In vitro culture of colony forming unit-megakaryocyte (CFU-MK) in fetal alloimmune thrombocytopenia

Neonatal Thrombocytopenia: Differing Characteristics of NAIT Versus Non-NAIT

While neonatal alloimmune thrombocytopenia (NAIT) is the most common cause of severe neonatal thrombocytopenia good clinical predictors are lacking. We analyzed cases of neonatal thrombocytopenia in Schneider Children's Medical Center of Israel to pinpoint qualifiers of NAIT (NAIT+) in comparison to non-NAIT (NAIT-) thrombocytopenia. Patient and maternal data were retrospectively collected on all thrombocytopenic newborns undergoing a workup for NAIT in our tertiary center between 2001 and 2016. Among 26 thrombocytopenic neonates, the mean nadir in NAIT+ patients (25×10 9 /L) was significantly lower than NAIT- patients (64×10 9 /L) ( P <0.001). 61.5% of NAIT+ infants required treatment compared with 23% of non-NAIT ( P =0.015). NAIT+ patients also required more therapeutic modalities than infants with NAIT- thrombocytopenia. Human platelet antigen (HPA)-1a and HPA-5b alloantibodies most frequently caused NAIT. In summary, thrombocytopenia in NAIT+ was significantly more severe compared with NAIT- and more likely to require treatment. In addition, despite the varied ethnic population in Israel, the HPA alloantibodies found in our population were most similar to those common in Western countries. In the absence of rigorous prenatal screening options, we suggest platelet counts below 40 to 50×10 9 /L in a healthy newborn be considered most suggestive for NAIT and warrant urgent NAIT-specific analysis.

Developmental differences between newborn and adult mice in response to romiplostim

Thrombocytopenia is frequent among sick neonates. While most cases are transient, some neonates experience prolonged and severe thrombocytopenia. These infants often pose diagnostic and therapeutic challenges, and may receive large numbers of platelet transfusions. Romiplostim (ROM) is a thrombopoietin (TPO)-receptor-agonist approved for treatment of adults with chronic immune thrombocytopenia (ITP). The immature platelet fraction (IPF) is a novel measure of newly produced platelets, which could aid with the diagnostic evaluation of thrombocytopenic neonates. This study had the following two objectives: (1) compare the response of newborn and adult mice to escalating doses of ROM in vivo and (2) assess the correlation between IPF and megakaryocyte (MK) mass in newborn and adult treated and untreated mice. In the first set of studies, newborn (day 1) and adult mice received a single subcutaneous (SC) dose of ROM ranging from 0 to 300 ng/g, and platelet counts were followed every other day for 14 days. Both sets of mice responded with dose-dependent platelet and IPF increases, peaking on days 5-7 post-treatment, but neonates had a blunted response (2.1-fold compared to 4.2-fold maximal increase in platelet counts, respectively). On day 5 post-treatment with 300 ng/g ROM, MKs in the bone marrow (BM) and spleen of adult mice were significantly increased in numbers and size (p < 0.0001 for both) compared to controls. MKs in the spleen and BM (but not liver) of treated neonates also increased in number, but not in size. The immature platelet count (IPC, calculated as IPF x platelet count) was highly correlated with the MK number and size in neonatal and adult BM and spleen, but not neonatal liver. The lack of response of neonatal liver MKs was not due to a cell-intrinsic reduced responsiveness to TPO, since neonatal liver progenitors were more sensitive to murine TPO (mTPO) in vitro than adult BM progenitor. In vivo treatment of newborn mice with high mTPO doses or with higher doses of ROM (900 ng/g) resulted in peak platelet counts approaching 3-fold of controls. Taken together, our data indicate that newborn mice are less responsive to ROM than adult mice in vivo, due to a combination of likely pharmacokinetic differences and developmental differences in the response of MKs to thrombopoietic stimulation, evidenced by neonatal MKs increasing in numbers but not in size. PK/PD studies in human infants treated with ROM are warranted.

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.

Suppression of in vitro megakaryopoiesis by maternal sera containing anti-HPA-1a antibodies

Incompatibility of the human platelet antigen-1 (HPA-1) system is the most common cause of fetal/neonatal alloimmune thrombocytopenia (F/NAIT) and is thought to be mediated by accelerated clearance of antibody-opsonized fetal platelets. We evaluated the effect of maternal sera containing anti-HPA-1a antibodies (F/NAIT sera) on in vitro megakaryopoiesis. Compared with control maternal sera, 14 out of 17 F/NAIT sera significantly reduced megakaryocyte (MK) number. This finding was associated with increased apoptosis and cell death of early MKs/MK progenitors, but normal maturation and differentiation of surviving MKs. An analysis of platelet counts in infants born to mothers following antenatal intravenous immunoglobulin (IVIG) ± prednisone therapy demonstrated a significant and moderately strong correlation between the MK growth in cultures and the infants' platelet counts at birth. These findings suggest that maternal anti-HPA-1a antibodies can suppress fetal megakaryopoiesis by inducing early cell death and that this influences the neonatal platelet count. Thus, the ability of maternal antibodies to suppress MK growth is a potential predictive factor for the fetal response to maternal IVIG therapy.

Longitudinal thrombopoietin plasma concentrations in fetuses with alloimmune thrombocytopenia treated with intrauterine PLT transfusions

BACKGROUND

The purpose of this study was to describe longitudinal thrombopoietin (TPO) plasma concentrations in fetuses with fetomaternal alloimmune thrombocytopenia (FMAIT).

STUDY DESIGN AND METHODS

Group 1 was the control group, 8 fetuses with normal hematopoiesis. Group 2 consisted of 4 nonthrombocytopenic fetuses with fetomaternal human PLT antigen incompatibility. Group 3 consisted of 14 fetuses with prenatal-diagnosed severe FMAIT owing to human PLT antigen-1a incompatibility. Fetal PLT counts, MoAb-specific immobilization of PLT antigen score, and TPO plasma concentrations were measured in a total number of 94 serial samples taken by cordocentesis before intrauterine PLT transfusion.

RESULTS

Normal fetal TPO plasma concentrations ranged between 15 and 119 pg per mL (Group 1 median, 67 pg/mL). In fetuses with risk of FMAIT but normal PLT counts, TPO concentrations were normal (Group 2 median, 72 pg/mL; range, <15-158 pg/mL). In FMAIT with thrombocytopenia, the median TPO concentration was significantly higher than in Groups 1 and 2 (Group 3 median, 172 pg/mL; range, 15-623 pg/mL; p < 0.001). In the longitudinal analysis, TPO concentrations remained constant (n = 8), peaked only transiently (n = 3), or increased at the end of gestation (n = 3). Elevated TPO concentrations (592 and 623 pg/mL) were detected in one patient, who already had intracranial hemorrhage in utero.

CONCLUSION

TPO concentrations are normal or slightly elevated in FMAIT. Further clinical information can be provided by the longitudinal analysis of TPO concentrations in severe FMAIT.

Fetal and neonatal thrombopoietin levels in alloimmune thrombocytopenia

Thrombopoietin (Tpo) is the main hematopoietic growth factor for platelet production. Plasma Tpo levels in autoimmune thrombocytopenic patients are normal or slightly elevated. Although thrombocytopenia exists, Tpo levels are not increased because the produced megakaryocytes and platelets can bind circulating Tpo, thereby normalizing Tpo levels. In this study, plasma samples from fetuses and neonates with neonatal alloimmune thrombocytopenia (NAIT), a different form of immune thrombocytopenia, were measured. Umbilical cord samples from 50 fetuses before treatment because of severe thrombocytopenia and 51 fetuses after treatment, and peripheral blood samples of 21 untreated newborns with NAIT were analyzed. As controls, plasma Tpo levels were determined in 21 umbilical cord samples of 14 nonthrombocytopenic fetuses with hemolytic disease resulting from red blood cell alloimmunization and in umbilical cord samples of 51 healthy newborns. The values were also compared with the plasma Tpo levels in 193 healthy adults. Mean Tpo levels from the groups of fetuses and neonates, including both NAIT and control plasma, were slightly but significantly elevated compared with levels in healthy adults. Tpo levels in NAIT samples were not significantly different from the levels in hemolytic disease samples or in samples from healthy newborns. Thus, as in autoimmune thrombocytopenic patients, normal Tpo levels are present in NAIT patients.

Fetomaternal alloimmune thrombocytopenia

Thrombocytopenia is the second commonest haematological abnormality in the neonatal period after anaemia due to iatrogenic blood letting. One to four percent of all newborn babies have a platelet count < 150 x 10(9)/l at birth and approximately 20-40% of neonates in intensive care units are affected by neonatal thrombocytopenia. The most common cause of severe neonatal thrombocytopenia is fetomaternal platelet incompatibility and subsequent alloimmunisation. During the last decade recent advances in molecular techniques have led to rapid and efficient methods for diagnosis. Progress in fetal medicine has enabled accurate determination of fetal status, allowing improvements in fetal diagnosis and therapy. Human platelet antigen (HPA)-1a is by far the most frequently involved platelet antigen system in Caucasians accounting for 90% of cases, followed at a much lower frequency by HPA-5b (5-15%) and HPA-3a. The incidence is estimated to be 1 per 2000 to 1 per 5000 live births, but this is low in comparison to the incidence of fetomaternal platelet antigen incompatibility especially for the HPA-1 alloantigen system in the Caucasian population in whom the estimated frequency of HPA-1b1b individuals is 2%. Retrospective and prospective studies have reported that the immunogenetic background is important, and the chance of HPA-1a alloimmunisation is strongly associated with maternal HLA class II DRB3*0101 (DR52a) type. A significant association (p = 0.004) between severe thrombocytopenia and a third trimester antiHPA titre >1:32 has been observed. It is now possible to genotype the fetus or neonate and the parents, which provides confirmation as to which HPA systems are incompatible between the mother and father. Simultaneous genotyping of HPA-1, 2, 3 and 5 can be carried out using the polymerase chain reaction-sequence specific primers (PCR-SSP) protocol, which has been widely used for HLA class II determination. The platelet count may continue to fall during the first 48 h after birth and the risk of intracranial bleeding is highest during this period. The best option is transfusion of specially selected antigen negative compatible donor platelets or if unavailable, maternal washed platelets. Antenatal screening for the most common form of fetomaternal alloimmune thrombocytopenia (FMAIT), due to antiHPA-1a is under consideration, but there is no established method at present. The Scottish National Blood Transfusion Service started a study in August 1999 on 25,000 pregnancies to carry out a cost benefit analysis of routine antenatal screening. The aims of the study are to determine the frequency of HPA-1b homozygosity; monitor antibody titres during pregnancy and confirm correlation of antibody emergence with HLA-DRB3*0101, and finally to access cost effectiveness of routine screening across Scotland. Of 26,509 women screened in three Scottish regions 501 (1.9%) are HPA-1b homozygous and about 9%, of the consented women are antibody positive.

Feto-maternal alloimmune thrombocytopenia: a literature review and statistical analysis

Exploring prognostic factors that determine outcomes in fetomaternal alloimmune thrombocytopenia (FMAIT), a search of Medline was performed covering the years 1966 to April 1998. 376 articles were collected and reviewed; 140 articles contained the case histories of 297 mothers and 433 pregnancies that fulfilled entry criteria. More than 30 data variables were sought from these cases. The data were analysed using SPSS and Arcus Quickstat Biomedical. Nineteen different antigen incompatibilities were documented, the majority being human platelet antigen (HPA)-1a (77.3%), HPA-3a (3.5%) and HPA-5b (3.5%). The relative risk reduction (RRR) in mortality with any intervention was 57% (0.19-0.77) p = 0.009. Treatment of HPA-1a (PlA1) pregnancies with intravenous immunoglobulin (IVIG) increased the likelihood of a neurologically normal outcome, relative risk (RR) 1.68, confidence interval (1.3-2.2) p = 0.0003. Treatment of HPA-1a (PlA1) pregnancies with only antenatal complementary platelet transfusions increased the likelihood of a neurologically normal outcome, RR 1.63 (1.1-2.1) p = 0.01. Despite reviews of more than 400 cases of FMAIT, few prognostic variables are identifiable. Although IVIG appears to reduce the risk of intracranial haemorrhage (ICH), the dosage and timing of IVIG treatment was varied. This study highlights the need for standardised and directed research.

The management of alloimmune neonatal thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAITP), defined as thrombocytopenia (platelet count < 150 x 10(9)/l) due to transplacentally acquired maternal platelet alloantibodies, occurs in approximately 1 per 1200 live births in a Caucasian population. In such a population, the majority (> 75 percent) of cases are due to fetomaternal incompatibility for the platelet specific alloantigen, HPA-1a (P1A1, Zwa). Incompatibility for the HPA-5b (Bra) alloantigen is the next most frequent cause of NAITP in Caucasians; much less common is NAITP due to incompatibility for HLA, blood group ABO or other platelet-specific antigens. In non-Caucasian populations (e.g. Orientals) HPA-1a incompatibility is a rare cause of NAITP and other alloantigens e.g. HPA-4b (Penb, Yuka) are implicated. The greatest clinical challenge relates to the antenatal management of pregnant women alloimmunized to the HPA-1a (P1A1, Zwa) antigen, and particularly the subset of such women who have a history of a previously affected infant with severe thrombocytopenia and/or intracranial hemorrhage (ICH). The risk of antenatal ICH in the fetus of such women is high enough to merit intervention, either weekly infusion of high-dose intravenous immunoglobulin G (IVIG) with or without corticosteroids given to the mother (the preferred approach in North American centres), or repeated in-utero fetal platelet transfusions (the preferred treatment approach in some European centres). Post-natal management of severely affected infants centres on the rapid provision of compatible antigen-negative platelets harvested from the mother or a phenotyped donor. The value of antenatal screening programs to detect 'at risk' alloimmunized women during pregnancy continues to be debated.

Plasma thrombopoietin concentrations in thrombocytopenic and non-thrombocytopenic patients in a neonatal intensive care unit

Thrombocytopenia is a frequent occurrence in the neonatal intensive care unit (NICU), but the role of thrombopoietin (Tpo) in the pathophysiology is unknown. We obtained serial plasma Tpo concentrations in 20 thrombocytopenic neonates in our NICU, and performed bone marrow studies in 15. The initial Tpo levels ranged from undetectable (<41 pg/ml) to 1112 pg/ml and did not correlate with gestational age or platelet count. Neonates with decreased marrow megakaryocytes did not have plasma Tpo levels as high as those reported in adults, particularly in small for gestational age infants (Tpo < 300 pg/ml). In 14/15 neonates followed until resolution, the Tpo concentration decreased as the platelet count increased.

The Natural History of Fetomaternal Alloimmunization to the Platelet-Specific Antigen HPA-1a (PlA1, Zwa) as Determined by Antenatal Screening

Immunization against the human platelet antigen (HPA)-1 alloantigen is the most common cause of severe fetal and neonatal thrombocytopenia. Fetal therapy has substantial risks and its indications need better definition. Of 24,417 consecutive pregnant women, 618 (2.5%) were HPA-1a negative of whom 385 entered an observational study. All were HLA-DRB3*0101 genotyped and screened for anti–HPA-1a. Their partners and neonates were HPA-1 genotyped and the latter were assessed by cord blood platelet counts and cerebral ultrasound scans. Anti–HPA-1a was detected in 46 of 387 pregnancies (12.0%; 95% CI 8.7%-15.2%). All but one were HLA-DRB3*0101 positive (odds ratio 140; 95% CI 19-1035;P< .00001). One baby died in utero, and of 26 HPA-1a–positive babies born to women with persistent antenatal antibodies, 9 were severely thrombocytopenic (8 with a count <10 × 109/L, 1 with a large porencephalic cyst), 10 were mildly thrombocytopenic, whereas 7 had normal platelet counts. Severe thrombocytopenia was significantly associated with a third trimester anti–HPA-1a titer ≥ 1:32 (P = .004), but was not observed in babies of women with either transient or postnatal-only antibodies. HPA-1a alloimmunization complicates 1 in 350 unselected pregnancies, resulting in severe thrombocytopenia in 1:1,200. HPA-1a and HLA-DRB3*0101 typing combined with anti–HPA-1a titration allows selection of the majority of pregnancies at risk of severe thrombocytopenia.

The Natural History of Fetomaternal Alloimmunization to the Platelet-Specific Antigen HPA-1a (PlA1, Zwa) as Determined by Antenatal Screening

Immunization against the human platelet antigen (HPA)-1 alloantigen is the most common cause of severe fetal and neonatal thrombocytopenia. Fetal therapy has substantial risks and its indications need better definition. Of 24,417 consecutive pregnant women, 618 (2.5%) were HPA-1a negative of whom 385 entered an observational study. All were HLA-DRB3*0101 genotyped and screened for anti–HPA-1a. Their partners and neonates were HPA-1 genotyped and the latter were assessed by cord blood platelet counts and cerebral ultrasound scans. Anti–HPA-1a was detected in 46 of 387 pregnancies (12.0%; 95% CI 8.7%-15.2%). All but one were HLA-DRB3*0101 positive (odds ratio 140; 95% CI 19-1035;P< .00001). One baby died in utero, and of 26 HPA-1a–positive babies born to women with persistent antenatal antibodies, 9 were severely thrombocytopenic (8 with a count <10 × 109/L, 1 with a large porencephalic cyst), 10 were mildly thrombocytopenic, whereas 7 had normal platelet counts. Severe thrombocytopenia was significantly associated with a third trimester anti–HPA-1a titer ≥ 1:32 (P = .004), but was not observed in babies of women with either transient or postnatal-only antibodies. HPA-1a alloimmunization complicates 1 in 350 unselected pregnancies, resulting in severe thrombocytopenia in 1:1,200. HPA-1a and HLA-DRB3*0101 typing combined with anti–HPA-1a titration allows selection of the majority of pregnancies at risk of severe thrombocytopenia.

Screening programmes for foetomaternal alloimmune thrombocytopenia

Screening to detect pregnancies at risk of severe thrombocytopenia due to Foetomaternal alloimmunisation to Human Platelet Alloantigens (FAIT) is not yet a standard of care. Screening may be performed antenatally with the possibility of early intervention/therapy. Post natal measurement of cord platelet count may not always prevent fetal damage but may detect severe thrombocytopenia from other causes. Assessment of FAIT screening against World Health Organisation standards for screening programmes helps define areas where knowledge is still lacking and where research effort should be directed. Limitations to screening have been lack of reliable assays for typing and antibody detection, a limited understanding of the relationship between the presence of alloantibodies and clinical disease, inability to predict non-invasively which cases will have a severe outcome, and controversy as to which antenatal therapy is optimal for cases at high risk of a poor outcome.

Maternal DRB1*1501, DQA1*0102, DQB1*0602 haplotype in fetomaternal alloimmunization against human platelet alloantigen HPA-6b (GPIIIa-Gln489)

Fetomaternal incompatibility of platelet alloantigens may lead to alloimmunization and neonatal alloimmune thrombocytopenia (NAIT). Human platelet alloantigen (HPA) 6b, which associates with residue Gln 489 of platelet membrane glycoprotein IIIa, has been described as a cause of NAIT. We have studied the MHC genes of all available family members in the six thus far reported families with a thrombocytopenic newborn and fetomaternal HPA-6b incompatibility. Maternal HPA-6b antibodies could be detected in five mothers to the altogether seven thrombocytopenic male infants. The MHC genes HLA-DRB, -DQA1, -DQB1, -DPB1, TAP1,2 and HSP70-Hom were studied by using polymerase chain reaction (PCR)-based DNA analysis methods. All five mothers with detectable circulating HPA-6b antibodies at the time of delivery shared an identical DRB1*1501, DQA1*0102, DQB1*0602 haplotype. The sixth, HPA antibody negative mother and a HPA-6b-negative mother to a healthy HPA-6b+ child were negative for this haplotype. The frequency of DRB1*15-positive haplotype was increased in immunized mothers (100%) as compared with the general Finnish population (27%), but the association was not statistically significant after correction. We conclude that there is a potential association between the MHC haplotype DRB1*1501, DQA1*0102, DQB1*0602 and alloimmunization to the HPA-6b antigen and that this alloimmunization probably involves different HLA class II molecules from immunization to HPA-1a.

Megakaryocytes and platelets in immune thrombocytopenic purpura

In idiopathic thrombocytopenic purpura, 90% of the patients have an elevated platelet-associated immunoglobulin. The most important related antigens are glycoprotein IIb-IIIa (GPIIb-IIIa) (16.7-83.3%), GPIb-IX (13.3-83%), GPIb (3.3-47.1%) and GPIIIa (21.6-33.3%), and less commonly GPIa-IIa, GPIV and GPV. Other related antigens can be platelet granule membrane protein, phospholipid, intraplatelet and cytoplasmic antigens, and rarely human platelet antigen (HPA) 1a and HLA-DR antigens. The marrow megakaryocytes are usually normal or increased in number with maturation impairment. There are discrepancies regarding megakaryocytopoiesis in vitro. A low dose of heparin could elevate the platelet number in certain cases. The expression of c-sis was reported to be inhibited in ITP, when the plasma beta-thromboglobin-platelet factor 4 level was elevated. In secondary immune thrombocytopenia, platelet antibodies can be (1) alloantibodies against the (HPA) system and (2) autoantibodies most commonly against platelet GPIIb-IIIa or GPIb-IX. Other antigens can be 30-52kDa proteins of the platelet membrane. Platelet survival is usually shortened, and marrow megakaryocytes are normal in number. Megakaryocyte colony-forming units could be reduced.