Subamniotic haemorrhage, a possible new presentation of fetal and neonatal alloimmune thrombocytopenia

Background: Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare fetal disease in which maternal antibodies directed towards fetal human platelet antigens (HPA) are formed during pregnancy and cause fetal thrombocytopenia. The diagnosis FNAIT is suspected when a fetus or neonate presents with signs of bleeding. Case: We describe a pregnancy complicated by a placental hematoma in the 20th week of gestation as the first manifestation of FNAIT. Further evaluation showed signs of germinal matrix haemorrhage and HPA-5b allo-antibodies. After the diagnosis, intravenous immunoglobulin was administered weekly and a healthy daughter was born at 37 weeks. Histopathological analysis revealed that the hematoma was caused by a subamniotic haemorrhage of fetal origin. Conclusion: A subamniotic hematoma appears to be the first manifestation of FNAIT.

Prevention of Fetal/Neonatal Alloimmune Thrombocytopenia in a Preclinical Model By Monoclonal Anti-HPA-1a Antibody Prophylaxis: Structural and Functional Properties of Antibody Variant Isoforms

Maternal alloantibodies to paternally inherited platelet antigens can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT), rendering the fetus or newborn prone to bleeding and intracranial hemorrhage (ICH) with risk of lifelong disabilities or death. In Caucasians, the vast majority of cases are due to antibodies to the Human Platelet Antigen (HPA)-1a epitope on glycoprotein (GP)IIIa. To date, there are neither any means to prevent or reduce the risk of alloimmunization and subsequent FNAIT, nor safe and efficient treatment during affected pregnancies. Thus, a prophylactic regimen by administration of monoclonal antibodies to women at risk would be highly beneficial. Knowledge about the optimal functional design of prophylactic monoclonal antibodies for antenatal or post-natal use is however still limited. We have previously isolated and characterized a fully human anti-HPA-1a monoclonal antibody, mAb26.4. In the current study we have explored the prophylactic potential of this antibody by testing a panel of different IgG1 designs, including the wild-type mAb26.4, variants with modified Fc-region N-glycans, as well as an effector silent variant. We performed analyses of properties relevant for immunosuppression: in vitro Fc-receptor binding and capacity to induce phagocytosis, in vivo half-life measurements in humanized FcRn mice and platelet clearance in a recently developed transgenic mouse strain with a recreated HPA-1a epitope on murine GPIIIa. The prophylactic capacity by antibody-mediated immune suppression in vivo, were further tested in the FNAIT pre-clinical murine model, in which BALB/c females can be immunized by transfusion of HPA-1a-expressing platelets from the transgenic mice and where subsequent breeding of pre-immunized mice with transgenic males cause thrombocytopenia in off-springs mimicking FNAIT. By intravenous administrations of mAb26.4 variants prior to platelet transfusions, the mice generated no or low anti-platelet responses compared to control mice, and normal platelet counts in pups upon subsequent breeding. Our data thus successfully demonstrates efficient immunosuppression and prevention of FNAIT by anti-HPA-1a human monoclonal variants, providing further support for potential use in humans. Disclosures Skogen: Prophylix Pharma AS: Current holder of individual stocks in a privately-held company. Newman: Rallybio: Consultancy, Research Funding.

Prophylactic Administration of Antibodies Specific for the HPA-1a Epitope Prevents Alloimmunization and Platelet Destruction in a Murine Model of Fetal Neonatal Alloimmune Thrombocytopenia

Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal alloantibodies directed against paternally inherited antigens present on the surface of fetal and neonatal platelets. The human platelet alloantigen HPA-1a (formerly known as the Pl A1 alloantigen), is the most frequently implicated HPA for causing FNAIT in Caucasians. A single Leu33Pro amino acid polymorphism near the amino terminus of the integrin b3 subunit (known as GPIIIa in the platelet literature) serves as the central target for alloantibody binding, leading to clearance of both fetal and neonatal platelets, thrombocytopenia, and in the most severe cases, spontaneous- or trauma-induced intracranial hemorrhage. Unlike hemolytic disease of the newborn, which occurs in pregnancies subsequent to parturition-induced alloimmunization, an estimated 25% to 50% of FNAIT cases occur without warning during gestation of the first pregnancy. Though long proposed, there are currently no approved therapies for the prevention of FNAIT. We recently described the development of transgenic mice expressing the human HPA-1a allogeneic epitope on a murine GPIIIa backbone. Transfusion of such platelets into wild-type female mice induced the generation of high-titer anti-HPA-1a alloantibodies that can cross the placenta and recapitulate many of the relevant clinical features of FNAIT. To test the hypothesis that rapid elimination of fetal HPA-1a positive platelets from the circulation of a mother who is HPA-1a negative might prevent maternal alloimmunization and the development of FNAIT, we administered either a hyperimmune plasma-derived polyclonal anti-HPA-1a antibody derived from females having previous cases of FNAIT (termed RLYB211), or a novel human monoclonal antibody directed against the HPA-1a epitope (termed RLYB212), to wild-type female mice prior to challenging them with HPA-1a-positive murine platelets. RLYB211 and RLYB212 were each able to effect the rapid removal of HPA-1a-positive platelets from murine circulation and prevent the development of anti-HPA-1a alloantibodies. Importantly, wild-type female mice pretreated with RLYB211 prior to exposure to HPA-1a-positive platelets, and then impregnated by HPA-1a-positive males, gave birth to HPA-1a-positive pups with significantly improved platelet counts and no bleeding symptoms. These preclinical data establish the potential for prophylactic polyclonal and monoclonal anti-HPA-1a antibody therapy for the prevention of FNAIT in humans. Disclosures Sheridan: Rallybio: Current Employment, Current equity holder in publicly-traded company. Newman: Rallybio: Consultancy, Research Funding.

Case Of NAIT Causing Severe Thrombocytopenia due to Anti-HLA Class I

Neonatal alloimmune thrombocytopenia (NAIT) is the leading cause of thrombocytopenia in otherwise healthy new-born. (1,2) Maternal antibodies raised against paternally inherited alloantigen carried on fetal platelet causing NAIT. Maternal IgG antibodies passed through to the fetal via the placenta, attack and cause the destruction of the fetal platelet. (3) We present a case of NAIT without any complications in a premature baby (35 weeks) with VACREL association, G6PD deficiency, left calcified cephalohaematoma, cardiomegaly and hypospadias with severe thrombocytopenia (platelet counts is 23 109/L) at day two of life and received twice platelet transfusion. Platelet count initially 123 109/L at birth but significantly drop and persistently less than 50 109/L until day 10 of life before it normalized. Maternal serum antibody screening was negative, but platelet immunology test detected maternal platelet-reactive antibody Anti-HLA Class I and correlates with incompatible parental crossmatch indicating that parent had “platelet-antigen incompatibility”. The goal of obstetric management is to identify pregnancies at risk and prevent intracranial haemorrhage. (4) There is no evidence to support routine screening for pregnancies as per current practice. (2, 5) The latest treatments include maternal administration of intravenous immunoglobulin to suppress maternal antibody production and or to reduce placental transfer of antibodies; with or without steroids during antepartum period besides planning of mode, timing and method of delivery. (2, 5, 6, 7) This is a rare and unique case of NAIT secondary to Anti-HLA Class I antibody and hence clinician should be au fait with the diagnosis and management as it is infrequent among Malaysian.International Journal of Human and Health Sciences Supplementary Issue-2: 2021 Page: S21

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.

Neonatal alloimmune thrombocytopenia: two different neonatal manifestations of the same disease

Fetal and neonatal alloimmune thrombocytopenia is a rare disorder in which maternal alloantibodies cross the placenta and cause fetal thrombocytopenia. Most cases are mild but can be potentially fatal if there is an intracranial haemorrhage. Usually, the mother is asymptomatic during pregnancy and no screening is routinely recommended unless there is obstetric or family history. After birth, prompt identification is crucial so that the newborn is closely monitored and treatment is given, if needed, to prevent serious complications. In this article we present two clinical cases of fetal and neonatal alloimmune thrombocytopenia in first-born children, after uneventful pregnancies, with different outcomes and discuss the obstetric management and follow-up in future pregnancies.