A human monoclonal antibody bivalently binding two different epitopes in streptococcal M protein mediates immune function

Multimodal Mass Spectrometry Identifies a Conserved Protective Epitope in S. pyogenes Streptolysin O

An important element of antibody-guided vaccine design is the use of neutralizing or opsonic monoclonal antibodies to define protective epitopes in their native three-dimensional conformation. Here, we demonstrate a multimodal mass spectrometry-based strategy for in-depth characterization of antigen-antibody complexes to enable the identification of protective epitopes using the cytolytic exotoxin Streptolysin O (SLO) from Streptococcus pyogenes as a showcase. We first discovered a monoclonal antibody with an undisclosed sequence capable of neutralizing SLO-mediated cytolysis. The amino acid sequence of both the antibody light and the heavy chain was determined using mass-spectrometry-based de novo sequencing, followed by chemical cross-linking mass spectrometry to generate distance constraints between the antibody fragment antigen-binding region and SLO. Subsequent integrative computational modeling revealed a discontinuous epitope located in domain 3 of SLO that was experimentally validated by hydrogen-deuterium exchange mass spectrometry and reverse engineering of the targeted epitope. The results show that the antibody inhibits SLO-mediated cytolysis by binding to a discontinuous epitope in domain 3, likely preventing oligomerization and subsequent secondary structure transitions critical for pore-formation. The epitope is highly conserved across >98% of the characterized S. pyogenes isolates, making it an attractive target for antibody-based therapy and vaccine design against severe streptococcal infections.

Pathogen-driven degradation of endogenous and therapeutic antibodies during streptococcal infections

Group A streptococcus (GAS) is a major bacterial pathogen responsible for both local and systemic infections in humans. The molecular mechanisms that contribute to disease heterogeneity remain poorly understood. Here we show that the transition from a local to a systemic GAS infection is paralleled by pathogen-driven alterations in IgG homeostasis. Using animal models and a combination of sensitive proteomics and glycoproteomics readouts, we documented the progressive accumulation of IgG cleavage products in plasma, due to extensive enzymatic degradation triggered by GAS infection in vivo. The level of IgG degradation was modulated by the route of pathogen inoculation, and mechanistically linked to the combined activities of the bacterial protease IdeS and the endoglycosidase EndoS, upregulated during infection. Importantly, we show that these virulence factors can alter the structure and function of exogenous therapeutic IgG in vivo. These results shed light on the role of bacterial virulence factors in shaping GAS pathogenesis, and potentially blunting the efficacy of antimicrobial therapies.

Group A streptococci induce stronger M protein-fibronectin interaction when specific human antibodies are bound

Group A streptococcus (GAS) is a highly adapted, human-specific pathogen that is known to manipulate the immune system through various mechanisms. GAS' M protein constitutes a primary target of the immune system due to its spatial configuration and dominance on the bacterial surface. Antibody responses targeting the M protein have been shown to favor the conserved C region. Such antibodies (Abs) circumvent antigenic escape and efficiently bind to various M types. The ability of GAS to bind to fibronectin (Fn), a high molecular weight glycoprotein of the extracellular matrix, has long been known to be essential for the pathogen's evolutionary success and fitness. However, some strains lack the ability to efficiently bind Fn. Instead, they have been found to additionally bind Fn via the A-B domains of their M proteins. Here, we show that human Abs can induce increased Fn-binding affinity in M proteins, likely by enhancing the weak A-B domain binding. We found that this enhanced Fn binding leads to a reduction in Ab-mediated phagocytosis, indicating that this constitutes a GAS immune escape mechanism. We could show that the Fc domain of Abs is necessary to trigger this phenomenon and that Ab flexibility may also play a key role. We, moreover, saw that our Abs could enhance Fn binding in 3 out of 5 emm type strains tested, belonging to different clades, making it likely that this is a more generalizable phenomenon. Together our results suggest a novel synergistic interplay of GAS and host proteins which ultimately benefits the bacterium.

Measurement of Antibody Binding Affinity on Bacterial Surfaces Using Flow Cytometry

Antibody binding to bacterial surfaces plays a crucial role in immunity, and a key characteristic of this protein-protein interaction is the binding affinity. Determining the affinity of an antibody binding to its antigen is the first step in predicting the function in a physiological environment where other competing protein interactions may be present. Antibody-antigen affinity is often evaluated with isolated proteins. It is informative to also be able to assess antibody binding to a bacterial surface where many antigens might be present, including multiple copies of the specific antigen the antibody recognizes, and in a context where the antigen might be in a more natural conformation. In this chapter, we present a flow cytometry-based assay to measure and calculate the cell surface binding affinity or avidity of any mono- or polyclonal antibody solution.