Background: Naturally occurring regulatory T (T(reg)) cells are functionally impaired in patients with relapsing-remitting multiple sclerosis. We recently showed that prevalences of newly generated CD31-coexpressing naive T(reg) cells (recent thymic emigrant-T(reg) cells) are critical for suppressive function of circulating T(reg) cells, and a shift in the homeostatic composition of T(reg)-cell subsets related to a reduced de novo generation of recent thymic emigrant-T(reg) cells may contribute to the multiple sclerosis (MS)-related T(reg)-cell dysfunction. Interferon beta, an immunomodulatory agent with established efficacy in MS, lowers relapse rates and slows disease progression. Emerging evidence suggests that T(reg)-cell suppressive capacity may increase in patients with MS undergoing treatment with interferon beta, although the mechanisms mediating this effect are uncertain.

Objective: To evaluate the effect of interferon beta treatment on the suppressive activity and the homeostasis of circulating T(reg) cells in patients with MS.

Participants: Twenty patients with relapsing-remitting MS and 18 healthy control subjects.

Interventions: Administration of interferon beta.

Main outcome measures: Effect of interferon beta on T(reg)-cell homeostasis and suppressive capacity.

Results: Suppressive capacities of T(reg) cells were consistently upregulated at 3 and 6 months after treatment with interferon beta. The restoration of T(reg)-cell function was paralleled by increased naive recent thymic emigrant-T(reg) cells and a coincidental reduction in memory T(reg) cells.

Conclusion: The increase in T(reg)-cell inhibitory capacity mediated by interferon beta treatment can be explained by its effect on the homeostatic balance within the T(reg) cell compartment.