Immunoglobulin M perception by FcμR

Area of Science:

• Immunology
• Structural Biology
• Molecular Mechanisms

Background:

• Immunoglobulin M (IgM) is a crucial antibody in early development and humoral immunity.
• IgM exists in diverse forms: monomeric, membrane-bound (B cell receptor), pentameric/hexameric (serum), and secretory (mucosal).
• FcμR is the sole IgM-specific receptor in mammals, regulating immune responses, but its interaction mechanisms are unclear.

Purpose of the Study:

• To elucidate the structural basis of the FcμR-IgM interaction.
• To understand how FcμR recognizes and binds different IgM forms.
• To reveal the molecular mechanisms underlying FcμR-mediated regulation of immune responses.

Main Methods:

• Crystallography
• Cryo-electron microscopy (cryo-EM)
• Structural analysis of FcμR-IgM complexes

Main Results:

• Two FcμR molecules bind a Fcμ-Cμ4 dimer, indicating a 2:1 stoichiometry for membrane-bound IgM.
• FcμR binding sites are accessible on the IgM B cell receptor.
• Pentameric IgM recruits four FcμR molecules on one side, forming an FcμR oligomer.
• Secretory IgM engages four FcμR molecules on the side opposite the secretory component, supporting retrotransport.

Conclusions:

• FcμR interacts with diverse IgM forms through distinct stoichiometric and structural arrangements.
• The structural insights explain FcμR's role in recognizing membrane-bound, serum, and secretory IgM.
• These findings uncover the intricate molecular mechanisms of IgM recognition by FcμR, essential for immune regulation.