As part of the innate immune system of vertebrates, macrophages and neutrophils migrate through interstitial tissues to monitor the environment, or directionally in response to tissue injury or infection. Migration occurs as amoeboid motility, which is fast and mainly adhesion independent, or as the slower mesenchymal type, with an establishment of strong adhesion and degradation of the extracellular matrix. Whereas neutrophils are thought to move as amoeboid cells and macrophages have the capacity to switch their migration mode in vitro, the behaviour of these cells in interstitial migration in vivo is unknown. Now, Anna Huttenlocher and co-workers (p. 3801) image neutrophil and macrophage migration in live zebrafish larvae. The authors find that the motility of these two cell types is different and that macrophages, indeed, migrate in a mesenchymal-like mode in interstitial tissue. In order to migrate, macrophages require protease activity; they also display structures that contain paxillin, the scaffold protein for focal adhesions. Moreover, neutrophils and macrophages have a different polarisation of F-actin dynamics but Rho-associated kinase signalling is important for migration in vivo of both cell types. Taken together, this study describes distinct in vivo mechanisms of interstitial tissue motility for neutrophils and macrophages in zebrafish.
