Cells in vivo respond to chemical and mechanical cues in the environment. In fact, it is the resulting migration of cells as a cohesive group that underlies embryonic morphogenesis, wound repair and cancer tumour development and invasion. Techniques have been developed to investigate chemotaxis, haptotaxis and mechanotaxis – the directional movement of cells in response to soluble chemical cues, substrate-bound chemical cues and mechanical cues respectively. Most of the existing tools however, have been designed for and applied to the investigation of single cell migration. Given its importance in vivo, there is a need for adapting these methods and applying them to characterize directed collective cell migration. The main objective of my thesis was to engineer tools and quantitative methods to investigate collective cell migration and use them to compare single and collective migration in response to mechanical cues and substrate-adhered chemical cues in vitro.

Cells in vivo respond to chemical and mechanical cues in the environment. In fact, it is the resulting migration of cells as a cohesive group that underlies embryonic morphogenesis, wound repair and cancer tumour development and invasion. Techniques have been developed to investigate chemotaxis, haptotaxis and mechanotaxis – the directional movement of cells in response to soluble chemical cues, substrate-bound chemical cues and mechanical cues respectively. Most of the existing tools however, have been designed for and applied to the investigation of single cell migration. Given its importance in vivo, there is a need for adapting these methods and applying them to characterize directed collective cell migration. The main objective of my thesis was to engineer tools and quantitative methods to investigate collective cell migration and use them to compare single and collective migration in response to mechanical cues and substrate-adhered chemical cues in vitro.

La capacité des leucocytes à se déplacer dans tout l’organisme est indispensable pour une réponse immunitaire rapide et efficace. Leur migration, dite amiboïde, est caractérisée par une vitesse importante (10-20 μm/min) et une grande adaptabilité face aux divers environnements qu’ils rencontrent, qu’ils soient bidimensionnels comme la paroi luminale endothéliale ou tridimensionnels (3D) comme les tissus. Telle qu'actuellement décrite, la migration amiboïde requiert de l’adhésion ou de la friction avec un support solide. Nous avons ici montré que les lymphocytes T effecteurs sont capables de nager sans interaction avec un support solide. Le mécanisme de propulsion est basé sur le flux rétrograde d’actine qui entraine une brosse protéique de molécules transmembranaires liées au cytosquelette entrant en interaction avec le medium. Par ailleurs, lors de leur migration sur la surface luminale des parois endothéliales, les leucocytes sont soumis à un flux important et s’orientent par rapport au flux via des mécanismes mal déterminés. Nous avons montré que l’orientation des lymphocytes et des neutrophiles respectivement dans le sens ou à contresens d’un flux peut s’expliquer sans détection moléculaire du stress hydrodynamique. Le lamellipode pour les neutrophiles et l’uropode pour les lymphocytes est non-adhérent et s’oriente dans le flux comme une girouette dans le vent. La polarisation avant-arrière réaligne l’ensemble de la cellule dans le même sens que l’extrémité orientée par le flux. Le mécanotactisme des leucocytes sous flux repose ainsi sur des mécanismes passifs, c’est-à-dire sans mécanotransduction. / A fast and efficient immunity response needs leukocytes’ability to migrate within the entire organism. Their migration, called amoeboid, is characterized by a high speed (10-20 μm.min-1) and a great adaptability to move through various environment, either two-dimensional as luminal endothelial surface or tri-dimensional (3D) environment as tissue. Since the observation of leukocytes migrating without adhesion through solid 3D medium, amoeboid migration is described as requiring either adhesion or friction with solid support to permit motility. We showed here that effector T lymphocytes are able to swim without any interaction with solid substrate. Propulsion is based on actin retrograde flow coupled with transmembrane proteins linked to cytoskeleton (like integrins) which drag a brush of polymeric molecules in interaction with the medium. Furthermore, cell guidance is required for many crucial functions as organism growth or immune system. However, when crawling on luminal endothelial surfaces, cells are exposed to blood flow and they robustly orient either with or against the flow with unknown mechanisms. We showed that lymphocytes and neutrophils flow orientation can be explain without any molecular flow sensor of shear stress. Lamellipodium for neutrophils and uropod for lymphocytes is non-adherent and orients in the direction of flow like a wind vane. Front-rear cell polarization aligns the axis of the whole cell with the non-adherent pole oriented by flow. Flow mechanotaxis of leukocytes relies on passive mechanisms without mechanotransduction.