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 directly contributes to CNS inflammation, and B cells play a central pathogenic role in NMO. However, where and how B cell response is altered remains controversial. In this study, we comprehensively analyzed with high-parameter flow cytometry the immune cell populations in the CSF samples obtained from first-episode acute-phase NMO patients, and compared to those from patients with acute-phase Multiple Sclerosis (MS) and other neurological diseases (OND). Among ten immune cell populations defined in the analysis, only the frequency of B cells and antibody-secreting cells (ASC) were higher in the CSF of acute-phase NMO compared to OND. Detailed assessments of B cell and ASC subsets in the CSF revealed differences in the dominant subsets between NMO and MS. In NMO, a series of CD21lo B cell subsets ranging from "activated" naïve B, double negative, and switched memory, thus 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 type1 and T follicular helper type1 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 contained AQP4-IgG-producing cells and displayed a high propensity to differentiate into ASCs. We further 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 significantly correlated with the annual relapse rate. Collectively, our study highly suggests that the mechanism to promote the generation of CD21lo B cells, likely via the extrafollicular pathway, becomes activated during the acute phase of NMO, and 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.