NMU promotes autoantibody-mediated arthritis. Serum from arthritic K/BxN mice was injected into NMU-KO mice or littermate control mice on days 0 and 2. The development of arthritis was assessed using (A) arthritis scores and (B) ankle thickening measurements. NMU-KO mice had significantly lower arthritis scores (P < 0.001, repeated-measures ANOVA) and ankle thickening (P = 0.001, repeated measures ANOVA), leading to the conclusion that arthritis severity increases more quickly in wildtype mice relative to the NMU-KO mice. Data plotted are means ± SEM from 10 mice per group compiled from three independent experiments. P-values for individual timepoints calculated using two-tailed Student's T-test are also depicted: *P < 0.05; **P < 0.01; ***P < 0.001. (C) Representative ankle sections from the indicated mice at the peak of arthritis severity were stained with H&E. Arrows indicate inflammatory infiltrates. Original magnification ×5. (D) Serum was collected from the mice at the end of the experiment, diluted 1:900, and assayed by ELISA for the presence of anti-GPI IgG. Data plotted are arbitrary ELISA absorbance values (means ± SEM) from seven mice per group compiled from two independent experiments; P = 0.78. ANOVA: analysis of variance; ELISA: enzyme-linked immunosorbent assay; H&E: hematoxylin and eosin stain; GPI: glucose-6-phosphate isomerase; IgG, immunoglobulin G; KO: knockout; NMU: neuromedin U; SEM: standard error of the mean.