Functional capacities of human IgM memory B cells in early inflammatory responses and secondary germinal center reactions

Abstract The generation and functions of human peripheral blood (PB) IgM+IgD+CD27+ B lymphocytes with somatically mutated IgV genes are controversially discussed. We determined their differential gene expression to naive B cells and to IgM-only and IgG+ memory B cells. This analysis revealed a high similarity of IgM+(IgD+)CD27+ and IgG+ memory B cells but also pointed at distinct functional capacities of both subsets. In vitro analyses revealed a tendency of activated IgM+IgD+CD27+ B cells to migrate to B-cell follicles and undergo germinal center (GC) B-cell differentiation, whereas activated IgG+ memory B cells preferentially showed a plasma cell (PC) fate. This observation was supported by reverse regulation of B-cell lymphoma 6 and PR domain containing 1 and differential BTB and CNC homology 1, basic leucine zipper transcription factor 2 expression. Moreover, IgM+IgD+CD27+ B lymphocytes preferentially responded to neutrophil-derived cytokines. Costimulation with catecholamines, carcinoembryonic antigen cell adhesion molecule 8 (CEACAM8), and IFN-γ caused differentiation of IgM+IgD+CD27+ B cells into PCs, induced class switching to IgG2, and was reproducible in cocultures with neutrophils. In conclusion, this study substantiates memory B-cell characteristics of human IgM+IgD+CD27+ B cells in that they share typical memory B-cell transcription patterns with IgG+ post-GC B cells and show a faster and more vigorous restimulation potential, a hallmark of immune memory. Moreover, this work reveals a functional plasticity of human IgM memory B cells by showing their propensity to undergo secondary GC reactions upon reactivation, but also by their special role in early inflammation via interaction with immunomodulatory neutrophils.