Abstract
Bacterial biofilms represent a challenge to the healthcare system because of their resilience against antimicrobials and immune attack. Biofilms consist of bacterial aggregates embedded in an extracellular polymeric substance (EPS) composed of carbohydrate polymers, nucleic acids and proteins. Carbohydrates within P. aeruginosa biofilms include neutral and mannose-rich Psl, and cationic Pel composed of N-acetyl-galactosamine and N-acetyl-glucosamine. Here we show that P. aeruginosa biofilms display ligands for the C-type lectin receptors mannose receptor (MR, CD206) and Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin (DC-SIGN, CD209). Binding of MR and DC-SIGN to P. aeruginosa biofilms is carbohydrate-and calcium-dependent and extends to biofilms formed by clinical isolates. Confocal analysis of P. aeruginosa biofilms shows abundant DC-SIGN ligands among bacteria aggregates while MR ligands concentrate into discrete clusters. DC-SIGN ligands are also detected in planktonic P. aeruginosa cultures and depend on the presence of the common polysaccharide antigen. Carbohydrates purified from P. aeruginosa biofilms are recognised by DC-SIGN and MR; both receptors preferentially bind the high molecular weight fraction (HMW; >132,000Da) with KDs in the nM range. HMW preparations contain 74.9-80.9% mannose, display α-mannan segments and alter the morphology of human dendritic cells without causing obvious changes in cytokine responses. Finally, HMW interferes with the endocytic activity of cell-associated MR and DC-SIGN. This work identifies MR and DC-SIGN as receptors for bacterial biofilms and highlights the potential for biofilm-associated carbohydrates as immunomodulators through engagement of C-type lectin receptors.
Author Summary Selective engagement of pattern recognition receptors during infection guides the decision-making process during induction of immune responses. This work identifies mannose-rich carbohydrates within bacterial biofilms as novel molecular patterns associated with bacterial infections. P. aeruginosa biofilms and biofilm-derived carbohydrates bind two important lectin receptors, MR (CD206) and DC-SIGN (CD209), involved in recognition of self and immune evasion. Abundance of MR and DC-SIGN ligands in the context of P. aeruginosa biofilms could impact immune responses and promote chronic infection.
