| Summary: | Cystic fibrosis patients endure serious lung infection caused by colonisation and persistent infection by a wide range of pathogens, most commonly Pseudomonas aeruginosa (PA). One of the factors that facilitates establishment of chronic lung infection is formation of biofilms which are structures resistant to antimicrobial drugs and immune attack. Biofilms are embedded within extracellular polymeric substance (EPS) that maintains the structure of biofilms. PA produces two important polysaccharides Pel and Psl, which have been implicated in promoting biofilm development and biofilm maintenance, respectively, as well as cell aggregation . To the best of our knowledge, there has not been any study showing the presence of specific immune receptors for the recognition of PA biofilms. C-type lectin receptors (CLRs) are pathogen-recognition receptors that contribute to the recognition of infectious agents through the detection of carbohydrates moieties representing a subset of pathogen-associated molecular patterns (PAMPs). We hypothesized that CLRs such as DC-SIGN (CD209) and mannose receptor (MR) (CD206) could play a crucial role in the immune recognition of PA biofilms through the binding to different carbohydrate-containing components. Investigating the CLR-PA cross-talk and the response of immune cells expressing CLRs to different PA components could lead to a novel strategy to eradicate infections. The main aim of this thesis is to determine the capability of CLRs, particularly MR and DC-SIGN, to interact with PA biofilms. We have shown that CTLD4-7, a region of MR, and DC-SIGN bind to PA biofilms with DC-SIGN binding significantly better than MR. Both lectins also recognised several independent preparations of EPS lacking Pel. Surprisingly, we found that DC-SIGN also binds to planktonic PA in the absence of Psl and Pel which indicates that DC-SIGN could recognize non-EPS carbohydrate-containing ligands in the bacteria. Further investigation unveiled that DC-SIGN requires the presence of the common polysaccharide antigen (CPA) which is shared among all PA serotypes to bind planktonic cells. These results indicate that CPA is a candidate ligand for DC-SIGN in PA. To determine the significance of these findings, assays incubating human dendritic cells with purified EPS and planktonic PA were performed but no definitive conclusion could be drawn. These findings shed light on the potential impact of PA Psl and CPA-LPS on the recognition of PA by immune cells expressing CLRs and might open new avenues for therapeutic approaches.
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