Measurement of wheat germ agglutinin binding with a fluorescence microscope

Signal intensity in fluorescence microscopy is often measured relative to arbitrary standards. We propose a calibration method based on a solution of the same fluorophore, whose binding to cells needs to be quantified. The method utilizes the low sensitivity of intensity to the object distance in wide-field imaging of uniform materials. Liquid layers of slowly varying depth were prepared by immersing a spherical lens into a drop of a fluorophore placed on a slide. Flatfield-corrected images of the contact and surrounding areas showed linear dependence of the gray level on the depth of fluorescent liquid. This allowed conversion of the measured intensity into the number of molecules per unit area. The method was applied to different cell types stained by WGA-Alexa 488 and WGA-TRITC. Consistent results were obtained by comparing microscopy with flow cytometry, comparing imaging through different objectives and comparing different WGA conjugates. Reproducibility of calibration was within 97% when low magnification was used. Fluorescence of free and bound WGA was found to be different, however, and therefore precise measurement of the number of cell-bound molecules was problematic in this particular system. We conclude that the method achieves reliable measurement of cellular staining in the units of soluble fluorophore. For probes whose fluorescent properties are unaffected by binding, quantification of staining in true molecular units should be possible.