Mechanisms and Prevention of Alloimmunization in Pregnancy

Rhesus D Prophylaxis: When and Why We Give Rhesus D Immunoglobulin

Importance

Hemolytic disease of the fetus and newborn (HDFN) is caused by maternal alloantibodies to fetal red blood cells and is associated with significant fetal and neonatal morbidity and mortality. Rhesus D antigen (RhD)-mediated HDFN is the only preventable cause of alloimmunization in pregnancy. Widespread utilization of RhD prophylaxis reduces the risk of RhD-mediated alloimmunization from 17% to <1% in at-risk pregnancies, although RhD-mediated HDFN still occurs.

Objective

To emphasize significance of RhD prophylaxis, outline current guideline-directed indications for administration, provide clarification in areas of uncertainty regarding prophylaxis administration, and review key concepts relevant to patient education and shared decision-making.

Evidence Acquisition

PubMed and Google Scholar literature search.

Results

Data over several decades have shown implementation of prenatal and postpartum RhD prophylaxis has significantly reduced incidence and morbidity of RhD-mediated HDFN. Most international guidelines recommend routine prophylaxis of Rh-negative mothers in the second trimester and peripartum, with additional prophylaxis following certain high-risk events. Recent shortages in RhD immunoglobulin (RhDIg) and new methods to determine fetal blood type have prompted renewed debate regarding criteria for prophylaxis during the first trimester.

Conclusion

Understanding indications for administration of RhD prophylaxis is essential for preventing RhD alloimmunization. Although uncertainty remains in some clinical scenarios, prophylaxis is strongly recommended in Rh-negative mothers in the second trimester, following events high-risk for sensitization in pregnancy, and postpartum.

Relevance

In this review, the etiology of alloimmunization and indications for RhDIg prophylaxis, current society recommendations, and areas of debate are summarized and discussed.

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

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.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.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.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.

Frequency of human platelet antigens (HPA) in the Greek population as deduced from the first registry of HPA-typed blood donors

BACKGROUND AND OBJECTIVES

Human platelet antigens (HPA) play a central role in foetal and neonatal alloimmune thrombocytopenia (FNAIT), post-transfusion purpura and some cases of platelet therapy refractoriness. The frequency distribution of HPA had not been studied in the Greek population before we started to create a registry of HPA-typed apheresis platelet donors. The aim of this study was the determination of the frequency of various HPA in the Greek population, through the establishment of a registry of typed donors.

MATERIALS AND METHODS

Here, we report on the first 1000 platelet donors of Greek origin who gave informed consent and were genotyped for 12 pairs of antithetical HPA by Single Specific Primer-Polymerase Chain Reaction (SSP-PCR), including HPA-1, HPA-3, HPA-5 and HPA-15. Antigen frequencies are reported, and allele frequencies were calculated and compared with other European and non-European populations. Tested donors cover all ABO and Rhesus D antigen spectrum.

RESULTS

Antigen and allele frequencies are very similar to other White populations. The frequency of HPA-1bb is 2.9% in our study, and the frequency of HPA-2b, HPA-4b, HPA-9b and HPA-15b is also slightly higher than in other literature reports, while the frequency of HPA-15b was found higher than that of HPA-15a.

CONCLUSION

We report antigen and allele frequencies for a large array of clinically significant HPA for the first time in the Greek population. Frequencies are consistent with other European populations. This registry of HPA-typed platelet donors, available to donate on demand, is an important asset for the treatment of FNAIT cases in Greece.

Informing pregnancy dose via target-mediated drug disposition modeling and simulations for a recombinant human monoclonal antibody

RLYB212 is a human monoclonal anti-human platelet antigen (HPA)-1a immunoglobulin gamma 1 in clinical development as a subcutaneous injection for the prevention of maternal alloimmunization to fetal HPA-1a leading to fetal and neonatal alloimmune thrombocytopenia (FNAIT). This analysis developed a target-mediated drug disposition (TMDD) model to simultaneously characterize RLYB212 pharmacokinetics (PK) and HPA-1a-positive platelet dynamics in HPA-1b/b (HPA-1a-negative) volunteers. The model was then used to perform simulations to inform a dosing regimen in a phase II clinical study in pregnant women, where simulations accounted for physiological changes throughout pregnancy. Allometric scaling (0.75) for clearance and intercompartment transfer rate and volume (1) was included in the base model to account for variations in body weight. A 0.06 mg RLYB212 dose with a loading dose of 0.12 mg was identified as the optimal dosing regimen of RLYB212, which maintained exposures below the target upper boundary of ~10 ng/mL throughout pregnancy. This work presents an application of the TMDD model that advances the quantitative clinical pharmacology toolkit to understand monoclonal antibody PK in pregnancy.

Noninvasive Prenatal Testing in Immunohematology-Clinical, Technical and Ethical Considerations

Hemolytic disease of the fetus and newborn (HDFN), as well as fetal and neonatal alloimmune thrombocytopenia (FNAIT), represent two important disease entities that are caused by maternal IgG antibodies directed against nonmaternally inherited antigens on the fetal blood cells. These antibodies are most frequently directed against the RhD antigen on red blood cells (RBCs) or the human platelet antigen 1a (HPA-1a) on platelets. For optimal management of pregnancies where HDFN or FNAIT is suspected, it is essential to determine the RhD or the HPA-1a type of the fetus. Noninvasive fetal RhD typing is also relevant for identifying which RhD-negative pregnant women should receive antenatal RhD prophylaxis. In this review, we will give an overview of the clinical indications and technical challenges related to the noninvasive analysis of fetal RBCs or platelet types. In addition, we will discuss the ethical implications associated with the routine administration of antenatal RhD to all pregnant RhD-negative women and likewise the ethical challenges related to making clinical decisions concerning the mother that have been based on samples collected from the (presumptive) father, which is a common practice when determining the risk of FNAIT.

Epidemiology and management of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a disease in pregnancy characterized by maternal alloantibodies directed against the human platelet antigen (HPA). These antibodies can cause intracranial hemorrhage (ICH) or other major bleeding resulting in lifelong handicaps or death. Optimal fetal care can be provided by timely identification of pregnancies at risk. However, this can only be done by routinely antenatal screening. Whether nationwide screening is cost-effective is still being debated. HPA-1a alloantibodies are estimated to be found in 1 in 400 pregnancies resulting in severe burden and fetal ICH in 1 in 10.000 pregnancies. Antenatal treatment is focused on the prevention of fetal ICH and consists of weekly maternal IVIg administration. In high-risk FNAIT treatment should be initiated at 12-18 weeks gestational age using high dosage and in standard-risk FNAIT at 20-28 weeks gestational age using a lower dosage. Postnatal prophylactic platelet transfusions are often given in case of severe thrombocytopenia to prevent bleedings. The optimal threshold and product for postnatal transfusion is not known and international consensus is lacking. In this review practical guidelines for antenatal and postnatal management are offered to clinicians that face the challenge of reducing the risk of bleeding in fetuses and infants affected by FNAIT.

Anemia in Pregnancy: A Pragmatic Approach

Importance

Anemia is common in pregnancy, ranging from 5.4% in developed countries to more than 80% in developing countries. Anemia in pregnancy has been associated with prematurity, low birth weight, and adverse pregnancy outcomes.

Objective

This review uses clinical vignettes to illustrate the clinical presentations, approach to diagnosis, maternal and fetal implications, and treatment for the common etiologies of anemia in pregnancy.

Evidence Acquisition

Literature review.

Results

Normal physiological changes in pregnancy result in alterations of hematological parameters particularly in a reduction of hemoglobin (Hb) concentration. Consequently, the Hb used to define anemia in pregnancy is lower than in nonpregnant patients. As there is an increased requirement of iron in pregnancy, it is not unexpected that iron deficiency remains the most common cause of anemia and warrants a preemptive approach to prevent a further reduction in Hb. The syndromes associated with microangiopathic hemolytic anemia may pose a diagnostic challenge, as there are several potential etiologies that may be difficult to differentiate, and microangiopathic hemolytic anemia can be associated with significant maternal and fetal morbidity andmortality. Anemia secondary to sickle cell disease and autoimmune hemolytic anemiamerit special attention because there are risks secondary to red blood cell transfusion and risks to withholding transfusion.

Conclusions

Anemia in pregnancy is potentially associated with maternal and fetal adverse outcomes. Providing evidence-based care is essential to achieving the best pregnancy outcomes.

Fatal delayed haemolytic transfusion reaction in a patient without previous transfusions but with an obstetric history of 13 pregnancies

Delayed haemolytic transfusion reaction is a rare, life-threatening complication of blood transfusion that has been typically described among patients with sickle cell disease (SCD) due to alloimmunisation induced by their exposure to red blood cell antigens through recurrent transfusions. We report the case of a patient who suffered from fatal delayed haemolytic transfusion reaction (DHTR) occurring 1 week after blood transfusion. Indirect antiglobulin testing confirmed the presence of anti-Kell antibodies that were absent in the pretransfusion sample. The patient did not receive blood transfusions in the past, but her obstetric history was remarkable for 13 pregnancies. Although DHTR occurs more commonly among patients with SCD, this type of reaction can occur in any patient who is able to mount an immune response. We would to like to draw the attention of physicians to this rare and potentially lethal complication of blood transfusion, especially in grand multiparous women.

AMIS and antigen modulation: of mice and men

In this issue of Blood, Liu et al investigated the mechanism of action in antibody-mediated immune suppression (AMIS) and the fate of antigen in a KEL mouse model system.1

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.

Platelets and platelet alloantigens: Lessons from human patients and animal models of fetal and neonatal alloimmune thrombocytopenia

Platelets play critical roles in hemostasis and thrombosis. Emerging evidence indicates that they are versatile cells and also involved in many other physiological processes and disease states. Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life threatening bleeding disorder caused by fetal platelet destruction by maternal alloantibodies developed during pregnancy. Gene polymorphisms cause platelet surface protein incompatibilities between mother and fetus, and ultimately lead to maternal alloimmunization. FNAIT is the most common cause of intracranial hemorrhage in full-term infants and can also lead to intrauterine growth retardation and miscarriage. Proper diagnosis, prevention and treatment of FNAIT is challenging due to insufficient knowledge of the disease and a lack of routine screening as well as its frequent occurrence in first pregnancies. Given the ethical difficulties in performing basic research on human fetuses and neonates, animal models are essential to improve our understanding of the pathogenesis and treatment of FNAIT. The aim of this review is to provide an overview on platelets, hemostasis and thrombocytopenia with a focus on the advancements made in FNAIT by utilizing animal models.