A 15-year-old girl presents with her second episode of neurologic dysfunction in the past year. Her first episode was unilateral optic neuritis, and she now presents with ataxia and dysarthria. A brain MRI shows multiple discrete, ovoid T2 lesions in the periventricular and juxtacortical regions. Which laboratory finding in the cerebrospinal fluid would most strongly support a diagnosis of multiple sclerosis (MS)?