Biology subjects

van Kessel, K. P. M.

Publications and source records attributed to van Kessel, K. P. M..

5 recordsLinked to original sources

bioRxiv2026

C1q-mediated steric hindrance limits FcγR contribution to antibody-driven phagocytosis by human neutrophils

Phagocytosis and killing of bacteria by neutrophils is a key driver of human immunity against invading infections. IgG antibodies targeting the bacterial surface can potently enhance phagocytic uptake by engaging Fc gamma receptors (Fc{gamma}Rs) on the neutrophil membrane. In addition, IgG-mediated complement activation triggers surface deposition of complement opsonins that engage complement receptors (CRs) to further enhance target cell clearance. In this paper, we reveal that complement deposition prevents engagement of Fc{gamma}Rs to the Fc-domain of surface-bound IgG molecules. When complement is present, we observe that blockade of Fc{gamma}Rs does not influence phagocytosis of both Gram-positive and -negative bacteria (using Staphylococcus aureus and Klebsiella pneumoniae as model organisms). Complement-mediated hindrance of Fc{gamma}R engagement was observed in different model conditions, for example using either pre-existing polyclonal antibodies in human serum or recombinant monoclonal antibodies (mAbs) targeting different bacterial antigens. Using Fc engineering, we found that inhibition of Fc{gamma}R binding is dependent on the ability of IgGs to form IgG hexamers. Mechanistically, we show that complement proteins, in particular C1q, directly prevents the interaction between IgG-Fc and Fc{gamma}R by steric hindrance. Together, these insights help us define the respective roles of Fc{gamma}R and CR in neutrophil phagocytosis, while also providing important fundamental insights for therapeutic antibody development.

immunology↗

bioRxiv2021

C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases

Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibodys 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-Fcs 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.

immunology↗

bioRxiv2020

Staphylococcal protein A inhibits complement activation by interfering with IgG hexamer formation

IgG molecules are essential players in the human immune response against bacterial infections. An important effector of IgG-dependent immunity is the induction of complement activation, a reaction that triggers a variety of responses that help to kill bacteria. Antibody-dependent complement activation is promoted by the organization of target-bound IgGs into hexamers that are held together via noncovalent Fc-Fc interactions. Here we show that Staphylococcal protein A (SpA), an important virulence factor and vaccine candidate of Staphylococcus aureus, effectively blocks IgG hexamerization and subsequent complement activation. Using native mass spectrometry and high-speed atomic force microscopy, we demonstrate that SpA blocks IgG hexamerization through competitive binding to the Fc-Fc interaction interface on IgG monomers. In concordance, we show that SpA interferes with the formation of (IgG)6:C1q complexes and prevents downstream complement activation on the surface of S. aureus. Lastly, we demonstrate that IgG3 antibodies against S. aureus can potently induce complement activation even in the presence of SpA. Altogether, this study identifies SpA as an immune evasion protein that specifically blocks IgG hexamerization.

microbiology↗

bioRxiv2020

Bacterial immune evasion proteins: The therapeutic potential of CHIPS

Bacterial pathogens have evolved to secrete strong anti-inflammatory proteins that target the immune system. It was long speculated whether these virulence factors could serve as therapeutics in diseases in which abnormal immune activation plays a role. We adopted the secreted Chemotaxis Inhibitory Protein of Staphylococcus aureus (CHIPS) as a model virulence factor-based therapeutic agent for diseases in which C5aR1 stimulation plays an important role. We show that administration of CHIPS in human C5aR1 knock-in mice successfully dampens C5a mediated neutrophil migration during immune complex-initiated inflammation. Subsequent CHIPS toxicology studies in animal models were promising. However, during a small phase-I trial, healthy human volunteers showed adverse effects directly after CHIPS administration. Subjects showed clinical signs of anaphylaxis with mild leukocytopenia and increased C-reactive protein concentrations, suggesting an inflammatory response, which are believed to be related to the presence of relatively high circulating anti-CHIPS antibodies. Even though our data in mice shows CHIPS as a potential anti-inflammatory agent, safety issues in human subjects temper the use of CHIPS in its current form as a therapeutic candidate. The use of staphylococcal proteins, or other bacterial proteins, as therapeutics or immune-modulators in humans is severely hampered by pre-existing circulating antibodies.

microbiology↗

bioRxiv2018

Staphylococcal superantigen-like protein 13 activates neutrophils via Formyl Peptide Receptor 2

Staphylococcal Superantigen-Like (SSL) proteins, one of major virulence factor families produced by Staphylococcus aureus, were previously demonstrated to be immune evasion molecules that interfere with a variety of innate immune defenses. However, in contrast to these characterized SSLs, that inhibit immune functions, we show that SSL13 is a strong activator of neutrophils via the formyl-peptide receptor 2 (FPR2). Moreover, our data show that SSL13 acts as a chemoattractant, induces degranulation and oxidative burst in neutrophils. As with many other staphylococcal immune evasion proteins, SSL13 shows a high degree of human specificity. SSL13 is not able to efficiently activate mouse neutrophils, hampering in vivo experiments.\n\nIn conclusion, SSL13 is a neutrophil chemoattractant and activator that acts via the FPR2. Therefore, SSL13 is a unique SSL member that does not belong to the immune evasion class, but is a pathogen alarming molecule.

microbiology↗