Editorial.

Mechanobiology is an emerging field at the crossroads between biology, physics, mechanics, bioengineering and materials science. It investigates how mechanics can influence cell function: how cells sense and respond to external mechanical properties and forces, and how they generate forces and adapt their mechanical properties to perform functions as varied as adhesion, migration, differentiation or immune response, to name a few. A growing interest for this field is establishing a corpus of evidence suggesting that potentially any cell, of any type, can sense mechanical inputs from its environment and adapt to them. A new horizon opens up for a more comprehensive description of biological processes that includes their mechanical component. Furthermore, because external mechanical cues are involved in many pathological contexts, understanding the interplay between mechanical inputs and cell response should bring new insights into many pathologies, including cancer, atherosclerosis or evasion from immune response. This themed issue on mechanobiology covers a variety of topics at the cellular and subcellular scale. Three contributions focus on immune cells. Since pioneering studies on the biophysics of leukocytes done decades ago, a growing corpus of knowledge has been accumulated on some myeloid cells such as neutrophils. However, surprising discoveries about these foot soldiers of innate immunity are yet to come, including the way they move to explore their environment. In this issue, Garcia-Seyda et al. (2021) lead the way by showing that neutrophils can swim to reach and phagocyte their target. Mechanics of myeloid cells other than neutrophils remain to be fully explored, and Bashant et al. (2020) review how mechanical properties of myeloid cells can be quantified using recently developed high-throughput deformability cytometry. The authors review how these mechanical properties can be influenced by several factors including: differentiation, priming by cytokines and other soluble molecules or mechanical stimulation, disease and pharmacological treatment. On another front of immunobiophysics, T cells attract a lot of attention given their central role in adaptive immunity and recent revolutions in cancer immunotherapy. T cells use a complex recognition machinery to identify presented antigens. This recognition is known to be mechanosensitive, but understanding the details of this process remains the focus of active work. Before forming a synapse, T cells need to arrest on an antigen-presenting cell (APC), which is yet another process where mechanics play a role. Chabaud et al. (2020) review how mechanical forces generated at the T cell–APC interface and beard by specific bonds between T cell receptors and antigens, and adhesive bonds, regulate the arrest of T cells. This themed issue goes also subcellular with the contribution of Allard et al. (2021), which describes how the shape of membrane tubules can be remodelled by the actin cytoskeleton. Finally, tackling another important aspect of mechanobiology that asks how cells react to themechanical properties of their environment, Cabriales et al. (2020) study how hepatic cells react to substrate stiffness and show that short-term response to soft substrate can be lost upon long (several week) treatment, calling for a more general investigation of the long-term stiffness sensing of cells.