Oral cladribine reshapes CSF immune cell composition in relapsing multiple sclerosis

BACKGROUND AND OBJECTIVES: Oral cladribine tablet (CladT) therapy is efficacious for relapsing multiple sclerosis (MS). However, the mechanisms by which cladribine exerts benefit in MS remain unclear, particularly regarding its effects on compartmentalized inflammation within the CSF. METHODS: Transcriptional profiles along with T and B lymphocyte receptor repertoires from CSF and blood were obtained by single-cell sequencing methods from a single site participating in a phase IV clinical trial investigating the impact of cladribine treatment for MS. Blood and CSF samples from patients were obtained immediately before starting CladT therapy, and they were randomized to also provide additional samples at either 5 weeks, 10 weeks, 1 year, or 2 years after CladT therapy. Thirty-four samples from 13 individuals with relapsing MS before and after treatment were available to test the hypothesis that CladT alters the composition and phenotype of lymphocytes in the CSF, including paired baseline and post-CladT CSF samples obtained from 4 unique participants. RESULTS: We found that treatment with CladT profoundly altered cellular composition, but not the transcriptional phenotype, of immune cells in the CSF. In particular, we identified a reduction in switched memory B cells but recovery of naive B cells in the CSF, similar to our findings in blood. In addition, populations of CD4 Treg cells emerged early after CladT therapy and remained elevated 1 year later in the CSF, but not in the blood. Antigen receptor sequencing revealed a moderate decrease in numbers of large clonally expanded CD8 T cell clones (>10 cells/clone) primarily in the CSF, but also in the blood after CladT treatment. DISCUSSION: Our results identified unique cellular dynamics and changes in T cell and B cell clonality in both tissues, which can potentially explain long-term beneficial effects of CladT therapy in MS, including preservation of immune function and a relatively low number of side effects. Altogether, this study demonstrates that CladT treatment had a substantial impact not only on blood but also on the CSF compartment, highlighting the importance of cross-tissue analysis for better understanding of effect and the mechanism of action of disease-modifying therapies.