Macrophages are Sensitive to Substrate Elasticity during Phagocytosis

Abstract

Phagocytosis, the internalization of objects like living bacteria and dead cells by macrophages is a main function of the innate immune system. After the detection of foreign objects by membrane receptors, this process is driven by the reorganization of the actin cytoskeleton, which leads to a protrusion of the membrane around the target. Although many molecular players have been identified in the past, there is still little known about the role of mechanics during this process in general and about the role of the mechanical cellular environment in particular.In this work we investigate the influence of the underlying substrate rigidity on the phagocytic uptake efficiency and the uptake velocity. We cultured murine J774 macrophages on thin polymer gels with different stiffnesses in the physiological range. The uptake efficiency of antibody-coated microparticles was quantified with secondary antibody staining and the uptake speed was measured in live cell experiments by using optical tweezers. We found that the uptake efficiency as well as the uptake velocity depend on the rigidity of the substrate. Furthermore, we observed that cells were able to adapt to the various substrate stiffnesses over time. In addition to these cellular measurements, we also present a novel calibration technique that enables the simultaneous characterization of the elastic modulus and the Poisson's ratio of thin gel layers. The technique requires only a standard epifluorescence microscope and spherical indenters.Our results on the phagocytic uptake velocity and efficiency support the hypothesis that phagocytosis is a mechanosensitive process. Our findings might contribute to an understanding of the complex interplay between the immune system and disease states that come along with changes in tissue rigidity like cancer and atherosclerosis.