Fingolimod Profoundly Reduces Frequencies and Alters Subset Composition of Circulating T Follicular Helper Cells in Multiple Sclerosis Patients

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Abstract
Fingolimod is an effective treatment for relapsing-remitting multiple sclerosis. It is well established that fingolimod, a modulator of the sphingosine-1-phosphate pathway, restrains the egress of CCR7+ lymphocytes from lymphatic tissues into the blood, thus resulting in reduced lymphocyte counts in peripheral blood. CXCR5+ T follicular helper (Tfh) cells provide help to B cells, are essential for the generation of potent Ab responses, and have been shown to be critically involved in the pathogenesis of several autoimmune diseases. Besides lymphoid tissue-resident Tfh cells, CXCR5+ circulating Tfh (cTfh) cells have been described in the blood, their numbers correlating with the magnitude of Tfh cells in lymphoid tissues. Although the effect of fingolimod on circulating lymphocyte subsets has been established, its effect on cTfh cells remains poorly understood. In this study, we found that although fingolimod strongly and disproportionally reduced cTfh cell frequencies, frequencies of activated cTfh cells were increased, and the composition of the cTfh cell pool was skewed toward a cTfh1 cell phenotype. The circulating T follicular regulatory cell subset and CXCR5+ CD8+ T cell frequencies were also strongly and disproportionally decreased after fingolimod treatment. In contrast, relative frequencies of CXCR5– memory Th cells as well as regulatory T and B cells were increased. In summary, these data provide new insights into fingolimod-induced compositional changes of lymphocyte populations in the blood, in particular cTfh cells, and thus contribute to a better understanding of the mechanism of action of fingolimod in multiple sclerosis patients. Key Points Circulating Tfh cell frequencies are reduced upon fingolimod treatment. Fingolimod reduces circulating Tfr cell and CXCR5+ CD8+ T cell frequencies. Blood CXCR5– Tmem, Treg, and Breg cell frequencies are elevated by fingolimod.

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