Quantifying Cell-Surface Marker Expression Through Imaging of Transient Interactions

Abstract

Characterizing cells in terms of their surface-marker expression is an indispensable tool for cell biology, disease diagnosis, and drug discovery. Flow cytometry, the most commonly used method, though invaluable, requires exogenous labelling of cells, is not tractable for small sample sizes, and is limited in the number of unique markers that can be measured simultaneously. Recently, node-pore sensing (NPS) has emerged as a label-free method for surface-marker characterization. In NPS, the Coulter principle is utilized to measure the transit time of cells as they flow through a microfluidic channel functionalized with antibodies for multiple surface-markers of interest. Cells transiting functionalized regions of the channel that express the corresponding surface-marker have longer transit times when compared against a control region due to transient and specific interactions with the functionalized surface. In this manner, NPS can screen label-free for multiple surface markers at the single-cell level. The sensitivity of NPS is dependent on the ratio of the cell's volume to the volume of the channel, which limits the length of the channel and thus the number of possible unique markers. Furthermore, since the current through the channel is dependent on its total resistance, only one cell may be present in the channel at a time. Here, we show that these limitations can be overcome if the transit time is measured optically rather than electrically. Imaging the channel allows for tracking individual cells, which enables multiple cells to be measured simultaneously. Moreover, the total number of markers is only limited by the size and speed of the CCD camera that is used for imaging. We utilize this approach to measure the surface-marker expression profile of single MCF-7 and SKBR3 cells.