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.