Central tolerance promoted by cell chimerism.

Historically, successful allotransplantation was only achieved by utilizing powerful immunosuppressive drugs that were exposing the patient to severe opportunistic infections. The thymus of the transplant recipient renders such therapy obligatory as it constitutively blocks self-reactive T cells while allowing alloreactive T cells to mature and populate the periphery. In 1992, a follow-up study revealed the presence of donor leukocytes in long-term transplant survivors. The stable persistence of recipient and donor leukocytes in the transplanted patient, referred to as "chimerism", was considered the reason why in some cases it was even possible to stop immunosuppressive treatment without damaging the transplanted organ. Unfortunately, it quickly became evident that stable, persistent allogeneic chimerism was not easily achievable by design. Recently, a novel approach has been identified to help address this clinical gap in knowledge: Cotransplantation of a donor graft with a thymic organoid populated with donor precursor cells generates stable, long-term chimerism in the recipient. In humanized mice, the implantation of thymic organoids, populated with human donor inducible pluripotent stem cell (iPSC)-derived thymic epithelial cells (TECs) and the same donor CD34+ bone marrow precursors, induces tolerance to human leukocyte antigen (HLA)-matched donor tissues/organs. This technology will allow successful allotransplantation of cells/organs even between Major Histocompatibility Complex (MHC)-noncompatible individuals and allow getting rid of immunosuppressive treatments reducing recipient morbidity.

Chemotherapy for a secondary malignancy nearly restores complete chimerism in an SCID-patient after HSCT.

For patients with inborn errors of immunity (IEI) and other inborn diseases, mixed donor chimerism is a well-accepted outcome of hematopoietic stem cell transplantation (HSCT). Cytoreductive chemotherapy for a secondary malignancy is a potential challenge for the stability of the graft function after HSCT. We report on a boy with X-SCID who developed Ewing sarcoma ten years after HSCT which was successfully treated with cytoreductive chemotherapy, surgery and local radiation. Surprisingly, this treatment had a positive impact on mixed chimerism with an increase of donor-cell proportions from 40% for neutrophils and 75% for non-T-mononuclear cells (MNCs) to >90% for both. T-cell counts remained stable with 100% of donor origin. This is -to our knowledge- the first report on the impact of cytoreductive chemotherapy on post-HSCT mixed chimerism and provides an important first impression for future patients.

Body-wide chimerism and mosaicism are predominant causes of naturally occurring ABO discrepancies.

Routine ABO blood group typing of apparently healthy individuals sporadically uncovers unexplained mixed-field reactions. Such blood group discrepancies can either result from a haematopoiesis-confined or body-wide dispersed chimerism or mosaicism. Taking the distinct clinical consequences of these four different possibilities into account, we explored the responsible cause in nine affected individuals. Genotype analyses revealed that more than three-quarters were chimaeras (two same-sex females, four same-sex males, one sex-mismatched male), while two were mosaics. Short tandem repeat analyses of buccal swab, hair root and nail DNA suggested a body-wide involvement in all instances. Moreover, genome-wide array analyses unveiled that in both mosaic cases the causative genetic defect was a unique copy-neutral loss of heterozygosity encompassing the entire long arm of chromosome 9. The practical transfusion- or transplantation-associated consequences of such incidental discoveries are well known and therefore easily manageable. Far less appreciated is the fact that such findings also call attention to potential problems that directly ensue from their specific genetic make-up. In case of chimerism, these are the appearance of seemingly implausible family relationships and pitfalls in forensic testing. In case of mosaicism, they concern with the necessity to delineate innocuous pre-existent or age-related from disease-predisposing and disease-indicating cell clones.

Immediate major dynamic changes in the T- and NK-cell subset composition after cardiac transplantation.

Early kinetics of lymphocyte subsets involved in tolerance and rejection following heart transplantation (HTx) are barely defined. Here, we aimed to delineate the early alloimmune response immediately after HTx. Therefore, blood samples from 23 heart-transplanted patients were collected before (pre-), immediately (T0), 24 hours (T24), and 3 weeks (3 wks) after HTx. Immunophenotyping was performed using flow cytometry. A significant increase was detected for terminally differentiated (TEMRA) CD4+ or CD8+ T cells and CD56dim CD16+ NK cells immediately after HTx linked to a decrease in naïve CD8+ and CM CD4+ T as well as CD56bright CD16- NK cells, returning to baseline levels at T24. More detailed analyses revealed increased CD69+ CD25- and diminished CD69- CD25- CD4+ or CD8+ T-cell proportions at T0 associated with decreasing S1PR1 expression. Passenger T and NK cells were found at low frequencies only in several patients at T0 and did not correlate with lymphocyte alterations. Collectively, these results suggest an immediate, transient shift toward memory T and NK cells following HTx. Opposite migratory properties of naïve versus memory T and NK cells occurring in the early phase after HTx could underlie these observations and may impinge on the development of allo-specific immune responses.

Molecular Analysis of Parthenogenetic Chimerism in a 46,XX/46,XY Patient with Idiopathic Oligoasthenoteratozoospermia.

INTRODUCTION: Parthenogenetic chimera is an extremely rare condition in human. Very few patients with parthenogenetic chimerism with XX/XY cells have been identified. CASE PRESENTATION: We report the clinical findings and molecular analysis of chimerism with a 46,XX/46,XY karyotype in a patient presenting idiopathic oligoasthenoteratozoospermia (OAT). To clarify the mechanism of chimera formation, short tandem repeat analysis using 21 loci was carried out. Quantitation of alleles in D6S1043, D12S391, fibrinogen alpha chain, and amelogenin revealed double paternal and one maternal genetic contribution to the patient, which is consistent with a parthenogenetic chimerism. The likely mechanism of chimerism formation was also discussed, followed by a literature review. CONCLUSION: This is the first documented case of parthenogenetic chimerism in an adult male with XX/XY cells presenting OAT. Improved cell sampling and more sensitive and specific detection methods are necessary to identify more patients with XX/XY chimerism for systematic studies on this condition in the future.

A case report of a normal fertile woman with 46,XX/46,XY somatic chimerism reveals a critical role for germ cells in sex determination.

Individuals with 46,XX/XY chimerism can display a wide range of characteristics, varying from hermaphroditism to complete male or female, and can display sex chromosome chimerism in multiple tissues, including the gonads. The gonadal tissues of females contain both granulosa and germ cells. However, the specific sex chromosome composition of the granulosa and germ cells in 46,XX/XY chimeric female is currently unknown. Here, we reported a 30-year-old woman with secondary infertility who displayed a 46,XX/46,XY chimerism in the peripheral blood. FISH testing revealed varying degrees of XX/XY chimerism in multiple tissues of the female patient. Subsequently, the patient underwent preimplantation genetic testing (PGT) treatment, and 26 oocytes were retrieved. From the twenty-four biopsied mature oocytes, a total of 23 first polar bodies (PBs) and 10 second PBs were obtained. These PBs and two immature metaphase I (MI) oocytes only displayed X chromosome signals with no presence of the Y, suggesting that all oocytes in this chimeric female were of XX germ cell origin. On the other hand, granulosa cells obtained from individual follicles exhibited varied proportions of XX/XY cell types, and six follicles possessed 100% XX or XY granulosa cells. A total of 24 oocytes were successfully fertilized, and 12 developed into blastocysts, where 5 being XY and 5 were XX. Two blastocysts were transferred with one originating from an oocyte aspirated from a follicle containing 100% XY granulosa cells. This resulted in a twin pregnancy. Subsequent prenatal diagnosis confirmed normal male and female karyotypes. Ultimately, healthy boy-girl twins were delivered at full term. In summary, this 46,XX/XY chimerism with XX germ cells presented complete female, suggesting that germ cells may exert a significant influence on the sexual determination of an individual, which provide valuable insights into the intricate processes associated with sexual development and reproduction.

Can novel methods replace the gold standard chimerism method after allogeneic hematopoietic stem cell transplantation?

After hematopoietic stem cell transplantation, chimerism assay is a useful approach to monitor the success of the transplant and to select the appropriate treatment strategy, such as donor leukocyte infusion or immunosuppressive drug dosage. Short tandem repeat PCR is the method that has been accepted as the gold standard for chimerism. However, it has not yet been sufficient to detect mixed chimerism in patients with minimal residual disease. Simultaneously, recent years have been marked by developing sensitive, high-throughput, and accurate molecular genetic assays. These novel methods have subsequently been adapted for the analysis of post-transplant chimerism. In this review, we discuss the technical features of both novel and conventional gold standard chimerism assays. We also discuss their advantages and disadvantages.

Allogeneic stem cell transplant in primary hemophagocytic lymphohistiocytosis - a single-center experience.

Hemophagocytic Lymphohistiocytosis (HLH) is a rare disorder of immune dysregulation characterized by fever, cytopenias, and splenomegaly. Its primary form poses a therapeutic challenge due to its high fatality when left untreated. We retrospectively analyzed 28 patients who underwent related-donor allogeneic stem cell transplant for primary HLH from 2010 to 2021. Among them were 10 cases of familial HLH, 8 cases of Griscelli syndrome type 2, and 1 case each with PRF1 and STX11 mutations. All the patients underwent transplants with reduced-intensity or myeloablative conditioning and 26 of them achieved neutrophil engraftment at a median of day + 14. The donors were either fully matched (68%) or haploidentical (32%). With a median follow-up of 1 year, overall survival was 68% (n = 19) and disease-free survival was 64.4% (n = 18). OS was better in patients transplanted with a sibling donor (compared to parent donor), who achieved complete donor chimerism, and those transplanted early in the course of the disease (diagnosis to transplant duration less than 6 months).

Cell-Based Therapies Induce Tolerance of Vascularized Composite Allotransplants: A Systematic Review.

INTRODUCTION: Vascularized composite allotransplantation (VCA) is the transplantation of multiple tissue types as a solution for devastating injuries. Despite the highly encouraging functional outcomes of VCA, the consequences of long-term immunosuppression remain the main obstacle in its application. In this review, we provide researchers and surgeons with a summary of the latest advances in the field of cell-based therapies for VCA tolerance. METHODS: Four electronic databases were searched: PubMed, Scopus, Cumulative Index to Nursing and Allied Health Literature , and Web of Science. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analysis as the basis of our organization. RESULTS: Hematopoietic stem cells prolonged VCA survival. A combination of immature dendritic cells and tacrolimus was superior to tacrolimus alone. T cell Ig domain and mucin domain modified mature dendritic cells increased VCA tolerance. Bone marrow-derived mesenchymal stem cells prolonged survival of VCAs. A combination of adipose-derived mesenchymal stem cells, cytotoxic T-lymphocyte antigen 4 immunoglobulin, and antilymphocyte serum significantly improved VCA tolerance. Ex-vivo allotransplant perfusion with recipient's bone marrow-derived mesenchymal stem cells increased VCA survival. Recipient's adipose-derived mesenchymal stem cells and systemic immunosuppression prolonged VCA survival more than any of those agents alone. Additionally, a combination of peripheral blood mononuclear cells shortly incubated in mitomycin and cyclosporine significantly improved VCA survival. Finally, a combination of donor recipient chimeric cells, anti-αß-T cell receptor (TCR), and cyclosporine significantly prolonged VCA tolerance. CONCLUSIONS: Evidence from animal studies shows that cell-based therapies can prolong survival of VCAs. However, there remain many obstacles for these therapies, and they require rigorous clinical research given the rarity of the subjects and the complexity of the therapies. The major limitations of cell-based therapies include the need for conditioning with immunosuppressive drugs and radiation, causing significant toxicity. Safety concerns also persist as most research is on animal models. While completely replacing traditional immunosuppression with cell-based methods is unlikely soon, these therapies could reduce the need for high doses of immunosuppressants and improve VCA tolerance.

A prenatal case misunderstood as specimen confusion: 46,XY/46,XY chimerism.

Chimerism results from the fusion of two zygotes in a single embryo, whereas mosaicism results from mitotic errors in a single zygote. True human chimerism is rare, with fewer than 100 cases reported in the literature. Here, we report a case in which the fetus was identified as having tetragametic chimerism based on short tandem repeat - polymerase chain reaction analysis of the family observed during amniocentesis for advanced maternal age. The chimerism occurred via the fertilization of two ova by two spermatozoa, followed by the fusion of early embryos. The genotypes of the two amniotic fluid samples obtained successively by one puncture were completely different, and the sex chromosomes were XY. Karyotyping and copy number variation sequencing showed no abnormalities. The fetus was delivered at term and the phenotype of the newborn was normal.

Mixed-field ABO front typing as an early sign of disease recurrence in ABO-matched stem cell transplantation.

ABO group testing is critical for allogeneic stem cell transplantation because mismatches can cause both transfusion and engraftment challenges. Even with ABO-matched donor-recipient pairs, ABO group determination may provide valuable insight into allograft status. Herein, we report a case of a 76-year-old female patient with myeloid neoplasm who underwent ABO-matched stem cell transplantation and in whom mixed-field ABO antigen expression during routine follow-up testing post-transplantation was the first sign of a change in transplant graft status; the mixed-field findings pre-dated changes in formal chimerism testing. This case underscores the potential of mixed-field ABO typing as an early indicator of disease recurrence in ABO-matched stem cell transplants and suggests that, in such cases, more sensitive forms of chimerism testing and/or closer monitoring for disease recurrence, particularly in the clinical setting of myeloid neoplasms, may be warranted.

Utility of Next-Generation Sequencing-Based Chimerism Analysis for Early Relapse Prediction following Allogenic Hematopoietic Cell Transplantation.

Chimerism monitoring following allogeneic hematopoietic cell transplantation (HCT) plays a pivotal role in evaluating engraftment status and identifying early indicators of relapse. Recent advancements in next-generation sequencing (NGS) technology have introduced AlloSeq HCT as a more sensitive alternative to short tandem repeat (STR) analysis. This study aimed to compare AlloSeq HCT with STR, focusing on the prediction of early relapse post-allogeneic HCT. Chimerism levels in 29 HCT recipients were assessed using both STR and NGS, employing a total of 125 whole blood or bone marrow aspirate samples (68 post-HCT and 57 pre-HCT samples from recipients or donors). AlloSeq HCT exhibited high concordance with STR and demonstrated the potential for early detection of chimeric changes, particularly at extremely low levels. The combined advantages of high sensitivity and automated data analysis offered by AlloSeq HCT substantiate its clinical adoption for effective chimerism monitoring.

Human kidney-derived hematopoietic stem cells can support long-term multilineage hematopoiesis.

Long-term multilineage hematopoietic donor chimerism occurs sporadically in patients who receive a transplanted solid organ enriched in lymphoid tissues such as the intestine or liver. There is currently no evidence for the presence of kidney-resident hematopoietic stem cells in any mammal species. Graft-versus-host-reactive donor T cells promote engraftment of graft-derived hematopoietic stem cells by making space in the bone marrow. Here, we report full (over 99%) multilineage, donor-derived hematopoietic chimerism in a pediatric kidney transplant recipient with syndromic combined immune deficiency that leads to transplant tolerance. Interestingly, we found that the human kidney-derived hematopoietic stem cells took up long-term residence in the recipient's bone marrow and gradually replaced their host counterparts, leading to blood type conversion and full donor chimerism of both lymphoid and myeloid lineages. Thus, our findings highlight the existence of human kidney-derived hematopoietic stem cells with a self-renewal ability able to support multilineage hematopoiesis.

Induced pluripotent stem cell-derived hematopoietic stem and progenitor cells induce mixed chimerism and donor-specific allograft tolerance.

In transplantation using allogeneic induced pluripotent stem cells (iPSCs), strategies focused on major histocompatibility complexes were adopted to avoid immune rejection. We showed that minor antigen mismatches are a risk factor for graft rejection, indicating that immune regulation remains one of the most important issues. In organ transplantation, it has been known that mixed chimerism using donor-derived hematopoietic stem/progenitor cells (HSPCs) can induce donor-specific tolerance. However, it is unclear whether iPSC-derived HSPCs (iHSPCs) can induce allograft tolerance. We showed that 2 hematopoietic transcription factors, Hoxb4 and Lhx2, can efficiently expand iHSPCs with a c-Kit+Sca-1+Lineage- phenotype, which possesses long-term hematopoietic repopulating potential. We also demonstrated that these iHSPCs can form hematopoietic chimeras in allogeneic recipients and induce allograft tolerance in murine skin and iPSC transplantation. With mechanistic analyses, both central and peripheral mechanisms were suggested. We demonstrated the basic concept of tolerance induction using iHSPCs in allogeneic iPSC-based transplantation.

Donor programmed cell death 1 ligand 1 is required for organ transplant tolerance in major histocompatibility complex-mismatched mixed chimeras although programmed cell death 1 ligand 1 and major histocompatibility complex class II are not required for inducing chimerism.

Induction of major histocompatibility complex (MHC) human leukocyte antigen (HLA)-mismatched mixed chimerism is a promising approach for organ transplantation tolerance; however, human leukocyte antigen-mismatched stable mixed chimerism has not been achieved in the clinic. Tolerogenic dendritic cell (DC) expression of MHC class II (MHC II) and programmed cell death 1 ligand 1 (PD-L1) is important for immune tolerance, but whether donor-MHC II or PD-L1 is required for the induction of stable MHC-mismatched mixed chimerism and transplant tolerance is unclear. Here, we show that a clinically applicable radiation-free regimen can establish stable MHC-mismatched mixed chimerism and organ transplant tolerance in murine models. Induction of MHC-mismatched mixed chimerism does not require donor cell expression of MHC II or PD-L1, but donor-type organ transplant tolerance in the mixed chimeras (MC) requires the donor hematopoietic cells and the organ transplants to express PD-L1. The PD-L1 expressed by donor hematopoietic cells and the programmed cell death 1 expressed by host cells augment host-type donor-reactive CD4+ and CD8+ T cell anergy/exhaustion and differentiation into peripheral regulatory T (pTreg) cells in association with the organ transplant tolerance in the MC. Conversely, host-type Treg cells augment the expansion of donor-type tolerogenic CD8+ DCs that express PD-L1. These results indicate that PD-L1 expressed by donor-type tolerogenic DCs and expansion of host-type pTreg cells in MHC-mismatched MCs play critical roles in mediating organ transplant tolerance.

Donor HLA mismatch promotes full donor T-cell chimerism in the allogeneic stem cell transplant with reduced-intensity conditioning and post-transplant cyclophosphamide GVHD prophylaxis.

Full donor T-cell chimerism (FDTCC) after allogeneic stem cell transplant (allo-SCT) has been associated with improved outcomes in hematologic malignancy. We studied if donor human leukocyte antigen (HLA) mismatch improves achievement of FDTCC because mismatched HLA promotes donor T-cell proliferation where recipient T-cells had been impaired by previous treatment. Patients (N = 138) received allo-SCT with reduced-intensity conditioning (RIC) from 39 HLA mismatched donors (16 unrelated; 23 haploidentical) with post-transplant cyclophosphamide (PTCy) or 99 matched donors (21 siblings; 78 unrelated) with PTCy (N = 18) or non-PTCy (N = 81). Achievement of FDTCC by day 100 was higher with HLA mismatched donors than matched donors (82.1% vs. 27.3%, p < 00,001), which was further improved with 200 cGy total body irradiation (87.9%) or lymphoid (versus myeloid) malignancy (93.8%). Since all mismatched transplants used PTCy, FDTCC was higher with PTCy than non-PTCy (68.4% vs. 25.7%, p < 0.00001), but not in the matched transplant with PTCy (38.9%), negating PTCy as the primary driver. Lymphocyte recovery was delayed with PTCy than without (median on day + 30: 100 vs. 630/µL, p < 0.0001). The benefit of FDTCC was not translated into survival outcomes, especially in myeloid malignancies, possibly due to the insufficient graft-versus-tumor effects from the delayed lymphocyte recovery. Further studies are necessary to improve lymphocyte count recovery in PTCy transplants.

Umbilical Cord Blood Cell Clearance Post-Infusion in Immune-Competent Children with Cerebral Palsy.

Umbilical cord blood cells have therapeutic potential for neurological disorders, through a paracrine mechanism of action. A greater understanding of the safety and immunological effects of allogeneic donor cord blood cells in the context of a healthy recipient immune system, such as in cerebral palsy, is needed. This study aimed to determine how quickly donor cord blood cells were cleared from the circulation in children with cerebral palsy who received a single intravenous infusion of 12/12 human leucocyte antigen (HLA)-matched sibling cord blood cells. Twelve participants with cerebral palsy aged 2-12 years received cord blood cell infusions as part of a phase I trial of umbilical blood infusion for cerebral palsy. Digital droplet PCR analysis of DNA copy number variants specific to donor and recipient was used to assess donor DNA clearance at five timepoints post-infusion, a surrogate measure of cell clearance. Donor cells were cleared by 3 months post-infusion in 11/12 participants. When detected, donor DNA was at a fraction of 0.01-0.31% of total DNA with no signs of graft-versus-host disease in any participant. The donor DNA clearance times provided by this study have important implications for understanding the safety of allogeneic cord blood cell infusion for cerebral palsy and translational tissue engineering or regenerative medicine research in other disorders.

Helical TomoTherapy Total Lymphoid Irradiation and Hematopoietic Cell Transplantation for Kidney Transplant Tolerance in Rhesus Macaques.

Development of a post-transplant kidney transplant tolerance induction protocol involving a novel total lymphoid irradiation (TLI) conditioning method in a rhesus macaque model is described. We examined the feasibility of acheiving tolerance to MHC 1-haplotype matched kidney transplants by establishing a mixed chimeric state with infusion of donor hematopoietic cells (HC) using TomoTherapy TLI. The chimeric state was hypothesized to permit the elimination of all immunosuppressive (IS) medications while preserving allograft function long-term without development of graft-versus-host-disease (GVHD) or rejection. An experimental group of 11 renal transplant recipients received the tolerance induction protocol and outcomes were compared to a control group (n = 7) that received the same conditioning but without donor HC infusion. Development of mixed chimerism and operational tolerance was accomplished in two recipients in the experimental group. Both recipients were withdrawn from all IS and continued to maintain normal renal allograft function for 4 years without rejection or GVHD. None of the animals in the control group achieved tolerance when IS was eliminated. This novel experimental model demonstrated the feasibility for inducing of long-term operational tolerance when mixed chimerism is achieved using a TLI post-transplant conditioning protocol in 1-haplotype matched non-human primate recipients of combined kidney and HC transplantation.

One fits all: a highly sensitive combined ddPCR/pyrosequencing system for the quantification of microchimerism after hematopoietic and solid organ transplantation.

OBJECTIVES: A combined digital droplet PCR (ddPCR)/pyrosequencing assay system was developed that demonstrated advantages applicable to multiple qualitative and quantitative molecular genetic diagnostic applications. Data for characterizing this combined approach for hematologic stem cell transplantation (HSCT) and allele quantification from graft-derived cell-free (cf) DNA in solid organ transplantation (SOT) is presented. METHODS: ddPCR and pyrosequencing assays targeting 32 SNPs/markers were established. ddPCR results from 72 gDNAs of 55 patients after allogeneic HSCT and 107 plasma-cfDNAs of 25 liver transplant recipients were compared with established methods/markers, i.e. short-tandem-repeat PCR and ALT, respectively. RESULTS: The ddPCR results were in good agreement with the established marker. The limit of detection was 0.02 % minor allele fraction. The relationship between ddPCR and STR-PCR was linear with R2=0.98 allowing to transfer previously established clinical STR-PCR cut-offs to ddPCR; 50-fold higher sensitivity and a variation coefficient of <2 % enable the use of low DNA concentrations (e.g. pre-sorted cells). ddPCR detected liver allograft injury at least as sensitive as ALT suggesting that ddPCR is a reliable method to monitor the transplant integrity, especially when other biomarkers are lacking (e.g. kidney). CONCLUSIONS: Combining pyrosequencing for genotyping and ddPCR for minor allele quantification enhances sensitivity and precision for the patient after HSCT and SOT. The assay is designed for maximum flexibility. It is expected to be suitable for other applications (sample tracking, prenatal diagnostics, etc.).

Increased heterozygosity in low-pass sequencing data allows identification of blood chimeras in cattle.

In about 90% of multiple pregnancies in cattle, shared blood circulation between fetuses leads to genetic chimerism in peripheral blood and can reduce reproductive performance in heterosexual co-twins. However, the early detection of heterosexual chimeras requires specialized tests. Here, we used low-pass sequencing data with a median coverage of 0.64× generated from blood samples of 322 F1 crosses between beef and dairy cattle and identified 20 putative blood chimeras through increased levels of genome-wide heterozygosity. In contrast, for 77 samples with routine SNP microarray data generated from hair bulbs of the same F1s, we found no evidence of chimerism, simultaneously observing high levels of genotype discordance with sequencing data. Fifteen out of 18 reported twins showed signs of blood chimerism, in line with previous reports, whereas the presence of five alleged singletons with strong signs of chimerism suggests that the in-utero death rate of co-twins is at the upper limit of former estimates. Together, our results show that low-pass sequencing data allow reliable screening for blood chimeras. They further affirm that blood is not recommended as a source of DNA for the detection of germline variants.