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Whether one considers swarming insects, flocking birds, or bacterial colonies, collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. In particular, in close confinements, such as those encountered by dense cell populations during development or regeneration, collective migration can only arise coordinately. Yet, how individuals unify their velocities is often not understood. Focusing on a finite number of cells in circular confinements, we identify waves of polymerizing actin that function as a pacemaker governing the speed of individual cells. We show that the onset of collective motion coincides with the synchronization of the wave nucleation frequencies across the population. Employing a simpler and more readily accessible mechanical model system of active spheres, we identify the synchronization of the individuals’ internal oscillators as one of the essential requirements to reach the corresponding collective state. The mechanical ‘toy’ experiment illustrates that the global synchronous state is achieved by nearest neighbor coupling. We suggest by analogy that local coupling and the synchronization of actin waves are essential for the emergent, self-organized motion of cell collectives. Collective motion arises from the coordination of individuals and entails the adjustment of their respective velocities. Yet, how individuals achieve this coordination is often not understood. For migrating cells and motorized agents, Riedl et al. show that the synchronization of the intrinsic oscillator through nearest neighbour coupling establishes the necessary feedback leading to a uniform speed within the collective.

Key Points The shuttling of leukocytes between the blood stream and interstitial tissues involves different locomotion strategies that are governed by locally presented soluble and cell-bound signals. There are key concepts in the regulation of leukocyte migration through venular walls and motility in the extravascular tissue, with common and distinct mechanisms mediating these responses. Integrin-mediated adhesion of leukocytes to endothelial cells lining venular walls is a prerequisite to leukocyte crawling over and migration through endothelial cells. These responses are associated with great morphological changes in both leukocytes and endothelial cells and can support leukocyte transendothelial cell migration through both paracellular and transcellular routes. Leukocyte migration through endothelial cells is dependent on signalling events in both leukocytes and endothelial cells, events that can regulate leukocyte–endothelial cell interactions, as well as contacts between adjacent endothelial cells and/or endothelial cell vesicular trafficking. After endothelial cell migration, leukocytes need to penetrate the pericyte sheath and the venular basement membrane in which pericytes are embedded. Breaching the pericyte layer may occur through gaps between adjacent cells or in a transcellular manner. Migration through the venular basement membrane occurs through regions that may be biochemically or biophysically permissive. Once detached from the perivascular basement membrane, leukocytes approach their final destination by crawling within the three-dimensional interstitial space, which can either be a fibrillar network or a cell-packed environment like many organ parenchymas or lymphatic tissues. Leukocyte migration in the interstitium is driven by actin protrusion at the leading edge and is occasionally supported by actomyosin contraction at the trailing edge. The cytoskeletal forces can be transduced onto the environment either by integrins or by direct physical interaction of the cell body with the extracellular environment. This flexible mode of migration renders leukocytes largely independent of the molecular composition of the interstitium. Leukocytes use different strategies to migrate through the endothelium of venular walls and in interstitial tissues. These strategies are regulated by soluble and cell-bound signals. Studies have identified many of the cellular and subcellular events that govern transendothelial migration and are beginning to elucidate the nature of leukocyte interstitial motility. The shuttling of leukocytes between the bloodstream and interstitial tissues involves different locomotion strategies that are governed by locally presented soluble and cell-bound signals. Recent studies have furthered our understanding of the rapidly advancing field of leukocyte migration, particularly regarding cellular and subcellular events at the level of the venular wall. Furthermore, emerging cellular models are now addressing the transition from an adherent mode to a non-adherent state, incorporating mechanisms that support an efficient migratory profile of leukocytes in the interstitial tissue beyond the venular wall.

During metazoan development, immune surveillance and cancer dissemination, cells migrate in complex three-dimensional microenvironments 1 – 3 . These spaces are crowded by cells and extracellular matrix, generating mazes with differently sized gaps that are typically smaller than the diameter of the migrating cell 4 , 5 . Most mesenchymal and epithelial cells and some—but not all—cancer cells actively generate their migratory path using pericellular tissue proteolysis 6 . By contrast, amoeboid cells such as leukocytes use non-destructive strategies of locomotion 7 , raising the question how these extremely fast cells navigate through dense tissues. Here we reveal that leukocytes sample their immediate vicinity for large pore sizes, and are thereby able to choose the path of least resistance. This allows them to circumnavigate local obstacles while effectively following global directional cues such as chemotactic gradients. Pore-size discrimination is facilitated by frontward positioning of the nucleus, which enables the cells to use their bulkiest compartment as a mechanical gauge. Once the nucleus and the closely associated microtubule organizing centre pass the largest pore, cytoplasmic protrusions still lingering in smaller pores are retracted. These retractions are coordinated by dynamic microtubules; when microtubules are disrupted, migrating cells lose coherence and frequently fragment into migratory cytoplasmic pieces. As nuclear positioning in front of the microtubule organizing centre is a typical feature of amoeboid migration, our findings link the fundamental organization of cellular polarity to the strategy of locomotion. Geometrically defined microenvironments are used to show that leukocytes migrate along chemokine gradients using the nucleus as a mechanical gauge to sample potential paths and identify the path of least resistance.

For innate and adaptive immune responses it is essential that inflammatory cells use quick and flexible locomotion strategies. Accordingly, most leukocytes can efficiently infiltrate and traverse almost every physiological or artificial environment. Here, we review how leukocytes might achieve this task mechanistically, and summarize recent findings on the principles of cytoskeletal force generation and transduction at the leading edge of leukocytes. We propose a model in which the cells switch between adhesion‐receptor‐mediated force transmission and locomotion modes that are based on cellular deformations, but independent of adhesion receptors. This plasticity in migration strategies allows leukocytes to adapt to the geometry and molecular composition of their environment. How do leukocytes efficiently infiltrate and traverse almost every physiological or artificial environment? The authors propose a model where the cells flexibly switch between adhesion receptor mediated force transmission and locomotion modes based on cellular deformations but independent of adhesion receptors.

We compare anti-parasite defences at the level of multicellular organisms and insect societies, and find that selection by parasites at these two organisational levels is often very similar and has created a number of parallel evolutionary solutions in the host's immune response. The defence mechanisms of both individuals and insect colonies start with border defences to prevent parasite intake and are followed by soma defences that prevent the establishment and spread of the parasite between the body's cells or the social insect workers. Lastly, germ line defences are employed to inhibit infection of the reproductive tissue of organisms or the reproductive individuals in colonies. We further find sophisticated self/non-self-recognition systems operating at both levels, which appear to be vital in maintaining the integrity of the body or colony as a reproductive entity. We then expand on the regulation of immune responses and end with a contemplation of how evolution may shape the different immune components, both within and between levels. The aim of this review is to highlight common evolutionary principles acting in disease defence at the level of both individual organisms and societies, thereby linking the fields of physiological and ecological immunology.

Directional guidance of cells via gradients of chemokines is considered crucial for embryonic development, cancer dissemination, and immune responses. Nevertheless, the concept still lacks direct experimental confirmation in vivo. Here, we identify endogenous gradients of the chemokine CCL21 within mouse skin and show that they guide dendritic cells toward lymphatic vessels. Quantitative imaging reveals depots of CCL21 within lymphatic endothelial cells and steeply decaying gradients within the perilymphatic interstitium. These gradients match the migratory patterns of the dendritic cells, which directionally approach vessels from a distance of up to 90-micrometers. Interstitial CCL21 is immobilized to heparan sulfates, and its experimental delocalization or swamping the endogenous gradients abolishes directed migration. These findings functionally establish the concept of haptotaxis, directed migration along immobilized gradients, in tissues.

Gradients of chemokines and growth factors guide migrating cells and morphogenetic processes. Migration of antigen-presenting dendritic cells from the interstitium into the lymphatic system is dependent on chemokine CCL21, which is secreted by endothelial cells of the lymphatic capillary, binds heparan sulfates and forms gradients decaying into the interstitium. Despite the importance of CCL21 gradients, and chemokine gradients in general, the mechanisms of gradient formation are unclear. Studies on fibroblast growth factors have shown that limited diffusion is crucial for gradient formation. Here, we used the mouse dermis as a model tissue to address the necessity of CCL21 anchoring to lymphatic capillary heparan sulfates in the formation of interstitial CCL21 gradients. Surprisingly, the absence of lymphatic endothelial heparan sulfates resulted only in a modest decrease of CCL21 levels at the lymphatic capillaries and did neither affect interstitial CCL21 gradient shape nor dendritic cell migration toward lymphatic capillaries. Thus, heparan sulfates at the level of the lymphatic endothelium are dispensable for the formation of a functional CCL21 gradient.

Current approaches for live imaging of cellular actin dynamics have several drawbacks. Now the use of Lifeact, a 17-aa actin-binding peptide from yeast that is not present in higher eukaryotes, allows imaging of actin dynamics in live mammalian cells without disruption of function and without competition with endogenous binding proteins. Live imaging of the actin cytoskeleton is crucial for the study of many fundamental biological processes, but current approaches to visualize actin have several limitations. Here we describe Lifeact, a 17-amino-acid peptide, which stained filamentous actin (F-actin) structures in eukaryotic cells and tissues. Lifeact did not interfere with actin dynamics in vitro and in vivo and in its chemically modified peptide form allowed visualization of actin dynamics in nontransfectable cells.

To navigate through diverse tissues, migrating cells must balance persistent self-propelled motion with adaptive behaviors to circumvent obstacles. We identify a curvature-sensing mechanism underlying obstacle evasion in immune-like cells. Specifically, we propose that actin polymerization at the advancing edge of migrating cells is inhibited by the curvature-sensitive BAR domain protein Snx33 in regions with inward plasma membrane curvature. The genetic perturbation of this machinery reduces the cells’ capacity to evade obstructions combined with faster and more persistent cell migration in obstacle-free environments. Our results show how cells can read out their surface topography and utilize actin and plasma membrane biophysics to interpret their environment, allowing them to adaptively decide if they should move ahead or turn away. On the basis of our findings, we propose that the natural diversity of BAR domain proteins may allow cells to tune their curvature sensing machinery to match the shape characteristics in their environment. Motile cells must navigate complex environments. Here the authors use state-of-the-art imaging, coarse-grained MD simulations and experimental biophysics to show that cells sense their plasma membrane curvature to circumvent obstacles.

Cell migration is hypothesized to involve a cycle of behaviours beginning with leading edge extension. However, recent evidence suggests that the leading edge may be dispensable for migration, raising the question of what actually controls cell directionality. Here, we exploit the embryonic migration of Drosophila macrophages to bridge the different temporal scales of the behaviours controlling motility. This approach reveals that edge fluctuations during random motility are not persistent and are weakly correlated with motion. In contrast, flow of the actin network behind the leading edge is highly persistent. Quantification of actin flow structure during migration reveals a stable organization and asymmetry in the cell-wide flowfield that strongly correlates with cell directionality. This organization is regulated by a gradient of actin network compression and destruction, which is controlled by myosin contraction and cofilin-mediated disassembly. It is this stable actin-flow polarity, which integrates rapid fluctuations of the leading edge, that controls inherent cellular persistence. Yolland et al. demonstrate persistent flow of the actin flow behind the leading edge and its impact on cell directionality during migration.