Enrichment of lymphocytes with cholesterol and its effect on lymphocyte activation

Membrane Cholesterol Depletion Enhances Enzymatic Activity of Cell-Membrane-Coated Metal-Organic-Framework Nanoparticles

Metal-organic-framework nanoparticles (MOF NPs) have been increasingly used to encapsulate therapeutic enzymes for delivery. To better interface these MOF NPs with biological systems, researchers have coated them with natural cell membranes, enabling biomimicking properties suitable for innovative biomedical applications. Herein, we report that the enzymatic activity of cell-membrane-coated MOF NPs can be significantly enhanced by reducing membrane cholesterol content. We demonstrate such cholesterol-enzymatic activity correlation using zeolitic imidazolate framework-8 MOF NPs to encapsulate catalase, horseradish peroxidase, and organophosphate hydrolase, respectively. MOF NPs coated with membranes of human red blood cells or macrophages show similar outcomes, illustrating the broad applicability of this finding. The mechanistic investigation further reveals that reducing cholesterol levels effectively enhances membrane permeability likely responsible for the increased enzymatic activity. These results also imply a facile approach to tailoring the enzymatic activity of cell-membrane-coated MOF NPs by simply tuning the membrane cholesterol level.

Critical Phenomena in Plasma Membrane Organization and Function

Lateral organization in the plane of the plasma membrane is an important driver of biological processes. The past dozen years have seen increasing experimental support for the notion that lipid organization plays an important role in modulating this heterogeneity. Various biophysical mechanisms rooted in the concept of liquid-liquid phase separation have been proposed to explain diverse experimental observations of heterogeneity in model and cell membranes with distinct but overlapping applicability. In this review, we focus on the evidence for and the consequences of the hypothesis that the plasma membrane is poised near an equilibrium miscibility critical point. Critical phenomena explain certain features of the heterogeneity observed in cells and model systems but also go beyond heterogeneity to predict other interesting phenomena, including responses to perturbations in membrane composition.

Cholesterol inhibits glutamine metabolism in LLC WRC256 tumour cells but does not affect it in lymphocytes: possible implications for tumour cell proliferation

The effect of cholesterol on proliferation and glutamine metabolism of lymphocytes and tumour cells was investigated. The addition of cholesterol to the culture medium did not cause a significant effect on [2-(14)C]-thymidine incorporation in lymphocytes. In the presence of concanavalin A, lymphocyte proliferation was increased by cholesterol (from 0.013 up to 1.3 microm). At high concentrations (234 and 468 microm), however, a marked inhibition of lymphocyte proliferation occurred. The same inhibitory effect was observed in the presence of lipopolysaccharides. Cholesterol also caused a marked decrease of LLC WRC256 tumour cell growth at 117 and 234 microm. The same findings were obtained by the measurement of [2-(14)C]-thymidine incorporation in these cells. The effect of cholesterol on phosphate-dependent glutaminase activity was then tested in cultured lymphocytes and LLC WRC256 tumour cells. Cholesterol at concentrations of 117 and 234 microm did not alter this enzyme activity in lymphocytes. However, this sterol, already at 26 microm, caused a 44 per cent reduction in glutaminase activity. Similar to the changes observed for glutaminase, cholesterol reduced glutamine oxidation in LLC WRC256 tumour cells, whereas no effect was observed on lymphocytes. Therefore, cholesterol might control lymphocyte and tumour cells proliferation by different mechanisms. The significance of these findings for the immune function in tumour-bearing patients remains to be investigated.

Liposome-incorporated dexamethasone palmitate inhibits in-vitro lymphocyte response to mitogen

The use of liposomes for the pulmonary delivery of corticosteroid is an area that is under active investigation. We have recently developed a novel liposomal corticosteroid preparation based on the incorporation of dexamethasone palmitate (DMP) within the bilayer of small unilamellar vesicles (SUVs) made of egg yolk phosphatidylcholine (EPC) and cholesterol; molar ratio EPCC:cholesterol: DMP, 4:3:0.3. In the present study, the biological activity of DMP-SUVs was evaluated using the lymphocyte transformation test with peripheral blood mononuclear cells (PBMCs) and a gamma-interferon production assay. Results showed that DMP-SUVs (but not empty SUVs) inhibited [3H]thymidine uptake and gamma-interferon production by concanavalin A-stimulated PBMCs by 94 and 96%, respectively, at a concentration corresponding to 10(-6) M dexamethasone. The inhibition by DMP-SUVs was found to require a 24-h pre-incubation with unstimulated PBMCs, suggesting that interaction of SUVs with lymphocytes may be altered by mitogen stimulation. We conclude that our DMP liposomal preparation is biologically active and may be considered a promising alternative to conventional local glucocorticoid therapy.

Effects of hypercholesterolemia on the nucleotide content in human blood cells

In hypercholesterolemia significant changes in the nucleotide pattern of erythrocytes and lymphocytes as determined by high performance liquid chromatography were found. The decrease in ATP of lymphocytes in hypercholesterolemia from 10.4 +/- 0.3 to 7.0 +/- 0.4 nmol mg-1 protein (n = 8) was associated with an increase in ADP from 2.2 +/- 0.2 to 4.0 +/- 0.2 nmol mg-1 protein (P less than 0.005). The pattern of guanosine phosphates likewise was found to be changed in hypercholesterolemia. Akin to lymphocytes, red blood cells displayed marked changes in nucleotide levels. No such changes were observed in platelets. Cultured lymphocytes incubated with human plasma low density lipoproteins (LDL) (140 mg cholesterol dl-1) displayed a reversible fall in ATP and an increase in ADP by about 40% and 160%, respectively, with high density lipoproteins (HDL) or very low density lipoproteins (VLDL) being essentially ineffectual. It is concluded that in hypercholesterolemia a significant change in the nucleotide pattern of blood cells is exerted by the increase in LDL. Possible pathophysiological implications are discussed.

Mitochondrial and microsomal cholesterol mobilization after oxidative stress induced by adriamycin in rats fed with dietary olive and corn oil

The influence of three different dietary fats (8%) and of endogenous lipid peroxidation with regard to cholesterol concentrations in liver mitochondria and microsomes and in serum has been investigated in the rat. Although the different diet fat used did not produce any effect on serum cholesterol, it was possible to show that each experimental diet differently influenced the microsomal and mitochondrial levels of cholesterol. The highest mitochondrial and microsomal cholesterol content was found in case of diet supplemented with virgin olive oil and the lowest with rectified olive oil. An endogenous oxidative stress induced by adriamycin was able to produce a clear decrease in microsomal and mitochondrial cholesterol level and a sharp increase in serum concentration in all three groups. However, dietary fats and adriamycin had no effect on the microsomal and mitochondrial membrane viscosity as detected by fluorescence polarization. These results are consistent with the hypothesis that mitochondrial and microsomal cholesterol can exchange with exogenous pools when phospholipid peroxidation occurs.

Food restriction slows down age-related changes in cell membrane parameters

The effect of undernutrition on some plasma membrane parameters has been analyzed. Diet restriction was applied to female Wistar rats on every-other-day schedule (EOD), starting from the age of 3.5 months. Membrane microviscosity of splenic lymphocytes was lower in EOD rats than in the ad libitum (AL) fed ones even if one assumes a decrease of body temperature of 2 degrees C. The decrease of membrane microviscosity due to diet restriction ran parallel with the improvement of proliferative response of lymphocytes. The analysis of Arrhenius plots of 1,6-diphenyl-1,3,5-hexatriene as well as of 5'-nucleotidase activity showed a diet-dependent improvement of membrane properties also of liver plasma membranes. Diet restriction was able to partially recover the age-dependent decrease of beta-adrenoceptor density of cerebellar membranes. On the contrary, beta-adrenoceptor density of lymphocytes, which did not show any age-dependent alteration, was not influenced by diet restriction. Present results support that undernutrition exerted a protective effect on cell membranes of old animals and it was able to improve those alterations which are related to aging.

Food restriction in female Wistar rats. I. survival characteristics, membrane microviscosity and proliferative response in lymphocytes

The effect of food restriction on the survival characteristics, membrane microviscosity and proliferative response in lymphocytes of female Wistar undernourished rats has been evaluated. Diet restriction was applied starting from the age of 3.5 months by feeding the animals on an every-other-day schedule (EOD). Diet restricted animals showed an increase of both mean, median and maximal life span as compared to the rats fed ad libitum (AL). Analyzing the survival curves by a parametric model, it emerged that undernutrition increased the individual resistance to environmental insults. In particular, it could be speculated that the positive influence was more pronounced in individuals with the lowest physiological capacities. The membrane microviscosity of lymphocytes was lower in EOD animals as compared to the AL ones even if one assumes a decrease in body temperature of 1-2 degrees C in EOD groups. The improvement of membrane microviscosity due to diet restriction may in part explain the improvement of proliferative response of lymphocytes from EOD groups.

Improvement of natural killer cell activity by in vitro active lipids (AL 721) administration in old mice

Results of several experiments have given rise to the hypothesis that the decline of the immunocompetence with aging is at least in part related to alterations of the lipid membrane composition and, consequently, to a decrease in membrane fluidity. The age-dependent decline of mitogen responsiveness can, in fact, be reversed by a special lipid mixture designated as active lipids (AL 721), which acts by means of its fluidizing action on the plasma membrane. The purpose of this study was to examine the possibility of raising the low endogenous levels of Natural Killer (NK) activity by in vitro AL administration in old mice. When spleen cells from old mice were incubated in vitro with AL, a significant increase in cytotoxic activity was obtained over control cultures, without reaching, however, the levels observed in young mice. In spleen cells from young mice, the AL administration causes a slight augment of NK basal activity. These results suggest that cell membrane fluidity plays an important role in the efficiency of NK cells, giving support to the hypothesis that a rectification of rigidified cell membranes may represent a valuable approach to restore proper physiological functions in old age.

Effects of preliminary culture on the membrane microviscosity of lymphocytes from young and old donors. Microviscosity correlates with mitogenic response

Membrane microviscosity was assessed by a fluorescence polarization technique in fresh and precultured human peripheral blood lymphocytes of young and old subjects. Membrane microviscosity was significantly higher in fresh, non-treated cells of old donors as compared to young adults. Preincubation of cells in culture medium supplemented with pooled human serum diminishes the original microviscosity difference between the age groups. The observed increase in membrane fluidity correlates with the improvement of the mitogen-induced proliferative response due to preculturing cells from aged subjects. The results support the suggestion that membrane microviscosity can affect the proliferative response of lymphocytes, and it may play a role in the decline of the immune responsiveness in the elderly.

Lipid requirements of human T lymphocytes stimulated with mitogen in serum-free medium. Membrane "fluidity" changes are an artefact of lipid (AL721) uptake by monocytes

A correlation has been sought between the effects of lipids on membrane fluidity and mitogen responsiveness of human peripheral blood lymphocytes (PBL). Cholesterol and the reputedly potent membrane fluidizing agent AL721 (1) were used for these studies. However, the large AL721 induced increase in membrane "fluidity", assessed by steady state polarization of the probe 1,6-diphenyl-1,3,5-hexatriene (DPH), was found to be an artefact arising from lipid uptake by monocytes. Mitogen responses were enhanced by AL721 but unaffected by cholesterol. It is concluded that AL721 does not exert its effect through enhanced triggering of cells by altered membrane fluidity but rather that lymphocytes require an exogenous source of phospholipids/triglycerides for optimal growth in vitro, although they can synthesize sufficient cholesterol to meet their own needs.

Augmentation of mitogen responsiveness in the aged by a special lipid diet AL 721

Diminished immune responses in aging may be partially due to alteration of the lipid composition of membrane and a decrease in membrane fluidity. The effect of a daily diet of 10 g active lipid (AL 721) on the mitogen responsiveness of peripheral blood mononuclear cells (PBMC) from aged volunteers was studied. AL 721 is a mixture of lipids from hen egg yolk which was formulated for in vivo rectification of rigidified cell membranes in an attempt to restore proper physiological function. After 3 weeks on the diet, six aged participants displayed a significant increase in lymphocyte responsiveness to mitogens, while in four the responsiveness was unaffected, or increased insignificantly. In all six participants, a decline in lymphocyte responsiveness was observed a week after termination of the diet. Our preliminary results indicate that improvement of immune functions in the aged may be achieved by dietary manipulation of lipids. This innocuous approach seems to be of potential value and merits further studies.

Lymphocyte membrane lipid composition and mitogen responsiveness in chickens: role of membrane "fluidity"

After establishing optimal conditions for measuring the membrane lipid packing density ("fluidity") of chicken peripheral blood lymphocytes, the fluidity was modulated in vitro by incubation in cholesterol or phospholipid ("active lipid", AL)-enriched serum-free tissue culture medium. The effect of these lipids on mitogen responsiveness was then investigated, the aim being to determine whether the observed enhancement/suppression was membrane mediated, i.e. explainable by fluidity changes. Chicken peripheral blood lymphocytes exhibited no requirement for exogenous cholesterol; low concentrations did not affect the mitogen response while the higher concentrations, which induced a measurable decrease in membrane fluidity, were usually mildly suppressive. Pre-incubation did not increase this suppressive effect and we believe it not to be membrane mediated. AL, at low concentrations which induced no changes in membrane fluidity, prolonged the phytohemagglutinin response, enhancement being evident only after the peak; we interpret this as a nutrient effect. At the higher concentrations, which induced large increases in fluidity, a transient enhancement was followed by suppression; suppression was delayed in onset when AL was added 4 h after phytohemagglutinin stimulation. It is therefore an early event which may be mediated through changes in membrane fluidity.

Lack of correlation between serum cholesterol levels, lymphocyte plasma membrane fluidity and mitogen responsiveness in young and aged chickens

Chickens were studied in an attempt to demonstrate correlations between serum lipid levels and peripheral blood lymphocyte (PBL) plasma membrane fluidity and mitogen responsiveness: (a) in the laying hen; (b) during aging; and (c) following dietary manipulation of serum cholesterol of young and aged chickens. The membrane fluidity of PBL from laying hens was significantly greater than that of immature birds. However, no direct correlation was found between serum lipid levels, nor the serum free cholesterol/phospholipid (FC/Pl) mole composition and PBL membrane fluidity in any of the age-groups tested. Likewise, no correlation was found either between serum FC/Pl mole ratio or membrane fluidity and mitogen responsiveness of PBL from birds up to 5 years of age nor was there any evidence for a decline in mitogen responsiveness up to this age. Supplementation of diets with 1% cholesterol induced hypercholesterolemia, mainly in the very low density lipoprotein (VLDL) fraction, but membrane fluidity and mitogen responsiveness remained unaffected.

Interactions of multilamellar phospholipid vesicles with bovine lymphocytes: effects of alpha-tocopherol on lymphocyte blastogenesis

Multilamellar lipid vesicles (MLV) composed of egg yolk lecithin (EYL) suppressed the response of bovine peripheral blood lymphocytes (BPBL) to phytohemagglutinin (PHA). EYL contains 18:1 and 18:2 as the major unsaturated phospholipids. Dioleoyllecithin (DOL; cis 9) MLV did not suppress BPBL blastogenesis. Dilinoeyllecithin (DLL; cis, cis 9, 12) MLV suppressed BPBL blastogenesis. The suppressive effect could be reversed by increasing the MLV DML concentration. The addition of alpha-tocopherol (alpha-T) at 10 mole% into MLV containing DLL reversed blastogenic suppression of BPBL. MLV composed of mixed saturated phospholipids (dimyristoyllecithin and dipalmitoyllecithin) and alpha-T enhanced the BPBL blastogenic response to PHA. BPBL incubated with varying PHA concentrations (11.82-375 mg/ml) and a constant concentration (2 mumoles/ml) of MLV composed of EYL remained suppressed either when PHA and MLV were added simultaneously or when MLV were incubated for 1 hr prior to the addition of PHA. This suggests that alpha-T may act as an immunomodulator in the blastogenic response to PHA. Results suggest that alpha-T reversion of EYL suppression of BPBL blastogenesis may be due to interactions of alpha-T with unsaturated acyl chains in EYL phospholipids.

Inhibition of lymphocyte capping by fatty acids in mouse and man

Lipid-enriched diets have been related to a high cancer incidence in experimental animals for many years, and more recently, to assorted defects on the immune response. We investigated the effect of incubating human or murine (C3H/HEJ) lymphocytes with saturated (16:0) and unsaturated (18:1, 18:2, 18:3, 20:4) fatty acids (12 micrograms for each 10(7) cells), on the ability to cap with antihuman or antimouse anti-IgM, mu-chain specific antibody. Capping was also tested in obese (ob/ob, C57BL/6J) mice. Capping at 30 and 60 min was reduced by fatty acid incubation to 10-30% of control values in humans (p less than .001), and to 30% of control values in mice (p less than .01), regardless of degree of unsaturation. ob/ob capped normally. A lymphocyte membrane effect caused by fatty acids is observed in these experiments. Whether this is related to the dysimmunity caused by lipid diets cannot be assessed from our data, especially since all fatty acids, regardless of unsaturation, reduced the capping phenomenon.

Preparation and properties of spin-labeled lecithin-cholesterol liposomes

Lecithin-cholesterol vesicles of various compositions containing membrane-bound spin-labeled cholestane can be prepared by appropriate choice of initial concentrations of components during sonication. Increasing incorporation of spin label increases incorporation of cholesterol and decreases incorporation of lecithin, with the result that liposomes with cholesterol-lecithin molar ratios larger than 2 can be obtained. Besides associating with cholesterol-lecithin complexes in the liposome, the spin label seems to associate with cholesterol. Changes of the paramagnetic resonance spectrum of the liposome-bound spin label due to changes in liposomal cholesterol and spin label mole fractions - assessed by three parameters - can be used in cell-liposome interaction studies.

Membrane fluidity and cholesterol in thymus and spleen cells from mice treated with immunomodulatory drugs

We have used spin labeling, fluorescence polarization, and chemical analysis to characterize membrane properties of thymocytes from mice treated with immunomodulatory drugs. The number of thymocytes was reduced 90-95% by treatment of 6-9 week old mice with hydrocortisone acetate (HCA) or methylprednisolone (both 125 mg/kg) or with cyclophosphamide (250 mg/kg). Electron spin resonance (esr) examination of thymocytes labeled with 5-nitroxyl stearic acid indicated that the membranes of cells remaining after treatment with any of these drugs were more rigid than those from saline-treated controls. The total cholesterol/phospholipid (C/PL) molar ratio of the HCA-resistant thymocytes was twice that of the control mice. Treatment of mice with other immunomodulatory drugs, cyclophosphamide, cytosine arabinoside (Ara-C), 2-amino-5-bromo-6-phenyl-4-(3H)-pyrimidinone (ABPP) and 15(S)-methyl prostaglandin E1 (15(S)-methyl PGE1), also altered the C/PL ratio in thymocytes and, in some cases, in spleen cells. Fluorescence polarization measurements of thymocytes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) did not reveal the differences between cells from HCA-and saline-treated mice that were detected by spin labeling and chemical analysis. Our results indicate that the greater rigidity detected by spin labeling of hydrocortisone-resistant thymocytes may be due, at least in part, to greater membrane cholesterol content. Of the methods employed, chemical analysis was the most sensitive in revealing drug-induced alterations in thymocyte populations.

Peculiar behaviour of rabbit thymocytes in interaction with liposomes of different compositions shown by fluorescence polarization studies, lipid analysis, and uptake of vesicle-entrapped carboxyfluorescein

In order to obtain more information on membrane phenomena occurring at the cell surface of rabbit thymocytes we have performed experiments aimed at altering the lipid composition of the plasma membrane. Thymocytes were incubated at 37 degree C with phospholipid vesicles of different compositions. Vesicle-cell interaction was followed by measuring the degree of fluorescence polarization and the uptake of vesicle-entrapped carboxyfluorescein. Neutral and negatively charged liposomes prepared from egg phosphatidylcholine are currently used in investigations of vesicle-cell interaction. In this report we show that these liposomes do not interact with rabbit thymocytes as is evident from unaltered lipid fluidity measured in whole cells and in isolated plasma membranes. This was confirmed by experiments with vesicle-entrapped carboxyfluorescein showing hardly any uptake of the fluorophor from neutral and negatively charged egg phosphatidylcholine liposomes. Using both techniques substantial interaction was found with positively charged egg phosphatidylcholine liposomes and with liposomes prepared from soybean lecithin which is composed of a variety of phospholipids. The results of these experiments were supported by lipid analysis of cells treated with soybean lecithin liposomes. Increase in phosphatidylcholine contents of mixed phospholipid vesicles was further shown to result in decreased vesicle-cell interaction. From measurements of the quantity of carboxyfluorescein inside cells and the total amount of cell-associated carboxyfluorescein it is concluded that adsorption plays a prominent role in interaction between liposomes and rabbit lymphocytes. The grade of maturation of lymphocytes was also found to affect vesicle-cell interaction. The more mature thymocytes took up more vesicle-entrapped carboxyfluorescein from soybean liposomes than immature thymocytes. Mesenteric lymph node cells exhibited a still stronger interaction. The role of vesicle and cell surface charge and membrane fluidity of both vesicles and cells in interaction between liposomes and rabbit thymocytes is discussed.

Effects of mitogen and liposome phospholipid concentration on the blastogenic response of bovine lymphocytes to phytohemagglutinin

Lymphocytes incubated with liposomes prior to the addition of phytohemagglutinin (PHA) exhibited a time dependent suppression of blastogenesis which was reversible for phosphatidylcholine (PC):cholesterol (Chol) alpha-tocopherol (alpha-T) (1:0.5:0.5), and PC: alpha-T (1:1) liposomes but not for PC and PC:Chol (1:1) liposomes. Incubation of PHA with lymphocytes prior to the addition of liposomes caused a time dependent, reversible suppression of blastogenesis for PC: alpha-T:Chol (1:0.5:0.5), PC: alpha-T (1:1) and PC:Chol (1:1) liposomes but not reversible for PC liposomes. The effects of phospholipid concentration on the blastogenic response to PHA exhibited a concentration dependent suppression from 1 to 2 mumoles phospholipid/ml which was reversible, to varying degrees, at a concentration of 4 mumoles phospholipid/ml.

Effects of enrichment of phosphatidylcholine liposomes with cholesterol or alpha-tocopherol on the response of lymphocytes to phytohemagglutinin

Studies were performed to determine the ability of alpha-tocopherol and cholesterol to influence the effect of phosphatidylcholine (PC) liposomes on the blastogenic response of bovine peripheral blood lymphocytes (BPBL) to phytohemagglutinin (PHA). BPBL were cultured with liposomes having a molar ratio of cholesterol to PC (C/P) ranging from 0 to 2.0, a molar ratio of alpha-tocopherol to PC (E/P) of 1.0 and a molar ratio of cholesterol + alpha-tocopherol to PC [(C + E)/P] of 2.0 and 4.0 PC liposomes significantly suppressed BPBL blastogenic response to PHA. Cholesterol-rich (C/P greater than or equal to 1.0) liposomes, alpha-tocopherol-rich (E/P = 1.0) liposomes and liposomes rich in cholesterol and alpha-tocopherol [(C + E)/P greater than or equal to 2.0] were able to completely reverse PC liposome suppression of BPBL. There was no molar ratio [C/P, E/P or (C + E)/P] that was able to enhance the blastogenic response of BPBL above the response obtained with PHA alone. The results suggest that the augmentation of PC liposomes rich in cholesterol, alpha-tocopherol and cholesterol with alpha-tocopherol (C/P and E/P greater than or equal to 1.0) was equally capable of restoring normal responses in BPBL but did not enhance or suppress the response to PHA.

Modulation of concanavalin A-induced lymphocyte stimulation by human low-density lipoproteins

Spleen lymphocytes and T cells were stimulated by concanavalin A in the presence of various concentrations of human low-density lipoproteins (LDL). The proliferative responses were measured by [14C]thymidine incorporation. Low LDL doses (5-50 microgram/ml) significantly enhanced the stimulation of splenic lymphocytes and T cells. Further, LDL had the capacity to partially relieve the suppression produced by supraoptimal doses of concanavalin A.

Correlations between membrane viscosity, serum cholesterol, lymphocyte activation and aging in man

Membrane microviscosity, serum cholesterol and lymphocyte stimulation in man were examined in relation to aging. A direct correlation was found between the mole ratio of cholesterol to phospholipids in the serum and the membrane microviscosity of the peripheral blood lymphocyte membranes, both of which increase progressively with age. The increase in membrane microviscosity with age is also concomitant with the decrease in response of the lymphocytes to concanavalin A stimulation. The data suggest that increase in lymphocyte membrane microviscosity is a prominent factor which may lead to the overt reduction of proliferative responses of lymphocytes with aging.

Alterations of the carrier-mediated transport of an anionic solute, methotrexate, by charged liposomes in Ehrlich ascites tumor cells

Interaction of positively charged liposomes with Ehrlich ascites tumor cells increases the bidirectional transmembrane fluxes of the anionic folic acid analog, methotrexate. Negative liposomes reduce methotrexate influx. Stimulation of methotrexate influx by positively charged liposomes is time and concentration dependent, requiring at least a 5-min incubation with 2.5 mM phosphatidylcholine containing 20% stearylamine for maximum effect. Stimulation is not appreciably reversed by washing the cells. Similar increases are observed for influx and efflux so that there is no change in the steady-state methotrexate electrochemical-potential difference across the cell membrane. The increase influx appears to be a stimulation of the carrier-mediated transport process for methotrexate since both control and stimulated influx are abolished by the competitive inhibitor, 5-formyltetrahydrofolate or the sulfhydryl group inhibitor, p-chloromercuriphenylsulfonic acid and the Q10 of the system remains unchanged. Influx of 5-methyltetrahydrofolate, which shares the same transport carrier as methotrexate, is also stimulated. However, the transport of folic acid, which is structurally similar to methotrexate but does not utilize the carrier, is unaffected. The kinetic change induced by positively charged liposomes is an increase in the Vma in, while the Kt in remains unchanged. Trans-stimulation of methotrexate influx by 5-formyltetrahydrofolate occurs to the same extent in the presence or absence of positively charged liposomes. The liposomes have no apparent effect on the intracellular water, the extracellular space, or the chloride distribution ratio. The data suggest that interaction of positively charged liposomes with Ehrlich ascites tumor cells accelerates the rate of transposition of the membrane carrier system for methotrexate, altering the kinetics of transport without a change in transport thermodynamics.

Viscosity of lymphocyte plasma membrane in aging mice and its possible relation to serum cholesterol

Lymphocyte membrane fluidity was examined in aged mice and characterized as a qualitative and effective change which takes place in the aging process. Fluorescence polarization of diphenylhexatriene-labelled mouse spleen cells is substantially higher in cells from old mice (20--36 months) than young mice (2--7 months). A similar difference was also observed with isolated plasma membranes from spleen cells of old and young mice. The overall estimate is that the lipid microviscosity in the lymphocyte plasma membrane from old mice is about 20% higher than that of young mice. The cholesterol/phospholipid ratio determined for the isolated plasma membrane preparations was 0.68 and 0.9, respectively, which is probably the main cause for the difference in membrane viscosity. An elevated cholesterol/phospholipid ratio was also observed in the blood serum of old mice. It is plausible that the source of excess membrane cholesterol in the old mouse lymphocytes originates in the high serum cholesterol.

Do plasma glycoproteins induce lymphocyte hyporesponsiveness and insulin resistance?

Depression of lymphocyte transformation and an increase in insulin resistance are common to pregnancy, oral contraceptive use, widespread malignancy, infection, and tissue destruction. We suggest that these abnormalities are caused by a rise in the plasma-glycoprotein level which is also common to these clinical states. There is evidence that glycoproteins can inhibit cell division, lymphocyte transformation, and the action of hormones on target cells. Because of the increase in plasma glycoprotein the cells in many organs and their hormone receptors may have a thicker coating of glycoproteins which blunts their response to variuos stimuli.

Peturbation of lymphocyte response to concanavalin A by exogenous cholesterol and lecithin

Incubation of lymphocytes with lecithin liposomes enriched with cholesterol, elevated the cholesterol level of the cells relative to phospholipids. Treatment of lymphocytes with pure lecithin liposomes resulted in the converse effect. Both these treatments resulted in suppression of the induction phase of the response to concanavalin A and were practically reversible. It is suggested that these changes induce modulations of the fluidity of the lymphocyte membrane which may also take place in vivo by serum lipoproteins. Based on this study, the possible effects of lipids on lymphocyte activation are discussed.

Interactions of phospholipid vesicles with murine lymphocytes. II. Correlation between altered surface properties and enhanced proliferative response

The effect of unilamellar lipid vesicles composed of dioleoyl lecithin (DOL), egg yolk lecithin (EYL), 1:1 EYL:cholesterol (Chol), dipalmitoyl lecithin (DPL), and dimyristoyl lecithin (DML) on the mitogenic response in mouse lymphocytes was tested. Cortisone-resistant thymocytes were briefly treated with lipid vesicles and subsequently stimulated with concanavalin A (con A). All of the lipid vesicles induced an enhanced mitogenic response on day 3 as tested by [3H]TdR incorporation and by counting total cells. The order of enchanced [3H]TdR incorporation (less than or equal to 5.3 times the control) was DML greater than DPL greater than 1:1 EYL:Chol greater than EYL congruent to DOL greater than untreated control cells. These increases were paralleled by increased numbers of total cells. The response of spleen cells to a B-cell mitogen, bacterial lipopolysaccharide, was similarly enhanced by vesicle pretreatments in the same order. Vesicle treatments alone were not mitogenic Pretreatment of cells with lipid vesicles modified lectin binding: DML and DPL increased the binding of [125I]con A by three to four times the control, whereas 1:1 EYL:Chol, EYL, or DOL had little or no effect. The binding of [125I]phytohemagglutinin-P (PHA-P) to vesicle-treated cells was indistinguishable from untreated cells. The lectin (con A; PHA-P)-induced agglutination of vesicle-treated cells was also modified by different lipid vesicles in the same order as the mitogenic response. Based on the results presented in the accompanying report [6], we find that the cell surface adsorption properties of the applied lipid vesicles correlate with their ability to enhance the mitogenic response, and that they modify agglutinability and lectin binding. These results are further discussed in terms of the possible alteration of membrane properties and subsequent cellular activity.

Interactions of phospholipid vesicles with murine lymphocytes. I. Vesicle-cell adsorption and fusion as alternate pathways of uptake

The interactions of mouse thymocytes with unilamellar phospholipid vesicles comprised of dimyristoyl lecithin (DML), dipalmitoyl (DPL), dioleoyl lecithin (DOL), and egg yolk lecithin (EYL) were examined in vitro. In cells treated with [3H]DML or [3H]DPL vesicles, electron croscope (EM) autoradiographic analysis showed most of the radioactive lipids to be confined to the cell surface. Transmission EM studies showed the presence of intact vesicles (DPL) and collapsed or ruptured vesicle fragments (DML) adsorbed to the surfaces of treated cells. In cells treated with DPL vesicles containing a water-soluble dye (6-carboxyfluorescein; 6-CF), most of the fluorescent vesicles were localized at the periphery of the treated cells. Furthermore, substantial fractions of the cell-associated DPL and DML could be released by a mild trypsinization without damaging the cells. These results suggest that the uptake of DML and DPL is primarily due to vesicle-cell adsorption. Such an adsorption process appears to be enhanced at or below the thermotropic-phase transition temperature of the vesicle lipid. Under certain conditions these adherent vesicles also formed patches or caps on the cell surface. In cells treated with DOL or EYL vesicles, transmission EM and EM autoradiography showed relatively little exogenous vesicle lipid located at the cell surface. Thymocytes incubated (37 degrees C) with [14C] EYL vesicles containing a trapped marker, [3H]inulin, incorporated both isotopes at identical rates. In separate experiments it was found that this marker was located inside the treated cells. Thymocytes treated with DOL vesicles containing 6-CF exhibited a uniform and diffuse distribution of dye in the internal volume of the cells. Little cell-associated EYL or DOL could be released by trypsinization. Evidence against endocytosis of intact vesicles as a major pathway of vesicle uptake is also presented. These observations, coupled with the demonstration of vesicle-cell lipid exchange as a minor component of vesicle uptake suggest that incorporation of EYL and DOL vesicles by thymocytes is primarily by vesicle-cell fusion.

Metabolic immunodepression which increases the risk of cancer

When some metabolic changes occur--namely, when blood levels of free fatty acids (F.F.A.), insulin, cholesterol, or triglycerides rise--cellular immunity becomes depressed. These metabolic changes also promote the division of somatic, non-lymphoid cells, which in turn increases the possibility of malignant transformation. The diametrically opposite effects of these metabolic changes on non-lymphoid cells, on the one hand, and on T-dependent lymphocytes and macrophages on the other, increase an individual's susceptibility to cancer. This normally occurs with pregnancy, rapid growth, stress, and ageing. Many chemical carcinogens raise the hypothalamic threshold to feedback control mechanisms, decrease glucose tolerance, and increase blood-insulin--i.e., all the main changes of "cancer-proneness". Changes in biogenic amine level in the hypothalamus during ageing and stress and after an exposure to some carcinogens are responsible for the elevation of hypothalamic threshold to feedback control.

The immunoregulatory role of cholesterol and other lipids: a hypothesis

Membrane lipids play an important role in cellular responses to exogenous signals. In immunocompetent lymphocytes, marked changes in the concentrations of membrane lipids occur following cell-antigen interaction. These changes lead to an increase in membrane fluidity, thus facilitating the microaggregation of receptor-antigen complexes. This event constitutes the inductive signal for lymphocytes. Lipid profile alterations leading to increased concentration of membrane cholesterol, of polyunsaturated lipids, or of both, bring about a decrease in membrane fluidity. The latter interferes with receptor displacement preventing delivery of an inductive signal to the responding cell. Interference with microaggregation is readily brought about in interactions involving low affinity antigens, such as tumoral antigens. We postulate that in hyperlipidemic and hypercholesterolemic states there is decreased immune responsiveness to weak antigens due to the aformentioned lipid profile alterations in the membranes of immunocompetent cells. The manner in which an increase in the concentration of the lipids mentioned can lead to decreased immune responsiveness and hence to an increased incidence of malignancies in hyperlipidemic and hypercholesterolemic states is the hypothesis presented in this paper.