Microfluidic measurement of antibody-antigen binding kinetics from low-abundance samples and single cells

We present a simple microfluidic fluorescence bead assay for accurately measuring antibody-antigen binding kinetics with a standard inverted fluorescent microscope. We measured association and dissociation rate constants from antibody-antigen interactions spanning nearly 4 orders of magnitude in equilibrium binding affinity (30 pM-100 nM). Two versions of this assay are presented, which allow for dissociation rate measurements either directly, by use of fluorescently labeled antigen, or indirectly, by use of unlabeled antigen. We also demonstrate simultaneous, multiplexed binding measurements of multiple antibody-antigen interactions using a combination of spectral separation and spatial localization. Complete antibody-antigen binding kinetics were measured for as little as 8 × 10⁴ antibody molecules (~132 zeptomoles) immobilized on a single bead and less than 2 × 10⁶ antibodies (~3 attomoles) loaded into the microfluidic device, a reduction in detection limit and sample consumption of 4 orders of magnitude when compared to surface plasmon resonance (SPR) spectroscopy and alternative measurement techniques. We show that the microfluidic bead assay, when combined with small volume compartmentalization, enables direct measurement of antigen binding kinetics of antibodies secreted from single hybridoma cells. We anticipate that this assay will be useful as a routine analytical tool for studying molecular interactions as well as for screening primary antibody-secreting plasma cells isolated from immunized animals.