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 (C1qr2s2) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r2s2). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r2s2 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 Staphylococcus aureus). The extent to which C1r2s2 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 C1r2s2. In addition, hexamer-enhanced IgGs targeting S. aureus 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.

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 (C1qr2s2) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r2s2). While interactions between C1 and IgG-Fc's are believed to be mediated by the globular heads of C1q, we here find that C1r2s2 proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various DNP-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r2s2 contribute 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 absence and presence of C1r2s2. In addition, hexamer-enhanced IgGs targeting S. aureus 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.

Abstract:Human hookworm Necator Americanus (NA) causes iron deficiency anemia, as the parasite ingests blood from the gastrointestinal tract of its human host. This bioinformatics-based study focuses on eight of the cathepsin B-like cysteine proteases (CPs) of the worm to explore their pathogenic potential. CP1 -CP6, which harbored the active site cysteine residue for enzymatic activity, were relevantly observed to have Nterminal signal peptide for extracellular localization. The secretory CPs could be releasing indigenous worm heparin at the host-pathogen interface for anticoagulation purposes. CP2 and CP3 showed a novel hemoglobinase motif that could be a prerequisite for hemoglobin degradation. CP1 and CP6 shared similar enzymatic-pocket features with cathepsin B and cruzain that cleave high molecular weight kininogen for blood-thinning activity. CP1, CP2, CP3, CP5 and CP6 were predicted to bind heparin, at their C terminal domain, like human cathepsin B and cruzain non-covalently bind heparin to enhance their activity. NA CPs' action in concert with heparin, have implications for anti-heparin and heparin analog design against hookworm infection.

No abstract