BACKGROUND: Fingolimod is an oral drug approved for multiple sclerosis (MS) with 
an ability to trap central memory T cells in secondary lymphoid tissues; 
however, its variable effectiveness in individual patients indicates the need to 
evaluate its effects on other lymphoid cells.
OBJECTIVE: To clarify the effects of fingolimod on B-cell populations in 
patients with MS.
METHODS: We analysed blood samples from 9 fingolimod-treated and 19 control 
patients with MS by flow cytometry, to determine the frequencies and activation 
states of naive B cells, memory B cells, and plasmablasts.
RESULTS: The frequencies of each B-cell population in peripheral blood 
mononuclear cells (PBMC) were greatly reduced 2 weeks after starting fingolimod 
treatment. Detailed analysis revealed a significant reduction in activated 
memory B cells (CD38(int-high)), particularly those expressing Ki-67, a marker 
of cell proliferation. Also, we noted an increased proportion of activated 
plasmablasts (CD138(+)) among whole plasmablasts, in the patients treated with 
fingolimod.
CONCLUSIONS: The marked reduction of Ki-67(+) memory B cells may be directly 
linked with the effectiveness of fingolimod in treating MS. In contrast, the 
relative resistance of CD138(+) plasmablasts to fingolimod may be of relevance 
for understanding the differential effectiveness of fingolimod in individual 
patients.