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

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. 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. 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. 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.