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

Significance Human IgM ⁺IgD ⁺CD27 ⁺ B lymphocytes represent a large subpopulation of the human B-cell pool, but their generation is debated and their immunological functions are poorly understood. This work shows that these lymphocytes possess typical memory B-cell expression patterns, enabling them to differentiate rapidly into plasma cells upon restimulation. Moreover, we reveal unique features of these IgM memory B cells, their potential to reenter germinal center reactions, and their specific interaction with immunomodulatory neutrophils in early inflammatory responses. Thus, key characteristics and functions of a major human B-cell subset are elucidated. 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 IgG ₂, 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.