Extracellular proteolysis in macrophage migration: losing grip for a breakthrough

Macrophage tissue infiltration is a hallmark of several pathological situations including cancer, neurodegenerative disorders and chronic inflammation. Hence, deciphering the mechanisms of macrophage migration across a variety of tissues holds great potential for novel anti-inflammatory therapies. Leukocytes have long been thought to migrate through tissues by using the amoeboid (protease-independent) migration mode; however, recent evidence indicates that macrophages can use either the amoeboid or the mesenchymal (protease-dependent) migration mode depending on the environmental constraints. Proteolytic activity is required for several key processes including cell migration. Paradoxically, the role of proteases in macrophage migration has been poorly studied. Here, by focusing on the best characterized extracellular protease families - MMPs, cathepsins and urokinase-type plasminogen activator - we give an overview of their probable involvement in macrophage migration. These proteases appear to play a role in all of the situations encountered by migrating macrophages, i.e. diapedesis, 2D and 3D migration. Migration of macrophages across tissues seems to proceed through an integrative analysis of numerous environmental clues allowing the cells to adapt their migration mode (amoeboid/mesenchymal) and secrete dedicated proteases to ensure efficient tissue infiltration, as discussed in this review. The role of proteases in macrophage migration is an emerging field of research, which deserves further work to allow a more precise understanding.