 phosphorylation in muscle biopsies of 50 patients with neuromuscular disease (17 with sIBM). We tested inhibition of activin receptors IIA and IIB (ActRII) in 14 patients with sIBM using one dose of bimagrumab (n 5 11) or placebo (n 5 3). The primary outcome was the change in right thigh muscle volume by MRI at 8 weeks. Lean body mass, strength, and function were secondary outcomes. Twelve of the patients (10 bimagrumab, 2 placebo) participated in a subsequent 16-week observation phase.Results: Muscle SMAD2/3 phosphorylation was higher in sIBM than in other muscle diseases studied (p 5 0.003). Eight weeks after dosing, the bimagrumab-treated patients increased thigh muscle volume (right leg 16.5% compared with placebo, p 5 0.024; left leg 17.6%, p 5 0.009) and lean body mass (15.7% compared with placebo, p 5 0.014). Subsequently, bimagrumabtreated patients had improved 6-minute walking distance, which peaked at 16 weeks (114.6%, p 5 0.008) compared with placebo. There were no serious adverse events; the main adverse events with bimagrumab were mild acne and transient involuntary muscle contractions.Conclusions: Transforming growth factor b superfamily signaling, at least through ActRII, is implicated in the pathophysiology of sIBM. Inhibition of ActRII increased muscle mass and function in this pilot trial, offering a potential novel treatment of sIBM. Classification of evidence:This study provides Class I evidence that for patients with inclusion body myositis, bimagrumab increases thigh muscle volume at 8 weeks. Neurology ® 2014;83:2239-2246 GLOSSARY ActRII 5 activin receptors IIA and IIB; DXA 5 dual-energy x-ray absorptiometry; LBM 5 lean body mass; pSMAD2/3 5 phosphorylated SMAD2/3; QMT 5 quantitative muscle testing; sIBM 5 sporadic inclusion body myositis; 6MWD 5 6-minute walking distance; TGFb 5 transforming growth factor b; TMV 5 thigh muscle volume.Sporadic inclusion body myositis (sIBM) is a slowly progressive degenerative and inflammatory skeletal muscle disease beginning in middle or later life.1 Its clinical features include a specific pattern of muscle involvement (preferential weakness of finger flexors and knee extensors) accompanied by progressive muscle atrophy, distinctive microscopic pathology including endomysial inflammation and rimmed vacuoles, and a recently identified serum autoantibody (against cytosolic 59-nucleotidase 1A) biomarker.2-4 Despite a prominent adaptive immune response characterized by antigen-stimulated B-and T-cell maturation and prominent infiltration into muscle of immune system cells, sIBM is highly refractory to immunosuppressive therapies studied to date. 2Members of the transforming growth factor b (TGFb) superfamily of ligands signal through a heterodimeric receptor system.5 They first bind a type II receptor, such as the TGFb...