Title: Dissecting complement resistance in Staphylococcus aureus
Funder: The Academy of Medical Sciences

Abstract: Staphylococcus aureus is a leading cause of severe invasive infection, reflected in the 20,000 people who died of staphylococcal related disease in 2017 in the United states alone (1). The aim of this proposal is to develop a detailed understanding of how S aureus evades a primary line of host immunity, the complement system. A significant hurdle in evaluating bacterial factors required for complement evasion is the absence of accurate technology to determine complement deposition. This is due to the expression of staphylococcal immunoglobulin-binding proteins (IgG-BP) that disrupt antibody-mediated detection of complement fixation. In Objective 1, I will develop and optimise a new method to measure complement deposition, assessing and comparing two different strategies to minimise risk in assay development. •Antibody-independent strategy: Using biophysical techniques and modelling, I have identified two novel peptides that interact with specific complement activation products that will be used to determine complement deposition •F(ab)’2 fragments strategy: In tandem, I will generate and label commercial complement-specific antibodies devoid of the IgG-BP binding (Fc) domain In Objective 2, this new transformative technology will be used to determine the physiological contribution of specific surface proteins predicted to prevent complement deposition. Objective 3 will discover the precise interaction of S aureus with host complement inhibitors and determine whether this is a conserved immune evasion strategy. Combined, this information will reveal essential S aureus immune evasion mechanisms and lay the foundation for future grant applications dedicated to developing therapies to enhance complement activity and promote pathogen elimination.

Lay abstract: Staphylococcus aureus is a member of a select group of pathogens that colonise a significant proportion of the human population without causing harm, while retaining the capacity to cause severe infection. Historically, the switch from coloniser to pathogen was attributed to the immune status of the host. However, in the last two decades, S aureus has been causing severe infections in healthy individuals, the reasons for which are poorly understood. Furthermore, the rise of antibiotic resistance and the lack of effective vaccines are further complicating staphylococcal infections. Importantly, a major contributor to vaccine failure is our incomplete understanding of how this pathogen manipulates the host immune response. The primary host response to S aureus infection occurs via complement. Complement is an elegant evolutionarily conserved system, playing essential roles in early defences by working in concert with immune cells to survey, label and destroy microbial intruders and coordinate inflammation. Currently the exact mechanisms employed by S aureus to manipulate and evade complement is not clear. In this proposal I aim to develop a novel technique that can accurately report on complement deposition on the bacterial surface, enabling us to dissect which bacterial surface proteins mediate complement resistance. In addition, I aim to determine the importance of recruitment of host complement inhibitors as a means of complement evasion and identify specific proteins associated with this mechanism. Unravelling S aureus complement evasive strategies will highlight novel targets for therapeutic intervention, with the long-term aim of effectively diminishing pathogen resistance to host immunity.

Research activity codes: 2.2 FACTORS RELATING TO PHYSICAL ENVIRONMENT;5.1 PHARMACEUTICALS
Health categories: INFECTION