Abstract
T cells are major players of adaptive immune response in mammals. Recognition of an antigenic peptide in association with the major histocompatibility complex at the surface of an antigen presenting cell (APC) is a specific and sensitive process whose mechanism is not fully understood. The potential contribution of mechanical forces in the T cell activation process is increasingly debated, although these forces are scarcely defined and hold only limited experimental evidence. In this work, we have implemented a biomembrane force probe (BFP) setup and a model APC to explore the nature and the characteristics of the mechanical forces potentially generated upon engagement of the T cell receptor (TCR) and/or lymphocyte function-associated antigen-1 (LFA-1). We show that upon contact with a model APC coated with antibodies towards TCR-CD3, after a short latency, the T cell developed a timed sequence of pushing and pulling forces against its target. These processes were defined by their initial constant growth velocity and loading rate (force increase per unit of time). LFA-1 engagement together with TCR-CD3 reduced the growing speed during the pushing phase without triggering the same mechanical behavior when engaged alone. Intracellular Ca2+ concentration ([Ca2+]i) was monitored simultaneously to verify the cell commitment in the activation process. [Ca2+]i increased a few tens of seconds after the beginning of the pushing phase although no strong correlation appeared between the two events. The pushing phase was driven by actin polymerization. Tuning the BFP mechanical properties, we could show that the loading rate during the pulling phase increased with the target stiffness. This indicated that a mechanosensing mechanism is implemented in the early steps of the activation process. We provide here the first quantified description of force generation sequence upon local bidimensional engagement of TCR-CD3 and discuss its potential role in a T cell mechanically-regulated activation process.
Highlights
T cell activation is a crucial event in the development of adaptive immune response to pathogens or tumor cells
T cell mechanical response to T cell receptor (TCR)-CD3 engagement In order to substantiate the idea of a defined T cell mechanical behavior associated with activation triggering, we brought into contact a primary CD4+ lymphocyte with a simplified model of an antigen presenting cell (APC) by using an adapted version of the biomembrane force probe (BFP)
In order to validate the potential of activation of the beads used in this study, we activated the CD4+ primary T cells with these beads and checked the phosphorylation of signaling molecules known to be involved in the TCR/CD3 signaling cascades
Summary
T cell activation is a crucial event in the development of adaptive immune response to pathogens or tumor cells. Activation is triggered as a T cell encounters an antigenic peptide associated with the major histocompatibility complex at the surface of an antigen presenting cell (APC) This encounter is followed by the formation of a dynamic contact zone called immunological synapse (IS). T cell activation triggering is a highly sensitive and specific process involving several pairs of ligands and receptors in addition to the central TCR-antigen engagement Among these molecules, the integrin lymphocyte function-associated antigen-1 (LFA-1) plays a crucial role, as it controls T cell adhesion to APC and formation of long lasting contacts [1,2]. The idea began to form that a link might be missing in the comprehensive vision of the process, due to almost complete oversight of its mechanical aspects This hypothesis was first evoked as a possible working hypothesis to reconcile binding data with activation profiles [5,6]. Up to now the sources of mechanical forces have only been hypothesized, supposedly originating from membrane tension due to bidimensional alignment of proteins of different size in the cell-cell contact zone [11] or from cytoskeletal activity associated with T cell motility [8]
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