Platelet activation induced by a murine monoclonal antibody directed against a novel tetra-span antigen

MAb 14A2.H1 identifies a novel low-abundance platelet surface antigen, PETA-3, which is a member of the tetra-span (TM4) family. This MAb brings about platelet aggregation and mediator release, which is completely inhibitable by prostaglandin E₁, and partially inhibitable by aspirin and ketanserin. Platelet activation by MAb 14A2.H1 is dependent on interaction with both the platelet Fc receptor, FcγRII, and the specific antigen as it was prevented by either a blocking MAb to FcγRII (IV.3) or F(ab')₂ fragments of 14A2.H1. The extent of platelet activation by the antibody varied considerably between donors, and is believed to reflect the polymorphism of FcγRII. Subaggregating concentrations of 14A2.H1 synergized with other platelet agonists, ADP, adrenaline, collagen and serotonin, indicating signalling via a pathway distinct from these activators. Synergy was also blocked by MAb rv.3, or F(ab')₂ fragments of 14A2.H1. The similar low copy number of PETA-3 and FcγRII in the platelet membrane (approximately 1000/platelet). together with the dependence on FcγRII for activation by MAb 14A2.H1, suggests that PETA-3 may be a component of the FcγRII signal transducing complex in platelets.     

Exposure of platelet fibrinogen receptors by a monoclonal antibody to CD9 antigen

We found that a monoclonal antibody to CD9 antigen, PMA2, induces fibrinogen binding to platelets and examined the mechanism for this. That PMA2 recognized the CD9 antigen was confirmed by its immunoblot-reactivity with a 24,000-dalton protein, reactivity with platelets and common acute lymphoblastic leukemia (ALL) cells, and competitive binding with the ALB6 antibody known as the CD9 antibody. At saturation, PMA2 bound to approximately 46,000 sites per platelet. The binding of 125I-fibrinogen to platelets occurred in a PMA2 concentration-dependent manner and was blocked by EDTA or an anti-glycoprotein (GP)IIb-IIIa monoclonal antibody. PMA2-stimulated platelets caused ATP secretion and thromboxane B2 synthesis under non-stirred conditions. The role of secreted ADP and thromboxane in fibrinogen-binding and subsequent platelet aggregation was studied using creatine phosphate/creatine phosphokinase (CP/CPK) and aspirin. CP/CPK or aspirin alone reduced fibrinogen binding to 20% to 30%; however, this binding was sufficient to support full platelet aggregation. Combined treatment with CP/CPK and aspirin abolished fibrinogen binding and aggregation. These results demonstrate that the binding of IgG molecules to the CD9 antigen exposes fibrinogen receptors through both secreted ADP and thromboxane and that either one of both can expose the receptors to an extent sufficient to aggregate platelets.     

Platelet activation by CD9 monoclonal antibodies is mediated by the FCγII receptor

The function of the human cell surface CD9 antigen is not known, yet monoclonal antibodies (mAbs) of the IgG1 subclass in the CD9 cluster induce activation of platelets. Previously it had been shown that this activation pathway is comparable both in kinetics and extent to physiological agonists such as thrombin. Here it is demonstrated that activation with CD9 mAbs depends on interaction of the Fc part of the CD9 antibody molecule with Fc receptors on the platelet surface, since: (i) mAb directed against the Fc receptor totally blocked the platelet response to CD9 mAb; and (ii) F(ab')2 fragments of the CD9 mAb SYB-1 which bound to platelets, as demonstrated by flow cytometry, failed to activate them. Furthermore, platelet activation by CD9 mAb closely paralleled the activation caused by cross-linking Fc receptors when comparing: (i) kinetics and extent of aggregation; (ii) thromboxane synthesis; (iii) calcium flux; and (iv) the cytoplasmic alkalinization response. Thus it is concluded that CD9 antigen itself does not necessarily participate in stimulus-response coupling leading to platelet activation by CD9 mAbs, and that this activation can be entirely accounted for by the Fc receptor pathway mechanism. The results suggest a possible novel mechanism for platelet consumption in cases of immune thrombocytopenia.     

Platelet function and activation markers in primary hypercholesterolemia treated with anti-PCSK9 monoclonal antibody: A 12-month follow-up

Background and aimsIn the association between hypercholesterolemia (HC) and thrombotic risk platelet hyper-reactivity plays an important role. The inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) to reduce plasma LDL-cholesterol merges as effective therapeutic strategy to prevent cardiovascular (CV) events. Aim of this study was to verify whether a treatment up to 12 months with the monoclonal antibodies (mAbs) anti-PCSK9 influences platelet function in primary HC.Methods and resultsIn patients affected by primary HC (n = 24), all on background of statin and 17 on acetyl salicylic acid (ASA), platelet function parameters were evaluated at baseline up to 12 months of treatment with the mAb anti-PCSK9 alirocumab or evolocumab.From baseline, the treatment with anti-PCSK9 mAbs: i) in ASA HC patients, significantly decreased platelet aggregation detected in platelet-rich plasma by light transmission aggregometry and in whole blood Platelet Function Analyzer-100 assay; ii) in all HC patients, significantly decreased platelet membrane expression of CD62P and plasma levels of the in vivo platelet activation markers soluble CD40 Ligand, Platelet Factor-4, and soluble P-Selectin. Furthermore, CD62P expression, and sP-Selectin, PF-4, sCD40L levels significantly correlated with serum PCSK9.ConclusionBesides markedly lowering LDL-c levels, our results suggest that HC patients benefit from anti-PCSK9 mAb treatment also for reducing platelet reactivity and increasing platelet sensitivity to the inhibitory effects of aspirin. These effects on platelets could play a role in the reduction of CV event incidence in patients treated with PCSK9 inhibitors.     

Platelet activation by a novel solid-phase agonist: effects of VWF immobilized on polystyrene beads

The interaction between platelets stirred in suspension and VWF immobilized on polystyrene beads was studied. Platelets aggregated and released ATP in response to stirring with VWF beads. Closer examination of the interaction using transmission electron microscopy revealed that the platelets did not simply aggregate with one another but initially adhered to the beads and spread. Platelets in suspension then bound to the bead-adherent platelets forming layers of platelets associated with each bead. The VWF bead-induced platelet activation was com-pletely inhibited by addition of monoclonal antibody (mAb) to GPIb or GPIIb/IIIa. In addition, the activation response was inhibited in the presence of aspirin, indomethacin or the thromboxane receptor antagonist BM13.177, demonstrat-ing a dependence on an intact cyclo-oxygenase pathway.   Platelet function studies were carried out on 30 patients with a history of mild bleeding using conventional optical aggregation and VWF bead-induced platelet activation. 12 patients were abnormal by conventional optical aggregometry, whereas 27 patients showed depressed ATP release in response to VWF beads. The results suggest that easily-bruised patients may have a platelet function defect rather than a vascular-based abnormality and that VWF bead-induced platelet activation is a more sensitive test for detecting platelet dysfunction.     

Platelet activation and inhibition of malarial cytoadherence by the anti-CD36 IgM monoclonal antibody NL07

The surface glycoprotein CD36 (GPIV) is known to mediate the adhesion of Plasmodium falciparum malaria-infected red blood cells and to be a receptor for extracellular matrix proteins such as collagen and thrombospondin. The murine monoclonal IgM antibody NL07, which is specific for CD36, has now been shown to also be a potent inhibitor of the adhesion of P falciparum malaria-infected red blood cells to C32 melanoma cells. Treatment of platelets with NL07 monoclonal antibody resulted in rapid degranulation, release of ATP and serotonin, increase in [Ca2+]i, and tyrosine phosphorylation of a substrate protein of 130 kD. In about one-half of the experiments, activation with NL07 resulted in the formation of small aggregates of 10 to 30 platelets, whereas in the other half of the experiments, large aggregates were seen similar to those induced by adenosine diphosphate (ADP) and these large aggregates could be converted to the small aggregates by ATP alpha S or by AP-2 or other antibodies against GPIIb and/or IIIa. Microaggregates of 2 to 5 platelets were seen with Glanzmann's platelets that constitutively lack GPIIb/IIIa. Aggregate formation was not seen with heat-treated serum, in the presence of anti C1q antibodies, or when using C5-, C8-, or C9-deficient human sera. Although activation of platelets with purified complement components results in a slow morphologic change without aggregation, involvement of CD36 results in rapid complement-mediated activation leading to formation of small aggregates that is largely independent of GPIIb/IIIa and that, under certain circumstances, proceeds to the formation of large ADP-dependent aggregates.     

Hedgehog signal transduction by Smoothened: Pharmacologic evidence for a 2-step activation process

The Hedgehog (Hh) signaling pathway controls growth, cell fate decisions, and morphogenesis during development. Damage to Hh transduction machinery can lead to birth defects and cancer. The transmembrane protein Smoothened (Smo) relays the Hh signal and is an important drug target in cancer. Smo enrichment in primary cilia is thought to drive activation of target genes. Using small-molecule agonists and antagonists to dissect Smo function, we find that Smo enrichment in cilia is not sufficient for signaling and a distinct second step is required for full activation. This 2-step mechanism—localization followed by activation—has direct implications for the design and use of anticancer therapeutics targeted against Smo.     

Induction of platelet Ca2+ influx and mobilization by a monoclonal antibody to CD9 antigen

We found that a monoclonal antibody (MoAb) to CD9 antigen, PMA2, induced a rise in cytosolic free calcium concentration ([Ca2+]i) in fura-2-loaded platelets, and we examined whether this response was due to direct action of PMA2 on CD9 antigen. The rise in [Ca2+]i was dependent on the PMA2 concentration, irrespective of the presence or absence of extracellular Ca2+. The role of secreted adenosine diphosphate (ADP) and thromboxane in the [Ca2+]i response to PMA2 was studied using creatine phosphate/creatine phosphokinase (CP/CPK) and aspirin. Combined treatment with CP/CPK and aspirin abolished the rise in [Ca2+]i, although either CP/CPK or aspirin alone produced only partial inhibition. Inhibition of adenosine triphosphate (ATP) secretion and thromboxane B2 synthesis by an MoAb to the glycoprotein IIb-IIIa complex, PMA1, resulted in little [Ca2+]i response to PMA2. In contrast, thrombasthenic platelets, in which ATP secretion and thromboxane B2 synthesis were normal, showed a normal [Ca2+]i response. When PMA2 was added to CD9+ mononuclear cells, no rise in [Ca2+]i was observed. Thus, we conclude that binding of monoclonal immunoglobulin G molecules to the CD9 antigen raises [Ca2+]i through the effect of secreted ADP and thromboxane on platelets, and that CD9 antigen is not directly involved in induction of Ca2+ influx and mobilization.     

Enhancement of T cell activation by immobilized hu5C8 (anti-CD40L) monoclonal antibody

Background: Soluble monoclonal antibodies (MoAb) targeting CD40L on T cells can partially block T cell alloreactivity by preventing the costimulatory signal of antigen presenting cells through CD40. However, it is not known if these MoAbs can also deliver inhibiting or stimulating signals through the CD40L receptor. Materials and methods: Blood mononuclear cells were stimulated by mitogens or allogeneic stimulator cells in the presence of hu5C8 MoAb, either in soluble form, or immobilized to the culture wells. T cell responses were evaluated by means of primary and secondary mixed lymphocyte culture (MLC), cytotoxic T lymphocyte (CTL) generation, immunophenotype, apoptosis assay and cytokine release. Also, the effect of hu5C8 on cells inhibited by CTLA4‐Ig was tested. Results: While the soluble hu5C8 inhibited T cell proliferation, the immobilized hu5C8 enhanced both mitogen and alloantigen‐induced proliferative and cytotoxic T cell responses, without inducing further apoptotic T cell death. In the presence of CTLA4‐Ig, immobilized hu5C8 increased the residual CD28‐independent proliferation of alloantigen‐specific T cells both in primary and secondary MLC, and prevented the inhibiting effect of CTLA4‐Ig on the generation of CTL. Immobilized hu5C8 MoAb‐stimulated T cells also showed a limited capacity of producing interleukin (IL)‐10, even in the presence of CTLA4‐Ig. Conclusions: We show that the hu5C8 MoAb has a strong mitogenic activity when immobilized, likely due to higher crosslinking capacity as compared to the soluble antibody. Strategies to induce ex‐vivo T cell responses against tumor or viral antigens by means of hu5C8 MoAb antibody will be exploited based on these findings.     

Cellular distribution of a colonic adenoma-associated antigen as defined by monoclonal antibody Adnab-9.

Adenomatous colonic polyps constitute a precursor for colorectal cancer. Antibodies to these precancerous lesions might identify specific early tumor antigens. Adnab-9 is a murine monoclonal antibody raised against membranes of colonic adenomas. Adnab-9 binding in colonic washings (effluent) correlates with the presence of colorectal cancer. Immunohistochemical staining with Adnab-9 shows cytoplasmic reactivity in scattered cells in 4 of 31 adenomatous tissue sections, 0 of 14 sections of colorectal cancer cells, and 1 of 8 normal-appearing colonic mucosa specimens examined. Adnab-9 recognized a dominant M(r) 87,000 protein species in tissue extracts in the membrane-bound fraction of effluent by Western blotting. Adnab-9 binding by enzyme-linked immunosorbent assay in adenomatous extracts is higher than cancer or normal tissue, is membrane-bound, and is absent from established colorectal cancer cell lines. This distribution and nature of immunostaining suggest that Adnab-9 recognizes a determinant associated with the membrane component of a subpopulation of adenoma cells which may have a role in early colorectal neoplasia.     

Chimeric CD7 monoclonal antibody therapy in rheumatoid arthritis.

Murine monoclonal antibody (Mab) therapy in patients with rheumatoid arthritis (RA) produces an antimouse immunoglobulin response by the recipient. We studied a chimeric (human/mouse) CD7 Mab, in a dose ranging tolerability study in 10 patients with RA. Modest improvements in disease activity occurred with frequent acute adverse effects of malaise, fever and nausea. After treatment, peripheral blood T lymphocyte numbers fell by 50% and CD7 expression fell by 97% for less than 7 days. Our study demonstrates chimeric Mab function in vivo and illustrates the influence of antibody isotype and patient characteristics on adverse effects.     

Flow cytometric platelet enumeration utilizing monoclonal antibody CD42a

Summary We investigated a flow cytometric method with monoclonal antibody CD42a as a potential reference method for platelet enumeration by blood count analysers. Using peripheral blood samples from 25 healthy individuals we obtained significant (P < 0.0001) correlations between the results obtained by a FACScan method and similar cell counters (TOA Medical Electronics, Kobe, Japan): r = 0.960 (FACScan vs SSF), r = 0.958 (FACScan vs SE-9000A,B), r = 0.949 (FACScan vs K-4500A) and r = 0.954 (FACScan vs K-4500B). This flow cytometric method for counting platelets using the monoclonal antibody CD42a can be used to check the calibration of the platelet count by blood cell analysers.     

Inhibition of cell surface mediated plasminogen activation by a monoclonal antibody against alpha-Enolase

Localization of plasmin activity on leukocyte surfaces plays a critical role in fibrinolysis as well as in pathological and physiological processes in which cells must degrade the extracellular matrix in order to migrate. The binding of plasminogen to leukocytic cell lines induces a 30- to 80-fold increase in the rate of plasminogen activation by tissue-type (tPA) and urokinase-type (uPA) plasminogen activators. In the present study we have examined the role of alpha-enolase in plasminogen activation on the cell surface. We produced and characterized a monoclonal antibody (MAb) 11G1 against purified alpha-enolase, which abrogated about 90% of cell-dependent plasminogen activation by either uPA or tPA on leukocytoid cell lines of different lineages: B-lymphocytic, T-lymphocytic, granulocytic, and monocytic cells. In addition, MAb 11G1 also blocked enhancement of plasmin formation by peripheral blood neutrophils and monocytes. In contrast, MAb 11G1 did not affect plasmin generation in the presence of fibrin, indicating that this antibody did not interact with fibrinolytic components in the absence of cells. These data suggest that, although leukocytic cells display several molecules that bind plasminogen, alpha-enolase is responsible for the majority of the promotion of plasminogen activation on the surfaces of leukocytic cells.     

Identification of a human erythroid cell surface antigen by monoclonal antibody HAE9

A noncytotoxic monoclonal antibody (IgM) HAE9 that selectively binds to 36% CFU-E and more than 90% nucleated erythroid cells in human bone marrow is described. This antibody recognizes a 70-kDa-membrane protein. It is suggested that HAE9 is directed to a human epitope of Ag-Eb, an interspecies mammalian erythroid-specific cell surface marker.     

Fc-independent cross-linking of a novel platelet membrane protein by a monoclonal antibody causes platelet activation

A monoclonal antiplatelet antibody (MA-13G8E1) is described that dose- dependently induces platelet aggregation and serotonin release in an Fc- independent fashion. Whereas platelets were equally aggregated by F(ab')2 fragments of this monoclonal antibody (MoAb), its Fab fragments, on the other hand, were inactive, indicating that divalent interaction is an essential requirement to induce platelet activation by MA-13G8E1. In addition, we could show that platelet epitope cross- linking by MA-13G8E1 occurred on the same platelet. MA-13G8E1 stimulated platelet phospholipase C (PLC) and induced activation of protein kinase C (PKC), both of which were almost unaffected by aspirin pretreatment. Furthermore, PLC activation appeared to be a direct antibody-mediated effect, since intracellular Ca2+ rises were not inhibited by EGTA, cytochalasin B, or aggregation-blocking MA-16N7C2 (antiglycoprotein [anti-GP]IIb/IIa). The MA-13G8E1 antigen is constitutively expressed on resting platelets of different species (7,100 +/- 800 molecules per human platelet), but not on other cell types tested. Both immunoprecipitation and affinity isolation by MA- 13G8E1 showed two low-molecular weight proteins (45 and 36 kD), having slightly acidic isoelectric pH levels (4.5 to 5.5) and forming multimolecular complexes. In conclusion, we found an MoAb that is able to induce platelet activation in an Fc-independent fashion. The mechanism involves cross-linking of a hitherto undescribed platelet membrane protein, leading to PLC and PKC stimulation.     

Platelet surface antigens: Analysis by monoclonal antibodies

Summary Monoclonal antibodies were produced against human platelets. Four antibodies (PA1, PA2, PA3 and PA4) reacted specifically with platelets and megakaryocytes, but not with peripheral blood lymphocytes, granulocytes, erythrocytes or monocytes. The antibodies belonged to the mouse IgG subclass 2a (PA1, PA2, PA3), or 1 (PA4) respectively. PA1 and PA4 did not precipitate, their antigens have not yet fully been characterized. PA3 was directed against the glycoprotein (Gp) complex IIb/IIIa; PA2 precipitated Gp IIb/IIIa, and, in addition, Gp Ia. PA4 revealed specificity against the human platelet alloantigen Zw(a).     

Platelet activation induced by an antiplatelet autoantibody against CD9 antigen and its inhibition by another autoantibody in immune thrombocytopenic purpura

Summary. In a patient with immune thrombocytopenic purpura (ITP), we found a novel platelet-activating IgG (act-IgG) and an inhibitory IgG (inhi-IgG) that prevented activation induced by both CD9 monoclonal antibody (mAb) and the act-IgG. Purified IgG from the patient plasma caused a rise in [Ca²⁺]_i and the aggregation of normal platelets, and bound to a 24 kD membrane protein. This aggregation was inhibited by aspirin, staurosporine, an inhibitor of protein kinase C, and F(ab')₂ fragments of MALL13, a CD9 mAb. When the platelet count of this patient rose to normal range, the act-IgG disappeared. About 2 weeks later, the relapse of thrombocytopenia was observed. The purified IgG obtained in this period did not activate platelets but inhibited both the rise in [Ca²⁺]_i and platelet aggregation stimulated by NNKY 1–19, a CD9 mAb, as well as the act-IgG, and bound to a 40 kD membrane protein. The inhi-IgG prevented the binding of IV-3, a mAb against Fcγ receptor II (FcγRII), but did not prevent the binding of NNKY 1–19 to its antigen. We suggest that the activating autoantibody recognized CD9 antigen and activated both the thromboxane- and phospholipase C-dependent pathways, while the inhibitory autoantibody recognized the FcγRII and inhibited CD9 antibody-induced platelet activation mediated via this receptor.     

Inhibition of zymosan activation of human neutrophil oxidative metabolism by a mouse monoclonal antibody

We have studied a neutrophil-specific murine monoclonal antibody, PMN7C3 (IgG3), which specifically alters PMN oxidative metabolism stimulated by serum-opsonized zymosan (STZ) or Candida albicans (STC). Polymorphonuclear cells (PMNs) exposed to PMN7C3 show a significant depression in O2- release (52.8% +/- 2.5% of control), H2O2 release (44.4% +/- 6.0% of control), and O2 consumption (73.9% +/- 2.6% of control) in response to STZ. O2 release in response to phorbol myristate acetate (PMA) was modestly reduced (78.4% +/- 3.7%) by PMN7C3 treatment, but not to the extent seen with STZ or STC. PMN7C3 did not affect O2 release by PMNs stimulated by zymosan opsonized with IgG or by S. aureus, A 23187, or FMLP. PMN7C3 was not cytotoxic, did not trigger oxidative metabolism when used as a stimulus, did not alter STZ- induced degranulation, and did not interfere with binding or uptake of STZ by PMNs. Exposure of PMNs to PMN7C3 decreased PMN rosette formation with erythrocytes coated with C3b (54% of control) or C3bi (63% of control), but had no affect on rosette formation with IgG-coated erythrocytes. PMN7C3 does not bind to monocytes and had no affect on rosette formation by this cell type. Binding of antibody PMN7C3 to the neutrophil surface inhibits the oxidative response to opsonized STZ or STC, possibly in part by altering the function or expression of C3b and C3bi receptors. Monoclonal antibodies such as PMN7C3 provide highly specific probes that may be used to define the molecular features of the stimulus-coupled response of PMN activation.     

An autoimmuno-dominant thyroglobulin epitope characterized by a monoclonal antibody.

It has become evident in recent years that autoimmune thyroglobulin (Tg) antibodies of Graves disease and Hashimoto's thyroiditis show a restricted epitope repertoire compared to Tg heteroantibodies. We have produced monoclonal antibodies (Mab) against human Tg by the hybridoma technique and the epitope specificity was determined by crossblocking experiments. Six noncrossreactive Mabs were used in a double determinant IRMA system for plasma Tg measurements. Sensitivity of the assays was between 1 and 2 ng/ml, intraassay variation less than 5%. Recovery experiments with added Tg were performed in 25 Graves sera with elevated Tg autoantibodies. Monoclonal antibody Tg13 showed an unusual strong interference with autoantibodies resulting in a very low recovery in all sera (median: less than 10%). In further studies Tg was digested by trypsin and after Western blotting, the resulting fragments were incubated with different Mab antibodies, a polyclonal antibody and 10 different Graves sera with high Tg autoantibodies. In contrast to all other mabs only Mab Tg13 showed several low molecular weight bands between 17 and 50 KD. The major bands recognized by Mab Tg13 corresponded to bands obtained by the autoimmune sera, which showed a very homogeneous band pattern. We conclude that Mab Tg13 is specific for an autoimmunodominant B cell epitope of human Tg.