The kinetics of a ligand-receptor interaction determine the responses of the receptor-expressing cell. One approach to experimentally and reversibly change this kinetics on demand is optogenetics. We have previously developed a system in which the interaction of a modified receptor with an engineered ligand can be controlled by light. In this system the ligand is a soluble Phytochrome B (PhyB) tetramer and the receptor is fused to a mutated PhyB-interacting factor (PIFS). However, often the natural ligand is not soluble, but expressed as a membrane protein on another cell. This allows ligand-receptor interactions in two dimensions. Here, we developed a strategy to generate cells that display PhyB as a membrane-bound protein by expressing the SpyCatcher fused to a transmembrane domain in HEK-293T cells and covalently coupling purified PhyB-SpyTag to these cells. As proof-of-principle, we use Jurkat T cells that express a GFP-PIFS-T cell receptor and show that these cells can be stimulated by the PhyB-coupled HEK-293T cells in a light dependent manner. Thus, we call the PhyB-coupled cells opto-antigen presenting cells (opto-APCs). Our work expands the toolbox of optogenetic technologies, allowing two-dimensional ligand-receptor interactions to be controlled by light.

Hydrogels with adjustable mechanical properties have been engineered as matrices for mammalian cells and allow the dynamic, mechano‐responsive manipulation of cell fate and function. Recent research yields hydrogels, where biological photoreceptors translated optical signals into a reversible and adjustable change in hydrogel mechanics. While their initial application provides important insights into mechanobiology, broader implementation is limited by a small dynamic range of addressable stiffness. Herein, this limitation is overcome by developing a photoreceptor‐based hydrogel with reversibly adjustable stiffness from ≈800 Pa to the sol state. The hydrogel is based on star‐shaped polyethylene glycol, functionalized with the red/far‐red light photoreceptor phytochrome B (PhyB), or phytochrome‐interacting factor 6 (PIF6). Upon illumination with red light, PhyB heterodimerizes with PIF6, thus crosslinking the polymers and resulting in gelation. However, upon illumination with far‐red light, the proteins dissociate and trigger a complete gel‐to‐sol transition. The hydrogel's light‐responsive mechanical properties are comprehensively characterized and it is applied as a reversible extracellular matrix for the spatiotemporally controlled deposition of mammalian cells within a microfluidic chip. It is anticipated that this technology will open new avenues for the site‐ and time‐specific positioning of cells and will contribute to overcome spatial restrictions.

Hydrogels with adjustable mechanical properties have been engineered as matrices for mammalian cells and allow the dynamic, mechano-responsive manipulation of cell fate and function. Recent research yielded hydrogels, where biological photoreceptors translated optical signals into a reversible and adjustable change in hydrogel mechanics. While their initial application provided important insights into mechanobiology, broader implementation is limited by a small dynamic range of addressable stiffness. Here, we overcome this limitation by developing a photoreceptor-based hydrogel with reversibly adjustable stiffness from the near kPa range to the sol state. The hydrogel is based on star-shaped polyethylene glycol, functionalized with the red/far-red light photoreceptor phytochrome B (PhyB), or phytochrome-interacting factor 6 (PIF6). Upon illumination with red light, PhyB heterodimerizes with PIF6, thus crosslinking the polymers and resulting in gelation. However, upon illumination with far-red light, the proteins dissociate and trigger a complete gel-to-sol transition. We comprehensively characterize the hydrogel's light-responsive mechanical properties and apply it as reversible extracellular matrix for the spatiotemporally controlled deposition of mammalian cells within a microfluidic chip. We anticipate that this technology will open new avenues in understanding cellular interpretation of dynamic mechanical cues as well as in engineering and controlling cell fate and function.