Physiological and Pathophysiological Roles of IgM Fc Receptor (FcµR) Isoforms

Author:

Kubagawa Hiromi1,Clark Caren2,Skopnik Christopher M.1ORCID,Mahmoudi Aliabadi Pedram1,Al-Qaisi Khlowd1,Teuber Ruth1,Jani Peter K.1,Radbruch Andreas1ORCID,Melchers Fritz1,Engels Niklas2,Wienands Jürgen2ORCID

Affiliation:

1. Deutsches Rheuma-Forschungszentrum, 10117 Berlin, Germany

2. Institute of Cellular & Molecular Immunology, University Medical Center, 37073 Göttingen, Germany

Abstract

IgM is the first antibody to emerge during phylogeny, ontogeny, and immune responses and serves as a first line of defense. Effector proteins interacting with the Fc portion of IgM, such as complement and its receptors, have been extensively studied for their functions. IgM Fc receptor (FcµR), identified in 2009, is the newest member of the FcR family and is intriguingly expressed by lymphocytes only, suggesting the existence of distinct functions as compared to the FcRs for switched Ig isotypes, which are expressed by various immune and non-hematopoietic cells as central mediators of antibody-triggered responses by coupling the adaptive and innate immune responses. Results from FcµR-deficient mice suggest a regulatory function of FcµR in B cell tolerance, as evidenced by their propensity to produce autoantibodies of both IgM and IgG isotypes. In this article, we discuss conflicting views about the cellular distribution and potential functions of FcµR. The signaling function of the Ig-tail tyrosine-like motif in the FcµR cytoplasmic domain is now formally shown by substitutional experiments with the IgG2 B cell receptor. The potential adaptor protein associating with FcµR and the potential cleavage of its C-terminal cytoplasmic tail after IgM binding are still enigmatic. Critical amino acid residues in the Ig-like domain of FcµR for interacting with the IgM Cµ4 domain and the mode of interaction are now defined by crystallographic and cryo-electron microscopic analyses. Some discrepancies on these interactions are discussed. Finally, elevated levels of a soluble FcµR isoform in serum samples are described as the consequence of persistent B cell receptor stimulation, as seen in chronic lymphocytic leukemia and probably in antibody-mediated autoimmune disorders.

Funder

Leibniz Collaborative Excellence

the Deutsche Forschungsgemeinschaft

Publisher

MDPI AG

Subject

Inorganic Chemistry,Organic Chemistry,Physical and Theoretical Chemistry,Computer Science Applications,Spectroscopy,Molecular Biology,General Medicine,Catalysis

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