Expression of HLA-B12 on platelets, on lymphocytes and in serum: a quantitative study

Human Leukocyte Antigen Alloimmunization and Alloimmune Platelet Refractoriness

Despite significant advancements in the production of platelet products, storage, and transfusion, transfusion refractoriness remains a significant clinical problem, affecting up to 14% of hematological patients receiving platelet transfusions. Human leukocyte antigen (HLA) alloimmunization is a major cause of immune platelet refractoriness, and its rate can be significantly reduced by implementation of leukoreduction. Despite promising preclinical results, pathogen reduction does not reduce HLA alloimmunization. Patients with HLA alloimmune refractoriness are usually managed with HLA-selected platelet transfusions. In this review, we describe the pathophysiology of HLA alloimmunization and alloimmune refractoriness, as well as options to prevent and treat these transfusion complications. We discuss the evidence supporting these options and point out the outstanding gaps. Finally, we review the possible future directions for prevention and treatment of alloimmune refractoriness.

Platelets from donors with consistently low HLA-B8, -B12, or -B35 expression do not undergo antibody-mediated internalization

HLA-B8, -B12, or -B35 expression on platelets varies significantly between donors and in certain donors is consistently low or undetectable. Antibody-mediated internalization of platelets correlates with antigen expression and is absent in platelets with low HLA expression.

The pig histocompatibility system SLA: serological study on a group of antigenic specificities

The report presents an analysis of a group of class I SLA reagents which were highly correlated within one cluster in a previous analysis. Further population and family studies, and selected purification of several of these reagents led to the identification of 4 distinct specificities, namely SLA A 15, B 18, C 1 and A 16. Three of them, SLA A 15, B 18 and C 1, are actually in strong linkage disequilibrium and represent the main SLA haplotype in the Large White breed. SLA A 16 is present essentially in the Landrace breeds. SLA A 16 displays a strong cross-reaction with SLA A 15 and there is another specificity in linkage disequilibrium with SLA A 16 which cross-reacts with SLA B 18. Altogether, the strong linkage disequilibrium and the high degree of cross-reactivity among the allelic products of the SLA complex explain the failure to detect the diversity of our reagents in previous studies.

Platelet alloantibodies in transfused patients

BACKGROUND

Patients receiving cellular blood components may form HLA antibodies and platelet-specific alloantibodies.

STUDY DESIGN AND METHODS

Serum samples from a cohort of 252 patients with hematologic or oncologic diseases who are receiving cellular blood components were studied for platelet-reactive antibodies. Specificity of platelet alloantibodies was determined with a panel of typed platelets

RESULTS

Platelet-reactive antibodies were detected in the sera of 113 patients (44.8% of 252), HLA antibodies in the sera of 108 (42.9%), and platelet-specific antibodies in the sera of 20 (8%). The following platelet-specific antibodies were identified: anti-HPA-5b (n = 10), anti-HPA-1b (n = 4), anti-HPA-5a (n = 2), anti-HPA-1a (n = 1), anti-HPA-2b (n = 1), anti-HPA-1b+5b (n = 1), and anti-HPA-1b+2b (n = 1). Fifteen sera from the 108 patients with anti-HLA (13.9%) contained additional platelet-specific alloantibodies, while in 5 sera, platelet-specific alloantibodies only were detected: anti-HPA-5b (n = 4) and anti-HPA-1a (n = 1). Of the 108 sera with HLA antibodies, 29 (26.9%) showed discordant results when studied with the lymphocytotoxicity test and the glycoprotein-specific immunoassay. Ten sera contained panreactive antibodies against platelet glycoproteins (GP) IIb/IIIa, GPIa/IIa, and/or GPIb/IX. Alloimmunization occurred in 58.3 percent of female patients with previous pregnancies, but in only 23.3 percent of those without previous pregnancies (p = 0.0049).

CONCLUSION

Platelet alloantibody specificities in transfused patients (predominantly anti-HPA-5b and -1b with antigen frequencies <30% among whites) differ significantly from those observed in patients with neonatal alloimmune thrombocytopenia or posttransfusion purpura, in whom anti-HPA-1a (antigen frequency >95%) is the most prevalent specificity. HLA antibody detection yields discordant results when the lymphocytotoxicity assay and a glycoprotein-specific immunoglobulin-binding assay are used.

Soluble HLA class I and Fas ligand molecules in blood components and their role in the immunomodulatory effects of blood transfusions

It has been known for many years that blood transfusions may have immunomodulatory effects, however an ultimate explanation of this phenomenon is lacking. In the present paper we report that the concentrations of soluble HLA class I (sHLA-I) and soluble Fas ligand (sFasL) molecules in supernatants of blood components which contain elevated numbers of residual donor leukocytes, like red blood cells and random-donor platelets, are significantly higher than in other blood components. Elevated amounts of sFasL molecules are also found in some commercial immunoglobulin preparations. sHLA-I and sFasL molecules in blood components and in immunoglobulin preparations are biologically active in vitro as they inhibit mixed lymphocyte responses and cytotoxic T cell activity in allogeneic and autologous combinations and induce apoptosis in Fas-positive cells. If these results are paralleled in vivo the amount of sHLA-I and sFasL molecules should be taken into account in clinical practice in order to select the blood component and the immunoglobulin preparation which could induce the desired immunomodulatory effect in the recipient.

Soluble HLA Class I, HLA Class II, and Fas Ligand in Blood Components: A Possible Key to Explain the Immunomodulatory Effects of Allogeneic Blood Transfusions

The immunomodulatory effect of allogeneic blood transfusions (ABT) has been known for many years. However, a complete understanding of the effects of ABT on the recipient’s immune system has remained elusive. Soluble HLA class I (sHLA-I), HLA class II (sHLA-II), and Fas ligand (sFasL) molecules may play immunoregulatory roles. We determined by double-determinant immunoenzymatic assay (DDIA) sHLA-I, sHLA-II, and sFasL concentrations in different blood components. sHLA-I and sFasL levels in red blood cells (RBCs) stored for up to 30 days and in random-donor platelets are significantly (P < .001) higher than in other blood components and their amount is proportionate to the number of residual donor leukocytes and to the length of storage. Blood components with high sHLA-I and sFasL levels play immunoregulatory roles in vitro as in allogeneic mixed lymphocyte responses (MLR) and antigen-specific cytotoxic T-cell (CTL) activity, and induce apoptosis in Fas-positive cells. These data suggest that soluble molecules in blood components are functional. If these results are paralleled in vivo, they should be taken into account in transfusion practice. Blood components that can cause immunosuppression should be chosen to induce transplantation tolerance, whereas blood components that lack immunosuppressive effects should be preferred to reduce the risk of postoperative complications and cancer recurrence.

Soluble HLA Class I, HLA Class II, and Fas Ligand in Blood Components: A Possible Key to Explain the Immunomodulatory Effects of Allogeneic Blood Transfusions

The immunomodulatory effect of allogeneic blood transfusions (ABT) has been known for many years. However, a complete understanding of the effects of ABT on the recipient’s immune system has remained elusive. Soluble HLA class I (sHLA-I), HLA class II (sHLA-II), and Fas ligand (sFasL) molecules may play immunoregulatory roles. We determined by double-determinant immunoenzymatic assay (DDIA) sHLA-I, sHLA-II, and sFasL concentrations in different blood components. sHLA-I and sFasL levels in red blood cells (RBCs) stored for up to 30 days and in random-donor platelets are significantly (P &lt; .001) higher than in other blood components and their amount is proportionate to the number of residual donor leukocytes and to the length of storage. Blood components with high sHLA-I and sFasL levels play immunoregulatory roles in vitro as in allogeneic mixed lymphocyte responses (MLR) and antigen-specific cytotoxic T-cell (CTL) activity, and induce apoptosis in Fas-positive cells. These data suggest that soluble molecules in blood components are functional. If these results are paralleled in vivo, they should be taken into account in transfusion practice. Blood components that can cause immunosuppression should be chosen to induce transplantation tolerance, whereas blood components that lack immunosuppressive effects should be preferred to reduce the risk of postoperative complications and cancer recurrence.

Genetic predetermination of quantitative expression of HLA antigens in platelets and mononuclear leukocytes

In view of the potential functional importance of quantitative expression of HLA antigens, a series of studies were conducted to determine the relative quantities of specific HLA-A and -B antigens expressed in MNLs and platelets of HLA-phenotyped family members and unrelated individuals. An mAb that reacts with a well-defined monomorphic epitope in the alpha 3 domain of the heavy chains of HLA molecules was developed and used to quantify each HLA-A or -B antigen on western blots of IEF gels. The results of these studies demonstrated that the relative quantities of HLA-A and -B antigens in platelets and MNLs of an individual did not change over time. Further studies showed that the relative quantities of HLA-A and -B antigens for haplotypes shared among first-degree relatives were always the same and followed Mendelian inheritance. In contrast, the relative quantities of HLA-A and -B antigens for a haplotype shared by unrelated individuals varied significantly. All these findings support the hypothesis that the quantitative expression of HLA antigens is genetically predetermined and may play important roles in determining disease susceptibility and severity.

Evaluation of individual specificities of class I HLA on platelets by a newly developed monoclonal antibody

In order to quantify each specific HLA-A or -B antigen on platelets, a monoclonal antibody against HLA heavy chains was developed and designated as 2F2 monoclonal antibody. This monoclonal antibody reacted on Western blot with platelet HLA from each of 10 individuals with different HLA phenotypes and precipitated all 35S-methionine-labeled HLA-A and -B antigens from three different Epstein-Barr Virus--transformed lymphoblastoid cell lines. The results indicate that the 2F2 monoclonal antibody recognizes an epitope shared by different HLA-A and -B antigens. The quantitative variation of specific HLA antigens on platelets was then studied in nine different donors by isoelectric-focusing gel electrophoresis and immunoblot using the 2F2 monoclonal antibody. The results of our studies showed that the shared HLA antigens such as A2, B35, and B62, varied three- to fivefold among different individuals and individual HLA-A or -B antigen was not equally expressed on a person's platelets. The relative quantities of specific HLA-A and -B antigens on lymphocytes were also noted to be the same as those on platelets. The finding suggests that differential expression of HLA specificities may not be restricted to platelets but is a more general phenomenon including other nucleated cells.

Quantification of soluble HLA class I gene products by an enzyme linked immunosorbent assay

A simplified enzyme linked immunosorbent assay utilizing an HLA class I framework-specific monoclonal antibody and a polyclonal enzyme linked beta-2 microglobulin specific antiserum has been established for the quantitative measurement of soluble HLA class I molecules. A total of 219 unrelated healthy individuals and 137 members of 28 families typed for HLA were analyzed for their non-membrane bound, i.e. soluble HLA-A,B,C antigens (sHLA-A,B,C). As reported by others, we observed associations of higher or lower sHLA-A,B,C values to particular HLA antigens: High plasma values were observed in probands positive for HLA-A23, A24, A29, Aw33, Bw65, and Cw8 and low values in HLA-B27 and B37 positive individuals. However, as shown by family studies, levels of sHLA-A,B,C were apparently not controlled by the MHC haplotypes alone, since no significant difference between HLA identical siblings and two haplotype different individuals could be detected. Thus, additional non-MHC linked gene(s) may be involved in the release of class I gene products.

Immunology of platelet disorders

Immune-mediated thrombocytopenias (IMTPs) are frequently-occurring haemostatic disorders in clinical medicine. They may be caused by allo-immunity, autoimmunity, or by drug-induced immune mechanism. All IMTPs are caused by antibodies, which may induce increased platelet destruction but may also hinder platelet function. Many different platelet membrane molecules (i.e. antigens) are involved in the immune processes that play a role in IMTP. Much is already known about the structure of these molecules. Notably the alloantigens involved in alloimmune-mediated thrombocytopenia have been quite well studied. Many of these antigens appear to be polymorphic determinants of adhesion molecules of the integrin superfamily, and are also present on cells other than platelets (endothelial cells, fibroblasts, smooth muscle cells). The methodology for studying platelet antigens and antibodies involved in IMTPs has markedly improved in recent years. This has not only led to better diagnostic tests but also to a better understanding of the immunopathogenesis of these diseases. Platelet immunology is scientifically very much alive, and is expected to remain so in the coming years.

An enzyme-linked immunosorbent assay for the detection of platelet antibodies using detergent-solubilized platelets immobilized on nitrocellulose discs

A method is detailed for the solubilization of human platelets using a dialyzable detergent, decanoyl-N-methylglucamide (Mega-10). At a detergent/protein ratio of 1:12, the efficiency of solubilization was 27%. This platelet lysate (PLy) was then bound to nitrocellulose (NC) discs to assay retention of the native immunological functions of the platelet membrane antigens. Using alkaline phosphatase-coupled anti-IgG, the major platelet membrane glycoproteins GPIb, GPIIb, and GPIIb/IIIa were detectable with as little as 20 ng of monoclonal antibody. Antisera to the class I histocompatibility antigens HLA-A1, B7, B8, the PlA1 allodeterminant, and serum from multiply transfused, alloimmunized patients were reactive even after 100 days storage of the discs at 4 degrees C, and with as little as 1.0 micrograms of NC-bound PLy. The binding of the same antisera to intact, immobilized platelets as well as specific complement-mediated lymphocytotoxicity was also inhibited by PLy. PLy from HLA-A3- or B44-positive donors, however, did not inhibit cytotoxicity of lymphocytes expressing either antigen using several different antisera. Our results indicate that Mega-10 is an excellent solubilizing agent for the immunological study of platelet membranes. The fact that clinically relevant platelet membrane antigens are preserved, immunologically reactive, and stable over long periods of storage, makes this assay amenable to a routine crossmatching procedure for platelet transfusions.

Relationship of HLA and platelet-reactive antibodies in alloimmunized patients refractory to platelet therapy

Platelet crossmatching assays have been used to predict the outcome of platelet transfusions in alloimmunized patients by detecting antibodies against platelets. The transfusion failure of HLA-matched platelets predicted by platelet crossmatching may be related to HLA antibodies undetected by lymphocytotoxicity but detected by platelet immunoglobulin-binding assays or platelet-specific antibodies (both antibodies defined here as platelet-reactive antibodies). To differentiate platelet-reactive antibodies from lymphocytotoxic HLA antibodies, we used HLA characterized lymphocytes in parallel with platelets from individuals to form separate frozen panels. Sera from 10 allosensitized patients were studied in the lymphocyte panel by lymphocytotoxicity and in the platelet panel by enzyme-linked immunoassay (ELISA). By comparing pattern and percent wells reacting in each panel, lymphocytotoxic HLA antibodies and antibodies reactive with platelets in ELISA were detected separately. In all 10 allosensitized patients, platelet-associated antibodies were present and 7 had additional lymphocytotoxic HLA antibodies. Using this double parallel panel technique, we found platelet-reactive antibodies important in platelet alloimmunization, unrecognized by lymphocytotoxicity. These data indicate platelet-crossmatching be solely used in the selection of platelets for allosensitized patients.

HLA antigens on platelet membranes. In vitro and in vivo studies

In order to determine whether HLA-A,B antigens of platelets are integral membrane constituents or rather represent adsorbed plasma proteins, their presence in plasma and their adsorbability onto platelet membranes was studied by in vitro and in vivo experiments. The amount of HLA antigens was quantitated by inhibition of lymphocytotoxicity (LCT) and by enzyme-linked immunosorbent assay (ELISA) using operationally monospecific polyclonal HLA antibodies or murine HLA-specific monoclonal antibodies, respectively. We found that in 11 out of 13 HLA-A2 and in 9 out of 10 HLA-B13 experiments, platelets from antigen-negative donors pretreated with plasma from the same number of antigen-positive donors inhibited LCT to the same extent as platelets from antigen-positive donors. Nevertheless, the in vitro adsorbed HLA antigens onto antigen-negative platelets were, unlike those on antigen-positive platelets or in plasma, not reactive with monoclonal antibodies as quantitated by ELISA. Similarly, infusion of HLA-A2-negative platelets from single donors into 3 HLA-A2-positive, thrombocytopenic patients with bone marrow failure led to a good platelet increment, but did not convert the HLA type of donor platelets, neither at 2 h nor at 18 h posttransfusion. On the basis of these results, we conclude that soluble HLA antigens can be taken up by human platelets from plasma in small amounts. However, the major portion of HLA antigens appears to be integral membrane constituents.

Quantitative determination of platelet surface alloantigens using a monoclonal probe

A monoclonal antibody with specificity for the Fc portion of IgG was used to determine the number of IgG alloantibody molecules bound at saturation to alloantigens of the PlA1 and HLA systems on normal human platelets. In preliminary studies, it was found that the number of cell-bound IgG molecules recognized by this probe correlates well with the number measured by electroimmunoassay, an independent measure of alloantibody binding. PlA1-positive platelets could be divided into two groups binding 34,000-43,000 or 19,000-24,000 alloantibody molecules. Family studies and studies with a cytolytic assay showed that the former group is homozygous and the latter heterozygous for PlA1. Because the number of glycoprotein IIIa (GPIIIa) molecules carrying the PlA1 determinant on the surface of normal platelets is thought to be about 40,000, these findings suggest that each GPIIIa molecule carries one PlA1 determinant. The number of class I HLA molecules expressed on normal platelets was considerably smaller than the number of PlA1 determinants, ranging from 4400 to 10,000 (HLA-A2), 870 to 8400 (Bw4), and 1300 to 5800 (Bw6). Preliminary analysis indicates that stronger or weaker expression of Bw4 and of Bw6 correlates with certain "private" HLA-B determinants carried on the HLA-B molecule as found in previous studies using an indirect method to measure alloantigen density. These findings appear to explain why antibodies reactive with platelet-specific antigens such as PlA1 react more strongly with platelets than HLA-specific antibodies in most serologic tests. The weak expression of HLA determinants on platelets of some subjects may account for the less than perfect correlation between in vitro compatibility tests and post-transfusion platelet survivals observed in most studies.

Post-transfusion purpura associated with alloimmunization against the platelet-specific antigen, Baka

Post-transfusion purpura (PTP) with severe thrombocytopenia occurred eight days after transfusion in a 28-year-old woman and responded to treatment with prednisone and plasma exchange. In contrast to nearly all previously studied cases of PTP, the patient's platelets were PlA1-positive and anti-PlA1 antibody could not be detected in serum obtained during the thrombocytopenic episode. Her serum was found to contain an antibody specific for a recently described platelet-specific alloantigen, Baka, in addition to multiple HLA-specific antibodies. The patient's platelets, typed following recovery, were Baka-negative. These findings indicate that post-transfusion purpura can occur in association with alloimmunization to platelet-specific antigens other than PlA1. In performing the serologic studies, a close relationship and possible identity between Baka and another recently reported platelet antigen, Leka, was observed. A method for analyzing mixtures of cytotoxic platelet-reactive antibodies without separating the individual antibodies is described.

The major histocompatibility complex of the cynomolgus monkey: absorption analysis of 24 CyLA antisera

The major histocompatibility complex (MHC) of the cynomolgus monkey consists of at least 35 serologically defined specificities, 14 of which have been assigned to the CyLA-A locus, 10 to the B locus, and six to the newly identified C locus. Selected specificities and antisera were further studied by the analysis of data obtained from absorptions with single donor platelets of known CyLA type. These data confirmed the existence of four groups of cross-reactive specificities identified by serological typing and revealed that three other well-defined specificities could be divided into two or more subtypes. Seven sera were shown to contain two or more populations of distinct, probably noncross-reactive antibodies that could be readily separated by absorption. Of the 24 sera analyzed, only three could be considered likely to contain antibodies reactive with a single CyLA specificity. At least three antibody specificities, after repeated absorption attempts, could not be readily removed by platelets and may reflect poor expression of the determinants on the platelet membrane. The complexity and degree of polymorphism of the CyLA system approaches that known for the human HLA complex. This homology of genetic organization and serologic features provide additional evidence in yet another species for the important role that these systems of alloantigens play in the organism.

Enzyme-linked immunosorbent assay for determination of HLA: gene dose effect

In a previous publication we demonstrated that polymorphic HLA antigens could be detected on fresh and dried peripheral blood lymphocytes using the enzyme-linked immunosorbent assay (ELISA) with HLA alloantisera (Bishara et al. 1983). In the present study we investigated whether the ELISA technique can be used in determination of gene-dose effect for antigens of the HLA-A and B loci. Lymphocytes from HLA-A1 and HLA-B14 homozygous individuals are shown to bind significantly more anti-HLA alloantibodies than their heterozygous siblings for the same HLA antigens. These results indicate that ELISA is a sensitive and reliable technique for the qualitative and quantitative assay of polymorphic HLA determinants.

Detection of platelet antibodies by anti-kappa light chain facilitation of C-FDA (KC-FDA) thrombocytotoxicity

Sensitivity of the carboxyfluorescein diacetate (C-FDA) thrombocytotoxicity technique for the detection of antiplatelet antibodies has been enhanced by the addition of an anti-Kappa light chain antibody facilitation step. This new technique, KC-FDA, was compared with the platelet suspension immunofluorescence test (PSIFT) by titering platelet-reactive allo-anti-PlAl and anti-HLA antibodies. The results show that compared to PSIFT, KC-FDA is more sensitive for detecting platelet specific antibodies (PlAl), is more or equally sensitive for detecting other antibodies (HLA), and is significantly faster and easier to perform.

Bgb expression in relation to the HLA-B17 antigen splits Bw57 and Bw58 and the cross-reactions of anti-Bgb antibodies

A substantial number of individuals typed as HLA-B17 do not have Bgb. To determine whether or not this discrepancy reflects genetic or ethnic group differences or the influence of B17 antigen subtypes, a large number of unrelated and related B17 individuals from the Caucasian, Hispanic, Black and Chinese ethnic groups were tested in parallel for Bgb and Bw57- or Bw58- B17 subtype. Higher Bgb expression was found in association with Bw57, but differences in expression of Bgb were also seen in different ethnic groups and in different related individuals carrying the same Bw57 or Bw58 haplotype. Genetic factors other than HLA thus influence the expression of HLa Antigens on red cells. Standard and antiglobulin lymphocytotoxicity tests indicate that all Bgb typing sera contain anti-Bw57 antilymphocyte antibodies and most also contain anti-Bw58 and cross-reacting anti-B12, B15 and/or Bw49 antibodies.

Technical and theoretical considerations in the HLA typing of amniotic fluid cells for prenatal diagnosis and paternity testing

HLA typing of amniotic fluid cells has been used for the prenatal diagnosis of the HLA linked diseases congenital adrenal hyperplasia (21-OH-deficiency (21-OH-def) type) and complement C4 deficiency and it has also been used for the prenatal determination of paternity. There are, however, technical difficulties in this test associated with the weak expression of some B locus antigens on amniotic fluid cells, and theoretical difficulties related to associations between particular HLA antigens and the 21-OH-def allele. Since certain HLA-B locus antigens are found in significantly increased frequencies among patients with 21-OH-def, there is a relatively high incidence of HLA-B homozygosity among the patients and over 40 per cent of the parents of these patients share one or more HLA-B locus antigens. Results of some prenatal HLA typing tests may thus be difficult to interpret, and supplementary tests should be used whenever possible. HLA typing of amniotic cells is, however, the only available procedure for prenatal diagnosis of C4 deficiency and it is the best available procedure for prenatal determination of paternity. A modification of our original procedure allows HLA typing to be performed with increased numbers of HLA typing sera, and sera with optimum reactivity for amniotic fluid cells have now been selected for the definition of most of the more commonly expressed HLA antigens. Although amniotic fluid cells do not express DR Antigens, amniotic fluid cells can be typed for the HLA-linked marker glyoxalase I (GLO) and this may be the informative for prenatal diagnosis in some cases.

A new micromethod for the in vitro detection of antiplatelet antibodies: C-FDA thrombocytotoxicity

A new microtechnique, C-FDA, for the in vitro detection of antiplatelet antibodies, is described. This technique is faster and simpler than either 51Cr thrombocytotoxicity or immunofluorescence (IF). C-FDA is more sensitive than 51Cr for all (anti-HLA, --P1A1, ABO, drug-related, and ITP-related) antibodies tested. Although IF was more sensitive for many types of antibodies, C-FDA was as good or better a clinical test method for all drug-related and isoimmune neonatal thrombocytopenia patient sera tested. Preliminary data also suggest that this method detects possible new non-HLA, non-ABO, nonP1A1 platelet antigens.

Adverse reactions due to leucocyte and platelet antibodies

Each blood transfusion exposes the recipient to the hazards of immunisation to histocompatibility antigens as well as white cell and platelet antigens. As the number of transfusions received by a patient increases so too does the likelihood of the development of antibodies which may by their multispecific character effectively prevent lifesaving therapy in the future. Special steps can and should be taken to minimise the risks of immunisation in the group of patients who will be dependent on long term transfusion therapy. An extension of knowledge about the antigenic systems of leucocytes and platelets, including HLA, and the introduction of more specific tests for the detection of antibodies directed against them should assist in the provision of more appropriately matched blood components.

HLA antigens of platelets. IV. Influence of "private" HLA--B locus specificities on the expression of Bw4 and Bw6 on human platelets

Quantities of the HLA--B antigens B5, B7, B8, B13, B27, and Bw44 were determined on platelets of subjects heterozygous for those markers using an assay based on inhibition of antibody-mediated 51Cr release from lymphocytes. In confirmation of previous studies, Bw44 was found to vary about 35-fold and B8 about 8-fold. In contrast, B5, B7, B27, and B13 varied only 2--3-fold. Expression of each of the six antigens was proportional to that of its associated broad specificity, Bw4 or Bw6. The antigens Bw4 and Bw6, when measured directly on platelets of 62 normal heterozygous subjects, were found to vary 32-fold and 20-fold, respectively. Bw4 was strongly expressed on platelets carrying B5 or B27 (P less than 0.001), was weakly expressed in the presence of B13 (P less than 0.001) and was variable, possibly bimodal, in the presence of Bw44. Bw6 was strongly expressed on platelets carrying B7, Bw15.1, or Bw35 (P less than 0.01) but was weak on platelets carrying B8 or B14 (P less than 0.01). Little or no variation in Bw4 or Bw6 was observed on lymphocytes of the same donor population. These findings are consistent with the possibility that private HLA--B locus specificities influence the expression of part or all the HLA--B molecule on the platelet surface through an undetermined mechanism. The atypical behavior of Bw44/Bw4 on platelets suggests the possibility that Bw44 consists of two subspecificities not yet detectable with available serologic reagents. Variability of HLA markers in tissues other than platelets could have important immunobiologic implications which seem deserving of further investigation.

Sera from volunteers immunized by planned blood transfusions as source of DR cytotoxic typing reagents

Sera from volunteers immunized with planned blood transfusions were tested for anti DR cytotoxic antibodies with a panel of HLA typed cultured human lymphoid cells using a variety of serological techniques. The majority of sera contained DR cytotoxic antibodies. The specificity of DR antibodies in seven sera was determined by testing them with a panel of B peripheral lymphocytes typed with DR alloantisera submitted to the 7th International Histocompatibility Workshop. The temporal evolution of DR and HLA-A and B cytotoxic antibodies was determined in two subjects by testing serial bleedings with B lymphoid cells, coated with Fab2 fragments from anti beta 2 mu and anti DR xenoantisera. Results indicated a parallel evolution of DR and HLA-A and B cytotoxic antibodies.

Deletion of the platelet-specific alloantigen PlA1 from platelets in Glanzmann's thrombasthenia

Expression of a Platelet-specific alloantigen (Pl(A1)) was studied in five unrelated patients with Glanzmann's thrombasthenia using immunologic techniques based on release of (51)Cr from tagged platelets by Pl(A1)-specific antibody. Less than 1% of the normal quantity of Pl(A1) could be detected on platelets of patients 1, 2, and 3; platelets from patients 4 and 5 contained 22 and 12% of normal levels, respectively. After treatment with bromelain, platelets from patients 4 and 5, but not those from patients 1, 2, and 3, released (51)Cr as well as normal Pl(A1)-positive platelets when exposed to anti-Pl(A1). Platelets from each of the five patients reacted normally with drug-dependent antibodies and with autoantibodies specific for platelets. Polyacrylamide gel electrophoresis of thrombasthenic platelets showed marked deficiencies of glycoproteins IIbalpha and III (P < 0.0005), confirming recent reports of others. Deficiency of the two proteins as determined by gel scanning was more pronounced in patients 1, 2, and 3 than in patients 4 and 5. Normal levels of glycoproteins IIbalpha and III were found in platelets from normal subjects negative for Pl(A1). These observations are consistent with the possibility that the Pl(A1) antigen is located on one or both of the glycoproteins lacking in Glanzmann's thrombasthenia, although other explanations are possible. They further suggest that patients with thrombasthenia may be heterogeneous in respect to the degree to which these glycoproteins are deleted. The Pl(A1) antigen can be measured with considerable precision and may provide a marker useful for the diagnosis and study of Glanzmann's disease.