Abstract
To clear pathogens from host tissues or biomaterial surfaces, phagocytes have to break the adhesive bacteria-substrate interactions. Here we analysed the mechanobiological process that enables macrophages to lift-off and phagocytose surface-bound Escherichia coli (E. coli). In this opsonin-independent process, macrophage filopodia hold on to the E. coli fimbriae long enough to induce a local protrusion of a lamellipodium. Specific contacts between the macrophage and E. coli are formed via the glycoprotein CD48 on filopodia and the adhesin FimH on type 1 fimbriae (hook). We show that bacterial detachment from surfaces occurrs after a lamellipodium has protruded underneath the bacterium (shovel), thereby breaking the multiple bacterium-surface interactions. After lift-off, the bacterium is engulfed by a phagocytic cup. Force activated catch bonds enable the long-term survival of the filopodium-fimbrium interactions while soluble mannose inhibitors and CD48 antibodies suppress the contact formation and thereby inhibit subsequent E. coli phagocytosis.
Highlights
To clear pathogens from host tissues or biomaterial surfaces, phagocytes have to break the adhesive bacteria-substrate interactions
To allow for co-adhesion of E. coli (UPEC strain J96) and macrophages (J774.1), we performed all phagocytosis experiments on glass substrates coated with a mixture of purified human plasma fibronectin (FN) and the glycoprotein Ribonuclease B (RNaseB)
With a filopodium contact formed (33 s), the macrophage locally protruded a lamellipodium towards the bacterium (33–57 s)
Summary
Correspondence and requests for materials should be addressed to V.V. While most strains of E. coli are harmless, enterohemorrhagic E. coli (EHEC) and uropathogenic E. coli (UPEC) can cause life-threatening infections upon entry into the blood circulation through lesions of the digestive track or the epithelium of the urinary tract[10] respectively Analysing this process is physiologically relevant as macrophages frequently encounter bacteria that are tightly bound to (engineered) surfaces[11] or to ECM fibrils[12]. While most receptor-ligand interactions are known to dissociate faster under tensile forces (slip bonds), it is well established that the E. coli adhesin FimH forms long-lived catch bonds with mannoses, i.e. bonds that are activated by mechanical force (for reviews see[14,15]) In this single-cell analysis study, we describe kinetic and mechanistic details of a multistep process that enables macrophages to pick up surface-adhering type 1 fimbriated E. coli in an opsonin-independent, but mannose-specific www.nature.com/scientificreports manner. We show here that filopodia retraction is not sufficient to lift-off surface bound E. coli and that the mechanical interplay of forming a long-term bond with a filopodium and subsequent lamellipodium protrusion is required for the pickup that initiates phagocytosis
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