In the present study we showed that transitional B cells of patients with clinically isolated syndrome (CIS) and relapsing-remitting multiple sclerosis (RR-MS) are reduced in the peripheral blood (PB) (5.5- and 3.7-fold, respectively). In addition, these cells appeared to up-regulate different integrins (α4 and β1). These observations were associated with a primed cellular status, confirmed by an increased proportion of circulating CD80(+) transitional B cells. Interestingly, these results correlate with presence of transitional B cells in the CSF. Furthermore, these cells were absent in the CSF of individuals with other inflammatory neurological disease, and their levels in paired PB and CD80 expression were normal. Altogether, our data revealed that a differential primed status of transitional B cells is a characteristic feature of early phases of MS disease, and this functional status is associated with the ability of these cells to cross the blood-CSF barrier.
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