Analysis of fluorescent low density lipoprotein uptake by lymphocytes. Paradoxical increase in the elderly

Passage of low-density lipoproteins through Bruch's membrane and choroid

Plasma lipoproteins are thought to transport cholesterol, vitamins and carotenoids to the retinal pigment epithelium (RPE) for ultimate use by the photoreceptors. However, to reach the RPE, these lipoprotein particles must cross Bruch's membrane. We examined the reflection coefficient of Bruch's membrane (BrM) to low-density lipoprotein (LDL). Bruch's membrane and choroid were removed from 47 bovine eyes. Specimens were placed in a Ussing chamber and perfused with phosphate-buffered saline (PBS) with (31 specimens) or without (16 specimens) fluorescent low-density lipoproteins (DiI-LDL). The hydraulic conductivity of the tissue was determined for both calf and cow eyes. In the perfusions with DiI-LDL, the fluorescence intensity emitted by DiI-LDL in the efflux was measured and the reflection coefficient of BrM/choroid preparations to DiI-LDL determined. Leakage tests were done to confirm tissue integrity. Several specimens were examined using scanning electron microscopy (SEM) to examine tissue integrity before and after perfusion. Leak testing confirmed that BrM was intact both before and after perfusion. The average hydraulic conductivity of BrM/choroid perfusion of calf eyes with PBS alone was 1.42 ± 0.55 × 10(-9) m/s/Pa (mean ± SD, n = 11). The average hydraulic conductivity of the cow eyes was 4.94 ± 1.48 × 10(-10) m/s/Pa (n = 5), nearly a 3-fold decrease with age. While the flow rate remained constant during the PBS perfusions, it decreased as a function of time during perfusion with DiI-LDLs. Our major finding was of fluorescence in the effluent collected in all perfusions with DiI-LDLs, demonstrating passage of LDL through the tissue. The average reflection coefficient of calf BrM/choroid preparations to DiI-LDL was 0.58 ± 0.25 (n = 23); a similar distribution of reflection coefficients was seen in tissue from cow eyes (0.51 ± 0.33, n = 8). Our data suggested that the DiI-LDL was modestly hindered and/or captured by the tissue. This might explain the progressive decrease of hydraulic conductivity with continued perfusion of DiI-LDL.

T-cadherin mediates low-density lipoprotein-initiated cell proliferation via the Ca(2+)-tyrosine kinase-Erk1/2 pathway

The GPI-anchored protein T-cadherin was found to be an atypical LDL binding site that is expressed in various types of cells, including endothelial cells, smooth muscle cells, and neurons. Notably, the expression of T-cadherin was reduced in numerous types of cancers, although it was up-regulated in tumor-penetrating blood vessels, atherosclerotic lesions, and during neointima formation. Despite these intriguing findings, our knowledge of the physiological role and the signal transduction pathways associated with this protein is limited. Therefore, T-cadherin was overexpressed in the human umbilical vein-derived endothelial cell line EA.hy926, the human embryonic kidney cell line HEK293, and LDL-initiated signal transduction, and its consequences were elucidated. Our data revealed that T-cadherin serves as a receptor specifically for LDL. Following LDL binding to T-cadherin, mitogenic signal transduction was initiated that involved activation of PLC and IP3 formation, which subsequently yielded intracellular Ca2+ mobilization. Downstream to these early phenomena, activation of tyrosine kinase(s) Erk 1/2 kinase, and the translocation of NF kappa B toward the nucleus were found. Finally, overexpression of T-cadherin in HEK293 cells resulted in accelerated cell proliferation in an LDL-dependent manner, although cell viability was not influenced. Because LDL uptake was not facilitated by T-cadherin, our data suggest that T-cadherin serves as a signaling receptor for LDL that facilitates an LDL-dependent mitogenic signal in the vasculature.

Current status of flow cytometry in cell and molecular biology

This review summarizes recent developments in flow cytometry (FC). It gives an overview of techniques currently available, in terms of apparatus and sample handling, a guide to evaluating applications, an overview of dyes and staining methods, an introduction to internet resources, and a broad listing of classic references and reviews in various fields of interest, as well as some recent interesting articles.

Flow cytometric assessment of LDL receptor activity in peripheral blood mononuclear cells compared to gene mutation detection in diagnosis of heterozygous familial hypercholesterolemia

BACKGROUND

Studies indicate that human peripheral blood mononuclear cells mirror low-density lipoprotein (LDL) receptor activity of other cells in the body. To measure LDL receptor activity in patients with heterozygous familial hypercholesterolemia (FH), we prepared peripheral blood mononuclear cells from individuals with molecularly verified LDL receptor defective (Trp66-Gly mutation, n = 18) or receptor negative (Trp23-stop mutation, n = 17) heterozygous FH and from healthy individuals (n = 24).

METHODS

The cells were stimulated to express maximum LDL receptor by preincubation in lipoprotein-free medium. They were then incubated at 4 degrees or 37 degrees C with fluorescently conjugated LDL (DiI-LDL). T-lymphocytes and monocytes were identified by fluorescently conjugated monoclonal antibodies. DiI-LDL bound (at 4 degrees C) or internalized (at 37 degrees C) by the cells was measured using flow cytometry. Knowing the LDL receptor gene mutation of the FH patients allowed us to compare the diagnostic capability of our functional assay with the DNA diagnosis.

RESULTS

The diagnostic accuracy did not allow our assay to be used for diagnosis of individual cases of heterozygous FH.

CONCLUSIONS

We suggest that our two-color fluorescence flow cytometry assay can be used to characterize functionally gene mutations causing LDL receptor dysfunction in patients with heterozygous FH.

Environmental influence on age-related changes of human lymphocyte membrane viscosity using severe combined immunodeficiency mice as an in vivo model

Peripheral blood lymphocytes (PBL) of healthy elderly people show increased plasma membrane viscosity compared to young subjects, that inversely correlates with lymphocyte proliferation after mitogen stimulation in vitro. Maintenance of a constant membrane viscosity, which is necessary for proper cell function, is crucially dependent on the membrane lipid composition. The cellular lipid metabolism, and thus lymphocyte function, may be subject to modulation by diet or drugs. To study the susceptibility of membrane viscosity to environmental conditions, we established an in vivo model using severe combined immunodeficiency (SCID) mice: human peripheral blood lymphocytes from healthy young and old subjects were engrafted for three days intraperitoneally into SCID mice to offer identical environmental conditions. First, we demonstrate that human lymphocytes can take up and utilize murine lipoproteins: engrafted human PBL can participate in the mouse lipid metabolism, and an exchange of membrane lipids in vivo is, therefore, possible. Second, plasma membrane viscosity was determined before and after engraftment: before engraftment, PBL from the elderly showed a significantly higher membrane viscosity than that from young controls, but this difference vanished during engraftment into SCID mice, wherein cells from both age groups exhibited nearly identical values. It was, therefore, concluded that lymphocyte membrane viscosity is influenced by environmental factors, and that the age-related increase is, in principle, reversible.

The aging immune system: primary and secondary alterations of immune reactivity in the elderly

The function of the immune system is known to decline in the elderly. The present communication will show that, similar to the situation in children, it is necessary to distinguish between primary and secondary alterations of the immune reactivity in the elderly. The primary immunological change in the elderly is the age-dependent intrinsic decline of immune responsiveness that is also observed in healthy persons, i.e., those selected according to the criteria of the SENIEUR Protocol of EURAGE (European Community Concerted Action on Aging). Secondary immunological changes are due to underlying diseases and various environmental factors, including diet, drug intake, physical activity, etc. While primary immunodeficiencies of the elderly are not, or only to a very minor extent, presently influenced by therapeutic measures, secondary alterations of the immune function offer further possibilities for corrective measures. Clinically, the consequences of impaired immune function in the elderly include increased susceptibility to infectious disease, the emergence of tumors, and increased autoimmune reactions, the latter albeit often without concomitant autoimmune disease. In fact, autoimmune diseases in generally begin to develop at younger ages, but their consequences are major factors affecting the quality of life of the elderly. Investigations of basic molecular and cellular aspects of the aging immune system are, therefore, of theoretical, clinical and socioeconomic interest.

Scavenger receptor activity and expression of apolipoprotein E mRNA in monocyte-derived macrophages of young and old healthy men

The aim of this study was to compare some aspects of lipid metabolism in monocyte-derived macrophages isolated from young males, aged 18-24 years, and old males, aged 74-90 years, who were found healthy in accordance with the Senieur protocol. The parameters tested were metabolism of 125I-acetylated low-density lipoproteins (LDL) and oxidized LDL, incorporation of [3H]cholesterol into cholesteryl ester and expression of apolipoprotein E (apo E) mRNA. Cell association and degradation of 125I-acetylated LDL by macrophages of old and young subjects, respectively, was 15,978 +/- 2492 and 9300 +/- 1416 ng/mg cell protein per 24 h. Incorporation of [3H]cholesterol into cellular [3H]cholesteryl ester in the presence of acetylated LDL in cells isolated from old subjects was twice that in cells from young subjects. The macrophages from both age groups metabolized less 125I-oxidized LDL than 125I-acetylated LDL. Cell association and degradation of 125I-oxidized LDL in cells from old and young subjects, respectively, was 6779 +/- 1398 and 3219 +/- 643 ng/mg cell protein per 24 h. Expression of apo E mRNA was determined by reverse transcriptase polymerase chain reaction. In the basal state, it was 5.8 +/- 0.4 and 2.4 +/- 0.2 photo-stimulated luminescence (PSL) units in cells from the old and young subjects, respectively, and increased after exposure to acetylated LDL. In conclusion, these findings suggest that a combination of higher scavenger receptor activity and increased expression of apo E mRNA in macrophages could contribute to (a) enhanced metabolism of modified LDL and (b) more efficient removal of cholesterol from arteries, thus leading to healthy old age.

Properties of low density lipoproteins relevant to oxidative modifications change paradoxically during aging

Atherosclerosis is a common problem among the elderly. Because lipid peroxidation is considered a contributor to the development of atherosclerosis, we compared oxidative properties of lipoproteins in an otherwise healthy (SENIEUR-classified) aged population (65-74 years) with young controls (18-30 years). Relative amounts of oxidatively altered low density lipoprotein (LDL), estimated by means of an antibody against LDL modified by 4-hydroxynonenal, a product of lipid peroxidation, were increased marginally in serum from the elderly (9.8 vs. 7.4%, P = 0.07). In contrast, isolated LDL from the elderly revealed a decreased susceptibility to in vitro oxidation: the lag time was increased (2.34 vs. 2.10 h, P < 0.01), and the maximal rate of LDL oxidation decreased (0.88 vs. 1.01 O.D./h, P = 0.001). However, there were no age-related changes in lipid composition of native LDL and consumption of fatty acids during in vitro oxidation. The serum concentrations of ascorbic acid and most lipophilic anti-oxidants (the latter expressed per g serum lipids) were significantly decreased in the elderly except tocopherols which tended to be higher. In conclusion, our data reveal paradox age-related alterations of LDL as to its behaviour in oxidation in vivo vs. in vitro.

The effect of age on hemopoiesis

Although several workers have described numerous changes affecting the hemopoietic system during senescence, the existence of univocal "hematological disease" closely related to the elderly is controversial. Many of the hematological changes described, such as sideropenic or megaloblastic anemia, are frequently the consequence of the different pathological conditions which often affect elderly patients. This review will consider the most important alterations of hemopoiesis and coagulation in the elderly, the causes capable of influencing hematological changes in old people, and their pathogenesis. Some of the major diagnostic problems encountered in the management of elderly subjects with hematological changes are also addressed. In the presence of an elderly patient with hematological alteration, it is necessary to follow a precise diagnostic schedule, which should first of all exclude the presence of a primary hematological disorder, and consider the different extrahematological conditions which frequently occur in elderly subjects (malignancies, malnutrition, chronic infections from immunological abnormalities, hormonal changes, deficiencies of various organs and systems etc.) and are responsible for many different hematological changes. These must be tackled rationally so that treatment may not only be symptomatic, but may also directly intervene on the cause of the disorder.

Standardization of a flow cytometric method for measurement of low-density lipoprotein receptor activity on blood mononuclear cells

Flow cytometric methods for measurement of low-density lipoprotein (LDL) receptor activity on peripheral blood mononuclear cells (PBMC) may be used to identify patients with familial hypercholesterolemia (FH). However, cellular LDL receptor activities measured in FH heterozygotes may overlap with those of healthy subjects. Analytical variation is probably responsible for some of this overlap. We have examined several technical details that may affect analytical variation. In each analysis, we included one standard and two control cell preparations. These were cells isolated from three donors and stored in aliquots at -135 degrees C. Use of standard cells reduced between-series analytical variation of the controls by approximately 50%. Preincubation-conditions used to induce the maximum number of receptors, the concentration of fluorochrome 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-perchlorate (DiI)-LDL, labelling time, and conditions during storage of labelled cells before flow cytometry were also examined in order to reduce analytical variation. Having standardized the assay, we found among 20 healthy subjects a median receptor activity of 100% vs. 51% among 26 patients who fulfilled clinical criteria for FH. However, four of the patients showed distinctly normal receptor activities, which may suggest either the presence of some other biochemical defect or that in vivo dysfunctional receptors may be measured as normal in some patients with our assay.

In vivo LDL receptor and HMG-CoA reductase regulation in human lymphocytes and its alterations during aging

The LDL receptor and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase play primary roles in the regulation of cellular cholesterol metabolism. To investigate the transcriptional regulation of lipid metabolism under physiological conditions ex vivo and its alterations during aging, we analyzed both the activity and mRNA concentration of the LDL receptor and HMG-CoA reductase in freshly isolated lymphocytes from healthy young and elderly donors. Data from fluorescent reverse transcriptase-polymerase chain reaction indicated that not only plasma LDL but also plasma HDL downregulates lymphocyte LDL receptor mRNA. Downregulation by HDL was three times more effective than that by LDL and presumably involved specific HDL binding sites. There was coordinate regulation of HMG-CoA reductase mRNA with LDL receptor mRNA that was independent of plasma lipoprotein concentrations. Despite elevated plasma concentrations of LDL, lymphocytes from elderly donors paradoxically expressed increased levels of the LDL receptor (P = .030) and HMG-CoA reductase mRNA (P = .062). The age-related dysregulation of the LDL receptor was predominantly due to impaired downregulation by plasma LDL rather than by HDL. Thus, not only LDL but also HDL and age significantly influences the transcriptional regulation of the LDL receptor in extrahepatic cells in vivo.

Evidence of non-deficient low-density lipoprotein receptor patients in a pool of subjects with clinical familial hypercholesterolemia profile

For this study, we selected 41 adult patients with the classic clinical diagnosis of heterozygous familial hypercholesterolemia (FH), which is characterized by a low-density lipoprotein (LDL) cholesterol level above the 95th percentile, xanthomas, and/or personal or familial cardiovascular history. We used an indirect immunocytofluorimetric assay to classify these 41 subjects according to LDL receptor function on lymphocytes. We found that LDL receptor activity was normal in nine patients. A large study of plasma lipid, lipoprotein, and apolipoprotein levels found no significant difference between patients with and without LDL receptor defect. Familial defective apolipoprotein (apo) B-100 (FDB) and LDL-binding defects were not found in the nine patients without LDL receptor defect. These results suggest that other defects in the regulation of lipoprotein metabolism are capable of giving rise to a clinical and biochemical disorder indistinguishable from classic FH.

Lipid utilization by human lymphocytes is correlated with high-density-lipoprotein binding site activity

The nature and physiological importance of high-density lipoprotein (HDL) binding sites on unstimulated (resting) and mitogen-activated (blast) human peripheral blood lymphocytes were investigated. Specific HDL binding on resting and blast T-lymphocytes was saturable at 50 micrograms of 125I-HDL/ml and of high affinity, with Kd values of 8.1 x 10(-8) M and 6.5 x 10(-8) M, respectively, and Bmax. values of 79 ng and 180 ng/mg of cell protein respectively at 4 degrees C. Binding of HDL double-labelled with fluorescent dioctadecylindocarbocyanine (Dil) and isotope (125I) as well as of single fluorescence- or isotope-labelled HDL was inhibited competitively by HDL apoproteins. Studies of the cholesterol flux between the cells and HDL showed that HDL, low-density lipoprotein (LDL) or BSA at a concentration of 100 micrograms/ml in the tissue culture medium did not result in a significant difference in exogenous [3H]cholesterol efflux from the cell membrane at 37 degrees C. Proliferating T-blasts incorporated more cholesterol from HDL or LDL than did resting lymphocytes. When the cells were pulsed with 125I-HDL and chased in fresh lipid-free medium, up to 80% of the radioactivity released was not precipitable with trichloroacetic acid. This percentage decreased in a competitive manner when unlabelled HDL was present in the chase incubation medium. Finally, cultivation of lymphocytes with conditioned medium from macrophages increased Dil-HDL binding/uptake, while it was decreased by mevinolin-induced inhibition of hydroxymethylglutaryl-coA reductase. In conclusion, human lymphocytes possess a HDL binding site (receptor) responsible for lipid binding/uptake and concomitant internalization and degradation of apoproteins from HDL, but not for reverse cell membrane cholesterol transport. The activity of the binding site is up-regulated during cell proliferation and down-regulated during cell growth suppression.

Correlation of lymphocyte lipid composition membrane microviscosity and mitogen response in the aged

Healthy aged and young blood donors were investigated for the role of membrane lipid composition in the age-related increase in membrane microviscosity and decline of mitogen responsiveness. Membrane microviscosity was shown to correlate positively with membrane cholesterol/phospholipid molar ratios, which were significantly elevated in the elderly. A positive correlation also was confirmed between lymphocyte membrane microviscosity, which was measured using the probe 1,6-diphenyl 1,3,5-hexatriene, and phytohemagglutinin responsiveness of cells from the same donor. Using stepwise regression statistical analysis, the variables age, cholesterol, cholesterol/total phospholipid and phosphatidyl ethanolamine/phosphatidyl choline molar ratios were all shown to have a significant positive influence on membrane microviscosity, whereas total phospholipids had a negative effect. No statistically significant difference was seen in content of any single saturated or unsaturated fatty acid between young and old donors. After pooling, however, the proportion of all unsaturated fatty acids was significantly higher in cells from the elderly as a consequence of an increase of n-6 and n-3 polyunsaturated fatty acids. Changes in lipid composition and physical properties of lymphocyte plasma membranes may, therefore, be responsible (at least partially) for the diminution of immune reactivity in old age.

The labeling of lipoproteins for studies of cellular binding with a fluorescent lipophilic dye

N,N-dipentadecylaminostyrylpyridinium iodide is a dye that is approximately 100-fold more intensely fluorescent in a lipid than aqueous environment. This observation suggests its potential as a fluorescence stain for lipoproteins. This work reports the staining of LDL with this dye for use in studies of cellular binding. The staining procedure is simple, resulting in stable attachment of the dye as determined by transfer experiments, physical properties essentially identical to native LDL as demonstrated by virtually identical electrophoretic mobility, and consistent results in studies of cellular binding using flow cytometry. Increased signal to noise ratio over other dyes used for lipoprotein staining including the widely used Dil (3,3'-dioctadecylindocarbocyanine iodide) allows determinations of greater sensitivity and precision to be made. This is demonstrated by the flow cytometric determination of the 4 degrees C binding curve of LDL with freshly isolated human peripheral blood lymphocytes (i.e., cells not LDL receptor upregulated). Mediation of binding by the LDL receptor is demonstrated by correspondence between the LDL receptor dissociation constant derived from this work and literature values; increased specific binding in lymphocytes cultured in lipoprotein-deficient media to up-regulate the LDL receptor; and decreased specific binding in lymphocytes cultured in the presence of 25-hydroxy cholesterol for 48 h to suppress the LDL receptor.

Lipoprotein interactions with T cells: an update

The role of plasma lipoproteins in atherogenesis is well recognized but the physiological relevance of their immunoregulatory properties is still questioned. Here Karine Traill and colleagues outline the recent advances that have been made towards unravelling the mechanisms of immunoregulation by lipoproteins in vitro and consider whether any of these mechanisms are operative in vivo. In particular they address the possible detrimental effects of high serum lipoprotein levels on immune function and the question of whether hyperlipidemia (or hypercholesterolemia) should be considered a risk factor for diminished immunity, for example in old age.

Augmentation of LDL receptor activities on lymphocytes by interleukin-2 and anti-CD3 antibody: a flow cytometric analysis

Dormant lymphocytes are known to show little LDL receptor (LDL-R) activities. The present study was designed to determine whether or not LDL-R activities of lymphocytes from normal subjects were high enough to be measured by flow cytometry after the cells had been stimulated with recombinant interleukin-2 (IL-2) and anti-CD3 monoclonal antibody (mAb). IL-2 or anti-CD3 mAb individually provokes proliferation of lymphocytes in a serum-free medium. Proliferation rate was accelerated when the two reagents were used in combination. Stimulated cells cultured for 5 days expressed more than 85% CD3 positive, less than 0.5% CD14 positive, and less than 1.5% CD20 positive. The LDL-R activities of the cells were examined by the uptake of a fluorescence probe, DiI-labeled LDL (DiI-LDL) and analyzed by flow cytometry. Stimulated cells showed increased uptake of DiI-LDL and 84 +/- 9% were positive, whereas only 3.0 +/- 2.5% of the cells without stimulation were positive (P less than 0.001). Under the same conditions stimulated lymphocytes from a homozygous familial hypercholesterolemia (FH) patient showed little LDL-R activities; 14% of the cells were positive. Displacement assays reveal that the uptake of LDL by these cells is occurring by way of its specific pathway. These data imply the lymphocytes stimulated with the reagents used in the study might be used for detecting defects in LDL-R, perhaps defects in other genomic systems as well.

Characterization of human monocytic cell line, U937, in taking up acetylated low-density lipoprotein and cholesteryl ester accumulation. A flow cytometric and HPLC study

The uptake of LDL and acetylated LDL and the ability of cholesteryl ester accumulation by cells of a human monocytic cell line, U937, has been characterized by flow cytometric assay using a fluorescent probe, DiI, and by high-performance liquid chromatography (HPLC). The increase of mean fluorescence intensity of U937 incubated with DiI-labeled lipoproteins demonstrates that this cell line could incorporate DiI-AcLDL, as well as DiI-labeled LDL. Competition and saturation studies indicate that the manner of taking up DiI-AcLDL is receptor-mediated. While differentiated U937 incubated with 16 nM phorbol myristate acetate for 24 h took up little DiI-AcLDL, HPLC analysis confirmed that intracellular free and esterified cholesterols significantly increase in the U937 cells incubated with AcLDL or LDL. The ability of mouse peritoneal macrophage to abundantly accumulate at least five kinds of cholesteryl ester were also shown in this analysis. In contrast, in U937 cells, free fatty acids are incorporated into various substances rather than into cholesteryl esters (as revealed by HPLC analysis), so that the cholesterol in AcLDL taken up by U937 cells is not synthesized into cholesteryl esters to any great extent.

Approach to define "normal aging" in man. Immune function, serum lipids, lipoproteins and neopterin levels

In 53 healthy women with mean age 25.1 years (age range 21-34 years) and in 51 healthy women with mean age 82.1 years (age range 75-91 years), a panel of immunological and biochemical tests was performed. These tests, comprising lymphocyte mitogen responsiveness, phenotyping of lymphocytes, uptake of low density lipoprotein by T cells, serum levels of neopterin, lipids and lipoproteins, as well as routine blood chemistry, were investigated for a possible effect of age and of the classification according to the SENIEUR Protocol of admission criteria by the European Economic Community's Concerted Action Program on Aging (EURAGE). A highly significant effect of age on serum levels of neopterin, lipids and lipoproteins was found. No clear effects, however, of SENIEUR status on these variables was detected. As expected, age had a significant impact on mitogen responsiveness of T cells. Proportional numbers of helper/inducer and cytotoxic/suppressor T cells (as well as antigen density on these cells) were not influenced by age. SENIEUR classification did not affect these immunologic variables. Thus, most of the tested variables that are not included in the SENIEUR admission criteria appear to present information not yet covered by the SENIEUR variables. Various ways for a possible revision or extension of the SENIEUR Protocol are discussed.

High density lipoprotein uptake by freshly isolated human peripheral blood T lymphocytes

A high density lipoprotein (HDL) receptor/binding site has been identified on peripheral blood lymphocytes, and some of its properties were compared with those of HDL receptors on other cell types. Binding studies were performed using fluorescent (dioctadecylindocarbocyanine)-labelled HDL (DiI-HDL) and analyzed by fluorescence microscopy and quantitative flow cytometry. Uptake of low levels of DiI-HDL during a 2 h incubation at 37 degrees C was a property of all lymphocytes, i.e. not of one particular subset only. Visual inspection of these cells in the fluorescence microscope revealed both membrane and cytoplasmic fluorescence, indicating that DiI-HDL become internalized during the 2 h incubation; internalization appeared to be a receptor-mediated process. In competitive binding studies, apo E-free HDL competed effectively for DiI-HDL binding, whereas LDL competed very weakly. Two features of DiI-HDL uptake are demonstrated which are unique to lymphocytes: 1) it was enhanced 3-6-fold by inclusion of EDTA in the incubation medium or by incubating in Ca2+/Mg2+ free medium, and 2) it was saturable at 37 degrees C.