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JC Virus Antibodies

Natalizumab (Tysabri®) is a monoclonal antibody therapy that blocks alpha-4 integrin and is used to treat relapsing multiple sclerosis. Natalizumab treatment is associated with a risk of progressive multifocal leukoencephalopathy (PML) of 2.1 per 1000 patients treated per year. PML is a fatal demyenating disease of the central nervous system caused by the JC polyoma virus. Three known risk factors for PML include anti-JC virus antibody, longer duration of treatment (2 more years) and prior immunosuppressive therapy. All patients treated with Natalizumab who developed PML have detectable anti-JC virus antibodies. The risk of PML is lowest (0.09 cases or less per 1000) in patients who are seronegative. The increased risk of PML has limited natalizumab’s use as a first line therapy.

Natalizumab's label now calls for repeat serology testing every 6 months because new JC virus infections may occur during treatment. A positive test result is not an absolute contraindication to natalizumab therapy, but the risks and benefits of continuing treatment with Tysabri should be carefully considered in patients who are found to be anti-JCV antibody positive and have one or more additional risk factors.

A two step assay has been developed for detecting and confirming the presence of anti-JC virus antibodies in human plasma. Patients who have detectable antibodies are at risk of developing PML, while those who test negative are not. Approximately 55% of patients test positive for anti-JC virus antibodies.

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