Rituximab treatment did not aggravate ongoing progressive multifocal leukoencephalopathy in a patient with multiple sclerosis.

A multiple sclerosis (MS) patient developed progressive multifocal leukoencephalopathy (PML) after 43 months of natalizumab treatment. New clinical and magnetic resonance imaging (MRI) findings were initially misinterpreted as breakthrough MS disease activity and natalizumab treatment was replaced by rituximab treatment. The patient had a single infusion of rituximab 1000 mg before a definite PML diagnosis was confirmed. Despite undetectable levels of B-cells, JC virus DNA became undetectable in the cerebrospinal fluid by quantitative polymerase chain reaction. The patient partially recovered without any clinical or MRI signs of new MS activity. These findings suggest that B-cell depletion in a non-immune compromised individual did not prevent the patient from clearing the JC virus infection.