Complement activation on platelets correlates with a decrease in circulating immature platelets in patients with immune thrombocytopenic purpura

The role of the complement system in immune thrombocytopenic purpura (ITP) is not well defined. We examined plasma from 79 patients with ITP, 50 healthy volunteers, and 25 patients with non‐immune mediated thrombocytopenia, to investigate their complement activation/fixation capacity (CAC) on immobilized heterologous platelets. Enhanced CAC was found in 46 plasma samples (59%) from patients with ITP, but no samples from patients with non‐immune mediated thrombocytopenia. Plasma from healthy volunteers was used for comparison. In patients with ITP, an enhanced plasma CAC was associated with a decreased circulating absolute immature platelet fraction (A‐IPF) (<15 × 10⁹/l) (P = 0·027) and thrombocytopenia (platelet count < 100 × 10⁹/l) (P = 0·024). The positive predictive value of an enhanced CAC for a low A‐IPF was 93%, with a specificity of 77%. The specificity and positive predictive values increased to 100% when plasma CAC was defined strictly by enhanced C1q and/or C4d deposition on test platelets. Although no statistically significant correlation emerged between CAC and response to different pharmacological therapies, an enhanced response to splenectomy was noted (P < 0·063). Thus, complement fixation may contribute to the thrombocytopenia of ITP by enhancing clearance of opsonized platelets from the circulation, and/or directly damaging platelets and megakaryocytes.     

Correlation between immature platelet fraction and reticulated platelets. Usefulness in the etiology diagnosis of thrombocytopenia

Background: Reticulated platelets (RP) are a surrogate marker for megakaryocytic activity, but the limitation of this determination is the lack of standardization of methodology. The determination of the immature platelet fraction (IPF) is performed in a simple, automated, and reproducible way between laboratories. We analyzed the correlation between IPF and RP, and usefulness of IPF in patients with thrombocytopenia. Methods: RP were determined by flow cytometry using double staining with thiazole orange and CD61 PerCP^®. IPF was performed with Sysmex XE2100 analyzer. We used a control group with normal platelets, and thrombocytopenic patients were classified into three groups: Group 1. Central thrombocytopenia, Group 2. Thrombocytopenia as a result of enhanced peripheral platelet destruction, and Group 3. Peripheral non‐immune thrombocytopenia by abnormal distribution. Results: Fourteen controls and 66 patients were analyzed. Group 1: 25 patients, they had mean and confidence interval 95% (95% CI) for IPF 8.67% (6.49–10.46%) and RP 4.08% (2.86–5.30%). Group 2: 20 patients, they had mean and 95%CI for IPF 16.80% (12.20–21.39%) and RP 16.14% (9.89–22.40%). Group 3: 21 patients, they had mean and 95% CI for IPF 9.04% (6.95–11.14%) and RP 5.23% (3.41–7.05%). The overall Pearson linear correlation between IPF and RP was r: 0.65. There were statistically significant differences in values of IPF and RP between Group 2 and the other two groups (P < 0.01). Conclusion: There is a good correlation between IPF and RP mainly in thrombocytopenia by peripheral destruction. Determination of IPF is an easy technique in their implementation, standardized and reproducible, so it could be a useful screening technique in patients with thrombocytopenia.     

Aberrant increase in the immature platelet fraction in patients with myelodysplastic syndrome: a marker of karyotypic abnormalities associated with poor prognosis

Objectives: Some patients with myelodysplastic syndrome (MDS) show a marked increase in the percentage of immature platelet fraction (IPF%) despite the absence of severe thrombocytopenia. To determine the significance of such an unbalanced increase in the IPF%, we investigated the IPF% and other laboratory findings of 51 patients recently diagnosed with MDS. Method: Subjects consisted of 80 healthy males, 90 healthy females, and 51 patients with MDS and 20 patients with idiopathic thrombocytopenic purpura (ITP). The IPF and IPF% were determined using a Sysmex XE‐2100 system loaded with IPF Master software (XE IPF Master, Sysmex). Platelet counts were measured simultaneously. Results: IPF% and platelet counts of these patients ranged from 1.1% to 25.1% (median, 5.3%) and from 6 to 260 × 10⁹/L (median, 71 × 10⁹/L), respectively. Twelve patients showed platelet counts more than 50 × 10⁹/L with 10% or more IPF%. All of the 12 patients had chromosome abnormalities including monosomy 7 and complex abnormalities involving 7 or 5q. In the other 39 patients who did not show the aberrant IPF% increase, chromosomal abnormalities were seen only in seven patients and none of them had chromosome 7 abnormalities. The IPF% of two patients increased to more than 10% in association with the appearance of monosomy 7. Conclusions: These findings suggest that a high IPF% in MDS patient may be a marker for karyotypic abnormalities with a poor prognosis, including chromosome 7 abnormalities.     

Evaluation of the immature platelet fraction in the diagnosis and prognosis of childhood immune thrombocytopenia

Rapid assessment of platelet production would distinguish between thrombocytopenia due to decreased platelet production or increased peripheral platelet destruction. We evaluated the value of immature platelet fraction (IPF) in differentiating immune thrombocytopenia (ITP) from thrombocytopenia secondary to bone marrow failure and its potential use as a prognostic marker. Forty-one young patients with ITP were compared with 14 patients with hematological malignancies under chemotherapy, representing a control group with thrombocytopenia due to bone marrow suppression and 30 age- and sex-matched healthy controls. Patients were studied stressing on bleeding manifestations, organomegaly/lymphadenopathy and therapy. Complete blood count including IPF was performed using Sysmex XE-2100. ITP patients were classified into two subgroups: acute ITP with spontaneous resolution within 3 months from diagnosis and chronic ITP that lasted ≥1 year from diagnosis. Median IPF was 11.8% in patients with ITP, 7% in those with hematological malignancy and 3% in the control group (p?<?0.001). ITP patients had significantly higher mean platelet volume (MPV), platelet distribution width (PDW), platelet large cell ratio (P-LCR) and IPF compared with patients with malignancy or healthy controls, while plateletcrit (PCT) was significantly lower in ITP patients than other groups (p?<?0.001). IPF was increased in patients with chronic ITP compared with acute ITP group (p?<?0.001). Patients with active ITP had the highest IPF followed by those in partial remission, while ITP patients in remission had the lowest IPF. IPF was positively correlated to the number of lines of treatment used, MPV, PDW and P-LCR, while negatively correlated to platelet count and PCT among ITP patients (p?<?0.001). Multiple regression analysis showed that platelet count and P-LCR were independently related to IPF. ROC curve analysis revealed that the cut-off value of IPF at 9.4% could be diagnostic for ITP patients with a sensitivity of 88% and a specificity of 85.7%. We suggest that IPF may be a rapid and inexpensive automated marker for etiology of thrombocytopenia and can be integrated as a standard parameter to evaluate the thrombopoietic state of the bone marrow. It may be considered as a potential prognostic marker for the development of chronic ITP.     

The beta 1 tubulin R307H single nucleotide polymorphism is associated with treatment failures in immune thrombocytopenia (ITP)

Predictive biomarkers are needed in immune thrombocytopenia (ITP). Single nucleotide polymorphisms (SNPs) in beta 1 tubulin are potential candidates, as beta 1 tubulin is integral for platelet production and function, and SNPs in beta 1 tubulin have been associated with distinct phenotypes in platelets. We investigated the most prevalent beta 1 tubulin SNP (R307H) as a biomarker in patients with ITP via a retrospective chart review. Allelic frequencies between a group of 191 ITP patients and a healthy control group showed no difference, suggesting no direct aetiological role for the SNP in ITP. However, over similar periods of follow‐up, both heterozygote and homozygote minor allele ITP patients were treated with significantly more treatment modalities and had significantly higher risk of failure to immune‐modulatory therapies [relative risk (RR) = 1·5, 95% confidence interval (CI) = 1·1–2·1; P = 0·01]; with rituximab, in particular, ITP patients with the SNP experienced a 58% failure rate (RR = 1·6, 95%CI = 1·03–2·5; P = 0·04). Analysis of the absolute immature platelet fraction (A‐IPF) as a marker of platelet production showed that SNP patients had significantly higher median A‐IPFs compared to non‐SNP patients when complete responses were achieved using immune modulatory therapies. The data suggest that the beta 1 tubulin R307H SNP has potential for use as a biomarker in ITP and may affect platelet turnover.     

Determinants of platelet count in pediatric patients with congenital cyanotic heart disease: Role of immature platelet fraction

Objectives

Congenital heart defects are common noninfectious causes of mortality in children. Bleeding and thrombosis are both limiting factors in the management of such patients. We assessed the frequency of thrombocytopenia in pediatric patients with congenital cyanotic heart disease (CCHD) and evaluated determinants of platelet count including immature platelet fraction (IPF) and their role in the pathogenesis of thrombocytopenia.

Methods

Forty‐six children and adolescents with CCHD during pre‐catheter visits were studied; median age was 20.5 months. Complete blood count including IPF as a marker of platelet production and reticulated hemoglobin content (RET‐He) as a marker of red cell production and iron status were done on Sysmex XE 2100 (Sysmex, Japan). C‐reactive protein, prothrombin time (PT), Activated partial thromboplastin time (APTT) were also assessed.

Results

Thrombocytopenia was found in 6 patients (13%). PT was prolonged (P = .016) and IPF was significantly higher in patients with thrombocytopenia compared with patients with normal platelet count (14.15 ± 5.2% vs 6.68 ± 3.39%; P = .003). Platelet count was negatively correlated with IPF while significant positive correlations were found between IPF and hemoglobin, red blood cells (RBCs) count, hematocrit (Hct), PT, reticulocytes count, and immature reticulocyte fraction.

Conclusions

We suggest that elevated IPF in CCHD patients with thrombocytopenia may denote peripheral platelets destruction as an underlying mechanism. Hemoglobin level, RBCs count, Hct, and RET‐He were not significant determinants for platelet count in CCHD.     

Platelet production and platelet destruction: assessing mechanisms of treatment effect in immune thrombocytopenia

This study investigated the immature platelet fraction (IPF) in assessing treatment effects in immune thrombocytopenia (ITP). IPF was measured on the Sysmex XE2100 autoanalyzer. The mean absolute-IPF (A-IPF) was lower for ITP patients than for healthy controls (3.2 vs 7.8 × 10?/L, P < .01), whereas IPF percentage was greater (29.2% vs 3.2%, P < .01). All 5 patients with a platelet response to Eltrombopag, a thrombopoietic agent, but none responding to an anti-FcγRIII antibody, had corresponding A-IPF responses. Seven of 7 patients responding to RhoD immuneglobulin (anti-D) and 6 of 8 responding to intravenous immunoglobulin (IVIG) did not have corresponding increases in A-IPF, but 2 with IVIG and 1 with IVIG anti-D did. This supports inhibition of platelet destruction as the primary mechanism of intravenous anti-D and IVIG, although IVIG may also enhance thrombopoiesis. Plasma glycocalicin, released during platelet destruction, normalized as glycocalicin index, was higher in ITP patients than controls (31.36 vs 1.75, P = .001). There was an inverse correlation between glycocalicin index and A-IPF in ITP patients (r2 = -0.578, P = .015), demonstrating the relationship between platelet production and destruction. Nonresponders to thrombopoietic agents had increased megakaryocytes but not increased A-IPF, suggesting that antibodies blocked platelet release. In conclusion, A-IPF measures real-time thrombopoiesis, providing insight into mechanisms of treatment effect.     

Low‐dose anti‐CD20 veltuzumab given intravenously or subcutaneously is active in relapsed immune thrombocytopenia: a phase I study

Low doses of the humanized anti‐CD20 monoclonal antibody, veltuzumab, were evaluated in 41 patients with immune thrombocytopenia (ITP), including 9 with ITP ≤1 year duration previously treated with steroids and/or immunoglobulins, and 32 with ITP >1 year and additional prior therapies. They received two doses of 80–320 mg veltuzumab 2 weeks apart, initially by intravenous (IV) infusion (N = 7), or later by subcutaneous (SC) injections (N = 34), with only one Grade 3 infusion reaction and no other safety issues. Thirty‐eight response‐assessable patients had 21 (55%) objective responses (platelet count ≥30 × 10⁹/l and ≥2 × baseline), including 11 (29%) complete responses (CRs) (platelet count ≥100 × 10⁹/l). Responses (including CRs) occurred with both IV and SC administration, at all veltuzumab dose levels, and regardless of ITP duration. Responders with ITP ≤1 year had a longer median time to relapse (14·4 months) than those with ITP >1 year (5·8 months). Three patients have maintained a response for up to 4·3 years. SC injections resulted in delayed and lower peak serum levels of veltuzumab, but B‐cell depletion occurred after first administration even at the lowest doses. Eight patients, including 6 responders, developed anti‐veltuzumab antibodies following treatment (human anti‐veltuzumab antibody, 19·5%). Low‐dose SC veltuzumab appears convenient, well‐tolerated, and with promising clinical activity in relapsed ITP.( Clinicaltrials.gov identifier: NCT00547066.)     

Platelets in idiopathic thrombocytopenic purpura are increased in size but are of normal density

To determine whether platelet size and volume are related to one another or to platelet age, subpopulations of platelets from patients with idiopathic thrombocytopenic purpura (ITP) have been produced on the basis of density using Percoll gradients. The density distribution of platelets from patients with ITP and from patients with other forms of thrombocytopenia (thought to be nonimmune in nature) was the same as in normal controls. However, the platelets in each density subpopulation from ITP patients were increased in size. beta-thromboglobulin (beta TG) content of platelets from each patient group and the normals increased with density and tended to be higher in ITP than in normal controls. beta TG concentration per unit platelet volume and its level in plasma were similar in ITP patients and in normal controls. This suggests that the apparently normal density of ITP platelets was not a result of degranulation of large, dense platelets. Thus platelet size and density are independently determined and the increased size of platelets in immune thrombocytopenia may be the result of abnormalities in their production.     

Detection of platelet autoantibodies to identify immune thrombocytopenia: state of the art

Immune Thrombocytopenia (ITP) is diagnosed by exclusion of other causes for thrombocytopenia. Reliable detection of platelet autoantibodies would support the clinical diagnosis of ITP and prevent misdiagnosis. We optimized our diagnostic algorithm for suspected ITP using the direct monoclonal antibody immobilization of platelet antigens assay (MAIPA), which evaluates the presence of platelet autoantibodies on the glycoproteins (GP) IIb/IIIa, Ib/IX and V bound on the patient platelets. The direct MAIPA was shown to be a valuable technique for the detection of platelet autoantibodies and could possibly become a guide for optimizing therapy towards a more personalized treatment of ITP.     

IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia

Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20 × 10⁹/L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20 × 10⁹/L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP.     

Assessment of an immature platelet fraction (IPF) in peripheral thrombocytopenia

A new automated method to reliably quantify reticulated platelets, expressed as the immature platelet fraction (IPF), has been developed utilizing the XE-2100 blood cell counter with upgraded software (Sysmex, Kobe, Japan). The IPF is identified by flow cytometry techniques and the use of a nucleic acid specific dye in the reticulocyte/optical platelet channel. The clinical utility of this parameter was established in the laboratory diagnosis of thrombocytopenia due to increased peripheral platelet destruction, particularly autoimmune thrombocytopenic purpura (AITP) and thrombotic thrombocytopenic purpura (TTP). Reproducibility and stability results over 48 h were good. An IPF reference range in healthy individuals was established as 1.1-6.1%, with a mean of 3.4%. Patients in whom platelet destruction might be abnormal, were studied and two of these patients followed serially during the course of treatment. The IPF was raised in several disease states. The most significant increases in IPF values were found in patients with AITP (mean 22.3%, range 9.2-33.1%) and acute TTP (mean 17.2%, range 11.2-30.9%). Following patients during treatment demonstrated that as the platelet count recovered the IPF% fell. These results show that a rapid, inexpensive automated method for measuring the IPF% is feasible and should become a standard parameter in evaluating the thrombocytopenic patient.     

Flow cytometric analysis of platelet cyclooxygenase-1 and -2 and surface glycoproteins in patients with immune thrombocytopenia and healthy individuals

Immature platelets may contain more platelet enzymes such as cyclooxygenase (COX)-1 and COX-2 than mature platelets. Patients with immune thrombocytopenia (ITP) have a higher fraction of immature platelets and can therefore be utilized as a biological model for investigating COX-1 and COX-2 platelet expression. The aims were to develop flow cytometric assays for platelet COX-1 and COX-2 and to investigate the COX-1 and COX-2 platelet expression, platelet turnover, and platelet glycoproteins in ITP patients (n = 10) compared with healthy individuals (n = 30). Platelet count and platelet turnover parameters (mean platelet volume (MPV), immature platelet fraction (IPF), and immature platelet count (IPC)) were measured by flow cytometry (Sysmex XE-5000). Platelet COX-1, COX-2, and the glycoproteins (GP)IIb, IX, Ib, Ia, and IIIa were all analyzed by flow cytometry (Navios) and expressed as median fluorescence intensity. COX analyses were performed in both whole blood and platelet rich plasma (PRP), whereas platelet glycoproteins were analyzed in whole blood only. ITP patients had significantly lower platelet count (55 × 10⁹/L) than healthy individuals (240 × 10⁹/L, p < 0.01), but a higher MPV (p = 0.03) and IPF (p < 0.01). IPC was similar for the two groups (p = 0.74). PRP had significantly lower MPV (p < 0.01) and significantly higher platelet count and IPC (both p-values <0.03) when compared with whole blood. IPF was similar for PRP and whole blood (p = 0.18). COX-1 expression was 10 times higher and COX-2 expression was 50% higher in PRP than in whole blood (p_COX-1 < 0.01, p_COX-2 < 0.01). Platelet COX-1 expression was higher in ITP patients than healthy individuals using whole blood (p_COX-1 < 0.01) and PRP, though this was nonsignificant in PRP (p_COX-1 = 0.17). In ITP patients, positive correlations were found between platelet turnover and COX-1 expression (all p-values <0.01, rho = 0.80–0.94), whereas healthy individuals showed significant though weaker correlations between platelet turnover and COX-1 and COX-2 expressions (all p-values <0.03, rho = 0.44–0.71). GPIIb, IX, and Ib expression was increased in ITP patients compared with healthy individuals (all p-values < 0.03). GPIIb, IX, Ib, and IIIa showed positive correlations with platelet turnover in ITP patients (all p-values <0.02, rho = 0.71–0.94), but weak and nonsignificant correlations in healthy individuals (all p-values >0.14, rho = 0.11–0.28). In conclusion, ITP patients expressed higher COX-1 and platelet glycoprotein levels than healthy individuals. COX-1 and platelet glycoproteins demonstrated positive correlations with platelet turnover in ITP patients. In healthy individuals, COX-1 and COX-2 expression correlated positively with platelet turnover. PRP was more sensitive compared with whole blood as regards determination of COX. Therefore, PRP is the recommended matrix for investigating COX-1 and COX-2 in platelets.     

Effect of pregnancy on the course of immune thrombocytopenia: a retrospective study of 118 pregnancies in 82 women

In women with pre‐existing immune thrombocytopenic purpura (ITP), the effect of pregnancy on the course of the disease is poorly known. We performed a dual‐centre retrospective cohort study of 118 pregnancies in 82 women with primary ITP. In early pregnancy, the platelet count was <100 × 10⁹/l in 35·6% of pregnancies. During pregnancy the median platelet count nadir was 66 × 10⁹/l (25th–75th percentile: 42–117), with platelet count <30 × 10⁹/l for 26 pregnancies (22%). In 49% of pregnancies, a significant decrease of the platelet count required treatment at least transiently in preparation for delivery. At the time of delivery, the median platelet count was 110 × 10⁹/l (77–155). Compared to before pregnancy, at 3 months post‐partum, only 11% of pregnancies [95% confidence interval (95% CI): 6·8–20·2] showed disease worsening. Previous splenectomy was the only factor significantly associated with ITP worsening after pregnancy (53·9% vs. 10·3%, P < 0·001). For 8·3% of the pregnancies (95% CI: 3·8–15·1), neonatal thrombocytopenia required treatment, especially in case of previous maternal splenectomy (adjusted odds ratio 16·7, 95% CI: 2·61–106). The overall risk of exacerbation of ITP and severe thrombocytopenia during pregnancy is acceptable.     

Chronic thrombocytopenia of childhood: use of non-invasive methods in clinical evaluation

OBJECTIVES: An unselected group of 21 children with chronic thrombocytopenia was investigated to understand the patients' platelet abnormality better. METHODS: Platelet counts, mean platelet volumes (MPV), membrane glycoproteins and Fcgamma receptor type IIA (FcgammaRIIA) polymorphism H131R, reticulated platelets (% RP), antiplatelet antibodies and plasma thrombopoietin (TPO) were measured. RESULTS: Sixteen patients had idiopathic thrombocytopenic purpura (ITP) (group 1: platelets < 50 x 10(9)/L, n = 6; group 2: 50-99 x 10(9)/L, n = 4; group 3: 100-149 x 10(9)/L, n = 4; group 4: splenectomised patients with normal platelet counts, n = 2). Five patients had familial thrombocytopenia. Six healthy children were studied as a reference. In the 19 thrombocytopenic patients, the platelets were significantly larger and % RP and TPO levels were significantly higher than those in the controls. Increased megakaryocytosis at diagnosis was associated with larger MPV and higher % RP but not with platelet level or TPO. The % RP was remarkably high in all ITP patients of group 1 indicating a brisk production of platelets despite low peripheral count. In all patients with familial thrombocytopenia, TPO was increased suggesting that the syndrome was not because of defective TPO production. The distribution of FcgammaRIIA alleles in the patients was similar to that in the controls. CONCLUSIONS: A combined application of % RP and TPO could be helpful in classifying patients with chronic thrombocytopenia into different categories. The observations may be of value in the clinical evaluation of ITP patients and lead to avoidance of invasive examinations at least in some patients.     

Platelet-bound complement (C3) in immune thrombocytopenia

The fixation of complement to the circulating platelet in immune thrombocytopenia was detected by measurement of one of the complement components, C3, on the surface of platelets from patients with idiopathic thrombocytopenic purpura (ITP) and systemic lupus erythematosus (SLE) using the anti-C3 consumption assay. The surface IgG was determined simultaneously using the previously described anti- IgG consumption assay. Washed platelets from normal controls had 3.5 fg (10(-15) g) of C3, or about 11,000 molecules, per platelet, an amount comparable to the IgG (4.1 FG, or 15,000 molecules, per platelet). For most patients with ITP both C3 and IgG were increased on the platelet surface, although for 5 of 16 patients only IgG was increased. Two patients with SLE and thrombocytopenia had an increase in both C3 and Ig, six patients with SLE who were not thrombocytopenic had normal amounts of membrane-bound C3 and IgG. In 5 patients, 3 with ITP and 2 with collagen vascular disease, both surface immunoproteins decreased with successful treatment of the thrombocytopenia.     

Platelet homeostasis is regulated by platelet expression of CD47 under normal conditions and in passive immune thrombocytopenia

Interaction between target cell CD47 and the inhibitory macrophage receptor signal regulatory protein alpha (SIRPalpha) counteracts macrophage phagocytosis of CD47-expressing host cells. As platelets also express CD47, we asked whether inhibitory CD47/SIRPalpha signaling regulates normal platelet turnover and clearance of platelets in immune thrombocytopenic purpura (ITP). CD47(-/-) mice had a mild spontaneous thrombocytopenia, which was not due to a decreased platelet half-life as a result of increased expression of P-selectin, CD61, or phosphatidylserine. In contrast, CD47(-/-) platelets were rapidly cleared when transfused into CD47(+/+) recipients, whereas CD47(+/-) platelets had a nearly normal half-life in CD47(+/+) mice under nonautoimmune conditions. CD47(-/-) mice were more sensitive to ITP, as compared with CD47(+/+) mice. In vitro, macrophage phagocytosis of immunoglobulin G (IgG)-opsonized CD47(-/-) platelets was significantly higher than that for equally opsonized CD47(+/+) platelets. However, when SIRPalpha was blocked, phagocytosis of CD47(+/+) platelets increased to the level of CD47(-/-) platelets. Phagocytosis of opsonized CD47(+/-) platelets was higher than that for CD47(+/+) platelets, but lower than that for CD47(-/-) platelets, suggesting a gene-dose effect of CD47 in this system. In conclusion, we suggest that inhibitory CD47/SIRPalpha signaling is involved in regulating platelet phagocytosis in ITP, and that targeting SIRPalpha may be a new means of reducing platelet clearance in ITP.     

Thrombocytopenia model with minimal manipulation of blood cells allowing whole blood assessment of platelet function

In vitro models of thrombocytopenia are useful research tools. Previously published models have shortcomings altering properties of platelets and other blood components. The aim of the present study was to develop a whole blood method to induce thrombocytopenia with minimal manipulation, and to describe platelet function in induced thrombocytopenia in individuals with healthy platelets. Hirudin anticoagulated blood was obtained from 20 healthy volunteers. One part of the blood was gently centrifuged at 130g for 15 minutes. The platelet-rich plasma was replaced with phosphate-buffered saline to establish thrombocytopenia. Various levels of thrombocytopenia were achieved by combining different volumes of baseline whole blood and thrombocytopenic blood. Platelet counts were measured by flow cytometry (Navios, Beckman Coulter) and routine haematological analyser (Sysmex XE-5000). Platelet function was analysed by impedance aggregometry (Multiplate® Analyzer, Roche) and by flow cytometry (Navios, Beckman Coulter) using collagen, adenosine diphosphate, thrombin receptor activating peptide-6 and ristocetin as agonists. Median baseline platelet count was 227×10⁹/l. The in vitro model yielded median platelet counts at 51×10⁹/l (range 26–93×10⁹/l). We observed minor, yet significant, changes in platelet size and maturity from baseline to modelled thrombocytopenia. In the thrombocytopenic samples, significant and positive linear associations were found between platelet count and platelet aggregation across all agonists (all p-values<0.001). Platelet function assessed by flow cytometry showed minimal alterations in the thrombocytopenic samples. A new whole blood-based model of thrombocytopenia was established and validated. This new model serves as a useful future tool, particularly to explore platelet function in patients with thrombocytopenia.     

Evaluation of platelet function in thrombocytopenia

Whole blood aggregometry is a functional assay for determination of platelet function. Until now, whole blood aggregometry has not been considered feasible at low platelet counts. Hence, the objectives of the present study were to explore platelet function in thrombocytopenia using a novel index of impedance aggregometry adjusted for platelet count and evaluate the association to platelet function assessed by flow cytometry. Hirudin anticoagulated blood was collected from 20 healthy volunteers, 20 patients with primary immune thrombocytopenia (ITP), and 17 hematological cancer patients. Platelet function was analyzed by impedance aggregometry and by flow cytometry. Collagen, adenosine diphosphate, thrombin receptor agonist peptide-6, and ristocetin were used as agonists for both analyses. Thrombocytopenia in healthy whole blood was induced in vitro employing a recently published method. Platelet aggregation of thrombocytopenic patients was evaluated relative to the aggregation of healthy volunteers at the same platelet count. In flow cytometry, platelet function was described as expression of the platelet surface glycoproteins: bound fibrinogen, CD63, and P-selectin. Similar platelet counts were obtained in the patient groups (p = 0.69) (range: 13–129 × 10⁹/l). Aggregation adjusted for platelet count was significantly increased in ITP patients compared to healthy platelets across all agonists. The platelet aggregation was high in the 95% prediction interval, with 18 ITP patients above the prediction interval in at least two agonists. In contrast, the platelet aggregation was low in the prediction interval in cancer patients, and three cancer patients with platelet aggregation below the prediction interval in at least one agonist. ITP patients displayed increased expression of bound fibrinogen and CD63 following activation, compared with particularly cancer patients, but also compared with healthy platelets. This study demonstrated the feasibility of a novel approach to perform platelet function analyses in thrombocytopenia using impedance aggregometry adjusted for platelet count.     

Immature platelet fraction measurement is influenced by platelet size and is a useful parameter for discrimination of macrothrombocytopenia

Abstract

Background

Reticulated platelets (RPs) as measured using flow cytometry are useful parameters of thrombopoiesis; however, difficulties remain with standardization between laboratories. On the other hand, immature platelet fraction (IPF) measurement, as determined using an automated hematology analyzer, is simple, reproducible, and displays a good correlation with RP, although specific factors may affect its value. We previously noticed that a small proportion of patients exhibit extremely high IPF values that do not correlate with flow cytometrically measured RP.

Objectives

We investigated the mechanism of the aberrant increase in IPF values of different types of macrothrombocytopenia.

Patients/methods

IPF, RP, and other platelet indexes were analyzed using samples from 15 congenital macrothrombocytopenic patients from 12 families, 150 immune thrombocytopenic patients, and 27 normal individuals. We further monitored the change in IPF values and morphology during platelet agglutination.

Results

IPF values were about five times higher in MYH9 disorders (IPF 48.6 ± 1.9%) and about twice as high in other macrothrombocytopenias (IPF 18.4 ± 2.1%) than in immune thrombocytopenic patients with similar platelet counts (IPF 9.2 ± 0.3%). We then examined changes in IPF values during ethylenediaminetetraacetic acid- and macroglobulinemia-induced platelet agglutination. The IPF value significantly increased in a time-dependent manner along with the formation of platelet clumps and was strongly influenced by a few tiny platelet aggregates.

Conclusions

These results suggested that IPF values are influenced by platelet size. Furthermore, IPF could be a useful and convenient parameter for screening of macrothrombocytopenia, which presents with a disproportionately high IPF value.