Methods for the analysis of cell secretions at the single-cell level only provide semiquantitative endpoint readouts. Researchers at the École Polytechnique Fédérale de Lausanne (EPFL) have developed a microwell array for the real-time spatiotemporal monitoring of extracellular secretions from hundreds of single cells in parallel. The microwell array incorporates a gold substrate with arrays of nanometric holes functionalized with receptors for a specific analyte, and is illuminated with light spectrally overlapping with the device’s spectrum of extraordinary optical transmission. Spectral shifts in surface plasmon resonance resulting from analyte–receptor bindings around a secreting cell are recorded by a camera as variations in the intensity of the transmitted light while machine-learning-assisted cell tracking eliminates the influence of cell movements. The researchers used the microwell array 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. High-throughput measurements of spatiotemporal secretory profiles at the single-cell level will aid the study of the physiological mechanisms governing protein secretion.
Plasmonic single-cell microwell array system
a,b, Schematic of the working principle of the plasmonic single-cell microwell array showing all four main elements: LED, PDMS micromesh, gold nanohole array substrate and sCMOS camera. Gold nanohole arrays are functionalized with desired receptors against a specific analyte secreted by the cells into the extracellular space. By illuminating the gold nanohole array substrate with the LED, for a secreting cell, the interactions between the analytes and receptors alter the refractive index on the close vicinity of the surface and consequently, the resonance peak redshifts. The sCMOS camera collects time-lapse images with a sub-minute temporal resolution to translate the spectral shifts into the intensity changes in the camera’s pixels over time. c, A 1D binding event sensogram at each camera pixel is obtained by measuring the intensity changes in real time. High-efficiency light transmission through the patterned gold surface provides high-contrast optical images enabling simultaneous analysis of secretion and morphology of the cell. d, Photograph of the integrated plasmonic gold nanohole array substrate with the PDMS micromesh featuring 20 × 20 arrays of microwells. e, Image of the plasmonic single-cell microwell array after deterministic loading of the individual cells using the piezoelectric liquid dispenser. f, Representative image of the single-cell microwell array showing the sensing wells containing individual cells and empty reference wells used for noise reduction. g, SEM image of a hybridoma cell on the nanohole array substrate. Nanoholes are patterned throughout the substrate, allowing secretion monitoring regardless of cell location. The inset shows nanoholes with a diameter of 200 nm and periodicity of 600 nm.