Exploring the Immunostimulatory Properties of GAS Pili

Abstract

The global burden of Streptococcus pyogenes (Group A Streptococcus, GAS) infection related diseases has resulted in widespread investigation into the virulence factors of the bacterium, in an effort to produce a prophylactic vaccine. One vaccine candidate is the GAS pilus, which is a long, filamentous cell surface anchored structure. GAS pili have been demonstrated to elicit adaptive immune response but little is known about the innate immune response to the structure. This study thus aimed to elucidate the interaction between GAS pili and components of the innate immune system, using recombinant pilus proteins and whole assembled pili expressed on the surrogate L. lactis bacterium to explore the immunomodulatory properties of the complex. Pili interaction with toll-like receptors (TLRs) and their ability to activate immune cells was explored in vitro with immunoassays, flow cytometry and reporter cell lines. The inflammatory response elicited by the structure was also modelled using wax moth larvae and the adjuvanting capacity of the pilus proteins was assessed using a mice intranasal immunisation model. Assays using TLR reporter cell lines depicted proteins physically interacting with and activating TLR2, an attribute further consolidated in competition assays using a TLR2 antagonist. Furthermore, specificity of the pilus proteins for TLR2 in the TLR2/6 heterodimeric form was revealed. Pilus proteins were also illustrated activating macrophages and inducing upregulation of proteins and pro-inflammatory cytokines associated with the enhanced ability of these cells to modulate the adaptive immune system. The wax moth larvae model indicated that this pili mediated stimulation of innate immunity did not appear to be a contributor to disease pathogenesis. Furthermore, immunisation of mice with pilus proteins conjugated to the low immunogenicity influenza antigen M2e resulted in enhanced M2e specific antibody production. These results uncovered the previously undiscovered characteristics of GAS pilus proteins being TLR2 agonists with the ability to prime innate immune cells to enhance adaptive immune responses, without exerting deleterious effects on the recipient. This emphasises the suitability of GAS pili as a vaccine candidate and highlights its potential as a vaccine adjuvant.