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Activatable Fluorescent Probes as Smart Diagnostic Tools for Microendoscopy Imaging

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Understanding the body’s immune responses with the help of fluorescent chemical probes

EU-funded scientists have been seeking to understand the behaviour of immune cells important for combating diseases such as cancer by using chemical probes that ‘switch on’ when the cells become active.

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Scientists working on the EU-funded ENDOIMAGE project have developed a new type of chemical probe that can provide information on how the body’s immune cells react to a range of diseases. The technology, which can be used for imaging key molecular events in cancer, is based on chemical compounds which emit fluorescent light called fluorophores. The ENDOIMAGE chemists fine-tuned the chemical structures of fluorophores so they only ‘switch on’ in an environment related to a particular disease. “We have done experiments that show these fluorophores can specifically detect active macrophages in living organisms,” says Marc Vendrell, a senior lecturer in biomedical imaging at the University of Edinburgh whose work at Edinburgh has been funded by a Marie Curie grant. A macrophage — from the Greek word phage or ‘to eat’ — is a type of immune cell that consumes debris in the body and is known to play a role in the progression of many diseases including cancer. “We have developed a toolbox of fluorophores to study macrophages in different diseases including cancer,” Dr Vendrell says. “We design the fluorophores and we tweak them so that they switch on when they find a specific environment associated with a disease or a biological process that we want to study.” It is a powerful tool for real-time imaging. “As soon as one of the cells is active and becomes fluorescent, the signal stands out more than if we were using a fluorophore that was always fluorescent,” Dr Vendrell explains. “And we can discriminate between different colours so we can see multiple signals at once.” Detecting fungal cells Fluorophores can also detect fungal cells. Unlike many bacteria, the human immune system is usually strong enough to kill most fungal pathogens on it own. However, when a patient’s immune system is very weak, a fungal infection can be fatal. “We’ve been working to develop new chemical probes that would allow us to see these fungal cells faster in human tissue,” Dr Vendrell says, adding this will enable swifter diagnosis and treatment. “We engineered a new method of putting fluorophores into antimicrobial peptides that can bind rapidly to fungal pathogens,” Dr Vendrell explains. The researchers made a fluorescent tryptophan (a type of amino acid) that can be inserted instead of the natural amino acid, ending up with “a peptide that is very similar to the natural one but with a fluorescent tag,” he explains. Wide range of clinical applications This type of fluorophore has been patented and a major global distributor of chemicals, Merck, is marketing it as a building block for other chemists to make fluorescent peptides. Fluorescent peptides can be used not only for imaging fungal cells but for any other cells “provided the peptide binds to a specific target,” Dr Vendrell says. Collaborators worldwide are working on different applications using these probes, he adds. As chemical fluorophores take just a few minutes to become fluorescent, damaged tissue could be sprayed to detect specific cells during surgery, for example. “During the course of an operation, surgeons could discriminate between active and inactive cells, and treat it accordingly,” Dr Vendrell says. Moving forward, DYNAFLUORS or Dynamically Activatable Fluorophores, a follow-on project to do further research on the applications of these chemical probes in tumour immunology, has been awarded an EU ERC consolidator grant to begin in 2018.

Keywords

ENDOIMAGE, DYNAFLUORS, immunology, fluorophores, fluorescence, biomarker, cancer, pathogens, fungi, medical imaging

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