Integrin ß1 demarks precursors of brain-residing antibody-secreting cells in multiple sclerosis

Multiple sclerosis (MS) is characterized by B cells that infiltrate the central nervous system (CNS) to mature into antibody-secreting cells (ASCs). To date, it remains to be determined how to benefit from this disease hallmark as a prognostic tool. We aimed to uncover markers that reflect B-cell entry and maturation in the CNS and on that basis explore associations with response to high efficacy therapies in MS. Single-cell RNA sequencing (scRNA-seq) revealed ITGB1 (integrin ß1/CD29) as a major discriminator of memory B cells expressing CXCR3, which was validated on protein level by spectral flow cytometry. Functional in vitro assays showed that CD29+CXCR3+ memory B cells preferentially cross a brain microvascular endothelial layer and are more receptive to T-cell help to become ASCs. In MS blood, CD29 and CXCR3 were particularly co-expressed on circulating memory B cells, but not on ASCs. In CNS compartments of MS donors, however, high co-expression of CD29 and CXCR3 was restricted to ex vivo ASCs, which was supported by a stronger upregulation of CD29 on in vitro-differentiated ASCs. In MS, while CD29 was effectively downregulated on CXCR3+ memory B cells that accumulated in the blood of natalizumab-treated patients, CD29 levels were only reduced on CXCR3+ memory B cells that repopulated in cladribine-treated patients without early breakthrough disease activity. Taken together, this study reveals that CD29 defines CXCR3+ B cells with a propensity to infiltrate and develop into ASCs in the CNS, which holds potential to be utilized as a cellular biomarker to predict treatment responses in people with MS. Overall design: Single-cell RNA-seq profiling of FACS-sorted CD19+ B cells from thawed PBMCs from healthy donors (n=3)