Neutrophils migrating through extravascular spaces must negotiate narrow matrix pores without losing directional movement. We investigated how chemotaxing neutrophils probe matrices and adjust their migration to collagen concentration ([col]) changes by tracking 20,000 cell trajectories and quantifying cell-generated 3D matrix deformations. In low-[col] matrices, neutrophils exerted large deformations and followed straight trajectories. As [col] increased, matrix deformations decreased, and neutrophils turned often to circumvent rather than remodel matrix pores. Inhibiting protrusive or contractile forces shifted this transition to lower [col], implying that mechanics play a crucial role in defining migratory strategies. To balance frequent turning and directional bias, neutrophils used matrix obstacles as pivoting points to steer toward the chemoattractant. The Actin Related Protein 2/3 complex coordinated successive turns, thus controlling deviations from chemotactic paths. These results offer an improved understanding of the mechanisms and molecular regulators used by neutrophils during chemotaxis in restrictive 3D environments.

The eggs of the parasitic blood fluke, Schistosoma, are the main drivers of the chronic pathologies associated with schistosomiasis, a disease of poverty afflicting approximately 220 million people worldwide. Eggs laid by Schistosoma mansoni in the bloodstream of the host are encapsulated by vascular endothelial cells (VECs), the first step in the migration of the egg from the blood stream into the lumen of the gut and eventual exit from the body. The biomechanics associated with encapsulation and extravasation of the egg are poorly understood. We demonstrate that S. mansoni eggs induce VECs to form two types of membrane extensions during encapsulation; filopodia that probe eggshell surfaces and intercellular nanotubes that presumably facilitate VEC communication. Encapsulation efficiency, the number of filopodia and intercellular nanotubes, and the length of these structures depend on the egg’s vitality and, to a lesser degree, its maturation state. During encapsulation, live eggs induce VEC contractility and membranous structures formation in a Rho/ROCK pathway-dependent manner. Using elastic hydrogels embedded with fluorescent microbeads as substrates to culture VECs, live eggs induce VECs to exert significantly greater contractile forces during encapsulation than dead eggs, which leads to 3D deformations on both the VEC monolayer and the flexible substrate underneath. These significant mechanical deformations cause the VEC monolayer tension to fluctuate with the eventual rupture of VEC junctions, thus facilitating egg transit out of the blood vessel. Overall, our data on the mechanical interplay between host VECs and the schistosome egg improve our understanding of how this parasite manipulates its immediate environment to maintain disease transmission.

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Three-dimensional (3-D) neutrophil migration is essential for immune surveillance and inflammatory responses. During 3-D migration, especially through extravascular spaces, neutrophils rely on frontal protrusions and rear contractions to squeeze and maneuver through extracellular matrices containing narrow pores. However, the role of matrix density and the cells’ ability to probe and remodel matrix pores during 3-D chemotaxis are far from being understood. We investigated these processes by tracking the trajectories of over 20,000 neutrophils in a 3-D migration device containing collagen matrices of varying concentrations and analyzing the shape of these trajectories at multiple scales. Additionally, we quantified the transient 3-D matrix deformations caused by the migrating cells. The mean pore size of our reconstituted collagen matrices decreased when the collagen concentration ([col]) was increased. In low-[col] matrices, neutrophils exerted large transient deformations and migrated in relatively straight trajectories. In contrast, they were not able to appreciably deform high- [col] matrices and adapted to this inability by turning more often to circumvent these narrow matrix pores. While this adaptation resulted in slower migration, the cells were able to balance the more frequent turning with the long-range directional bias necessary for chemotaxis. Based on our statistical analysis of cell trajectories, we postulate that neutrophils achieve this balance by using matrix obstacles as pivoting points to steer their motion towards the chemoattractant. Inhibiting myosin-II contractility or Arp2/3-mediated pseudopod protrusions not only compromised the cells’ ability to deform the matrix, but also made them switch to increased turning in more restrictive matrices when compared to untreated control cells. Both myosin-II contractility and Arp2/3-mediated branched polymerization of actin played a role in fast migration, but Arp2/3 was also crucial for neutrophils when coordinating the orientations of successive turns to prevent veering away from the chemotactic path. These results may contribute to an improved understanding of the mechanisms employed by migrating neutrophils in confined 3-D environments, as well as the molecular and environmental regulators for maintaining persistent motion.