Despite the global health burden posed by Group A Streptococcus (GAS) infections and complications, there is currently no available vaccine against this pathogen. One potential vaccine candidate is the GAS pilus (plural, pili), a long, hair-like structure expressed on the cell surface which involves in the initiation of infection. While current research mainly focuses on pili’s function and their potential as a vaccine target, little is known about the interaction between the pilus and host immune system. Accordingly, the interaction between GAS pili and components of the innate immune system was explored to characterise the implication of pili-based vaccines on the immune system.
Recombinant forms of pilus proteins and L. lactis gain-of-function strains expressing GAS pili on the cell surface were generated to investigate the immunomodulatory properties of the structure. Pili mediated interaction with toll-like receptors (TLRs), cytokine production, and immune cell activation was investigated using immunoassays and flow cytometry experiments. Pili induced inflammation was also studied in moth larvae and a mice model was utilised to study pilus based vaccine.
Pili and pilus proteins induced upregulation of proteins and pro-inflammatory cytokines in innate immune cells associated with adaptive immune modulation. Assays using TLR reporter cell lines indicated pilus specificity to TLR2 and binding assays confirmed physical interactions with the receptor. This was confirmed by cytokine measurement, where production of downstream cytokine was inhibited in the presence of a TLR2 antagonist. Additionally, the TLR2/6 heterodimer was pinpointed as the TLR2 heterodimer recognising pili. Further experiments indicated that pili mediated inflammation stimulated antibody production in mice and did not correlate with disease severity in wax worms.
The GAS pilus proteins were shown to be highly immunostimulatory ligands of TLR2, with the ability to prime the immune system for enhanced antibody production. The insight gained into the immunomodulatory characteristics of GAS pili emphasise the pilus proteins’ potential as a GAS vaccine candidate and as an adjuvant in other vaccine formulations