TNFSF13B and primary progressive multiple sclerosis: The present study results did not support the hypothesis that MRZR positivity might be an indicator of a higher B cell activity, as MRZR+ and MRZR- PPMS patients neither differed in the level of any of the four B cell biomarkers studied (sBCMA, sTACI, CXCL-13, and BAFF), nor in respect of other signs of generally more pronounced B cell activity (such as higher CSF cell count or higher intrathecal Ig synthesis).