Although the maternal and placental/fetal blood circulations are physically separate, there is bidirectional exchange of small numbers of cells between the mother and fetus. Cell-free fetal DNA also circulates in maternal plasma during pregnancy. Certain cell types can reside in maternal organs for decades (long-term fetal microchimerism), but the majority of circulating fetal cells disappear after delivery (short-term fetal microchimerism). Noninvasive prenatal genetic diagnosis by isolating rare circulating fetal cells and analyzing their DNA content is a promising area of intense investigation but is not yet optimized for clinical use because of the challenges with cell purification. Cell-free fetal DNA in maternal plasma, thought to derive from apoptotic trophoblasts, is already clinically used for noninvasive screening for fetal chromosomal abnormalities and single gene disorders. Other sources of maternal chimerism, such as iatrogenic chimerism from organ and stem-cell transplants, can affect the efficacy and interpretation of cell-free fetal DNA-based screening and testing. Chimerism found in very rare dizygotic monochorionic twins with placental vascular anastomoses can complicate interpretation of prenatal genetic testing results, especially from cord blood samples. These possibilities should be considered when prenatal genetic screening and testing yield unexpected and difficult to interpret results.

Haplo-cord transplant has emerged as a feasible and reliable approach for haematopoietic stem cell transplant in patients who are unable to find matched-donor. This approach provides fast myeloid recovery, low incidence of graft vs host disease (GVHD) and favorable graft versus leukemia (GVL) effects. T cell recovery plays an important role in preventing infectious complications; it also mediates the GVHD and the GVL effects. Here, we utilized a novel RNA-based sequencing approach to quantitatively characterize the T cell receptor (TCRs) repertoire in patients underwent haplo-cord transplant in comparison with those underwent matched-donor transplant. Our study shows that higher percentage of cord cells early post transplant were associated with significantly higher TCR diversity. TCR diversity was significantly lower in patients with GVHD and in relapsed patients. A larger cohort study is needed to validate these data and to provide useful information on the specific TCR clones correlated with clinical outcome.

This article interweaves a history of Ray Owen's early work with a broader account of the conceptual landscape of immunology in the mid 1950's. In particular, Owen's openness to the very possibility of chimeric phenomena is recognized.

For over 50 y the association between hematopoietic chimerism and tolerance has been recognized. This originated with the brilliant observation by Dr. Ray Owen that freemartin cattle twins that shared a common placental blood supply were red blood cell chimeras, which led to the discovery that hematopoietic chimerism resulted in actively acquired tolerance. This was first confirmed in neonatal mice by Medawar etal. and subsequently in adult rodents. Fifty years later this concept has been successfully translated to solid organ transplant recipients in the clinic. The field is new, but cell-based therapies are being used with increasing frequency to induce tolerance and immunomodulation. The future is bright. This review focuses on chimerism and tolerance: past, present and prospects for the future.

Previous studies have shown that a stable presence of both donor and recipient haematopoietic derived cells after allogeneic haematopoietic stem cell transplantation (HSCT) occurs in approximately ten percent of the patients affected by β-Thalassemia. Once achieved this condition, defined as persistent mixed chimerism (PMC), the patients do not require additional red blood cells (RBCs) support and, regardless of the presence in some cases of an extremely low percentage of donor-derived nucleated cells, they are clinically cured by an incomplete, but functional graft. Most of the published papers have, however, investigated the impact of donor engraftment in the nucleated cells rather than in the mature erythrocytes. We have recently published a paper showing that in four long-term transplanted patients affected by hemoglobinopathies, characterized by the presence of few donor engrafted nucleated cells-both in the peripheral blood and in the bone marrow-the majority of the erythrocytes were of donor origin. Moreover we showed that the proportion of donor-derived erythroid precursors, determined by analyzing singularly picked-up burst-forming unit erythroid colonies, was equivalent to that observed in the mature nucleated cells rather than in the red blood cells. These results suggest that in patients characterized by the presence of PMC after HSCT a selective advantage of the donor erythroid precursors maturation might successfully contrast the problems bound to the recipient ineffective erythropoiesis. When genetically modified HSCT will be a possible option for treating Thalassemia Major, the co-existence of the repaired cells with those still expressing the genetic defect will be an expected scenario, not in an allogeneic, but in an autologous environment.

Studies on both circulating and tissue fetal cell microchimerism (FCM) favored its protective role in thyroid cancer, consistent with findings in other malignancies. Nevertheless, scanty data were available on the possible impact on the outcome of the disease. We demonstrated that FCM has a positive effect on thyroid cancer presentation and outcome. We also excluded that the better clinical features observed were due to the effect of pregnancy per se. In conclusion, FCM may have not only a protective role toward the onset of thyroid cancer, but also a positive effect on its progression. These findings give novel insights into the identification of the role of FCM in oncology and, consequently, in the potential therapeutic application of this physiological phenomenon.

Allogeneic hematopoietic stem cell transplantation (HSCT) is a widely applied treatment for disorders mainly involving the hematopoietic system. The success of this treatment depends on many different patient- and donor-specific factors. Based on higher CD34+ yields and superior clinical outcomes associated with the use of male donors, males are generally seen as the preferred HSCT donor. In addition, female donors are notorious for bearing memory type lymphocytes induced by previous pregnancies; such alloimmune cells may provoke unwanted immune reactions such as graft-vs.-host disease in transplant recipients. Consequently, many transplant centers try to avoid parous donors, particularly when searching the best unrelated donor for a male patient. We recently showed that parous women with female offspring have an anti-male directed tolerogenic immune status comparable to that of nulliparous donors. As discussed in this article addendum, the sex of the donor's offspring combined with the presence of HY-specific T regulator cells are possibly better selection criteria than parity status per se.

The incidence and potentially different biological impact of naturally acquired maternal and fetal cells were enthusiastically and extensively discussed when basic scientists and clinicians met at the Medical University of Graz in Austria for an International Symposium on Chimerism (Fig. 1). The two day program featured presentations by speakers from 4 different continents, intermingled with short presentations focusing on technical aspects of chimerism analysis by representatives from the companies who sponsored the meeting. Details on the program and speakers list are available at the meeting’s website (http://chimerism.medunigraz.at/cms/website.php). As also exemplified by the various different types of studies published in the recently released journal Chimerism, this field is slowly but steadily expanding as new research teams working outside and within the transplantation field have become interested in the biologically intriguing phenomena of chimerism and microchimerism (Mc). The highlights of the meeting are summarized in this meeting report. Figure 1. The Symposium on Chimerism. Photo courtesy of Harry Schiffer, Copyright Graz Tourism. Technical Aspects of Chimerism Analysis Since the 1979 paper by Herzenberg and colleagues reporting enrichment of cells from pregnant women presumed to be fetal using flow-sorting techniques,1 many different studies have been published describing fetal or maternal cells in diverse hematopoietic cell types or tissues and organs. During the past decade, major technical improvements regarding the detection of these generally rare cells have been made. This has resulted in a shift from the more traditional XY fluorescent in situ hybridization (FISH) staining technique applied to tissue sections to more refined technologies such as real-time PCR for the detection of male-specific DNA sequences in genomic DNA extracted from peripheral blood cells or processed tissues. Real-time PCR reactions for mismatched HLA alleles as well as for other genetic markers (insertion/deletion) have been developed in the laboratories of meeting participants Dr Nelson (Fred Hutchinson Cancer Research Center, Seattle, USA), Dr Lambert (INSERM UMRs 1097, Marseille, France) and Dr van Halteren (Leiden University Medical Center, Leiden, the Netherlands). Unfortunately, many researchers are faced by the problem that this approach requires the availability of DNA from mother or offspring in order to find informative genetic markers. Of note, the newly developed QuantiChimera platform established at the laboratory of Dr Lambert can be contacted for technical support regarding state-of-the art PCR technology for Mc detection. Particularly relevant for investigators who are restricted to using FISH techniques for the identification of rare maternal or fetal chimeric cells is the collective observation that this technique does not always give consistent staining results. This may be explained by the fact that archived tissue samples are often subjected to different fixation protocols. As discussed by Dr Sedlmayr and Dr Kroneis (Medical University, Graz, Austria), FISH by itself is a less than optimal technique for the analysis of rare microchimeric cells given the frequency of false positive cells. This issue can in part be addressed by using two reverse sets of differently labeled X and Y probes. Also biochemical markers such as unique cell surface molecules or markers that can be used to distinguish fetal cells from adult cells are not perfectly reliable in the setting of analysis of extremely rare cells. Unambiguous identification of individual microchimeric cells can be done by combining either FISH analysis or analysis of biochemical markers with DNA genotyping of candidate cells isolated by laser microdissection.2 The introduction of a whole genome amplification step subsequently even allows combining DNA genotyping of single microchimeric cells with sequencing or comparative genome hybridization.3This exciting new approach is expected to further boost the chimerism field.

Accurate quantification of chimerism and microchimerism is proving to be increasingly valuable for hematopoietic cell transplantation as well as non-transplant conditions. However, methods that are available to quantify low-level chimerism lack accuracy. Therefore, we developed and validated a method for quantifying chimerism based on digital PCR technology. We demonstrate accurate quantification that far exceeds what is possible with analog qPCR down to 0.01% with the potential to go even lower. Also, this method is inherently more informative than qPCR. We expect the advantages of digital PCR will make it the preferred method for chimerism analysis.