Mechanobiology of Phagocytosis

Macrophages are immune cells that act as one of the first lines of defense against bacteria, viruses, cancer cells and other pathogens. Macrophages wrap themselves around a foreign particle, engulfing and digesting it. This process, called phagocytosis, involves both chemistry and physical forces. Though the process of phagocytosis has been well studied biochemically, this project takes a different perspective: it treats the macrophage as a machine and the phagocytic process as a mechanical event. What are the forces? How do they coordinate? How does the machine work? Without understanding forces, we lack a complete scientific description of phagocytosis and, more broadly, the immune response itself. The project employs both advanced microscopic techniques as well as unique force measurement methods to investigate the three-dimensional structure of the macrophage in real time while simultaneously measuring the forces it generates. The data generated from this project will provide a more comprehensive model of this fundamental biological process that integrates the biochemistry, the structure, and the biophysical forces. Another critical component of the overall project is bringing the research to the public to help inform folks on the latest exciting science that is happening in the lab. Broader impact projects include a virtual-reality-based exhibit that will be developed at a local museum, and a week-long biophysics 'mini-term' that will be taught at a local high school.

The overarching goal of this project is to use advanced imaging and force measurement tools to reveal the mechanisms of force generation and mechanosensing in phagocytosis. Phagocytosis is the process through which macrophages, neutrophils and other cells engulf large foreign pathogen particles as a first line of immune system defense. To engulf a target particle, one or more distinct actin-driven mechanical processes draws in the target, surrounds and encloses it. Along with biochemical signaling dynamics, phagocytosis is also driven by fundamentally mechanical processes involving forces and structural dynamics. The most fundamental questions are unanswered: What forces does a macrophage produce? What actin dynamics and associated cell morphological dynamics give rise to them? How do external forces and mechanical environment affect decision making in phagocytosis? Measurements directly associating macrophage-target forces with actin dynamics are a crucial missing piece to developing models of the engulfment process. Within this project, a set of hypotheses will be tested that are guided by analogous mechanisms that have been proposed for other cell systems such as the 'molecular clutch' mechanism in mesenchymal motility which provides force generation as well as mechanosensing. Measurements of macrophage force generation during engulfment will be performed along with accompanying high quality, live cell volumetric imaging of cell morphology and cytoskeletal dynamics. This will be complemented by use of soft beads as targets to measure local compressive and shear forces being imposed by the macrophage on the target. Quantitative spatio-temporal correlation analysis of volumetric actin dynamics and force measurements will also be performed. This will provide rich detailed data informing location and timing of the local dynamic actin remodeling that produce a specific macrophage force event. If successful, the project will produce a completely new dynamical atlas of the macrophage that maps the relationship of dynamic 3D cell morphology and actin dynamics with local force generation and mechanosensing.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.