High-throughput measurements of intra-cellular and secreted cytokine from single spheroids using anchored microfluidic droplets

While many single-cell approaches have been developed to measure secretion from anchorage- independent cells, these protocols cannot be applied to adherent cells, especially when these cells requires to be cultured in 3D formats. Here we demonstrate a platform to measure the secretions from individual spheroids of human mesenchymal stem cells, cultured within microfluidic droplets. The platform allows us to quantify the secretion from hundreds of individual spheroids in each device, by using a secondary droplet to bring functionalized micro-beads into proximity with each spheroid. We focus on vascular endothelial growth factor (VEGF) and measure a distribution of secretion levels that presents broad heterogeneity within the population of spheroids. Moreover, the intra-cellular level of VEGF-A on each spheroid, measured through immuno-staining, corre- lates well with the extra-cellular measurement, indicating that the heterogeneities observed at the spheroids level result from variations at the scale of individual cells. Finally, we model the molecular accumulation within the droplets and find that physical confinement is crucial for measurements of protein secretions. The model predicts the time to achieve a measurement, which scales with droplet volume. Therefore these first measurements of secretions from individual spheroids provide several new biological insights.