Abstract:

Phagocytosis is the mechanism of internalization and degradation of large particles, such as microorganisms and cell debris. This process is crucial for host defense against invading pathogens, as well as for tissue remodeling. Phagocytosis starts with the recognition of a particle by phagocytic receptors, which trigger signaling cascades resulting in actin polymerization that drives the extension of the plasma membrane around the target. It ultimately seals to form a closed compartment called the phagosome. The phagosome then undergoes a process called maturation, which leads to the degradation of the cargo.

Macrophages are professional phagocytic cells that are present in virtually all tissues of the body. Tissues are complex environments, where the extracellular matrix provides a wide variety of both chemical and mechanical cues that affect macrophage behavior. Mechanical cues, in particular, are altered in many disease conditions. In addition, human rhinoviruses (HRVs) are known to impair macrophage functions in chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD). Thus, we asked if phagocytosis by macrophages can be modulated by mechanical properties of the microenvironment and by HRV16.

We found that cell adhesion and the ability of primary human macrophages to perform phagocytosis varied with the stiffness of their substrate. To understand if phagocytosis influences how phagocytic cells interact with their environment as well, we used live traction force microscopy and showed that phagocytosing macrophages have more dynamic interactions with their substrate. Furthermore, phagocytosis triggered a transient loss of podosomes concomitantly with actin polymerization at the phagocytic cup, and this was associated with decreased degradation of the extracellular matrix. Overall, these results highlight the existence of a crosstalk between phagocytosis and cell adhesion in macrophages. Mechanical properties of the microenvironment influence phagocytosis, which, in turn, impacts how macrophages interact with their immediate surroundings.

Furthermore, we showed that HRV16 impairs phagosome maturation in human macrophages. Small GTPase ARL5b was found to be up-regulated by the virus and depletion of this protein restored normal phagosome maturation in macrophages upon HRV16 exposure. To better understand how HRV16 might affect macrophage functions, we then studied whether HRV16 could actually enter and infect these cells. We showed that HRV16 was internalized by human macrophages through receptor-mediated uptake but that it cannot efficiently replicate inside these cells. Macrophages also mounted an antiviral response to the virus.

Taken together, these results shed new light on how phagocytosis by human macrophages is uniquely affected by mechanical cues from the microenvironment and by human rhinoviruses.