A Bistable Mechanism Mediated by Integrins Controls Mechanotaxis of Leukocytes

The recruitment of leukocytes from blood vessels to inflamed zones is guided by biochemical and mechanical stimuli, with mechanisms only partially deciphered. We studied here the guidance by flow of primary human effector T lymphocytes crawling on substrates coated with ligands of integrins LFA-1 (L{beta}2) and VLA-4 (4{beta}1), and showed that cells segregated in two populations of opposite orientation for combined adhesion. Sharp decisions of orientation were shown to rely on a bistable mechanism between LFA-1-mediated upstream and VLA-4-dominant downstream phenotypes. At the molecular level, bistability results from a differential front-rear polarization of both integrins affinity, combined with an inhibiting crosstalk of LFA-1 toward VLA-4. At the cellular level, directivity with or against the flow is mechanically mediated by the passive orientation of detached uropod or lamellipod by flow. This complete chain of logical events provides a unique mechanistic picture of a guiding mechanism, from stimuli to cell orientation.\n\nSignificanceCellular guidance is crucial to many biological functions, but the precise mechanisms remain unclear. We have analyzed here an original phenotype of flow-guided cells mimicking leukocytes crawling into the blood vessels and showed that the controlling parameter of cells decision to migrate upstream or downstream was the relative number of two specific adhesion molecules, the integrins LFA-1 and VLA-4. The spatial polarisation of integrins affinity and an intermutually feedback of their activation create a bistable system where cells adhere either by their tip or their tail and orient respectively downstream or upstream. This mechanism therefore proposes a complete chain of event from stimuli to cell orientation and differs strongly from the chemotaxis paradigm because stimuli trigger no signaling.