CD21lo B-cell subsets are recruited to the central nervous system in acute neuromyelitis optica

<jats:title>Abstract</jats:title> <jats:p>Neuromyelitis optica (NMO) is an acute inflammatory demyelinating disease of the CNS. The presence of astrocyte-targeted AQP4-immunoglobulin G (IgG) in peripheral blood is a major factor in its diagnosis. Previous studies show that AQP4-IgG contributes directly to CNS inflammation and that B cells play a central pathogenic role in NMO. However, where and how the B-cell response is altered remains controversial.</jats:p> <jats:p>In this study, we used high-parameter flow cytometry to carry out a comprehensive analysis of the immune cell populations in the CSF samples obtained from first-episode acute-phase NMO patients, compared with those from patients with acute-phase multiple sclerosis and other neurological diseases. Among 10 immune cell populations defined in the analysis, the frequency only of B cells and antibody-secreting cells (ASC) was higher in the CSF of acute-phase NMO compared with other neurological diseases. Detailed assessments of B-cell and ASC subsets in the CSF revealed differences in the dominant subsets between NMO and multiple sclerosis. In NMO, a series of CD21lo B-cell subsets, including ‘activated’ naïve B, double-negative and switched memory subsets, considered as ASC precursors, were dominant. A majority of these CD21lo B-cell subsets expressed CD69 and CXCR3, suggesting their CNS residency. An increase of CD21lo B-cell subsets was also observed in the CSF of treatment-refractory NMO patients. Furthermore, two B-helper T-cell subsets, T peripheral helper type 1 and T follicular helper type 1 cells, both highly expressing CD69 and CXCR3, were enriched in the CSF of NMO patients, suggesting their interactions with ASC precursors in the CNS.</jats:p> <jats:p>In vitro culture experiments using blood samples from patients with NMO showed that CD21lo B cells included AQP4-IgG-producing cells and displayed a high propensity to differentiate into ASCs. We also found that CD21lo B-cell subsets in NMO upregulated the expression of C5a receptors, and C5a signals promoted their differentiation into ASCs. ASCs derived from CD21lo B cells expressed high levels of CXCR3 and CD138. The increase in CD21lo B-cell subsets was significantly correlated with the annual relapse rate.</jats:p> <jats:p>Collectively, our study strongly suggests that the mechanism to promote the generation of CD21lo B cells, probably via the extrafollicular pathway, becomes activated during the acute phase of NMO and that the generated CD21lo B-cell subsets contribute to the pathogenesis. Targeting CD21lo B-cell subsets might be useful for the development of novel therapeutic approaches.</jats:p>