A unique algorithm for the determination of peptide-carrier protein conjugation ratio by amino acid analysis using intrinsic internal standard

Soluble prefusion-closed HIV-envelope trimers with glycan-covered bases

Soluble HIV-1-envelope (Env) trimers elicit immune responses that target their solvent-exposed protein bases, the result of removing these trimers from their native membrane-bound context. To assess whether glycosylation could limit these base responses, we introduced sequons encoding potential N-linked glycosylation sites (PNGSs) into base-proximal regions. Expression and antigenic analyses indicated trimers bearing six-introduced PNGSs to have reduced base recognition. Cryo-EM analysis revealed trimers with introduced PNGSs to be prone to disassembly and introduced PNGS to be disordered. Protein-base and glycan-base trimers induced reciprocally symmetric ELISA responses, in which only a small fraction of the antibody response to glycan-base trimers recognized protein-base trimers and vice versa. EM polyclonal epitope mapping revealed glycan-base trimers -even those that were stable biochemically- to elicit antibodies that recognized disassembled trimers. Introduced glycans can thus mask the protein base but their introduction may yield neo-epitopes that dominate the immune response.

Carriers and Antigens: New Developments in Glycoconjugate Vaccines

Glycoconjugate vaccines have proven their worth in the protection and prevention of infectious diseases. The introduction of the Haemophilus influenzae type b vaccine is the prime example, followed by other glycoconjugate vaccines. Glycoconjugate vaccines consist of two components: the carrier protein and the carbohydrate antigen. Current carrier proteins are tetanus toxoid, diphtheria toxoid, CRM197, Haemophilus protein D and the outer membrane protein complex of serogroup B meningococcus. Carbohydrate antigens have been produced mainly by extraction and purification from the original host. However, current efforts show great advances in the development of synthetically produced oligosaccharides and bioconjugation. This review evaluates the advances of glycoconjugate vaccines in the last five years. We focus on developments regarding both new carriers and antigens. Innovative developments regarding carriers are outer membrane vesicles, glycoengineered proteins, new carrier proteins, virus-like particles, protein nanocages and peptides. With regard to conjugated antigens, we describe recent developments in the field of antimicrobial resistance (AMR) and ESKAPE pathogens.

Streamlining Peptide Mapping LC-MS Approach for Studying Fusion Peptide-Conjugated Vaccine Immunogens

A newly introduced HIV-1 vaccination utilizes a fusion peptide (FP)-based immunogen-carrier conjugate system, where the FP is coupled to a protein carrier via a bifunctional linker. Such heterogeneous materials present a challenge for the routine product quality assessment. Peptide mapping LC-MS analysis has become an indispensable tool for assessing the site-specific conjugation ratio, estimating site occupancy, monitoring conjugation profiles, and analyzing post-translational modifications (PTMs) and disulfide bonds as well as high-order protein structures. To streamline the peptide mapping approach to match the needs of a fast-paced conjugate vaccine product characterization, a selection of signature fragment ions generated by MS^E fragmentation was successfully applied to assess the product quality at the different stages of a conjugates' manufacturing process with an emphasis on monitoring the amount of a reactive linker. This technique was employed in different conjugation studies of the protein carriers, linkers, and FP compositions as well as the cross-linked species formed during stress-degradation studies. Multiple derivatives of the intermediate and final conjugated products formed during a multistaged synthesis were monitored by means of the sensitive extracted-ion chromatogram (XIC) profiling and were included in the estimation of the site-specific conjugation loads. Differentiation of the conjugates with various FP compositions was demonstrated. The conjugation site occupancy was evaluated with respect to the solvent exposure of Lys residues. The findings of these LC-MS studies greatly aided in choosing the best conjugation strategy to ensure that the final recombinant tetanus toxoid heavy chain (rTTHc) product is chemically inert and represents a safe vaccine candidate for clinical evaluation.