Antenatal intravenous immunoglobulins in pregnancies at risk of fetal and neonatal alloimmune thrombocytopenia: comparison of neonatal outcome in treated and nontreated pregnancies.

BACKGROUND: Maternal alloantibodies to human platelet antigen-1a can cause severe intracranial hemorrhage in a fetus or newborn. Although never evaluated in placebo-controlled clinical trials, most Western countries use off-label weekly administration of high-dosage intravenous immunoglobulin in all pregnant women with an obstetrical history of fetal and neonatal alloimmune thrombocytopenia. In Norway, antenatal intravenous immunoglobulin is only recommended in pregnancies wherein a previous child had intracranial hemorrhage (high-risk) and is generally not given in other human platelet antigen-1a alloimmunized pregnancies (low-risk). OBJECTIVE: To compare the frequency of anti-human platelet antigen-1a-induced intracranial hemorrhage in pregnancies at risk treated with intravenous immunoglobulin vs pregnancies not receiving this treatment as a part of a different management program. STUDY DESIGN: This was a retrospective comparative study where the neonatal outcomes of 71 untreated human platelet antigen-1a-alloimmunized pregnancies in Norway during a 20-year period was compared with 403 intravenous-immunoglobulin-treated pregnancies identified through a recent systematic review. We stratified analyses on the basis of whether the mothers belonged to high- or low-risk pregnancies. Therefore, only women who previously had a child with fetal and neonatal alloimmune thrombocytopenia were included. RESULTS: Two neonates with brain bleeds were identified from 313 treated low-risk pregnancies (0.6%; 95% confidence interval, 0.2-2.3). There were no neonates born with intracranial hemorrhage of 64 nontreated, low-risk mothers (0.0%; 95% confidence interval, 0.0-5.7). Thus, no significant difference was observed in the neonatal outcome between immunoglobulin-treated and untreated low-risk pregnancies. Among high-risk mothers, 5 of 90 neonates from treated pregnancies were diagnosed with intracranial hemorrhage (5.6%; 95% confidence interval, 2.4-12.4) compared with 2 of 7 neonates from nontreated pregnancies (29%; 95% confidence interval, 8.2-64.1; P=.08). CONCLUSION: The most reliable data hitherto for the evaluation of intravenous immunoglobulins treatment in low-risk pregnancies is shown herein. We did not find evidence that omitting antenatal intravenous immunoglobulin treatment in low-risk pregnancies increases the risk of neonatal intracranial hemorrhage.

A combined effect of anti-HPA-1a and anti-HLA Class I in pregnancy?

BACKGROUND: Maternal anti-human leukocyte antigen (HLA) Class I is commonly detected alongside anti-human platelet antigen (HPA)-1a in fetal and neonatal alloimmune thrombocytopenia (FNAIT). Little is known regarding whether the presence of anti-HLA Class I may exert an additive effect on the risk and severity of FNAIT. METHODS AND MATERIALS: We reanalyzed samples originally collected as part of a large Norwegian screening study on FNAIT during 1995-2004. This study identified and managed 170 pregnancies where the mother was HPA-1a negative and had detectable anti-HPA-1a during pregnancy. Maternal samples from 166 of these pregnancies were rescreened for anti-HLA Class I, revealing 111 (67%) that were antibody positive. Various regression models were used to assess if and how maternal anti-HLA Class I influenced the neonatal platelet count. RESULTS AND CONCLUSIONS: Unadjusted neonatal platelet counts and the frequency of neonatal thrombocytopenia was not significantly affected by the presence of anti-HLA Class I alongside anti-HPA-1a, but results from regression analyses revealed a possible increased risk when the mother was nulliparous. These results warrant further investigation.

Fetal and neonatal alloimmune thrombocytopenia - The Norwegian management model.

In Norway, the management strategy for fetal and neonatal alloimmune thrombocytopenia (FNAIT) has for more than two decades differed from most other countries. The focus of this paper is to describe and discuss the Norwegian FNAIT management program. We recommend antenatal IVIg to women who previously have had a child with FNAIT-induced ICH, and usually not to HPA-1a alloimmunized pregnant women where a previous child had FNAIT, but not ICH. When deciding management strategy, we use not only the obstetric history but also the antenatal anti-HPA-1a antibody level as a tool for risk stratification. The Norwegian National Unit for Platelet Immunology (NNUPI) at the University Hospital of North Norway in Tromsø provides diagnostic and consulting service for the clinicians and the blood banks all over the country, and serves as a national reference laboratory for FNAIT investigations.

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 ß3 that is associated with integrin αIIb (the fibrinogen receptor) on platelets and megakaryocytes. Integrin ß3 is also associated with integrin αV forming the αVß3 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.

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

Human platelet Ag (HPA)-1a, located on integrin ß3, is the main target for alloantibodies responsible for fetal and neonatal alloimmune thrombocytopenia (FNAIT) in the white population. There are ongoing efforts to develop an Ab prophylaxis and therapy to prevent or treat FNAIT. In this study, an mAb specific for HPA-1a, named 26.4, was derived from an immortalized B cell from an alloimmunized woman who had an infant affected by FNAIT. It is the only HPA-1a-specific human mAb with naturally paired H and L chains. Specific binding of mAb 26.4, both native and recombinant forms, to platelets and to purified integrins αIIbß3 (from platelets) and αVß3 (from trophoblasts) from HPA-1a(+) donors was demonstrated by flow cytometry and surface plasmon resonance technology, respectively. No binding to HPA-1a(-) platelets or integrins was detected. Moreover, the Ab binds with higher affinity to integrin αVß3 compared with a second HPA-1a-specific human mAb, B2G1. Further in vitro experimentation demonstrated that mAb 26.4 can opsonize HPA-1a(+) platelets for enhanced phagocytosis by monocytes, inhibit binding of maternal polyclonal anti-HPA-1a Abs, and weakly inhibit aggregation of HPA-1a-heterozygous platelets, the latter with no predicted clinical relevance. Thus, mAb 26.4 is highly specific for HPA-1a and could potentially be explored for use as a prophylactic or therapeutic reagent for FNAIT intervention and as a phenotyping reagent to identify women at risk for immunization.

Fetal and Neonatal Alloimmune Thrombocytopenia: Management and Outcome of a Large International Retrospective Cohort.

OBJECTIVE: To evaluate the management and outcome of a large international cohort of cases of pregnancies complicated by fetal and neonatal alloimmune thrombocytopenia (FNAIT). METHODS: This was an observational prospective and retrospective cohort study of all cases of FNAIT entered into the international multicentre No IntraCranial Haemorrhage (NOICH) registry during the period of 2001-2010. We evaluated human platelet antigen (HPA) specificity, the antenatal and postnatal interventions performed, and clinical outcome. RESULTS: A total of 615 pregnancies complicated by FNAIT from 10 countries were included. Anti-HPA-1a was the most commonly implicated antibody. Antenatal treatment was administered in 273 pregnancies (44%), varying from intrauterine platelet transfusion to maternal administration of immunoglobulins, steroids, or a combination of those. Intracranial haemorrhage was diagnosed in 23 fetuses or neonates (3.7%). Overall perinatal mortality was 1.14% (n = 7). CONCLUSION: This study presents the largest cohort of cases of FNAIT published. Our data show that antenatal treatment for FNAIT results in favourable perinatal outcome. Over time, in most centres, treatment for FNAIT changed from an invasive to a complete non-invasive procedure.

Pregnant Women at Low Risk of Having a Child with Fetal and Neonatal Alloimmune Thrombocytopenia Do Not Require Treatment with Intravenous Immunoglobulin.

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare condition in which maternal alloantibodies to fetal platelets cause fetal thrombocytopenia that may lead to intracranial hemorrhage (ICH). Off-label intravenous immunoglobulin (IVIg) has for 30 years been the standard of care for pregnant women who previously have had a child with FNAIT. The efficacy of this treatment has never been tested in a placebo-controlled clinical trial. Although IVIg treatment may improve the neonatal outcome in women who previously have had a child with FNAIT-associated ICH, the question is whether IVIg is necessary for all immunized pregnant women at risk of having a child with FNAIT. The results from some recent publications suggest that antenatal IVIg treatment is not necessary for women who are (1) HPA-1a-immunized and HLA-DRB3*01:01-negative, (2) HPA-1a-immunized with a previous child with FNAIT but without ICH or (3) HPA-5b-immunized. If IVIg is not used for these categories of pregnant women, the amount of IVIg used in pregnant women with platelet antibodies would be reduced to less than » of today's use. This is important because IVIg is a scarce resource, and the collection of plasma for the treatment of one pregnant woman is not only extremely expensive but also requires tremendous donor efforts.

What's with the boys? Lower birth weight in boys from HPA-1a alloimmunized pregnancies - New insights from a large prospective screening study in Poland.

Fetomaternal incompatibility in human platelet antigens (HPAs) can cause maternal alloimmunization, which in turn may lead to thrombocytopenia with or without intracranial hemorrhage (ICH) in the fetus or newborn. Retrospective studies suggest that boys from alloimmunized mothers may have higher risk of ICH and lower birth weight than girls. The objective of this study was to assess how maternal HPA-1a alloimmunization, sex of the neonate and birth weight relates in a large prospective cohort. Through a national screening study in Poland (PREVFNAIT) involving HPA-1 typing of 24,259 pregnant women during 2013-2017, 606 HPA-1a negative pregnant women and their offspring were identified and included. Various multivariate models were used to assess if and how maternal HPA-1a alloimmunization status was associated with birth weight and risk of having a small for gestational age (SGA) neonate, and if and how sex of the neonate mattered. Most immunized pregnancies had male fetuses (69 %). Women carrying a male fetus had increased likelihood of having an SGA newborn if they were HPA-1a alloimmunized compared to non-immunized mothers. Increasing maternal anti-HPA-1a antibody levels were significantly associated with reduced birth weight and SGA risk among male-fetus pregnancies, but not if the fetus was female. In conclusion, anti-HPA-1a antibodies in a male fetus pregnancy is associated with increased risk of SGA and lower birth weight, especially if the antibody level is high. Sex of the fetus may therefore be considered as a new clinical predictor of more severe FNAIT neonatal outcome.

Risk factors for anxiety and depression among pregnant women during the COVID-19 pandemic: Results of a web-based multinational cross-sectional study.

OBJECTIVE: To assess risk factors for anxiety and depression among pregnant women during the COVID-19 pandemic using Mind-COVID, a prospective cross-sectional study that compares outcomes in middle-income economies and high-income economies. METHODS: A total of 7102 pregnant women from 12 high-income economies and nine middle-income economies were included. The web-based survey used two standardized instruments, General Anxiety Disorder-7 (GAD-7) and Patient Health Questionnaire-9 (PHQ-9). RESULT: Pregnant women in high-income economies reported higher PHQ-9 (0.18 standard deviation [SD], P < 0.001) and GAD-7 (0.08 SD, P = 0.005) scores than those living in middle-income economies. Multivariate regression analysis showed that increasing PHQ-9 and GAD-7 scales were associated with mental health problems during pregnancy and the need for psychiatric treatment before pregnancy. PHQ-9 was associated with a feeling of burden related to restrictions in social distancing, and access to leisure activities. GAD-7 scores were associated with a pregnancy-related complication, fear of adverse outcomes in children related to COVID-19, and feeling of burden related to finances. CONCLUSIONS: According to this study, the imposed public health measures and hospital restrictions have left pregnant women more vulnerable during these difficult times. Adequate partner and family support during pregnancy and childbirth can be one of the most important protective factors against anxiety and depression, regardless of national economic status.

Toward a prophylaxis against fetal and neonatal alloimmune thrombocytopenia: induction of antibody-mediated immune suppression and prevention of severe clinical complications in a murine model.

BACKGROUND: Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a severe bleeding disorder caused by maternal antibody-mediated destruction of fetal or neonatal platelets (PLTs). Results from our recent large screening study suggest that the pathophysiology of FNAIT is more similar to hemolytic disease of the fetus and newborn (HDFN) than previously thought. Immunization against HPA-1a might therefore be preventable by a prophylactic regimen of inducing antibody-mediated immune suppression (AMIS), which has been documented to be a useful prophylaxis against HDFN. This preclinical proof-of-concept study investigated whether passive administration of anti-ß3 integrin could induce AMIS and thereby prevent clinical complications of FNAIT. STUDY DESIGN AND METHODS: A murine model of FNAIT using ß3 integrin (GPIIIa)-deficient (ß3-/-) mice was employed for this study. AMIS in ß3-/- mice was induced by intravenous administration of human anti-HPA-1a immunoglobulin G or murine anti-ß3 antisera given as prophylaxis after transfusion of HPA-1a-positive human PLTs or murine wild-type PLTs, respectively. RESULTS: AMIS against both human and murine PLT antigens was induced using this prophylactic approach, reducing the amount of maternal PLT antibodies by up to 90%. Neonatal PLT counts were significantly increased and pregnancy outcome was improved in a dose-dependent manner. The incidence of intracranial hemorrhage, miscarriage, and dead-born pups in mice receiving high-dose prophylaxis was reduced to that of normal controls. We also observed that the severity of thrombocytopenia inversely correlated with birth weight. CONCLUSION: This work conceptually proves that prophylactic administration of PLT antibodies induces AMIS and prevents poor pregnancy outcome in FNAIT.

Platelet antibodies and fetal growth: maternal antibodies against fetal platelet antigen 1a are strongly associated with reduced birthweight in boys.

OBJECTIVE: To assess whether maternal HPA 1a alloimmunization is associated with birthweight. DESIGN: A retrospective observational cohort study. SETTING: The national reference laboratory for clinical platelet immunology at a university hospital. POPULATION: 165 HPA 1a incompatible pregnancies identified from a recent screening study of 100 448 women (124 pregnancies) and the national reference laboratory for clinical platelet immunology (41 pregnancies). METHODS: A linear mixed model analysis was used to assess whether maternal anti-HPA 1a antibodies were associated with birthweight. A generalized linear model was used to assess maternal anti-HPA 1a antibodies as risk factor for small-for-gestational age neonates. Both models were adjusted for gestational age at time of delivery, maternal age, parity, smoking habits during pregnancy, preeclampsia, diabetes mellitus and fetal sex. MAIN OUTCOME MEASURES. Maternal anti-HPA 1a antibody as risk factor of reduced birthweight and small-for-gestational age neonates. RESULTS: The level of maternal anti-HPA 1a antibodies was significantly associated with birthweight and risk of small-for-gestational age neonates after correcting for confounding variables (p<0.001). However, this association was only significant for boys. When the mother had high levels of anti-HPA 1a antibodies during pregnancy, the adjusted mean birthweight in boys was 530g lower compared with anti-HPA 1a antibody negative pregnancies (p<0.001). CONCLUSIONS: A linear relation between maternal anti-HPA 1a antibody levels and reduced birthweight in boys was demonstrated. Reduced birthweight should be considered a possible complication of fetal and neonatal alloimmune thrombocytopenia.

Postnatal treatment for children with fetal and neonatal alloimmune thrombocytopenia: a multicentre, retrospective, cohort study.

BACKGROUND: Children affected by fetal and neonatal alloimmune thrombocytopenia (FNAIT) are at risk of severe intracranial haemorrhage. Management in the postnatal period is based on sparse evidence. We aimed to describe the contemporary management and outcomes of patients with FNAIT in high-income countries. METHODS: In this multicentre, retrospective, cohort study, we set up a web-based registry for the collection of deidentified data on the management and course of neonates with FNAIT. Eight centres from seven countries (Australia, Norway, Slovenia, Spain, Sweden, the Netherlands, and the USA) participated. Eligibility criteria comprised neonates with FNAIT being liveborn between Jan 1, 2010, and Jan 1, 2020; anti-human platelet antigen (HPA) alloantibodies in maternal serum; confirmed maternal and fetal HPA incompatibility; and bleeding detected at antenatal ultrasound, neonatal thrombocytopenia (<150 × 109 platelets per L), or both in the current or previous pregnancy. Clinical data were retrieved from local medical records of the first neonatal admission and entered in the registry. The key outcome was the type of postnatal treatment given to neonates with FNAIT. Other outcomes were daily median platelet counts in the first week of life, median platelet count increment after first unmatched versus first matched transfusions, and the proportion of neonates with mild or severe bleeding. FINDINGS: 408 liveborn neonates with FNAIT were entered into the FNAIT registry, of whom 389 from Australia (n=74), Norway (n=56), Slovenia (n=19), Spain (n=55), Sweden (n=31), the Netherlands (n=138), and the USA (n=16) were included in our analyses. The median follow-up was 5 days (IQR 2-9). More neonates were male (241 [64%] of 379) than female (138 [36%]). Severe thrombocytopenia (platelet count <50 × 109 platelets per L) was reported in 283 (74%) of 380 neonates, and extreme thrombocytopenia (<10 × 109 platelets per L) was reported in 92 (24%) neonates. Postnatal platelet count nadir was higher in the no-treatment group than in all other groups. 163 (42%) of 389 neonates with FNAIT received no postnatal treatment. 207 (53%) neonates received platelet transfusions, which were either HPA-unmatched (88 [43%] of 207), HPA-matched (84 [41%]), or a combination of both (35 [17%]). The proportion of neonates who received HPA-matched platelet transfusions varied between countries, ranging from 0% (Slovenia) to 63% (35 of 56 neonates; Norway). Postnatal intravenous immunoglobulin treatment was given to 110 (28%) of 389 neonates (alone [n=19] or in combination with platelet transfusions [n=91]), with the proportion receiving it ranging from 12% (17 of 138 neonates; the Netherlands) to 63% (ten of 16 neonates; the USA) across countries. The median platelet increment was 59 × 109 platelets per L (IQR 35-94) after HPA-unmatched platelet transfusions and 98 × 109 platelets per L (67-134) after HPA-matched platelet transfusions (p<0·0001). Severe bleeding was diagnosed in 23 (6%) of 389 liveborn neonates, with one having a severe pulmonary haemorrhage and 22 having severe intracranial haemorrhages. Mild bleeding was diagnosed in 186 (48%) neonates. INTERPRETATION: Postnatal management of FNAIT varies greatly between international centres, highlighting the absence of consensus on optimal treatments. Our data suggest that HPA-matched transfusions lead to a larger median platelet count increment than HPA-unmatched transfusions, but whether HPA matching is also associated with a reduced risk of bleeding remains unknown. FUNDING: Sanquin.

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.

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.

Fetal exposure to maternal human platelet antigen-1a does not induce tolerance. An analytical observational study.

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a disease that may cause severe bleeding complications with risk of perinatal death or lifelong disability. The main cause of FNAIT is maternal antibodies against human platelet antigen (HPA)-1a. Both fetomaternal bleeding and transplacental trafficking of fetal cells during pregnancy could be the cause of alloimmunization. Persistence of fetal cells in the mother (fetal microchimerism) and maternal cells in the child (maternal microchimerism) are well-recognized phenomena. Thus, it could be envisaged that fetal exposure to the HPA-1a antigen could tolerize an HPA-1a negative female fetus and prevent production of anti-HPA-1a antibodies later in life if she becomes pregnant with an HPA-1a positive fetus. The objective of the current study was to assess if the risk of producing anti-HPA-1a antibodies and the severity of neonatal thrombocytopenia in HPA-1a negative women with HPA-1a positive mothers (i.e. the mother is HPA-1a/b), was lower than in HPA-1a negative women with HPA-1a negative mothers. HPA-1a negative women with HPA-1a antibodies, identified from a Norwegian screening study (1996-2004), where HPA-1 genotype of their mothers was available, were included in the study. The frequency of HPA-1a positive mothers to HPA-1a immunized daughters were compared to the calculated frequency in the general population. We did not find any difference in the frequency of HPA-1ab among mothers to daughters with HPA-1a antibodies as compared with the general population. Furthermore, acknowledging sample-size limitations, we neither found an association between the mothers' HPA type and their daughters' anti-HPA-1a antibody levels or any difference between the two groups of mothers (HPA-1ab vs HPA-1bb), with respect to frequency of thrombocytopenia in the children of their daughters with HPA-1a antibodies. Hence, there was no indication of tolerance against fetal HPA-1a antigen in HPA-1bb women who had been exposed to HPA-1a antigen during fetal development.

Unraveling the role of maternal anti-HLA class I antibodies in fetal and neonatal thrombocytopenia-Antibody specificity analysis using epitope data.

Anti-HLA class I antibodies have been suggested as a possible cause of fetal and neonatal alloimmune thrombocytopenia (FNAIT). The aim of this study was to characterize maternal anti-HLA class I alloantibodies in suspected cases of FNAIT. The study population consisted of all nationwide referrals of neonates with suspected FNAIT to the National Unit for Platelet Immunology in Tromsø, Norway, during 1998-2009 (cases), and 250 unselected pregnancies originally included in a prospective study (controls). Inclusion criterion was a positive screening for maternal anti-HLA class I antibodies. Neonates with other identifiable causes of thrombocytopenia, including maternal anti-human platelet antigens (HPA) antibodies, were excluded. Ultimately, 50 cases with suspected FNAIT were compared with 60 controls. The median neonatal platelet count nadir among cases was 24×109/L (range 4-98×109/L). Five children (10%) were reported to have intracranial hemorrhage. Maternal and neonatal HLA class I genotype was available for 33 mother/child pairs (66%). Immunization was not tied to any particular HLA class I antigen. Using epitope mapping, we could demonstrate that the maternal anti-HLA class I antibodies were specific towards mismatched paternally-inherited fetal epitopes, with little reactivity towards any third-party epitopes. Antibody reactivity patterns were similar to those found among controls, although the mean fluorescence intensities (MFI) among cases were significantly higher. This study demonstrates the value of using data on HLA epitope expression, instead of HLA antigens, to examine alloimmune responses in connection with neonatal thrombocytopenia. Our findings support the idea that maternal anti-HLA class I antibodies are involved in FNAIT.

Maternal anti-platelet ß3 integrins impair angiogenesis and cause intracranial hemorrhage.

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening disease in which intracranial hemorrhage (ICH) is the major risk. Although thrombocytopenia, which is caused by maternal antibodies against ß3 integrin and occasionally by maternal antibodies against other platelet antigens, such as glycoprotein GPIbα, has long been assumed to be the cause of bleeding, the mechanism of ICH has not been adequately explored. Utilizing murine models of FNAIT and a high-frequency ultrasound imaging system, we found that ICH only occurred in fetuses and neonates with anti-ß3 integrin-mediated, but not anti-GPIbα-mediated, FNAIT, despite similar thrombocytopenia in both groups. Only anti-ß3 integrin-mediated FNAIT reduced brain and retina vessel density, impaired angiogenic signaling, and increased endothelial cell apoptosis, all of which were abrogated by maternal administration of intravenous immunoglobulin (IVIG). ICH and impairment of retinal angiogenesis were further reproduced in neonates by injection of anti-ß3 integrin, but not anti-GPIbα antisera. Utilizing cultured human endothelial cells, we found that cell proliferation, network formation, and AKT phosphorylation were inhibited only by murine anti-ß3 integrin antisera and human anti-HPA-1a IgG purified from mothers with FNAIT children. Our data suggest that fetal hemostasis is distinct and that impairment of angiogenesis rather than thrombocytopenia likely causes FNAIT-associated ICH. Additionally, our results indicate that maternal IVIG therapy can effectively prevent this devastating disorder.