Alteration of platelet-associated membrane glycoproteins during extracorporeal apheresis of peripheral blood progenitor cells

Monocytes complexed to platelets differentiate into functionally deficient dendritic cells

In addition to their role in hemostasis, platelets store numerous immunoregulatory molecules such as CD40L, TGFβ, β2-microglobulin, and IL-1β and release them upon activation. Previous studies indicate that activated platelets form transient complexes with monocytes, especially in HIV infected individuals and induce a proinflammatory monocyte phenotype. Because monocytes can act as precursors of dendritic cells (DCs) during infection/inflammation as well as for generation of DC-based vaccine therapies, we evaluated the impact of activated platelets on monocyte differentiation into DCs. We observed that in vitro cultured DCs derived from platelet-monocyte complexes (PMCs) exhibit reduced levels of molecules critical to DC function (CD206, dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin, CD80, CD86, CCR7) and reduced antigen uptake capacity. DCs derived from PMCs also showed reduced ability to activate naïve CD4+ and CD8+ T cells, and secrete IL-12p70 in response to CD40L stimulation, resulting in decreased ability to promote type-1 immune responses to HIV antigens. Our results indicate that formation of complexes with activated platelets can suppress the development of functional DCs from such monocytes. Disruption of PMCs in vivo via antiplatelet drugs such as Clopidogrel/Prasugrel or the application of platelet-free monocytes for DCs generation in vitro, may be used to enhance immunization and augment the immune control of HIV.

Using Dendritic Cell-Based Immunotherapy to Treat HIV: How Can This Strategy be Improved?

Harnessing dendritic cells (DC) to treat HIV infection is considered a key strategy to improve anti-HIV treatment and promote the discovery of functional or sterilizing cures. Although this strategy represents a promising approach, the results of currently published trials suggest that opportunities to optimize its performance still exist. In addition to the genetic and clinical characteristics of patients, the efficacy of DC-based immunotherapy depends on the quality of the vaccine product, which is composed of precursor-derived DC and an antigen for pulsing. Here, we focus on some factors that can interfere with vaccine production and should thus be considered to improve DC-based immunotherapy for HIV infection.

Acoustophoretic purification of platelets: feasibility and impact on platelet activation and function

Purity, limited platelet activation, and preservation of platelet function are important stakes of preparation of platelet concentrates (PC) for clinical use. In fact, contaminating red blood cells and leukocytes, as well as activated and/or poorly functional platelets in PC, represents a risk of poor efficiency and adverse side effects during platelet transfusion. Therefore, optimization of preparation and storage of PC is still an active field of research. Shear-induced platelet activation is an unwanted side effect of the hard-spin (up to 5000g) step of centrifugation-based methods currently used in blood banks to prepare PC from whole blood samples. Here, we evaluated the effectiveness of an acoustic-based fractionation device for the isolation of human platelets from whole blood bags. The purity, activation status, and functionality of platelets isolated by acoustopheresis were compared with those of platelets isolated using a reference protocol known to produce limited platelet activation and consisting of two consecutive soft-spin centrifugations (120g and 1200g). Platelet concentration and purity were determined using an automated hematology analyzer. Platelet activation status and platelet reactivity to collagen and thrombin were assessed in flow cytometry by measurement of surface expression of P-selectin and activated integrin αIIbβ3. The ability of isolated platelets to incorporate into a thrombus when transfused to NOD/SCID mice was investigated by intravital microscopy using the ferric chloride-induced thrombosis model. Blood fractionation by acoustophoresis led to the elimination of more than 80% of red blood cells and leukocytes from the platelet fraction, whose mean purity was of 92.8 ± 12.8%. The activation status and reactivity to collagen and thrombin of acoustophoresis-isolated platelets were similar to those of platelets isolated by soft-spin centrifugation. Finally, acoustophoresis-isolated platelets were tethered, adhered to the vessel wall, and incorporated into a growing thrombus following ferric chloride-induced vascular injury. Together, our results indicate that acoustophoresis is a suitable method for the isolation of human platelets with minimal platelet activation and preservation of platelet function.

Efficient removal of platelets from peripheral blood progenitor cell products using a novel micro-chip based acoustophoretic platform

Background

Excessive collection of platelets is an unwanted side effect in current centrifugation-based peripheral blood progenitor cell (PBPC) apheresis. We investigated a novel microchip-based acoustophoresis technique, utilizing ultrasonic standing wave forces for the removal of platelets from PBPC products. By applying an acoustic standing wave field onto a continuously flowing cell suspension in a micro channel, cells can be separated from the surrounding media depending on their physical properties.

Study Design and Methods

PBPC samples were obtained from patients (n = 15) and healthy donors (n = 6) and sorted on an acoustophoresis-chip. The acoustic force was set to separate leukocytes from platelets into a target fraction and a waste fraction, respectively. The PBPC samples, the target and the waste fractions were analysed for cell recovery, purity and functionality.

Results

The median separation efficiency of leukocytes to the target fraction was 98% whereas platelets were effectively depleted by 89%. PBPC samples and corresponding target fractions were similar in the percentage of CD34+ hematopoetic progenitor/stem cells as well as leukocyte/lymphocyte subset distributions. Median viability was 98%, 98% and 97% in the PBPC samples, the target and the waste fractions, respectively. Results from hematopoietic progenitor cell assays indicated a preserved colony-forming ability post-sorting. Evaluation of platelet activation by P-selectin (CD62P) expression revealed a significant increase of CD62P+ platelets in the target (19%) and waste fractions (20%), respectively, compared to the PBPC input samples (9%). However, activation was lower when compared to stored blood bank platelet concentrates (48%).

Conclusion

Acoustophoresis can be utilized to efficiently deplete PBPC samples of platelets, whilst preserving the target stem/progenitor cell and leukocyte cell populations, cell viability and progenitor cell colony-forming ability. Acoustophoresis is, thus, an interesting technology to improve current cell processing methods.

Platelet-rich-plasmapheresis for minimising peri-operative allogeneic blood transfusion

BACKGROUND

Concerns regarding the safety of transfused blood have generated considerable enthusiasm for the use of technologies intended to reduce the use of allogeneic blood (blood from an unrelated donor). Platelet-rich plasmapheresis (PRP) offers an alternative approach to blood conservation.

OBJECTIVES

To examine the evidence for the efficacy of PRP in reducing peri-operative allogeneic red blood cell (RBC) transfusion, and the evidence for any effect on clinical outcomes such as mortality and re-operation rates.

SEARCH STRATEGY

We identified studies by searching MEDLINE (1950 to 2009), EMBASE (1980 to 2009), The Cochrane Library (Issue 1, 2009), the Internet (to March 2009) and the reference lists of published articles, reports, and reviews.

SELECTION CRITERIA

Controlled parallel group trials in which adult patients, scheduled for non-urgent surgery, were randomised to PRP, or to a control group which did not receive the intervention.

DATA COLLECTION AND ANALYSIS

Primary outcomes measured were: the number of patients exposed to allogeneic RBC transfusion, and the amount of RBC transfused. Other outcomes measured were: the number of patients exposed to allogeneic platelet transfusions, fresh frozen plasma, and cryoprecipitate, blood loss, re-operation for bleeding, post-operative complications (thrombosis), mortality, and length of hospital stay. Treatment effects were pooled using a random-effects model. Trial quality was assessed using criteria proposed by Schulz et al (Schulz 1995).

MAIN RESULTS

Twenty-two trials of PRP were identified that reported data for the number of patients exposed to allogeneic RBC transfusion. These trials evaluated a total of 1589 patients. The relative risk (RR) of exposure to allogeneic blood transfusion in those patients randomised to PRP was 0.73 (95%CI 0.59 to 0.90), equating to a relative risk reduction (RRR) of 27% and a risk difference (RD) of 19% (95%CI 10% to 29%). However, significant heterogeneity of treatment effect was observed (p < 0.00001; I² = 79%). When the four trials by Boldt are excluded, the RR is 0.76 (95% CI 0.62 to 0.93). On average, PRP did not significantly reduce the total volume of RBC transfused (weighted mean difference [WMD] -0.69, 95%CI -1.93 to 0.56 units). Trials provided inadequate data regarding the impact of PRP on morbidity, mortality, and hospital length of stay. Trials were generally small and of poor methodological quality.

AUTHORS' CONCLUSIONS

Although the results suggest that PRP is effective in reducing allogeneic RBC transfusion in adult patients undergoing elective surgery, there was considerable heterogeneity of treatment effects and the trials were of poor methodological quality. The available studies provided inadequate data for firm conclusions to be drawn regarding the impact of PRP on clinically important endpoints.

Platelet growth factors suppress ex vivo expansion and enhance differentiation of umbilical cord blood CD133+ stem cells to megakaryocyte progenitor cells

BACKGROUND AND OBJECTIVES

Umbilical cord blood (UCB) is a rich source of hematopoietic cells. Here, for the first time, we surveyed the effects of different concentrations of platelet growth factors and cytokine cocktail (CC) on the expansion and differentiation of UCB CD133(+) stem cells into megakaryocyte progenitors.

MATERIALS AND METHODS

UCB CD133(+) cells were separated by magnetic cell sorting and cultured in different concentrations of platelet growth factors in combination with a CC containing interleukins 3 and 6, stem cell factor, and thrombopoietin. Cell expansion and differentiation were assessed using mononuclear cell count and flow cytometry.

RESULTS

The results show that either activated platelet-rich plasma or the platelet supernatant, when added in the first day of culture, significantly suppress the expansion of CD133(+) cells after 7 days in culture (p < 0.05). By contrast, the expression of CD41, CD61, and CD42b markers in the presence of all platelet growth factors increased compared with that of the control (p < 0.05).

CONCLUSION

Taken together, platelet growth factors in the presence of CC suppress ex vivo expansion of UCB CD133(+) cells and enhance their differentiation into megakaryocytic progenitor cells in a dose- and time-dependent manner.

Platelet-rich-plasmapheresis for minimising peri-operative allogeneic blood transfusion

BACKGROUND

Concerns regarding the safety of transfused blood have generated considerable enthusiasm for the use of technologies intended to reduce the use of allogeneic blood (blood from an unrelated donor). Platelet-rich plasmapheresis (PRP) offers an alternative approach to blood conservation.

OBJECTIVES

To examine the evidence for the efficacy of PRP in reducing peri-operative allogeneic red blood cell (RBC) transfusion, and the evidence for any effect on clinical outcomes such as mortality and re-operation rates.

SEARCH STRATEGY

Studies were identified by: computer searches of MEDLINE, EMBASE, Current Contents, and the Cochrane Library (to June 2001). These searches were supplemented by checking the reference lists of published articles, reports, and reviews.

SELECTION CRITERIA

Controlled parallel group trials in which adult patients, scheduled for non-urgent surgery, were randomised to PRP, or to a control group who did not receive the intervention.

DATA COLLECTION AND ANALYSIS

Main outcomes measured were: the number of patients receiving an allogeneic RBC transfusion, and the amount of RBC transfused. Trial quality was assessed using criteria proposed by Schulz et al. (Schulz 1995) and Jadad et al. (Jadad 1996).

MAIN RESULTS

Nineteen trials of PRP were identified that reported data for the number of patients exposed to allogeneic RBC transfusion. These trials evaluated a total of 1452 patients. The pooled relative risk (RR) of exposure to allogeneic blood transfusion in those patients randomised to PRP was 0.71 (95%CI: 0.56, 0.90), equating to a relative risk reduction (RRR) of 29%; the average absolute risk reduction (ARR) was 19% (RD = -0.19: 95%CI: -0.29, -0.09). On average, PRP did not significantly reduce the total volume of RBC transfused (weighted mean difference [WMD] = -0.69: 95%CI: -1.93, 0.56 units). Substantial statistical heterogeneity was observed (p < 0.001). Trials provided inadequate data regarding the impact of PRP on morbidity, mortality, and hospital length of stay. The majority of trials were small and of poor methodological quality.

REVIEWER'S CONCLUSIONS

Although the results suggest that PRP is effective in reducing allogeneic RBC transfusion in adult patients undergoing elective surgery, there was considerable heterogeneity in treatment effects and the trials were of poor methodological quality. As the majority of trials were unblinded, transfusion practices may have been influenced by knowledge of the patient's allocation status, potentially exaggerating the true magnitude of the beneficial effect of PRP. The available studies provided inadequate data for firm conclusions to be drawn regarding the impact of PRP on clinically important endpoints.

Mobilization of allogeneic peripheral blood progenitor cells

It is possible to reliably obtain sufficient PBSC from most normal donors to perform allogeneic transplantation. The mobilization regimen, usually administration of a single daily dose of G-CSF at 7.5 to 10 micrograms/kg subcutaneously for 4 to 6 days, is tolerable with acceptable side effects. However, there is wide variability among individuals with respect to the extent of mobilization achieved by the regimen and the optimal timing of apheresis. Studies suggest that the likelihood of obtaining an adequate harvest of CD34+ cells, as defined locally may be enhanced by employing higher doses or different schedules of G-CSF, monitoring the mobilization and/or collection of PBPC, and using apheresis procedures processing 2 or more times blood volume. However, an optimal regimen for mobilization and harvesting for all donors has not yet been identified and a small percentage of donors may not mobilize adequately with G-CSF. Alternative regimens employing combinations of G-CSF and GM-CSF are available that may prove useful in such cases and novel cytokines that are even more effective than G-CSF in mobilizing stem cells are eagerly awaited. Based on currently available experience with normal donors, the short-term safety of G-CSF appears to be acceptable, however there exist several scenarios in which marrow harvesting may be preferable to G-CSF mobilization and apheresis collection of PBPC.

Peripheral blood progenitor cell grafts contain high levels of platelet-secreted mediators

BACKGROUND

The cytokine network in peripheral blood progenitor cell (PBPC) grafts may affect hematopoietic reconstitution or the risk of postransplant relapse of malignant disorders through effects on normal progenitor cells or contaminating malignant cells. Whether thrombopoietin (TPO), SCF, and platelet-secreted mediators are parts of this network was investigated.

STUDY DESIGN AND METHODS

Peripheral blood and PBPC plasma samples were collected consecutively from patients with malignant disorders who underwent PBPC harvest. Blood samples were collected immediately before and after apheresis. Patients underwent mobilization by chemotherapy plus G-CSF, except for one patient who received only G-CSF. Plasma levels were also determined for healthy controls.

RESULTS

PBPC grafts had greater levels of platelet-secreted platelet factor 4 (PF4), beta-thromboglobulin, and platelet-derived growth factor isoform AB, as compared with venous levels in patients and controls. Although platelet and PF4 levels in autografts were significantly correlated, the graft:blood ratio was higher for PF4 than for platelets. In both the patients' blood and the autografts, TPO levels were increased from the levels in normal controls. Blood and graft levels of SCF were within the normal range.

CONCLUSION

The cytokine network of PBPC autografts includes increased levels of TPO and several platelet-derived mediators.

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+ granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Platelet-derived growth factor (PDGF) in human acute myelogenous leukemia: PDGF receptor expression, endogenous PDGF release and responsiveness to exogenous PDGF isoforms by in vitro cultured acute myelogenous leukemia blasts

We investigated effects of Platelet-derived growth factor (PDGF) and Platelet factor 4 (PF-4) on the functional characteristics of native, human acute myelogenous leukemia (AML) blasts. AML blast expression of the PDGF-receptor alpha-chain was detected for a subset of patients (45%), whereas PDGF-receptor beta-chain expression was detected for most patients (90%). Constitutive AML blast release of the PDGF-AB isoform (the major form also derived from normal platelets) was detected for 43% of patients, whereas PDGF-BB release was not detected for any patient. The PDGF isoforms AA, AB and BB had dose-dependent and divergent effects on spontaneous and cytokine-dependent AML blast proliferation, whereas for constitutive cytokine secretion (IL-1beta, IL-6, TNF-alpha) inhibitory effects were rare and all three isoforms usually had no effect or enhanced the constitutive secretion. The PDGF effects were caused by a direct effect on the AML blasts and were not dependent on the presence of serum. The PDGF effects could also be detected after in vitro culture of AML cells in the presence of IL-4+granulocyte-macrophage colony stimulating factor. PF-4 had divergent effects on proliferation and cytokine secretion by native AML blasts. Our results suggest that exogenous (e.g. platelet-secreted) PDGF and PF-4 can function as regulators of leukemic hematopoiesis and possibly also modulate the function of residual AML cells in peripheral blood stem cell grafts. On the other hand, endogenous release of PDGF-AB by native blasts may modulate the function of normal cells in the bone marrow microenvironment (e.g. bone marrow stromal cells).

Effects of vascular endothelial growth factor on acute myelogenous leukemia blasts

Vascular endothelial growth factor (VEGF) and its specific receptors are expressed by various malignant cells, including acute myelogenous leukemia (AML) blasts. In this study we performed a detailed characterization of VEGF effects on native human AML blasts derived from a large group of consecutive AML patients with high blast counts in peripheral blood. Exogenous VEGF had divergent effects on spontaneous proliferation and cytokine-dependent (GM-CSF, G-CSF, IL-3) proliferation. Increased, decreased, or unaltered proliferation was observed in the presence of VEGF for various patients, and the VEGF effect differed even in the same patient depending on which exogenous cytokine being present together with VEGF. Similarly, increased, decreased or unaltered interleukin-1beta (IL-1beta) and IL-6 secretion was detected when VEGF was added, and for certain patients the effect of VEGF differed between IL-1beta and IL-6. Exogenous VEGF could also modulate proliferation and differentiation of clonogenic AML progenitors. Constitutive AML blast secretion of VEGF was detected for 40% of patients. Leptin, Flt3-L, IL-4, IL-10, and IL-13 had divergent effects on VEGF release by AML blasts. These results suggest that VEGF can modulate AML blast functions in vivo for a subset of patients. Furthermore, the detection of VEGF in peripheral blood stem cell (PBSC) autografts suggests that VEGF may influence the proliferation and possibly also the survival of contaminating AML cells in PBSC autografts. We conclude that VEGF may influence the functional characteristics of AML cells. Our results suggest that VEGF is important in leukemic hematopoiesis, and the detection of VEGF in PBSC autografts indicates that VEGF may influence the functional phenotype of contaminating AML cells in these grafts.

Collection of MNCs with two cell separators for adoptive immunotherapy in patients with stage IV melanoma

BACKGROUND

MNCs for adoptive immunotherapy may be collected by leukocytapheresis with a cell separator.

STUDY DESIGN AND METHODS

Six healthy cytapheresis donors donated two MNC concentrates on a cell separator (AS.TEC 204, Fresenius): one on the standard MNC program and one on a modified MNC program with reduced centrifuge velocity that leads to a lower platelet contamination. Seventeen patients with malignant melanoma donated 26 MNC concentrates: 5 on the AS.TEC 204 MNC program, 9 on the modified AS.TEC 204 MNC program, and 12 on another modified MNC program (Spectra, COBE).

RESULTS

In the course of cultivation of MNCs to dendritic cells (DCs), the donor MNC concentrates with the lower platelet contamination (475 +/- 85 x 10(9)/L) had a significantly higher relative DC yield (low platelet contamination: 3.9 +/- 1.6% of the plated cells; high platelet contamination: 2.5 +/- 1.8% of the plated cells; p = 0.019) than the concentrates with the higher platelet contamination (2364 +/- 448 x 10(9)/L). No significant difference was found in the yields of MNCs and CD14+ cells in the three protocols used for the collection of MNCs from patients with melanoma. The components obtained by the standard AS.TEC 204 MNC program had a significantly higher platelet contamination (1768 +/- 994 x 10(9)/L) than the components obtained by the modified AS.TEC MNC program (360 +/- 98 x 10(9)/L; p<0.05) and the modified Spectra MNC program (636 +/- 266 x 10(9)/L); p<0.05). Because of the low number of investigated components, no significant difference in the DC yield of the three protocols could be detected (mean DC yield after cultivation: 746 +/- 429 x 10(6)).

CONCLUSION

A high platelet contamination of MNC concentrates intended for adoptive immunotherapy can lead to a significant impairment of the DC yield after cultivation. Both the modified AS.TEC 204 and the modified Spectra MNC programs are well suited for collecting MNC concentrates with high MNC yields and low platelet contamination from patients with malignant melanoma.

Autologous stem cell transplantation as post-remission therapy in adult acute myelogenous leukemia: does platelet contamination of peripheral blood mobilized stem cell grafts influence the risk of leukemia relapse?

Conventional chemotherapy of acute myelogenous leukemia (AML) results in an overall long-term disease-free survival of less than 50%, but for selected subsets of younger patients the prognosis can be improved by allogeneic stem cell transplantation. The use of autologous stem cell transplantation is now investigated as an alternative to allotransplantation due to its lower risk of serious complications. However, autotransplantation is associated with a relatively high risk of post-transplant AML relapse that can be derived from contaminating leukemia cells in the autograft. Peripheral blood mobilized stem cell (PBSC) grafts usually contain a higher number of platelets. The degree of platelet contamination is determined by the peripheral blood platelet count at the time of harvesting, and the platelets become activated and release soluble mediators during the ex vivo handling of PBSC grafts. Many of these platelet-derived mediators can bind to specific receptors expressed by AML blasts, and the platelet contamination may then alter AML blast survival and thereby influence the risk of post-transplant leukemia relapse. Therefore, we conclude that the platelet contamination of autologous stem cell grafts is possibly of clinical importance, but the effect of this nonstandardized parameter is difficult to predict in individual patients because the number of graft-contaminating platelets, the degree of platelet activation, and the effects of platelet-derived mediators on AML blasts differ between patients.

Apheresis-induced platelet activation:comparison of three types of cell separators

BACKGROUND

Platelet-harvesting technology differs in various cell separators. Alteration in shear stress and biocompatibility of surfaces may give variable platelet activation and thereby affect the quality of the component.

STUDY DESIGN AND METHODS

Four groups (n = 10) of single-needle apheresis procedures using three cell separators, were compared: 1) Spectra LRS, 90-minute harvesting time; 2) MCS+, 90-minute harvest; 3) Amicus, 90-minute; and 4) Amicus, 45-minute. Whole-blood samples were collected from the donors as were samples from the final components at intervals during the first 4 hours after cessation of the apheresis. Platelet activation status and platelet activation capacity after agonist stimulation were assessed by flow cytometry.

RESULTS

No activated platelets were found in preapheresis and postapheresis samples from the donors. The platelets in the components from the Amicus (90-min) were significantly more activated than those in the other groups of components: that is, there was increased size of platelet aggregates, increased fraction of microparticles, increased degranulation, increased fibrinogen receptor activation, and decreased von Willebrand factor receptor expression. Moreover, the response of these platelets to agonist stimulation was reduced for all activation variables.

CONCLUSIONS

After 90 minutes' processing time, platelets obtained with the Amicus cell separator were significantly more activated than platelets harvested with the Spectra and the MCS+.

The role of flow cytometry in improving biocompatibility in transfusion medicine

In transfusion medicine, blood and blood components, donors and patients are increasingly confronted with biomaterials. The need to understand the response of human blood to contact with these artificial surfaces has led to multiple studies on the biocompatibility of biomaterials. Up to this time, these investigations have predominantly been performed using physical, immunological and biochemical methods. Many of these approaches are useful in investigating the multiple factors involved in blood-biomaterial interactions. However, they always reflect the overall behaviour of whole cellular populations in local or systemic reactions. The application of multiparameter flow cytometry, on the other hand, provides insight into antigenic expression and changes at the single-cell level. Therefore, the technique of flow cytometry represents a new and powerful way of analysing and improving the biocompatibility of these materials in blood-contacting applications in this field.