Red-cell and platelet alloimmunisation in pregnancy

Red Blood Cell Alloimmunization in Pregnancy: A Review of the Pathophysiology, Prevalence, and Risk Factors

This review paper provides an overview of the risk factors and laboratory testing for red blood cell (RBC) alloimmunization in pregnancy. RBC alloimmunization is a significant medical issue that can cause haemolytic disease of the fetus and newborn (HDFN), leading to neonatal morbidity and mortality. Current HDFN prophylaxis targets only Rhesus D (RhD) alloimmunization, with no effective measures to prevent alloimmunization to other RBC antigen groups. Several factors can increase the risk of developing RBC alloimmunization during pregnancy, including fetomaternal haemorrhage, RBC and maternal genetic status, and previous transfusions. Identifying these risk factors is essential to execute the appropriate management strategies to minimize the risk of HDFN. The review also discusses the laboratory methods and overview of pregnancy management. The paper highlights the importance of identifying and managing the risk factors for RBC alloimmunization in pregnancy to minimize the risk of HDFN and improve neonatal outcomes.

The immune system and microbiome in pregnancy

Hormonal changes during pregnancy instigate numerous physiological changes aimed at the growth and delivery of a healthy baby. A careful balance between immunological tolerance against fetal antigens and immunity against infectious agents needs to be maintained. A three-way interaction between pregnancy hormones, the immune system and our microbiota is now emerging. Recent evidence suggests that microbial alterations seen during pregnancy may help maintain homeostasis and aid the required physiological changes occurring in pregnancy. However, these same immunological and microbial alterations may also make women more vulnerable during pregnancy and the post-partum period, especially regarding immunological and infectious diseases. Thus, a further understanding of the host-microbial interactions taking place during pregnancy may improve identification of populations at risk for adverse pregnancy outcomes.

Efficacy of Antenatal Intravenous Immunoglobulin Treatment in Pregnancies at High Risk due to Alloimmunization to Red Blood Cells

Background

Alloimmunization to red blood cells (RBCs) may result in fetal anemia prior to 20 weeks gestation. The question as to whether early commencement of antenatal treatment with high-dose intravenous immunoglobulins (IVIG) may prevent or at least delay the development of fetal anemia in the presence of alloantibodies to RBCs is highly relevant.

Patients and Results

Here we describe a patient with high-titer anti-K and two other severely affected pregnant women with a history of recurrent pregnancy loss due to high-titer anti-D or anti-D plus anti-C. Early commencement of treatment with IVIG (1 g/kg/week) resulted in prevention of intrauterine transfusion (IUT) in the former two cases, and in a significant delay of development of fetal anemia in the remaining case (26 weeks gestation).

Conclusion

Based on our findings and of previously published cases, early initiation of treatment of severely alloimmunized women with IVIG (1 g/kg/week) could potentially improve the outcome of fetuses at risk.

Outcomes of subsequent pregnancy following obstetric transfusion in a first birth

Background

Increasing rates of postpartum haemorrhage and obstetric transfusion mean that more women are entering subsequent pregnancies with a history of blood transfusion. This study investigates subsequent pregnancy outcomes of women with a prior obstetric red cell transfusion, compared to women without a transfusion.

Methods

All women with a first pregnancy resulting in a liveborn singleton infant of at least 20 weeks gestation delivering in hospitals in New South Wales, Australia, between 2003 and 2012 were included in the study, with followup for second births until June 2015. Linked hospital and births data were used to identify women with a transfusion and/or postpartum haemorrhage in their first birth, time to second pregnancy and adverse birth outcomes (including transfusion, postpartum haemorrhage and severe morbidity) in their subsequent birth.

Results

There were 358,384 singleton births to primiparous women, with 1.4% receiving an obstetric blood transfusion. Sixty-three percent of women had at least one subsequent birth. The relative risk (RR) of requiring a transfusion in a second birth was 4.9 (95% CI 4.1,6.1) for women with a previous transfusion compared with women without. The risk (RR) of severe morbidity in a second birth was 4.1 times higher (95% CI 2.2,7.4) for those receiving a transfusion without haemorrhage in their first birth compared with women with neither haemorrhage nor transfusion.

Conclusion

It is important to consider a woman’s history of transfusion and/or haemorrhage as part of her obstetric history to ensure management in a manner that minimises risk in subsequent pregnancies.

[Anti-D prophylaxis in fetal-maternal erythrocyte incompatibility in Tunisia]

Generalization of postnatal prophylaxis using anti-D immunoglobulins decreased the incidence of erythrocyte fetal-maternal incompatibility (EFMI) in the Rhesus system. Few recent studies have investigated the situation of anti-D prophylaxis in Tunisia and its effects on maternal and neonatal health. The aim of this study was therefore to analyze the situation of anti-D prophylaxis in Tunisia to detect defects and propose solutions. We conducted a retrospective descriptive study of IFME cases in the rhesus system in the Department of Medicine and Neonatal Resuscitation of the Tunis Maternity and Neonatology Center (CMNT) during an 8-year period from 1 January 2006 to 31 December 2015. We collected 51 cases of IFME. The prevalence of IFME was 3.4 per 10 000 live births; 41 % of the patients were transferred in utero and they were from the northwest of the country (17 %). The rate of women with a history of at least one spontaneous miscarriage (SCF) was 45 %. In 42 % of the cases, pregnancies were monitored at local clinics. The search for irregular agglutinins (RAI) was performed in 86 % of the women studied. RAI was positive in 97 % of the cases. Anti-D prophylaxis was correctly performed in only 27 % of the cases. The systematic prophylaxis of the third trimester and a systematic Kleihauer test must be combined with postpartum prophylaxis to better identify dysfunctions and improve the application of the recommendations.

Harmless Pregnancy-Induced Warm Autoantibodies to Red Blood Cells

Background

There is little information concerning the development and significance of autoantibodies (aab) to red blood cells (RBCs) during pregnancy.

Methods

Unselected pregnant women were routinely screened for the presence of unexpected antibodies to RBCs using standard techniques.

Results

Between 2009 and 2013, 153,612 pregnant women were tested. The antibody screening test was positive in 1,721 women (1.12%). In 1,602 (1.04%) cases, immune and/or non-immune alloantibodies and cold-reactive aab were detected, whereas warm-reactive aab were found in 119 women (0.08%). In almost all cases, warm-reactive aab belonged to the IgG class. No evidence of the presence of significant haemolysis in affected women was observed.

Conclusion

Pregnant women may rarely develop aab to RBCs, which do not appear to cause haemolytic anaemia. Further clarification is required on the reasons behind the development of these aab and their clinical insignificance.

Reassessing critical maternal antibody threshold in RhD alloimmunization: a 16-year retrospective cohort study

OBJECTIVE

To determine the critical maternal antibody threshold for specialist referral in cases of RhD alloimmunization.

METHODS

This was a retrospective cohort study, covering a 16-year period at the national tertiary fetal medicine center for management of alloimmunization. Data concerning RhD alloimmunized pregnant women were extracted from an institutional database and maternal anti-D antibody levels were cross-checked with the national reference laboratory. Fetal hemoglobin (Hb) levels were determined only at the first intrauterine transfusion (IUT) and were compared with the pretransfusion maternal anti-D antibody level (IU/mL). Sensitivity, specificity and positive and negative predictive values of maternal antibody thresholds for detecting moderate to severe (Hb ≤ 0.64MoM) fetal anemia were calculated.

RESULTS

Between 1996 and 2011, 66 women underwent a first IUT for RhD alloimmunization at our institution. The highest serum anti-D antibody level was extracted for 208 RhD alloimmunized women who did not require IUT during the last 10 years of the study period. The traditional maternal antibody threshold of > 15 IU/mL failed to detect 20% of cases of moderate to severe fetal anemia. The ≥ 4 IU/mL threshold had 100% sensitivity but a 45% false-positive rate. The optimal anti-D antibody threshold for specialist referral in our population was ≥ 6 IU/mL; at this level, no case of moderate to severe anemia was missed and specificity was 61%. Use of this threshold would have eliminated 10% of referrals to our fetal medicine unit without compromising fetal outcomes.

CONCLUSIONS

Setting the critical maternal RhD antibody level at >15 IU/mL does not provide sufficient sensitivity. The lower threshold of ≥ 4 IU/mL, though sensitive, is associated with a 45% false-positive rate. In our population, a threshold of ≥ 6 IU/mL minimizes false-positive referrals while maintaining 100% sensitivity for moderate to severe fetal anemia.

Genetics of transfusion recipient alloimmunization: can clues from susceptibility to autoimmunity pave the way?

The search for genetic determinants of alloimmunization in sickle cell disease transfusion recipients was based on two premises: i) that polymorphisms responsible for stronger immune and/or inflammatory responses and hemoglobin β(S) mutation were co-selected by malaria; and ii) that stronger responder status contributes to development of lupus. We found a marker of alloimmunization in the gene encoding for Ro52 protein, also known as Sjögren syndrome antigen 1 (SSA1) and TRIM21. Surprisingly, the nature of the association was opposite of that with lupus; the same variant of a polymorphism (rs660) that was associated with lupus incidence was also associated with induction of tolerance to red blood cell antigens during early childhood. The dual function of Ro52 can explain this apparent contradiction. We propose that other lupus/autoimmunity susceptibility loci may reveal roles of additional molecules in various aspects of alloimmunization induced by transfusion as well as during pregnancy.

Thrombocytopenia, bleeding, and use of platelet transfusions in sick neonates

Survival rates for infants born prematurely have improved significantly, in part due to better supportive care such as RBC transfusion. The role of platelet transfusions in neonates is more controversial. Neonatal thrombocytopenia is common in premature infants. The primary causal factors are intrauterine growth restriction/maternal hypertension, in which the infant presents with thrombocytopenia soon after birth, and sepsis/necrotizing enterocolitis, which are the common morbidities associated with thrombocytopenia in neonates > 72 hours of age. There is no evidence of a relationship between platelet count and occurrence of major hemorrhage, and cardiorespiratory problems are considered the main etiological factors in the development of intraventricular and periventricular hemorrhage in the neonatal period. Platelet transfusions are used commonly as prophylaxis in premature neonates with thrombocytopenia. However, there is widespread variation in the pretransfusion thresholds for platelet count and evidence of marked disparities in platelet transfusion practice between hospitals and countries. Platelet transfusions are biological agents and as such are associated with risks. Unlike other patient groups, specifically patients with hematological malignancies, there have been no recent clinical trials undertaken comparing different thresholds for platelet transfusion in premature neonates. Therefore, there is no evidence base with which to inform safe and effective practice for prophylactic platelet transfusions. There is a need for randomized controlled trials to define the optimal use of platelet transfusions in premature neonates, who at present are transfused heavily with platelets.

Solving the dilemma of prevention of red cell alloimmunization

Evaluation of: Stephen J, Cairns LS, Pickford WJ, Vickers MA, Urbaniak SJ, Barker RN. Identification, immunomodulatory activity and immunogenicity of the major helper T cell epitope on the K blood group antigen. Blood 119(23), 5563-5574 (2012). Alloimmunization to blood group antigens is a major concern in transfusion medicine. This occurs when antigen-mismatched blood is transfused into a recipient lacking a red blood cell antigen that is expressed on the donor red blood cells. Alloimmunization in this case can result in future problems in finding compatible blood for transfusion and can cause hemolytic transfusion reactions. Alloimmunization can also occur in instances where a mother lacks a red blood cell antigen that is carried by the fetus. In these cases, alloimmunization can result in an antibody that can cross the placenta and cause moderate-to-severe problems in the fetus or newborn due to hemolytic anemia and/or inhibition of hematopoiesis. This is called hemolytic disease of the fetus and newborn. Stephen et al. describe a unique approach to producing a peptide tolerogen to prevent alloimmunization to a specific blood group antigen, K, in the Kell blood group system. They identify an immunodominant K peptide and use this peptide to show that it strongly stimulates human T helper cells from K-immunized people in vitro and that it shows efficacy when used as a nasal tolerogen to suppress immunization with K protein in a mouse model. These results open the door for therapies aimed at the prevention and/or treatment of alloimmunization in both a transfusion setting and, importantly, in hemolytic disease of the fetus and newborn.