Characterisation of the Novel Staphylococcal Superantigen-Like Related Protein: FPRL1-Inhibitory Protein (FLIPr) and its Potential Glycan Binding Site

Abstract

Having Staphylococcus aureus as a commensal bacterium can come with some serious side-effects such as sepsis and even death if this opportunistic pathogen is able to breach the host barrier defences. This is due to its many secreted virulence factors. Both the innate immune system and the adaptive immune system are involved in the host immune responses to infection caused by S. aureus. To survive against detection and elimination by the host immune system, S. aureus has evolved a complex set of mechanisms to evade innate immune components and resist phagocytosis by neutrophils through the secretion of numerous proteins and molecules. The staphylococcal superantigen–like (SSL) family is one such virulence factors. FLIPr is a novel SSL-related protein with a predicted β-grasp topology and conservation to the SSL glycan binding site. It is reported in the literature that FLIPr can bind to FPRL1, FcγR II and C1q to inhibit their functions. This research therefore looked at the characterisation of FLIPr and its potential glycan binding site in its interaction with these known receptors/molecules. In this thesis, FLIPr was found to bind to human leukocytes primarily granulocytes, monocytes, T lymphocytes, B lymphocytes and NK cells by flow cytometry. Variants of FLIPr mutants in the potential glycan binding site showed reduced binding to some of these leukocyte populations. Western blot analysis of FLIPr and its glycan binding site-mutants showed that the binding to FPRL1, FcγR II and C5aR were not dependent in this conserved binding site. However, analysis of the binding to C1q/C1 confirmed that the potential glycan binding site played a role in FLIPr-C1q/C1 interaction suggesting a plausible inhibitory function of the binding site in the complement classical pathway. The results have shed light on FLIPr and its potential glycan binding site and their roles in the immune evasion mechanisms of S. aureus. In-depth understanding of the complex pathogenesis of S. aureus, not only promotes understanding of the organism but also aids the development of novel prophylactics by providing a new research targets such as FLIPr.