Following 2 baseline assessments including Magnetic Resonance Imaging (MRI) of the spinal cord and a lumbar puncture for examination of the fluid around the brain (CSF) participants will be treated with a total of 7 infusions of the anti-TNF-alpha antibody infliximab over a period of 48 weeks. After the last on therapy assessment at 48 weeks participants will be followed up for a further 24 weeks. Study assessments will be clinical, virological, immunological and radiological. MRIs of the spinal cord will be obtained at weeks 12 and 72. CSF will be examined, on therapy, at week 12.