Department of Physics, University of Bayreuth, Bayreuth, Germany.
Department of Physics, University of Bayreuth, Bayreuth, Germany.
Biophys J. 2021 Aug 17;120(16):3527-3537. doi: 10.1016/j.bpj.2021.04.034. Epub 2021 Jun 26.
Phagocytosis is an important part of innate immunity and describes the engulfment of bacteria and other extracellular objects on the micrometer scale. The protrusion of the cell membrane around the bacteria during this process is driven by a reorganization of the actin cortex. The process has been studied on the molecular level to great extent during the past decades. However, a deep, fundamental understanding of the mechanics of the process is still lacking, in particular because of a lack of techniques that give access to binding dynamics below the optical resolution limit and cellular viscoelasticity at the same time. In this work, we propose a technique to characterize the mechanical properties of cells in a highly localized manner and apply it to investigate the early stages of phagocytosis. The technique can simultaneously resolve the contact region between a cell and an external object (in our application, a phagocytic target) even below the optical resolution limit. We used immunoglobulin-G-coated microparticles with a size of 2 μm as a model system and attached the particles to the macrophages with holographic optical tweezers. By switching the trap on and off, we were able to measure the rheological properties of the cells in a time-resolved manner during the first few minutes after attachment. The measured viscoelastic cellular response is consistent with power law rheology. The contact radius between particle and cell increased on a timescale of ∼30 s and converged after a few minutes. Although the binding dynamics are not affected by cytochalasin D, we observed an increase of the cellular compliance and a significant fluidization of the cortex after addition of cytochalasin D treatment. Furthermore, we report upper boundaries for the length- and timescale, at which cortical actin has been hypothesized to depolymerize during early phagocytosis.
吞噬作用是先天免疫的重要组成部分，描述了细菌和其他细胞外物体在微米尺度上的吞噬作用。在这个过程中，细胞膜在细菌周围的突起是由肌动蛋白皮层的重排驱动的。在过去的几十年里，这个过程在分子水平上已经被广泛研究。然而，对这个过程的力学的深入、基本的理解仍然缺乏，特别是因为缺乏同时能够获得低于光学分辨率限制的结合动力学和细胞粘弹性的技术。在这项工作中，我们提出了一种技术，可以高度局部地表征细胞的力学性质，并将其应用于研究吞噬作用的早期阶段。该技术甚至可以在低于光学分辨率限制的情况下，同时分辨细胞与外部物体（在我们的应用中，是吞噬目标）之间的接触区域。我们使用大小为 2 微米的免疫球蛋白 G 包被的微球作为模型系统，并使用全息光镊将颗粒附着到巨噬细胞上。通过打开和关闭陷阱，我们能够在附着后的最初几分钟内以时间分辨的方式测量细胞的流变学性质。测量得到的粘弹性细胞响应与幂律流变学一致。颗粒和细胞之间的接触半径在约 30 秒的时间尺度上增加，并在几分钟后收敛。尽管结合动力学不受细胞松弛素 D 的影响，但我们观察到细胞顺应性增加，并且在用细胞松弛素 D 处理后皮层明显流化。此外，我们报告了皮层肌动蛋白在早期吞噬作用中假设解聚的长度和时间尺度的上限。
