Regulatory T cells (Tregs) provide an essential tolerance mechanism to suppress the immune response. Under normal conditions, Tregs reduce reaction to self-antigens, and conversely, lack of Treg function leads to autoimmune diseases. Reengineering of the immune system with regards to Tregs, such as through adoptive immunotherapy, holds great therapeutic promise for treating a range of diseases. These approaches require production of Tregs, which can be induced from conventional, reactive T cells. This thesis is driven by the concept that changing the mechanical stiffness of biomaterials can be used to direct and optimize this induction process. It is known that T cells sense their extracellular environment, and that T cell activation can be modulated by mechanical cues. However, it is still unclear whether or not human Treg induction is sensitive to material stiffness. We studied this phenomenon by replacing the stiff plastic supports commonly used for T cell activation with planar, elastic substrates — specifically polyacrylamide (PA) gels and polydimethylsiloxane (PDMS) elastomer.

Treg induction, as measured by expression of FOXP3, a master transcription factor, was sensitive to stiffness for both materials. Substrate stiffness also modulated the suppressive function and epigenetic profiles of these cells, demonstrating that substrate rigidity can direct Treg induction, complementing the use of chemical and genetic tools. Delving deeper into the mechanisms of T cell mechanosensing, single-cell transcriptomic analysis revealed that substrate rigidity modulates the trajectory of Treg induction from conventional T cells, altering a host of functions including metabolic profile. Together, these studies introduce the use of substrate stiffness and T cell mechanosensing towards directing and optimizing regulatory T cell production. Further development of cell culture systems around this discovery is critical for emerging T cell-based therapies, targeting cancer but also a broad range of diseases.