The fc fragment of IgMs binds C1q to activate the first step of the classical complement pathway, while inhibiting complement-dependent cytotoxicity.

Soluble IgMs, among the most potent activators of the classical pathway, are key mediators of complement-dependent cytotoxicity, which render them as promising drug candidates for the development of alternative drugs in treating autoimmune or inflammatory diseases. In the present study, we investigated the biochemical and in vitro functional properties of recombinant fragments from IgMs corresponding to the fragment crystallizable region (Fc)-core in their pentameric or hexameric forms. Biophysical experiments confirmed the crucial role of the IgM Joining (J) chain in favoring homogeneous pentamers, whereas its absence led to a 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 an 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, which is likely made possible by the absence of fragment antigen binding region. These findings support a possible mechanism of C1 sequestration in plasma by IgM cores and consumption of the initial complement component C4. Our data thus provide important information for the development of IgM-based anti-inflammatory molecules that target specifically complement activation.