Background: Following fingolimod cessation, immune reconstitution or lack thereof may have consequences for disease rebound or safety of commencing alternative therapies. Objective: To examine the degree and profile of peripheral blood lymphocyte reconstitution following fingolimod withdrawal. Methods: Total lymphocyte counts (TLC) and CD4+/CD8+ T-cell counts were measured in 18 multiple sclerosis (MS) patients pre-treatment, on fingolimod, and up to 8–9 months post-cessation. T-cell subsets were analyzed using flow cytometry. Results: At 2-week post-fingolimod cessation, TLC reconstitution was variable and not correlated with age, treatment duration, pre-, or on-treatment TLC. Despite normalization of TLC and CD4+:CD8+ ratios over months, naive subsets remained lower and effector memory subsets higher in frequency compared with pre-treatment. Drug-induced increases in ratios of regulatory to pathogenic Th17-containing central memory populations appeared to rapidly return to baseline. Conclusion: Early peripheral lymphocyte reconstitution after fingolimod withdrawal remains partial and heterogeneous. Relative frequencies of circulating naive and memory T-cell subsets may not recover for many months, even when clinical laboratory tests have normalized. Analyzing specific components of the peripheral immune repertoire helps define the overall immune status of patients. To be determined is whether assessment of such immune measures will have implications for the timing and safety of commencing alternative therapies.
