Immune reconstitution inflammatory syndrome and natalizumab-Is it possible before removing the drug?

Multiple sclerosis (MS) patients treated with natalizumab have a significant reduction in annualized relapse rate; in these patients, a relapse is uncommon but not unexpected. In contrast, the appearance of a severe exacerbation is striking and requires the differential diagnosis with progressive multifocal leukoencephalopathy. Here, we describe a case of a 22-year-old woman with relapsing-remitting MS who developed an unexpected response after the patient׳s fifth natalizumab infusion with an aggressive radiological and clinical evolution. Changing the patient׳s treatment to fingolimod resulted in the absence of new clinical relapses and the absence of active lesions on brain magnetic resonance images (MRI) during the first 12 months of follow-up. We hypothesize that the appearance of natalizumab antibodies in this patient triggered lymphocyte migration to the central nervous system in a rebound phenomenon; this is similar to what occurs during immune reconstitution inflammatory syndrome (IRIS) after removal of natalizumab.