Autoantibody against integrin αv β3 contributes to thrombocytopenia by blocking the migration and adhesion of megakaryocytes

Essentials The pathogenesis of immune thrombocytopenia (ITP) has not been fully clarified. We analyzed the role of anti-αvβ3 autoantibody in the pathogenesis of ITP in patients. Anti-αvβ3 autoantibody impeded megakaryocyte migration and adhesion to the vascular niche. Anti-αv β3 autoantibody potentially contributes to the pathogenesis of refractory ITP.

SUMMARY

Background The pathogenesis of immune thrombocytopenia (ITP) has not been fully clarified. Anti-αvβ3 integrin autoantibody is detected in chronic ITP patients, but its contribution to ITP is still unclear. Objectives To clarify the potential role of anti-αvβ3 integrin autoantibody in chronic ITP and the related mechanism. Methods Relationship between levels of anti-αvβ3 autoantibody and platelets in chronic ITP patients was evaluated. The influence of anti-αvβ3 antibody on megakaryocyte (MK) survival, differentiation, migration and adhesion was assessed, and the associated signal pathways were investigated. Platelet recovery and MKs' distribution were observed in an ITP mouse model pretreated with different antibodies. Result In this study, we showed that the anti-αvβ3 autoantibody usually coexists with anti-αIIbβ3 autoantibody in chronic ITP patients, and patients with both autoantibodies have lower platelets. In in vitro studies, we showed that the anti-αvβ3 antibody had no significant effect on the survival and proliferation of MKs, whereas it decreased formations of proplatelet significantly. Anti-αvβ3 antibody impeded stromal cell derived facor-1 alpha (SDF-1α)- mediated migration and inhibited the phosphorylation of protein kinase B. Anti-αvβ3 antibody significantly inhibited MKs' adhesion to endothelial cells and Fibrogen. The phosphorylation of focal adhesion kinase and proto-oncogene tyrosine-protein kinase Src induced by adhesion was inhibited when MKs were pretreated with anti-αvβ3 antibody. In in vivo studies, we showed that injection with anti-αv antibody delayed platelet recovery in a mouse model of ITP. Conclusions These findings demonstrate that the autoantibody against integrin αv β3 may aggravate thrombocytopenia in ITP patients by impeding MK migration and adhesion to the vascular niche, which provides new insights into the pathogenesis of ITP.

Icaritin Improves Antibody-Induced Thrombocytopenia in a Mouse Model by Regulating T-cell Polarization

Previous studies have shown that icaritin (ICT) has significant protective effects on immune thrombocytopenia (ITP), and the present study aimed to discuss the mechanism of this protective effect from the aspect of regulating T-cell polarization by an antibody-induced ITP mice model. Mice were given rat anti-mouse CD41 antibody (MWReg30) by intraperitoneal injection for 7 d to produce ITP model. At the same time, ICT was administrated at 10 mg/kg/d orally for 9 d. Peripheral blood platelets were counted by hematology analyzer. Spleen index was also tested. Spleen T-helper cell (Th), cytotoxic T-cell (CTL), Th1, Th2, Th17, regulatory T-cell (Treg), and follicular helper T-cell (Tfh) were quantified by flow cytometry. Serum Th1/Th2/Th17 cytokines were tested by mouse Th1/Th2/Th17 cytometric bead array (CBA) kit and transforming growth factor beta (TGF-β) were analyzed by enzyme-linked immunosorbent assay (ELISA) kit. The results indicated that ICT (10 mg/kg) protected against MWReg30-induced ITP, as evidenced by increased blood platelets and decreased spleen index. In addition, the imbalance of Th/CTL in ITP mice spleen was regulated by ICT. Meanwhile, ICT inhibited Th1, Th17, and Tfh and improved Th2 and Treg in ITP mice spleen. Furthermore, the results of CBA and ELISA suggested that ICT decreased serum Th1- and Th17-related cytokines and increased Th2 cytokines, as well as promoted the release of TGF-β. These results demonstrated that the protective effect of ICT on ITP was mediated by regulating T-cell polarization.

Integrin-Alpha IIb Identifies Murine Lymph Node Lymphatic Endothelial Cells Responsive to RANKL

Microenvironment and activation signals likely imprint heterogeneity in the lymphatic endothelial cell (LEC) population. Particularly LECs of secondary lymphoid organs are exposed to different cell types and immune stimuli. However, our understanding of the nature of LEC activation signals and their cell source within the secondary lymphoid organ in the steady state remains incomplete. Here we show that integrin alpha 2b (ITGA2b), known to be carried by platelets, megakaryocytes and hematopoietic progenitors, is expressed by a lymph node subset of LECs, residing in medullary, cortical and subcapsular sinuses. In the subcapsular sinus, the floor but not the ceiling layer expresses the integrin, being excluded from ACKR4⁺ LECs but overlapping with MAdCAM-1 expression. ITGA2b expression increases in response to immunization, raising the possibility that heterogeneous ITGA2b levels reflect variation in exposure to activation signals. We show that alterations of the level of receptor activator of NF-κB ligand (RANKL), by overexpression, neutralization or deletion from stromal marginal reticular cells, affected the proportion of ITGA2b⁺ LECs. Lymph node LECs but not peripheral LECs express RANK. In addition, we found that lymphotoxin-β receptor signaling likewise regulated the proportion of ITGA2b⁺ LECs. These findings demonstrate that stromal reticular cells activate LECs via RANKL and support the action of hematopoietic cell-derived lymphotoxin.

Identification of CDCA1-derived long peptides bearing both CD4+ and CD8+ T-cell epitopes: CDCA1-specific CD4+ T-cell immunity in cancer patients

We recently identified a novel cancer-testis antigen, cell division cycle associated 1 (CDCA1) using genome-wide cDNA microarray analysis, and CDCA1-derived cytotoxic T lymphocyte (CTL)-epitopes. In this study, we attempted to identify CDCA1-derived long peptides (LPs) that induce both CD4+ helper T (Th) cells and CTLs. We combined information from a recently developed computer algorithm predicting HLA class II-binding peptides with CDCA1-derived CTL-epitope sequences presented by HLA-A2 (A*02:01) or HLA-A24 (A*24:02) to select candidate CDCA1-LPs encompassing both Th cell epitopes and CTL-epitopes. We studied the immunogenicity of CDCA1-LPs and the cross-priming potential of LPs bearing CTL-epitopes in both human in vitro and HLA-class I transgenic mice in vivo. Then we analyzed the Th cell response to CDCA1 in head-and-neck cancer (HNC) patients before and after vaccination with a CDCA1-derived CTL-epitope peptide using IFN-γ enzyme-linked immunospot assays. We identified two CDCA1-LPs, CDCA1(39–64)-LP and CDCA1(55–78)-LP, which encompass naturally processed epitopes recognized by Th cells and CTLs. CDCA1-specific CTLs were induced through cross-presentation of CDCA1-LPs in vitro and in vivo. In addition, CDCA1-specific Th cells enhanced induction of CDCA1-specific CTLs. Furthermore, significant frequencies of CDCA1-specific Th cell responses were detected after short-term in vitro stimulation of peripheral blood mononuclear cells (PBMCs) with CDCA1-LPs in HNC patients (CDCA1(39–64)-LP, 74%; CDCA1(55–78)-LP, 68%), but not in healthy donors. These are the first results demonstrating the presence of CDCA1-specific Th cell responses in HNC patients and underline the possible utility of CDCA1-LPs for propagation of both CDCA1-specific Th cells and CTLs.

Calreticulin-independent regulation of the platelet integrin αIIbβ3by the KVGFFKR αIIb-cytoplasmic motif

The platelet integrin alphaIIbbeta3 alters conformation in response to platelet activation and ligand binding, although the molecular mechanisms involved are not known. We previously showed that a lipid modified peptide, corresponding to the membrane proximal 989KVGFFKR995 portion of the alphaIIb cytoplasmic tail, independently activates platelet alphaIIbbeta3. Calreticulin (CRT) is a potential integrin regulatory protein based on its interaction with the highly conserved alpha-integrin sequence KxGFFKR. We therefore examined the possible interaction of calreticulin and alphaIIbbeta3 in human platelets. We demonstrate that calreticulin in platelets is localised to the granulomere. In contrast, the known integrin-binding protein talin accumulates at the periphery of spreading platelets and colocalises with alphaIIbbeta3 during the process of adhesion. An interaction between calreticulin and alphaIIbbeta3 could not be demonstrated using co-immunoprecipitation techniques under various platelet activation states, even in the presence of covalent chemical crosslinkers. Thus, calreticulin does not functionally interact with the major integrin in human platelets. In order to identify proteins that interact with the integrin KVGFFKR motif we then used a peptide 'pull-down' assay from platelet lysates with biotinylated peptides and demonstrate that only the alphaIIb and beta3 subunits selectively and individually interact with this sequence. This interaction is divalent cation-dependent, has high-affinity, and occurs both with purified alphaIIbbeta3 complex and with electroeluted alpha and beta subunits. Thus, our data show that the conserved integrin KVGFFKR domain interacts primarily with the alpha and beta cytoplasmic tails and not with CRT in human platelets.

[Generation of human Fab antibody against platelet membrane glycoprotein IIb/IIIa and its effect on platelet aggregation]

AIM

To generation of human Fab fragment against GP GPIIb/IIIa from phage display library and to study its effect on platelet aggregation.

METHODS

An antibody phage display library was constructed from spleen cells from a donor with idiopathic thrombocytopenic purpura(ITP) whose anti-GPIIb/IIIa antibody was positive. High-affinity human mAbs was selected by panning against GPIIb/IIIa expressed on CHO123 cells. The binding specificity of phage antibody to GPIIb/IIIa was detected by ELISA and Western blot. And the effect of antibody on platelet aggregation was analyzed.

RESULTS

After three rounds of panning with CHO123, phage antibodies against GPIIb/IIIa were enriched specifically. 2 positive phage antibodies with high specific for GP IIb/IIIa were verified, and the purified antibody can inhibited ADP induced platelet aggregation.

CONCLUSION

Human antibodies against GPIIb/IIIa are obtained from phage display library.

[hNUDC promotes proliferation and differentiation of megakaryocytopoiesis on human CD341+ cells]

AIM

aTo study the effect of human nuclear distribution C èhNUDCé on human megakaryocyte proliferation and differentiation from cord blood CD34(+) cells in vitro.

METHODS

aHuman CD34(+) cells were isolated using the Dynal CD34 Progenitor Cell Selection System from umbilical cord blood. The CD34(+) cells were then cultured in serum free methylcellulose semi-solid media, the morphologic aspects and number of small, medium and large CFU-MK colonies were observed and scored on the day12 by microscopy analysis. The CD34(+) cells were cultured in serum free liquid media, cells were removed on day 10 and formation of CD41(+) in human megakaryocyte and its DNA polyploidization of nuclear were analyzed on a FACsort flowcytometer.

RESULTS

ahNUDC supported the formation of small and medium CFU-MK colony in serum free semi-solid media. Furthermore, hNUDC induced a remarkable increase in expression of the megakaryocyte cell surface marker CD41(+) and stimulated the CD41(+) DNA polyploidization more effectively than TPO.

CONCLUSION

hNUDC may play an important role in megakaryocyte proliferation and differentiation.

Reticulated platelet counts correlate with treatment response in patients with idiopathic thrombocytopenic purpura and help identify the complex causes of thrombocytopenia in patients after allogeneic hematopoietic stem cell transplantation

BACKGROUND

In thrombocytopenic conditions of unknown origin, quantification of reticulated platelets (RP) in the peripheral blood by flow cytometry has been shown to differentiate increased platelet (Plt) turnover from insufficient Plt production.

METHODS

We used a whole blood flow cytometry method combining thiazole orange and anti-CD41a-staining to assess RP in 71 healthy subjects, six with thrombocytopenic myelodysplastic syndrome (MDS), nine with liver cirrhosis, 14 patients with idiopathic thrombocytopenic purpura (ITP), and 12 patients who had undergone hematopoietic stem cell transplantation (HSCT).

RESULTS

Patients with MDS had normal, patients with liver cirrhosis had slightly elevated RP counts compared to healthy subjects. ITP patients had elevated RP counts, and RP >15% were associated with treatment response (P = 0.015). In 7/10 patients after HSCT, an increase of RP preceded Plt recovery, whereas in patients with secondary thrombocytopenia after normal regeneration, the assessment of RP allowed the differentiation between conditions with high Plt turnover, such as GvHD and microangiopathy, indicated by high RP counts, and graft failure, indicated by low RP counts.

CONCLUSIONS

Our data provide the rationale for prospective studies on the diagnostic and prognostic value of RP counts in larger patient populations with ITP and after HSCT.

[3 polymorphisms of gene GP IIb and the function of GP IIb T13959 G in the platelet transfusion refractoriness]

OBJECTIVE

To investigate the relationship among 3 polymorphisms of GP IIb and the function of GP IIb T13959 G in the platelet transfusion refractoriness(PTR).

METHODS

The 26th exon, the 30th exon and the 21st intron of gene GP IIb in 110 individuals were amplified by polymerase chain reaction (PCR), and the PCR products were analyzed with single-strand conformation polymorphism(SSCP) and sequenced to investigate whether there was linkage among the polymorphisms of the gene. Human platelet antigen-3 (HPA-3) gene frequency was detected by Fok I enzyme in 147 patients with hematologic diseases, and was compared with that in 110 normal individuals. Forty-four patients who received apheresis platelet transfusion repeatedly were randomly divided into the HPA-3 homotype group and the control group. The antibodies of the platelet were detected after 3 times of platelet transfusion.

RESULTS

There were polymorphisms of gene GP IIb in the 26th, 30th exon and the 21st intron, and the mutations were: T changed into G in 13,959 th of the 26th exon; C changed into T in 16,997 th of the 30th exon; the 9 bps deletion occurred in 11,996-12,004 th of the 21st intron. The 3 polymorphisms had synchronization in the individuals. The results of Fok I enzyme indicated that the frequency of HPA-3a was 83.6% (92/110)and 81.9%(119/147), and that of HPA-3b was 16.4%(18/110) and 19.1%(28/147) in the normal individuals and the patients respectively. There was no significant difference between the patients and normal individuals (P>0.05). After the platelet transfusion, the antibodies of all the cases of the homotype platelet transfusion were negative, but the antibodies in 2 cases of the control group were positive, and there was antibody to HPA-3a in one of the antibodies positive cases.

CONCLUSION

(1)There is close linkage among the polymorphisms of gene GP IIb, which is T->G in 13 959 th of the 26th exon, C->T in 16,997 th of the 30th exon, and the 9 bps deletion in 11,996-12,004 th in the 21st intron. (2)The gene frequency of HPA-3a/3b is similar in the normal individuals and patients with hematologic diseases. (3) HPA-3 system may be one of the reasons for PTR in Chinese.

A unique T cell subset described as CD4loCD40+ T cells (TCD40) in human type 1 diabetes

Human T1D pancreatic lymph nodes contain diabetes-autoantigen responsive T cells but identification of such T cells in the periphery has proven difficult. Here we describe a unique T cell subset defined by CD4(lo) and CD40 expression (T(CD40)) that is significantly expanded in peripheral blood of T1D but not control or T2D subjects. The HLA-DR3 and DR4 alleles are considered high risk factors for T1D and T(CD40) expansion occurs in T1D subjects carrying HLA DR3 or DR4 haplotypes but, T1D subjects who do not carry either DR3 or DR4 haplotypes still have an expanded percentage of T(CD40) cells. Non-autoimmune subjects, even DR3(+) and DR4(+), do not have elevated percentages of T(CD40) cells. The majority of T(CD40) cells in T1D carry a memory phenotype and a portion of those proliferates when exposed to diabetes-associated self-antigens. A greater number of memory T(CD40) cells express CXCR3 when compared to CD40(-) memory cells and that number is significantly expanded in T1D compared to control subjects. If only total CD4(+) T cells are compared no difference in CXCR3 is seen. Furthermore, T(CD40) cells produce a Th1, pro-inflammatory cytokine profile. In healthy controls, T(CD40) cells have equally Th1 and Th2 profiles.

Distribution and dynamic changes of sphingolipids in blood in response to platelet activation

BACKGROUND

Sphingolipids are signaling molecules in a range of biological processes. While sphingosine-1-phosphate (S1P) is thought to be abundantly stored in platelets and released upon stimulation, knowledge about the distribution and function of other sphingolipids in blood is lacking.

OBJECTIVES

To analyze the sphingolipid content of blood components with special emphasis on dynamic changes in platelets.

METHODS

Blood components from mice and humans were prepared by gradient centrifugation and analyzed by liquid chromatography-mass spectrometry. Additionally, murine platelets were activated in vitro and in vivo.

RESULTS

Isolated non-activated platelets of mice were devoid of S1P, but instead contained dihydrosphingosine-1-phosphate (dhS1P), along with a high concentration of ceramide. Activation of platelets in vitro led to a loss of dhS1P and an increase in sphingosine, accompanied by a reduction of ceramide content. Platelet activation in vivo led to an immediate and continuous rise of dhS1P in plasma, while S1P remained stable. The sphingolipid distribution of human blood was markedly different from mice. Human platelets contained dhS1P in addition to S1P.

CONCLUSIONS

Mouse platelets contain dhS1P instead of S1P. Platelet activation causes loss of dhS1P and breakdown of ceramide, implying ceramidase activation. Release of dhS1P from activated platelets might be a novel signaling pathway. Finally, the sphingolipid composition of mouse and human blood shows large differences, which must be considered when studying sphingolipid biology.

Ex vivo large-scale generation of human platelets from cord blood CD34+ cells

In the present investigation, we generated platelets (PLTs) from cord blood (CB) CD34(+) cells using a three-phase culture system. We first cultured 500 CB CD34(+) cells on telomerase gene-transduced human stromal cells (hTERT stroma) in serum-free medium supplemented with stem cell factor (SCF), Flt-3/Flk-2 ligand (FL), and thrombopoietin (TPO) for 14 days. We then transferred the cells to hTERT stroma and cultured for another 14 days with fresh medium containing interleukin-11 (IL-11) in addition to the original cytokine cocktail. Subsequently, we cultured the cells in a liquid culture medium containing SCF, FL, TPO, and IL-11 for another 5 days to recover PLT fractions from the supernatant, which were then gel-filtered to purify the PLTs. The calculated yield of PLTs from 1.0 unit of CB (5 x 10(6) CD34(+) cells) was 1.26 x 10(11) - 1.68 x 10(11) PLTs. These numbers of PLTs are equivalent to 2.5-3.4 units of random donor-derived PLTs or 2/5-6/10 of single-apheresis PLTs. The CB-derived PLTs exhibited features quite similar to those from peripheral blood in morphology, as revealed by electron micrographs, and in function, as revealed by fibrinogen/ADP aggregation, with the appearance of P-selectin and activated glycoprotein IIb-IIIa antigens. Thus, this culture system may be applicable for large-scale generation of PLTs for future clinical use.

[The immunoreactivity of IgG and its fragments from ITP patients and their effects on platelet aggregation function]

OBJECTIVE

To prepare ITP plasma IgG and its F(ab')2 fragments and investigate their immunoreactivity to platelet GPIIb/IIIa and/or GPIb/IX and their effects on platelet aggregation function.

METHODS

The ITP patients having inhibitory autoantibody to the platelet aggregation were selected by modified MAIPA and platelet aggregation test with turbidimetry. Plasma IgG and its F(ab')2 fragments were prepared by streptococcal protein A affinity column and pepsin digestion. The immunoreactivity and the effects on platelet aggregation function of the whole antibody and its fragments were detected by modified MAIPA and platelet aggregation test, respectively.

RESULTS

(1) Anti-platelet GPIIb/IIIa and/or GPIb/IX autoantibodies were detected in 34 of 68 (53.6%) ITP patients' plasmas and that from 5 patients significantly inhibited the platelet aggregation induced by ADP or ristocetin. (2) By using protein A column combined with protease digestion, pure IgG and its F(ab')2 fragments were successfully obtained. (3) The purified IgG and its F(ab')2 fragments retained the ability to bind to their respective glycoproteins and inhibited the platelet aggregation function, whereas the IgG depleted plasma lost the ability of binding to the platelet GPs.

CONCLUSIONS

F(ab')2 fragment of the IgG antibody is a functional fragment, which not only has the binding ability to the platelet GPs but also inhibits the platelet aggregation function in a dose-dependent manner.

Cell- rather than antibody-mediated immunity leads to the development of profound thrombocytopenia during experimental Plasmodium berghei malaria

Experimental malarial thrombocytopenia can reach life-threatening levels and is believed to be due to Abs targeting platelets for destruction by the reticuloendothelial system. However, we report that Abs account for at most 15% of platelet destruction as Plasmodium berghei-infected B cell-deficient mice exhibited profound thrombocytopenia (83%) as did C57BL/6 controls (98%). Further, no significant increase in Abs bound to intact platelets was observed during infection. P. berghei infection can enhance the activity of anti-platelet Abs as indicated by a significantly (p < 0.005) increased thrombocytopenia on day 4 of infection in mice that were administered a low dose anti-CD41 mAb compared with rat IgG1-injected controls. RAG1-/- and CD4- plus CD8-deficient mice were markedly protected from thrombocytopenia (p < 0.005) and malarial pathogenesis. CD8- or TCRgammadelta-deficient mice were not protected from thrombocytopenia and CD4-deficient mice were modestly protected. RAG1-/- mice exhibited significantly (p < 0.05) lower levels of plasma TNF, IFN-gamma, and IL-12 during infection. IFNgamma-/- and IL-12-/- mice exhibited increased survival but similar thrombocytopenia to C57BL/6 controls. Collectively, these data indicate that thrombocytopenia is necessary but not sufficient for malarial pathogenesis and Abs are not the major contributors to malarial thrombocytopenia. Rather, we propose that both CD4+ and CD8+ T cell populations play key roles in malarial thrombocytopenia; a complex bidirectional interaction between cell-mediated immunity and platelets exists during experimental severe malaria that regulates both responses.

Agonists of toll-like receptor (TLR)2 and TLR4 are unable to modulate platelet activation by adenosine diphosphate and platelet activating factor

Inappropriate platelet activation is a feature of acute and chronic diseases such as disseminated intravascular coagulation (DIC) and atherosclerosis. Since proinflammatory microbial-derived agonists can be involved in the pathogenesis of these diseases, we examined the potential role of TLR4 (mediating responses to LPS) and TLR2 (which responds to bacterial lipopeptides) in platelet activation. Our data suggested low-level expression of TLR2 and TLR4 on platelets, determined by flow cytometry, and we also observed expression of TLR4 on a megakaryocytic cell line by both flow cytometry and immunohistochemistry. Stimulation of the platelets with the TLR4 agonist LPS, and the synthetic TLR2 agonist Pam3CSK4, resulted in no platelet aggregation, no increase in CD62P surface expression and no increase in the cytosolic concentration of Ca2+. The TLR agonists were also unable to directly activate platelets primed with epinephrine, or pretreated with a low concentration of ADP or PAF. Pretreatment of platelets with LPS or Pam3CSK4 also failed to modulate the platelet response to submaximal concentrations of the classical platelet agonists ADP and PAF. We conclude that the TLR agonists LPS and Pam3CSK4 have no direct effect on platelet activation and that platelet TLRs may be a remnant from megakaryocytes. TLR2 and TLR4 agonists are thought to have a significant role in diseases such as atherosclerosis and DIC, but our research suggests that this is through a mechanism other than direct platelet activation or by modification of platelet responses to other agonists.

[Results of clinical trials of a novel glycoprotein IIb-IIIa antagonist framon in high-risk coronary angioplasty]

New glycoprotein (GP) IIb-IIIa antagonist preparation framon (Monafram), is the F(ab')(2) fragment of a monoclonal antibody FRaMon directed against GP IIb-IIIa. This preparation blocks GP IIb-IIIa binding with fibrinogen and inhibits platelet aggregation both in vitro and upon intravenous administration. Safety and ability of framon to prevent thrombotic complications in high risk coronary angioplasty (CA) was evaluated in the present study. FRAMON was injected intravenously into 153 patients just before the start of procedure as a single bolus at the dose of 0.25 mg/kg. Control group was formed of 126 patients who underwent angioplasty without GP IIb-IIIa blockers. After framon administration there were no allergic reactions or major bleedings, deep thrombocytopenia (< 50000/microl) developed in 1 patient (< 1%), and antibodies against framon were detected in less than 5% of patients. Number of unfavorable outcomes (cardiovascular death, myocardial infarction, angina recurrence) within 1 month after CA was 3 times higher in control group than in the group of patients treated with framon (11.4% and 3.3%, respectively, p = 0.018). The effect of framon was most strongly pronounced within the first day after procedure -- administration of the drug reduced number of acute thromboses from 6.5% to 0.7% (p = 0.013). Significant differences between numbers of end points was still preserved at 6 months after procedure (25.7 and 14.2% in control and framon groups, respectively, p = 0.023). The data obtained proved safety and clinical efficacy of framon administration in coronary angioplasty with high risk of thrombotic complications.

Platelet depletion by anti-CD41 (alphaIIb) mAb injection early but not late in the course of disease protects against Plasmodium berghei pathogenesis by altering the levels of pathogenic cytokines

Accumulating evidence indicates that platelets play a critical role in the pathogenesis of experimental severe malaria (ESM) elicited by infection with Plasmodium berghei. Mice injected on day 1 of P berghei infection (early) with either anti-CD41 or anti-CD61 monoclonal antibodies (mAbs) exhibited significantly (P<.001) increased survival from ESM compared with infection controls, indicating that platelets function early in the disease. In contrast, groups of mice treated on days 4, 5, and 6 (late) with anti-CD41 mAb exhibited similar mortality as controls. Because platelet depletion by anti-CD41 mAb on day 4 of infection did not protect mice, and platelet adherence occurs on day 6, platelet adherence to endothelium is not required to mediate malarial pathogenesis. Few platelet microparticles were detected in the blood during the course of malaria, but large numbers of erythrocyte vesicles, microparticles, and debris were detected. The protective effect of early anti-CD41 mAb treatment was independent of the number of platelets, platelet microparticles, erythrocyte-platelet conjugates, and erythrocyte vesicles. Mice treated early with anti-CD41 mAb exhibited markedly altered cytokine production on day 4 of P berghei infection (increased interleukin 10 [IL-10], IL-1alpha, IL-6, interferon-gamma [IFN-gamma], and tumor necrosis factor alpha [TNF-alpha]; decreased IL-2) but no decline in coagulation factors compared with rat immunoglobulin G (IgG)-treated controls, indicating that platelets regulate the levels of pathogenic cytokines.

Intense exercise increases shear-induced platelet aggregation in men through enhancement of von Willbrand factor binding, glycoprotein IIb/IIIa activation, and P-selectin expression on platelets

Vigorous exercise transiently enhances the risk of primary cardiac arrest. Shear-induced platelet aggregation (SIPA) is an important mechanism in arterial thrombogenesis. This study investigates whether intense exercise affects SIPA, and elucidates mechanisms that underlie SIPA. Eighteen sedentary healthy men engaged in intense exercise (about 80% of maximal oxygen consumption) for 40 min on a bicycle ergometer. Platelet aggregation, binding of von Willebrand factor (vWF) to platelets, and activation of glycoprotein (GP) IIb/IIIa and expression of P-selectin on platelets induced by shear stress were analyzed both before and immediately after exercise. Analytical results demonstrated that: (1). the levels of plasma vWF antigen and activity were enhanced after intense exercise, (2). intense exercise increased either shear- or ristocetin-induced platelet aggregation and was accompanied by an increase in vWF binding to platelets and vWF-mediated GP IIb/IIIa activation at high shear flow, and (3). shear-induced P-selectin expression in the absence or the presence of exogenous vWF was enhanced by intense exercise. Therefore, we conclude that intense exercise promotes the extent of SIPA, possibly by enhancing the ability of vWF to bind to platelets and the subsequent activation of GP IIb/IIIa complexes, as well as the expression of P-selectin in response to shear stress, which in turn may augment the risk of vascular thrombosis.

Antibodies against human cell receptors, CD36, CD41a, and CD62P crossreact with porcine platelets

BACKGROUND

A limitation in platelet study has been the availability of platelet function-specific membrane receptor antibodies for use in the various animal species that are currently used in the study of hemostasis and other phenomena.

METHODS

Platelets were isolated from human and porcine blood. Resting and activated platelets were incubated with antibodies against the human cell surface receptors CD36 (clone CB38), CD41a (clone HIP8), CD62P (clone AK4), and CD63 (clone H5C6). Antibody titration and ligand blocking studies also were performed.

RESULTS

Binding of anti-CD41a and anti-CD62P were similar for human and porcine platelets in percentage of platelets labeled and in number of receptors per cell. Binding of anti-CD36 was similar between species, with fewer receptors present in porcine cells. Anti-CD63 and anti-CD107a did not bind specifically to porcine platelets.

CONCLUSION

The anti-CD36, anti-CD41a, and anti-CD62P antibodies studied crossreact with porcine platelets and will be useful in the investigation of platelet function in porcine models.

The activity of alpha1,6-fucosyltransferase during human megakaryocytic differentiation

alpha1,6-Fucosyltransferase (6FucT, E.C. 2.4.1.68) is one of the enzymes involved in the synthesis of N-linked glycans of the GpIIb/IIIa complex (CD41a) which is present on megakaryocytes (MKs) and platelets. In this study, we examined 6FucT activity in ex vivo cultures of immunoselected cord blood CD34(+) cells grown in a medium promoting megakaryocytopoiesis. Our results show that the activity of 6FucT increased ahead of, and thereafter concomitantly with, cells expressing the CD41a antigen. When the CD41a(+) subpopulation of cells was immunoselected (using anti-CD61 i.e. anti-GpIIIa antibodies), its 6FucT activity increased proportionally to the yield of CD61(+)(+)(+) cells. Taking into account the heavy load of 6FucT in platelets and megakaryocytes, we regard this enzyme as a candidate for the earliest marker of MK-commitment in cultured hematopoietic stem cells. Such a marker should allow an earlier detection and earlier transplantation of patients' own, ex vivo expanded, Mk progenitors.

P-selectin and leukocyte microparticles are associated with venous thrombogenesis

OBJECTIVES

P-selectin inhibition has been found to limit venous thrombosis. We hypothesize that elevated levels of P-selectin will amplify thrombosis, mediated by procoagulant microparticles (MPs).

METHODS

Male mice (Mus musculus, n659), 20 to 25 grams, underwent IVC ligation to induce thrombosis. Groups consisted of wild type (WT) C57BL/6 controls, mice with high circulating levels of soluble P-selectin (CT), P-selectin gene-interrupted knockout mice (PKO), and E- and P-selectin gene-interrupted mice (EPKO). Additional groups were used to evaluate the ability of a P-sel antagonist (rPSGL-Ig) and an antibody directed against PSGL-1 to downregulate the effects of P-sel in CT mice and WT mice administered soluble P-sel at time of thrombosis. Animals were sacrificed on days 2 and 6 after IVC ligation. Thrombus mass (TM), vein wall morphometrics, and serum leukocyte/platelet microparticles (MPs) were evaluated by means of double-stained fluorescence-activated cell scanning analysis, and soluble P- and E-sel protein determination by ELISA.

RESULTS

At days 2 and 6 in phase I of the experiment, significant differences (P <.01) in TM were noted between groups, with CT animals having the largest thrombi (50% and 57% increase in TM compared to WT at days 2 and 6) while EPKO mice had the smallest thrombi. Statistically, greater levels of neutrophils and total inflammatory cells were noted in the vein walls of CT animals at day 2 compared with WT and PKO animals. A significant difference was noted between CT and EPKO for neutrophils, monocytes, and total inflammatory cells, also at day 2. At day 6, the only statistically significant difference was found for monocytes, with a higher number in the CT animals than in WT animals. The evaluation of MPs revealed that the CT mice had a mixed leukocyte (MAC-1) and platelet (CD41) MP population that was also present in WT and PKO mice on day 2 and day 6. EPKO mice revealed a primarily platelet-derived MP population. Of interest, the CT mice with the highest TM showed a high amount of mean channel fluorescence for MAC-1 (phycoerythrin) antibody, indicative of leukocyte MPs. CT mice revealed statistically higher levels of soluble P-selectin at days 2 and 6. In phase 2, an antibody directed against PSGL-1 was more effective than rPSGL-Ig in decreasing TM and limiting leukocyte-derived MP fluorescence.

CONCLUSIONS

This study demonstrates that high circulating levels of P-selectin are associated with increased thrombosis, whereas a lack of P-selectin and E-selectin is associated with a lessening of thrombosis. Additionally, leukocyte MPs are associated with venous thrombus formation. These data suggest the importance of selectins to venous thrombogenesis and show that P-selectin and leukocyte-derived MPs should be good targets to limit venous thrombus formation.

Pathogenic effects of anti-glycoprotein Ib antibodies on megakaryocytes and platelets

Antibodies directed against the glycoprotein (GP) Ib have been identified as the potential cause of various platelet disorders: Immune thrombocytopenic purpura (ITP) may be caused by such autoantibodies; Anti-thrombotic drugs targeting GPIb also induce thrombocytopenia. In order to elucidate the potential mechanism(s) of the anti-GPIb effects, we have examined by electron microscopy (EM) the effect of several antibodies directed against GPIb and GPIIb-IIIa on human culture megakaryocytes (MK). Virtually all antibodies to GPIb enhanced the interaction of newly formed platelets with MK when compared to other antibodies. These effects were retrieved when antibodies were tested on platelets. We conclude that antibodies to GPIb can potentially inhibit platelet release by MK, and can also induce homotypic platelet adhesion. These results may have implications in the pathophysiology of thrombocytopenia and platelet recovery in ITP, and shed light on the pathological effect of anti-GPIb antibodies used as antithrombotic drugs.