We surveyed the T cell receptor repertoire in three separate compartments (brain, cerebrospinal fluid, and blood) of two multiple sclerosis patients who initially had diagnostic brain biopsies to clarify their unusual clinical presentation but were subsequently confirmed to have typical multiple sclerosis. One of the brain biopsy specimens had been previously investigated by microdissection and single-cell PCR to determine the clonal composition of brain-infiltrating T cells at the single-cell level. Using complementarity-determining region 3 spectratyping, we identified several identical, expanded CD8 ؉ (but not CD4 ؉ ) T cell clones in all three compartments. Some of the expanded CD8 ؉ T cells also occurred in sorted CD38 ؉ blood cells, suggesting that they were activated. Strikingly, some of the brain-infiltrating CD8 ؉ T cell clones persisted for >5 years in the cerebrospinal fluid and͞or blood and may thus contribute to the progression of the disease.

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