Site‐Selective Functionalization of Flagellin by Steric Self‐Protection: A Strategy To Facilitate Flagellin as a Self‐Adjuvanting Carrier in Conjugate Vaccine

Abstract

Flagellin (FliC) can act as a carrier protein in the preparation of conjugate vaccines to elicit a T-cell-dependent immune response and as an intrinsic adjuvant to activate the toll-like receptor 5 (TLR5) to enhance vaccine potency. To enable the use of FliC as a self-adjuvanting carrier, an effective method for site-selective modification (SSM) of pertinent amino-acid residues in the D2 and D3 domains of FliC is explored without excessive modification of the D0 and D1 domains, which are responsible for activating and binding with TLR5. In highly concentrated Na SO solution, FliC monomers form flagellar filaments, in which the D0 and D1 domains are situated inside the tubular structure. Thus, the lysine residues (K219, K224, K324, and K331) in the D2 and D3 domains of flagellin are selectively modified by a diazo-transfer reaction with imidazole-1-sulfonyl azide. The sites with azido modification are confirmed by MALDI-TOF-MS, ESI-TOF-MS, and LC-MS/MS analyses along with label-free quantitation. The azido-modified filament dissolves to give FliC monomers, which can conjugate with alkyne-hinged saccharides by the click reaction. Transmission electron microscopy imaging, dynamic light scattering measurements, and the secreted embryonic alkaline phosphatase reporter assay indicate that the modified FliC monomers retain the ability either to bind with TLR5 or to reassemble into filaments. Overall, this study establishes a feasible method for the SSM of FliC by steric self-protection of the D0 and D1 domains.

Constructing conjugate vaccine against Salmonella Typhimurium using lipid-A free lipopolysaccharide

“…FliC monomers are directly subjected to chemical modification with imidazole-1-sulfonyl azide by the conventional method [ 45 ], so that the lysine residues in all domains may be modified in non-selective manner. Since such excessive modification of FliC may cause steric hindrance and interfere with its binding to TLRs as to reduce its adjuvant effect, a site-selective modification method was also carried out [ 46 ]. FliC monomers are first treated in high concentration of Na 2 SO 4 solution to afford flagellar filaments, so that only the lysine residues of the exposed D2 and D3 domains can be easily modified.…”

Site-Specific Conjugation for Fully Controlled Glycoconjugate Vaccine Preparation

“…However, if second mechanism has to be considered, the selection of the glycosylation sites is equally important. Along this line, Peng et al have taken advantage of the propensity of flagellin to self-assemble in a supercoiled structure to selectively modify the sole lysines exposed to the solvent and thus preserving the protein properties to activate immune response (Peng et al, 2018). Stefanetti et al recently prepared a series of glycoconjugates made of CRM 197 and Salmonella O -antigen as the carrier protein and the carbohydrate antigen, respectively (Stefanetti et al, 2015).…”