Fetal cells in the maternal circulation: isolation by multiparameter flow cytometry and confirmation by polymerase chain reaction

Single Circulating Fetal Trophoblastic Cells Eligible for Non Invasive Prenatal Diagnosis: the Exception Rather than the Rule

Non-Invasive Prenatal Diagnosis (NIPD), based on the analysis of circulating cell-free fetal DNA (cff-DNA), is successfully implemented for an increasing number of monogenic diseases. However, technical issues related to cff-DNA characteristics remain, and not all mutations can be screened with this method, particularly triplet expansion mutations that frequently concern prenatal diagnosis requests. The objective of this study was to develop an approach to isolate and analyze Circulating Trophoblastic Fetal Cells (CFTCs) for NIPD of monogenic diseases caused by triplet repeat expansion or point mutations. We developed a method for CFTC isolation based on DEPArray sorting and used Huntington’s disease as the clinical model for CFTC-based NIPD. Then, we investigated whether CFTC isolation and Whole Genome Amplification (WGA) could be used for NIPD in couples at risk of transmitting different monogenic diseases. Our data show that the allele drop-out rate was 3-fold higher in CFTCs than in maternal cells processed in the same way. Moreover, we give new insights into CFTCs by compiling data obtained by extensive molecular testing by microsatellite multiplex PCR genotyping and by WGA followed by mini-exome sequencing. CFTCs appear to be often characterized by a random state of genomic degradation.

Fetal Gender Determination and BclI Polymorphism Using Nucleated Erythrocytes in Maternal Blood

This study demonstrated determination of fetal gender from nucleated red blood cells (NRBCs) in maternal blood and attempted to apply prenatal diagnosis of hemophilia A using BclI DNA polymorphism. Venous blood was drawn from 20 pregnant women, and NRBCs were recovered by magnetic activated cell sorting and anti-GPA (glycophorin A) immunostaining. After microdissector isolation of the NRBCs, primer extension preamplification (PEP) and nested PCR of the amelogenin gene were performed to determine fetal gender. We also performed PEP and nested PCR of BclI polymorphism to verify the validity of prenatal diagnosis of hemophilia A. DNA amplification was achieved in 107 cells (51.9%) and fetal gender determined with 65.0% accuracy. Unfortunately, we could not verify the validity within the scope of this study. However, in a larger number of cases that are informative in BclI polymorphism, we will be able to identify patients affected by hemophilia A using fetal NRBCs in maternal blood.

The promise of fetal cells in maternal blood

Delaying childbirth increases the proportion of advanced maternal age pregnancies. This increases the number of pregnancies requiring invasive prenatal testing. Prenatal diagnosis of chromosomal aneuploidies and monogenic disorders requires fetal cells obtained through invasive procedures (i.e. chorionic villus sampling and amniocentesis). These procedures carry a risk of fetal loss, which causes anxiety to at-risk couples. Intact fetal cells entering maternal circulation have raised the possibility of non-invasive prenatal diagnosis. Rarity of fetal cells, however, has made it challenging. Fetal nucleated red blood cells are ideal candidate target cells because they have limited lifespan, contain true representation of fetal genotype, contain specific fetal cell identifiers (embryonic and fetal globins), and allow interrogation with chromosomal fluorescence in-situ hybridisation and possibly with array comparative genomic hybridisation. The utility of fetal nucleated red blood cells in non-invasive prenatal diagnosis has not reached clinical application because of the inconsistencies in enrichment strategies and rarity of cells.

Prenatal diagnosis: update on invasive versus noninvasive fetal diagnostic testing from maternal blood

The modern obstetrics care includes noninvasive prenatal diagnosis testing such as first trimester screening performed between 11 and 14 weeks' gestation and second trimester screening performed between 15 and 20 weeks. In these screening tests, biochemical markers are measured in the maternal blood with or without ultrasound for fetal nuchal translucency with reported accuracy of up to 90%. Invasive procedures, including amniocentesis or chorionic villi sampling, are used to achieve over 99% accuracy. During these procedures direct fetal material is examined and, therefore, these tests are highly accurate with the caveat of a small risk for pregnancy loss. Much research now focuses on other noninvasive highly accurate and risk-free tests that will identify fetal material in the maternal blood. Fetal cells and fetal DNA/RNA provide fetal information but are hard to find in an overwhelming background of maternal cells and in the absence of specific fetal cell markers. The most experience has been accumulated with fetal rhesus and fetal sex determination from maternal blood, with an accuracy of up to 100% by using gene sequences that are absent from maternal blood. Although not clinically applicable yet, fetal cells, fetal DNA/RNA and fetal proteomics in combination with cutting edge technology are described to prenatally diagnose aneuploidies and single-gene disorders.

Pregnancy IFN-gamma responses to foetal alloantigens are altered by maternal allergy and gravidity status

BACKGROUND

During pregnancy, variations in maternal-foetal cellular interactions may influence immune programming. This study was carried out to determine if maternal responses to foetal alloantigens are altered by maternal allergic disease and/or previous pregnancies.

METHODS

For this cohort study, peripheral blood was collected from allergic (n = 69) and nonallergic (n = 63) pregnant women at 20, 30, 36-week gestation and 6-week postpartum (pp). Cord blood was collected at delivery. Mixed lymphocyte reactions were used to measure maternal cytokine responses [interleukin-6 (IL-6), IL-10, IL-13 and (interferon-gamma) IFN-gamma] at each time point towards foetal mononuclear cells.

RESULTS

Maternal cytokine responses during pregnancy (20, 30 and 36 weeks) were suppressed compared to the responses at 6-week pp. The ratio of maternal IFN-gamma/IL-13 and IFN-gamma/IL-10 responses were lower during pregnancy. Allergic mothers had lower IFN-gamma responses at each time-point during pregnancy with the greatest difference in responses observed at 36-week gestation. When allergic and nonallergic women were further stratified by gravidity group, IFN-gamma responses of allergic multigravid mothers were significantly lower than nonallergic multigravid mothers during pregnancy.

CONCLUSIONS

During normal pregnancy, peripheral T-cell cytokine responses to foetal alloantigens may be altered by both allergic status of the mother and previous pregnancies. These factors could influence the cytokine milieu experienced by the foetus and will be further explored in the development of allergic disease during early life.

Clinical Potential for Noninvasive Prenatal Diagnosis Through Detection of Fetal Cells in Maternal Blood

Fetal cells circulate in maternal blood and are considered a suitable means by which to detect fetal genetic and chromosomal abnormalities. This approach has the advantage of being noninvasive. Since the early 1990s, nucleated erythrocytes (NRBCs) have been considered good target cells for a number of techniques, including fluorescence-activated cell sorting and magnetic cell sorting, using antibodies such as anti-transferrin receptor and anti-gamma-hemoglobin antibodies, followed by analysis with fluorescence in situ hybridization or polymerase chain reaction. In the late 1990s, the National Institute of Child Health and Human Development Fetal Cell Isolation Study assessed the reliability of noninvasive prenatal diagnosis of fetal aneuploidy using NRBCs isolated from maternal circulation. This study revealed the limitations of NRBC separation using antibodies specific for NRBC antigens. A more recent study has demonstrated the efficiency and success of recovery of NRBCs using a galactose-specific lectin, based on the observation that erythroid precursor cells have a large quantity of galactose molecules on their cell surface. Thus, recent advances in this field enhance the feasibility of this diagnostic method. This review article focuses on various methods of detection of fetal cells within the maternal circulation, as well as the status of previous and current studies and the prospective view for noninvasive prenatal diagnosis using fetal cells from the maternal circulation.

Re-evaluation of risk for Down syndrome by means of the combined test in pregnant women of 35 years or more

OBJECTIVE

Evaluation of combined test in pregnant women 35 years of age and over to detect fetal Down syndrome.

MATERIALS AND METHODS

The study population included 408 pregnant women of 35 years and over, who requested the combined test (nuchal translucency, PAPP-A, free beta hCG, maternal age, cut-off 1:250) before deciding whether to undergo amniocentesis.

RESULTS

The test was positive in 66 women who then requested amniocentesis for fetal karyotype determination; the other women had a negative test and declined amniocentesis. False-positives increased with maternal age from 6.6% at 35 years to about 50% at 40 to 41 and 100% in women over 41. Six cases of Down syndrome and two cases of trisomy 18 were detected. Not a single case of Down syndrome or trisomy 18 was missed, and other chromosome abnormalities were detected as well.

CONCLUSIONS

The application of the combined test reduced the need for invasive testing to only 14% of the studied pregnant population, without missing any of the fetuses with trisomy 21 or 18.

Assessment of efficacy of cell separation techniques used in the enrichment of foetal erythroblasts from maternal blood: triple density gradient vs. single density gradient

The aim of this study was to determine the efficacy of cell separation with single density and triple density-gradient techniques in the yield of foetal erythroblasts isolated from maternal blood. Maternal blood was obtained from 20 singleton pregnancies at 11-14 weeks of gestation immediately before foetal karyotyping by chorionic villus sampling. In each woman, the blood sample was divided into two portions; one portion was used for single density-gradient separation and the other, for triple density-gradient separation. Magnetic cell sorting (MACS) was subsequently performed with anti-CD71/antiglycophorin-A. The enriched erythroblasts were stained with Kleihauer-Giemsa and with fluorescent antibodies for the gamma, epsilon and zeta globin chains. The percentage of foetal cells positive for each stain was calculated. Fluorescence in situ hybridization (FISH) for X- and Y-chromosomes was also performed. Comparison was made in the proportion of enriched foetal cells between the two separation methods for each CD71 and glycophorin-A (GPA) antibody. The percentage of erythroblasts enriched from maternal blood that stained positive for gamma, epsilon and zeta globin chains and with Kleihauer-Giemsa was significantly higher in the triple density-gradient separation fractions compared with the single density-gradient fractions with both anti-CD71 and GPA MACS. FISH analysis for the Y-chromosome confirmed the increase in foetal cell proportion in the triple density-gradient samples. Isolation of foetal erythroblasts from maternal blood using triple density-gradient separation and MACS is more effective with regard to foetal cell yield and purity than single density-gradient separation and MACS.

Long-term feto-maternal microchimerism: nature's hidden clue for alternative donor hematopoietic cell transplantation?

During pregnancy, fetal hematopoietic cells carrying paternal human leukocyte antigens (HLA) migrate into maternal circulation, and, vice versa, maternal nucleated cells can be detected in fetal organs and umbilical cord blood, indicating the presence of bidirectional cell traffic between mother and fetus. By taking advantage of fluorescence in-situ hybridization or polymerase chain reaction-based techniques, researchers recently found that postpartum persistence of such reciprocal chimerism was common among healthy individuals and may sometimes cause tissue chimerism. Although the biological significance of long-lasting feto-maternal microchimerism is unknown, a number of investigations have suggested its association with the development of "autoimmune" diseases such as systemic sclerosis. However, the very common presence of feto-maternal microchimerism among subjects without any autoimmune attack may allow us the more appealing hypothesis that it is an indicator for the acquired immunological hyporesponsiveness to noninherited maternal or fetal HLA antigens. An offspring's tolerance to noninherited maternal antigens has been clinically suggested by the retrospective analysis of renal transplantations or haploidentical hematopoietic stem cell transplantations, and whether postpartum mothers can tolerate paternally derived fetal antigens is an intriguing question. Although an exact linkage between microchimerism and transplantation tolerance is yet to be elucidated, long-term acceptance of a recipient's cell in the donor may have a favorable effect on preventing the development of severe graft-versus-host disease, and the donor cell microchimerism in the recipient might facilitate the graft acceptance. If this concept holds true, HLA-mismatched hematopoietic stem cell transplantation would be more feasible among haploidentical family members mutually linked with feto-maternal microchimerism. Further studies are warranted to investigate the potential role of feto-maternal microchimerism in human transplantation medicine.

Enrichment of rare fetal cells from maternal peripheral blood

The use of fetal cells enriched from maternal peripheral blood for genetic analysis would be a significant advancement for prenatal diagnosis. For over two decades, researchers have employed different techniques (including density gradients, red blood cell lysis, fluorescence-activated cell sorter and magnetic cell separation) to enrich fetal cells that exist in low levels in maternal blood. Despite extensive attempts by various groups and companies, reliable isolation of a sufficient amount of fetal nucleated cells remains a monumental challenge. AVIVA Biosciences Corp. has been developing a technology that allows significant enrichment and recovery of rare fetal cells from maternal peripheral blood. This review will characterize fetal cell types present in maternal blood, technologies used to enrich for these cells and the issues involved with rare fetal cell separation.

Maternal or fetal origin of rhesus monkey (Macaca mulatta) amniotic fluid leukocytes can be identified by polymerase chain reaction using the zinc finger Y gene

Leukocytes can be found in substantial numbers within the intrauterine tissues and amniotic fluid of women, and play a central role in the pathophysiology of infection-related preterm labor by their production of proinflammatory mediators. It remains unclear whether these leukocytes represent a fetal immune response, a maternal response, or a combination of the two. The objective of this study was to develop a test in the rhesus monkey (Macaca mulatta) suitable for determining the percentage of male fetal cells present in a population of leukocytes recovered from blood or amniotic fluid. We found inadequate specificity for rhesus monkey cells using commercial human Y-chromosome paint kits (fluorescence in situ hybridization (FISH)). Human-specific primers for the repetitive Y chromosome DYZ-1 locus employed in the polymerase chain reaction (PCR) produced an unacceptable percentage of false positives. However, we successfully developed a PCR-based test using rhesus-specific primers for the zinc finger Y (ZFY) locus. Densitometry of PCR products from known ratios of male and female adult peripheral leukocytes generated a linear standard curve which provided quantitative results and required only 400 cells per sample. The rhesus beta globin (RBG) gene served as an internal control. The PCR test correctly discriminated the sex of peripheral leukocytes in 20 adult males, 20 adult females, two male fetuses, and one female fetus. Serial samples of amniotic fluid from four chronically catheterized rhesus monkeys bearing male fetuses were used to confirm the utility of this assay for quantifying fetal cells in amniotic fluid. In conclusion, we have developed a PCR test which is suitable for distinguishing male from female cells in adult and fetal blood and in amniotic fluid, which lends itself to a variety of diagnostic and biologic applications in the rhesus monkey and potentially in other nonhuman primates.

[Detection of fetal cells in maternal blood: myth or reality?]

Fetal cells exist in maternal blood and can be utilized for prenatal diagnosis. These cells are present from the sixth week of gestation, with frequency increasing as gestation advances, to many years after the birth. Enrichment of trophoblast cells, erythroblasts and lymphocytes was performed with various density gradient techniques and either magnetic activated or fluorescent activated sorting techniques. The abnormalities were detected by fluorescent in-situ hybridation (FISH) with chromosome-specific DNA probes for the detection of trisomy 21, trisomy 18, Klinefelter syndrome 47 XXY, or by polymerase chain reaction (PCR) for the detection of fetal sex, certain Mendelian disorders (as beta-globin mutations), HLA polymorphisms and fetal Rhesus D blood type. However others studies were necessary to determine the sensitivity and specificity of this technique as a noninvasive alternative to conventional methods of prenatal cytogenetic diagnosis.

Immunoglobulin-secreting cells of maternal origin can be detected in B cell-deficient mice

It is well known that the transfer of immunoglobulins (Igs) from mother to young via milk contributes to the offspring's immune defense. The present study suggests that not only is IgG transmitted to progeny, but that functional maternal Ig-secreting cells (or B cells) can also be transferred to the neonate. We have used B cell-deficient (micro(-/-)) mice and found that a high proportion of them obtain long-lasting, partial reconstitution of their serum Ig levels if born to micro(+/-) mothers. In some of these serum IgG-positive micro(-/-) mice, Ig-secreting cells were detected in spleen and bone marrow. To ensure that cells of maternal origin were present in the progeny, micro(-/-) offspring born to micro(+/-) dams transgenic for green fluorescent protein (GFP) were used. In spleens and bone marrow from some of these micro(-/-)GFP(-/-) offspring, GFP-positive cells were detected, which demonstrated that cells of maternal origin could infiltrate the progeny. In addition, splenic Ig-secreting cells were detected in micro(-/-) mice that were born to micro(-/-) dams and transferred to a lactating micro(+/+) foster dam at birth. This indicates that maternal Ig-secreting cells can be transferred postnatally via milk.

Two independent pathways of maternal cell transmission to offspring: through placenta during pregnancy and by breast-feeding after birth

Cell transmission from mother to offspring was demonstrated using mice with green fluorescent protein (GFP) transgenic markers. GFP transgene heterozygous (+/-) females were mated with GFP (-/-) males, and GFP(+) cells in the GFP (-/-) fetuses generated between them were analysed to assess maternal blood cell transmission to conceptuses in utero. The GFP+ maternal cells were observed throughout the body of the fetuses, as shown by fluorescence stereomicroscopy. Cell entrance into the fetal immune system was shown by histochemical and flow cytometric analyses of fetal organs such as thymus, spleen and liver. The GFP(+) maternal cells persisted in the offspring until postpartum. Next, GFP (-/-) neonates fed by GFP(+) foster mothers were examined to study the transfer of maternal milk leucocytes to offspring through breast-feeding. GFP(+) leucocytes that had infiltrated through the wall of the digestive tract were mainly localized in the livers of neonates. Their accumulation in the livers reached a maximum on days 5 or 6, and these cells became undetectable, as assessed by either histochemistry or flow cytometry, after day 9 of starting foster nursing. Collectively, the present results demonstrate two independent pathways of maternal cell transmission to offspring: transplacental passage during pregnancy and breast-feeding after birth.

First trimester prenatal diagnosis: fetal cells in the maternal circulation

The recovery of fetal cells from the maternal circulation represents a promising approach to noninvasive prenatal diagnosis. Advances in techniques of sensitive molecular genetic analysis have enabled the conclusive demonstration of the presence of fetal cells in maternal blood. In most pregnancies, there are few fetal cells detectable. In some abnormal pregnancies, there appears to be increased fetomaternal transfusion, which facilitates recognition of aneuploid fetal cells. This review article describes general strategies of fetal cell isolation, current technical challenges, and clinical applications that are envisioned for the future. The increased appreciation of fetal cell microchimerism, and its association with complications of pregnancy and the postpartum development of autoimmune disease, is also discussed.

Development, Characterization, and Use of Monoclonal Antibodies Made to Antigens Expressed on the Surface of Fetal Nucleated Red Blood Cells

Background: Current methods for obtaining fetal cells for prenatal diagnosis are invasive and carry a small (0.5–1.0%) but definite risk of miscarriage. An attractive alternative would be isolation of fetal cells from peripheral maternal blood using antibodies with high specificity and avidity.

Methods: To generate antibodies, we purified nucleated red blood cells (NRBCs) from fetal livers and used them as the immunogen to generate monoclonal antibodies (mAbs) directed against surface antigens.

Results: The four antibodies recognized at least two conformationally sensitive epitopes of the transferrin receptor. Isolation of NRBCs from 252 maternal blood samples using these antibodies in magnetic activated cell sorting after an initial density gradient centrifugation yielded 0–419 NRBCs per 25 mL of maternal blood. One antibody, 2B7.4, not only isolated the highest number of NRBCs (>10 in 90% of the samples) but also isolated these NRBCs in 78 consecutive maternal samples.

Conclusion: Antibody 2B7.4 shows promise for the isolation of NRBCs from maternal blood and should allow studies concerning the source of these cells, fetal vs maternal, and the factors controlling their prevalence.

Investigation of maternal blood enriched for fetal cells: role in screening and diagnosis of fetal trisomies

Prenatal diagnosis of chromosomal abnormalities relies on assessment of risk followed by invasive testing in the group with highest risk. Assessment of risk by a combination of maternal age and fetal nuchal translucency and invasive testing in the 5% of the population with the highest risk would identify about 80% of trisomy 21 pregnancies. Preliminary reports suggest that chromosomal abnormalities can also be diagnosed by fluorescent in situ hybridization (FISH) in maternal blood enriched for fetal cells. This study examines the potential role of this method on the prenatal diagnosis of fetal trisomies. Maternal blood was obtained before invasive testing in 230 pregnancies at 10-14 weeks of gestation. After enrichment for fetal cells, by triple density centrifugation and anti-CD71 magnetic cell sorting, FISH was performed and the proportion of cells with positive signals in the chromosomally normal and abnormal groups was determined. Fetal karyotype was normal in 150 cases and abnormal in 80 cases, including 36 with trisomy 21. Using a 21 chromosome-specific probe, three-signal nuclei were present in at least 5% of the enriched cells from 61% of the trisomy 21 pregnancies and in none of the normal pregnancies. For a cut-off of 3% of three-signal nuclei the sensitivity for trisomy 21 was 97% for a false positive rate of 13%. Similar values were obtained in trisomies 18 and 13 using the appropriate chromosome-specific probe. Examination of fetal cells from maternal blood may provide a noninvasive prenatal diagnostic test for trisomy 21 with the potential of identifying about 60% of affected pregnancies. Alternatively, this technique can be combined with maternal age and fetal nuchal translucency as a method of selecting the high-risk group for invasive testing. Potentially, 80% of trisomy 21 pregnancies could be identified after invasive testing in less than 1% of the pregnant population.

The application of individualised fetal growth curves

Individually adjusted or 'customised' growth charts aim to optimise the assessment of fetal growth by taking individual variation into account, and by projecting an optimal curve which delineates the potential weight gain in each pregnancy. This results in an increased detection rate of true growth restriction and a reduction in false positive diagnoses for IUGR. An adjustable standard can apply across geographical boundaries, as individual variation exceeds that between different maternity populations.

Prenatal diagnosis with use of fetal cells isolated from maternal blood: five-color fluorescent in situ hybridization analysis on flow-sorted cells for chromosomes X, Y, 13, 18, and 21

OBJECTIVE

Currently, prenatal diagnosis of chromosome abnormalities requires invasive techniques such as amniocentesis and chorionic villus sampling that carry small but finite risks of fetal loss. A noninvasive approach is to isolate fetal cells from maternal blood by flow sorting followed by genetic interphase analysis with fluorescence in situ hybridization. Because the ratio of fetal to maternal cells is relatively low after flow sorting and to detect 90% to 95% of fetal aneuploidies associated with serious birth defects, a 5-color fluorescent in situ hybridization strategy is necessary for simultaneous detection of chromosomes X, Y, 13, 18, and 21 in all flow-sorted nuclei recovered from a specimen.

STUDY DESIGN

Fetal nucleated red blood cells were isolated from maternal blood in 40 cases (10.4 to 27.0 weeks' gestation) by flow cytometry on the basis of positive selection of CD71+ (transferrin receptor), CD45-, and LDS751 staining. Each case was evaluated for 5-color fluorescent in situ hybridization efficiency by determining the percentage of flow-sorted nuclei containing 8 hybridization signals for chromosomes X, Y, 13, 18, and 21.

RESULTS

A total of 42,312 flow-sorted nuclei from maternal blood samples were analyzed. In 5 of 16 (31%) cases with a male fetus, 0.16% of nuclei scored were identified as fetal by the presence of 1 signal each for chromosomes X and Y. Fetal trisomy 21 nuclei were accurately detected in 2 cases with a female fetus, each of which was subsequently confirmed.

CONCLUSIONS

Five-color interphase fluorescent in situ hybridization analysis can be used to effectively analyze rare fetal aneuploid nuclei in enriched flow-sorted cells isolated from maternal blood.

Influence of gestational age on fetal deoxyribonucleic acid retrieval in maternal peripheral blood

OBJECTIVE

We wanted to verify whether gestational age influences the retrieval of fetal deoxyribonucleic acid in maternal blood to identify the best period for maternal blood sampling for a future noninvasive prenatal diagnoses.

STUDY DESIGN

We amplified 81 deoxyribonucleic acid samples extracted from the peripheral blood of 27 pregnant women (18 bearing male fetuses and 9 bearing females) by nested polymerase chain reaction of the Y-specific sequence DYS14. We obtained three blood samples (one per gestational trimester) from each woman. Statistical evaluation was assessed by the McNemar test of symmetry.

RESULTS

Polymerase chain reaction results in male-bearing pregnancies differed significantly between the first and second trimesters and between the second and third trimesters (p < 0.025) in parallel with a decrease in sensitivity in the second trimester (67%) compared with the first (94%) and third trimesters (100%).

CONCLUSIONS

The drop in sensitivity from the first to the second trimester witnesses a variable concentration of fetal cells in maternal blood, with a negative balance in the second trimester. Therefore, to achieve an adequate polymerase chain reaction accuracy, the choice of gestational age is relevant and the first trimester seems to be more suitable than the second trimester.

Frequency of Fetal Cells in Sorted Subpopulations of Nucleated Erythroid and CD34+ Hematopoietic Progenitor Cells From Maternal Peripheral Blood

Fetal cells that circulate in maternal peripheral blood (PB) during pregnancy offer a potential source of nucleated fetal material for noninvasive prenatal diagnosis. Fluorescence-activated cell sorting was used to target two populations of fetal cells: nucleated erythroid cells (NECs; CD71/glycophorin-A+ CD45lo-int CD34−) and hematopoietic progenitor cells (CD34+ cells; CD34++ CD71/glycophorin-A− CD45int). Fetal cells were detected by fluorescence in situ hybridization (FISH) using directly conjugated chromosome X and Y probes in 65% (13 of 20) of the maternal PBs (fetal karyotype 46,XY). The frequency of fetal cells isolated from the NEC and CD34+ fractions was, respectively, 0 to 14 and 0 to 7 cells per 2 × 107 previously frozen maternal cells (≈20 mL of blood). In nonfrozen samples, the yield and recovery of fetal cells was moderately improved. Culturing the CD34+ sorted fractions in serum-free media with cytokines improved the quality of the FISH preparations and resulted in a slight expansion in detectable fetal cells. The frequency of fetal cells isolated from cultured CD34+ fractions was 0 to 35 and 0 to 93 cells per 2 × 107 previously frozen and nonfrozen maternal PB cells, respectively. These results document the isolation, characterization, and enumeration of fetal cells from the maternal periphery that appear to be present in most, but not all, samples analyzed.

Prenatal diagnosis using fetal cells isolated from maternal peripheral blood: a review

Many questions remain about the feasibility of using fetal cells from maternal blood for prenatal diagnosis. Although recently there has been more focus on clinically relevant methods, many studies have been performed using blood drawn after invasive procedures, and over a wide range of gestational ages. For methods to be applicable to clinical use, more work is needed on isolating cells early in pregnancy, when termination is still an option for parents who are found to have an affected pregnancy. It is generally agreed that fetal nucleated erythrocytes are the most efficacious cell type for prenatal diagnosis, but it has not yet been shown definitively whether there is an ideal gestational age for sampling, whether ABO incompatibility might limit availability of fetal cells, or whether the number of cells present might be different in normal versus abnormal pregnancies. PCR has been shown to be a powerful tool in allowing amplification and identification of very small amounts of fetal DNA. However, this is limited to cases in which a specific and unique gene from the father is sought. This means that there is the potential to diagnose many paternally inherited autosomal dominant diseases and some autosomal recessive diseases, in which the parents have different and identifiable mutations. However, when parents are both carriers of the same autosomal recessive mutations, or when the disease is X linked, PCR will not aid in prenatal diagnosis. Cytogenetic analysis of fetal cells by FISH after cell sorting is another potentially useful method of prenatal diagnosis, but requires relatively pure samples of fetal cells or an independent marker that allows easy microscopic identification. The latter might be accomplished by identifying fetal cells through their expression of embryonic hemoglobins or because they contain HLA-G mRNA. In addition, current techniques of cell sorting must be improved so that a higher percentage of fetal cells can be isolated. Currently, the best cell sorting techniques usually produce a maximum purity of 10% fetal cells. Commonly, in normal pregnancies, fewer than 0.1% of the cells isolated after sorting are fetal in origin. Improving the concentration and quantity of fetal cells will improve the accuracy of FISH. Methods such as immunophenotyping that allow the selective identification of fetal cells by microscopy, and can be used in conjunction with FISH, may be extremely valuable because they may allow the genetic analysis of only the few fetal cells within a background preponderance of maternal cells. Although the retrieval of fetal cells from maternal blood is an attractive concept, it must be clearly stated that presently it is only in the investigational phase because of the low sensitivity and specificity. There is no current application for these methods in clinical practice. It remains to be determined whether testing maternal blood for fetal cells or DNA will be used as a screening tool, similar to the maternal serum screening currently in use, or whether the accuracy can be improved to a level such that the techniques can be used diagnostically. Although there are many questions that remain unanswered at this time, the outlook for noninvasive prenatal genetic testing in the future is optimistic.

Improved fetal nucleated erythrocyte sorting purity using intracellular antifetal hemoglobin and Hoechst 33342

Fetal nucleated erythrocytes (FNRBC) flow sorted from maternal peripheral blood, using monoclonal antibodies (mAb) that bind fetal cell surface antigens, are a noninvasive source of fetal DNA for prenatal diagnosis. These mAbs, however, also bind antigens shared by maternal cells. In sorted populations, this results in maternal cell contamination and low fetal cell purities, which complicates genetic analysis by fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR). Fetal hemoglobin, (alpha 2 gamma 2), has been proposed as a useful fetal marker. To improve fetal cell enrichment from maternal blood, we developed an intracellular staining protocol that combines anti-gamma mAb with Hoechst 33342 to identify and flow sort FNRBC. Artificial mixtures of male umbilical cord cells (as a source of fetal hemoglobin) and female adult, non-pregnant peripheral blood mononuclear cells were stained and flow sorted using this protocol. FISH analysis was performed using chromosome X and Y specific probes. Fetal cell purities were calculated by microscope confirmation of anti-gamma staining and counting the number of X and Y signals present after FISH. Results from microscope analyses showed a fetal cell yield of 39-100% and fetal cell purities of 59-73%. These purities are significantly higher than the .001-4.8% previously reported by us in maternal samples using cell surface staining protocols. FISH results demonstrated that 83-100% (mean = 98%) of anti-gamma positive cells were male, whereas 82-100% (mean = 92.5%) of anti-gamma negative cells were female. These results confirmed that the anti-gamma mAb is highly fetal specific. When applied to maternal blood samples, this protocol should lead to increased sensitivity for prenatal diagnosis.

A rapid combined immunocytochemical and fluorescence in situ hybridisation method for the identification of human fetal nucleated red blood cells

Fetal nucleated red blood cells are found in the maternal circulation during pregnancy. If a simple routine method of detection of these cells was developed, it could be used as the basis of non-invasive prenatal diagnosis of fetal genetic disorders. Fetal male and adult female blood were mixed to mimic maternal blood in pregnancy and used to establish a simple technique to unequivocally detect fetal nucleated red blood cells. These were identified by combined immunocytochemistry using a human fetal haemoglobin antibody and a rapid and simple-to-use fluorescence in situ hybridisation method using X and Y chromosome probes. Initial studies using the alkaline phosphatase anti-alkaline phosphatase technique as the first procedure showed that the stain was unstable and unsuitable for in situ hybridisation. An immunoperoxidase technique was found to produce a stable stain resistant to harsh fixation steps required in subsequent in situ hybridisation. This enabled the simultaneous visualisation of immunopositivity and in situ hybridisation signals on the same cell with neither procedure affecting the other's signal quality. We are currently using this procedure to detect a range of endoplasmic reticulum proteins in fetal nucleated red blood cells from maternal blood in an attempt to diagnose disorders of liver protein expression in early pregnancy.

Prenatal diagnosis by analysis of fetal cells in maternal blood

The data accumulated thus far indicate that fetal NRBCs are the target cell type of choice in maternal blood for most investigators, although some groups continue to work with the trophoblast. Reports of persistent circulation of hematopoietic stem cells, lymphoid/myeloid progenitors, and lymphocytes mandate that removal of these cell types must occur before clinical diagnosis of the current pregnancy can be made. In selected cases, accurate detection of fetal aneuploidy has been made from fetal cells in maternal blood; the clinical evaluation sponsored by the National Institute of Child Health and Human Development will determine the sensitivity and specificity of cytogenetic diagnosis in a larger group of pregnant women, but this information will not be available for several years. At present, detection of uniquely fetal, paternally inherited gene polymorphisms or mutations such as the Rh(D) antigen is possible only because the mother lacks these genes; hence, maternal cell contamination does not hinder diagnosis. Currently the presence of large numbers of maternal cells in enriched samples precludes single-gene diagnosis for conditions in which the mother carries a mutant gene, because her cells are preferentially amplified and difficult to distinguish from those of the fetus. It is likely, however, that as techniques of individual fetal cell isolation are perfected, maternal cell contamination will no longer be an issue, and the entire fetal genome will become available for diagnosis and therapy. Pediatricians need to be aware of the progress of research in this field, because fetal cell isolation from maternal blood not only could change prenatal diagnosis but would change the amount of genetic information that arrives with a newborn infant at birth. The ultimate goal of this work is to diagnose noninvasively, in the first trimester, the common fetal aneuploidies and single-gene disorders, to permit in utero treatment, or to allow low-risk pregnant women carrying an abnormal fetus an opportunity for reproductive choice.

Flow sorting of fetal erythroblasts using intracytoplasmic anti-fetal haemoglobin: preliminary observations on maternal samples

Monoclonal antibody to fetal haemoglobin (alpha 2 gamma 2) has been proposed as a fetal-specific reagent. We developed an intracellular staining protocol that combines fluorescein isothiocyanate or phycoerythrin conjugated anti-gamma with the DNA binding dye Hoechst 33342 to identify and flow sort fetal erythroblasts from maternal blood. Our preliminary observations on anti-gamma-positive cells sorted from four different pregnant women are described here, using fluorescence in situ hybridization (FISH) with chromosome-specific probes to identify fetal cells. Our data demonstrate that far fewer candidate fetal cells are sorted with this protocol than by current cell surface staining methods that employ the monoclonal antibody CD71. This results in increased fetal cell sorting purities. With this protocol, standard FISH techniques require modification due to the rigorous fixation with 4 per cent paraformaldehyde. Our initial data indicate the promise of this approach.

Isolating fetal nucleated red blood cells from maternal blood: the Baylor experience--1995

In our previous work we have isolated fetal cells from maternal blood and used fluorescent in situ hybridization (FISH) for chromosome-specific probes to detect aneuploidy. Current efforts in the Baylor College of Medicine programme are focusing on obtaining consistency in flow-sorting methodology and on determining sensitivity and specificity. To this end, systematic evaluation of five glycophorin A (gly A) antibodies all produced agglutination, leading us to abandon the use of gly A antibodies for positive selection of fetal cells. Conversely, we have found LDS-751 to be useful for nuclear selection. CD45 negative selection can best be accomplished by the use of flasks coated with goat antibodies against mouse antibodies. Positive selection by flow sorting for either CD71+ cells or gamma-globin-positive cells seems to be successful. Using these two approaches, we have recently detected male (fetal) cells in pregnancies in which the fetus was 46,XY in 10 of 18 and in 12 of 14 cases, respectively.

Screening for Down's syndrome

Down's syndrome (DS) is the commonest cause of severe mental retardation in children. It is the result of trisomy of chromosome 21 which is usually a random event though it is commoner in older mothers. DS can be diagnosed by chorionic villus sampling (CVS) and amniocentesis followed by karyotyping. Because of the risks associated with these invasive procedures, they can only be offered to a high-risk group. At one time the sole basis for identifying this increased risk was maternal age, but within the past ten years a series of biochemical and ultrasound abnormalities have been shown in DS pregnancies. The biochemical abnormalities include changes in the levels of most fetal and placental products in the maternal circulation. The best-known of these changes are the reduced levels of alphafetoprotein (AFP) and oestriol (E3) and increased levels of human chorionic gonadotrophin (hCG). The mechanism underlying these biochemical phenomena is unknown. Screening programmes involving the measurement of hCG and AFP, with or without additional parameters such as E3, at 15-18 weeks of pregnancy can typically identify 60% or more of cases of DS with a screen-positive rate of 5%. The combined risk derived from the various biochemical parameters, together with maternal age, is calculated by one of a number of computer programmes which have been developed for this purpose. There has been considerable discussion as to the exact biochemical tests which should be used for DS screening. This had led to controversy as to whether measurement of E3 has a place, and whether or not measurement of the free beta-subunit of hCG should replace measurement of the intact molecule. A notable recent development is the suggestion that measurement of the urinary beta-core of the hCG could be a highly discriminatory marker. A number of factors can affect the results of biochemical screening for DS. These include maternal weight, gestational age, ethnic origin, smoking, and diabetes. In addition, abnormal levels of the biochemical products may be found in other chromosome abnormalities.

Mid-trimester fetal sex determination from maternal peripheral blood by fluorescence in situ hybridization without enrichment of fetal cells

To determine the fetal sex on 30 women who were 16-20 weeks pregnant, about 100,000 maternal blood nucleated cells were analysed by means of fluorescence in situ hybridization (FISH) with a Y-chromosome-specific DNA probe. Cells with the hybridization signal were detected in 12 of the 30 women. All the 12 mothers gave birth to a male child. Of the other 18 women who had no Y-positive cells in the peripheral blood, 14 gave birth to a female child and four gave birth to a male child. These false-negative results probably occurred because the number of cells examined was inadequate. The data obtained in this study suggest that fetal sex determination using maternal peripheral blood with FISH is possible and that this diagnostic method will be clinically useful when more cells are analysed.

Isolating fetal cells in maternal circulation for prenatal diagnosis

Fetal cells unequivocally exist in and can be isolated from maternal blood. Erythroblasts, trophoblasts, granulocytes and lymphocytes have all been isolated by various density gradient and flow sorting techniques. Chromosomal abnormalities detected on isolated fetal cells include trisomy 21, trisomy 18, Klinefelter syndrome (47,XXY) and 47,XYY. Polymerase chain reaction (PCR) technology has enabled the detection of fetal sex, Mendelian disorders (e.g. beta-globin mutations), HLA polymorphisms, and fetal Rhesus (D) blood type. The fetal cell type that has generated the most success is the nucleated erythrocyte; however, trophoblasts, lymphocytes and granulocytes are also considered to be present in maternal blood. Fetal cells circulate in maternal blood during the first and second trimesters, and their detection is probably not affected by Rh or ABO maternal-fetal incompatibilities. Emphasis is now directed toward determining the most practical and efficacious manner for this technique to be applied to prenatal genetic diagnosis. Only upon completion of clinical evaluations could it be considered appropriate to offer this technology as an alternative to conventional invasive and non-invasive methods of prenatal cytogenetic diagnosis.

Fetal cells in maternal blood: determination of purity and yield by quantitative polymerase chain reaction

OBJECTIVE

The detection of fetal aneuploidy and gene mutations by analysis of fetal cells in maternal blood has demonstrated the feasibility of noninvasive prenatal diagnosis. Fetal cells are rare in the maternal circulation; all current methods used for their isolation also yield maternal cells. We developed a method that permits a quantitative assessment of the relative numbers of fetal and maternal cells.

STUDY DESIGN

Samples from 40 pregnant women were flow sorted with different monoclonal antibodies. Deoxyribonucleic acid was subsequently purified from candidate fetal cells; polymerase chain reaction was performed with synthetic primers specific for sequences on chromosomes Y and 7.

RESULTS

The maximum number of fetal cells detected was 52 in 1080 maternal cells. Fetal cell purity ranged from 0.001% to 4.8%. Fetal cells were detected with antibodies to CD71, CD36, and glycophorin A.

CONCLUSION

Quantitative polymerase chain reaction enables the determination of the purity and yield of fetal cells remaining after isolation from maternal blood, facilitating rapid comparisons between different cell separation techniques.

Clinical trials and experience: Boston

Our cumulative experience continues to validate the fetal nucleated erythrocyte as the target fetal cell type of choice, primarily because it reflects the cytogenetic status of the current pregnancy. Additional cell types, such as the granulocyte, await further study. Quantitative PCR is a sensitive and useful new method that can facilitate rapid comparisons between cell separation methods or different monoclonal antibodies. It can also be used on patient material to determine final purity of the enriched maternal samples. If the purity is too low, FISH studies will be complicated by the presence of thousands of maternal cells. Our planned studies include an analysis of why aneuploid pregnancies appear to have a higher number of fetal cells in the maternal circulation. We are also studying the timing of the fetomaternal transfer of cells with qPCR analysis of sorted maternal samples drawn weekly from well-dated women. We are continuously improving our methods (both in separations and antibodies) to reach a fetal cell purity of at least 20% for cytogenetic diagnosis by FISH studies. With the knowledge obtained thus far by us and by others, such a goal appears to be achievable within the near future.

Transferrin receptor (CD71) expression on circulating mononuclear cells during pregnancy

OBJECTIVE

We studied transferrin receptor (CD71) expression in peripheral blood mononuclear cells from healthy pregnant women, to determine if a relationship existed between gestational age and circulating CD71+ mononuclear cells.

STUDY DESIGN

Cell suspensions were prepared from venous blood from 139 pregnant women (7 to 26 weeks of gestation), incubated with monoclonal anti-CD71 antibody, and analyzed by flow cytometry.

RESULTS

When only the first sample from each woman was analyzed, extensive biologic variation between women was shown. An apparent biphasic increase in the percentage of CD71+ cells with advancing gestation was suggested. A subgroup of 13 women studied on multiple occasions demonstrated linear increases in CD71+ cells as pregnancy progressed.

CONCLUSIONS

Pregnant women, when compared with each other, may have differences in the baseline number of circulating CD71+ cells. The increases seen in individuals studied repeatedly are likely to reflect maternal hematopoiesis and current fetomaternal transfusion.

Prenatal diagnosis from maternal blood: simultaneous immunophenotyping and FISH of fetal nucleated erythrocytes isolated by negative magnetic cell sorting

Fetal nucleated cells in the maternal circulation constitute a potential source of cells for the non-invasive prenatal diagnosis of fetal genetic abnormalities. We have investigated the use of the Magnetic Activated Cell Sorter (MACS) for enriching fetal nucleated erythrocytes. Mouse monoclonal antibodies specific for CD45 and CD32 were used to deplete leucocytes from maternal blood using MACS sorting, thus enriching for fetal nucleated erythrocytes which do not express either of these antigens. However, significant maternal contamination was present even after MACS enrichment preventing the accurate analysis of fetal cells by interphase fluorescence in situ hybridisation (FISH). To overcome this problem, we used simultaneous immunophenotyping of cells with the mouse antifetal haemoglobin antibody, UCH gamma, combined with FISH analysis using chromosome X and Y specific DNA probes. This approach enables selective FISH analysis of fetal cells within an excess of maternal cells. Furthermore, we have confirmed the potential of the method for clinical practice by a pilot prospective study of fetal sex in women referred for amniocentesis between 13 and 17 weeks of gestation.