MS is a chronic inflammatory disease of the CNS involving T cell and B cell responses. Recently, several studies have described modifications of specific bacterium abundances of gut microbiota in patients with remitting-relapsing MS compared with healthy individuals (see for review 1 ). This was often associated with an increase in Akkermansia muciniphila bacteria. In experimental autoimmune encephalomyelitis, transfer of gut microbiota from patients with MS to mice induced proinflammatory responses and exacerbation of the disease, whereas microbiota from healthy volunteers (HVs) were less inflammatory. 2,3 Because bacteria in the gut modulate immune responses, we assessed the antibody production against A muciniphila in patients with MS. In CSF, levels of anti-A muciniphila immunoglobulin G (IgG) were increased in patients with MS compared with controls, whereas no difference was found for levels of IgG against Escherichia coli, Fusobacterium necrophorum, Acinetobacter baumannii, Prevotella melaninogenica, and Bacteroides fragilis. MethodsPatients with relapsing-remitting MS (RRMS, n = 62) were enrolled in the neurology department of CHU de Nantes. Sex and age-matched patients (HVs, n = 41), patients with noninflammatory neurologic disease (e.g., suffering from sudden headaches or idiopathic intracranial hypertension) (noninflammatory neurologic disease [NIND], n = 23), and patients with inflammatory neurologic disease (peripheral neuropathy, brain lymphoma) (inflammatory neurological disease [IND], n = 10) were used for comparison. Informed written consent was obtained from all the patients before any study-related procedure was performed. Patients had not received any disease-modifying drugs before the sampling.IgG concentration was measured with an immunonephelometric assay performed using a Beckman Immage Analyzer (Beckman Coulter). To detect antibody against bacteria, ELISA tests were performed using serum and CSF samples. Briefly, bacteria lysates from A muciniphila, F necrophorum, A baumannii, P melaninogenica, and E coli were coated on plates (Nunc) at 1 μg/mL of proteins in phosphate buffer saline (PBS). Bovine serum albumin (Sigma Aldrich) at 1% in PBS was used for blocking. Patient samples were incubated for 2 hours at 37°C in PBS (dilutions 1/ 100 for serum, 1/10 for CSF) and 1% bovine serum albumin. Antihuman IgG antibodies coupled with horseradish peroxidase (Bethyl Laboratories) at 1/5,000, 1 hour at 37°C, were used. The reaction with the substrate (3,3',5,5'-Tetramethylbenzidine, BD Biosciences) was stopped with sulfuric acid (0.18M, Sigma Aldrich). Plates were read at 450 nm using a Spark 10M multimode