Flow cytometric measurement of ceroid accumulation in macrophages

Characterization of viable autofluorescent macrophages among cultured peripheral blood mononuclear cells

BACKGROUND

Inflammatory macrophages that demonstrate intense autofluorescence have been isolated directly from alveolar and peritoneal tissues, but their generation in vitro remains vague. We use flow cytometry to identify a population of autofluorescent macrophages as they arise among nonadherent populations of cultured blood mononuclear cells.

METHODS

Cells were obtained from donated blood buffy coats and placed in culture for 14 days. Unstained populations from the cells remaining in suspension were sampled daily using flow cytometry. During the first 5 culture days, a distinct population of autofluorescent cells arose and comprised an average of < or =14% of the total cell population. This population declined to less than 6% by culture day 8.

RESULTS

The cells were identified as viable macrophages expressing CD68, lysozyme, and HLA-DR. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) demonstrated a unique cytokine profile with IL-1 alpha expression levels 138-fold higher than those measured in uncultured monocytes. No significant elevation in the levels of other cytokines was identified. Upon replating, the sorted populations became readherent, were able to ingest plastic beads, and remained viable for 6 or more additional weeks in culture without evidence of proliferation or multinucleation.

CONCLUSION

Viable autofluorescent macrophage populations arising among cultured peripheral blood may be easily identified and isolated for further study using flow cytometry. Cytometry 44:38-44, 2001. Published 2001 Wiley-Liss, Inc.

The inhibition of foam cell formation by sodium diethyldithiocarbamate

A prominent feature of human atherosclerosis is the lipid-laden foamy macrophage, which often also contains the insoluble pigment, ceroid. The culture of macrophage-like cells, P388D1s, with artificial lipoproteins composed of cholesteryl linoleate (CL) and bovine serum albumin (BSA) results in foam cell formation with lipoprotein uptake and the intracellular accumulation of ceroid. Ceroid accumulation is accompanied by the oxidation of the cholesterol ester as monitored by gas chromatography. The sodium salt of diethyldithiocarbamic acid (DDC) at 1-5 microM effectively inhibited lipoprotein uptake, cholesteryl linoleate oxidation and ceroid accumulation in cultures of P388D1. Further studies showed that intracellular ceroid accumulation appeared to require the presence of cystine in the medium. Lipoprotein oxidation by this macrophage-like cell therefore appears to involve a mechanism dependent on cystine metabolism which is consistent with previous reports of macrophage-mediated lipoprotein oxidation. Studies on CL/BSA-induced ceroid accumulation in human monocytes also showed that DDC behaved in much the same manner. This inhibitory effect of DDC on foam cell formation, often considered a primary event of atherosclerosis, at concentrations as low as 1 microM, suggests the need for further, more comprehensive, studies on this compound's activities.

Evidence that the death of macrophage foam cells contributes to the lipid core of atheroma

Sections of human atherosclerotic lesions of different stages show that, in early lesions, the acellular lipid core is usually immediately adjacent to the deepest edge of a collection of macrophage foam cells. Advanced lesions with a large lipid core have variable numbers of macrophage foam cells, close to the lateral edges, or shoulders, of the core. In both early and advanced lesions, some of the macrophages nearest the core appear to be dying. Lipid cores contain two materials which in earlier lesions are found only in macrophages, namely ceroid and CD68 antigen, but do not contain recognisable smooth muscle cell actin. It is concluded that death of macrophage foam cells contributes to the origin and slow enlargement of the lipid core. The cause of macrophage death is not yet certain, but is under investigation.

Lipid oxidation, lipoprotein cell-association and ceroid accumulation in P388D1 macrophage-like cells

Flow cytometry can be used to quantify the accumulation of ceroid in macrophages, the result of cellular handling of certain lipoproteins. Using P388D1 cells, a murine-derived macrophage-like cell line, the effect of the lipophilic antioxidant, DL-alpha-tocopherol, upon the uptake and accumulation of ceroid by the cells was monitored on culture with artificial lipoproteins containing a single lipid species. Ceroid accumulation was greater for artificial lipoprotein composed of BSA complexed with cholesteryl arachidonate, than with cholesteryl linoleate. alpha-Tocopherol inhibited the ceroid accumulation, which was also dependent upon cell density. Thus, since these findings are similar to recent observations in primary cultures of murine peritoneal macrophages, it would appear that macrophage-like cell lines such as P388D1 cells are appropriate for the study of potential agonists and antagonists of lipid oxidation. Culture of P388D1 cells with oxidised human low-density lipoprotein (LDL) also resulted in ceroid formation, shown to be dependent upon the level of LDL oxidation as assessed by thiobarbituric acid-reactivity, the xylenol orange assay of peroxides and gas chromatographic analysis of cholesterol and fatty acid content. Ceroid accumulation reflected changes in the level of LDL oxidation better than did the cell association of oxidised radiolabelled LDL, monitored as that bound and retained by the cell.

Glucose oxidation and low-density lipoprotein-induced macrophage ceroid accumulation: possible implications for diabetic atherosclerosis

The exposure of proteins to high concentrations of glucose in vitro is widely considered a relevant model of the functional degeneration of tissue occurring in diabetes mellitus. In particular, the enhanced atherosclerosis in diabetes is often discussed in terms of glycation of low-density lipoprotein (LDL), the non-enzymic attachment of glucose to apolipoprotein amino groups. However, glucose can undergo transition-metal-catalysed oxidation under near-physiological conditions in vitro, producing oxidants that possess a reactivity similar to the hydroxyl radical. These oxidants can fragment protein, hydroxylate benzoic acid and induce lipid peroxidation in human LDL. In this study, glycation of LDL in vitro is accompanied by such oxidative processes. However, the oxidation of LDL varies with glucose concentration in a manner which does not parallel changes in protein glycation. Glycation increases in proportion to glucose concentration, whereas in our studies maximal oxidation occurs at a glucose concentration of approx. 25 mM. The modification of LDL resulting from exposure to glucose alters macrophage ceroid accumulation, a process which occurs in the human atherosclerotic plaque. The accumulation of ceroid in macrophages is shown to be related to LDL oxidation rather than LDL glycation, per se, as it too occurs at a maximum of approx. 25 mM. Oxidative sequelae of protein glycation appear to be a major factor in LDL-macrophage interactions, at least with respect to ceroid accumulation. Our observations are discussed in the context of the observed increase in the severity of atherosclerosis in diabetes.

Measurement of ceroid accumulation in macrophages by flow cytometry

The accumulation in macrophages of ceroid, an autofluorescent polymer composed of oxidised protein and lipid, can be monitored semiquantitatively by staining techniques. However, such methods are crude and give little information about the amount of ceroid within cells. Flow cytometry, however, can give a quantitative assessment of cellular ceroid accumulation in vitro. Recently, flow cytometry was explored as a method for measurement of the accumulation in macrophages of ceroid. The accumulation appeared to be diminished in the presence of the antioxidant, alpha-tocopherol. This is consistent with the role of lipoprotein oxidation in ceroid accumulation. Here the optimum wavelengths of emission and excitation, using both conventional fluorescence spectroscopy of cellular ceroid and flow cytometric measurements with a number of optical filters, are determined. The use of optimal wavelengths determined in these studies enhances overall sensitivity. The findings are discussed in the context of future investigation of cell-mediated lipid oxidation and its potential antagonists.

Differing effects of probucol and vitamin E on the oxidation of lipoproteins, ceroid accumulation and protein uptake by macrophages

Studies using 125I-low density lipoprotein (125I-LDL) show that probucol (10 microM) and alpha-tocopherol (100 microM) inhibit protein degradation in LDL exposed to Cu (II) in vitro. The inhibitory effect of alpha-tocopherol on protein fragmentation exceeded that of probucol. On the other hand, probucol was more able to inhibit lipid peroxidation. The subsequent uptake of Cu (II)-oxidised 125I-LDL by murine peritoneal macrophages (MPM) was virtually unaffected by the presence of probucol during LDL oxidation. The same was not true for alpha-tocopherol which led to lower levels of 125I-LDL uptake by MPM. Thus, it appears that although the antioxidant activity of probucol exceeds that of alpha-tocopherol for lipid oxidation, the reverse is true for protein degradation and, perhaps more significantly, for subsequent macrophage uptake. Further studies used artificial lipoproteins composed of cholesteryl linoleate or cholesteryl arachidonate complexed with bovine serum albumin. Culture of these artificial lipoproteins with MPM resulted in protein uptake, protein degradation, cholesterol oxidation to cholest-5-en-3 beta,7 beta-diol and the intracellular accumulation of ceroid in MPM. The presence of alpha-tocopherol (0-100 microM) inhibited all of these processes. Probucol (0-10 microM) inhibited ceroid accumulation and cholesterol oxidation to the same degree as alpha-tocopherol (0-100 microM) but had no effect upon protein degradation and protein uptake. Control studies of lipoproteins incubated without cells showed that protein degradation by cell-independent processes was also inhibited by alpha-tocopherol, but not by probucol. These observations are discussed in the context of the role of lipoprotein oxidation in atherogenesis.