Poster V12 - Imaging Phagocytic Force Over Surface-bound Particles by Macrophages

Introduction: In metazoan life, surface-bound pathogens and cell debris are ubiquitous, necessitating the efficient clearance by phagocytes. Macrophages, located throughout tissues, serve as the frontline defense by internalizing these particles through phagocytosis. However, it is not well understood how macrophages overcome the attachment of surface-bound particles to tissue surfaces. We conducted a comprehensive investigation to elucidate the mechanism specialized for the phagocytosis of surface-bound particles.

Materials and

Methods: We employed a wide array of experimental tools and techniques including (i) cellular force sensors and force modulators to probe the forces exerted by macrophages during the process of particle detachment from the substrate, (ii) surface functionalization to control particle attachment strength to substrates, enabling the investigation of phagocytic mechanisms under varying adhesion conditions, and (iii) immunostaining combined with advanced imaging modalities for the visualization of molecular forces and protein dynamics at the cell-substrate interface. These approaches were applied to study various macrophages originating from fish, mouse and human.

Results, Conclusions, and Discussions: We unveiled a novel mechanism of the phagocytosis of surface-bound particles by macrophages (Fig. 1A). We showed that instead of directly binding to the particles, macrophages assemble unique cellular structures termed phagocytic adhesion rings (PARs) on the substrate, specifically encircling these particles (Fig. 1A and Fig. 1B). These PARs contain integrins and exhibit similar protein compositions to other integrin-mediated adhesion structures, such as focal adhesions. By calibrating the integrin-transmitted forces within PARs at the cell-substrate interface, we showed that these single integrin forces are within the range of 10-60 pN. These forces are crucial for stabilizing ring-shaped actin structures on PARs (Fig. 1C), which constrict into the particle-substrate cleavages and lift off the surface-bound particles, thereby overcoming the attachment of particles to the substrates (Fig. 1D and Fig. 1E).
In summary, this study has discovered a unique cellular adhesion structure, the PAR, and revealed a novel mechanism of phagocytosis employed by macrophages to dislodge particles from surfaces. These discoveries have deepened our understanding of macrophage-mediated clearance of surface-bound particles, providing new perspective for the development of therapeutic strategies by modulating phagocytic pathways to clear surface-bound pathogens.

Acknowledgements (Optional): This research was supported by National Institute of General Medical Sciences (1R35GM128747).