Analysis of receptor clustering on cell surfaces by imaging fluorescent particles

Fluorescently labeled low density lipoproteins (LDL) and influenza virus particles were bound to the surface of human fibroblasts and imaged with a cooled slow-scan CCD camera attached to a fluorescence microscope. Particles were also imaged after attachment to polylysine-coated microscope slides. The digital images were analyzed by fitting data points in the region of fluorescent spots by a two-dimensional Gaussian function, thus obtaining a measure of spot intensity with correction for local background. The intensity distributions for particles bound to polylysine slides were mainly accounted for by particle size distributions as determined by electron microscopy. In the case of LDL, the intensity distributions for particles bound to fibroblasts were considerably broadened, indicative of clustering. The on-cell intensity distributions were deconvolved into 1-particle, 2-particle, 3-particle, etc. components using the data obtained with LDL bound to polylysine-coated slides as an empirical measure of the single particle intensity distribution. This procedure yielded a reasonably accurate measure of the proportion of single particles, but large errors were encountered in the proportions of larger cluster sizes. The possibility of studying the dynamics of clustering was investigated by binding LDL to cells at 4 degrees C and observing changes in the intensity distribution with time after warming to 20 degrees C.