AFM studies unravel how glycan nanodomains on pathogenic bacteria drive selective recognition by host immune cells
Résumé
Adhesion to host cells and tissues is an important first step in infection employed by many bacterial pathogens. In mycobacteria, surface hydrophobic properties and specialized receptor-ligand bonds determine how these pathogens adhere to host immune cells. We recently used atomic force microscopy (AFM) to quantitatively map hydrophobic properties of mycobacterial pathogens, at high spatial resolution. We discovered that abundant polar outer membrane lipids modulate the nanoscale distribution of hydrophobicity on the bacterial surface, which could be modulated by treatments with specific antibiotics. This led us to hypothesize that the presence of nanodomains enriched in specific surface molecules have a functional role in pathogen-host interactions. By probing live mycobacteria with AFM tips modified with single tetramers of the soluble extracellular domain of the C-type lectin pathogen recognition receptor DC-SIGN we observed that the receptor forms complexes with ligands on pathogenic and nonpathogenic mycobacteria. Moreover, the complexes are mechanically weak, rupturing at ~30 pN. However, by relying on the specific force-extension signatures generated we could map the locations of single DC-SIGN ligands at a resolution of ~20 nm, which revealed concentration of ligands into dense nanodomains on pathogenic mycobacteria exclusively. Upon bacteria-host cell contact, ligand nanodomains induce the recruitment and clustering of DC-SIGN, resulting in mechanically stable adhesion. This work highlights the key role of clustering of both ligands on pathogenic bacteria and DC-SIGN host receptors in pathogen recognition, a mechanism that might be widespread in host-pathogen interactions. Our ongoing work is dedicated to unraveling the composition, formation and dynamics of glycan nanodomains functioning in mycobacterial recognition by the innate immune system.