Antigenic mapping of human low density lipoprotein with monoclonal antibodies

Low density lipoprotein-containing circulating immune complexes: role in atherosclerosis and diagnostic value

It has been suggested that low density lipoprotein-containing circulating immune complexes (LDL-CIC) play a role in atherogenesis and are involved in the formation of early atherosclerotic lesion. These complexes, as well as anti-LDL autoantibodies, have been found in the blood and in the atherosclerotic lesions of patients with different cardiovascular diseases, as well as in the blood of animals with experimental atherosclerosis. It can be suggested that the presence of anti-LDL antibodies in the blood is a result of immune response induced by lipoprotein modification. LDL-CIC differs from native LDL in many aspects. It has much lower sialic acid content, smaller diameter, and higher density and is more electronegative than native LDL. Fraction of LDL-CICs is fundamental to the serum atherogenicity manifested at the cellular level. LDL-CIC, unlike native LDL, is able to induce intracellular accumulation of neutral lipids, especially esterified cholesterol, in cells cultured from uninvolved human aortic intima and in macrophage cultures. After removal of LDL-CIC, the CHD patient's sera lose their atherogenic properties. Titer of LDL-CIC in blood serum significantly correlates with progression of atherosclerosis in human in vivo and has the highest diagnostic value among other measured serum lipid parameters. Elevated CIC-cholesterol might well be a possible risk factor of coronary atherosclerosis.

An immunoenzymatic procedure for human apo B-containing particles quantification using monoclonal antibodies

The present report describes a new monoclonal antibody-based enzyme immunoassay (ELISA) for the quantification of apolipoprotein (apo) B-containing particles in plasma. Native low-density lipoprotein (LDL) and a reference serum were utilized to prepare the standard curve. Three different antibodies to apo B-100--4A6E3, 6A10B10, and 2D9--all produced in our laboratory, were examined. The apparent affinity constants of the monoclonal antibodies (MAbs) 4A6E3, 6A10B10, and 2D9 were 2.9 x 10(9), 1.74 x 10(9), and 0.63 x 10(8), respectively. The standard curve was generated for an apo B-LDL range of 0.1 to 4.0 microg/ml. Evaluating the concentration of apo B-containing particles in plasma may allow for a more accurate assessment of the risk of coronary artery disease.

Two new immunogenetic polymorphisms of the apoB gene and their effect on serum lipid levels and responses to changes in dietary fat intake

In previous studies, apoB polymorphisms have been shown to modify serum lipid responses to changes in dietary fat intake. The functionally important apoB DNA change or changes underlying these effects have, however, remained unknown. Using a single-strand conformation polymorphism analysis-based screening method, we identified two previously unreported apoB polymorphisms located close to each other in the 5' region of apoB gene exon 26. This DNA segment corresponds to the binding site of monoclonal anti-apoB antibody D7.2. The two A-->G changes at apoB cDNA nucleotides 5869 and 5896 produced an Asn-->Ser change at amino acid 1887 and a His-->Arg change at amino acid 1896. In the Finnish population, allele frequencies of the rare alleles of the apoB 1887 (Asn-->Ser) and apoB 1896 (His-->Arg) polymorphisms were .02 and .11, respectively. Both polymorphisms were shown to have an independent effect on the binding affinity of LDL with monoclonal antibody D7.2. The effect of these polymorphisms on serum lipid levels and responses to changes in dietary fat intake in 102 healthy free-living subjects was assessed. The apoB 1896 Arg allele was associated with a higher serum LDL cholesterol level during a low-fat, low-cholesterol diet in men.

Moderate oxidation of hypertriglyceridemic low-density lipoprotein causes apolipoprotein B epitope change and enhances its uptake by macrophages

We prepared monoclonal antibody (MabB4) that selectively binds to acetylated low-density lipoprotein (LDL). Native hypertriglyceridemic LDL (HT-LDL) obtained from IIb and native normotriglyceridemic LDL (NT-LDL) from type IIa scarcely bound with MabB4. When these LDL were oxidized moderately by incubation with copper ions, the binding of MabB4 to HT-LDL was enhanced compared to that of NT-LDL, although the contents of the hydroperoxide they produced were the same. The incorporation of moderately oxidized HT-LDL into macrophages was enhanced compared to that of NT-LDL, and the rate of incorporation parallel the binding of LDL for MabB4. These results suggested that moderate oxidation of HT-LDL expressed some apolipoprotein B epitope on the surface of acetylated LDL to a much greater degree than NT-LDL, and that this expressed epitope might work as a ligand of moderately oxidized HT-LDL for the recognition by macrophages.

Low density lipoprotein derivatization by acetaldehyde affects lysine residues and the B/E receptor binding affinity

Acetaldehyde (AcA), the first metabolite in ethanol oxidation, forms covalent adducts with the free amino groups of various proteins. In this study, we examined how acetaldehyde modification affects the chemical and biological properties of the atherogenic low density lipoprotein (LDL). AcA modification did not alter the protein and lipid composition of LDL, but the AcA concentration used in the incubation correlated strongly with the electrophoretic mobility of acetaldehyde-treated LDL (AcA-LDL) (r = 0.97, p less than 0.001) and the percentage of the free amino groups in AcA-LDL (r = -0.90, p less than 0.01). Amino acid analysis of AcA-LDL showed that lysine was the predominant residue in LDL modified by AcA. Assays with monoclonal antibodies (MB47, 2b, 4G3, and C1.1) directed against different epitopes of the LDL apoprotein B suggested that AcA modification reduced the immunological recognition of the LDL receptor binding region and its vicinity. Also, the binding affinity of AcA-LDL to B/E receptors correlated negatively with the percentage of modified lysine residues in AcA-LDL (r = -0.96, p less than 0.001). The results suggest that AcA derivatizes the lysine residues of LDL, and thus decreases the B/E receptor binding affinity of LDL. However, major changes in LDL receptor binding were produced only with non-physiologically high concentrations of AcA, and, therefore, the role of the present findings in vivo remains uncertain.

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.

Monoclonal antibody studies of the antigenic determinants of human plasma retinol-binding protein

A battery of monoclonal antibodies (MoAbs) against human retinol-binding protein (RBP) was produced to obtain useful probes for the study of the antigenic determinants of RBP. The 12 antibodies all reacted with human RBP by immunoblotting. Based on antibody cross-competition radioimmunoassays, four distinct and different groups of antibodies were identified: group I, 1A4 and 2F4; group II, 1G10, 5C5, 6F4, and 7G3; group III, 5H6, 6C7, 10G5, and 14E3; and group IV, 5H9 and 13A1. Information about the epitopes of RBP recognized by these MoAbs was obtained by testing the reactivity of each antibody with human, rabbit, and rat RBPs by immunoblotting. Group I and group IV antibodies reacted to a similar extent with human, rabbit, and rat RBPs. Group II antibodies reacted strongly with human and rabbit RBPs, but reacted very weakly with rat RBP. Group III antibodies reacted strongly with human RBP, but did not react with rabbit or rat RBP. Thus, the epitopes for group I and group IV antibodies appear to be regions of the RBP molecule that are conserved across the three species, whereas group III antibodies recognized only human RBP. In a preliminary study, the reactivity of each antibody with purified cyanogen bromide fragments of RBP was tested by slot immunoblotting. None of the MoAbs reacted with any of the cyanogen bromide fragments. This study shows that MoAbs specific for at least four different regions of the RBP molecule can be produced; hence, RBP contains at least four major antigenic domains.

Correlation between cholesterol content in circulating immune complexes and atherogenic properties of CHD patients' serum manifested in cell culture

Blood serum of most patients with coronary heart disease (CHD) caused a 2-5-fold increase in the total cholesterol content of smooth muscle cells cultured from unaffected human aortic intima, i.e. possessed an atherogenic potential manifested in culture. Removal of immunoglobulins G and M from an atherogenic serum brought about a fall in its atherogenic potential. The serum deficient in immunoglobulins A retained its ability to induce the cholesterol accumulation in cells. 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 atherogenicity, i.e. failed to increase the cholesterol content in cultured cells. Incubation of smooth muscle cells derived from human aortic intima with circulating immune complexes isolated from an atherogenic patients' serum caused a 1.5-3-fold rise in the intracellular cholesterol. Blood sera of most (89%) CHD patients was characterized by a high cholesterol content in circulating immune complexes. More than 75% of healthy subjects and patients without stenosis of coronary arteries had low level of cholesterol in immune complexes. Blood sera atherogenicity manifested in culture directly correlated with the cholesterol level of circulating immune complexes (r = 0.90). These findings suggest that the atherogenicity of CHD patients blood serum is due to cholesterol-containing immune complexes.

A high carbohydrate-fat free diet alters the proportion of heparin-bound VLDL in plasma and the expression of VLDL-apoB-100 epitopes

High carbohydrate-fat free diets (CHO-diet) induce the secretion of increased numbers of very-low-density lipoprotein (VLDL) particles and alter the composition and metabolism of VLDL. The aims of this study were to examine VLDL in greater detail, specifically to document any CHO-diet-induced alterations of apolipoprotein B-100 (apoB-100) epitope expression of VLDL, and any changes induced in subclasses of VLDL, as defined by heparin Sepharose chromatography. Fifteen normolipidemic subjects participated in the study by eating a basal typical American diet for 7 days and high carbohydrate diet (85% carbohydrate, less than 1% fat) for another 7 days. The sequence was changed in seven subjects. Fasting blood samples were analyzed for lipoprotein lipid and apoprotein concentrations. Heparin affinity VLDL subclasses were characterized chemically and electrophoretically [sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE)]. Immunoreactivities of apoB in VLDL were tested in solid phase competitive-binding radioimmunoassays (RIAs) using five monoclonal anti-B antibodies that react with defined epitopes of apoB-100. The CHO diet produced consistent increases of plasma triglycerides in all subjects by a mean of 66% and decreases in plasma cholesterol by 18%. ApoB in plasma decreased by 21% and apoA-I by 17%; however, apoE and ApoA-II did not change. VLDL was enriched with triglycerides (55.0% +/- 0.8 v 57.0% +/- 0.7, P less than .05) and apoE (3.7% +/- 0.5 to 5.9% +/- 0.7, P less than .007) and the ratio between apoE and apoC in VLDL increased (0.15 +/- 0.03 to 0.25 +/- 0.03, P less than .002).(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibody 5A binds apolipoprotein B-48 and inhibits the low density lipoprotein-receptor interaction

In a panel of 10 monoclonal antibodies raised against human LDL we detected three antibodies (named 5A, 6B, and 6E) which recognize both apolipoprotein B-100 and B-48. Antibody 5A inhibited, in a dose dependent manner, the interaction of 125I-LDL with their receptor on human skin fibroblasts. Using thrombolytic fragments, the epitope of antibody 5A was mapped to the carboxy terminal region of apo B-48. MAB 5A was equipotent with MAB Mb 47, an inhibitory antibody whose epitope lies near a putative receptor binding domain of apo B in thrombolytic fragment T2. These findings suggest that areas other than the carboxy terminal portion of apo B-100 may participate in the LDL-receptor interaction, either directly or by determining the exposition of high affinity sites of apo B-100.

Two monoclonal antibodies that discriminate between allelic variants of human low density lipoprotein

Human low density lipoprotein shows a genetic polymorphism, the so-called Ag-system. it consists of 5 pairs of allelic epitopes, x/y, al/d, c/g, t/z, and h/i, which are localized on apolipoprotein B. We have generated a large number of monoclonal antibodies against low density lipoprotein. Two of them, D2E1 and H11G3, recognize epitopes related to this genetic polymorphism. Direct ELISA and ELISA inhibition experiments with different low density lipoproteins of known phenotype showed that D2E1 is directed against the allelic epitope c and H11G3 against d. The two antibodies were used for the characterization of low density lipoprotein in sera from different blood donors and the results compared to those obtained by passive hemagglutination using human allotypic anti-sera. Sera from homo- or heterozygous donors (which display the relevant epitope) could be distinguished from the sera of homozygous donors (which lack the epitope) with the monoclonal antibodies described.

Detection of two apolipoprotein B species (apoBc and apoBg) by a monoclonal antibody

A monoclonal antibody (MB-19) was used to investigate the polymorphism of apolipoprotein B in a large East Finnish family and in unrelated subjects. Apolipoprotein B was shown to exhibit high, intermediate or low affinity binding to this antibody. Thus, MB-19 bound strongly to the Ag(c) epitope, an Ag antigenic domain previously characterized by human antisera, while it bound only weakly to the allelic epitope Ag(g). It proved useful for the detection of the two corresponding allelic apoB species designated apoBc (= high affinity binding) and apoBg (= low affinity binding), and for confirming their co-dominant transmission. Intermediate binding resulted from the presence of a mixture of both apoB populations in heterozygous subjects.

Immunogenicity of malondialdehyde-modified low density lipoproteins. Studies with monoclonal antibodies

Malondialdehyde (MDA)-modified low density lipoprotein (LDL) can stimulate the accumulation of cholesteryl esters in cultured macrophages through its interaction with specific scavenger receptors. It has been speculated that such interaction occurs in vivo thus contributing to the formation of foam cells within atherosclerotic lesions. This report describes the development of new tools in the form of a specific assay for MDA-LDL to investigate this hypothesis. We have immunized BALB/c mice with malondialdehyde mouse low density lipoproteins and antibodies against malondialdehyde human low density lipoproteins were generated. Monoclonal antibodies were produced using hybridoma techniques and one particular clone (EB 7-3) was expanded for further studies. The immunoreactivity of several antigens was tested using antibody EB 7-3 in an enzyme-linked immunosorbent assay (ELISA). In a typical assay malondialdehyde human LDL (with at least 40% of lysines modified) was coated (2 micrograms/ml, 100 microliter) in 96-well microtiter plates. Antibody plus one of several antigens were then added and the interaction between the antibody and coated antigen was measured using alkaline phosphatase-conjugated affinity purified goat anti-mouse immunoglobulin. The binding of antibody EB 7-3 to wells coated with malondialdehyde-LDL was competitively inhibited by malondialdehyde-LDL added in solution, with half maximal inhibition occurring at 150 +/- 80 ng/ml. In addition, the ability of malondialdehyde-LDL to inhibit this interaction was proportional to the degree of modification: the more lysines were modified the more did malondialdehyde-LDL inhibit the binding of antibody EB 7-3 to coated malondialdehyde-LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunogenetic polymorphism of apolipoprotein B in humans: studies with a monoclonal anti-Ag(c) antibody

In studies that use a monoclonal antibody (MB-19), apolipoprotein B exhibited one of three immunophenotypes: high, intermediate, or low affinity binding to this antibody. The distribution of these immunophenotypes (allotypes) in families was compatible with a codominant transmission of two alleles, one coding for the high and the other for the low affinity binding allotype. The high affinity binding allotype coincided with antigen Ag(c) and the low affinity binding allotype with Ag(g), two allelic antigenic determinants previously defined by human antisera. Preliminary studies did not reveal differences in plasma lipid levels in association with apolipoprotein B allotypes. Young Finnish men with low affinity binding apolipoprotein B had slightly lower plasma apolipoprotein B levels than those with the intermediate affinity binding phenotype.

Identification of surface-exposed segments of apolipoprotein B-100 in the LDL particle

The isolation and amino acid sequence of eleven peptides liberated by tryptic treatment from surface-exposed regions of apolipoprotein B-100 in the native low-density lipoprotein particle are described. These peptides represent eight segments in the sequence of the B-100 protein, one of which was localised to the amino-terminal thrombolytic fragment T4 (1297 amino acids), four to the T3 fragment (2052 residues) and three to the carboxylterminal fragment T2 (1287 residues). An exposed segment was identified on each side of the T2/T3 cleavage site, in close proximity to two segments enriched in basic amino acids (residues 3147-3157 and 3359-3367 respectively). The surface exposure of this region is consistent with its contribution to the putative apo-B,E receptor binding domain. Four of the eight tryptic segments contribute to regions of proline-rich clusters. Homology between the sequence of the tryptic peptides and those predicted by cDNA cloning was complete.

Human apolipoprotein B: chromosomal mapping and DNA polymorphisms of hepatic and intestinal species

Apolipoprotein B (apoB) is the major protein component of low-density and very-low-density lipoproteins. We have recently isolated nonoverlapping cDNA clones for apoB and confirmed their identity by sequence comparisons. We now report the mapping of the human apoB gene (APOB) to the p23-p24 region of chromosome 2 by examination of human-mouse somatic cell hybrids and by in situ hybridization to human chromosomes. Thus, APOB is unlinked to members of the dispersed gene family encoding other apolipoprotein species or to the gene encoding the low-density lipoprotein receptor. Hybridization analysis with genomic DNA and liver and intestinal mRNA suggests that APOB encodes both the high-molecular-weight form of apoB (apoB100) incorporated into very-low-density lipoproteins in liver and the lower-molecular-weight form (apoB48) incorporated into chylomicrons in intestine. Restriction fragment length polymorphisms of APOB have been identified and should prove useful in examining the possibility that genetic variations of APOB are involved in dyslipoproteinemias and atherosclerosis.

Isolation and characterization of three monoclonal antibodies to human serum low density lipoprotein apoprotein B

Human serum low density lipoprotein (LDL) is a large (Mr = 2-3 X 10(6), complex particle composed of lipid, protein and carbohydrate. We obtained about 40 mouse spleen-myeloma hybrid cell lines which produce antibodies against LDL. Three of them, SC2, SC3 and SC10, have been cloned and subcloned and their antibody products characterized. They recognize three non-overlapping epitopes in native LDL. Two of them, SC3 and SC10, also are capable of recognizing very low density lipoprotein, (VLDL), whereas SC2 reacts only weakly with VLDL. All three antigenic determinants remain intact, and accessible to antibodies on the LDL protein apo B, prepared by delipidation in a 'non-denaturing' detergent, sodium deoxycholate. However, apo B prepared by organic solvent, ether-ethanol, or sodium dodecyl sulfate (SDS) delipidation, while reacting strongly with SC10, is only poorly recognized by SC2 or SC3. Proteolysis of LDL with trypsin, chymotrypsin, Staphylococcus aureus protease, papain or thermolysin gives, in each case, several non-identical protein fragments which are separable by SDS-polyacrylamide gel electrophoresis. Upon immunoblotting, some of these fragments are now recognized by either SC3 or SC10 but not SC2, some are recognized by both SC3 and SC10, and others are immunologically unreactive. The protein bands that are separated by SDS gel electrophoresis are composed of several non-identical fragments and contain the antigenic sites to differing degrees. Some of the immunologically reactive fragments do not appear to contain carbohydrate. Reduction and carboxymethylation do not destroy the immunoreactivity of LDL toward any of the antibodies; however, modification of lysine residues by citraconic anhydride markedly diminishes the reactivity of LDL toward SC3. It is likely that the two antibodies SC3 and SC10 are directed against different linear amino acid sequences or very stable domains, whereas the third, SC2, is directed against a more fragile conformational domain of apo B.

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

A stable mouse hybridoma cell line has been developed that produces monoclonal antibody to human plasma apolipoprotein B. This antibody was proven to be specific for apolipoprotein B immunoblotting and an enzyme immunoassay using apolipoprotein B and other apolipoproteins. The antibody bound with comparable affinities to soluble apolipoprotein B, chylomicrons, very-low-density (VLDL) and low-density lipoproteins (LDL). Coupled to agarose, this antibody allowed complete removal of apolipoprotein B-containing lipoproteins from normolipidemic, hypertriglyceridemic and hypercholesterolemic plasma. Desialyzation and deglycosylation had no effect on its binding to LDL. The described antibody had no effect on the receptor-mediated binding of radiolabeled LDL to the human hepatoma cells (HepG2) in culture. Analysis of 25 different samples of human plasma indicated identical expression of the corresponding epitope in these individuals. The described monoclonal antibody, most likely, binds to a rather stable domain of apolipoprotein B that is not altered by the interaction with lipids or polymorphism of the apolipoprotein B. We propose that this antibody be called 'Pan B' antibody.

Conservation of the low density lipoprotein receptor-binding domain of apoprotein B. Demonstration by a new monoclonal antibody, MB47

The fact that low density lipoprotein (LDL) from multiple animal species binds to the human LDL receptor suggested that the LDL-receptor binding domain of apoprotein (apo)B must be evolutionarily conserved. To determine if a common receptor domain epitope existed on apo B, we generated a monoclonal antibody that was specific for the LDL-receptor domain of apo B. This was accomplished by using a screening procedure that selected for a hybridoma supernatant that could block specific cellular uptake and degradation of LDL. Western blots showed that this antibody, termed MB47, was specific for apo B-100. Fluid phase assays indicated a high binding affinity (Ka = 4 X 10(9) M-1) and demonstrated that all human LDL particles expressed the MB47 epitope. Scatchard analysis indicated that a maximum of one MB47 molecule bound to each LDL particle. In solid phase assays, antibody MB47 bound to plasma or LDL of multiple mammalian species, including guinea pig, rabbit, pig, dog, cat, seal, whale, bear, and lion, but it did not bind to mouse or rat LDL. In contrast, a rabbit antiserum to LDL and two other anti-apo B monoclonal antibodies, MB3 and MB19, which do not bind to the receptor domain, were specific only for human LDL. LDL from multiple species, including mouse LDL, competed effectively with 125I-human LDL for binding to human fibroblasts. MB47 effectively inhibited uptake and degradation of labeled human, guinea pig, and rabbit LDL by both human and guinea pig fibroblasts. We conclude that antibody MB47 binds to a single receptor domain on LDL and identifies a vital region conserved through mammalian evolution.

Lipoprotein-antibody immune complexes. Their catabolism and role in foam cell formation

Immune complexes consisting of human [125I]LDL or [125I]VLDL and anti-apo B IgG were prepared in vitro. After intravenous administration of these complexes or free LDL to normal rabbits the elimination rate for complexes was 2-3-fold higher than for free lipoproteins. The liver/spleen radioactivity ratio after administration of immune complexes was 34% less than after administration of free lipoproteins. Investigations carried out on the cell cultures have shown that during 4-h incubation the uptake of [125I]LDL-anti-apo B IgG complex by human lung fibroblasts was lower than uptake of free [125I]LDL, whereas mouse peritoneal macrophages took up immune complex more actively than free LDL. During 3 days of incubation of macrophages with LDL-anti-apo B IgG the transformation of macrophages into foam cells was observed. This process was accompanied by almost a 60-fold increase of cholesteryl ester content in the cell. It is suggested that excessive uptake of lipoprotein-antibody complexes by macrophages leading to formation of foam cells may play an important role in atherogenesis.

The recognition domain for the low density lipoprotein cellular receptor is expressed once on each lipoprotein particle

The stoichiometry of binding of monoclonal antibodies and Fab fragments to LDL was assessed. Increasing amounts of two [125I]-labelled antibodies which define epitopes at or near the LDL-receptor recognition domains of apoB were incubated with fixed amounts of LDL and antibody-LDL complexes were separated from free antibodies by heparin-MnCl2 precipitation. Saturation kinetics were obtained and data were analyzed according to Scatchard. One antibody or Fab fragment was bound per LDL particle. Homogeneity of binding was indicated by straight Scatchard lines and by the binding of virtually all LDL particles by an antibody affinity chromatographic column.

Use of monoclonal antibodies in studies of lipoprotein structure and function

In order to probe the structure and function of apoB, monoclonal antibodies directed against human LDL were produced. The seven antibodies tested were found to be directed against five individual antigenic determinants of apoB in holo-LDL. One of the antibodies inhibited the binding of 125I-LDL to the apoB, E receptor of cultured human fibroblasts. Limited proteolysis of LDL results in the production of relatively stable lipoprotein "core" particles that retain many physical, chemical and immunological properties of the native lipoprotein. Perturbations of the structure of apoB by limited proteolysis of LDL had selective effects on the expression of various epitopes in the resulting LDL "cores" - the expression of some was unaffected, others were abolished. In addition, the various epitopes could be localized by immunoblotting to different proteolytic fragments of apoB present in the LDL "cores". Limited proteolysis also provided water soluble cleavage peptides, some of which exhibited immunoreactivity. Digested LDL "core" particles and cleavage peptides both interacted with the LDL receptors of cultured human fibroblasts. These experiments suggest that apoB may consist of repeating subunits of similar structure. The immunoreactivity of LDL may be determined not only by the structure of the protein moiety itself, but also by other lipoprotein constituents. Studies utilizing both polyclonal and monoclonal anti-apoB antibodies have shown that apoB reacts differently in different types of lipoprotein particles (i.e., VLDL, IDL and LDL). We studied the immunological reactivities of apoB within the LDL class. The results showed that the expression of epitopes in different LDL preparations indeed varied, and that the number of epitopes expressed, appeared to be influenced by the lipid composition of LDL.