Complement C1q subcomponent subunit B
symbiont cell surface
C1q binding to surface-bound IgG is stabilized by C1r&lt;sub&gt;2&lt;/sub&gt;s&lt;sub&gt;2&lt;/sub&gt; proteases.
Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody's constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr<sub>2</sub>s<sub>2</sub>) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r<sub>2</sub>s<sub>2</sub>). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r<sub>2</sub>s<sub>2</sub> proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic <i>Staphylococcus aureus</i>). The extent to which C1r<sub>2</sub>s<sub>2</sub> contributes to C1q-IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q-IgG stability, both in the absence and presence of C1r<sub>2</sub>s<sub>2</sub> In addition, hexamer-enhanced IgGs targeting <i>S. aureus</i> mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.