Mammalian cells developed two main migration modes. The slow mesenchymatous mode, like crawling of fibroblasts, relies on maturation of adhesion complexes and actin fibre traction, while the fast amoeboid mode, observed exclusively for leukocytes and cancer cells, is characterized by weak adhesion, highly dynamic cell shapes, and ubiquitous motility on 2D and in 3D solid matrix. In both cases, interactions with the substrate by adhesion or friction are widely accepted as a prerequisite for mammalian cell motility, which precludes swimming. We show here experimental and computational evidence that leukocytes do swim, and that efficient propulsion is not fuelled by waves of cell deformation but by a rearward and inhomogeneous treadmilling of the cell external membrane. Our model consists of a molecular paddling by transmembrane proteins linked to and advected by the actin cortex, whereas freely diffusing transmembrane proteins hinder swimming. Furthermore, continuous paddling is enabled by a combination of external treadmilling and selective recycling by internal vesicular transport of cortex-bound transmembrane proteins. This mechanism explains observations that swimming is five times slower than the retrograde flow of cortex, and also that lymphocytes are motile in non-adherent confined environments. Resultantly, the ubiquitous ability of mammalian amoeboid cells to migrate in 2D or 3D, and with or without adhesion, can be explained for lymphocytes by a single machinery of heterogeneous membrane treadmilling. SIGNIFICANCE STATEMENT Leukocytes have a ubiquitous capacity to migrate on or in solid matrices, and with or without adhesion, which is instrumental to fight infections. The precise mechanisms sustaining migration remain however arguable. It is for instance widely accepted that leukocytes cannot crawl on 2D substrates without adhesion. In contrast, we showed that human lymphocytes swim on nonadherent 2D substrates and in suspension. Furthermore, our experiments and modelling suggest that propulsion rely hardly on cell body deformations and predominantly on molecular paddling by transmembrane proteins protruding outside the cell. For physics, this study reveals a new type of micro-swimmer, and for biology it suggests that leukocytes ubiquitous crawling may have evolved from an early machinery of swimming shared by various eukaryotic cells.

Recruitment of leukocytes from blood vessels to inflamed zones is guided by biochemical and mechanical stimuli, with mechanisms only partially deciphered. Here, we studied the guidance by flow of primary human effector T lymphocytes crawling on substrates coated with ligands of integrins LFA-1 (L2) and VLA-4 (41). We reveal that cells segregate in two populations of opposite orientation for combined adhesion, and show that decisions of orientation rely on a bistable mechanism between LFA-1-mediated upstream and VLA-4-mediated 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 towards VLA-4. At the cellular level, direction is determined by the passive, flow-mediated orientation of the non-adherent cell parts, the rear uropod for upstream migration and the front lamellipod for downstream migration. This chain of logical events provides a comprehensive mechanism of guiding, from stimuli to cell orientation. STATEMENT OF SIGNIFICANCECell 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 onto blood vessels, and show that the controlling parameter of cells decision to migrate upstream or downstream is the relative number of two specific adhesion molecules, the integrins LFA-1 and VLA-4.The spatial polarization of these integrins affinity plus a feedback loop between them creates a bistable system, where cells adhere either by their front or their tail to orient upstream or downstream, respectively. This mechanism proposes a complete chain of events from stimuli to cell orientation which differs strongly from the chemotaxis paradigm, because the external stimuli triggers no signaling.