Cells can sense and respond to the physical environment through generation and transmission of mechanical forces from the surroundings to the cell interior and from one cell to another. This dissertation focuses on mechanosensing by T cells, key players in the adaptive immune system, which form a strong line of defense against infections by their ability to recognize foreign molecules and develop an appropriate response. T cells form close contact with an opposing antigen presenting cell upon recognition of protein fragments derived from infecting pathogens (antigens). Recent studies have shown that externally applied forces can trigger biochemical signaling in T cells. How forces are internally generated by T cells, involved in signaling and transmitted at the level of the cell interface, remains unclear. In this thesis, we investigate the molecular mechanisms of force generation by T cells and their response to forces and the stiffness of the opposing surface.

We have quantitatively characterized the initial phase of T cell contact with a model of antigen-bearing surfaces. We observe that T cells spread on such substrates and that the kinetics of spreading follows a universal function, with the spreading rate dependent on actin polymerization and myosin II activity. Altering cell-substrate adhesions leads to qualitative changes in cell spreading dynamics and wave-like patterns of actin dynamics. We then used soft elastic substrates with stiffness comparable to that of antigen presenting cells, to measure the forces generated by T cells during activation.

Perturbation experiments reveal that these forces are largely due to actin assembly and dynamics, with myosin contractility contributing to the development of traction forces but not its maintenance. We find that Jurkat T-cells are mechanosensitive, with both traction forces and signaling dynamics exhibiting sensitivity to the stiffness of the substrate. We further demonstrate that dynamics of the T cell microtubule cytoskeleton also participates in regulating forces at the cell-substrate interface, through the Rho/ROCK pathway which regulates myosin II light chain phosphorylation.

Overall, this work highlights physical force as an essential mediator that connects stiffness sensing to intracellular signaling, which then directs gene expression and eventually the immune response in T cells.