Proceedings of the National Academy of Sciences | 2019
DNA probes that store mechanical information reveal transient piconewton forces applied by T cells
Abstract
Significance To defend against cancer and viral infections, the T cell receptor (TCR) must recognize antigens on the surface of target cells. TCR antigen recognition involves the transmission of forces which are often weak, infrequent, and short lived, and hence difficult to study. We solve this challenge by developing molecular probes that store mechanical information. This approach reveals the TCR forces when encountering different antigens, and this mechanical sampling is correlated with antigen potency. Since coreceptors are heavily involved in tuning immune function, we investigate the programmed cell death receptor 1 (PD1) and show that T cells transmit forces to this coinhibitory receptor, thus suggesting that mechanics may play a role in this important checkpoint pathway. The advent of molecular tension probes for real-time mapping of piconewton forces in living systems has had a major impact on mechanobiology. For example, DNA-based tension probes have revealed roles for mechanics in platelet, B cell, T cell, and fibroblast function. Nonetheless, imaging short-lived forces transmitted by low-abundance receptors remains a challenge. This is a particular problem for mechanoimmunology where ligand–receptor bindings are short lived, and a few antigens are sufficient for cell triggering. Herein, we present a mechanoselection strategy that uses locking oligonucleotides to preferentially and irreversibly bind DNA probes that are mechanically strained over probes at rest. Thus, infrequent and short-lived mechanical events are tagged. This strategy allows for integration and storage of mechanical information into a map of molecular tension history. Upon addition of unlocking oligonucleotides that drive toehold-mediated strand displacement, the probes reset to the real-time state, thereby erasing stored mechanical information. As a proof of concept, we applied this strategy to study OT-1 T cells, revealing that the T cell receptor (TCR) mechanically samples antigens carrying single amino acid mutations. Such events are not detectable using conventional tension probes. Each mutant peptide ligand displayed a different level of mechanical sampling and spatial scanning by the TCR that strongly correlated with its functional potency. Finally, we show evidence that T cells transmit pN forces through the programmed cell death receptor-1 (PD1), a major target in cancer immunotherapy. We anticipate that mechanical information storage will be broadly useful in studying the mechanobiology of the immune system.