A Self-Adjuvanting Vaccine Platform: Optimization of Site-Specific Sortase A Mediated Conjugation of Toll-Like Receptor 2 Ligands onto the Carboxyl or Amino terminus of Recombinant Protein Antigens

Abstract

Self-adjuvanting vaccines, consisting of recombinant protein antigens and covalently attached Toll-like receptor (TLR) agonists, have the ability to simultaneously and efficiently deliver antigen and TLR adjuvant to antigen presenting cells (APCs). Here, an enzyme-mediated ligation approach was used to overcome difficulties in producing homogeneous, molecularly defined self-adjuvanting vaccine products under native conditions. This process was optimized to allow the incorporation of the lipopeptide TLR2 agonist fibroblast-stimulating lipopeptide (FSL)-1 onto the N- or C-termini of recombinant protein antigens, employing the enzyme Staphylococcus aureus sortase A (SrtAsa) penta mutant. In addition, because SrtAsa-mediated ligations are reversible, a tryptophan zipper derived sequence was introduced into both reactants, which was demonstrated to improve ligation efficiency through the formation of a β-hairpin structure that hinders the reverse reaction. Finally, it was demonstrated that N- or C-terminal conjugation, and the incorporation of the β-hairpin structure, did not affect the TLR2-agonist activities of protein antigen TLR agonist conjugates. Overall, this SrtAsa-mediated ligation platform enabled production of antigen TLR2 agonist conjugates with enhanced ligation efficiency, with the conjugates demonstrating potent TLR2 signaling activation (EC₅₀ <1nM).

Keywords: conjugation; delivery; self-adjuvanting vaccine; sortase A mediated ligation; toll-like receptors.