Hair follicle: a reliable source of recipient origin after allogeneic hematopoietic stem cell transplantation

Male microchimerism in females: a quantitative study of twin pedigrees to investigate mechanisms

STUDY QUESTION

Does having a male co-twin, older brothers, or sons lead to an increased probability of persistent male microchimerism in female members of twin pedigrees?

SUMMARY ANSWER

The presence of a male co-twin did not increase risk of male microchimerism and the prevalence of male microchimerism was not explained by having male offspring or by having an older brother.

WHAT IS KNOWN ALREADY

Microchimerism describes the presence of cells within an organism that originate from another zygote and is commonly described as resulting from pregnancy in placental mammals. It is associated with diseases with a female predilection including autoimmune diseases and pregnancy-related complications. However, microchimerism also occurs in nulliparous women; signifying gaps in the understanding of risk factors contributing to persistent microchimerism and the origin of the minor cell population.

STUDY DESIGN, SIZE, DURATION

This cross-sectional study composed of 446 adult female participants of the Netherlands Twin Register (NTR).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Participants included in the study were female monozygotic (MZ) twins, female dizygotic same-sex twins and females of dizygotic opposite-sex twin pairs, along with the mothers and sisters of these twins. Peripheral blood samples collected from adult female participants underwent DNA extraction and were biobanked prior to the study. To detect the presence of male-origin microchimerism, DNA samples were tested for the relative quantity of male specific Y chromosome gene DYS14 compared to a common β-globin gene using a highly sensitive quantitative PCR assay.

MAIN RESULTS AND THE ROLE OF CHANCE

We observed a large number of women (26.9%) having detectable male microchimerism in their peripheral blood samples. The presence of a male co-twin did not increase risk of male microchimerism (odds ratio (OR) = 1.23: SE 0.40, P = 0.61) and the prevalence of male microchimerism was not explained by having male offspring (OR 0.90: SE 0.19, P = 0.63) or by having an older brother (OR = 1.46: SE 0.32, P = 0.09). The resemblance (correlation) for the presence of microchimerism was similar (P = 0.66) in MZ pairs (0.27; SE 0.37) and in first-degree relatives (0.091; SE 0.092). However, age had a positive relationship with the presence of male microchimerism (P = 0.02).

LIMITATIONS, REASONS FOR CAUTION

After stratifying for variables of interest, some participant groups resulted in a low numbers of subjects. We investigated microchimerism in peripheral blood due to the proposed mechanism of cell acquisition via transplacental blood exchange; however, this does not represent global chimerism in the individual and microchimerism may localize to numerous other tissues.

WIDER IMPLICATIONS OF THE FINDINGS

Immune regulation during pregnancy is known to mitigate allosensitization and support tolerance to non-inherited antigens found on donor cells. While unable to identify a specific source that promotes microchimerism prevalence within pedigrees, this study points to the underlying complexities of natural microchimerism in the general population. These findings support previous studies which have identified the presence of male microchimerism among women with no history of pregnancy, suggesting alternative sources of microchimerism. The association of detectable male microchimerism with age is suggestive of additional factors including time, molecular characteristics and environment playing a critical role in the prevalence of persistent microchimerism. The present study necessitates investigation into the molecular underpinnings of natural chimerism to provide insight into women’s health, transplant medicine and immunology.

STUDY FUNDING/COMPETING INTEREST(S)

This work is funded by Royal Netherlands Academy of Science Professor Award (PAH/6635 to D.I.B.); The Netherlands Organisation for Health Research and Development (ZonMw)—Genotype/phenotype database for behavior genetic and genetic epidemiological studies (ZonMw 911-09-032); Biobanking and Biomolecular Research Infrastructure (BBMRI–NL, 184.021.007; 184.033.111); The Netherlands Organisation for Scientific Research (NWO)—Netherlands Twin Registry Repository (NWO-Groot 480-15-001/674); the National Institutes of Health—The Rutgers University Cell and DNA Repository cooperative agreement (NIMH U24 MH068457-06), Grand Opportunity grants Integration of genomics and transcriptomics in normal twins and major depression (NIMH 1RC2 MH089951-01), and Developmental trajectories of psychopathology (NIMH 1RC2 MH089995); and European Research Council—Genetics of Mental Illness (ERC 230374). C.B.L. declares a competing interest as editor-in-chief of Human Reproduction and his department receives unrestricted research grants from Ferring, Merck and Guerbet. All remaining authors have no conflict-of-interest to declare in regards to this work.

TRIAL REGISTRATION NUMBER

N/A.

Case Report: Calpainopathy Presenting After Bone Marrow Transplantation, With Studies of Donor Genetic Content in Various Tissue Types

We present a patient who had two allogeneic bone marrow transplantations for acute lymphocytic leukemia. She developed slowly progressive limb-girdle weakness in the context of other symptoms of graft-vs.-host disease (GVHD). Her myopathy symptoms had been initially attributed to GVHD, but when she progressed despite immunotherapy, genetic testing was requested. Initial testing was performed on a blood sample, identifying a variant of unknown significance in DMD. Subsequent testing of DNA from the patient's muscle tissue identified two pathogenic variants in CAPN3, with absence of the DMD variant (this latter variant presumed to have been received from the donor). Allele-specific digital droplet qPCR permitted the quantification of the donor variant in various tissues from the patient (whole skin, isolated fibroblasts, whole blood, saliva, buccal cells, urine sediment, and two muscle biopsies taken at a 2 year interval). This report emphasizes that genetic disease should still be considered in the context of presumably acquired disease, and also demonstrates the extent of transdifferentiation of donor cells into other tissues.

Chimeric status of biological samples after HSCT for personal identification: Y-STR based DNA analysis in sex mismatch cases

Identification of an individual is the prime object in forensic case works both in civil or criminal situations like paternity/maternity disputes, sexual assaults, murder, mass disaster victims etc. STR analysis has already proved its potential to give accurate results. In addition to autosomal chromosomes, sex determination at many times is crucial in forensic situations, especially in situations like rape cases or in cases of missing persons. The chances of wrong interpretations may arise due to false detection (or non-detection) of STR fragments overall or only at amelogenin-specific fragments, in situation like mutations, intersex conditions, trans-sexualism etc., due to natural or artificial chimersim. The forensic relevance of the possible misinterpretation of STR's or amelogenin should never be underestimated. The present study was carried out to identify an individual using Y-STR in sex mismatch patients who received hematopoietic stem cell transplantation. Hematopoietic stem cell transplantation is a method to replace patient's stem cell with the stem cell donated by the donor preferably biological related in order to cure malignant and non malignant diseases. This study enrolled ten female patients of HSCT. Samples were collected as pre and post transplant after 15 days, 30 days, 60 days, and 90 days of time interval from sex mismatch patient (female) and from donor (male) and chimeric status of the patient was analyzed using Y-STR markers (23 loci). Results demonstrated that donor genotype existed in blood and buccal swab of the recipient but no genetic profile existed for Y-STR in hair follicle of the recipient. This study suggests that only hair follicle out of three biological samples tested serves as reliable source of recipient's origin after HSCT for accurate personal identification especially in forensic situations.

Isolation and Characterization of a Fetal-Maternal Microchimeric Stem Cell Population in Maternal Hair Follicles Long after Parturition

Fetal-maternal microchimerism describes the acquisition of fetal stem cells (FSC) by the mother during pregnancy and their long-term persistence after parturition. FSC may engraft in a variety of maternal tissues especially if there is organ/tissue injury, but their role and mechanism of persistence still remains elusive. Clinical applications due to their pluripotency, immunomodulatory effects and accessibility make them good candidates for ex-vivo manipulation and autologous therapies. The hair follicles contain a distinctive niche for pluripotent stem cells (PSC). To date, there is no published evidence of fetal microchimerism in the hair follicle. In our study, follicular unit extraction (FUE) technique allowed easy stem cell cultures to be obtained while simple hair follicle removal by pull-out technique failed to generate stem cells in culture. We identified microchimeric fetal stem cells within the primitive population of maternal stem cells isolated from the hair follicles with typical mesenchymal phenotype, expression of PSC genes and differentiation potential towards osteocytes, adypocites and chondrocytes. This is the first study to isolate fetal microchimeric stem cells in adult human hair long after parturition. We presume a sanctuary partition mechanism with PSC of the mother deposited during early embryogenesis could explain their long-term persistence.

Prepregnancy Genetic Carrier Screening in Recipients of Hematopoietic Stem Cell Transplants

BACKGROUND

Genetic carrier screening of individuals with a history of hematopoietic stem cell (HSC) transplant is not always straightforward.

CASE

We present the case of a couple who underwent prepregnancy genetic screening before fertility treatment. The genetics laboratory flagged the male partner's blood sample because there was a discrepancy between the sex of the patient and the sex chromosome markers, ultimately leading to the discovery of a prior HSC transplant.

CONCLUSION

A history of HSC transplant may confound results of serum-based genetic carrier screening. Hematopoietic stem cell transplant recipients are at risk for misinterpreted genetic carrier screening because the results of such screening may not actually be reflective of their own DNA.

Use of low-level laser therapy in treatment of the androgenic alopecia, the first systematic review

Alopecia is a common disease affecting more than half of the world total number of people. Alopecia exists in different types, but one of the most common of these types is the Androgenic Alopecia which has affected approximately 51% of the total number of males ranging between the age bracket of 40 years and 75 years. This type of alopecia is more common in females who are above the age of 65 years and above. Despite this widespread effect, much has not been done regarding identifying the possible drugs for treating this disease. At present, there exist only two possible medications that have been scientifically approved to cure this disease, include finasteride and minoxidil. Also, another possible form of treatment has been the case of hair transplantation. Despite the new possible treatment options available for treatment of different types of hair loss, there is a need for the invention for more efficient management and treatment options that are less costly, environmentally friendly, and most importantly human consumption friendly. Due to the recent evaluation that low-level laser therapy stimulated hair growth. This systematic review and meta-analysis was to determine whether the use of low-level laser therapy is an effective therapy for treatment of the Androgenic alopecia and also to some degree we reviewed the level of the patient's satisfaction. Some earlier studies had shown that the use of low-level laser therapy stimulated the hair growth when mice were treated with chemotherapy which was induced by the alopecia and also the other type of alopecia called alopecia areata. The researchers hypothesized that the primary mechanism of treating Androgenic alopecia to be the stimulation of the epidermal stem cells which are in the hair follicle making them bulge and shift the follicles into the anagen phase.

Isolated extramedullary cutaneous relapse despite concomitant severe graft-vs.-host disease and tissue chimerism analysis in a patient with acute lymphoblastic leukemia after allogeneic hematopoietic stem cell transplantation: A case report

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative treatment option for patients with acute lymphoblastic leukemia (ALL). The curative potential of allo-HSCT for ALL is, in part, due to the graft-vs.-leukemia (GVL) effect, in addition to the intensive conditioning chemo-radiotherapy. However, relapse remains the major cause of treatment failure following allo-HSCT for ALL. In the allo-HSCT setting, testing for genetic markers of hematopoietic chimerism has become a part of the routine diagnostic program. Routine chimerism analysis is usually performed in peripheral blood or bone marrow; in fact, little is known about the value of tissue chimerism in patients with extramedullary relapse (EMR) after the allo-HSCT setting. The present study reports on, a case of a patient with ALL who experienced isolated cutaneous EMR despite ongoing graft-vs.-host disease (GVHD), and the results of peripheral blood and skin tissue chimerism studies using multiplex polymerase chain reaction (PCR) of short tandem repeats (STR-PCR). The present case demonstrates that, although complete remission and/or chimerism may be achieved in the bone marrow, chimerism achieved at the tissue level, and the subsequent GVL effect, may be limited, despite concomitant severe GVHD following allo-HSCT. Our tissue chimerism analysis results provide a good example of how skin tissue may be a ‘sanctuary’ site for effector cells of GVL, despite active GVHD and complete hematopoetic chimerism.

Evaluation of blood, buccal swabs, and hair follicles for DNA profiling technique using STR markers

AIM

To study the short tandem repeat (STR) pattern of DNA from the blood, buccal swabs, and hair follicles of the recipients of allogenic hematopoietic stem cell transplantation to examine whether these tissues contain donor derived cells.

METHODS

The study enrolled 25 patients who sustained engraftment. Peripheral blood, buccal swabs, and hair follicles were collected on days 21-30, 90, and 180 after transplantation and the chimeric status of the recipients was evaluated.

RESULTS

Donor derived cells existed in the blood and buccal swabs, but not in hair follicles, which can be used to obtain the pre-transplant sample of the recipient after transplant.

CONCLUSION

Peripheral blood and buccal swab do not serve as a reliable source of recipient's origin for DNA analysis of individuals who underwent allogenic hematopoietic stem cell transplantation at least within 6 months after transplant.

Quantitative evaluation of the transplanted lin(-) hematopoietic cell migration kinetics

Stem cells take part in organogenesis, cell maturation and injury repair. The migration is necessary for each of these functions to occur. The aim of this study was to investigate the kinetics of transplanted hematopoietic lin(-) cell population (which consists mainly of the stem and progenitor cells) in BALB/c mouse contact hypersensitivity model and quantify the migration to the site of inflammation in the affected foot and other healthy organs. Quantitative analysis was carried out with the real-time polymerase chain reaction method. Spleen, kidney, bone marrow, lung, liver, damaged and healthy foot tissue samples at different time points were collected for analysis. The quantitative data normalization was performed according to the comparative quantification method. The analysis of foot samples shows the significant migration of transplanted cells to the recipient mice affected foot. The quantity was more than 1000 times higher, as compared with that of the untreated foot. Due to the inflammation, the number of donor origin cells migrating to the lungs, liver, spleen and bone marrow was found to be decreased. Our data shows that transplanted cells selectively migrated into the inflammation areas of the foot edema. Also, the inflammation caused a secondary migration in ectopic spleen of hematopoietic stem cell niches and re-homing from the spleen to the bone marrow took place.

DNA profiling in peripheral blood, buccal swabs, hair follicles and semen from a patient following allogeneic hematopoietic stem cells transplantation

Allogeneic peripheral blood stem cells transplantation (allo-PBSCT) or allogeneic bone marrow transplantation (allo-BMT) have been widely used to treat patients exhibiting certain severe illnesses. However, previous studies have shown that the biological materials of allo-PBSCT or allo-BMT recipients may not constitute credible materials for personal identification. In the present study, four types of commonly used samples were collected from a male individual following gender-matched allo-BMT. Autosomal short tandem repeat (STR) and Y-STR markers analysis, based on polymerase chain reaction, were used to evaluate the chimerism status. The results showed that the blood sample were all donor type, the buccal swab sample were mixed chimerism, and the sperm and hair follicle samples maintained a recipient origin of 100%. In conclusion, identical results were obtained by the two methods and it was confirmed that DNA extracted from hair follicles and sperm can be used as a reference for the pre-transplant genotype DNA profile of the recipient in the gender-match allo-BMT or -PBSCT.

Utility of saliva and hair follicles in donor selection for hematopoietic stem cell transplantation and chimerism monitoring

Selection of an HLA identical donor is a critical pre-requisite for successful hematopoietic stem cell transplantation (HSCT). Most transplant centers utilize blood as the most common source of DNA for HLA testing. However, obtaining blood through phlebotomy is often challenging in patients with conditions like severe leucopenia or hemophilia, pediatric and elderly patients. We have used a simple in-house protocol and shown that HLA genotypes obtained on DNA extracted from saliva or hair are concordant with blood and hence can be used for selection of donors for HSCT or organ transplantation. Similarly, for post-HSCT chimerism monitoring, non-availability of pre-transplant DNA samples poses a major limitation of reference STR fingerprints. This study shows that DNA obtained post-HSCT from hair follicles can be used to generate pre-transplant patient specific fingerprints while the STR profiles obtained in saliva samples cannot as these display a mixed state of chimerism.

Chimerism analysis in transplant patients: a hypothesis-free approach in the absence of reference genotypes

INTRODUCTION

During routine analysis of chimerism in bone marrow transplant patients pre-transplant genotype of the recipient or the donor might lack. We aimed to develop a new method to analyze DNA results suitable when reference genotypes are not available.

METHODS

The method was based on the balance between heterozygotes. It was implemented in a standard computer spreadsheet, and considered the hypothetical donor-recipient genotype combinations. Hypotheses with peak height ratios and allele sharing tendency above a critical threshold were accepted. The results were compared with those obtained with prior knowledge of reference genotypes.

RESULTS

The algorithm predicted correctly the proportion of donor/recipient chimerism, even in the absence of reference genotypes. In fact, the predicted values were closely correlated (r(2)>0.98) and free of systematic bias (slope 0.98-1.04), in comparison with the reference values obtained with prior knowledge of the donor and recipient genetic profiles.

CONCLUSIONS

This study constitutes a proof-of-concept of the application of the heterozygote balance for the quantitative study of chimerism. The algorithm computes post-transplant chimerism in an easy and time-efficient way, even when the donor and recipient reference genotypes are unavailable. Therefore, it can be a useful tool for laboratories involved in chimerism analysis.

Avoiding pitfalls in bone marrow engraftment analysis: a case study highlighting the weakness of using buccal cells for determining a patient's constitutional genotype after hematopoietic stem cell transplantation

BACKGROUND AIMS

An accurate and reliable assessment of bone marrow engraftment (BME) after hematopoietic stem cell transplantation (HSCT) is based on the ability to distinguish between recipient and donor cells at selected polymorphic short tandem repeat (STR) DNA loci. Buccal cells are an important source of DNA for determining the recipient's constitutional genotype, particularly in patients transplanted before the STR evaluation.

METHODS

Genomic DNA was extracted from the recipient buccal cells and from isolated CD3+ (T-cell lymphocyte) and CD33+ (myelocyte) cells after HSCT. BME analysis was performed using a STR-based polymerase chain reaction amplification method followed by fragment-size analysis for assessing the recipient-derived or donor-derived composition of cell lineage-specific peripheral blood DNA.

RESULTS

We identified three cases of complete buccal epithelial cell engraftment after HSCT detected by BME analysis, potentially leading to misinterpretation of testing results if these cells were used as the sole source for determining the recipient's genotype.

CONCLUSIONS

These cases suggest that complete engraftment of buccal epithelial cells may be a common finding in patients receiving HSCT, drawing attention to important issues such as the type of samples used for determining a patient's constitutional genotype that may confound testing results. This study also highlights the need for careful interpretation of the BME testing results in the context of the clinical findings.

Genetic investigation of biological materials from patients after stem cell transplantation based on autosomal as well as Y-chromosomal markers

The authors presented the results of DNA polymorphism investigation of blood, buccal swabs and hair follicles originating from patients after allogeneic hematopoietic stem cell transplantation. The real-time and multiplex assays based on polymerase chain reaction within the range of autosomal as well as Y-chromosomal markers were applied to assess the possible dangers arising from investigation of these materials in forensic genetics. The results revealed that not only post-transplant blood and buccal swab, but also recipient hair, up to now regarded as devoid of any donor’s cells, do not constitute entirely safe material for forensic purposes. Their analysis can lead to the false identification of gender or male haplotype. The investigation of sex-determining region Y and Y-chromosome short tandem repeats performed in female recipients with male donors resulted in the designation of donor’s DNA in hair cells as well as in blood and buccal swabs. Therefore, biological stains gathered from crime scenes should not be analysed exclusively based on the investigation of male-specific markers.

Successful bone marrow transplantation in a patient with Diamond-Blackfan anemia with co-existing Duchenne muscular dystrophy: a case report

Introduction

Diamond-Blackfan anemia and Duchenne muscular dystrophy are two rare congenital anomalies. Both anomalies occurring in the same child is extremely rare. Allogeneic hematopoietic stem cell transplantation is a well-established therapy for Diamond-Blackfan anemia. However, in patients with Duchenne muscular dystrophy, stem cell therapy still remains experimental.

Case presentation

We report the case of a nine-year-old boy of north Indian descent with Diamond-Blackfan anemia and Duchenne muscular dystrophy who underwent successful allogeneic hematopoietic stem cell transplantation. He is transfusion-independent, and his Duchenne muscular dystrophy has shown no clinical deterioration over the past 45 months. His creatine phosphokinase levels have significantly decreased to 300 U/L from 14,000 U/L pre-transplant. The patient is 100% donor chimera in the hematopoietic system, and his muscle tissue has shown 8% to 10.4% cells of donor origin.

Conclusion

Our patient's Diamond-Blackfan anemia was cured by allogeneic hematopoietic stem cell transplantation. The interesting clinical observation of a possible benefit in Duchenne muscular dystrophy cannot be ruled out. However, further clinical follow-up with serial muscle biopsies and molecular studies are needed to establish this finding.

DNA profiling in blood, buccal swabs and hair follicles of patients after allogeneic peripheral blood stem cells transplantation

To investigate the possibility that buccal swab and hair follicle samples could be used to detect recipients' personal identification or kinship study in forensic cases, who have received allogeneic peripheral blood stem cells transplantation (allo-PBSCT). Seven casework examples of materials collected from individuals after allo-PBSCT and their donors were investigated. Detection of genotypes was preformed by PCR analysis of short tandem repeats (STRs) in blood, buccal swab and hair follicle samples. Donor genotypes were detected in all blood samples of the patients. Case 2 recipient presented obvious chimeric DNA patterns (54% donor) and case 3 showed almost donor genotype (90% donor) in buccal swab samples. Other cases exhibited much lower chimeric DNA patterns in some alleles (from 2-16% donor). The average time after allo-PBSCT in case 2 and 3 recipients (123 months) was much longer than that of others (11 months). All of 7 recipients did not show donor-derived DNA patterns in hair samples. Therefore, this study suggests that sometimes buccal swab contains DNA from donor and yet hair follicle is a better source of personal identification or kinship study of recipients.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Isolation of epithelial cells from acrylic removable dentures and gender identification by amplification of SRY gene using real time PCR

This study evaluates the usefulness of acrylic dentures as the source of DNA for forensic analysis. Thirty-eight samples (21 males and 17 females) were collected and stored for different time periods. The epithelial cells adhered to the dentures were retrieved and the genomic DNA was extracted. All the samples yielded sufficient amount of DNA for analysis irrespective of the storage time. Gender determination was done by amplification of the sex determining region on the Y chromosome (SRY) using real-time polymerase chain reaction with 100% accuracy, within minimal time. With this study, we conclude that saliva-stained acrylic dentures can act as a source of forensic DNA and co-amplification of SRY gene with other routine sex typing markers will give unambiguous gender identification.

Mixed chimerism in SCT: conflict or peaceful coexistence?

Stem cell transplants that follow both myeloablative and non-myeloablative conditioning regimens can result in states of mixed chimerism, which can be stable over time. With widespread availability of Y chromosome FISH in sex-mismatched transplantation and DNA-based methodologies for analysis of chimerism in other donor-recipient pairs, further insights have been gained regarding the implications of the mixed chimeric state. In transplants performed for inherited and acquired marrow failure disorders, disease status can be improved with only 10-20% donor cells, and it appears that stable mixed chimerism at that level is an acceptable outcome often leading to a state of tolerance, but an increasing level of recipient cells often precedes graft rejection. In transplants performed for malignant conditions, increasing levels of mixed chimerism may indicate disease relapse, but some cases with stable levels of mixed chimerism have been compatible with prolonged remission states. Understanding when mixed chimerism is an indication of secondary graft failure or impending graft rejection vs a state of tolerance and ongoing propensity for the establishment of a graft-vs-tumor effect is often difficult with currently available technologies and immunologic assays. The ability to understand the implication of mixed chimerism of multiple cell lineages and of varied lymphocyte subsets will remain important areas for future research to best harness the immunologic and other therapeutic benefits of allogeneic transplantation.

Plasticity after allogeneic hematopoietic stem cell transplantation

The postulated almost unlimited potential of transplanted hematopoietic stem cells (HSCs) to transdifferentiate into cell types that do not belong to the hematopoietic system denotes a complete paradigm shift of the hierarchical hemopoietic tree. In several studies during the last few years, donor cells have been identified in almost all recipient tissues after allogeneic HSC transplantation (HSCT), supporting the theory that any failing organ could be accessible to regenerative cell therapy. However, the putative potential ability of the stem cells to cross beyond lineage barriers has been questioned by other studies which suggest that hematopoietic cells might fuse with non-hematopoietic cells and mimic the appearance of transdifferentiation. Proof that HSCs have preserved the capacity to transdifferentiate into other cell types remains to be demonstrated. In this review, we focus mainly on clinical studies addressing plasticity in humans who underwent allogeneic HSCT. We summarize the published data on non-hematopoietic chimerism, donor cell contribution to tissue repair, the controversies related to the methods used to detect donor-derived non-hematopoietic cells and the functional impact of this phenomenon in diverse specific target tissues and organs.