Exploring the Effect of Conjugation Site and Chemistry on the Immunogenicity of an anti-Group B Streptococcus Glycoconjugate Vaccine Based on GBS67 Pilus Protein and Type V Polysaccharide

Abstract

We have recently described a method for tyrosine-ligation of complex glycans that was proven efficient for the site selective coupling of GBS capsular polysaccharides (PSs). Herein, we explored the effect of conjugation of type V polysaccharide onto predetermined lysine or tyrosine residues of the GBS67 pilus protein with the dual role of T-cell carrier for the PS and antigen. For the preparation of a conjugate at predetermined lysine residues of the protein, we investigated a two-step procedure based on microbial Transglutaminase (mTGase) catalyzed insertion of a tag bearing an azide for following copper-free strain-promoted azide-alkyne [3 + 2] cycloaddition (SPAAC) with the polysaccharide. Two glycoconjugates were obtained by tyrosine-ligation through the known SPAAC and a novel thiol-maleimide addition based approach. Controls were prepared by random conjugation of PSV to GBS67 and CRM197, a carrier protein present in many commercial vaccines. Immunological evaluation in mice showed that all the site-directed constructs were able to induce good levels of anti-polysaccharide and anti-protein antibodies inducing osponophagocytic killing of strains expressing individually PSV or GBS67. GBS67 randomly conjugated to PSV showed carrier properties similar to CRM197. Among the tested site-directed conjugates, tyrosine-directed ligation and thiol-malemide addition was elected as the best combination to ensure production of anti-polysaccharide and anti-protein functional antibodies (in vitro opsonophagocytic killing titers) comparable to the controls made by random conjugation, while avoiding anti-linker antibodies. Our findings demonstrate that (i) mTGase based conjugation at lysine residues is an alternative approach for the synthesis of large capsular polysaccharide-protein conjugates; (ii) GBS67 can be used with the dual role of antigen and carrier for PSV; and (iii) thiol-maleimide addition in combination with tyrosine-ligation ensures the production of anti-polysaccharide and anti-protein functional antibodies while maintaining low levels of anti-linker antibodies. Site-specific conjugation methods aid in defining conjugation site and chemistry in carbohydrate-protein conjugates.