Coupled mechanical mapping and interference contrast microscopy reveal viscoelastic and adhesion hallmarks of monocytes differentiation into macrophages

Monocytes in the blood torrent, when activated by pro-inflammatory signals, adhere to the vascular endothelium and migrate into the tissue for ultimately differentiate into macrophages. Mechanics and adhesion play a crucial role in macrophage functions, such as migration and phagocytosis. However, how monocytes change their adhesion and mechanical properties upon differentiation into macrophages is still not well understood. In this work, we combined atomic force microscopy (AFM) viscoelastic mapping with interference contrast microscopy (ICM) to simultaneously probe, at the single-cell level, viscoelasticity and adhesion during monocyte differentiation. THP-1 monocytic cells were differentiated into macrophages through phorbol 12-myristate 13-acetate (PMA). Morphological quantification was achieved using holographic tomography imaging and the expression of integrin subunit CD11b was tracked as a marker of differentiation. Holographic tomography proved to be a quantitative in vivo technique, revealing a dramatic increase in macrophage volume and surface area and two subpopulations, spread and round macrophages. AFM viscoelastic mapping revealed an increased stiffness and more solid-like behavior of differentiated macrophages, especially in the lamellipodia and microvilli regions. Differentiated cells revealed an important increase of the apparent Youngs modulus (E0) and a decrease of cell fluidity ({beta}) on differentiated cells, which correlated with an increase in adhesion area. Macrophages with a spreading phenotype enhanced these changes. Remarkably, when adhesion was eliminated, differentiated macrophages remained stiffer and more solid-like than monocytes, suggesting a permanent reorganization of the cytoskeleton. We speculate that the more solid-like microvilli and lamellipodia might help macrophages to minimize energy dissipation during mechanosensitive activity, such as phagocytosis, making it more efficient. Our proposed approach revealed viscoelastic and adhesion hallmarks of monocyte differentiation that may be important for biological function.