Platelet-specific antibodies in transfused patients

Modification of solid phase red cell adherence assay for the detection of platelet antibodies in patients with thrombocytopenia

Platelet refractoriness is caused by HLA antibodies and platelet-specific antibodies. Current methods used to detect antiplatelet antibodies have limitations. Solid phase red cell adherence (SPRCA) lacks sensitivity and requires a second assay using chloroquine-treated intact platelets to specify the response due to anti-HLA. We modified SPRCA by using 2 types of antihuman platelet antibodies with different specificities toward platelet lysate and tested samples from 361 patients (69 with unexplained thrombocytopenia and 292 with poor response to platelet transfusions not explicable by alloimmunization or the clinical situation) and 50 from healthy volunteers. Our method compared favorably with platelet suspension direct immunofluorescence. All samples from healthy volunteers were negative; of the samples from the patient population, 240 were positive (147 samples had only antiplatelet and 3 samples had only anti-HLA antibodies). This modified technique had a sensitivity of 98% and a specificity of 91%.

Comparison of acute non-haemolytic transfusion reactions in female and male patients receiving female or male blood components

To study the relationship between antibodies detected in patients’ and/or donors’ sera and the clinical features of acute non-haemolytic transfusion reactions (ANHTRs), and to determine any gender-related difference. ANHTRs range from urticaria to transfusion-related acute lung injury (TRALI). Antibodies to human leukocyte antigen (HLA), granulocytes, platelets, and/or plasma proteins are implicated in some of the ANHTRs. A higher antibody positivity is expected for females than for males. A comparative study of ANHTRs for antibody positivity and their clinical features between females and males for both patients and donors is helpful for characterizing ANHTRs including TRALI more clearly, but such studies are few and outdated. Two hundred and twenty-three ANHTR cases reported by 45 hospitals between October 2000 and July 2005 were analysed. The patients and 196 donors of suspect blood products were screened for antibodies to HLA Class I, HLA Class II, granulocytes, and platelets. The patients were also screened for anti-plasma protein antibodies. The types and severity of ANHTR did not differ significantly between female and male patients. The frequency of the anti-HLA antibodies, but not that of the non-HLA antibodies, was significantly higher in females. Non-HLA antibodies were significantly associated with severe reactions in females. All the TRALI cases had predisposing risk factors for acute lung injury, and 60% of the cases showed anti-leucocyte antibodies. Although the anti-HLA antibodies were detected more frequently in females than males, no significant association of ANHTRs including TRALI with gender, not only for patients, but also for donors, could be shown in this study.

New paradigms in the management of alloimmune refractoriness to platelet transfusions

PURPOSE OF REVIEW

Following transfusion or pregnancy, a significant number of patients develop antibodies to class I human leukocyte antigen. Some will exhibit platelet transfusion refractoriness, defined as inappropriately low platelet count increments after two or more consecutive transfusions. Unfortunately, failure of at least two products is required before an immunologic work-up is undertaken. Among those diagnosed with immune refractoriness, there is no standard method for identifying platelet products likely to be effective.

RECENT FINDINGS

Recent advances in detection and identification of human leukocyte antigen antibody may permit pretransfusion screening of selected patients and provide guidance in choosing the optimal product. An approach more like that for red cell alloimmunized patients, in which one provides products guided solely by the antibody profile, is preferable to selection based on educated guesswork when human leukocyte antigen identical units are unavailable, and offers some advantages over platelet crossmatching.

SUMMARY

This review presents a literature-based algorithm with which to approach the management of platelet refractory individuals, focusing on newer technology to maximize the post-transfusion yield of matched units. Strategies are presented that allow selection of more effective products for difficult, broadly alloimmunized individuals, including patients who have developed antibodies to human platelet antigens.

Thrombopoietin as a drug: biologic expectations, clinical realities, and future directions

After the cloning of thrombopoietin (c-mpl ligand, Tpo) in 1994, 2 recombinant thrombopoietic growth factors, full-length glycosylated recombinant human Tpo (reHuTPO) and polyethylene glycol conjugated megakaryocyte growth and development factor (PEG-reHuMGDF), have been studied in humans in a variety of clinical settings. Both thrombopoietins are generally well tolerated if administered intravenously (IV). The c-mpl ligands produce a dose-related enhancement of platelet levels, reduce nonmyeloablative chemotherapy-induced mild thrombocytopenia, and mobilize hematopoietic progenitors. On September 11, 1998, the development of PEG-reHuMGDF was suspended in the U.S., due to formation of the neutralizing anti-Tpo antibody. Those neutralizing antibodies lead to thrombocytopenia and pancytopenia in some patients receiving subcutaneous (SC) PEG-reHuMGDF. Japanese investigators indicate that the probability of antibody formation against PEG-reHuMGDF is low when the drug is administered IV instead of SC. reHuTPO has a more favorable safety profile from the point of antibody production. The c-mpl ligands can improve apheresis yields when administered to normal platelet donors. Preliminary data about the use of PEG-reHuMGDF in myelodysplasia, aplastic anemia, and immune thrombocytopenic purpura are promising. Tpo is usually not effective in myeloablative thrombocytopenia when bone marrow hematopoietic progenitors are not present. The major obstacle for the thrombopoietins is their delayed action for managing clinical thrombocytopenia. This review will focus on the biologic basis, current clinical experience, and future directions for the use of thrombopoietic molecules as drugs. The identification of a safe, effective, and potent pharmacologic platelet growth factor could significantly improve the management of thrombocytopenia-induced bleeding.

Post-transfusion alloimmunization to granulocytes and platelets in Japanese patients as determined by the MPHA method

The current occurrence of alloimmunization to granulocytes and platelets after blood transfusion is unclear due to the fact that antibody assays are cumbersome. Using the MPHA method with extracted granulocyte and platelet antigens, a randomized, blinded trial was conducted to investigate three types of alloantibodies in 431 Japanese patients receiving leukocyte-depleted blood transfusions prepared with or without our latest leukocyte-reduction filter. The frequency of granulocyte, platelet and HLA class I alloantibodies was 0.44%, 0.44% and 16.74%, respectively, in patients receiving non-filtered products and 0%, 0% and 0.49%, respectively, in patients receiving filtered products. The granulocyte antibody reacted with an antigen approximately of 51 KDa. The platelet-specific alloantibody was associated with GPIIb/IIIa and GPIa/IIa. The important factors affecting alloimmunization were the transfusion dose and the use of unfiltered platelet products.

Platelet-specific antibodies in HLA-immunized patients receiving chronic platelet support

BACKGROUND

In HLA-alloimmunized patients, the unexpected failure of HLA-matched platelet transfusions usually raises the suspicion about concomitant platelet-specific antibodies. As the reported frequency of platelet-specific antibodies in multitransfused patients varies widely, the aim of this study was to determine the prevalence of such antibodies in a population of chronic thrombocytopenic patients with HLA antibodies.

STUDY DESIGN AND METHODS

From 1985 to 1997, 11,777 determinations of HLA antibodies were performed in 1330 hematologic patients receiving chronic platelet support. Fifty-two patients with HLA alloimmunization that lasted more than 1 month were selected. The search for platelet-specific antibodies was performed by using a monoclonal antibody immobilization of platelet antigens assay, thus allowing the identification of platelet-specific antibodies directed against the platelet glycoproteins (GP) Ib/IX, GPIIb/IIIa, and GPIa/IIa. Specificity of the platelet-specific antibodies was further investigated by using a solid-phase assay with chloroquine-treated platelets.

RESULTS

Only 2 (3.8%) of the 52 patients had platelet-specific antibodies. One antibody reacted with an epitope of the GPIIb/IIIa that was present in all the panel platelets, and that probably was an autoantibody. The other was an anti-HPA-5b.

CONCLUSIONS

The prevalence of platelet-specific antibodies in patients with HLA alloimmunization is very small. The search for concomitant platelet-specific antibodies would be indicated only when other causes of refractoriness to HLA-matched platelets are ruled out.

Selecting donors of platelets for refractory patients on the basis of HLA antibody specificity

BACKGROUND

Patients who are refractory to platelet transfusion as a result of HLA alloimmunization are generally given HLA-matched or crossmatched platelets. However, HLA-matched platelets that are matched at HLA-A and -B loci (A-matched) or those without any mismatched or cross-reactive antigens (BU-matched) are frequently unavailable. A disadvantage of crossmatching is that crossmatched platelets have a shelf life of only 5 days, so that crossmatch tests must be performed frequently for patients requiring long-term platelet transfusions. An alternative method is the selection of platelets according to the patient's HLA antibody specificity, called the antibody specificity prediction (ASP) method.

STUDY DESIGN AND METHODS

An anti-human globulin-enhanced microlymphocytotoxicity test modified by a double addition of serum and a computer program were used to determine the specificity of patients' HLA antibodies. Platelet crossmatching was performed with a solid-phase adherence assay. The percentage of platelet recovery (PPR) was determined in 1621 platelet transfusions in an observational study in 114 patients, and the PPR of platelets selected by the ASP method was compared with the PPR of those that were HLA-matched, crossmatched, or randomly selected. The numbers of potential donors in files of HLA-typed donors as identified by HLA matching vs. the ASP method were determined.

RESULTS

After adjustments for covariates, the mean +/- SEM PPR was similar for HLA-matched (21 +/-4%), cross-matched (23+/-4%), and ASP-selected (24+/-3%) platelets and was significantly lower for randomly selected (15+/-1.4%) platelets. For 29 alloimmunized HLA-typed patients, the mean number of potential donors found in a file of 7247 HLA-typed donors was 6 who were an HLA-A match (median = 1), 33 who were an HLA-BU match (median = 20), and 1426 who were identified by the ASP method (median = 1365).

CONCLUSION

The ASP method of donor selection for refractory alloimmunized patients appears as effective as HLA matching or crossmatching. Far more donors are identified in a file of HLA-typed donors by the ASP method than by HLA matching, and this indicates that the ASP method provides important advantages regarding the availability of compatible platelet components.

Clinical and laboratory factors that affect the post-transfusion platelet increment

Transfusion of platelet concentrates (PC) reduced the incidence of fatal hemorrhages in several thrombocytopenic conditions. Unfortunately, long-term platelet supportive care may be complicated by the development of a state of refractoriness, resulting in inadequate recovery of functional platelets. PC handling, clinical conditions of the patients and alloimmunization are the main factors affecting refractoriness. We evaluated the post-transfusion platelet increase in 25 patients (M=6, F=19) with hypomegakaryocytic thrombocytopenia receiving random ABO-compatible PC within 24 h after collection. Quality of PC was assessed by platelet count, pH measuring, LDH release, glycocalicin levels, CD-62 and CD-42b expression. Besides history, clinical status and therapy, we searched for the presence of anti-HLA class 1 and anti-HPA 1-4-5 antibodies. Only six patients (24%) were refractory to PC transfusion, as assessed by a corrected count increment (CCI)<5000. Four of such six patients (67%) had anti-HLA antibodies, as compared to zero of 19 responders (P<0.02). No other investigated clinical or laboratory feature was significantly different in refractory and responsive patients. Although post-transfusion bleeding time was shorter in responders than in refractory patients (297.33+/-249.95 versus 673.33+/-409.96; P<0.02), it did not significantly change even in patients with adequate correct count increment. Our data confirm the importance of anti-HLA antibodies in determining adequate post-transfusion recovery or refractoriness.

Algorithm for managing the platelet refractory patient

Refractoriness to platelet transfusions continues to be a major problem for many thrombocytopenic patients. A proposed algorithm for managing these patients is presented which proceeds from easily instituted changes in platelet transfusion therapy such as provision of ABO-compatible and "fresh" platelet transfusions to the more difficult and costly process of selecting compatible platelets for patients who are documented to be alloimmunized. For nonimmunized platelet refractory recipients, multiple clinical and drug factors that may adversely effect transfusion responses have been identified. Identifying which of these factors are causally associated with poor platelet responses in any given patient remains a substantial challenge.

The number of platelet glycoprotein Ia molecules is associated with the genetically linked 807 C/T and HPA-5 polymorphisms

BACKGROUND

The neutral 807 C/T (Phe224) polymorphism (807 C/T polymorphism) of the glycoprotein (GP)Ia gene has been recently associated with the number of GPIa molecules on the platelet surface. The association of the number of GPIa molecules with other GPIa polymorphisms, such as HPA-5 (Glu/Lys505) (HPA-5 polymorphism), involved in alloimmune thrombocytopenias is unknown.

STUDY DESIGN AND METHODS

The association of the HPA-5 polymorphism with the number of GPIa molecules on the platelet surface in 159 white blood donors was investigated. The genetic linkage between the HPA-5 and the 807 C/T polymorphisms in 316 individuals was also determined.

RESULTS

Both the 807 C/T and HPA-5 polymorphisms correlate with the number of GPIa molecules on the platelet surface. The 807 T and HPA-5b alleles are associated with increased numbers of GPIa molecules on the platelet surface. Moreover, the HPA-5b allele is genetically linked to 15.8 percent of the 807 C alleles. Therefore, the number of GPIa molecules on the platelet surface is dependent on both GPIa polymorphisms as follows: 807 T/T, HPA-5 a/a > 807 C/T, HPA-5 a/b > 807 C/T, HPA-5 a/a > 807 C/C, HPA-5 a/b > 807 C/C, HPA-5 a/a.

CONCLUSION

Two GPIa polymorphisms (807 C/T and HPA-5) responsible for the variability in the numbers of GPIa/IIa molecules on the platelet surface in whites have been identified. Despite the genetic linkage between the two polymorphisms, their influence on the number of GPIa molecules on the platelet surface may occur through different mechanisms.