The Fc fragment of soluble IgMs binds C1q to activate the classical complement pathway, while inhibiting complement-dependent cytotoxicity

Soluble type-M immunoglobulins (IgMs), among the most potent activators of the classical pathway, are key mediators of complement-dependent cytotoxicity, which render them promising candidates for developing alternative drugs in treating autoimmune or inflammatory diseases. In this study, we investigated the biochemical and in vitro functional properties of recombinant fragments from IgMs corresponding to the Fc-core in their two pentameric or hexameric forms. Biophysical experiments confirmed the crucial role of the IgM Joining chain (J) in favoring homogenous pentamers, while its absence led to heterogeneous population with a mixture of oligomeric forms. By combining size-exclusion chromatography with mass photometry, isolation of enriched samples with IgM hexamers or IgM pentamers without the J chain was possible. Biolayer interferometry demonstrated that both IgM-Fc forms bind C1q and ELISA showed that they induce the in vitro C4b deposition when in solid phase. Additionally, our data confirmed the higher efficacy of IgM hexamers compared to pentamers in activating the first component of the classical pathway. Finally, hemolytic assays demonstrate the ability of IgM-Fc constructs to inhibit Ig-induced complement-dependent cytotoxicity. These findings provide important information for the ultimate development of IgM-based molecules that target specifically complement activation.