Development of Synthetic Conjugate Vaccines Against Clinically Urgent Pathogens

Abstract

The rising threat of antimicrobial resistance (AMR) and the rapid evolution of viral pathogens present critical challenges to global public health. Klebsiella pneumoniae (Kp), particularly the hypervirulent K1 and K2 serotypes, has emerged as a leading cause of drug-resistant infections, while the ongoing SARS-CoV-2 pandemic continues to be shaped by the emergence of immune-evasive variants. This thesis addresses both bacterial and viral threats through a unified synthetic vaccinology approach, aiming to develop structure-defined conjugate vaccines that can overcome current limitations in traditional vaccine platforms.

Focusing first on K. pneumoniae, the work targeted the K1 and K2 capsular polysaccharides (CPS), which are highly prevalent and cover more than 70% clinical cases of hvKp infection. Through chemical synthesis, structurally defined CPS fragments were prepared with strategic frameshifts to identify optimized minimal protective epitopes—the smallest glycan motifs capable of eliciting protective immunity. We synthesized hvKp K2 serotype derived three tetrasaccharide epitopes K2A to K2C and hvKp K1 serotype derived six trisaccharide epitopes K1A to K1C bearing 2-NHAc and 3-NHAc on fucose respectively. The NHAc can serve as the surrogate of OAc avoiding the possibility of acyl migration and ester hydrolysis. These synthetic fragments were then conjugated to carrier protein to create vaccine candidates. The result of mice immunization study indicated that K2B was the optimal epitope, which can be used for anti-hvKp glycoconjugate vaccine development and will be used for preparing monoclonal antibody for passive immunotherapy. The immunological evaluation of the K1 CPS derived synthetic glycoconjugates showed that K1A-Fuc3NAc demonstrated the strongest protective response against the K1 strain, and epitopes bearing Fuc-3NAc generally produced higher immune activity than those with Fuc-2NAc. These works establish a systematic synthetic and immunological platform for structure–activity analysis of K1 and K2 CPS epitopes, providing valuable insights for the rational design of conjugate vaccines against multidrug-resistant bacteria.

For SARS-CoV-2 vaccine development, the thesis explored a synthetic vaccine strategy against SARS-CoV-2 by designing glycopeptide conjugates that target conserved and surface-exposed regions of the viral spike (S) protein. Unlike traditional subunit vaccines that may lose efficacy due to antigenic drift, this platform incorporates glycosylation patterns and conserved sequences to produce a vaccine cocktail aimed at generating broad and variant-resilient immunity. Nine peptide conjugates were designed, including one glycopeptide, along with their mutants targeting SARS-CoV-2 variants. The mice immunization results demonstrated that the conjugates of PB4 and PB2PS5 and their variants can induce high IgG titers. Meanwhile, SCT-glycosylated PS3 and its variants can elicit robust immune response, highlighting the critical role of N-glycosylation in antigen design. Neutralization assays further identified a post-sera mixture of PB2PS5 and its two variants retained activity across variants, suggesting utility against diverse variants.

By connecting the development of vaccines for a multidrug-resistant bacterium and a rapidly evolving pathogenic virus, this thesis demonstrates the potential of synthetic conjugate platforms as a versatile solution for modern vaccinology. The outcomes offer not only promising vaccine candidates against K. pneumoniae (K1 and K2) and SARS-CoV-2, but also a scalable framework for addressing emerging infectious threats.

Date of Award	28 Oct 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Runsheng LI (Supervisor), Sheng CHEN (External Co-Supervisor) & Fuyong LI (Supervisor)