Matched Platelet Transfusions Research Articles

Should HLA and HPA-matched platelet transfusions for patients with Glanzmann Thrombasthenia or Bernard-Soulier syndrome be standardized care? A Dutch survey and recommendations.

Glanzmann thrombasthenia (GT) and Bernard-Soulier syndrome (BSS) patients require frequent platelet transfusions and hence have an increased risk for alloimmunization against donor Human Leukocyte Antigens (HLA) when no HLA-matching is performed. Knowing that Human Platelet Antigens (HPA) are located on the platelet glycoproteins that can be absent in these patients, preventive HPA-matching may also be considered. Uniform recommendations on this topic lack in transfusion guidelines making standard practice unclear, therefore, we aimed to provide a framework for matched platelet transfusions. We conducted a targeted literature search and a national survey of Dutch (pediatric) hematologists from July to September 2021. We found 20 articles describing platelet transfusion policies in 483 GT-patients and 29 BSS-patients, both adults and children. Twenty surveys were returned for full analysis. All responders treated patients with platelet disorders, including GT (n = 36 reported) and BSS (n = 29 reported). Of respondents, 75% estimated the risk of antibody formation as "likely" for HLA and 65% for HPA. Formation of HLA antibodies was reported in 5 GT and in 5 BSS-patients, including one child. Fifteen respondents gave preventive HLA-matched platelets in elective setting (75%). Three respondents additionally matched for HPA in GT-patients (15%). Main argument for matched platelet transfusions was preventing alloimmunization to safeguard the effectivity of 'random' donor-platelets in acute settings. Elective HLA-matching for GT and BSS-patients is already conducted by most Dutch (pediatric) hematologists. HPA-matching is mainly applied when HPA-antibodies are formed. Based on the current literature and the survey, recommendations are proposed.

Matched platelet transfusions in alloimmunization

Matched platelet transfusions in alloimmunization

Ensuring HLA-matched platelet support requires an ethnic diverse donor population.

BACKGROUNDPatients refractory for platelet transfusions benefit from human leukocyte antigen (HLA)‐matched platelet transfusions. Differences in ethnic background of patients and donors could hamper the availability of sufficient numbers of HLA‐matched donors for all patients. We evaluated our HLA‐matched donor program and explored the role of ethnic background of patients related to the number of available donors.METHODSWe performed a cohort study among consecutive patients who received HLA‐matched platelet concentrates in the Netherlands between 1994 and 2017. The number of available matched donors was determined per patient. Haplotypes were constructed from genotypes with computer software (PyPop). Based on haplotypes, HaploStats, an algorithm from the National Marrow Donor Program, was used to assess the most likely ethnic background for patients with 5 or fewer and 30 or more donors.RESULTSHLA typing was available for 19,478 donors in September 2017. A total of 1206 patients received 12,350 HLA‐matched transfusions. A median of 83 (interquartile range, 18‐266) donors were available per patient. For 95 (10.3%) patients, 5 or fewer donors were available. These patients were more likely to have an African American background, whereas patients with 30 or more donors were more often from Caucasian origin, compared with Caucasian origin for patients with 30 donors.CONCLUSIONAdequate transfusion support could be guaranteed for most but not all refractory patients. More non‐Caucasian donors are required to ensure the availability of HLA‐matched donors for all patients in the Netherlands.

Impact of Human Leucocyte Antigen epitope matched platelet transfusions in alloimmunised aplastic anaemia patients

To explore the impact of Human Leucocyte Antigen (HLA)-A and B epitope-matched platelets on the outcome of platelet transfusions in alloimmunised patients with aplastic anaemia (AA). The relevance of HLA-C epitope mismatches was also investigated. Patients who become immunologically refractory (IR) to random platelet transfusions can experience an adequate rise in platelet count through the provision of HLA-compatible platelets using an antigen-matching algorithm. This approach has been shown to be effective in patients with a low calculated reaction frequency, but it is not always successful in highly sensitised patients. The use of HLA epitopes-selected platelets has been suggested as an alternative to the antigen matching approach. The effect of HLA epitope matching (both Eplets and Triplets) on the outcome of platelet transfusion was analysed in 37 highly immunised AA patients previously transfused with HLA-A and B antigen-matched platelets. Epitope matching was determined using the HLAMatchmaker programme. The outcome of the transfusions was assessed by the platelet count increments (PCIs) obtained 1 and 24 hours post-transfusions. HLA-A and B epitope matching was equivalent to HLA antigen matching in raising platelet counts. There was no significant difference in PCI when HLA-C epitope mismatches were considered. In addition, transfusions with fewer than two antigen mismatches resulted in significantly higher PCIs compared to transfusions with more than two antigen mismatches. HLA epitope-matched platelet provision may represent a clinically effective transfusion strategy for patients IR to random platelet transfusions. Further prospective studies are required.

Platelet harvest, cryopreservation and infusion; re-expanding the resources of the BMT lab

A 53 year old multiparous woman with a hepatic mass was found to have AML. She proceeded to induction chemotherapy resulting in complete remission by day 28. Between cytarabine consolidation cycles, she received 3 TACE procedures for presumed hepatocellular carcinoma. During first cycle, she developed platelet refractoriness with no platelet increment one hour post transfusion. Testing confirmed the presence of an HLA antibody. HLA matched platelet transfusions did not give a satisfactory platelet increment. The decision was made to harvest and cryopreserve her platelets, then reinfuse post chemotherapy. The patient was scheduled to have plateletpheresis with a goal of 10 x 1011platelets to be harvested and cryopreserved, allowing for 4 infusions as support over the nadir post-chemotherapy. This was to be repeated to support the final round of chemotherapy. Once platelet count reached>150, 4 collections were performed over 2.5 months yielding between 3.1 to 5.5 x 1011 platelets per harvest, irradiated at 25Gy, and cryopreserved in 5% DMSO using a controlled rate freezer. Studies on her post-thaw platelets using PRP impedance indicated a 50% reduction in functionality compared with fresh platelets. Four infusions were performed 11-20 days post consolidation 2 chemotherapy and three infusions 11-19 days post consolidation 3. No adverse reactions were reported. Platelet counts increased typically from 11-29 (1.6 to 2.6 fold) within 24 hours of infusion. HLA matched platelets gave no increase (pre-infusion count of 8, to post of 9, within 24 hours). There were no clinically significant bleeding complications and she was able to finish her scheduled rounds of chemotherapy. We have presented a case study requiring harvest, cryopreservation and infusion of autologous platelets. Incorporating archived platelet protocols into current practices was easily and safely achieved by the BMT laboratory. The methodology, equipment and processes involved required little change from the traditional role of progenitor cell work and this can safely extend the services offered by BMT laboratories with minimal additional costs for equipment or consumables.

Efficacy of HLA‐matched platelet transfusions for patients with hypoproliferative thrombocytopenia: a systematic review

HLA-matched platelets (PLTs) are widely used to transfuse patients but the effectiveness of HLA matching has not been well defined and the cost is approximately five times the cost of preparing the random-donor PLTs. The objective of this systematic review was to determine whether HLA-matched PLTs lead to a reduction in mortality; reduction in frequency or severity of hemorrhage; reduction in HLA alloimmunization, refractoriness, or PLT utilization; or improvement in PLT count increment in patients with hypoproliferative thrombocytopenia. We conducted a literature search of MEDLINE, Cochrane Controlled Register of Clinical Trials, EMBASE, and PubMed databases to April 2012. A total of 788 citations were reviewed and 30 reports were included in the analysis. Most studies did not include technologies currently in use for HLA typing or detection of HLA antibodies as 75% were conducted before the year 2000. None of the studies were adequately powered to detect an effect on mortality or hemorrhage. HLA-matched PLTs did not reduce alloimmunization and refractoriness rates beyond that offered by leukoreduction, and utilization was not consistently improved. HLA-matched PLTs led to better 1-hour posttransfusion count increments and percentage of PLT recovery in refractory patients; however, the effect at 24 hours was inconsistent. The correlation of the PLT increment with other clinical outcomes and the effect of leukoreduction on HLA-matched PLT transfusion could not be determined. Prospective studies utilizing current technology and examining clinical outcomes are necessary to demonstrate the effectiveness of HLA-matched PLT transfusion.

Titers of ABO antibodies in group O blood donors

Overt hemolysis from the transfusion of ABO incompatible plasma containing high titer anti-A or anti-B is a rare event. Plasma from group O donors and the transfusion of single donor platelets are most often associated with these reactions when transfused to group A or AB recipients. Several studies over the years have evaluated donor ABO antibody class, titer, and subclass in an effort to understand the breadth of the problem and reduce or eliminate this potentially serious immune complication of transfusion. In this regard, de Franca et al.(1) in this issue of the Revista Brasileira de Hematologia e Hemoterapia underscore the lack of a national standard of practice to guide the transfusion of incompatible ABO plasma transfusions. They point out that seminal work, started more than 2 decades ago, has yet to set policy and safety guidelines with the use of plasma that potentially contain life-threatening levels of anti-A or anti-B.(2) de Franca et al. examined over 5% of their blood donors included males and females across all age groups in an effort to understand which donors pose the biggest risk for the transfusion of high-titer anti-A or anti-B. Not surprising they found that ABO antibody titers decrease with age, with older males having the lowest titers by age 50. Cumulatively, nearly 1 in 10 plasma contain anti-A titers ≥ 128. Their work suggests that targeting ABO incompatible plasma and platelets from older males would lower the risk, if one assumes titers over 64 by the doublingdilution method (generally titers = 100 are regarded as critical) pose significant risk of hemolysis. However, there is still a 1:20 (5.3%) chance that plasma from males over 50 will contain a titer ≥ 128. Thus, targeting sex and specific age groups as surrogate markers for the transfusion of ABO incompatible platelets could hardly be considered safe practice, although it would be a step in the right direction. The lack of a standardized test and the impact on blood product availability is having an effect on the ability to drive policy and develop meaningful guidelines. Methodology appears to be a major issue. de Franca et al. used direct agglutination, however, other studies have used a test tube indirect antiglobulin test (IAT), gel microcolumn agglutination in an IAT, and flow cytometry to measure the amount of anti-A,B, anti-A and anti-B. The measurement of anti-A,B does not appear to provide any additional information on titers when compared to anti-A (de Franca et al.(1) Table 1). One problem with defining a standard may be the lack of studies necessary to correlation these methods and thus an agreement on the critical titer for which non-ABO matched platelet transfusions should be avoided. European blood centers have addressed the issue of titers and have identified low-risk titers for ABO incompatible plasma transfusions. It is important to establish critical ABO antibody titers by population (national), using the most clinically relevant method, since it appears that diet and ethnicity may result in significant differences in titers.(3,4) Performing population estimates for anti-A and anti-B levels across blood donors determines the frequency of high titers and help estimate the cost of implementing a nationalized testing policy. de Franca et al. have made a significant contribution in this regard. However, the challenge remains to quantify the impact that such a policy will have on the provision of plasma containing platelet products to recipients.(5)

In Patients with Aplastic Anaemia, Conditioning with Reduced Intensity Followed by Infusion of T-Cell Depleted Grafts Is Associated with Limited Transplant Related Morbidity and Universal Survival.

Introduction: While the survival of patients receiving allogeneic stem cell transplantation for aplastic anaemia has improved significantly, a substantial number of patients develop extensive graft vs. host disease (GvHD), with the associated morbidity and mortality. T-cell depletion of grafts decreases the incidence and severity of GvHD but may result in greater risk of graft failure.Patients and methods: Between 2004 and 2007, seventeen patients with marrow aplasia were conditioned with fludarabine 30 mg/m2 for 5 consecutive days (150 mg/m2) followed by cyclophosphamide 120 mg/kg. In addition 2 (one with Fanconi's anaemia) received busulfan 8 mg/kg. All had received more than 30 units of blood products and 2 had lymphocytotoxic antibodies that required HLA matched platelet transfusions. HLA identical sibling donors were mobilised with filgrastim (5–10 ug/kg) for 5 days and underwent large volume apheresis. Donor cells were quantitated and the CD34+ cell number determined. PBPC grafts were then incubated “in the bag” with alumtuzumab (1 mg/1010 mononuclear cells) at 20°C for 30 minutes and infused without further processing. Patients commenced cyclosporin from day −1 and continued until day 90 post transplantation. End points were engraftment, OS, DFS, incidence and severity of GvHD.Results: Median age was 22 (7–49) years and 5/17 were females. Median CD34+ cell infused was 3.93 x 106/kg, containing median of 22.2 x 14/kg CFU-GM. The median “ex vivo” alumtuzumab dose employed was 10 mg (range 5–10 mg). There was universal engraftment and leukocytes reached 0.5 x 109/L at a median of 14 days (8–17) post transplantation. Other than haematological suppression, the toxicity of the conditioning was limited to grade I–II intestinal side effects. Seven individuals did not develop pyrexia following infusion of the graft. No patient developed GvHD. Two subjects had CMV reactivation (pp65+) and responded to therapy with gancyclovir. Another two developed haemorrhagic cystitis due to BK virus. Donor-recipient sex mismatch was present in 8 instances and post transplant bone marrow karyotype analyisis showed that haematopoiesis was of donor's origin in each case. One patient developed delayed graft failure 6 months post transplantation and had a satisfactory recovery following a second T- cell depleted transplant with similar conditioning. Another developed mixed chimerism at 8 months and received DLI. All patients survive at a median of 630 days (range 60–1433) with normal blood parameters.Conclusion: We conclude that purine analogues in combination with alkylators are a sufficiently intense immunosuppressive conditioning to allow successful engraftment of T-cell depleted PBPC grafts. Alumtuzumab “in the bag” led to excellent GvHD prophylaxis, leading to minimal morbidity of the procedure.

Structural epitope matching for HLA‐alloimmunized thrombocytopenic patients: a new strategy to provide more effective platelet transfusion support?

During the past three decades, HLA matching for platelet (PLT) transfusion of refractory thrombocytopenic patients has been based on serologic cross-reactivity between HLA-A and HLA-B antigens. Although many blood banks are using this matching strategy, the general experience is that such matched PLT transfusions are often ineffective. This report describes a new HLA matching algorithm that considers structurally defined epitopes recognized by antibodies. HLAMatchmaker is a computer program that determines histocompatibility at the amino acid level initially designed as triplets (i.e., linear sequences of three residues in molecular surface-exposed positions) but now updated as eplets representing patches of antibody-accessible polymorphic residues surrounded by residues within a 3-A radius. The eplet version of HLAMatchmaker is also useful in the analysis of HLA antibody reactivity patterns of alloimmunized patients so that acceptable mismatches can be identified. An HLA epitope-based matching protocol is proposed that may permit a more effective PLT transfusion management of refractory patients. This protocol includes high-resolution HLA-A, -B, and -C typing of patients and donors, serum screening to identify acceptable mismatches, and the identification of suitable donors in a donor database that incorporates HLAMatchmaker as a search engine. HLAMatchmaker programs can be downloaded from the Web site http://tpis.upmc.edu/tpis/HLAMatchmaker/.

Postnatal management of fetal and neonatal alloimmune thrombocytopenia: the role of matched platelet transfusion and IVIG

We evaluated the effects of platelet transfusions and intravenous immunoglobulin (IVIG) in neonates with fetal and neonatal alloimmune thrombocytopenia (FNAIT) with and without antenatal treatment with IVIG. Records of neonates with FNAIT admitted between January 2000 and November 2005 were reviewed. The patients were divided into group I, treated antenatally with IVIG for known FNAIT, and group II, postnatally diagnosed with FNAIT. The primary outcome was the time interval to reach a platelet level above 100 x 10(9)/L in relation to the type of treatment. Nineteen neonates with FNAIT were identified, 13 in group I and 6 in group II. In group I, four children were born with a platelet count above 100 x 10(9)/L and never needed treatment, and four received a single matched platelet transfusion at birth with a maintained response. Five neonates received IVIG and one matched transfusion, with all but one rapidly responding. In antenatally treated cases, postnatal IVIG had no apparent effect on the platelet count. In group II, two neonates died on day 1 with severe intracranial hemorrhage. Two of the four other patients responded to a number of unmatched platelet transfusions, with one neonate rapidly responding after one matched transfusion, while another needed nine matched transfusions before a persistent adequate platelet count was reached after 9 weeks. Postnatal IVIG had no apparent effect on the platelet count in any of our cases. In neonates with FNAIT treated antenatally with IVIG, neonatal management using a single matched platelet transfusion was adequate in all cases. In neonatally diagnosed cases not treated before birth, multiple matched platelet transfusions may be required. We found no evidence to support the use of IVIG in neonates with FNAIT.

A randomized trial of washed red blood cell and platelet transfusions in adult acute leukemia [ISRCTN76536440

BackgroundPlatelet transfusion is universally employed in acute leukemia. Platelet concentrate supernatants contain high concentrations of biologic mediators that might impair immunity. We investigated whether washed platelet and red cell transfusions could improve clinical outcomes in adult patients with acute leukemia.MethodsA pilot randomized trial of washed, leukoreduced ABO identical transfusions versus leukoreduced ABO identical transfusions was conducted in 43 adult patients with acute myeloid or lymphoid leukemia during 1991–94. Primary endpoints to be evaluated were platelet transfusion refractoriness, infectious and bleeding complications and overall survival.ResultsThere were no significant differences in infectious or major bleeding complications and only one patient required HLA matched platelet transfusions. Minor bleeding was more frequent in the washed, leukoreduced arm of the study. Confirmed transfusion reactions were more frequent in the leukoreduced arm of the study. Overall survival was superior in the washed arm of the study (40% versus 22% at 5 years), but this difference was not statistically significant (p = 0.36). A planned subset analysis of those ≤50 years of age found that those in the washed, leukoreduced arm (n = 12) had a 75% survival at five years compared with 30% in the leukoreduced arm (n = 10) (p = 0.037)ConclusionThis study provides the first evidence concerning the safety and efficacy of washed platelets, and also raises the possibility of improved survival. We speculate that transfusion of stored red cell and platelet supernatant may compromise treatment, particularly in younger patients with curable disease. Larger trials will be needed to assess this hypothesis.

Hlamatchmaker-driven analysis of response to HLA matched platelet transfusions

Hlamatchmaker-driven analysis of response to HLA matched platelet transfusions

Prevention and Management of Platelet Transfusion Refractoriness

AbstractPlatelet transfusion refractoriness is a major complication of long‐term platelet supportive care. Refractoriness may lead to fatal bleeding complications in thrombocytopenic patients. Major factors involved are factors related to the clinical condition of the patient as well as HLA alloimmunisation. Non‐alloimmune factors may occur in up to 80% of the patients. However, platelet transfusion outcome is impaired in only 50% of the patients having these conditions. HLA alloimmunisation has been convincingly reduced by the use of leucocytedepleted transfusions. UV‐B irradiation of platelet transfusions may be alternatively used to reduce HLA alloimmunisation. Despite these measures, patients with a history of pregnancy or non‐leucocyte‐depleted transfusions form HLA antibodies in a high proportion (up to 50%). HPA antibodies play a minor but relatively important role in patients with HLA antibodies. ABO antibodies may play a role in refractoriness, which can be abolished by transfusion of ABO‐identical platelets. Screening for the presence of HLA and/or HPA antibodies is indicated in case of transfusion failure after ABO‐identical or HLA‐matched platelets. If no alloantibodies are detected, further analysis to define a role of drugrelated or autoantibodies is required. In case of HLA and/or HPA alloimmunisation associated with refractoriness, matched platelet transfusions are indicated. In case of non‐alloimmune factors associated with increased platelet consumption, increasing the transfusion frequency can be considered. Additional investigations are still necessary to define risk factors for secondary HLA alloimmunisation and refractoriness due to non‐immune factors to further decrease the incidence of refractoriness.

A flow cytometric platelet immunofluorescence crossmatch for predicting successful HLA matched platelet transfusions

Platelet crossmatching may provide a useful way of selecting donors for effective platelet transfusions in patients refractory to random donor platelet concentrates due to alloimmunization. We assessed the predictive value of a flow cytometric platelet immunofluorescence crossmatch test for the outcome of HLA matched platelet transfusions in a group of alloimmunized patients. Platelet immunofluorescence (PIFT) crossmatches were performed for 104 HLA-matched platelet transfusions administered to 30 patients. A negative PIFT crossmatch correctly predicted a successful platelet transfusion (1 h post-transfusion platelet recovery >20%) in 56/75 (75%) cases. We also considered non-immunological factors that, in combination with alloimmunization, might have contributed to an unsuccessful transfusion result, i.e. fever, septicaemia, splenomegaly, disseminated intravascular coagulation and bleeding. The predictive value of a negative PIFT crossmatch was better when these non-immunological factors were absent [48/59 (81%) correct predictions] than when these factors were present [8/16 (50%) correct predictions] (P=0.01; chi-square test). The effect of ABO incompatibility between donor and recipient on the predictive value of the PIFT crossmatch was also analysed. Positive PIFT crossmatches occurred more frequently in ABO incompatible donor-recipient combinations [in 18/28 (64%) cases] than in ABO-compatible donor-recipient combinations [in 11/76 cases (14%)] (P<0.001, chi-square test). Successful platelet transfusions were observed on 53/76 (70%) occasions in ABO compatible transfusions as compared to 16/28 (57%) in ABO incompatible transfusions. This difference was not statistically significant (P=0.23; chi-square test). Consequently, a negative PIFT crossmatch appeared to be non-predictive for the transfusion outcome in cases of ABO incompatibility between donor and recipient. We conclude that the PIFT crossmatch for platelet donor selection in addition to matching for HLA antigens, is predictive for the outcome of ABO compatible transfusions in alloimmunized recipients and prediction levels are increased when non-immunological causes for platelet refractoriness are absent.

The definition of refractoriness to platelet transfusions.

The relationship between the 1 and 20 h post-transfusion platelet count and three parameters used to define refractory transfusions, namely the corrected increment (CI), platelet increment (PI), and percentage platelet recovery (%REC), was studied in 437 non-HLA matched platelet transfusions given to 102 patients with bone marrow failure. The percentage agreement between common definitions of refractoriness was calculated based on these parameters. As the maintenance of platelet counts above 20 x 10(9)/l is a relevant clinical goal for platelet support, the values of the CI, PI and %REC, which best corresponded to 1- and 20-h post-transfusion counts of 20 x 10(9)/l, were identified. A 1-h post-transfusion CI < 3 (PI < 7 x 10(9)/l or % REC < 8%) corresponded to clinically unsuccessful transfusions with a 1-h platelet count < 20 x 10(9)/l. A 1-h CI > or = 5.5 (PI > or = 12 x 10(9)/l or %REC > or = 14%) corresponded to clinically successful transfusions with a 20-h post-transfusion count of > or = 20 x 10/l. These data tie together the end points reported in the literature for defining refractory transfusions.

Human Platelet-Specific Antigen, Siba, Is Associated With the Molecular Weight Polymorphism of Glycoprotein Ibα

AbstractPlatelet-specific antigen Siba has been highly implicated in the pathogenesis of refractoriness to human leukocyte antigen (HLA)-matched platelet transfusions in Japan. We provide evidence that the Siba antigen is located on the glycoprotein (GP) Ibα and has a close association with the molecular weight (mol wt) polymorphism of GPIb. In modified antigen-capture ELISA (MACE), anti-Siba antibody reacted only with GPIb/IX held by a murine anti-GPIb/IX monoclonal antibody (MoAb). The reactivity of anti-Siba antibody to Siba-positive (Siba+) platelets was abolished after they were treated with Serratia marcescens protease. Platelets from 50 healthy volunteers were semiquantitatively phenotyped for Siba antigen by MACE and divided into three distinct groups: strongly positive, positive, and negative. They were also analyzed by sodium dodeyl sulfate-poiyacrylamide gel electrophoresis (SDS-PAGE) and periodic acid-silver staining for mol wt polymorphism of GPIb, phenotyped as A, B, C, or D. Without exception. Siba+ platelets showed larger phenotypes (A or B). Removal of sialic acid from Siba+ platelets did not reduce the binding of anti-Siba. Finally, anti-Siba antibody specifically immunoprecipitated A and B phenotypes of GPIb from Siba+ platelets. Thus, Siba antigen evidently is located in the region of glycocalicin that is present only on the A and B phenotypes of GPIb.

Human platelet-specific antigen, Siba, is associated with the molecular weight polymorphism of glycoprotein Ib alpha

Platelet-specific antigen Sib(a) has been highly implicated in the pathogenesis of refractoriness to human leukocyte antigen (HLA)-matched platelet transfusions in Japan. We provide evidence that the Sib(a) antigen is located on the glycoprotein (GP) Ib alpha and has a close association with the molecular weight (mol wt) polymorphism of GPIb. In modified antigen-capture ELISA (MACE), anti-Sib(a) antibody reacted only with GPIb/IX held by a murine anti-GPIb/IX monoclonal antibody (MoAb). The reactivity of anti-Sib(a) antibody to Sib(a)-positive (Sib(a+)) platelets was abolished after they were treated with Serratia marcescens protease. Platelets from 50 healthy volunteers were semiquantitatively phenotyped for Sib(a) antigen by MACE and divided into three distinct groups: strongly positive, positive, and negative. They were also analyzed by sodium dodeyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and periodic acid-silver staining for mol wt polymorphism of GPIb, phenotyped as A, B, C, or D. Without exception, Sib(a+) platelets showed larger phenotypes (A or B). Removal of sialic acid from Sib(a+) platelets did not reduce the binding of anti-Sib(a). Finally, anti-Sib(a) antibody specifically immunoprecipitated A and B phenotypes of GPIb from Sib(a+) platelets. Thus, Sib(a) antigen evidently is located in the region of glycocalicin that is present only on the A and B phenotypes of GPIb.

Vincristine Therapy for Severe Platelet Alloimmunization

A 19-month-old girl with idiopathic severe aplastic anemia refractory to multi-agent immunosuppressive therapy developed severe platelet alloimmunization following several months of platelet transfusions. She became refractory to human leukocyte antigen (HLA)-matched platelet transfusions and experienced frequent episodes of bleeding. She was treated with intravenous vincristine administered weekly for three doses and showed marked improvement in both clinical and laboratory response to platelet transfusions. When vincristine was held for 3 weeks, she again became refractory to HLA-matched platelet transfusion. Reinstitution of vincristine resulted in cessation of clinical bleeding and improved response to platelet transfusion. The mechanism of response likely involves selective delivery of cytotoxic drug to macrophages. To our knowledge this is the first reported case of alloimmune thrombocytopenia responsive to vincristine.

Identification of the platelet-specific alloantigen, Naka, on platelet membrane glycoprotein IV

We describe the membrane localization of a new platelet-specific alloantigen, designated Naka, that is involved in refractoriness to HLA- matched platelet transfusions. By indirect immunoprecipitation, anti- Naka antibody precipitated a single, radiolabeled platelet membrane protein with a molecular weight (mol wt) of 91 Kd from Naka-positive platelets. When radiolabeled Naka-negative platelets were used as a source of target antigens, no radiolabeled proteins were precipitated. The analyses using nonreduced-reduced two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and using rabbit antiglycoprotein (GP)IV demonstrated that this protein corresponds to GPIV (alternatively GPIIIb). Furthermore, in dot immunobinding, anti- Naka antibody bound to purified GPIV. Our results provide definitive evidence that the Naka alloantigen is carried on GPIV. These results also demonstrate that, on occasion, antibodies against GPIV may play an important role in refractoriness to platelet transfusions.

Disappearance of HLA and platelet-specific antibodies in acute leukaemia patients alloimmunized by multiple transfusions.

Alloimmunization by platelet transfusions was studied in 154 patients with acute leukaemia. 17 patients had HLA antibodies at initial presentation induced by previous transfusions or pregnancies; one of these also had platelet-specific antibodies and one other patient had platelet-specific antibodies alone. A further 38 patients developed HLA antibodies during therapy; three also had platelet-specific antibodies and two patients developed platelet-specific antibodies alone. Of these, 37 patients with HLA antibodies including three with platelet-specific antibodies and one patient with platelet-specific antibodies alone survived their initial therapy and formed the basis of this study. Antibodies once detected persisted throughout the study in seven of the 37 patients with HLA antibodies including one patient with platelet-specific antibodies and in the patient with platelet-specific antibodies alone. HLA antibodies disappeared after discontinuation of transfusions in six patients, and after switching to HLA matched platelet transfusions and leucocyte-poor blood in eight patients; two of the latter patients also had platelet-specific antibodies which disappeared. The other 16 patients with HLA antibodies lost their antibodies despite continued transfusions.