Soluble IgM antibodies, the first to respond to antigens, are promising therapeutic targets due to their broad pathogen reactivity, including viruses and tumors, and their potent but complex roles in complement activation and inflammation. IgMs mainly exist as pentamers or hexamers, with superior binding and complement activation compared to IgGs, making them attractive for therapeutic development. However, challenges in production and understanding their function have hindered clinical progress. This study comprehensively characterized recombinant IgM oligomers, focusing on structural and functional differences between constructs with and without the J chain. Structural analysis using cryo-electron microscopy (cryo-EM) and functional assays such as Biolayer Interferometry (BLI), ELISA, liposome-based lysis, and hemolytic tests, revealed that IgM constructs with the J chain formed homogeneous populations, while those without the J chain displayed a mix of pentamers, hexamers, and smaller oligomers. Cryo-EM at 4 Å resolution uncovered that the IgM-Fc hexamer adopts a compact, planar structure with pseudo-hexamer symmetry, distinct from the pentameric form. In the functional assays, surprisingly, constructs without antigen-binding domains demonstrated complement activation. Additionally, carbon nanotube templates were proposed as a novel antigen template for structural studies with IgM, surpassing existing antigen platforms. This work underscores the unique structure and enhanced function of hexameric IgM, highlighting its potential as a therapeutic candidate and paving the way for designing targeted IgM-based treatments for immune diseases.​
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