Characterization of a human platelet antigen-1a-specific monoclonal antibody derived from a B cell from a woman alloimmunized in pregnancy

A Phase 1b PK/PD Study to Demonstrate Antigen Elimination with RLYB212, a Monoclonal Anti-HPA-1a Antibody for FNAIT Prevention

BACKGROUND

 Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare bleeding disorder of the fetus/newborn caused by development of maternal alloantibodies against fetal human platelet antigens (HPAs), predominantly HPA-1a. Currently there are no treatments available to prevent maternal alloimmunization to HPAs or FNAIT.

METHODS

 This proof-of-concept study (EudraCT Number: 2021-005380-49) was designed to assess the ability of subcutaneous (SC) RLYB212, a monoclonal anti-HPA-1a antibody, to eliminate HPA-1a-positive platelets in an antigen challenge model of a 30 mL fetal-maternal hemorrhage. Subjects were randomized to receive a single SC dose of RLYB212 or placebo on day 1 in a single-blinded manner, followed by transfusion of 10 × 109 HPA-1a-positive platelets on day 8.

RESULTS

 Four subjects received 0.09 mg SC RLYB212, five received 0.29 mg SC RLYB212, and two received placebo. RLYB212 achieved rapid elimination of HPA-1a-positive platelets in a concentration-dependent manner, with concentrations as low as 3.57 ng/mL meeting the prespecified proof-of-concept criterion of ≥90% reduction in platelet elimination half-life versus placebo. Following HPA-1a-positive platelet transfusion, a rapid decline was observed in the concentration of RLYB212 over a period of 2 to 24 hours, corresponding to the time needed for RLYB212 to bind to ∼10% of HPA-1a on cell surfaces. RLYB212 was well tolerated with no reports of drug-related adverse events.

CONCLUSION

 The data from this study are consistent with preclinical efficacy data and support the potential use of RLYB212 as a prophylactic treatment for FNAIT that prevents maternal HPA-1a alloimmunization during at-risk pregnancies.

Generation of human antibodies targeting human platelet antigen (HPA)-1a

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a condition during pregnancy, which can lead to thrombocytopenia and a bleeding tendency with intracranial hemorrhage (ICH) being the most concerning complication in the fetus or neonate. An incompatibility between human platelet antigen (HPA)-1a accounts for the majority of FNAIT cases. Binding of HPA-1a-specific alloantibodies to their target on fetal platelets and endothelial cells can induce apoptosis of megakaryocytes, disrupt platelet function, and impair angiogenesis. Currently, there is no screening program to identify pregnancies at risk for severe disease. A better understanding of HPA-1a-specific antibody heterogeneity in FNAIT could aid in identifying pathogenic antibody properties linked to severe disease.

STUDY DESIGN AND METHODS

This study aimed to isolate HPA-1a-specific B-cells from an HPA-1a-alloimmunized pregnant woman. Using fluorescently labeled HPA-1a-positive platelets, single B-cells were sorted and cultured for 10 days to stimulate antibody production. Subsequently, supernatants were tested for the presence of antibodies by enzyme-linked immunosorbent assay and their reactivity towards HPA-1a-positive platelets. Amplification and sequencing of variable regions allowed the generation of monoclonal antibodies using a HEK-Freestyle-based expression system.

RESULTS

Three platelet-specific B-cells were obtained and cloned of which two were specific for HPA-1a, named D- and M-204, while the third was specific for HLA class I, which was named L-204.

DISCUSSION

This study outlined an effective method for the isolation of HPA-1a-specific B-cells and the generation of monoclonal antibodies. Further characterization of these antibodies holds promise for better understanding the pathogenic nature of alloantibodies in FNAIT.

A novel monoclonal antibody with improved FcγR blocking ability demonstrated non-inferior efficacy compared to IVIG in cynomolgus monkey ITP model at considerably lower dose

Intravenous immunoglobulin (IVIG) is a well-established treatment for various autoimmune and inflammatory diseases. However, the standard dose prescribed for autoimmune diseases, including immune thrombocytopenic purpura (ITP), is 2 g/kg, which is markedly high and leads to a high treatment burden. In this study, we generated fragment crystallizable (Fc)-modified anti-haptoglobin (Hp) monoclonal antibodies with non-inferior efficacy compared to IVIG at considerably lower doses than IVIG, as shown by in vitro experiments. We evaluated binding activity of anti-Hp antibodies to Fc gamma receptors (FcγRs) with ELISA and inhibitory activity against the ADCC reaction. Furthermore, we successfully established a novel cynomolgus monkey ITP model and demonstrated that the anti-Hp antibody exerted its effect in this model with only a single dose. This Fc-modified anti-Hp monoclonal antibody could be a valuable therapeutic replacement for IVIG for the treatment of ITP.

Prophylactic administration of HPA-1a-specific antibodies prevents fetal/neonatal alloimmune thrombocytopenia in mice

Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal alloantibodies directed against paternally inherited human platelet alloantigens (HPAs) present on the surface of fetal and neonatal platelets. There are currently no approved therapies for the prevention of FNAIT. We report herein the ability of 2 human HPA-1a-specific therapeutic candidates, one a polyclonal, and the other a monoclonal antibody, to prevent alloimmunization in a novel preclinical mouse model of FNAIT. Both antibody preparations effected the rapid and complete elimination of HPA-1a+ platelets from circulation and prevented the development of HPA-1a alloantibodies. HPA-1a- female mice treated prophylactically with anti-HPA-1a antibody prior to exposure to HPA-1a+ platelets gave birth to HPA-1a+/- pups with significantly improved platelet counts and no bleeding symptoms. These preclinical data establish both the potential and threshold exposure targets for prophylactic treatment with HPA-1a-specific antibodies for the prevention of FNAIT in humans.

Preclinical evaluation of immunotherapeutic regimens for fetal/neonatal alloimmune thrombocytopenia

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 when bred with HPA-1a+ males, gave birth to severely thrombocytopenic pups that exhibited an accompanying bleeding phenotype. Administering either polyclonal intravenous immunoglobulin G or a human monoclonal blocking antibody specific for the HPA-1a epitope into pregnant female mice resulted in significant elevation of the neonatal platelet count, normalized hemostasis, and prevented bleeding. The establishment of an alloantigen-specific murine model that recapitulates many of the clinically important features of FNAIT should pave the way for the preclinical development and testing of novel therapeutic and prophylactic modalities to treat or prevent FNAIT in humans.

Platelet alloimmunization is associated with low grade chronic histiocytic intervillositis - A new link to a rare placental lesion?

INTRODUCTION

Maternal alloimmunization against human platelet antigen (HPA)-1a has been implied to mediate both reduced birth weight and chronic placental inflammation. Fetal growth restriction is associated with different types of chronic inflammation in the placenta, mainly chronic histiocytic intervillositis and chronic villitis. The aim of this prospective study was to do a systematic examination of placentas from HPA-1a alloimmunized pregnancies, with focus on the histopathological and immunohistochemical diagnosis of variants of chronic inflammation.

MATERIAL AND METHODS

In a Polish-Norwegian study, 48 placentas were examined. The histopathology of placentas from 27 HPA-1a immunized women was compared with 21 placentas from non-immunized HPA-1a negative women (controls). In the group of alloimmunized women, ten received antenatal intravenous immunoglobulin G (IVIg). Tissue sections from formalin fixed paraffin embedded placental tissue were stained with hematoxylin and eosin and microscopically examined with focus on various types of chronic placental inflammations.

RESULTS

Chronic histiocytic intervillositis was observed in 40.7% of placentas from HPA-1a alloimmunized pregnancies, compared to none in the control group (p = 0.001). Chronic villitis of unknown etiology was more frequently found in the alloimmunized group, however this difference was not statistically significant. Maternal administration of IVIg did not seem to protect against chronic inflammatory lesions.

DISCUSSION

Placentas with detectable maternal anti-HPA-1a antibodies are associated with highly increased risk of low-grade chronic histiocytic intervillositis.

Naturally occurring point mutation Cys460Trp located in the I-EGF1 domain of integrin β3 alters the binding of some anti-HPA-1a antibodies

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by the destruction of platelets in the fetus or newborn by maternal platelet alloantibodies, mostly against human platelet antigen (HPA)-1a. Recent studies indicate that two anti-HPA subtypes exist: Type I reacts with epitopes residing on the plexin-semaphorin-integrin (PSI) and type II with plexin-semaphorin-integrin/integrin epidermal growth factor 1 (I-EGF1) domains of the β3 integrin. Here, we evaluated whether a Cys460Trp mutation in the I-EGF1 domain found in a patient with Glanzmann thrombasthenia can alter the binding of anti-HPA-1a.

METHODS

Stable HEK293 cell lines expressing wild-type and mutant αIIbβ3 and αvβ3 were generated to prove the reactivity of different antibodies against HPA-1a.

RESULTS

Flow cytometry analysis of wild-type (Cys460) and mutant (Trp460) expressed on HEK293 cells showed equal surface expression of αIIbβ3 and αvβ3. When tested with mutant αIIbβ3 cells, reduced binding was observed in Type II but not in Type I anti-HPA-1a. These results could be confirmed with platelets carrying Cys460Trp mutation. Interestingly, reduced binding of Type I antibodies was detected with mutant αvβ3 cells. Both antibody types were found in maternal sera from FNAIT cases by an antigen-capture assay with use of HEK293 transfected cells.

CONCLUSIONS

These observations confirm the existence of Type I and Type II anti-HPA-1a. Furthermore, this study underlines different immunogenicity of HPA-1a antigen(s) residing on either αIIbβ3 or αvβ3. Further analysis of FNAIT cases from mothers having a fetus with and without intracranial bleedings with use of such an approach may highlight the functional relevance of different anti-HPA-1a subtypes.

Strategies to develop a prophylaxis for the prevention of HPA-1a immunization and fetal and neonatal alloimmune thrombocytopenia

Anti-HPA-1a-antibodies are the main cause of fetal and neonatal alloimmune thrombocytopenia (FNAIT) which may result in intracranial hemorrhage (ICH) and death among fetuses and newborns. Advances in understanding the pathogenesis of FNAIT and proof of concept for prophylaxis to prevent immunization suggest that development of hyperimmune anti-HPA-1a IgG aimed at preventing immunization against HPA-1a and FNAIT is feasible. Anti-HPA-1a IgG can be obtained either by isolating immunoglobulin from already-immunized women or by development of monoclonal anti-HPA-1a antibodies. Here we discuss recent advances that may lead to the development of a prenatal and postnatal prophylactic treatment for the prevention of HPA-1a-associated FNAIT and life-threatening FNAIT-induced complications.

Current Anti-HPA-1a Standard Antibodies React with the β3 Integrin Subunit but not with αIIbβ3 and αvβ3 Complexes

BACKGROUND

 Fetal/neonatal alloimmune thrombocytopenia (FNAIT) results from maternal alloantibodies (abs) reacting with fetal platelets expressing paternal human platelet antigens (HPAs), mostly HPA-1a. Anti-HPA-1a abs, are the most frequent cause of severe thrombocytopenia and intracranial hemorrhage (ICH).

OBJECTIVES

 Titration of anti-HPA-1a in maternal serum using standard National Institute for Biological Standards and Control (NIBSC) 03/152 is one diagnostic approach to predict the severity of FNAIT. Recently, we found three anti-HPA-1a subtypes reacting with the β3 subunit independently or dependently from complexes with αIIb and αv. Endothelial cell-reactive anti-αvβ3 abs were found predominantly in cases with ICH. Our aim was to assess whether available standard material represents all anti-HPA-1a subtypes.

MATERIALS AND METHODS

 In this study, anti-HPA-1a sera (NIBSC 03/152) and human monoclonal antibodies (moabs) against HPA-1a (moabs 26.4 and 813) were evaluated using transfected cell lines expressing αIIbβ3, αvβ3 or monomeric cβ3.

RESULTS

 Flow cytometry analyses with well-characterized murine moabs recognizing αIIbβ3, αvβ3, or β3 alone demonstrated that AP3 reacts compound-independently, whereas compound-dependent moabs Gi5 and 23C6 reacted only with complexes. NIBSC 03/152, moabs 26.4, and 813 against HPA-1a reacted like AP3, same results were obtained with monomeric cβ3 in immunoblotting. Antigen capture assay targeting endothelial cells showed anti-HPA-1a reactivity disappearance after cβ3 beads adsorption. Furthermore, in contrast to anti-HPA-1a abs from ICH cases, none of NIBSC 03/152, 26.4, and 813 inhibited tube formation.

CONCLUSION

 These results suggest that current anti-HPA-1a standard material contains only the anti-β3 subtype. The absence of anti-αvβ3 makes NIBSC 03/152 less suitable as standard to predict the severity of FNAIT.

High-resolution mapping of the polyclonal immune response to the human platelet alloantigen HPA-1a (PlA1)

Antibodies to platelet-specific antigens are responsible for 2 clinically important bleeding disorders: posttransfusion purpura and fetal/neonatal alloimmune thrombocytopenia (FNAIT). The human platelet-specific alloantigen 1a/1b (HPA-1a/1b; also known as PlA1/A2) alloantigen system of human platelet membrane glycoprotein (GP) IIIa is controlled by a Leu33Pro polymorphism and is responsible for ∼80% of the cases of FNAIT. Local residues surrounding polymorphic residue 33 are suspected to have a profound effect on alloantibody binding and subsequent downstream effector events. To define the molecular requirements for HPA-1a alloantibody binding, we generated transgenic mice that expressed murine GPIIIa (muGPIIIa) isoforms harboring select humanized residues within the plexin-semaphorin-integrin (PSI) and epidermal growth factor 1 (EGF1) domains and examined their ability to support the binding of a series of monoclonal and polyclonal HPA-1a-specific antibodies. Humanizing the PSI domain of muGPIIIa was sufficient to recreate the HPA-1a epitope recognized by some HPA-1a-specific antibodies; however, humanizing distinct amino acids within the linearly distant but conformationally close EGF1 domain was required to enable binding of others. These results reveal the previously unsuspected complex heterogeneity of the polyclonal alloimmune response to this clinically important human platelet alloantigen system. High-resolution mapping of this alloimmune response may improve diagnosis of FNAIT and should facilitate the rational design and selection of contemplated prophylactic and therapeutic anti-HPA-1a reagents.

Adding to the complexity of fetal and neonatal alloimmune thrombocytopenia: Reduced fibrinogen binding in the presence of anti-HPA-1a antibody and hypo-responsive neonatal platelets

BACKGROUND

In fetal and neonatal alloimmune thrombocytopenia (FNAIT), maternal alloantibodies directed against paternally-derived platelet antigens are transported across the placenta to the fetus, where they may cause thrombocytopenia. The most serious complication of FNAIT is an intracranial hemorrhage (ICH), which may cause death or life-long disability of the child. Apart from alloantibody-mediated platelet destruction, the clinical outcome in FNAIT may be affected by properties of neonatal platelets and possible functional effects on platelets caused by maternal alloantibodies.

METHODS AND RESULTS

The function of umbilical cord blood platelets was compared with adult platelets in two assays, impedance aggregometry (Multiplate) and rotational thromboelastometry (Rotem). Both revealed a decreased in vitro neonatal platelet function compared to adult platelets. Consistent with this finding, activation using TRAP revealed less pronounced changes in the expression of CD62P, PAC-1, CD41 and CD42a in umbilical cord blood platelets compared to adult platelets. Furthermore, a monoclonal anti-HPA-1a antibody, derived from an immunized mother of two children with FNAIT, blocked fibrinogen binding to resting and activated umbilical cord blood and adult HPA-1aa and HPA-1ab platelets, interfered with platelet activation by TRAP, and impaired the function of umbilical cord blood HPA-1aa platelets in rotational thromboelastometry.

DISCUSSION AND CONCLUSIONS

Reduced fibrinogen binding in the presence of anti-HPA-1a antibodies may disturb the neonatal hemostatic balance, characterized by poorly responsive platelets. This effect may operate in parallel to platelet destruction and contribute to the clinical outcome in FNAIT.

Anti-human platelet antigen (HPA)-1a antibodies may affect trophoblast functions crucial for placental development: a laboratory study using an in vitro model

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a bleeding disorder caused by maternal antibodies against paternal human platelet antigens (HPAs) on fetal platelets. Antibodies against HPA-1a are accountable for the majority of FNAIT cases. We have previously shown that high levels of maternal anti-HPA-1a antibodies are associated with clinically significant reduced birth weight in newborn boys. Chronic inflammatory placental lesions are associated with increased risk of reduced birth weight and have previously been reported in connection with FNAIT pregnancies. The HPA-1a epitope is located on integrin β3 that is associated with integrin αIIb (the fibrinogen receptor) on platelets and megakaryocytes. Integrin β3 is also associated with integrin αV forming the αVβ3 integrin heterodimer, the vitronectin receptor, which is expressed on various cell types, including trophoblast cells. It is therefore thinkable that maternal anti-HPA-1a antibodies present during early pregnancy may affect placenta function through binding to the HPA-1a antigen epitope on invasive throphoblasts. The aim of the study was to examine whether interaction of a human anti-HPA-1a monoclonal antibody (mAb) with HPA-1a on trophoblast cells affect adhesion, migration and invasion of extravillous trophoblast cells.

METHODS

An in vitro model with human anti-HPA-1a mAb, clone 26.4, and the first trimester extravillous trophoblast cell line HTR8/SVneo was employed. The xCELLigence system was utilized to assess the possible effect of anti-HPA-1a mAb on adhesion and migration of HTR8/SVneo cells. Specially designed chambers precoated with Matrigel were used to assess the effect on the invasive capacity of cells.

RESULTS

We found that human anti-HPA-1a mAb 26.4 partially inhibits adhesion and migratory capacity of HTR8/SVneo cells.

CONCLUSIONS

Our findings suggest that anti-HPA-1a antibodies may affect trophoblast functions crucial for normal placental development. Future studies including primary throphoblast cells and polyclonal anti-HPA-1a antibodies are needed to confirm these results.

Current perspectives on fetal and neonatal alloimmune thrombocytopenia - increasing clinical concerns and new treatment opportunities

Differences in platelet type between the fetus and the mother can lead to maternal immunization and destruction of the fetal platelets, a condition named fetal and neonatal alloimmune thrombocytopenia (FNAIT). FNAIT is reported to occur in ~1 per 1,000 live born neonates. The major risk is intracranial hemorrhage in the fetus or newborn, which is associated with severe neurological complications or death. Since no countries have yet implemented a screening program to detect pregnancies at risk, the diagnosis is typically established after the birth of a child with symptoms. Reports on broader clinical impact have increased clinical concern and awareness. Along with new treatment options for FNAIT, the debate around antenatal screening to detect pregnancies at risk of FNAIT has been revitalized.

Recent progress in understanding the pathogenesis of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs in c. 1 in 1000 births and is caused by maternal antibodies against human platelet alloantigens that bind incompatible fetal platelets and promote their clearance from the circulation. Affected infants can experience bleeding, bruising and, in severe cases, intracranial haemorrhage and even death. As maternal screening is not routinely performed, and first pregnancies can be affected, most cases are diagnosed at delivery of a first affected pregnancy. Unlike its erythrocyte counterpart, Haemolytic Disease of the Fetus and Newborn, there is no prophylactic treatment for FNAIT. This report will review recent advances made in understanding the pathogenesis of FNAIT: the platelet alloantigens involved, maternal exposure and sensitization to fetal platelet antigens, properties of platelet Immunoglobulin G antibodies, maternal-fetal antibody transport mechanisms and efforts to develop an effective FNAIT prophylaxis.