Fetomaternal trafficking in the mouse increases as delivery approaches and is highest in the maternal lung

A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics

There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.

Fetal microchimerism and the two-stage model of preeclampsia

Fetal cells cross the placenta during pregnancy and some have the ability to persist in maternal organs and circulation long-term, a phenomenon termed fetal microchimerism. These cells often belong to stem cell or immune cell lineages. The long-term effects of fetal microchimerism are likely mixed, potentially depending on the amount of fetal cells transferred, fetal-maternal histocompatibility and fetal cell-specific properties. Both human and animal data indicate that fetal-origin cells partake in tissue repair and may benefit maternal health overall. On the other hand, these cells have been implicated in inflammatory diseases by studies showing increased fetal microchimerism in women with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. During pregnancy, preeclampsia is associated with increased cell-transfer between the mother and fetus, and an increase in immune cell subsets. In the current review, we discuss potential mechanisms of transplacental transfer, including passive leakage across the compromised diffusion barrier and active recruitment of cells residing in the placenta or fetal circulation. Within the conceptual framework of the two-stage model of preeclampsia, where syncytiotrophoblast stress is a common pathophysiological pathway to maternal and fetal clinical features of preeclampsia, we argue that microchimerism may represent a mechanistic link between stage 1 placental dysfunction and stage 2 maternal cardiovascular inflammation and endothelial dysfunction. Finally, we postulate that fetal microchimerism may contribute to the known association between placental syndromes and increased long-term maternal cardiovascular disease risk. Fetal microchimerism research represents an exciting opportunity for developing new disease biomarkers and targeted prophylaxis against maternal diseases.

Microchimerism as a source of information on future pregnancies

Small numbers of fetal cells cross the placenta during pregnancy turning mothers into microchimeras. Fetal cells from all previous pregnancies accumulate forming the mother's fetal microchiome. What is significant about microchimeric cells is that they have been linked to health problems including reproductive and autoimmune diseases. Three decades after the discovery of fetal microchimerism, the function of these cells remains a mystery. Here, we contend that the role of microchimeric cells is to inform the fetus about the likelihood that its genes are present in future pregnancies. We argue that, when genes are more likely than average to be in future maternal siblings, fetuses will send a fixed number of cells that will not elicit a maternal immune response against them. However, when genes are less likely to be in future maternal siblings, fetuses will send an ever-increasing number of cells that will elicit an ever-stronger maternal immune response. Our work can explain the observed clinical association between microchimeric cells and pre-eclampsia. However, our work predicts that this association should be stronger in women with a genetically diverse microchiome. If supported by medical tests, our work would allow establishing the likelihood of pregnancy or autoimmune problems advising medical interventions.

Contribution of fetal microchimeric cells to maternal wound healing in sickle cell ulcers

Leg ulcers are a major complication of sickle cell disease (SCD). They are particularly challenging to treat and innovative therapies are needed. We previously showed that the healing of SCD ulcers is delayed because of decreased angiogenesis. During pregnancy, fetal microchimeric cells (FMC) transferred to the mother are recruited to maternal wounds and improve angiogenesis. After delivery, FMC persist in maternal bone marrow for decades. Here, we investigated whether fetal cells could also improve SCD ulcers in the post-partum setting. We found that skin healing was similarly improved in post-partum mice and in pregnant mice, through increased proliferation and angiogenesis. In a SCD mouse model that recapitulates refractory SCD ulcers, we showed that the ulcers of post-partum SCD mice healed more quickly than those of virgin mice. This was associated with the recruitment of fetal cells in maternal wounds where they harbored markers of leukocytes and endothelial cells. In a retrospective cohort of SCD patients, using several parameters we found that SCD women who had ever had a baby had less of a burden related to leg ulcers compared to nulliparous women. Taken together, these results indicate that healing capacities of FMC are maintained long after delivery and may be exploited to promote wound healing in post-partum SCD patients.

Fetal cell microchimerism and susceptibiltiy to COVID-19 disease in women

PURPOSE

The clinical outcome of COVID-19 disease is worse in males, and the reasons of this gender disparity are currently unclear, though evidences point to a combination of biological and gender-specific factors. A phenomenon unique to the female gender is the fetal cell microchimerism (FCM), defined as the presence of fetal microchimeric cells in maternal organs and in the circulation for years after delivery and usually evaluated by assessing the presence of male cells or DNA in a woman. In the present case-control study, we aimed to evaluate the possible effect of pregnancy and related FCM on the susceptibility to SARS-CoV-2 infection and on the clinical course and outcome of COVID-19.

METHODS

One hundred twenty-three women with a previous male pregnancy, comprising 63 COVID-19 cases and 60 healthy controls were enrolled. The presence of blood male DNA was assessed by the amplification of the Y-chromosome specific gene SRY.

RESULTS

The prevalence of male DNA of presumed fetal origin was significantly higher in healthy controls than in COVID-19 cases (70 vs 44.4%, P = 0.0044; OR 0.3429, 95% CI 0.1631-0.7207, P = 0.0047). Among women affected with COVID-19, the presence of male FCM did not significantly influence the severity of the disease, though the 8 deceased women studied were all FCM negative.

CONCLUSION

This is the first case-control study reporting the prevalence of FCM in COVID-19 and healthy women. Overall, our data seem to suggest a role for FCM in the protection towards the SARS-CoV-2 infection with a possible positive impact on clinical outcome.

Fetomaternal microchimerism in tissue repair and tumor development

In various placental mammals, the bidirectional exchange of cells during pregnancy can lead to the acquisition of genetically unique cells that can persist in both mother and child for decades. Over the years, it has become increasingly clear that this phenomenon, termed fetomaternal microchimerism may play key roles in a number of biological processes. In this perspective, we explore the concept of fetomaternal microchimerism and outline how fetal microchimeric cells are detected and immunologically tolerated within the maternal setting. Moreover, we discuss undertakings in the field that hint at the significant plasticity of fetal microchimeric cells and their potential roles in promoting maternal wound healing. Finally, we explore the multifaceted roles of fetal microchimeric cells in cancer development and progression. A deeper understanding of fetomaternal chimerism in healthy and diseased states will be key toward developing more efficient anti-cancer treatments and regenerative therapies.

Feto-maternal microchimerism: Memories from pregnancy

There is a bidirectional transplacental cell trafficking between mother and fetus during pregnancy in placental mammals. The presence and persistence of fetal cells in maternal tissues are known as fetal microchimerism (FMc). FMc has high multilineage potential with a great ability to differentiate and functionally integrate into maternal tissue. FMc has been found in various maternal tissues in animal models and humans. Its permanence in the maternal body up to decades after delivery suggests it might play an essential role in maternal pathophysiology. Studying the presence, localization, and characteristics of FMc in maternal tissues is key to understanding its impact on the woman's body. Here we comprehensively review the existence of FMc in different species and organs and tissues, aiming to better characterize their possible role in human health and disease. We also highlight several methodological considerations that would optimize the detection, quantification, and functional determination of FMc.

Integrins mediate placental extracellular vesicle trafficking to lung and liver in vivo

Membrane-bound extracellular vesicles (EVs) mediate intercellular communication in all organisms, and those produced by placental mammals have become increasingly recognized as significant mediators of fetal-maternal communication. Here, we aimed to identify maternal cells targeted by placental EVs and elucidate the mechanisms by which they traffic to these cells. Exogenously administered pregnancy-associated EVs traffic specifically to the lung; further, placental EVs associate with lung interstitial macrophages and liver Kupffer cells in an integrin-dependent manner. Localization of EV to maternal lungs was confirmed in unmanipulated pregnancy using a transgenic reporter mouse model, which also provided in situ and in vitro evidence that fetally-derived EVs, rarely, may cause genetic alteration of maternal cells. These results provide for the first time direct in vivo evidence that placental EVs target maternal immune cells, and further, that EVs can alter cellular phenotype.

Forever Connected: The Lifelong Biological Consequences of Fetomaternal and Maternofetal Microchimerism

BACKGROUND

Originally studied as a mechanism to understand eclampsia-related deaths during pregnancy, fetal cells in maternal blood have more recently garnered attention as a noninvasive source of fetal material for prenatal testing. In the 21st century, however, intact fetal cells have been largely supplanted by circulating cell-free placental DNA for aneuploidy screening. Instead, interest has pivoted to the ways in which fetal cells influence maternal biology. In parallel, an increasing appreciation of the consequences of maternal cells in the developing fetus has occurred.

CONTENT

In this review, we highlight the potential clinical applications and functional consequences of the bidirectional trafficking of intact cells between a pregnant woman and her fetus. Fetal cells play a potential role in the pathogenesis of maternal disease and tissue repair. Maternal cells play an essential role in educating the fetal immune system and as a factor in transplant acceptance. Naturally occurring maternal microchimerism is also being explored as a source of hematopoietic stem cells for transplant in fetal hematopoietic disorders.

SUMMARY

Future investigations in humans need to include complete pregnancy histories to understand maternal health and transplant success or failure. Animal models are useful to understand the mechanisms underlying fetal wound healing and/or repair associated with maternal injury and inflammation. The lifelong consequences of the exchange of cells between a mother and her child are profound and have many applications in development, health, and disease. This intricate exchange of genetically foreign cells creates a permanent connection that contributes to the survival of both individuals.

Pathogenesis of preterm birth: bidirectional inflammation in mother and fetus

Preterm birth (PTB) complicates 5–18% of pregnancies globally and is a leading cause of maternal and fetal morbidity and mortality. Most PTB is spontaneous and idiopathic, with largely undefined causes. To increase understanding of PTB, much research in recent years has focused on using animal models to recapitulate the pathophysiology of PTB. Dysfunctions of maternal immune adaptations have been implicated in a range of pregnancy pathologies, including PTB. A wealth of evidence arising from mouse models as well as human studies is now available to support that PTB results from a breakdown in fetal-maternal tolerance, along with excessive, premature inflammation. In this review, we examine the current knowledge of the bidirectional communication between fetal and maternal systems and its role in the immunopathogenesis of PTB. These recent insights significantly advance our understanding of the pathogenesis of PTB, which is essential to ultimately designing more effective strategies for early prediction and subsequent prevention of PTB.

Multiomic immune clockworks of pregnancy

Preterm birth is the leading cause of mortality in children under the age of five worldwide. Despite major efforts, we still lack the ability to accurately predict and effectively prevent preterm birth. While multiple factors contribute to preterm labor, dysregulations of immunological adaptations required for the maintenance of a healthy pregnancy is at its pathophysiological core. Consequently, a precise understanding of these chronologically paced immune adaptations and of the biological pacemakers that synchronize the pregnancy "immune clock" is a critical first step towards identifying deviations that are hallmarks of peterm birth. Here, we will review key elements of the fetal, placental, and maternal pacemakers that program the immune clock of pregnancy. We will then emphasize multiomic studies that enable a more integrated view of pregnancy-related immune adaptations. Such multiomic assessments can strengthen the biological plausibility of immunological findings and increase the power of biological signatures predictive of preterm birth.

The Cellular Transcriptome in the Maternal Circulation During Normal Pregnancy: A Longitudinal Study

Pregnancy represents a unique immunological state in which the mother adapts to tolerate the semi-allogenic conceptus; yet, the cellular dynamics in the maternal circulation are poorly understood. Using exon-level expression profiling of up to six longitudinal whole blood samples from 49 pregnant women, we undertook a systems biology analysis of the cellular transcriptome dynamics and its correlation with the plasma proteome. We found that: (1) chromosome 14 was the most enriched in transcripts differentially expressed throughout normal pregnancy; (2) the strongest expression changes followed three distinct longitudinal patterns, with genes related to host immune response (e.g., MMP8, DEFA1B, DEFA4, and LTF) showing a steady increase in expression from 10 to 40 weeks of gestation; (3) multiple biological processes and pathways related to immunity and inflammation were modulated during gestation; (4) genes changing with gestation were among those specific to T cells, B cells, CD71+ erythroid cells, natural killer cells, and endothelial cells, as defined based on the GNF Gene Expression Atlas; (5) the average expression of mRNA signatures of T cells, B cells, and erythroid cells followed unique patterns during gestation; (6) the correlation between mRNA and protein abundance was higher for mRNAs that were differentially expressed throughout gestation than for those that were not, and significant mRNA-protein correlations were observed for genes part of the T-cell signature. In summary, unique changes in immune-related genes were discovered by longitudinally assessing the cellular transcriptome in the maternal circulation throughout normal pregnancy, and positive correlations were noted between the cellular transcriptome and plasma proteome for specific genes/proteins. These findings provide insights into the immunobiology of normal pregnancy.

Fetal microchimerism and implications for maternal health

This review paper outlines the definition, pathophysiology, and potential maternal health consequences of cellular fetal microchimerism, the maternal acquisition of intact cells of fetal origin during pregnancy. Increased rates and amounts of cellular fetal microchimerism are associated with several placental syndromes, including preeclampsia and fetal growth restriction. The discovery of cellular fetal microchimerism and methods of detection are briefly outlined, and we present the mechanisms hypothesized to govern pregnancy-related and long-term maternal health effects of cellular fetal microchimerism. Specifically, we discuss the potential implications of cellular fetal microchimerism in wound healing, autoimmunity, cancer, and possibly cardiovascular disease. Cellular fetal microchimerism represents a novel area of research on maternal and transgenerational health and disease, providing exciting opportunities for developing new disease biomarkers and precision medicine with targeted prophylaxis against long-term maternal disease.

Immunological implications of pregnancy-induced microchimerism

Immunological identity is traditionally defined by genetically encoded antigens, with equal maternal and paternal contributions as a result of Mendelian inheritance. However, vertically transferred maternal cells also persist in individuals at very low levels throughout postnatal development. Reciprocally, mothers are seeded during pregnancy with genetically foreign fetal cells that persist long after parturition. Recent findings suggest that these microchimeric cells expressing non-inherited, familially relevant antigenic traits are not accidental 'souvenirs' of pregnancy, but are purposefully retained within mothers and their offspring to promote genetic fitness by improving the outcome of future pregnancies. In this Review, we discuss the immunological implications, benefits and potential consequences of individuals being constitutively chimeric with a biologically active 'microchiome' of genetically foreign cells.

Multiparity improves outcomes after cerebral ischemia in female mice despite features of increased metabovascular risk

Females show a varying degree of ischemic sensitivity throughout their lifespan, which is not fully explained by hormonal or genetic factors. Epidemiological data suggest that sex-specific life experiences such as pregnancy increase stroke risk. This work evaluated the role of parity on stroke outcome. Age-matched virgin (i.e., nulliparous) and multiparous mice were subjected to 60 min of reversible middle cerebral artery occlusion and evaluated for infarct volume, behavioral recovery, and inflammation. Using an established mating paradigm, fetal microchimeric cells present in maternal mice were also tracked after parturition and stroke. Parity was associated with sedentary behavior, weight gain, and higher triglyceride and cholesterol levels. The multiparous brain exhibited features of immune suppression, with dampened baseline microglial activity. After acute stroke, multiparous mice had smaller infarcts, less glial activation, and less behavioral impairment in the critical recovery window of 72 h. Behavioral recovery was significantly better in multiparous females compared with nulliparous mice 1 mo after stroke. This recovery was accompanied by an increase in poststroke angiogenesis that was correlated with improved performance on sensorimotor and cognitive tests. Multiparous mice had higher levels of VEGF, both at baseline and after stroke. GFP+ fetal cells were detected in the blood and migrated to areas of tissue injury where they adopted endothelial morphology 30 d after injury. Reproductive experience has profound and complex effects on neurovascular health and disease. Inclusion of female mice with reproductive experience in preclinical studies may better reflect the life-long patterning of ischemic stroke risk in women.

Novel insights into the link between fetal cell microchimerism and maternal cancers

INTRODUCTION

Fetal cell microchimerism (FCM) is defined as the persistence of fetal cells in the mother for decades after pregnancy without any apparent rejection. Fetal microchimeric cells (fmcs) engraft the maternal bone marrow and are able to migrate through the circulation and to reach tissues. In malignancies, the possible role of fmcs is still controversial, several studies advising a protective and repairing function, and other postulating a beneficial role in the progression of the disease. At the peripheral blood level, FCM is less frequently observed in women with several solid and hematological neoplasia with respect to healthy controls, suggesting a beneficial role in cancer surveillance. At the tissue level, fmcs were documented in neoplastic lesions of thyroid, breast, cervix, lung and melanoma, displaying epithelial, hematopoietic, mesenchymal and endothelial lineage differentiation. Fmcs expressing hematopoietic markers were hypothesized to have a role in the attack to neoplastic cells, whereas those expressing epithelial or mesenchymal antigens could be involved in repair and replacement of damaged cells. On the other hand, fetal cells showing an endothelial phenotype could have a role in tumor evolution and progression. The positive effect of FCM is supported by findings in animal models.

CONCLUSIONS

This review provides an extensive overview of the link between fetal cell microchimerism and maternal cancers. Moreover, biological mechanisms by which fetal cell microchimerism is believed to modulate the protection against cancer development or tumor progression will be discussed, together with findings in animal models.

The Number of Endovascular Trophoblasts in Maternal Blood Increases Overnight and after Physical Activity: An Experimental Study

INTRODUCTION

Fetal cells in maternal blood may be used for noninvasive prenatal diagnostics, although their low number is a challenge. This study's objectives were to evaluate whether physical activity, transabdominal and transvaginal ultrasound scans of the uterus, as well as overnight or day-to-day variation affect the number of isolated fetal cells, more specifically the presumed endovascular trophoblast (pEVT).

MATERIAL AND METHODS

In each of 3 different experiments, 10 normal singleton pregnant women (gestational age 10+4-14+4 weeks) participated. The number of pEVTs was assessed in 30-36 ml blood using specific markers for enrichment and identification.

RESULTS

The number of pEVTs increased overnight (p = 0.001) from a median of 1.5 to 3.5 and even further to a median of 6.0 after 30 min of physical activity (p = 0.04) but was not affected by transabdominal and transvaginal ultrasound scans. Repeated sampling showed that the interindividual variation of pEVTs was higher than the intraindividual variation (p < 0.001). However, even in pregnant women with a consistently low number of pEVTs, isolation of the pEVTs for prenatal diagnoses was possible in all cases by doing 2 separate blood samplings a few days apart.

DISCUSSION

The number of pEVTs identified in maternal blood can be increased by presampling conditions or repeated sampling.

Fetal microchimerism and maternal health: a review and evolutionary analysis of cooperation and conflict beyond the womb

The presence of fetal cells has been associated with both positive and negative effects on maternal health. These paradoxical effects may be due to the fact that maternal and offspring fitness interests are aligned in certain domains and conflicting in others, which may have led to the evolution of fetal microchimeric phenotypes that can manipulate maternal tissues. We use cooperation and conflict theory to generate testable predictions about domains in which fetal microchimerism may enhance maternal health and those in which it may be detrimental. This framework suggests that fetal cells may function both to contribute to maternal somatic maintenance (e.g. wound healing) and to manipulate maternal physiology to enhance resource transmission to offspring (e.g. enhancing milk production). In this review, we use an evolutionary framework to make testable predictions about the role of fetal microchimerism in lactation, thyroid function, autoimmune disease, cancer and maternal emotional, and psychological health.

Also watch the Video Abstract.

Cigarette smoke exposure during pregnancy alters fetomaternal cell trafficking leading to retention of microchimeric cells in the maternal lung

Cigarette smoke exposure causes chronic oxidative lung damage. During pregnancy, fetal microchimeric cells traffic to the mother. Their numbers are increased at the site of acute injury. We hypothesized that milder chronic diffuse smoke injury would attract fetal cells to maternal lungs. We used a green-fluorescent-protein (GFP) mouse model to study the effects of cigarette smoke exposure on fetomaternal cell trafficking. Wild-type female mice were exposed to cigarette smoke for about 4 weeks and bred with homozygote GFP males. Cigarette smoke exposure continued until lungs were harvested and analyzed. Exposure to cigarette smoke led to macrophage accumulation in the maternal lung and significantly lower fetal weights. Cigarette smoke exposure influenced fetomaternal cell trafficking. It was associated with retention of GFP-positive fetal cells in the maternal lung and a significant reduction of fetal cells in maternal livers at gestational day 18, when fetomaternal cell trafficking peaks in the mouse model. Cells quickly clear postpartum, leaving only a few, difficult to detect, persisting microchimeric cells behind. In our study, we confirmed the postpartum clearance of cells in the maternal lungs, with no significant difference in both groups. We conclude that in the mouse model, cigarette smoke exposure during pregnancy leads to a retention of fetal microchimeric cells in the maternal lung, the site of injury. Further studies will be needed to elucidate the effect of cigarette smoke exposure on the phenotypic characteristics and function of these fetal microchimeric cells, and confirm its course in cigarette smoke exposure in humans.

Preclinical in vivo modeling of cytokine release syndrome induced by ErbB-retargeted human T cells: identifying a window of therapeutic opportunity?

The ErbB network is dysregulated in many solid tumors. To exploit this, we have developed a chimeric Ag receptor (CAR) named T1E28z that targets several pathogenetically relevant ErbB dimers. T1E28z is coexpressed with a chimeric cytokine receptor named 4αβ (combination termed T4), enabling the selective expansion of engineered T cells using IL-4. Human T4(+) T cells exhibit antitumor activity against several ErbB(+) cancer types. However, ErbB receptors are also expressed in several healthy tissues, raising concerns about toxic potential. In this study, we have evaluated safety of T4 immunotherapy in vivo using a SCID beige mouse model. We show that the human T1E28z CAR efficiently recognizes mouse ErbB(+) cells, rendering this species suitable to evaluate preclinical toxicity. Administration of T4(+) T cells using the i.v. or intratumoral routes achieves partial tumor regression without clinical or histopathologic toxicity. In contrast, when delivered i.p., tumor reduction is accompanied by dose-dependent side effects. Toxicity mediated by T4(+) T cells results from target recognition in both tumor and healthy tissues, leading to release of both human (IL-2/IFN-γ) and murine (IL-6) cytokines. In extreme cases, outcome is lethal. Both toxicity and IL-6 release can be ameliorated by prior macrophage depletion, consistent with clinical data that implicate IL-6 in this pathogenic event. These data demonstrate that CAR-induced cytokine release syndrome can be modeled in mice that express target Ag in an appropriate distribution. Furthermore, our findings argue that ErbB-retargeted T cells can achieve therapeutic benefit in the absence of unacceptable toxicity, providing that route of administration and dose are carefully optimized.

Pregnancy-acquired fetal progenitor cells

The transfer and persistence of fetal progenitor cells into the mother throughout pregnancy has sparked considerable interest as a trafficking stem cell and immunological phenomenon. Indeed, the intriguing longevity of semi-allogeneic fetal microchimeric cells (FMC) in parous women raises questions over their potential clinical implications. FMC have been associated with both immune-modulatory roles and participation in maternal tissue repair. Although their influence on maternal health is as yet unresolved, FMC selectively home to damaged maternal tissues and often integrate, adopting site-appropriate phenotypes. FMC features, such as plasticity and persistence in their maternal host, suggest that they likely include pluripotent, or various multipotent and committed stem and progenitor cells. Recent efforts to determine what cell types are involved have established that FMC include cells of ectodermal, endodermal, mesodermal, and perhaps trophectodermal lineages. This review details FMC phenotypes and discusses how FMC themselves may be considered a naturally occurring stem cell therapy.

Distant mesenchymal progenitors contribute to skin wound healing and produce collagen: evidence from a murine fetal microchimerism model

The contribution of distant and/or bone marrow-derived endogenous mesenchymal stem cells (MSC) to skin wounds is controversial. Bone marrow transplantation experiments employed to address this have been largely confounded by radiation-resistant host-derived MSC populations. Gestationally-acquired fetal MSC are known to engraft in maternal bone marrow in all pregnancies and persist for decades. These fetal cells home to damaged maternal tissues, mirroring endogenous stem cell behavior. We used fetal microchimerism as a tool to investigate the natural homing and engraftment of distant MSC to skin wounds. Post-partum wild-type mothers that had delivered transgenic pups expressing luciferase under the collagen type I-promoter were wounded. In vivo bioluminescence imaging (BLI) was then used to track recruitment of fetal cells expressing this mesenchymal marker over 14 days of healing. Fetal cells were detected in 9/43 animals using BLI (Fisher exact p = 0.01 versus 1/43 controls). These collagen type I-expressing fetal cells were specifically recruited to maternal wounds in the initial phases of healing, peaking on day 1 (n = 43, p<0.01). This was confirmed by detection of Y-chromosome+ve fetal cells that displayed fibroblast-like morphology. Histological analyses of day 7 wounds revealed vimentin-expressing fetal cells in dermal tissue. Our results demonstrate the participation of distant mesenchymal cells in skin wounds. These data imply that endogenous MSC populations are likely recruited from bone marrow to wounds to participate in healing.

High male chimerism in the female breast shows quantitative links with cancer

Clinical observations suggest that pregnancy provides protection against cancer. The mechanisms involved, however, remain unclear. Fetal cells are known to enter the mother's circulation during pregnancy and establish microchimerism. We investigated if pregnancy-related embryonic/fetal stem cell integration plays a role in breast cancer. A high-sensitivity Y-chromosome assay was developed to trace male allogeneic cells (from male fetus) in females. Fixed-embedded samples (n = 206) from both normal and breast cancer patients were screened for microchimerism. The results were combined with matching clinicopathological and histological parameters and processed statistically. The results show that in our samples (182 informative) more than half of healthy women (56%) carried male cells in their breast tissue for decades (n = 68), while only one out of five in the cancer sample pool (21%) (n = 114) (odds ratio = 4.75, CI at 95% 2.34-9.69; p = 0.0001). The data support the notion that a biological link may exist between chimerism and tissue-integrity. The correlation, however, is non-linear, since male microchimerism in excess ("hyperchimerism") is also involved in cancer. The data suggest a link between hyperchimerism and HER2-type cancers, while decreased chimerism ("hypochimerism") associates with ER/PR-positive (luminal-type) breast cancers. Chimerism levels that correlate with protection appear to be non-random and share densities with the mammary progenitor components of the stem cell lineage in the breast. The results suggest that protection may involve stem/progenitor level interactions and implicate novel quantitative mechanisms in chimerism biology.

Generation of human-to-pig chimerism to induce tolerance through transcutaneous in utero injection of cord blood-derived mononuclear cells or human bone marrow mesenchymals cells in a preclinical program of liver xenotransplantation: preliminary results

OBJECTIVE

Using a percutaneous ecoguided injection system to obtain chimeric piglets through a less invasive and traumatic technique than previously reported.

MATERIALS AND METHODS

The two types of human cells included umbilical cord blood mononuclear elements and mesenchymal stem cells cultured from bone marrow. Four sows at gestational day 50 were anesthetized. A needle was inserted through the skin and uterine wall to reach the peritoneal cavity of the fetuses under continuous ultrasound guidance. Fourteen piglets were injected with various cell concentrations.

RESULTS

All sows carried pregnancies to term yielding 69 piglets, among which 67 were alive and two mummified. Two piglets died during the first 48 hours of life. Chimerism was detected using flow cytometry and by quantitative polymerase chain reaction (q-PCR) to detect Alu gene in blood or tissues samples. The analysis detected blood chimerism in 13 piglets (21%) by flow cytometry and the presence of the human Alu gene in 33 (51%) by q-PCR. The results suggest cell trafficking between littermates after in utero injection.

CONCLUSIONS

Transcutaneous echo-guided injection succeeded to produce chimeric piglets without disadvantages to the sow or the fetuses and avoiding abortions or fetal death.

Microchimerism in the human brain: more questions than answers

Recently, our group reported the presence of microchimerism (Mc) in the human brain by performing quantitative PCR on female human brain tissues to amplify male DNA. We found brain Mc to be relatively frequent in humans and widely distributed in this organ. Our data also suggested a lower prevalence of brain Mc in women without Alzheimer disease than women without neurological disease. Altogether, these findings suggest that Mc could sometimes influence health and disease of the brain. As further research will be required to clarify this issue, here we discuss some of the questions that could be addressed to improve our understanding.

Male microchimerism in the human female brain

In humans, naturally acquired microchimerism has been observed in many tissues and organs. Fetal microchimerism, however, has not been investigated in the human brain. Microchimerism of fetal as well as maternal origin has recently been reported in the mouse brain. In this study, we quantified male DNA in the human female brain as a marker for microchimerism of fetal origin (i.e. acquisition of male DNA by a woman while bearing a male fetus). Targeting the Y-chromosome-specific DYS14 gene, we performed real-time quantitative PCR in autopsied brain from women without clinical or pathologic evidence of neurologic disease (n=26), or women who had Alzheimer's disease (n=33). We report that 63% of the females (37 of 59) tested harbored male microchimerism in the brain. Male microchimerism was present in multiple brain regions. Results also suggested lower prevalence (p=0.03) and concentration (p=0.06) of male microchimerism in the brains of women with Alzheimer's disease than the brains of women without neurologic disease. In conclusion, male microchimerism is frequent and widely distributed in the human female brain.

Comprehensive analysis of genes expressed by rare microchimeric fetal cells in the maternal mouse lung

During pregnancy, cells from each fetus travel into the maternal circulation and organs, resulting in the development of microchimerism. Identification of the cell types in this microchimeric population would permit better understanding of possible mechanisms by which they affect maternal health. However, comprehensive analysis of fetal cells has been hampered by their rarity. In this study, we sought to overcome this obstacle by combining flow cytometry with multidimensional gene expression microarray analysis of fetal cells isolated from the murine maternal lung during late pregnancy. Fetal cells were collected from the lungs of pregnant female mice. cDNA was amplified and hybridized to gene expression microarrays. The resulting fetal cell core transcriptome was interrogated using multiple methods including Ingenuity Pathway Analysis, the BioGPS gene expression database, principal component analysis, the Eurexpress gene expression atlas, and primary literature. Here we report that small numbers of fetal cells can be flow sorted from the maternal lung, facilitating discovery-driven gene expression analysis. We additionally show that gene expression data can provide functional information about fetal cells. Our results suggest that fetal cells in the murine maternal lung are a mixed population, consisting of trophoblasts, mesenchymal stem cells, and cells of the immune system. Detection of trophoblasts and immune cells in the maternal lung may facilitate future mechanistic studies related to the development of immune tolerance and pregnancy-related complications, such as pre-eclampsia. Furthermore, the presence and persistence of mesenchymal stem cells in maternal organs may have implications for long-term postpartum maternal health.

Fetal microchimeric cells in a fetus-treats-its-mother paradigm do not contribute to dystrophin production in serially parous mdx females

Throughout every pregnancy, genetically distinct fetal microchimeric stem/progenitor cells (FMCs) engraft in the mother, persist long after delivery, and may home to damaged maternal tissues. Phenotypically normal fetal lymphoid progenitors have been described to develop in immunodeficient mothers in a fetus-treats-its-mother paradigm. Since stem cells contribute to muscle repair, we assessed this paradigm in the mdx mouse model of Duchenne muscular dystrophy. mdx females were bred serially to either ROSAeGFP males or mdx males to obtain postpartum microchimeras that received either wild-type FMCs or dystrophin-deficient FMCs through serial gestations. To enhance regeneration, notexin was injected into the tibialis anterior of postpartum mice. FMCs were detected by qPCR at a higher frequency in injected compared to noninjected side muscle (P=0.02). However, the number of dystrophin-positive fibers was similar in mothers delivering wild-type compared to mdx pups. In addition, there was no correlation between FMC detection and percentage dystrophin, and no GFP+ve FMCs were identified that expressed dystrophin. In 10/11 animals, GFP+ve FMCs were detected by immunohistochemistry, of which 60% expressed CD45 with 96% outside the basal lamina defining myofiber contours. Finally we confirmed lack of FMC contribution to statellite cells in postpartum mdx females mated with Myf5-LacZ males. We conclude that the FMC contribution to regenerating muscles is insufficient to have a functional impact.

The natural history of fetal cells in postpartum murine maternal lung and bone marrow: a two-stage phenomenon

During pregnancy, fetal cells cross into the maternal organs where they reside postpartum. Evidence from multiple laboratories suggests that these microchimeric fetal cells contribute to maternal tissue repair after injury. In mouse models, most injury experiments are performed during pregnancy; however, in a clinical setting most injuries or diseases occur postpartum. Therefore, experiments using animal models should be designed to address questions in the time period following delivery. In order to provide a baseline for such experiments, we analyzed the natural history of fetal cells in the postpartum maternal organs. Female C57BL/6J mice were mated to males homozygous for the enhanced green fluorescent protein gene. Fetal cells in the maternal lungs and bone marrow were identified by their green fluorescence using in a high-speed flow cytometer and their counts were compared between the lung and bone marrow. Spearman correlation analysis was used to identify relationships between the duration of time postpartum and the cell counts and ratio of live and dead cells. Our results show that fetal cells persist in these organs until at least three months postpartum in healthy female mice. We show a two-stage decline, with an initial two and a half-week rapid clearance followed by a trend of gradual decrease. Additionally, an increase in the ratio of live to dead cells within the lung over time suggests that these cells may replicate in vivo. The results presented here will inform the design of future experiments and may have implications for women's health.

A mouse model for fetal maternal stem cell transfer during ischemic cardiac injury

Fetal cells enter the maternal circulation during pregnancies and can persist in blood and tissues for decades, creating a state of physiologic microchimerism. Microchimerism refers to acquisition of cells from another individual and can be due to bidirectional cell traffic between mother and fetus during pregnancy. Peripartum cardiomyopathy, a rare cardiac disorder associated with high mortality rates has the highest recovery rate amongst all etiologies of heart failure although the reason is unknown. Collectively, these observations led us to hypothesize that fetal cells enter the maternal circulation and may be recruited to the sites of myocardial disease or injury. The ability to genetically modify mice makes them an ideal system for studying the phenomenon of microchimerism in cardiac disease. Described here is a mouse model for ischemic cardiac injury during pregnancy designed to study microchimerism. Wild-type virgin female mice mated with eGFP male mice underwent ligation of the left anterior descending artery to induce a myocardial infarction at gestation day 12. We demonstrate the selective homing of eGFP cells to the site of cardiac injury without such homing to noninjured tissues suggesting the presence of precise signals sensed by fetal cells enabling them to target diseased myocardium specifically.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

Pro-inflammatory effector Th cells transmigrate through anti-inflammatory environments into the murine fetus

The presence of maternal DNA or even maternal cells within the offspring (microchimerism) has been reported for many fetal tissues, including the liver, heart, and spleen. Microchimerism is believed to be involved in the pathogenesis of autoimmune diseases; however, the cellular origin of this phenomenon remains unknown. Here, we determined whether differentiated T lymphocytes could transmigrate through the immunosuppressive environment of the placenta to reach the fetus. In vitro-differentiated effector/memory Th1 and Th17 cells from OVA₃₂₃₋₃₃₉-specific TCR(tg) T cells of OT-II mice were adoptively transferred (i.v.) into the tail veins of pregnant Ly5.1 mice at d15 and d19 of gestation. Mice were then sacrificed 40 h after adoptive cell transfer. Using radioactive labeling of T cells with sodium chromate [Cr⁵¹] prior to adoptive transfer, we observed that homing of pro-inflammatory Th cells was equally efficient in both pregnant and non-pregnant mice. Transmigration of Th1- and Th17-like cells through the highly immunosuppressive environment of the placenta into the fetus was significantly enhanced in experimental mice compared to control mice (P < 0.0001). In addition, a substantial amount of effector Th cells accumulated in the placenta. Finally, we found that treatment with Pertussis Toxin resulted in a 3-fold increase in the transmigration of effector Th17 cells into the fetus (P < 0.0001). When pro-inflammatory Th1-or Th17-like cells were injected into syngeneic mothers, almost all of the fetuses analyzed exhibited radioactivity, suggesting that transmigration of effector T cells occurs frequently. Our results suggest the possibility of novel roles for these maternal effector cells in the pathogenesis or reduction of disease.

Fetal cells in the murine maternal lung have well-defined characteristics and are preferentially located in alveolar septum

The transfer of fetal cells to maternal organs occurs in mouse and human pregnancy. Techniques such as polymerase chain reaction and flow cytometry do not permit study of fetal cell morphology or anatomic location. Using a green fluorescent protein (GFP) transgenic mouse model, our objective was to determine whether GFP+ signal emanates from intact or degraded fetal cells, and whether they have a characteristic appearance and location within maternal lung. Four wild-type female mice were mated to males homozygous for the Gfp transgene and studied at days e16-18. Controls were 2 females mated to wild-type males. Morphologic appearance and anatomic position of each GFP+ object within maternal lung was recorded. GFP signals were sufficiently bright to be visualized without anti-GFP antibody and were confirmed by confocal microscopy to be separate from fluorescent artifact. Of 438 GFP+ objects detected, 375 (85.6%) were from intact cells, and 63 (14.4%) were acellular. Four distinct categories of intact cells were observed. Of these, 23.2% had mononuclear morphology with a relatively large nucleus and GFP+ cytoplasm (Group A). An additional group of cells (10.1%) had mononuclear morphology and podocyte extensions (Group B). The remainder of cells had fragmented nuclei or cytoplasm. Both intact cells and acellular fragments were predominantly localized to the maternal alveolar septum (P<0.0001). This study demonstrates that fetal GFP+ cells are predominantly located in the alveolar septum and have characteristic morphologies, although it remains unclear whether these represent distinct categories of cells or degrading cells. Nevertheless, this naturally acquired population of fetal cells in maternal lung should be considered in studies of lung biology and repair.

Maternal background strain influences fetal-maternal trafficking more than maternal immune competence in mice

The objective of this study was to determine if fetal-maternal cell trafficking is affected by maternal immune competence and/or parental background strain using fluorescence-activated cell sorting (FACS). In our experience the sensitivity of FACS allows for the detection of 5 fetal in 10(7) maternal cells and assessment of cell surface phenotype. Wild-type C57BL/6J (n=18), FVB/NJ (n=15), and immunodeficient B6129S7-Rag1(tm1Mom)/J (n=16) female mice were mated to C57BL/6J males homozygous for the green fluorescent protein (GFP) transgene. Single cell suspensions of maternal lung, liver, spleen, bone marrow, and blood were analyzed between late gestation (day e16-18) and 1 day post-partum for the number of GFP-positive fetal cells in relation to 10(7) maternal cells and the percentage of GFP-positive cells that expressed the surface markers CD11b, CD29, CD34, CD44, or CD105. The highest relative proportions of GFP-positive fetal cells were observed in maternal lungs and livers from immunocompetent allogenic females. Among congenic matings, fetal cell microchimerism was higher in immunodeficient compared with immunocompetent females. Maternal strain and strain differences between the mother and father statistically significantly affected both the numbers of fetal cells and the relative distribution of cell types in maternal organs. The highest relative proportion of fetal cells was observed in allogenic matings with immunocompetent females. Since allogenic matings are more similar to those that occur in humans, future studies using animal models of microchimerism should consider incorporating this type of experimental design.

Pregnancy-associated progenitor cells: an under-recognized potential source of stem cells in maternal lung

Novel therapies are needed for the treatment of acute and chronic lung diseases, many of which are incurable. The use of exogenous stem cells has shown promise in both animal models and clinical trials. However, to date, the stem cell literature has under-recognized naturally acquired pregnancy-associated progenitor cells (PAPCs). These cells are found at sites of injury or disease in female tissues. They persist for decades after parturition in maternal blood and organs, with the largest number being found in the maternal lungs. Their presence there may be one explanation for the sex differences observed in the prevalence and prognosis of some lung diseases. Although the clinical significance of these cells is as yet unknown, the literature suggests that some of the PAPCs are stem cells or have stem cell-like properties. PAPCs harvested from the blood or organs of parous women could potentially be used as an alternate source of cells with regenerative properties for the woman herself or her children. Because PAPCs preferentially traffic to the maternal lung they may play a significant role in recovery or protection from lung disease. In this review article, we discuss ongoing research investigating the administration of both adult and placenta-derived stem cells to treat lung disease, and how PAPCs may also play an important future therapeutic role.

Fetal cell microchimerism and cancer: a nexus of reproduction, immunology, and tumor biology

Fetal cell microchimerism (FCM) is the persistence of fetal cells in the maternal circulation and organs following pregnancy. Proposed hypotheses about the function of fetal cells in the pathogenesis of maternal cancer include promotion of tumorigenesis, protection by providing immunosurveillance, and participation in tissue repair. To date, studies of FCM and cancer have been primarily descriptive and quantitative. More research is needed to understand the cellular phenotype of the microchimeric cells in maternal tumors and whether they have a functional role. This research will require further study using a multidisciplinary approach, incorporating knowledge of the fetomaternal relationship, tumor biology, immunology, and clinical oncology.

Fetal microchimerism as an explanation of disease

Fetal cell microchimerism is defined as the persistence of fetal cells in the mother after birth without any apparent rejection. Fetal microchimeric cells (FMCs) engraft into the maternal bone marrow for decades after delivery and are able to migrate to blood and tissues. This phenomenon was hypothesized to have a detrimental role in autoimmune diseases, but data are still controversial and debated. In malignant tumors, fetal cell microchimerism has been postulated to have a positive effect on tumor burden, although some evidence suggests that FMCs may be involved in neoplastic progression. At the peripheral level, circulating FMCs are less frequently detected in patients with thyroid cancer, breast cancer or other solid, hematologic malignancies than in healthy individuals, which suggests a protective role for fetal cell microchimerism. In tissues, FMCs have been found in tumor sections from malignancies such as thyroid, breast, cervix, lung cancers and melanomas and have been shown to differentiate into epithelial, hematopoietic, endothelial and mesenchymal cells. FMCs with hematopoietic differentiation have been postulated to have a role in destroying the tumor, whereas mesenchymal and epithelial cells could participate in repair processes. Endothelial cells, on the other hand, are believed to play a part in tumor progression. This Review provides an overview of the role of fetal cell microchimerism in autoimmune and benign or malignant nonautoimmune diseases. Moreover, the mechanisms by which fetal cell microchimerism is believed to modulate the protection against cancer or tumor progression will be discussed, together with future research directions.

Transplacental traffic after in utero mesenchymal stem cell transplantation

Transplacental traffic of fetal progenitor and differentiated cells is a well-known phenomenon in pregnancies. We hypothesize that intrauterine stem cell transplantation leads to microchimerism in the dams and that this is gestational age-dependent. EGFP+ fetal liver-derived mesenchymal stem cell (MSC) (10(5) per fetus) were injected intraperitoneally into congeneic and allogeneic recipient fetuses at E12 versus E13.5 of murine pregnancy (56 dams). Engraftment in maternal organs was evaluated using TaqMan quantitative polymerase chain reaction (PCR) and fluorescence microscopy during pregnancy (1, 3, and 7 days after in utero transplantation [IUT]) and after delivery (1 and 4 weeks after delivery). One day after IUT donor cells were mainly found in the placenta (E12: 9/10 dams vs. E13.5: 4/8 dams) and laparotomy site (E12: 5/10 dams vs. E13.5: 4/8 dams). Three days after IUT these probabilities decreased significantly in the placenta to 3/8 and 1/3, respectively, whereas it was increased within the surgical wound to 8/8 and 2/4. One week after IUT donor cells could be detected in other single maternal organs, such as bone marrow or spleen. The surgical wound was chimeric in all dams. One week after delivery the surgical wound was still a major site of engraftment in both groups. E12 IUT resulted in detectable donor cell microchimerism in the maternal bone marrow (3/4), liver (2/4), lungs (1/4), spleen (1/4), and thymus (1/4), whereas engraftment probabilities were lower following E13.5 IUT (BM: 1/4, liver: 2/4, lungs: 1/4, spleen: 1/4, thymus: 0/4). At 4 weeks after delivery persistent microchimerism was found only after E12 IUT in various maternal organs (BM: 1/4, spleen: 1/4, lungs: 1/4) and within newly created surgical wounds (3/4), but completely not in the E13.5 group. Allogeneic IUT did also not result in any detectable long-term fetal microchimerism. An earlier IUT might lead to a higher transplacental traffic of donor MSC and persistent microchimerism within maternal tissues. Even 4 weeks after delivery, these cells are present in surgical wounds.

Fetal microchimerism and women's health: a new paradigm

Pregnancy is associated with transfer of maternal cells to the fetus and fetal cells to the mother. In both cases, the transferred cells are described as microchimeric. Fetal microchimeric cells include semi-allogeneic stem cells, which are few in number and are capable of long-term survival in the "foreign" host. They are recognized by the maternal immune system but not rejected or attacked. These cells appear to survive and even thrive for years in a mother's body, perhaps for her lifetime. Previously regarded as potentially dangerous interlopers that might propagate autoimmune and even malignant disease, fetal microchimeric cells are now increasingly being recognized and analyzed for their healing, reparative, and perhaps regenerative roles. Fetal microchimerism (MC) may make significant and previously unknown positive contributions to women's health, longevity, and risk of disease. This article reviews the history, major discoveries, and current concepts and gaps in knowledge in the field of fetal MC.

Pregnancy-associated progenitor cells differentiate and mature into neurons in the maternal brain

Bidirectional cell trafficking between fetus and mother during pregnancy is a well-established phenomenon observed in placental vertebrates including humans. Although studies have shown that transmigratory fetal cells, also termed pregnancy-associated progenitor cells (PAPCs), can integrate into multiple maternal organs, the integration, long-term survival, and differentiation of PAPCs in the brain has not been extensively studied. Using a murine model of fetomaternal microchimerism, we show that PAPCs integrated and persisted in several areas of the maternal brain for up to 7 months postpartum. Besides expressing neural stem cell or immature neuronal markers, PAPCs were observed to express mature neuronal markers, indicating that PAPCs adopted a neuronal fate. Further, PAPCs also displayed morphologically neuronal maturation by an increasing axonal/dendritic complexity over time. Therefore, PAPCs seem to undergo a molecular and morphological maturation program similar to that observed during adult neurogenesis. We provide evidence that neuronal gene expression of PAPCs was not a consequence of cell fusion with maternal neurons. In addition, in mothers with experimentally induced Parkinson's disease (PD), the frequency of PAPCs within the hippocampus initially increased whereas long-term presence of PAPCs was compromised. Also, the spatial distribution of PAPCs within the hippocampus was altered in mothers with PD. Thus, the disease context influenced the initial attraction, long-term survival, and spatial distribution of PAPCs, which may have wider implications on cell replacement strategies in human neurodegenerative diseases such as PD.

Fetal-maternal exchange of multipotent stem/progenitor cells: microchimerism in diagnosis and disease

The biological concept of microchimerism, the bidirectional trafficking and stable long-term persistence of small numbers of allogeneic (fetal and maternal) cells in a genetically different organ, has gained considerable attention. Microchimerism is a common phenomenon in many species, including humans, and microchimeric cells can modify immunological recognition or tolerance, affect the course and outcome of various diseases and demonstrate stem cell-like or regenerative potential. Here, we review current knowledge of the biology of microchimerism and show how long-term allogeneic co-existence within an organism can impact on existing paradigms in chronic disease, cancer biology, regenerative medicine and fetal-maternal immunology. We discuss diagnostic challenges, clinical applications and future research directions in this exciting and rapidly emerging field of allogeneic fetal-maternal cell exchange.

Increased fetal cell trafficking in murine lung following complete pregnancy loss from exposure to lipopolysaccharide

To determine whether chemically induced miscarriage affects fetomaternal trafficking in a mouse model, we measured the amount of fetal DNA present in various maternal organs by polymerase chain reaction amplification following exposure to lipopolysaccharide (LPS). As the frequency of fetal cells and the number of animals with detectable microchimerism following LPS injection were significantly increased, particularly in lung tissue compared to controls, with no signs of an inflammatory response, we conclude that LPS-induced miscarriage results in increased murine fetomaternal cell trafficking, supporting a relationship between fetal loss and the establishment of fetal cell microchimerism.

Fetal cells in the pregnant mouse are diverse and express a variety of progenitor and differentiated cell markers

To better understand fetomaternal cell trafficking during pregnancy, we used a mouse model to determine the cell surface markers expressed on fetal cells, based on the hypothesis that fetal progenitor cells have the capacity to repair maternal organs, whereas more differentiated cells might initiate graft versus host disease. Wild-type females were mated to either homozygous or hemizygous transgenic males and euthanized in the peripartum period. Using dual color flow cytometry, we analyzed fetal transgene positive cells for the presence of nine markers (ITGAM, ITGB1, PECAM, CD34, CD44, PTPRC, ENG, SLAMF1, and CXCR4) to begin to identify the phenotype and degree of differentiation of fetal cells in nine maternal organs (lung, liver, spleen, blood, bone marrow, kidney, heart, thymus, and brain). Fetal cells were found in all maternal organs following either type of mating, albeit always at a higher frequency following mating with homozygous males. Some organs (e.g., lung and liver) had a wide variety of fetal cell markers present, while other organs (e.g., bone marrow and spleen) had a skewed distribution of fetal cell markers. Fetal cells in the murine pregnant female are diverse. Our results suggest that the fetal cells comprise a mixed population of progenitor and differentiated cells, with different relative proportions in different maternal organs. Future studies will address whether fetal cells cross the placental barrier in a differentiated state or as a homogenous population and subsequently differentiate in target maternal organs.