EPFL monitors single-cell secretions in real-time

12 Apr 2023

Technique using plasmon resonance could show details of immune response and cancers.

Proteins, antibodies and neurotransmitters secreted from cells are a key aspect of metabolism and cell communication, but most methods for monitoring them report only the quantity detected, rather than where and when they were produced.

A project at Switzerland's EPFL research center has now developed a novel optical approach that gives a four-dimensional view of cell secretions in both space and time.

Reported in Nature Biomedical Engineering, the technique provides an unprecedentedly detailed view of how cells function and communicate, according to the project team. This will be valuable in both fundamental research and pharmaceutical development.

"A key aspect of our work is that it allows us to screen cells individually in a high-throughput fashion," said Hatice Altug from EPFL's Bionanophotonic Systems (BIOS) laboratory.

"Collective measurements of the average response of many cells do not reflect their heterogeneity; and in biology, everything is heterogeneous, from immune responses to cancer cells. This is why cancer is so hard to treat."

The EPFL approach builds on research into how surface plasmon resonance (SPR) can be used to monitor biomolecules and their interactions. If cells are placed where they will encounter the charge oscillations that characterize a plasmon moving in a substrate, details of cell makeup and behavior can be deduced from changes in the optical properties.

This approach has been of limited use with single-cell secretions, however, with previous attempts hindered by small fields of view, low throughput, and difficulty in quantifying the secretions being assessed.

EPFL designed a microwell array, with multiple nanometric holes 200 nanometers in size and 600 nanometers apart fabricated in a gold substrate. Each microwell was functionalized with receptors for a specific secretion of interest from cells placed inside.

Spotting the signatures of cell death

Illuminating the array with 670-nanometer light from a LED creates the desired SPR effect in the microwells, which alters the refractive index in close proximity to the cells. A sCMOS camera collects time-lapse images and translates the spectral shifts into a record of intensity changes over time.

A machine-learning algorithm then recreates secretion maps, showing the secretion of the chosen analyte from the cell as well as the cell's movement and morphology.

In trials the microwell array was used to characterize the antibody-secretion profiles of hybridoma cells, and of a rare subset of antibody-secreting cells sorted from human donor peripheral blood mononuclear cells.

"We saw the cell content released during two forms of cell death, apoptosis and necroptosis," commented Hatice Altug. "In the latter, the content is released in an asymmetric burst, resulting in an image signature or fingerprint. This has never before been shown at the single-cell level."

EPFL believes that the versatility and performance of the system can pave the way towards a comprehensive understanding of single-cell secretory behaviours, for applications ranging from basic research to drug discovery and personalized cell therapy.

"Cell secretions are like the words of the cell; they spread out dynamically in time and space to connect with other cells," said EPFL's Saeid Ansaryan. "Our technology captures key heterogeneity in terms of where and how far these 'words' travel."