Uncovering the glycan-mediated host-pathogen interactions in cystic fibrosis

Pseudomonas aeruginosa is a prevalent pathogen in cystic fibrosis (CF) lungs with elaborate defence mechanisms serving to counter the host immune response. This thesis hypothesises a previously unstudied defence mechanism involving a P. aeruginosa lectin (LecB) that proposedly acts to neutralise host microbicidal glycoproteins. Multi-pronged approaches involving methods in microbiology and biochemistry were employed to produce robust structural and functional data supporting the hypothesis by firstly showing the binding of LecB to the N-glycans of human neutrophil elastase (HNE) and, importantly, that LecB fully neutralises the potent microbicidal activity of HNE against P. aeruginosa. In-silico 3D structure analyses indicated spatial compatibility and bivalent binding between two neighbouring carbohydrate binding domains (CBDs) of tetrameric LecB and two HNE N-glycans. Moreover, complete LecB sequence conservation from genotypically/phenotypically diverse clinical P. aeruginosa strains was identified suggesting essential roles of LecB. Excitingly, a LecB-mediated inhibition of the HNE enzyme activity easily disrupted with D-mannose was shown. Finally, advanced mass spectrometry supported the existence of a LecB-HNE complex. This thesis has generated novel data suggesting that P. aeruginosa LecB carry out other roles than acting as the glue in biofilms by proposing a completely new pathogen defence mechanism involving LecB-mediated neutralisation of host microbicidal glycoproteins.