A RAPID FLOW CYTOMETRIC ASSAY FOR LOW-DENSITY-LIPOPROTEIN RECEPTORS IN HUMAN PERIPHERAL-BLOOD MONONUCLEAR-CELLS

Receptor-dependent uptake of low density lipoproteins (LDLs) labeled w ith the fluorescent probe 3,3'-dioctadecylindocarbocyanine iodide (Dil ) by human lymphocytes was measured by flow cytometry. Freshly isolate d lymphocytes showed very little uptake, whereas the cells cultured in lipoprotein-free medium for 48 hours showed about a 10-fold increase in the uptake of DiI-LDL. The uptake was inhibited by excess unlabeled LDL, heparin, and ethyleneglycol-bis-(beta-aminoethylether)-N,N, N',N '-tetraacetic acid (EGTA) but not by acetylated LDL, and therefore it appears to be very specific. Comparative studies with iodine 125-label ed LDL showed that there is correlation between the uptake of DiI-LDL and degradation of I-125-labeled LDL by the lymphocytes. The assay was carried out with lymphocytes from several normal healthy individuals and some clinically diagnosed heterozygous familial hypercholesterolem ic patients. The results indicate that this assay can be used to disti nguish patients with LDL receptor-defective familial hypercholesterole mia from normal individuals.