The success of B cell-targeted therapies such as Rituximab and Ocrelizumab suggests that B cells play a prominent role in the pathogenesis of MS. However, not all B cell subsets are pathogenic with some B cells having immune regulatory properties that make them protective from MS disease. Identifying which B cell subsets are associated with active disease will allow for the design of more targeted therapies that inhibit these B cells specifically.

Mass cytometry combines flow cytometry with mass spectrometry allowing for the simultaneous analysis of over 30 parameters. Peripheral blood mononuclear cells from 7 active (disease activity within 6 months of blood collection, confirmed by MRI) and 8 inactive MS patients were stained with our panel of 35 heavy-metal isotope-conjugated antibodies. Data was analysed using the clustering algorithm FlowSOM.

Mass cytometry allowed us to analyse and phenotype B cell subsets with unparalleled depth, revealing that three major subsets of CD24⁺ CD38^– IgG₃⁺ switched memory B cells were significantly elevated in active MS disease: CD21⁺ CD27^– (p = 0.0228, Kruskal-Wallis test with Dunn’s multiple comparisons test); CD21⁺ CD27⁺ (p = 0.0465); and CD21^– CD27⁺ (p = 0078).

IgG3 antibodies are potent activators of the classical complement pathway and may indicate a previously unrecognised role for IgG₃⁺ B cells in MS pathogenesis. If this proves to be the case, then IgG3⁺ B cells, or their products, may be a biomarker of MS relapse and/or offer a specific target for future disease modifying immunotherapeutic strategies.