Activatable Fluorophores for Imaging Immune Cell Function

CONSPECTUS:Optical imaging has become an essential tool tostudy biomolecular processes in live systems with unprecedentedspatial resolution. Newfluorescent technologies and advances inoptical microscopy have revolutionized the ways in which we canstudy immune cells in real time. For example, activatablefluorophores that emit signals after target recognition have enableddirect imaging of immune cell function with enhanced readoutsand minimal background. In this Account, we summarize recentadvances in the chemical synthesis and implementation ofactivatablefluorescent probes to monitor the activity and therole of immune cells in different pathological processes, frominfection to inflammatory diseases or cancer. In addition to thecontributions that our group has made to thisfield, we review themost relevant literature disclosed over the past decade, providingexamples of different activatable architectures and their applicationin diagnostics and drug discovery. This Account covers theimaging of the three major cell types in the immune system, that is,neutrophils, macrophages, and lymphocytes. Attracted by the tunability and target specificity of peptides, many groups have designedstrategies based onfluorogenic peptides whosefluorescence emission is regulated by the reaction with enzymes (e.g., MMPs,cathepsins, granzymes), or through Fo''rster resonance energy transfer (FRET) mechanisms. Selective imaging of immune cells hasbeen also achieved by targeting different intracellular metabolic routes, such as lipid biogenesis. Other approaches involve theimplementation of diversity-orientedfluorescence libraries or the use of environmentally sensitivefluorescent scaffolds (e.g.,molecular rotors). Our group has made important progress by constructing probes to image metastasis-associated macrophages intumors, apoptotic neutrophils, or cytotoxic natural killer (NK) cells against cancer cells, among other examples. The chemical probescovered in this Account have been successfully validated in vitro in cell culture systems, and in vivo in relevant models ofinflammation and cancer. Overall, the range of chemical structures and activation mechanisms reported to sense immune cellfunction is remarkable. However, the emergence of new strategies based on new molecular targets or activatable mechanisms that areyet to be discovered will open the door to track unexplored roles of immune cells in different biological systems. We anticipate thatupcoming generations of activatable probes willfind applications in the clinic to help assessing immunotherapies and advanceprecision medicine. We hope that this Account will evoke new ideas and innovative work in the design offluorescent probes forimaging cell function

Automated summary

Optical imaging has become an essential tool for studying biomolecular processes in live systems with unprecedented spatial resolution. In this Account, recent advances in the synthesis and implementation of activatable fluorescent probes for monitoring immune cell activity are summarized. The Account covers the imaging of the three major cell types in the immune system: neutrophils, macrophages, and lymphocytes.