Characterization of a monoclonal antibody that binds equally to all apolipoprotein and lipoprotein forms of human plasma apolipoprotein B. I. Specificity and binding studies

Human plasma trans-sialidase causes atherogenic modification of low density lipoprotein

In earlier studies we have found that incubation of low density lipoprotein (LDL) with autologous blood plasma-derived serum leads to a loss of sialic acid from lipoprotein particles. In this study we demonstrated that sialic acid removed from LDL was transferred to glycoconjugates of lipoproteins, glycoproteins and sphingolipids of human serum. This showed that human serum contained the trans-sialidase activity. Gel-filtration chromatography of human blood serum demonstrated the presence of trans-sialidase activity in lipoprotein subfractions as well as in lipoprotein-deficient serum. Trans-sialidase (about 65 kDa) was isolated from lipoprotein-deficient serum using affinity chromatography carried out with Neu5Acalpha2-8Neu5Ac-sepharose FF-6. Optimal pH values for the trans-sialidase were 3.0, 5.0 and 7.0. Calcium and magnesium ions stimulated the enzyme activity at millimolar concentrations. Isolated enzyme can remove sialic acid from LDL, IDL, VLDL, and HDL particles (in decreasing rate order). Serum trans-sialidase transferred sialic acid from glycoconjugates of plasma proteins (fetuin, transferrin) and gangliosides (GM3, GD3, GM1, GD1a, GD1b). Sialylated glycoconjugates of human blood erythrocytes also served as substrate for serum trans-sialidase. We have found that sialic acid can also be removed from N- and O-linked glycans, sialylated Le(x) and Le(a), oligosialic acids, and sphingolipid carbohydrate chains. The rate of sialic acid release decreased in the following order: alpha2,6>alpha2,3>>alpha2,8. Transferred molecule of sialic acid can form alpha2,6, alpha2,3 and to a lesser degree alpha2,8 linkage with galactose, N-acetyl-galactosamine or sialic acid of acceptors. The glycoconjugates of erythrocytes, lipoprotein particles, plasma proteins, neutral sphingolipids and gangliosides may serve as acceptors of transferred sialic acid. Trans-sialidase-treated native LDL becomes desialylated and then can induce cholesteryl ester accumulation in human aortic intimal smooth muscle cells. Thus, trans-sialidase may be involved in the early stages of atherogenesis characterized by foam cell formation.

Monoclonal antiidiotypic antibodies neutralize cytotoxic effects of anti-alphaGal antibodies

The aims of this study were to produce and characterize mouse monoclonal antiidiotypic antibodies (AIAs) that specifically bind human antipig (anti-alphaGal) antibodies and to select those AIAs that neutralize the cytotoxicity of human or baboon serum to pig (PK15) cells. Mice were immunized with human anti-pig antibodies, and hybridomas were produced using conventional techniques. From a total of 480 clones, 11 produced AIAs that bound with high affinity to human anti-alpha Gal IgG and F(ab')2 fragments, and individually reduced serum cytotoxicity to pig cells by 40 - 90%. Seven of the AIAs also bound to human peripheral B lymphocytes (that express the same idiotypes as the antibodies produce). Several combinations of two or three AIAs provided 100% protection of PK15 cells. Selected AIAs injected intravenously into baboons reduced the cytotoxicity of subsequently drawn sera by 50--80% for >48 hr. The cytotoxicity studies also indicated that there are at least two dominant idiotypes expressed in the human anti-alphaGal population. We conclude that AIAs against anti-alphaGal antibodies could be successfully used in 1) preoperative characterization of a recipient's anti-alphaGal profile, (2) prevention of hyperacute rejection of a pig organ, and (3) specific immunosuppression through elimination of anti-alphaGal-producing B lymphocytes.

Characteristics of low density lipoprotein isolated from circulating immune complexes

Circulating immune complexes (CIC) containing low density lipoprotein (LDL) were recently found in the blood of patients with coronary atherosclerosis. In the present study, we investigated the chemical composition and physical characteristics of the lipoprotein constituents of these CIC. CIC were isolated from the blood of atherosclerotic patients by affinity chromatography using anti-human immunoglobulin G-agarose. Low density lipoprotein of these complexes (CIC-LDL) was obtained by ultracentrifugation. CIC-LDL was compared with free circulating LDL isolated from the blood plasma of the same patients. Plasma LDL was fractionated by lectin-chromatography on RCA120-agarose to obtain desialylated LDL (atherogenic) and sialylated LDL (nonatherogenic). Both CIC-LDL and desialylated LDL, but not native (sialylated) lipoprotein, induced a 1.8- to 3-fold increase in the intracellular contents of free and esterified cholesterol of cells cultured from grossly normal areas of human aorta. The sialic acid level in CIC-LDL was 1.3- and 2.1-fold lower than in desialylated or native LDL, respectively. The neutral lipid and phospholipid contents of CIC-LDL and desialylated LDL were reduced as compared to native LDL. The levels of lipid-oxidation products, thiobarbituric acid-reactive substances and hydroperoxides, were similar in all lipoprotein preparations. However, desialylated LDL and CIC-LDL had an elevated oxysterol content. Gradient ultracentrifugation revealed that CIC-LDL particles had a higher density than native LDL. The mean diameters of native, desialylated and CIC-LDL accounted for 24.0, 21.3 and 19.5 nm, respectively. Like desialylated LDL, CIC-LDL displayed a higher electrophoretic mobility compared with that of native LDL. Thus, LDL obtained from circulating immune complexes appears to be a multiple-modified lipoprotein possessing many similarities to desialylated LDL. It was also found that the LDL content of circulating immune complexes correlates well with the desialylated LDL level in human plasma but not with the total LDL concentration. We believe that desialylated LDL predominately interacts with antibodies forming immune complexes. Taken together, our findings suggest that multiple-modified desialylated LDL is the circulating autoantigen for anti-LDL autoantibodies.

Chromatographic techniques for the isolation and purification of lipoproteins

Various modes of chromatography are available for lipoprotein separation. Gel permeation and affinity chromatography are used for preparative purposes and to separate lipoproteins according to size and apolipoprotein content, respectively. Development of rigid supports for gel permeation has led to large improvements in speed and resolution. Reversed-phase high-performance liquid chromatography (HPLC) of apolipoproteins offers the best performance in terms of speed and resolution of structural variants. Due to its high speed and superior resolving power, the recently developed technique of capillary electrophoresis should emerge as an important method for lipoprotein analysis.

Cholesterol in remnant-like lipoproteins in human serum using monoclonal anti apo B-100 and anti apo A-I immunoaffinity mixed gels

We have developed a simple, rapid assay method for apo E-rich lipoproteins (d < 1.006 g/ml), using an immunoaffinity gel mixture of anti apo B-100 and apo A-I antibodies coupled to Sepharose 4B. The immunoaffinity mixed gels adsorb normal lipoproteins containing apo A-I quantitatively as well as most lipoproteins containing apo B-100. Unbound lipoproteins are quantified by assay of cholesterol. Characterization of the unbound lipoproteins of d < 1.006 g/ml (J Lipid Res 1992; 33: 369-380) has shown that they represent chylomicron and VLDL remnant-like particles (RLP). RLP-Cholesterol(C) levels in plasma have been determined in 363 male and female normolipidemic subjects (mean +/- S.D.: 72 +/- 16 mg/l) and have been found to be higher in patients with coronary heart disease and familial dysbetalipoproteinemia. Triglyceride-rich lipoproteins may well contain both atherogenic and non-atherogenic particles that can be separated by this simple immunoadsorption assay.

Methylamine-treated low density lipoproteins elicit different responses in HepG2 cells and macrophages

Recent results from this laboratory have demonstrated the existence of labile thiolester bonds in apolipoprotein B (ApoB). Thiolester bonds can be cleaved with nucleophiles such as methylamine, resulting in conformational change. The purpose of this study was to explore whether the cellular interactions would be altered after methylamine treatment of low density lipoproteins (LDL). Human hepatoma cells, HepG2, and human monocyte derived macrophages were used for these studies. Fresh LDL were incubated with methylamine under mild alkaline conditions under N2 and with preservatives for 24 h. The methylamine-treated LDL showed particle size and net charge identical to fresh native LDL. In addition, no oxidative modification of LDL occurred under the experimental conditions. The methylamine-treated LDL were indistinguishable from native LDL in HepG2 cells as judged by binding, degradation, cholesterol accumulation and de novo sterol synthesis. However, methylamine-treated LDL caused an increased accumulation of cholesteryl esters in macrophages which was comparable to the accumulation caused by acetylated LDL. Dual color digital imaging fluorescence microscopy revealed no competition between acetylated and methylamine-treated LDL, suggesting that the excessive uptake of methylamine-treated LDL was not mediated by the 'scavenger' receptor. The increased accumulation of cholesteryl ester in macrophages also did not appear to stem from the classical LDL receptor. These results suggest that a new receptor binding domain is exposed due to the conformational change upon treatment of LDL with methylamine.

Apolipoprotein A-I and apolipoprotein B containing lipoprotein particles in coronary patients treated with extracorporal low density lipoprotein precipitation (HELP)

Evidence for chemical and biological heterogeneity of human plasma lipoprotein density classes has been steadily accumulating over the last 15 years. Furthermore, several recent reports have indicated potential clinical significance of certain lipoprotein subspecies as either atherogenic or antiatherogenic. It is generally accepted that lipid lowering treatments can retard or even reverse development of atherosclerotic lesions. However, very little is known about effects of various lipid lowering treatments on specific lipoprotein particles. The purpose of this study was to explore the effects of heparin induced extracorporal low density lipoprotein precipitation (HELP) on various subspecies of plasma lipoprotein particles defined primarily by their apolipoprotein composition. Using particle specific enzyme immunoassays, the immediate changes in lipoprotein particle profiles were analyzed after a single HELP treatment in 12 patients with angiographically documented coronary artery disease. In a separate group of 6 patients, particles were repeatedly measured over a period of 96 h following a HELP treatment. Single HELP treatment caused an immediate and highly significant decrease (67%) in the concentration of simple lipoprotein particles containing apolipoprotein B (apo B) as a sole apolipoprotein (LP-B). Various subspecies of complex particles containing apo B and other apolipoproteins (Lp-B-complex) were also decreased although to a lesser degree (44-53%). HELP treatment caused an insignificant, 3% decrease of lipoprotein particles containing apo A-I but no apo A-II (Lp-A-I) and a 6% decrease in the concentration of particles containing both apo A-I and apo A-II (Lp-A-I:A-II). During the 96-h period following HELP treatment various apo B containing particles recovered at different rates in different patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and partial characterization of lipoprotein A-II (LP-A-II) particles of human plasma

High density lipoproteins (HDL) consist of a mixture of chemically and functionally distinct families of particles defined by their characteristic apolipoprotein (Apo) composition. The two major lipoprotein families are lipoprotein A-I (LP-A-I) and lipoprotein A-I:A-II (LP-A-I:A-II). This study describes the isolation of a third minor HDL family of particles referred to as lipoprotein A-II (LP-A-II) because it lacks ApoA-I and contains ApoA-II as its main or sole apolipoprotein constituent. Because ApoA-II is an integral protein constituent of three distinct lipoprotein families (LP-A-I:A-II, LP-A-II: B:C:D:E and LP-A-II), LP-A-II particles were isolated from whole plasma by sequential immunoaffinity chromatography on immunosorbers with antisera to ApoA-II, ApoB and ApoA-I, respectively. In normolipidemic subjects, the concentration of LP-A-II particles, based on ApoA-II content, is 4-18 mg/dl accounting for 5-20% of the total ApoA-II not associated with ApoB-containing lipoproteins. The lipid composition of LP-A-II particles is characterized by low percentage of triglycerides and cholesterol esters and a high percentage of phospholipids in comparison with lipid composition of LP-A-I and LP-A-II: A-II. The major part of LP-A-II particles contain ApoA-II as the sole apolipoprotein constituent; however, small subsets of LP-A-II particles may also contain ApoD and other minor apolipoproteins. The lipid/protein ratio of LP-A-II is higher than those of LP-A-I and LP-A-I:A-II. In homozygous ApoA-I and ApoA-I/ApoC-III deficiencies, LP-A-II particles are the only ApoA-containing high density lipoprotein with levels found to be within the same range (7-13 mg/dl) as those of normolipidemic subjects. However, in contrast to normal LP-A-II, their lipid composition is characterized by higher percentages of triglycerides and cholesterol esters and a lower percentage of phospholipids and their apolipoprotein composition by the presence of ApoC-peptides and ApoE in addition to ApoA-II and ApoD. These results show that LP-A-II particles are a minor HDL family and suggest that, in the absence of ApoA-I-containing lipoproteins, they become an efficient acceptor/donor of ApoC-peptides and ApoE required for a normal metabolism of triglyceride-rich lipoproteins. Their other possible functional roles in lipid transport remain to be established in future experiments.

Low density lipoprotein-containing circulating immune complexes and coronary atherosclerosis

Blood serum of most patients with coronary heart disease (CHD) caused a 2- to 5-fold increase in the lipid content of smooth muscle cells cultured from unaffected human aortic intima, i.e., possessed an atherogenic potential manifested in culture. Treatment of the CHD patients' serum with 2.5% polyethylene glycol 6000 removed the circulating immune complexes. The serum subjected to this treatment lost its atherogenic properties, i.e., failed to increase the content of lipids in cultured cells. Incubation of smooth muscle cells derived from human aortic intima with circulating immune complexes isolated from an atherogenic patient's serum caused a 1.5- to 3-fold rise in the intracellular cholesterol. Circulating immune complexes contained apolipoprotein B (apo B), but not apolipoproteins A1 and E. The apo B content strongly correlated with the total cholesterol content. The cholesterol/apo B ratio of the complexes was characteristic of low density lipoproteins (LDL), but not of very low density lipoproteins or intermediate density lipoproteins. The composition of the main lipid classes in these complexes was similar to that in LDL. Blood sera of most (90%) CHD patients was characterized by a high cholesterol and apolipoprotein B content in circulating immune complexes. The ability of these sera to induce lipid accumulation in cultured cells directly correlated with the cholesterol and apolipoprotein B level of circulating immune complexes (r = 0.91). These findings suggest that the atherogenic potential of CHD patients' blood serum is due to LDL-containing immune complexes.

Identification and partial characterization of discrete apolipoprotein A-containing lipoprotein particles secreted by human hepatoma cell line HepG2

The purpose of this study was to identify the apolipoprotein A-containing lipoprotein particles produced by HepG2 cells. The apolipoprotein A-containing lipoproteins separated from apolipoprotein B-containing lipoproteins by affinity chromatography of culture medium on concanavalin A were fractionated on an immunosorber with monoclonal antibodies to apolipoprotein A-II. The retained fraction contained apolipoproteins A-I, A-II and E, while the unretained fraction contained apolipoproteins A-I and E. Both fractions were characterized by free cholesterol as the major and triglycerides and cholesterol esters as the minor neutral lipids. Further chromatography of both fractions on an immunosorber with monoclonal antibodies to apolipoprotein A-I showed that 1) apolipoprotein A-II only occurs in association with apolipoprotein A-I, 2) apolipoprotein A-IV is only present as part of a separate lipoprotein family (lipoprotein A-IV), and 3) apolipoprotein E-enriched lipoprotein A-I:A-II and lipoprotein A-I are the main apolipoprotein A-containing lipoproteins secreted by HepG2 cells.

Monoclonal antibodies to human A-I apolipoprotein and characterisation of cyanogen bromide fragments of apoA-I

Several monoclonal antibodies to human A-I apolipoprotein were produced after immunising mice with pure delipidated apoA-I. These monoclonal antibodies were characterised for their ability to react with whole lipoproteins, apolipoproteins and fragments of apoA-I generated by cleavage with cyanogen bromide. The data suggest that production of monoclonal antibodies using apoA-I as antigen was influenced by two major epitopes subsequently localised to cyanogen bromide fragments 1 and 3, and have been designated antibodies 1----5 A-IB and 6----10 A-IB, respectively. Cyanogen bromide fragments were first purified to homogeneity before screening by competitive displacement or immunoblotting procedures. Definitive characterisation of one antibody series (1----5 A-IB) depended ultimately on Western blotting following isoelectric focusing of purified apoA-I fragments. This technique identified the epitope for these antibodies to fragment 1, an identification not fully concluded from competitive displacement studies. These studies have also revealed the presence of microheterogeneity in fragment 1 (as well as in fragment 4) of apoA-I, suggesting that structural variations in several regions may account for the polymorphism observed in this apolipoprotein.

Identification of Escherichia coli DNA helicase IV with the use of a DNA helicase activity gel

A DNA helicase activity gel was developed based on the assumption that DNA helicases could unwind double-stranded DNA in a polyacrylamide matrix. The production of single-stranded DNA was detected by staining the activity gel with acridine orange and visualizing the gel under long-wave UV light. The products of DNA helicase activities appeared as red bands within a green fluorescent background. A novel DNA helicase, called helicase IV, was detected in crude extracts of Escherichia coli with the use of the helicases activity gel assay. The new DNA helicase was purified to near homogeneity. The chromatographic properties and the sequence of its 11 amino-terminal residues proved that helicase IV was distinct from all of the previously described DNA helicases from E. coli.

Characterization of a monoclonal antibody that binds equally to all apolipoprotein and lipoprotein forms of human plasma apolipoprotein B. II. Isolation of apolipoprotein B-containing lipoproteins from human plasma

Monoclonal antibody ('Pan B' antibody) that binds equally to all major forms of human plasma apolipoprotein B was used in an immunoaffinity chromatography procedure to isolate apolipoprotein B-containing lipoproteins from hyperlipidemic human plasma. These lipoproteins were compared with lipoproteins in native plasma, with lipoproteins isolated by polyclonal antibodies and with lipoproteins isolated by the conventional ultracentrifugational method. Judged by the apolipoprotein and lipid composition, lipoproteins isolated with 'Pan B' antibody were virtually identical to those isolated by ultracentrifugation or polyclonal antibodies. Lipoproteins isolated by 'Pan B' antibody were comparable in size and shape to the lipoproteins in native plasma and to the lipoproteins isolated by polyclonal antibodies or ultracentrifugation. The immunoaffinity column with monoclonal 'Pan B' antibody retained all apolipoprotein B-containing lipoproteins and showed significantly higher capacity than polyclonal immunoaffinity column. The column with the highest capacity allowed the isolation from whole plasma of 0.144 mg of apolipoprotein B per ml of gel in less than 2 h.