Antimicrobial resistant bacteria account for high mortality rates due to the lack of effective methods to combat the pathogens. Carbapenem-resistant Klebsiella pneumoniae (CRKP) is a bacterial group of huge concern that is resistant to nearly all available antibiotics. CRKP has been classified as an “urgent threat” to public health with a dire need to develop new drugs alternative treatments. Antibody therapy or even better, a vaccine, targeting this group would be a viable alternative to resolve this threat. Current strategies to target CRKP rely mostly on the bacterial capsular polysaccharides as antigens. However, the high variability in the CPS composition and low epidemiological correlation of clinical isolates would lead to vaccines with limited target spectrum and numerous antibodies for each CPS variant would be needed for acute treatment, making it a very costly approach. Contrary, O-antigens that are part of the bacterial LPS, account with just four serotypes for the majority of clinically relevant strains, with the O2afg serotype as the most frequent in the CRKP group. The use of native LPS-based antigens for vaccine development is hampered by the low immunogenicity, the lack of a T-cell dependent immune answer, and endotoxin contaminations in LPS batches, which can lead to lethal side effects. Thus, no LPS based vaccines are currently in development. In my thesis I address this bottleneck by exploring a semi-synthetic glycoconjugate vaccine using a highly pure and well-characterized synthetic oligosaccharide that mimics the native O2afg antigen of CRKP. The antigen was designed based on a hexasaccharide repeating unit of O2afg and synthesized by Dr. Dacheng Shen. The glycan was subsequently conjugated to CRM197, a carrier protein, and adsorbed into alum as adjuvant, creating a semi-synthetic glycoconjugate vaccine against CRKP based on approved and validated technology. I provide conclusive evidence that the novel glycoconjugate vaccine induces a strong T-cell dependent supported by an immune response with a long-term memory effect at very low antigen concentrations. The resulting antibodies activate complement deposition, thereby activating the opsophagocytic pathway. The generated antibodies are well suited for passive immunization and result in a significant improvement of several physiological parameters in a mouse model for acute pneumonia. This included lower levels of cytotoxic enzymes, neutrophil infiltration and the reduction of permeability and lung edema. Hospital-acquired pneumonia is the major problem caused by K. pneumoniae, especially among individuals with critical illness, for example HIV and cancer in intensive care units. Recently, the pathogen has also been associated with SARS-CoV-2 pulmonary co-infections contributing to chronic obstructive pulmonary diseases, a severe COVID-19 condition with high mortality rates. Thus, the antibodies would significantly reduce the burden as a result of co-infection and increase the chances of survival. I show that a highly effective and low cost glycoconjugate vaccine targeting conserved LPS structures can be developed based on proven technology and opens up for a safer alternative to fight antimicrobial resistant pathogens without the effect of evolutionary selection of resistant strain caused by the currently treatment with antibiotics.
