Seroconversion of JCV antibodies is strongly associated to natalizumab therapy.

Previous infection with John Cunningham virus (JCV) increases the risk of progressive multifocal leukoencephalopathy in patients with multiple sclerosis (MS) undergoing treatment with natalizumab. Patients who test negative for JCV antibodies must be assessed every six months due to the risk of seroconversion. Data from the United States of America, Portugal, Holland, France, United Kingdom and Sweden have shown a strong correlation between the use of natalizumab and JCV seroconversion. The authors present now data on patients from Brazil, as there are no data from Latin American countries published on this subject yet. A group of 86 patients with MS with negative results for antibodies against JCV were included in this analyses with at least two JCV antibodies testing. Twenty-five patients (29% of the total group) did not use natalizumab at any time, while the remaining 71% used natalizumab for a median period of 800 days (equivalent to 28 monthly infusions). Seroconversion was observed in 19 patients (22.1%). There was no association of seroconversion with gender, age, previous pulses of corticosteroid or specific MS-modifying drugs. The use of natalizumab was strongly associated to seroconversion (p < 0.0001). The present results confirm the influence of natalizumab therapy on JCV antibodies in several countries and continents.