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. 2021 Sep 23;12:652252. doi: 10.3389/fimmu.2021.652252

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Copyright © 2021 Labarrere and Kassab

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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IgM-mediated immune response in coronavirus infection. [1] Most of natural (innate) immunoglobulin M (IgM) natural antibodies (NAbs) are secreted by B-1 cells (CD5⁺ B-1a and CD5^- B1b) without previous exposure to any antigen or pathogen. B-1a cells are critical in early protection during influenza infections through their IL-17A-driven differentiation into high-rate natural IgM producing plasma cells. Marginal zone (MZ) B cells can recognize IgM NAbs bound to RNA viruses and apoptotic cells through Fcα/μ receptors (Rs) and enhance interleukin-10 (IL-10) release. Innate IgM Nab-secreting cells (CD5⁺ B-1a and CD5^- B1b cells; plasmablasts/plasma cells) recognize virus-bound IgM NAbs through membrane FcμRs. Innate IgM NAbs facilitate lysis of viruses and virus-infected apoptotic cells through activation of the complement cascade (C1q-C5b-9). [2] Natural IgM-secreting B1a cells and human B1 cells, identified in the umbilical cord and in adult peripheral blood and expressing the novel CD20⁺CD27⁺CD43⁺CD70^- phenotype contribute to natural (innate) IgM secretion. Polyreactive IgM monomers on B cells (newly formed naïve B-cells, memory B1b cells) bind repetitive antigenic determinants on bacteria and viruses and induce immune (adaptive) antibody production without the involvement of T-cells. [3] Immune (adaptive) IgM are produced by adaptive B2 cells following exposure to an antigen or pathogen. B2-secreting cells and plasmablasts/plasma cells can produce immune IgM following antibody recognition by FcμRs. [4] B2 lymphocytes can secrete adaptive IgM antibodies following virus recognition by membrane IgM monomers and FcμR recognition of virus bound IgM. [5] Follicular dendritic cells, dendritic cells and macrophages can phagocytose viruses and apoptotic cells bound to innate or adaptive IgM through Fcα/μRs and C1qRs and enhance IL-10 and transforming growth factor (TGF) ß release to reduce inflammation.