The B1-cell subpopulation is diminished in patients with relapsing-remitting multiple sclerosis

•The fraction of CD20+ B cells was significantly increased in MS.•The mean frequency of B1 cells was significantly decreased among MS patients.•In MS, B1-cell frequencies were inversely correlated with time since last attack.•The presence of CD3+CD20+ cells was demonstrated by confocal microscopy.

B cell subsets in newly diagnosed untreated, relapsing-remitting multiple sclerosis (MS) patients were examined. The fraction of CD20+ B cells was significantly increased in MS. Among subsets of B cells, MS patients had increased frequency of naïve cells, but reduced frequency of memory and B1 cells. The frequencies of B1 cells were inversely correlated with the time since last attack. B1 cells resembled the phenotype of either lymphocytes (CD11b− B1 cells) or monocytes (CD11b+ B1 cells) and a small fraction of cells was CD3+CD20+ by confocal microscopy.