Despite the global disease burden imposed 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 on the cell surface key for the initiation of infection. Pili based vaccines have been demonstrated to stimulate robust production of protective antibodies, but the innate immune responses involved remain undefined. Thus the interaction between GAS pili and components of innate immunity was investigated to characterise the implication of pili-based-vaccines on the immune system.
Pilus recombinant proteins and recombinant L. lactis strains expressing GAS pili on the cell surface were utilised to investigate the immunomodulation capacity of this surface structure. Furthermore, interactions between pili and toll-like receptors (TLRs) was studied in HEK239 reporter cell lines expressing various human TLRs. Cytokine production in response to pili was analysed by a cell-based ELISA using human monocytic THP-1 cells.
Protein production downstream of TLRs in the HEK239 cell lines indicated the GAS pilus proteins are ligands of TLR2. Additionally, the interaction between TLR2 and pilus proteins was confirmed via binding assays as well as flow cytometry. Furthermore, the TLR2/6 heterodimer was pinpointed as the TLR2 heterodimer recognising pili. In the THP-1 cells, strong pili induced production of pro-inflammatory cytokines such as TNF was observed. This pili induced cytokine release decreased with the introduction of a TLR2 antagonist, consolidating the pili signalling pathway. Whilst both the tip and backbone subunit appeared to be involved in the innate immune response, the tip subunit was found to have higher affinity binding to receptor and induced higher levels of cell stimulation. Interestingly, differences in the levels of cellular response were seen between different GAS pilus types.
The GAS pilus was shown to be highly immunostimulatory ligands of TLR2, alluding to its ability to achieve a desirable immunisation outcome. This helps solidify the pili as a GAS vaccine candidate, and warrants further investigation into its use as an adjuvant.