To measure the prevalence of JCV-specific antibodies in a French cohort of MS patients treated with natalizumab and to identify risk factor(s) of JCV seropositivity. Progressive multifocal leukoencephalopathy (PML) risk may be stratified by anti-JCV antibody status, duration of natalizumab therapy (≥24 months) and prior exposure to immunosuppressive (IS) drugs. No data are available in France on the prevalence of anti-JCV antibodies and distribution of PML risk factors in patients treated with natalizumab. Sera of 361 patients under natalizumab therapy in two MS centers were analyzed using a previously validated ELISA test. We studied different characteristics: demographic, ethnic, radiological, clinical, prior use of immunomodulatory (IM) or IS drugs and natalizumab exposure duration. The JCV seropositivity rate was 51 % for the whole cohort. Mean natalizumab exposure duration was 27.27 months ± 15.57 (mean ± SD), and prior use of IS drugs was observed in 15.24 % of patients. Twenty-three patients (6.4 %) presented the three PML risk factors. By multivariate analysis, presence of anti-JCV antibodies was significantly linked to age, North African origin and natalizumab exposure duration. Anti-JCV antibody prevalence was similar to previously published data. Anti-JCV antibody status was linked to age. We also suggested that anti-JCV antibody status could be linked to natalizumab exposure duration and ethnic characteristics.

"Disease activity free" in relapsing-remitting multiple sclerosis (RRMS) is a new concept introduced by the results of the AFFIRM study. Our objective was to analyze the clinical and radiological efficacy of natalizumab treatment in actual clinical practice and compare it with the post hoc analysis of the AFFIRM study. All patients with RRMS who began treatment with natalizumab at our two French MS centres between April 2007 and May 2008 were included and followed-up for at least 2 years. No measurable disease activity ("disease activity free") was defined as no activity on clinical measures (no relapses and no sustained disability progression) and radiological measures (no gadolinium-enhancing lesions and no new T2-hyperintense lesions on cerebral MRI). A total of 193 patients were included. Natalizumab was discontinued in 25.9% of cases before the completion of 2 years of treatment. In our cohort, we observed patients with more severe disease than in the AFFIRM study. The proportion of patients remaining free of clinical activity during 2 years of treatment was lower than in the AFFIRM study (37.8% vs. 64.3%). The proportion of patients remaining free of radiological activity during 2 years of treatment was higher than in the AFFIRM study (68.9% vs. 57.7%), while the proportion of patients remaining free of disease activity during 2 years of treatment was comparable to the AFFIRM study (33.3% vs. 36.7%). Natalizumab seems to be as effective in a real-life setting as in pivotal and post hoc studies. The confirmation of such benefits is important because of the progressive multifocal leukoencephalopathy risk.

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