Thrombin induces neoangiogenesis in the chick chorioallantoic membrane.

Most tumors have constitutively active tissue factor on their surface, capable of generating thrombin in the surrounding environment, and thrombosis is associated with cancer. Thrombin is known to induce a malignant phenotype by enhancing tissue adhesion and cell growth in vitro and in vivo in mice. Because tumors require angiogenesis for growth, we examined whether thrombin induces neoangiogenesis in a physiologically intact in vivo model. Thrombin (0.1 U mL-1) induced neoangiogenesis in the chick chorioallantoic membrane over a 24-72-h period by approximately 2-3-fold. This was inhibited by the potent thrombin inhibitor, hirudin and shown to have its mode of action by ligation of the thrombin protease-activated receptor, PAR-1. The thrombin receptor activation peptide, SFLLRNPNDKYEPF (200 microm) also enhanced neoangiogenesis c. 2-3-fold. Thrombin-induced neoangiogenesis was accompanied by the induction of vascular endothelial growth factor (VEGF) and angiopoietin-2 (Ang-2) mRNA at 24-48 h (approximately 2-fold) as determined by semi-quantitative reverse transcriptase-polymerase chain reaction. Thrombin-induced neoangiogenesis was inhibited to baseline level by the specific angiogenesis receptor inhibitors KDR-Fc (vs. VEGF) and Tie-2-Fc (vs. Ang-1 and Ang-2), as well as the non-specific angiogenesis inhibitor thrombospondin-1. Thrombin-induced neoangiogenesis was also inhibited to baseline level by agents known to inhibit thrombin receptor signaling in other cells: G-coupled protein receptor inhibitor, pertussis toxin (40 pg per egg), protein kinase C inhibitor, bisindolylmaleimide (1 microm per egg), MAP kinase inhibitor, PD980598 (10 microm per egg) and PI3 kinase inhibitor, LY294002 (0.25 microm per egg). Thus angiogenesis is stimulated by thrombosis, which could help explain the enhancement of experimental tumorigenesis by thrombin.

Thrombin induces increased expression and secretion of VEGF from human FS4 fibroblasts, DU145 prostate cells and CHRF megakaryocytes.

Angiogenesis is required for tumor growth and metastasis. It has recently been suggested that thrombin is a potent promoter of angiogenesis. We therefore examined the possibility that thrombin could be inducing the expression of vascular endothelial growth factor (VEGF), which promotes endothelial growth. Primary human FS4 fibroblasts as well as tumor cell lines: prostate DU145 and megakaryocyte CHRF were incubated with thrombin (0.25-1 unit/ml) for 1-8 hrs and then examined for mRNA by Northern Analysis. Enhanced mRNA (approximately 3-4 fold over base line) was noted at 2-4 hrs, with 0.5 u/ml thrombin. The effect was specific for thrombin activity on its PAR-1 receptor, since equal units of hirudin completely inhibited the response and the thrombin effect could be mimicked with the 14 mer thrombin receptor activation peptide (TRAP). Upregulation of mRNA was associated with enhanced VEGF protein synthesis and secretion as assayed by immunoblot. Enhanced expression of VEGF mRNA was not secondary to enhanced transcription (nuclear run on experiments), but due to an >3 fold stabilization of mRNA (Actinomycin D chase experiment). Enhanced VEGF mRNA stabilization is promoted by the PI3Kinase and serine/threonine kinase pathways, since thrombin-induced mRNA expression is inhibited by Wortmanin and H7. No effect was noted with the MAPKinase inhibitor, PD98059. Thus, thrombin-induced tumorigenesis and metastasis is associated with enhanced VEGF protein synthesis and secretion via the stabilization of VEGF mRNA promoted by the PI3Kinase and serine/threonine kinase pathways. This could help explain how thrombin promotes angiogenesis.

Complement-independent, peroxide-induced antibody lysis of platelets in HIV-1-related immune thrombocytopenia.

Immunologic thrombocytopenia is seen commonly in HIV-1 infection. The pathogenesis of this problem has been unclear, but it is associated with circulating immune complexes that contain platelet membrane components and anti-platelet membrane GPIIIa49-66 IgG antibodies. These antibodies cause acute thrombocytopenia when injected into mice. We now show that purified anti-GPIIIa49-66 causes platelet fragmentation, in vitro in the absence of complement, and in vivo in wild-type and C3-deficient mice. The mechanism of complement-independent platelet lysis is shown to be caused by the antibody-induced generation of H202, as indicated by in vitro experiments with inhibitors of reactive oxygen species, and in vivo studies carried out with p47phox-deficient mice. Thus, a novel mechanism of immunologic platelet clearance is described in which an anti-platelet IgG causes platelet fragmentation via the induction of reactive oxygen species.

Thrombin induces the release of angiopoietin-1 from platelets.

Blood platelets contain angiopoietin-1, a growth factor essential for blood vessel development via stabilization of proliferating endothelial cells. It has recently been reported that angiopoietin-1 can act as a vascular stability factor (Nature Medicine 6:460, 2000). In investigating the normal tissue distribution of angiopoietin-1 from surgically-removed frozen specimens by RT-PCR, we found it consistently present in platelets and megakaryocytes, usually absent in relatively non-vascular tissue: breast, colon, lung, skin, kidney, thyroid, testicle, cervix and occasionally present in tissue enriched with vasculature: prostate, endometrium, ovary, under conditions in which mRNA stability was verified by the positive detection of internal control, actin mRNA. The consistent distribution in platelets and relatively absent distribution in non-vascular normal tissue suggested that the well-known role of platelets in maintaining vascular stability, may in part be due to platelet release of angiopoietin-1 following platelet activation. In this communication we report the incidence of Ang-1 in various normal tissues and demonstrate that thrombin-treated human platelets release angiopoietin-1 in vitro.

Antiidiotype antibody against platelet anti-GPIIIa contributes to the regulation of thrombocytopenia in HIV-1-ITP patients.

Patients with human immunodeficiency virus 1-associated immunological thrombocytopenia (HIV-1-ITP) have markedly elevated platelet-bound immunoglobulin (Ig)G, IgM, and C3C4, as well as serum circulating immune complexes (CICs) composed of the same. Affinity purification of IgGs from their CICs with fixed platelets reveals high-affinity antibody (Ab) against platelet glycoprotein (GP)IIIa 49-66, which correlates inversely with their platelet count. However, sera from these patients have little to no anti-GPIIIa activity. To investigate this, we assayed serum, purified serum IgG, and CIC-Ig from these patients. This revealed approximately 150-fold greater Ab activity in purified serum IgG, and approximately 4,000-fold greater reactivity in CIC-IgG. This was shown to be associated with the presence of antiidiotype Ab2 (both IgG and IgM) sequestered in the CIC-IgG. The IgM antiidiotype was predominantly blocking Ab, as demonstrated by specificity for F(ab')(2) fragments of anti-GPIIIa 49-66 of HIV-1-ITP patients and inhibition of reactivity with peptide GPIIIa 49-66, not with a control peptide. The IgM antiidiotype was not polyreactive. Similar measurements were made in nonthrombocytopenic HIV-1-infected patients. Their serum reactivity was not measurable, but serum Ig and CIC-IgG against platelet GPIIIa 49-66 was present, although considerably lower than that found in HIV-1-ITP patients (26- and 35-fold lower, respectively). In addition, their IgM antiidiotype reactivity was 12-fold greater than that found in HIV-1-ITP patients. The IgM antiidiotype Ab titer of both cohorts correlated with in vivo platelet count (r = 0.7, P = 0. 0001, n = 32). To test the in vivo effectiveness of the IgM antiidiotype, thrombocytopenia was induced in mice with 25 microgram of affinity-purified anti-GPIIIa 49-66 (mouse GPIIIa has 83% homology with human GPIIIa and Fc receptors for human IgG1). Maximum effect was obtained at 4-6 h after intraperitoneal injection into Balb/c mice with a platelet count of approximately 30% baseline value. Preincubation of the anti-GPIIIa Ab with control IgM at molar ratios of IgM/IgG of 1:7 before intraperitoneal injection had no effect on the in vivo platelet count, whereas preincubation with patient IgM antiidiotype improved the platelet count to 50-80% of normal. Thrombocytopenia could be reversed after addition of IgM antiidiotype 4 h after induction of thrombocytopenia. Thus, CICs of HIV-1-infected patients contain IgM antiidiotype Ab against anti-GPIIIa, which appears to regulate their serum reactivity in vitro and their level of thrombocytopenia in vivo.

Concentration-dependent dual effect of thrombin on impaired growth/apoptosis or mitogenesis in tumor cells.

Because thrombin-treated tumor cell-induced metastasis increases tumor nodule volume(12) greater than nodule number, we studied the effect of thrombin on tumor cell growth in vitro and in vivo (murine B16F10 melanoma, human HCT8 colon carcinoma, DU145 prostate carcinoma). Tumor cell growth was measured after 3 to 7 days in 1% fetal calf serum (FCS) + RPMI 1640. We found that, whereas relatively low concentrations of thrombin, 0.1 to 0.5 U/mL (1-5 nmol/L) enhance tumor cell growth in vitro approximately 2- to 3-fold, higher concentrations, 0.5 to 1 U/mL (5-10 nmol/L) impaired cell growth approximately 2- to 4-fold. Impaired cell growth was associated with cell cycle arrest at G(2)M and increased pre-G(o) DNA, as well as apoptosis, measured by tumor cell binding to Annexin V and propidium iodide. Apoptosis was reversed with the general caspase inhibitor, FK-011. The enhancing and inhibiting effects were specific for thrombin (reversed with inactive diisopropyl-fluorophosphate [DFP]-thrombin) and mediated via the protease-activated receptor 1 (PAR-1). PAR-1 activation was demonstrated by (1) use of a cell line, B16F10, devoid of the 3 other thrombin receptors, PAR-3, PAR-4, and GPIb; and (2) greater sensitivity of PAR-1 transfected B16F10 and HCT8 cells to impaired cell growth/apoptosis, 3- and 14-fold, respectively. Thus, thrombin has a bimodal effect on PAR-1 in tumor cells: enhanced growth at low concentration, impaired growth/apoptosis at higher concentration.

Identification of a family of alternatively spliced mRNA species of angiopoietin-1.

Angiopoietin-1 (Ang-1) is required for developing vessels, and its absence leads to defects in vessel remodeling. Ang-1 has been identified as the ligand for the tyrosine kinase receptor Tie-2, which is expressed specifically on endothelial cells and early hematopoietic cells. In studying the role of Tie-2 and Ang-1 in megakaryocytopoiesis, 3 alternatively spliced species of Ang-1 mRNA (Ang-1.3 kb, Ang-0.9 kb, and Ang-0.7 kb) were identified in addition to the full-length Ang-1 (Ang-1.5 kb), in the megakaryocyte cell line CHRF by reverse transcription-polymerase chain reaction (RT-PCR), and then cloned and sequenced. The expression of 3 alternatively spliced isoforms of Ang-1 was confirmed by RT-PCR using specific primer pairs derived from junction sites and the 3' end of Ang-1 cDNA, and it was further demonstrated by nuclease protection assay, Northern blotting, and immunoblotting in CHRF cells. Expression of the Ang-1.3 kb isoform was also detected in human primary fibroblast cell line FS4, breast cancer cell line MDAMB-468, and CD34(+)CD41(+) cells of fetal liver and platelets. The function of the 1.5-kb, 1.3-kb, and 0.9-kb isoforms was examined. Recombinant proteins Ang-1.5 and 0.9 kb bind strongly to the recombinant Tie-2 receptor (Tie-2-Fc), whereas the 1.3-kb isoform does not. The Ang-1.3 kb isoform binds to the 1.5-kb isoform. Ang-1. 5 kb, but not the 1.3-kb and 0.9-kb isoforms, induces tyrosine phosphorylation of Tie-2 in human umbilical vein endothelial cells. These data suggest that isoforms 1.3 kb and 0.9 kb could serve as dominant negative molecules for the full-length Ang-1. The possible involvement of the newly identified Ang-1 isoforms in angiogenesis and in growth and differentiation of hematopoietic progenitor cells provides a greater complexity to these processes. (Blood. 2000;95:1993-1999)

Thrombin inhibits tumor cell growth in association with up-regulation of p21(waf/cip1) and caspases via a p53-independent, STAT-1-dependent pathway.

Thrombin, a multifunctional protein, has been found to be involved in cellular mitogenesis, tumor growth, and metastasis, in addition to its well known effects on the initiation of platelet aggregation and secretion and the conversion of fibrinogen to fibrin to form blood clots. These properties of thrombin rely on its action as a serine protease, which cleaves the N-terminal region of a 7-transmembrane G protein receptor (protease-activated receptor, PAR-1), thus exposing a tethered end hexapeptide sequence capable of activating its receptor. Little is known about its effect on genes that regulate the cell cycle. This study was undertaken to investigate the possible mechanisms by which thrombin regulates tumor cell growth in several tumor cell lines: human CHRF megakaryocyte, DU145 prostate, MDAMB231 and MCF7 breast, U3A fibrosarcoma, and 2 murine fibroblast cell lines, MEFp53(-/-) and CD STAT(-/-). We have found that thrombin under the conditions of culture employed inhibits cell growth by both up-regulation of p21(waf/cip1) and induction of caspases via its PAR-1 receptor. The increased expression of p21(waf/cip1) by thrombin was p53 independent, STAT1 dependent, and protein synthesis independent. This was associated with tyrosine phosphorylation of JAK2 and STAT1, and nuclear translocation of STAT1. Induction of apoptosis is also PAR-1-specific, STAT1-dependent, and associated with up-regulation of caspases 1, 2, and 3. Our study establishes, for the first time, a link between PAR-1 receptor activation with the STAT signal pathway, which leads to cell cycle control and apoptosis. This observation broadens our understanding of the mechanism of PAR-1 activation and its effect on cell growth, and could possibly lead to therapeutic approaches for the treatment of cancer.

Expression of CD41 and c-mpl does not indicate commitment to the megakaryocyte lineage during haemopoietic development.

Haemopoietic progenitors with the phenotype expected of early megakaryocyte precursors (CD34+ CD41+) were isolated from normal human bone marrow or induced in culture from CD34+ CD41- bone marrow cells by treatment with thrombopoietin (TPO) or IL-3. We found that although this population included the majority of cells that can form CFU-MK in culture, it also contained both erythroid and myeloid progenitors. The clonogenic potential of the CD34+ CD41+-induced cells was greater than that of isolated CD34+ CD41+ cells in that the isolated cells only formed CFU-MK and BFU-e, whereas the induced cells formed myeloid colonies as well. Glycophorin was found on isolated CD34+ CD41+ cells, not on induced cells. Its presence distinguished between MK and erythroid progenitors. Separation of a CD34+ CD41+ glycophorin A+ population resulted in the isolation of a highly purified population of BFU-e. A major portion of the cells that expressed CD34+ CD41+, in either cohort, were of the erythroid lineage. True MK progenitors were present in the CD34+ population in greater proportion than in whole marrow and were further enriched amongst CD34+ populations that expressed CD41. The presence of the thrombopoietin (TPO) receptor, c-mpl, did not correlate with inducibility of the gpIIbIIIa complex since essentially all CD34+ progenitors, including the earliest identifiable human haemopoietic progenitors (CD34+ CD38- cells), expressed c-mpl mRNA detectable by PCR regardless of their ultimate fate. Thus neither the expression of CD41 nor the expression of c-mpl was predictive of commitment to the MK lineage.

Differentiation of autoimmune thrombocytopenia from thrombocytopenia associated with immune complex disease: systemic lupus erythematosus, hepatitis-cirrhosis, and HIV-1 infection by platelet and serum immunological measurements.

A method and approach are described to differentiate classic autoimmune thrombocytopenia (ATP) from immune complex-associated thrombocytopenia in systemic lupus (SLE), hepatitis/chronic liver disease (LIV-ITP) and HIV-1 related thrombocytopenia (HIV-1-ITP). The platelet immunologic profile of IgG, C3C4 and IgM was measured with a solid-phase ELISA, employing 125I-staphylococcal protein A to detect indicator antibody binding. Polyethylene glycol was employed to precipitate immune complexes (PEG-IC). Platelet-associated IgG (PAIgG) was 2.8-, 5.6- and 5.8-fold higher in SLE, LIV-ITP and HIV-1-ITP patients respectively compared to ATP patients: platelet C3C4 was 3.2-, 4.8- and 4.5-fold higher respectively; platelet IgM was 2.2-, 3.7- and 3.8-fold higher respectively; serum PEG-IC levels were 4.2-, 4.8- and 2.1-fold higher respectively. With all parameters measured, there was no overlap between the 75th percentile for ATP patients and the 25th percentile for all three cohorts. The likelihood of having a platelet C3C4 level higher than the highest ATP level was 69% for SLE, 90% for LIV-ITP and 94% for HIV-1-ITP respectively; with PEG-IC measurements the likelihood was 83%, 100% and 100% respectively. Serum IgG, C3, C4, IgM and PEG-IC were examined for a possible relationship with platelet measurements. Except for a positive correlation between serum and platelet IgM in ATP, r = 0.5, P < 0.04, there was no positive correlation with any of the parameters measured. An inverse correlation was noted between PEG-IC level and platelet C3C4 in SLE, r = 0.7, P < 0.04. Thus platelet immunologic profile and serum PEG-IC level measurements differentiated classic ATP from immune complex-associated thrombocytopenias (SLE, LIV-ITP, HIV-1-ITP). Except for IgM measurements in ATP, platelet measurements could not be attributed to their respective serum concentration.

Protease-activated receptor 1 (PAR-1) is required and rate-limiting for thrombin-enhanced experimental pulmonary metastasis.

Thrombin-treated tumor cells induce a metastatic phenotype in experimental pulmonary murine metastasis. Thrombin binds to a unique protease-activated receptor (PAR-1) that requires N-terminal proteolytic cleavage for activation by its tethered end. A 14-mer thrombin receptor activation peptide (TRAP) of the tethered end induces the same cellular changes as thrombin. Four murine tumor cells (Lewis lung, CT26 colon CA, B16F10 melanoma, and CCL163 fibroblasts) contain PAR-1, as detected by reverse transcriptase-polymerase chain reaction (RT-PCR). B16F10 cells did not contain the two other thrombin receptors, PAR-3 and glycoprotein Ib. TRAP-treated B16F10 tumor cells enhance pulmonary metastasis 41- to 48-fold (n = 17). Thrombin-treated B16F10 cells transfected with full-length murine PAR-1 sense cDNA (S6, S7, S14, and S22) enhanced their adhesion to fibronectin 1.5- to 2.4-fold (n = 5, P <.04), whereas thrombin-treated wild-type cells do not. S6 (adhesion index, 1.5-fold) and S14 (index, 2.4-fold) when examined by RT-PCR and Northern analysis showed minimal expression of PAR-1 for S6 over wild-type and considerable expression for S14. Immunohistochemistry showed greater expression of PAR-1 for S14 compared with wild-type or empty-plasmid transfected cells. In vivo experiments with the thrombin-treated S14 transfectant showed a fivefold to sixfold increase in metastases compared with empty-plasmid transfected thrombin-treated naive cells or S6 cells (n = 20, P =.0001 to .02). Antisense had no effect on thrombin-stimulated tumor mass. Thus, PAR-1 ligation and expression enhances and regulates tumor metastasis.

Treatment of thrombocytopenia in chimpanzees infected with human immunodeficiency virus by pegylated recombinant human megakaryocyte growth and development factor.

Three chimpanzees experimentally infected with human immunodeficiency virus (HIV) developed significant chronic thrombocytopenia after 5, 4, and 2 years, with peripheral platelet counts averaging 64 +/- 19 x 10(3)/microL (P = .004 compared with 228 +/- 92 x 10(3)/microL in 44 normal control animals), mean platelet volumes of 11.2 +/- 1.8 fL (P > .5 compared with 10.9 +/- 0. 7 fL in normal controls), endogenous thrombopoietin (TPO) levels of 926 +/- 364 pg/mL (P < .001 compared with 324 +/- 256 pg/mL in normal controls), uniformly elevated platelet anti-glycoprotein (GP) IIIa49-66 antibodies, and corresponding viral loads of 534, 260, and 15 x 10(3) RNA viral copies/mL. Pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) was administered subcutaneously (25 microg/kg twice weekly for 3 doses) to determine the effects of stimulating platelet production on peripheral platelet concentrations in this cohort of thrombocytopenic HIV-infected chimpanzees. PEG-rHuMGDF therapy increased (1) peripheral platelet counts 10-fold (from 64 +/- 19 to 599 +/- 260 x 10(3) platelets/microL; P = .02); (2) marrow megakaryocyte numbers 30-fold (from 11.7 +/- 6.5 x 10(6)/kg to 353 +/- 255 x 10(6)/kg; P = .04); (3) marrow megakaryocyte progenitor cells fourfold (from a mean of 3.6 +/- 0.6 to 14.1 x 10(3) CFU-Meg/1, 000 CD34(+) marrow cells); and (4) serum levels of Mpl ligand from 926 +/- 364 pg/mL (endogenous TPO) to predosing trough levels of 1, 840 +/- 353 pg/mL PEG-rHuMGDF (P = .02). The peripheral neutrophil counts were also transiently increased from 5.2 +/- 2.6 x 10(3)/microL to 9.9 +/- 5.0 x 10(3)/microL (P = .01), but neither the erythrocyte counts nor the reticulocyte counts were altered significantly (P > .1). The serum levels of antiplatelet GPIIIa49-66 antibodies exhibited reciprocal reductions during periods of thrombocytosis (P < .07). PEG-rHuMGDF therapy did not increase viral loads significantly (395, 189, and 53 x 10(3) RNA viral copies/mL; P > .5 compared with baseline values). The striking increase in peripheral platelet counts produced by PEG-rHuMGDF therapy implies that thrombocytopenia in HIV-infected chimpanzees is attributable to insufficient compensatory expansion in platelet production resulting from HIV-impaired delivery of platelets despite stimulated megakaryocytopoiesis. These data suggest that PEG-rHuMGDF therapy may similarly correct peripheral platelet counts in thrombocytopenic HIV-infected patients.

Ineffective platelet production in thrombocytopenic human immunodeficiency virus-infected patients.

Thrombocytopenia has been characterized in six patients infected with human immunodeficiency virus (HIV) with respect to the delivery of viable platelets into the peripheral circulation (peripheral platelet mass turnover), marrow megakaryocyte mass (product of megakaryocyte number and volume), megakaryocyte progenitor cells, circulating levels of endogenous thrombopoietin (TPO) and platelet TPO receptor number, and serum antiplatelet glycoprotein (GP) IIIa49-66 antibody (GPIIIa49-66Ab), an antibody associated with thrombocytopenia in HIV-infected patients. Peripheral platelet counts in these patients averaged 46 +/- 43 x 10(3)/microL (P = . 0001 compared to normal controls of 250 +/- 40x 10(3)/microL), and the mean platelet volume (MPV) was 10.5 +/- 2.0 fL (P > 0.3 compared with normal control of 9.5 +/- 1.7 fL). The mean life span of autologous 111In-platelets was 87 +/- 39 hours (P = .0001 compared with 232 +/- 38 hours in 20 normal controls), and immediate mean recovery of 111In-platelets injected into the systemic circulation was 33% +/- 16% (P = .0001 compared with 65% +/- 5% in 20 normal controls). The resultant mean peripheral platelet mass turnover was 3.8 +/- 1.5 x 10(5) fL/microL/d versus 3.8 +/- 0.4 x 10(5) fL/microL/d in 20 normal controls (P > .5). The mean endogenous TPO level was 596 +/- 471 pg/mL (P = .0001 compared with 95 +/- 6 pg/mL in 98 normal control subjects), and mean platelet TPO receptor number was 461 +/- 259 receptors/platelet (P = .05 compared with 207 +/- 99 receptors/platelet in nine normal controls). Antiplatelet GPIIIa49-66Ab levels in sera were uniformly increased in HIV thrombocytopenic patients (P < .001). In this cohort of thrombocytopenic HIV patients, marrow megakaryocyte number was increased to 30 +/- 15 x 10(6)/kg (P = .02 compared with 11 +/- 2.1 x 10(6)/kg in 20 normal controls), and marrow megakaryocyte volume was 32 +/- 0.9 x 10(3) fL (P = .05 compared with 28 +/- 4.5 x 10(3) fL in normal controls). Marrow megakaryocyte mass was expanded to 93 +/- 47 x 10(10) fL/kg (P = .007 compared with normal control of 31 +/- 5.3 x 10(10) fL/kg). Marrow megakaryocyte progenitor cells averaged 3.3 (range, 0.4 to 7.3) CFU-Meg/1,000 CD34(+) cells compared with 27 (range, 0.1 to 84) CFU-Meg/1,000 CD34(+) cells in seven normal subjects (P = .02). Thus, thrombocytopenia in these HIV patients was caused by a combination of shortening of platelet life span by two thirds and doubling of splenic platelet sequestration, coupled with ineffective delivery of viable platelets to the peripheral blood, despite a threefold TPO-driven expansion in marrow megakaryocyte mass. We postulate that this disparity between circulating platelet product and marrow platelet substrate results from direct impairment in platelet formation by HIV-infected marrow megakaryocytes.

Complementary and antagonistic effects of IL-3 in the early development of human megakaryocytes in culture.

The effect of IL-3 on the early steps in the growth and development of megakaryocytes (MK) in culture has been studied. Although thrombopoietin (TPO) by itself could support the development of mature CD41+ MK from pre-MK, the number of cells produced was greatly augmented by the addition of IL-3 and SCF. IL-3 was also able to support the growth of MK colonies in semi-solid media (CFU-MK). The CD41+ cells that developed in suspension cultures containing IL-3 differed phenotypically from those that developed without this agent. Cells grown in the presence of IL-3 lost CD34 expression more rapidly, expressed lower levels of the platelet glycoproteins gpIIb-IIIa and Ib and achieved lower degrees of polyploidy than in the absence of IL-3. The inhibitory effects of IL-3 were not a consequence of the dilution of the mature cells by increased numbers of immature cells since it was observed under conditions in which IL-3 did not stimulate MK growth. The results obtained in these cultures suggest that IL-3 plays an important role in early MK development, but inhibits further maturation after endoreduplication begins. Thus, prolonged contact with IL-3 results in the appearance of cells that do not mature normally.

GPIIIa-(49-66) is a major pathophysiologically relevant antigenic determinant for anti-platelet GPIIIa of HIV-1-related immunologic thrombocytopenia.

High-affinity (Kd = 1 x 10(-9) M) anti-platelet GPIIIa has been isolated from serum immune complexes of immunologic thrombocytopenic HIV-1-infected patients (HIV-1-ITP). Affinity-purified anti-platelet antibody reacted with a recombinant GPIIIa-(1-200) and -(1-66) fusion peptide and with an 18-mer GPIIIa-(49-66) peptide but not with seven other GPIIIa peptides spanning the length of GPIIIa. Most of the anti-platelet antibody ( approximately 85%) could be adsorbed to and eluted from a GPIIIa-(49-66) affinity column. Binding of antibody to platelets could be inhibited by GPIIIa-(49-66) or an equimolar peptide-albumin conjugate (IC50 = 2 microM). Sera from 7 control subjects and 10 classic autoimmune thrombocytopenic patients gave background reactivity with GPIIIa-(49-66). HIV-1-ITP sera from 16 patients reacted with a mean OD 6-fold greater than background (range, 4- to 9-fold). Serum anti-GPIIIa-(49-66) concentration correlated inversely with platelet count, R2 = 0.51, n = 31, P < 0. 0001. Because mouse platelet GPIIIa-(49-66) has 83% homology with human GPIIIa and mouse monocytes contain Fc receptors for the human IgG1-kappa/lambda antibody, we determined the in vivo effect of human anti-GPIIIa on mouse platelets. Affinity-purified antibody, 25-50 microg given i.p., resulted in a precipitous drop in platelet count to 30% of baseline, with nadir at 4 hr and return to normal in 36 hr. No effect was noted with control IgG. Acute thrombocytopenia could be prevented or reversed by the injection of the GPIIIa-(49-66) albumin conjugate at zero time or 2 hr after antibody, respectively, but not with a scrambled peptide-albumin conjugate. Thus HIV-1-ITP patients have high-affinity anti-platelet GPIIIa against a major antigenic determinant, GPIIIa-(49-66), which correlates inversely with platelet count and induces thrombocytopenia in mice.

The development of human megakaryocytes: III. Development of mature megakaryocytes from highly purified committed progenitors in synthetic culture media and inhibition of thrombopoietin-induced polyploidization by interleukin-3.

Megakaryocyte (MK) progenitors, CD34+CD41+ cells, were isolated from human bone marrow with a purity greater than 98% and a viability of 95%, using affinity techniques with magnetic beads followed by fluorescence-activated cell sorting. These cells were incubated in synthetic media containing the cytokines thrombopoietin (TPO), interleukin-3 (IL-3), stem cell factor (SCF), and IL-6, obviating the confounding effects of serum growth factors or cytokine secretions of non-MK cells on MK maturation. MK number, MK colony-forming units (CFU-MK), and MK ploidy and phenotype were examined during 7 days in culture. TPO in serum-free cultures without any other exogenously added cytokine supported MK growth and maturation. SCF synergized with TPO to augment MK production and maturation and could partially replace it under some conditions. Both TPO and IL-3 alone increased MK number (12- and 5-fold, respectively) and CFU-MK (approximately 15-fold each). SCF alone had no effect on MK proliferation in the absence of TPO, but increased both MK number and CFU-MK by 1.5- to 2.0-fold in the presence of TPO. When combined with IL-3, SCF increased both MK number and CFU-MK by 15- to 20-fold in the absence of TPO. In the presence of TPO, the combination of IL-3 and SCF produced only modest increases (1.5- to 2.0-fold) in both MK number and CFU-MK. The proportion of polyploid MK increased greater than fivefold in the presence of TPO. SCF had little effect on MK ploidy in the presence of TPO, but enhanced ploidy twofold to threefold in the absence of TPO. IL-3 alone never increased the level of polyploidization. Rather, it consistently inhibited TPO- and SCF-induced polyploidization of MK. This inhibition was observed in cultures with or without SCF or IL-6. Although IL-3 also supported the proliferation of CD41+ cells and CFU-MK production, the cells that developed under the influence of IL-3 were phenotypically unusual (CD41dim, CD42dim) and of relatively low ploidy. Mature MK were not produced. When added with TPO, IL-3 suppressed polyploidization. Therefore, TPO stimulates MK growth and maturation, whereas IL-3 stimulates growth without maturation and may serve to conserve the immature MK compartment.