Promoting Fc-Fc interactions between anti-capsular antibodies provides strong immune protection against Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and an important cause of childhood mortality. Despite the introduction of successful vaccines, the global spread of both non-vaccine serotypes and antibiotic-resistant strains reinforce the development of alternative therapies against this pathogen. One possible route is the development of monoclonal antibodies (mAbs) that induce killing of bacteria via the immune system. Here we investigate whether mAbs can be used to induce killing of pneumococcal serotypes for which the current vaccines show unsuccessful protection. Our study demonstrates that when human mAbs against pneumococcal capsule polysaccharides (CPS) have a poor capacity to induce complement activation, a critical process for immune protection against pneumococci, their activity can be strongly improved by hexamerization-enhancing mutations. Our data indicate that anti-capsular antibodies may have a low capacity to form higher-order oligomers (IgG hexamers) that are needed to recruit complement component C1. Indeed, specific point mutations in the IgG-Fc domain that strengthen hexamerization strongly enhance C1 recruitment and downstream complement activation on encapsulated pneumococci. Specifically, hexamerization-enhancing mutations E430G or E345K in CPS6-IgG strongly potentiate complement activation on S. pneumoniae strains that express capsular serotype 6 (CPS6), and the highly invasive serotype 19A strain. Furthermore, these mutations improve complement activation via mAbs recognizing CPS3 and CPS8 strains. Importantly, hexamer-enhancing mutations enable mAbs to induce strong phagocytosis and intracellular killing by human neutrophils. Finally, passive immunization with CPS6-IgG1-E345K protected mice from developing severe pneumonia. Altogether, this work provides insights that are crucial for optimizing antibody therapies against encapsulated S. pneumoniae.

Monoclonal antibodies effectively potentiate complement activation and phagocytosis of Staphylococcus epidermidis in neonatal human plasma

“…In the presence of complement, uptake was greatly enhanced. This is in agreement with previous work on Streptococcus pneumoniae, where IgG1 mAbs directed against capsule polysaccharide CPS6 were completely dependent on complement deposition (24). In contrast, we previously observed that naturally occurring IgG, IVIG (45) and a monoclonal antibody against WTA (23) can induce Fc receptor mediated phagocytosis of S. aureus in absence of complement.…”

“…Hexamer enhancing mutations have already been shown to enhance complement mediated lysis of Neisseria gonorrhoeae (47) and tumor cells (26) via the formation of membrane attack complex pores. Together with data on S. aureus (23) and S. pneumoniae (24), our study provides an important proof of concept that hexamer enhancing mutations can also potentiate opsonization and phagocytic killing of Gram-positive bacteria. This indicates that hexamer enhancing mutations could be applied to a broad range of pathogens and diseases.…”

“…We then studied the immune activating potential of rF1-, CR5133-and CR6453-IgG1 and found that activation of the complement system is essential to induce efficient phagocytosis of S. epidermidis. As shown before on Staphylococcus aureus (23) and Streptococcus pneumoniae (24), phagocytosis and killing of S. epidermidis could be further enhanced by Fc-mutations that improve IgG hexamerization, which is needed for efficient activation of the classical pathway (25)(26)(27). Finally, we collected human cord blood from neonates to study the ability of the mAbs to activate the neonatal complement system.…”

“…We introduced mutation E345K in the Fc backbone of rF1-IgG1, CR5133-IgG1, CR6453-IgG1. This mutation was selected based on previous results obtained with Streptococcus pneumoniae (24). After confirming that introduction of the E345Kmutations did not affect antibody binding to S. epidermidis (Figure S4), we tested their ability to induce C3b deposition (Figure 3A), phagocytosis (Figure 3B) and killing (Figure 3C).…”

Neonatal sepsis and transient immunodeficiency: Potential for novel immunoglobulin therapies?

“…Antibodies with hexamerization-enhancing mutations (hexabodies) activate complement more efficiently compared to wild-type IgG. These hexabodies showed more efficient C3b activation and phagocytosis of Staphylococcus aureus in vitro, Streptococcus pneumoniae in vitro and more efficient MAC activation and bacterial lysis of Neisseria gonorrhoeae in vitro and in vivo (54)(55)(56). Recently, it was shown that hexabodies can also enhance C3b activation and phagocytosis of Staphylococcus epidermidis in the context of neonatal plasma in vitro (42).…”