Every cell of our body is surrounded by a plasma membrane that separates the inside of a cell from the outside. This is not just a static barrier but the stage of an intricate interplay of invisibly small structures, proteins and even smaller lipids, that move and assemble themselves in complex ways to mediate membrane function. A special type of cells, T-cells, are major players in our immune system. One protein in their plasma membrane, the T-cell receptor (TCR), recognizes a part of a pathogen (the antigen), which is presented by another cell, that has previously found this intruder in our body. Upon this recognition process, the T-cell becomes activated and a series of events is initiated ultimately leading to an immune response. We have already a good understanding of the protein players involved in this process, but the role of lipids is largely unclear. This is because lipids are extremely hard to study, particularly in living cells. Lipids and proteins are much too small to be directly imaged using optical microscopic methods, and they are constantly moving around. However, by labelling only a tiny subset of these molecules with fluorescent dyes they can be detected as individual diffraction limited spots and we can follow their movement over time and watch, how they interact with each other. Still, this is not good enough for lipids, because their interactions with proteins are often too short. Further, they cannot only influence proteins by direct, one-on-one interaction, but they also make up the 2-dimensional matrix of the plasma membrane itself. There are thousands of different lipid species with very different properties and their organization and dynamics shape the properties of the membrane.

Here, a novel experimental approach will be employed and will allow, for the first time, to interrogate the interaction of lipids with the TCR. T-cells will be interfaced with microstructured substrates which mimic antigen presenting cells; this allows to immobilize and enrich the TCR specifically within certain areas of the T-cell plasma membrane in live cells. Thus, it becomes possible to extract quantitative parameters on the interaction of the immobile interaction partner (the TCR) with a mobile interaction partner (the lipid). Several powerful single molecule fluorescence techniques will be used to for the first time observe the behavior of the lipids with respect to the TCR to elucidate their role in the process of T-cell activation.