Characterization of a Human Platelet Antigen-1a-Specific Monoclonal Antibody Derived from a B Cell from a Woman Alloimmunized in Pregnancy (original) (raw)
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Journal of Clinical Investigation, 2008
A SNP in the gene encoding integrin β3 causes a clinically important maternal-paternal antigenic difference; Leu33 generates the human platelet antigen 1a (HPA-1a), whereas Pro33 generates HPA-1b. As a potential treatment to prevent fetal intracranial hemorrhage in HPA-1a alloimmunized pregnancies, we generated an antibody that blocks the binding of maternal HPA-1a-specific antibodies to fetal HPA-1a1b platelets by combining a high-affinity human HPA-1a-specific scFv (B2) with an IgG1 constant region modified to minimize Fcγ receptor-dependent platelet destruction (G1Δnab). B2G1Δnab saturated HPA-1a + platelets and substantially inhibited binding of clinical HPA-1a-specific sera to HPA-1a + platelets. The response of monocytes to B2G1Δnab-sensitized platelets was substantially less than their response to unmodified B2G1, as measured by chemiluminescence. In addition, B2G1Δnab inhibited chemiluminescence induced by B2G1 and HPA-1a-specific sera. In a chimeric mouse model, B2G1 and polyclonal Ig preparations from clinical HPA-1a-specific sera reduced circulating HPA-1a + platelets, concomitant with transient thrombocytopenia. As the Δnab constant region is uninformative in mice, F(ab′) 2 B2G1 was used as a proof of principle blocking antibody and prevented the in vivo platelet destruction seen with B2G1 and polyclonal HPA-1a-specific antibodies. These results provide rationale for human clinical studies.
Transfusion, 2009
Background-Neonatal alloimmune thrombocytopenia (NAIT) is most commonly caused by transplacental passage of maternal human platelet-specific alloantigen (HPA)-1a antibodies that bind to fetal platelets (PLTs) and mediate their clearance. SZ21, a monoclonal antibody (MoAb) directed against PLT glycoprotein IIIa, competitively inhibits the binding of anti-HPA-1a alloantibodies to PLTs in vitro. The purpose of this investigation was to determine whether SZ21 F(ab′) 2 fragments might be therapeutically effective in inhibiting or displacing maternal HPA-1a antibodies from the fetal PLT surface and preventing their clearance from circulation. Study Design and Methods-Resting human PLTs from HPA-1ab heterozygous donors were injected into nonobese diabetic/severe combined immunodefi-cient (NOD/SCID) mice. Purified F(ab′) 2 fragments of SZ21 or control immunoglobulin G (IgG) were injected intraperitoneally 30 minutes before introduction of HPA-1a antibodies. Blood samples were taken periodically and analyzed by flow cytometry to determine the percentage of circulating human PLTs. Results-Anti-HPA-1a IgG from NAIT cases were able to efficiently clear HPA-1a-positive PLTs from murine circulation. Administration of SZ21 F(ab′) 2 fragments not only inhibited binding of HPA-1a antibodies to circulating human PLTs, preventing their clearance, but also displaced bound HPA-1a antibodies from the PLT surface. Conclusion-F(ab′) 2 fragments of HPA-1a-selective MoAb SZ21 effectively inhibit anti-HPA-1a-mediated clearance of human PLT circulating in an in vivo NOD/SCID mouse model.
Annals of Blood, 2019
Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a devastating disorder affecting approximately 0.5-1.5/1,000 live neonates. It occurs due to maternal immune responses against paternally inherited human platelet antigens (HPAs), resulting in low platelet counts, severe bleeding (e.g., intracranial hemorrhage; ICH), intrauterine growth restriction (IUGR), and miscarriage. There are currently 37 known HPAs; and 15 HPAs are located on the integrin β3 subunit. Fetomaternal incompatibilities in this protein have been most frequently reported, in which HPA-1 system accounts for more than 75% of FNAIT cases. The data from our animal models and from human anti-HPA-1a demonstrate that maternal anti-β3 antibodies have an anti-angiogenic effect, and that anti-angiogenesis, not thrombocytopenia, may be the key cause of ICH, suggesting that fetal platelet transfusion may have limited clinical benefits. Anti-β3 antibodies may also damage antigen positive trophoblasts via natural killer (NK) cells, causing placental dysfunction and miscarriage. Notably, anti-GPIbα alloantibody (e.g., anti-HPA-2) may induce platelet cellbased thrombin generation and thrombosis in placenta, leading to miscarriage. The consequences of antiangiogenesis and pathology in placenta have not been adequately explored but may cause symptoms beyond thrombocytopenia and bleeding disorders, termed non-classical FNAIT. Meanwhile, maternal intravenous IgG (IVIG) transfusion is likely able to block pathogenic antibody transport across placenta, and ameliorate thrombocytopenia in fetal reticuloendothelial system (RES), although it is currently unclear whether IVIG has equal efficacy for all anti-HPAs or other platelet antigens such as CD36. Further research is required to define standard treatment protocols and explore new treatment options, such as anti-HPA-1a prophylaxis, anti-neonatal Fc receptor (FcRn) and anti-NK therapies. In this review, we summarize the current state of literature comprising platelet versatilities and hemostasis, and integrate new discoveries related to FNAIT etiological factors in order to develop better diagnostic and therapeutic strategies against this life-threatening disease.
Preclinical evaluation of immunotherapeutic regimens for fetal/neonatal alloimmune thrombocytopenia
Blood Advances, 2021
Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal antibodies directed against paternally inherited antigens present on the surface of fetal platelets. The human platelet alloantigen HPA-1a (formerly known as the PlA1 alloantigen), is the most frequently implicated HPA for causing FNAIT in Whites. A single Leu33Pro amino acid polymorphism residing within the ∼50-amino-acid plexin-semaphorin-integrin domain near the N-terminus of the integrin β3 subunit (platelet membrane glycoprotein IIIa [GPIIIa]) is responsible for generating the HPA-1a and HPA-1b epitopes in human GPIIIa and serves as the central target for alloantibody-mediated platelet destruction. To simulate the etiology of human FNAIT, wild-type female mice were pre-immunized with platelets derived from transgenic mice engineered to express the human HPA-1a epitope on a murine GPIIIa backbone. These mice developed a strong alloimmune response specific for HPA-1a, and...
Transfusion, 2009
BACKGROUND: Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is mostly caused by maternal alloantibodies directed against the human platelet alloantigen (HPA)-1a. Currently, the serologic diagnosis of FNAIT is based on the characterization of the HPA alloantibodies in monoclonal antibody-based antigencapture assays (e.g., MAIPA assay). Accumulated current evidence indicated that such assays may overlook some HPA-1a antibodies. STUDY DESIGN AND METHODS: This study employed surface plasmon resonance (SPR) technology using immunoaffinity-purified glycoprotein IIb/IIIa isoforms immobilized on biosensor chips to study the binding kinetics of HPA-1a alloantibodies from different FNAIT cases in real time. RESULTS: Analysis of HPA-1a alloantibodies from FNAIT cases (n = 9) in SPR showed a moderate relative response (22.2-69.7 resonance units [RU]) and slow antibody dissociation. After the dissociation phase, varying amounts of bound antibodies (41%-79%) remained on the chip. In contrast in HPA-1a alloantibodies from a patient suffering from posttransfusion purpura, a high relative response (~490 RU) was observed at the end of the association phase and no dissociation of antibody binding was detectable. Of particular relevance, by the use of this SPR technique, HPA-1a alloantibodies were detected in two severe FNAIT cases that had determined as false negative by MAIPA assay. In SPR, these HPA-1a alloantibodies showed low-avidity nature characterized by gradual dissociation of antibody during the association phase and complete detachment of antibody binding after the dissociation phase. This high "off-rate" character of lowavidity HPA-1a alloantibodies indicates that such antibody binding is easily detachable by the extensive washing procedure of the MAIPA. CONCLUSIONS: Our results demonstrated that the SPR method can facilitate the diagnosis of clinically relevant low-avidity HPA-1a antibodies. F etal and neonatal alloimmune thrombocytopenia (FNAIT) is induced by maternal immunization against a fetal human platelet alloantigen (HPA). 1 In Caucasians, FNAIT has an incidence of 1:1000 to 1:2000 births. 2,3 Approximately 75 percent of serologically verified cases of alloantibodies against HPA-1a are responsible for the disease. 4 Antibodies against HPA-1a react with the Leu33 but not Pro33 isoform of platelet (PLT) glycoprotein (GP)IIIa or integrin b3 subunit. 5 Recently, the three-dimensional structure of the b3 integrin has been elucidated; 6,7 however, the precise binding site(s) of HPA-1a alloantibodies is not known. A previous study suggested that the amino terminal PSI domain containing the polymorphic residue 33 is sufficient to form HPA-1a epitopes. 8 However, various studies have shown that other residues located in the hybrid and epidermal growth factor domains of the b3 ABBREVIATIONS: D-PBS = Dulbecco's phosphate-buffered saline; FNAIT = fetal and neonatal alloimmune thrombocytopenia; GP(s) = glycoprotein(s); MAIPA assay = monoclonal antibody-based antigen-capture assay; PAIFT = platelet adhesion immunofluorescence test; PTP = posttransfusion purpura; RU = resonance units; SPR = surface plasmon resonance.
Transfusion, 2007
BACKGROUND: The antenatal management of fetomaternal alloimmune thrombocytopenia (FMAIT) due to HPA-1a antibodies remains controversial, and a test identifying pregnancies that do not require therapy would be of clinical value. STUDY DESIGN AND METHODS: The statistical correlation was analyzed between clinical outcome and 1) anti-HPA-1a potency in maternal serum samples determined by a monoclonal antibody immobilization of platelet (PLT) antigen assay with an international anti-HPA-1a potency standard and 2) anti-HPA-1a biological activity measured by a monocyte chemiluminescence (CL) assay. RESULTS: A total of 133 pregnancies with FMAIT due to anti-HPA-1a were analyzed. In 97 newly diagnosed cases, there was no difference in antibody potency or CL signal between cases with intracranial hemorrhage (ICH; n = 15), those with no ICH but a PLT count of less than 20 ¥ 10 9 per L (n = 52), and those with a PLT count of at least 20 ¥ 10 9 per L (n = 30). In 22 previously known pregnancies, the positive predictive value of maternal anti-HPA-1a of greater than 30 IU per mL for a PLT count of less than 20 ¥ 10 9 per L was 90 percent, but the negative predictive value was only 66 percent. Antibody potency tended to stay stable throughout pregnancy (n = 16) and from one pregnancy to the next (n = 16). CONCLUSION: Neither severe thrombocytopenia nor ICH in HPA-1a-alloimmunized pregnancies can be predicted with sufficient sensitivity and specificity for clinical application from maternal anti-HPA-1a potency or bioactivity.