Exploring Monoclonal Antibody Action Against the Group A Streptococcal M Protein

Group A Streptococcus (GAS) is a significant human pathogen that has developed multiple
immune evasion mechanisms to counter the host immune response. One of these mechanisms involves
the production of the M protein, which, amongst other things, acts as an anti-phagocytic factor and can
bind host proteins. Another is the ability of M protein to bind a human protein called fibronectin (Fn). This
protein plays a key role in a number of physiological processes and can be used by GAS to evade the
immune system. In this PhD dissertation, we aimed to assess the binding efficacy and function of
monoclonal antibodies targeting the GAS M protein.
In the first paper we start by developing a robust method to assess phagocytosis. This method highlights
the importance of factors such as volume, time, and the ratio of phagocyte to prey on the phagocytic
process. It has allowed us to, henceforth, attain precise, high quality phagocytosis data and has been a
major driving force for other projects within the lab – especially the three other papers included in this
thesis.
In the second paper we discovered a novel form of antibody binding whereby a monoclonal binds the
GAS M protein in a bivalent dual-Fab cis mode. This means that both Fab arms of the Ab bind to distinct
epitopes on the target molecule simultaneously. Even so this antibody bound to a region of the M protein
associated with non-opsonic antibodies we found that this Ab could enhance phagocytosis suggesting
that this novel binding form can circumvent the M protein's anti-phagocytic properties.
In the third paper we investigated the M protein’s ability to bind fibronectin. While this function was
described in previous studies, we found it could only do so with very low affinity. We found that the binding
of antibodies from the blood of donors who had recently recovered from a severe GAS infection could
greatly enhance this fibronectin binding. We show that same occurs with certain anti-M monoclonals and
that this mechanism leads to a reduction in opsonophagocytosis. Moreover we find that Ab flexibility may
play a role and that Ab Fc domains are a crucial factor in mechanism.
In the fourth paper we further explore this anti-phagocytic effect. Here we assess the effects of varying
concentrations of Fn since this can differ greatly within the human body. We found that both very low and
high concentrations of Fn, corresponding with the nasopharyngeal niche and blood respectively, led to a
substantial reduction in phagocytosis. We moreover found that this reduction in phagocytosis is likely
linked to a modulation of integrins. Overall, this work provides insights into immune evasion mechanisms
developed by GAS and highlights how this remarkable pathogen always seems to be one step ahead of
us.