Traction force microscopy (TFM) is a popular technique for studying cellular stresses; however, the reproducible fabrication of ultrasoft substrates for the reliable detec-tion of weak cellular stresses (below 100 Pa) remains a challenge. Here, we describe a simple in vitro TFM protocol using such ultrasoft protein-coated polyacrylamide gels and wide-field fluorescence microscopy. We complement the protocol with open-source and in-house scripts for data analysis for the easy quantification of trac-tion stresses, which is demonstrated here using peripheral blood mononuclear cells.

For more than a couple of decades now, "force" has been recognized as an important physical parameter that cells employ to adapt to their microenvironment. Whether it is externally applied, or internally generated, cells use force to modulate their various actions, from adhesion and migration to differentiation and immune function. T lymphocytes use such mechano-sensitivity to decipher signals when recognizing cognate antigens presented on the surface of antigen presenting cells (APCs), a critical process in the adaptive immune response. As such, many techniques have been developed and used to measure the forces felt/exerted by these small, solitary and extremely reactive cells to decipher their influence on diverse T cell functions, primarily activation. Here, we focus on traction force microscopy (TFM), in which a deformable substrate, coated with the appropriate molecules, acts as a force sensor on the cellular scale. This technique has recently become a center of interest for many groups in the "ImmunoBiophysics" community and, as a consequence, has been subjected to refinements for its application to immune cells. Here, we present an overview of TFM, the precautions and pitfalls, and the most recent developments in the context of T cell immunology.

The fate of the adult human body, in terms of tissue development and homeostasis, is governed by how well its cells interact with one another, and with their environment.While the biochemical aspect of such interactions has been extensively studied for decades, their mechanical features, have only more recently captured the attention of cell biologists.Such an over sight becomes particularly notable when studying immune cells that experience different mechanical milieus during their life cyclesfrom primary/secondary/tertiary lymphoid organs and peripheral tissues displaying variable substrate rigidities, to the blood and lymphatic circulatory systems presenting complex hydrodynamic forces-and that are capable of exerting a substantial amount of force against their interacting surfaces.Indeed, mechanical cues, both dynamic forces and spatial features, have been shown to regulate the development, activation, differentiation and expansion of immune cells.T cells in specific, however, depict a unique paradigm of mechano-immunomodulation as the T cell receptor (TCR) itself has been shown to both sense and convert forces into biochemical signals, as well as induce force exertion following triggering.Consequently, it is only reasonable to imagine that incorporating mechanical cues into our "classical" view of T cell biology will help us better understand and manipulate their behavior, and more importantly, address the still unresolved mystery of their activation.In this chapter, we will review the existing body of knowledge showcasing the influence www.videleaf.com of mechanical forces on certain T cell surface and cytoplasmic proteins, the process of force generation during T cell interactions, how these forces come into play in T cell biology, and finally the ability of T cells to sense and respond to substrate stiffness and ligand mobility.

T cells use forces to read out and act on the mechanical parameters of their microenvironment, which includes antigen presenting cells (APCs). Here we explore the early interaction of T cells with an APC-mimicking ultra-soft polymer gel exhibiting physiologically relevant stiffness in the range of 350-450 Pa. We quantify the dependence of cell spreading and stiffness on gel elasticity, and measure early time traction forces. We find that coating the surface with an antibody against the CD3 region of the TCR-complex elicits small but measurable gel deformation in the early recognition phase, which we quantify in terms of stress or energy. We show that the time evolution of the energy follows one of three distinct patterns: active fluctuation, intermittent, or sigmoidal signal. Addition of either anti-CD28 or anti-LFA1 has little impact on the total integrated energy or the maximum stress. However, the relative distribution of the energy patterns does depend on the additional ligands. Remarkably, the forces are centrifugal at very early times, and only later turn into classical in-ward pointing centripetal traction. Graphical abstract

To accomplish their critical task of removing infected cells and fighting pathogens, leukocytes activate by forming specialized interfaces with other cells. The physics of this key immunological process are poorly understood, but it is important to understand them because leukocytes have been shown to react to their mechanical environment. Using an innovative micropipette rheometer, we show in three different types of leukocytes that, when stimulated by microbeads mimicking target cells, leukocytes become up to 10 times stiffer and more viscous. These mechanical changes start within seconds after contact and evolve rapidly over minutes. Remarkably, leukocyte elastic and viscous properties evolve in parallel, preserving a well-defined ratio that constitutes a mechanical signature specific to each cell type. Our results indicate that simultaneously tracking both elastic and viscous properties during an active cell process provides a new, to our knowledge, way to investigate cell mechanical processes. Our findings also suggest that dynamic immunomechanical measurements can help discriminate between leukocyte subtypes during activation.

The key function of T lymphocytes during an immune response is to scan the surface of surrounding cells and detect, via the membrane T cell receptor (TCR), the presence of foreign peptide antigens on antigen presenting cells (APC) among the many self-peptides presented by the Major Histocompatibility Complexes (MHC). A TCRpeptide-MHC (pMHC) interaction is required for the activation of T-cells and subsequent actions, such as proliferation, which is the essence of the adaptive immune response. In addition, TCR-pMHC interactions constantly provide "survival signals" in order to maintain a steady population of memory cells, which constitute our long-term immunity. [1]