Membrane proteins on human megakaryocytes and platelets identified by monoclonal antibodies

Developmental Endothelial Locus-1 (Del-1) Mediates Clearance of Platelet Microparticles by the Endothelium

Background—

Phosphatidylserine-expressing microparticles circulate in blood with a short half-life of <10 minutes. We tested the role of an endothelium-derived phosphatidylserine-binding opsonin, developmental endothelial locus-1 (Del-1), in the uptake of platelet microparticles.

Methods and Results—

Cultured human umbilical vein and microvascular endothelial cells avidly engulf BODIPY (4,4-difluoro-4-bora-3a,4a-diaza- s -indacene)-maleimide–labeled platelet microparticles. Microparticle uptake was inhibited by a monoclonal antibody to Del-1 ( P =0.027) and by annexin A5 ( P =0.027), abciximab ( P =0.027), a monoclonal antibody to integrin αVβ3 ( P =0.027), and chlorpromazine ( P =0.027). These results suggest that Del-1 mediates phosphatidylserine- and integrin-dependent endothelial uptake of microparticles by endocytosis. To assess the in vivo significance, we infused fluorescent platelet microparticles into the inferior vena cava of mice and harvested endothelial cells from the pulmonary and systemic circulation. Compared with their wild-type littermates, Del-1–deficient mice had decreased uptake in endothelial cells in lung (3.07±1.9 versus 1.09±1.3, P =0.02) and liver (2.85±1.1 versus 1.35±0.92, P =0.01). Furthermore, after endotoxin administration, Del-1–deficient mice displayed an increase in the level of microparticles compared with wild-type mice ( P =0.02).

Conclusions—

These studies show a physiological role for Del-1 in the clearance of phosphatidylserine-expressing microparticles by endothelium.

Cardiolipin binding a light chain from lupus-prone mice

Autoantibodies in systemic lupus erythematosus react with multiple epitopes on highly conserved molecules such as nucleic acids, cytoskeletal proteins, phospholipids, and phospholipid-binding proteins. Analysis of the heavy- and light-chain variable sequences (VH and VL) has shown that a restricted set of V genes gives rise to these autoantibodies. Several monoclonal antibodies were developed from a strain of mouse prone to lupus (F1 male NZW x BXSB). Two of these antibodies, A1.72 and A1.84, reacted directly with cardiolipin and their VH and VL sequences were analyzed. Surprisingly, these two antibodies had identical light-chain variable sequences despite having substantially different heavy-chain variable sequences. This VL sequence, VL 72/84 was 97% identical with the germ-line sequences with only four single nucleotide substitutions. When this VL sequence was shuffled with the VH sequence of other monoclonal antibodies and expressed as single chain variable fragment (scFv) in Escherichia coli, it imparted cardiolipin-binding activity to the hybrids. Furthermore, the VL 72/84 sequence, when expressed alone without any VH sequence, also bound to cardiolipin. The antibodies and their recombinant fragments were immunoaffinity-purified on cardiolipin liposomes. The dissociation constant of the light chain for cardiolipin was similar to the intact molecule (21 +/- 0.01 vs 20 +/- 0.03 nM). These studies demonstrate that the VL sequence alone, in the absence of any other immunoglobulin domains, can mediate cardiolipin binding, raising the possibility that antigen specificity of certain antibodies may exclusively reside in their light-chain sequences.

Characterization of beta2-glycoprotein I-dependent and -independent "antiphospholipid" antibodies from lupus-prone NZW/BXSB F1 hybrid male mice

Male (NZW x BXSB)F1 (W/BF1) mice develop a systemic lupus-like syndrome characterized by thrombocytopenia, coronary vascular disease, nephritis, and anticardiolipin antibodies. Three stable hybridoma cell lines secreting monoclonal anticardiolipin antibodies were developed from these mice by fusing their splenic lymphocytes with nonsecreting myeloma cell line, NS-1. Monoclonal antibody A1.17 reacted with cardiolipin in a beta2-Glycoprotein I-dependent manner. The epitope for this antibody consisted of beta2-glycoprotein I bound to cardiolipin or immobilized on plastic plates. Other anionic phospholipid-binding proteins, such as prothrombin or annexin V, had no significant effect in the reactivity of these antibodies. The specificity is similar to the autoimmune anticardiolipin antibodies described in patients with systemic lupus erythematosus and other infectious diseases. In contrast, monoclonal antibodies A1.72 and A1.84 reacted with cardiolipin in the absence of beta2-glycoprotein I. Beta2-glycoprotein I, either in the fluid phase or bound to cardiolipin, inhibited the binding of these antibodies. The specificity of the latter two antibodies was similar to that described in patients with syphilis and allied disorders. Both types of antibodies had lupus anticoagulant properties. Thus lupus-prone male (NZW x BXSB)F1 (W/BF1) mice develop both beta2-glycoprotein I-dependent and beta2-glycoprotein I-independent anticardiolipin antibodies.

Biochemical and functional characterization of a new murine monoclonal antibody against human platelet glycoprotein IIIa

A new murine monoclonal antibody, MDP-1, specific for human platelet glycoprotein IIIa has been produced and characterized. Following SDS-polyacrylamide gel electrophoresis, MDP-1 reacted with a 94kDa protein immobilized on a nitrocellulose membrane. Upon reduction, MDP-1 no longer bound to the 94kDa protein indicating an epitope requiring at least one disulfide bond. On crossed immunoelectrophoresis MDP-1 reacted to the same peak as the GP IIb-IIIa complex-specific antibody AP-2. After dissociation of the GP IIb-IIIa complex with EDTA, AP-2 showed no reactivity while MDP-1 bound to a new peak that was broader and anodal to the original GP IIb-IIIa peak, consistent with GP IIIa. MDP-1 inhibited ADP and thrombin induced aggregation. In addition, MDP-1 inhibited ADP induced release of ATP, but did not inhibit thrombin stimulated ATP release. Following chymotrypsin digestion, MDP-1 bound to a cleaved GP IIIa protein (nonreduced M, = 122 kDa) consistent with opening of the major disulfide loop. A second cleavage resulted in a 63 kDa species that reacted with MDP-1. Scatchard analysis revealed 22 000 molecules of MDP-1 bound per platelet, and indicated a type of binding consistent with positive cooperativity. The antibody bound equally well to stimulated and unstimulated platelets. MDP-1 binding was inhibited by a polyclonal anti-PI(A1) antibody, but bound to platelets from a PI(A1) negative individual indicating a binding site close to but not identical to the PI(A1) epitope. In addition, MDP-1 binding was not inhibited by Arg-Gly-Asp-Ser (RGDS) suggesting that it is not directed to the RGD binding site on GP IIIa.

Simultaneous measurements of megakaryocyte-associated IgG (MAIgG) and platelet-associated IgG (PAIgG) in chronic idiopathic thrombocytopenic purpura

We have simultaneously measured platelet-associated IgG (PAIgG) and megakaryocyte-associated IgG (MAIgG) in 30 untreated patients with chronic idiopathic thrombocytopenic purpura (CITP). Megakaryocytes were purified from bone marrow by 35% Percoll gradient centrifugation, followed by negative immunopanning using magnetic immunobeads. The normal range of MAIgG in 30 healthy donors was 15.5 +/- 10.0 ng/10(5) megakaryocytes, whereas MAIgG in the 30 CITP patients was 140 +/- 59.3 ng/10(5) megakaryocytes, although the values were widely distributed. From the PAIgG and MAIgG data, CITP patients were classified into three types; type I (PAIgG < 200 ng/10(7) platelets and MAIgG < 150 ng/10(5) megakaryocytes), type II (PAIgG > 200 ng and MAIgG > 150 ng), and type III (PAIgG < 200 ng and MAIgG > 150 ng). Patients with types I and III had good clinical courses, but, in contrast, patients with type II responded poorly to steroid therapy followed by splenectomy or became refractory to treatment. In splenectomized patients, MAIgG of responder was promptly decreased to normal range and, in contrast, that of non-responder was persistently elevated. These results indicate that anti-platelet autoantibodies are able to bind with megakaryocytes in the bone marrow as well as with platelets in the peripheral blood, and the results also suggest that megakaryopoiesis in CITP is heterogeneous. Simultaneous measurement of PAIgG and MAIgG may predict the clinical outcome of CIPT.

Activation of human platelets by exposure to a monoclonal antibody, PM6/248, to glycoprotein IIb-IIIa

Monoclonal antibody PM6/248, which recognizes the GPIIb-IIIa complex on human platelets, causes platelet aggregation in platelet-rich plasma or in gel-filtered platelet suspensions. Aggregation follows a concentration-dependent lag phase and reaches a maximum at 8 micrograms/ml. High concentrations of antibody (less than 30 micrograms/ml) produce complete inhibition of the aggregation response. Aggregation is accompanied by serotonin secretion and thromboxane A2 synthesis, neither of which are inhibited by high concentrations of antibody, and by the mobilization of intracellular Ca2+. The F(ab')2 fragment of PM6/248 does not cause platelet activation and pre-incubation of platelets with this fragment inhibits all platelet responses stimulated by the whole antibody. Pre-incubation with the F(ab')2 fragment of the anti-Fc gamma RII Mab, IV. 3, also inhibits all responses to PM6/248. These data indicate that platelet activation stimulated by PM6/248 is caused by cross-linking of GPIIb-IIIa to the Fc gamma RII which stimulates signal transduction across the plasma membrane through a conformational change in the Fc gamma RII.

The use of the anti-factor VIII method on trephine biopsies of the bone marrow for the identification of immature and atypical megakaryocytes in myeloproliferative diseases and allied disorders. A morphometric study

A morphometric analysis was performed on trephine biopsies of the bone marrow to identify atypical megakaryocyte proliferation following PAS staining and the immunohistological demonstration of factor VIII. This study includes nine patients with a megakaryoblastic crisis in chronic myeloid leukemia (CML), four with acute megakaryoblastic leukemia (AM) and three with myeloid dysplasia later evolving into overt acute leukemia. Comparison and statistical evaluation of the PAS reaction with anti-factor VIII staining reveals that the latter technique not only facilitates the recognition of immature and abnormal megakaryocytes, but leads to a significantly increased count for all megakaryocytic elements in the bone marrow. Thus our retrospective investigation of routinely processed and paraffin-embedded trephine biopsies shows that the diagnosis of a megakaryoblastic crisis in CML as well as AM may be easily established with the aid of the anti-factor VIII method. In all cases of megakaryoblastic proliferation in CML and AM, the appearance of blasts was associated with moderate to pronounced myelofibrosis which could be also determined by morphometry.

Mechanisms of thrombocytopenia in chronic autoimmune thrombocytopenic purpura. Evidence of both impaired platelet production and increased platelet clearance

Mechanisms of thrombocytopenia were studied in 38 patients with mild to moderately severe chronic autoimmune thrombocytopenia (AITP). 51Cr and 111In-labeled autologous platelet turnover studies and in vitro analysis of committed megakaryocyte progenitors (CFU-Meg) were used as independent measures of platelet production. Autologous 111In-labeled platelet localization studies were performed to assess platelet clearance. Although there was no increase in the frequency of marrow CFU-Meg, a specific increase in the CFU-Meg [3H]TdR suicide rate was seen which was inversely correlated with the platelet count (P less than 0.001). Platelet turnover studies showed significant numbers of patients had inappropriate thrombopoietic responses to their reduced platelet counts. Platelet-associated antibody levels correlated inversely with platelet turnover suggesting that antiplatelet antibody impairs platelet production. The circulating platelet count was best predicted by an index relating platelet production (i.e., turnover) to the spleen-liver platelet clearance that correlated directly with platelet survival (P less than 0.001). In summary, both depressed platelet production and increased platelet clearance by the liver and spleen contribute to the thrombocytopenia of AITP.

A human erythroleukemia cell line synthesizes a functionally active glycoprotein IIb-IIIa complex capable of binding fibrinogen

The human erythroleukemia (HEL) cell line is known to express a number of platelet-megakaryocyte markers, including glycoproteins IIb and IIIa. Using [35S]methionine as well as monoclonal and polyclonal antibodies to glycoprotein IIb and IIIa, we have demonstrated synthesis of these two glycoproteins by this cell line. When Triton X-100-solubilized membranes were subjected to cross immunoelectrophoresis in 1% agarose, using a mixture of polyclonal antibodies to glycoproteins IIb and IIIa, a single peak was seen in the presence of Ca2+, whereas two distinct peaks were seen in the presence of 5 mM EDTA. When these crossed immunoelectrophoresis were overlaid with 125I-labelled fibrinogen, binding of fibrinogen to the glycoprotein IIb and IIIa complex peak was observed. When either the electrophoresis or the subsequent overlay was done in the presence of 5 mM EDTA, no 125I-fibrinogen binding was observed. These studies demonstrate that HEL cells contain glycoproteins IIb and IIIa capable of forming a Ca2+-dependent complex and capable of binding fibrinogen in a Ca2+-dependent reaction. Nevertheless, glycoprotein IIb- and IIIa-specific fibrinogen binding, of the type observed in platelets, could not be demonstrated in 'resting' or 'stimulated' intact HEL cells. The mechanism giving rise to this difference is currently unknown.

Constitutive and inducible secretion of platelet-derived growth factor analogs by human leukemic cell lines coexpressing erythroid and megakaryocytic markers

We have examined the constitutive and inducible secretion of platelet-derived growth factor (PDGF)-like proteins in a variety of human hemopoietic cell lines. The highest levels of secreted protein were noted in four human erythroleukemia lines which, in addition to erythroid lineage markers, express one or more megakaryocytic lineage markers. Induction of these lines by 12-O-tetradecanoylphorbol-13-acetate enhanced the expression of megakaryocytic markers and increased secretion of PDGF-like proteins several fold. In concert with these changes, there was significant induction of c-sis/PDGF-B messenger RNA (mRNA) expression in all lines, whereas one line showed significant concurrent induction of PDGF-A mRNA expression. Whether PDGF-like secretion is part of the stem cell-like phenotype displayed by these lines or is secondary to their leukemic transformation remains to be determined. Nevertheless, these lines provide new cellular models for studying the expression and function of PDGF analogs in hemopoietic cells.

Multipotent human hematopoietic cell line K562: lineage-specific constitutive and inducible antigens

K562 cells have been reported to display a variety of non-erythroid properties. Using 28 lineage-specific monoclonal antibodies, we analysed which antigens are present spontaneously and which are inducible by a variety of agents. The data suggest that (1) antigens of a given lineage are preferentially responsive to certain inducers, e.g. megakaryocytic antigens to phorbol ester, and (2) a given inducer may influence antigens of different lineages in opposite directions, e.g. phorbol dibutyrate, not only induces megakaryocytic antigens, but also decreases granulocyte and erythroid antigens. We conclude that the K562 cell, despite its malignant origin, retains some capacity for expression of alternative programs of differentiation, a characteristic of the normal multipotent hematopoietic stem cell.

Monoclonal antibodies for specific cell labeling: considerations, preparations and preliminary evaluation

Two types of monoclonal antibodies (MAb), B79.7 and B59.2 determined to be specific for human platelet glycoprotein III.a and III.a.II.b respectively, were labeled with 125I and 111In via diethylenetriaminepentaacetic acid (DTPA) bifunctional chelation technique and evaluated as agents for labeling platelets. Under the chemical conditions chosen one DTPA molecule was bound to one MAb molecule. The specific activities of 111In and 125I preparation averaged 5 and 2.5 Ci/mumol respectively. Both agents showed saturable binding to human platelets. Scatchard analysis showed that the Kd values for 111In-B79.7 and 125I-B79.7 were 83.3 X 10(-9) M and 113.3 X 10(-9) M respectively. The corresponding protein molecules bound per human platelet were 16.75 X 10(3) and 29.5 X 10(3). The Mab B59.2 also reacted with canine platelets and at 50% antigen saturation 49.5% of the added 111In-DTPA-B59.2 was bound to platelets. Using these platelets experimental thrombi were imaged. At 2 h post injection, the thrombi to blood ratios were 15.8. These radiolabeled MAbs are worthy of further evaluation.

The platelet protein phosphorylation induced by a monoclonal antibody against human platelets (TP82)

In human platelets, a monoclonal anti-human platelet antibody (TP82) induced platelet aggregation and release of granules (i.e., serotonin, platelet factor 4, N-acetyl-beta-D-glucosaminidase). The release reaction occurred even in the absence of aggregation and was preceded by not only the protein phosphorylation, but the transient formation of endogenous diacylglycerol (DG). These results suggest that polyphosphoinositide breakdown plays an essential role in antibody-induced release of platelet granules.

A monoclonal antibody to human platelet glycoprotein IIIa detects a related protein in cultured human endothelial cells

We have previously described a series of monoclonal antibodies against platelet membrane glycoproteins. Two of the antibodies, B59.2 and B2.12, recognize the glycoprotein IIb-IIIa complex. These two antibodies react specifically with glycoprotein (GP) IIIa, as shown by immunoblotting of sodium dodecyl sulfate-polyacrylamide gels of solubilized platelet membranes. Monoclonal B2.12, but not B59.2, binds to cultured human endothelial cells obtained from umbilical vein, internal iliac artery, and inferior vena cava. At saturation approximately 100,000 binding sites were detected per human umbilical vein endothelial cell. When solubilized radioiodinated cells were chromatographed on a column of agarose-bound B2.12, a single radiolabeled protein was obtained whose apparent molecular weight is slightly larger than that of platelet GP IIIa. This protein incorporated [35S]methionine when endothelial cells were labeled metabolically. These results demonstrate that human endothelial cell membranes synthesize a protein immunologically related to platelet GP IIIa.

Human megakaryocytes. V. Changes in the phenotypic profile of differentiating megakaryocytes

Human megakaryocytes were studied for phenotypic changes occurring throughout differentiation using a panel of monoclonal antibodies raised against marrow megakaryocytes and blood platelets. 11 monoclonal antibody preparations were selected for restricted specificity against megakaryocytes and/or platelets after screening by immunofluorescence, complement-mediated cytolysis, and solid phase enzyme-linked immunosorbent assay. The expression of the cellular epitopes recognized by these reagents enabled the identification of three levels of megakaryocyte maturation characterized by distinct immunologic phenotypes. Based upon their reactivities against megakaryocytic cells at different ontogenetic levels, monoclonal antibodies were operationally categorized into three groups. Group A consisted of six different monoclonal antibodies that recognized antigens on the colony-forming unit-megakaryocyte (CFU-Mk), in vitro grown colony megakaryocytes, and early immature marrow megakaryocytes, only, and did not detect their respective epitopes on either mature megakaryocytes or platelets. A monoclonal antibody categorized in group B detected a cell antigen expressed by megakaryocytic cells at all maturational levels, but which is lost or suppressed during terminal differentiation and is not expressed on blood platelets. Group C included four different monoclonal antibodies raised against platelets that recognized antigenic determinants expressed on the CFU-Mk, colony megakaryocytes, early and mature megakaryocytes, and platelets. Three group C monoclonal antibodies (PC-1, PC-3, and PC-4) were specific for platelet glycoprotein IIb/IIIa. Additionally, group C monoclonal antibody PC-2 was unique in that it showed partial reactivity against the clonable progenitor for the erythroid series (BFU-E). Recognition of discrete phenotypic changes in differentiating megakaryocytes will enable multiparameter analyses of these cells as well as the study of factors regulating the dynamics of megakaryocytopoiesis in health and disease.

Novel peripheral blood-derived human cell lines with properties of megakaryocytes

For 18 mo, we derived 18 cell lines from 11 donors with various clinical profiles ranging from normal to leukemic. Suspension cultures were initiated with 1 X 10(6) mononuclear blood cells/ml of nutrient medium containing 10% human serum and 10% lectin-stimulated human lymphocyte conditioned medium. The cultures were monitored weekly by morphological analyses of Wright-Giemsa-stained cell preparations. All successful cultures showed a significant decline in viability during the first 3-4 wk with rate "lymphoid" cells observed in mitosis. Within the next 2 wk, the proliferating cells gave rise to a rapidly expanding population of mononuclear cells. As the cultures expanded, cell morphology became heterogeneous with respect to cell size and nuclear ploidy, with an accumulation of giant multinuclear cells that were suggestive of megakarocytes. Even though the cells did not have the classical morphology of mature platelet-forming megakaryocytes, 90% of the cells within a cell line were positive by direct or indirect immunofluorescence for the platelet membrane glycoproteins IIb and IIIa; for surface markers HLA-Dr and B2-microglobulin; for intracellular platelet-derived growth factor and platelet factor IV; and for membrane affinity or binding with serum platelet-derived growth factor and platelet factor IV. These results suggest that a blood precursor cell, most likely a primitive megakaryoblast, was isolated from the peripheral blood and was provided with an optimal culture environment for sustained growth. These cells did not mature to a more differentiated stage, perhaps owing to regulatory factor deficiencies in this in vitro system. The remarkable frequency of obtaining cell lines with megakaryocyte properties from normal peripheral blood and the capacity of some normal donors to repeatedly yield these cell lines make this cell culture system indeed unique by being selective for putative megakaryocyte precursors.

Distribution of glycoprotein IIb-IIIa complexes in the surface membranes of human platelets and megakaryocytes

The distribution of glycoprotein (GP) IIb-IIIa complexes in the surface membranes of human platelets and megakaryocytes was investigated by transmission electron microscopy of cells that had been incubated with Fab fragments of a human alloantibody (IgG L) specific for the complex. Binding was visualized by a second antibody conjugated to peroxidase or adsorbed onto gold particles. Initial studies showed that the peroxidase reaction product and the gold particles were to be found at the outer surface of unactivated platelets. The occasional small cluster of particles was present. A positive reaction, more apparent with peroxidase labelling, was also seen in the channels of the platelet open canalicular system. Gold particles were abundant on the outer surface of mature megakaryocytes, and their distribution resembled that on unactivated platelets. As with platelets, peroxidase-labelled antibodies penetrated better, and revealed GP IIb-IIIa in the demarcation membrane system. A double immunofluorescence study, involving Fab fragments of IgG L and rhodamine-conjugated antibodies to factor VIII R:Ag, demonstrated the presence of GP IIb-IIIa in megakaryocyte precursor cells. Our results show that the GP IIb-IIIa complex is present in megakaryocyte membranes and that it appears at the same time as the other platelet antigens.

A monoclonal antibody binding to human medulloblastoma cells and to the platelet glycoprotein IIB-IIIA complex

A monoclonal antibody, designated M148, produced by the hybridoma technique from spleen cells of mice immunized with human medulloblastoma, was found by indirect immunofluorescence to bind to normal human platelets (both PlA1 positive and PlA1 negative) and megakaryocytes, as well as to some medulloblastoma and neuroblastoma cells and cell lines and certain other solid tumours. No binding was observed to other marrow constituents, nor to any other normal tissue examined. The antibody bound to platelets from a patient with the Bernard-Soulier syndrome but not to thrombasthenic platelets. It immunoprecipitated glycoproteins IIb and IIIa from 125I-labelled normal platelet membranes, and completely inhibited ADP-induced fibrinogen binding and aggregation of platelets. Aggregation was also inhibited in response to adrenaline, collagen, thrombin, sodium arachidonate and the ionophore A23187; clot retraction was partially inhibited. The antibody was without effect on thromboxane formation or 5-hydroxytryptamine (5HT) secretion in response to thrombin, but inhibited 5HT secretion in response to arachidonate. It did not inhibit factor VIII binding or agglutination in response to ristocetin, but completely inhibited factor VIII binding in response to thrombin. These findings suggest that the epitopes are close to the fibrinogen and factor VIII binding sites on glycoproteins IIb/IIIa, and that the lack of these glycoproteins is sufficient explanation for the pattern of dysfunction observed in thrombasthenic platelets, without invoking any other membrane abnormality.