Microchimerism in human diseases

Self or nonself: end of a dogma?

Immunologists generally view the notion of self and non-self as part of a broader, more contextual understanding of immune function, rather than a rigid dogma. While the classical paradigm that the primary role of the immune system is to recognize and eliminate anything foreign once provided a unifying basis for explaining tolerance and rejection, numerous discoveries have focused attention on how immune responses are finely tuned by a range of contextual cues, including tissue signals, hygienist theory, molecular mimicry, symbiotic microbes, metabolic factors and epigenetic modifications. Maternal-fetal tolerance and the persistence of microchimeric cells in adults demonstrate that genetically foreign cells can be actively integrated into the host, challenging the simple assumption that 'foreign' equals unconditional attack. Similarly, research into the microbiome, the virome and the phenomenon of trained innate immunity has shown that there can be beneficial or even essential relationships between the body and what has traditionally been labelled 'non-self'. Over the last decade, the idea that the immune system strictly enforces a binary distinction has instead evolved towards a model in which it continuously interprets signals of damage or perturbation, manages complex ecological relationships with commensal or latent organisms, and recalibrates according to the organism's life stage and environment. There remains a recognition that clonal deletion and negative selection in the thymus, together with MHC-bound peptide recognition, still underlie many core processes, and in certain clinical contexts, such as acute transplant rejection or the prevention of autoimmunity, an approximate self-non-self-categorization is directly relevant. Overall, however, the field recognizes that 'self' is not a static attribute defined once and for all, but rather a dynamic and context-dependent state that continues to be shaped by microbial symbioses, epigenetic reprogramming and immunoregulatory networks throughout an individual's lifespan.

Fetal microchimeric cells in autoimmune thyroid diseases: harmful, beneficial or innocent for the thyroid gland?

Autoimmune thyroid diseases (AITD) show a female predominance, with an increased incidence in the years following parturition. Fetal microchimerism has been suggested to play a role in the pathogenesis of AITD. However, only the presence of fetal microchimeric cells in blood and in the thyroid gland of these patients has been proven, but not an actual active role in AITD. Is fetal microchimerism harmful for the thyroid gland by initiating a Graft versus Host reaction (GvHR) or being the target of a Host versus Graft reaction (HvGR)? Is fetal microchimerism beneficial for the thyroid gland by being a part of tissue repair or are fetal cells just innocent bystanders in the process of autoimmunity? This review explores every hypothesis concerning the role of fetal microchimerism in AITD.

Quantitative analysis of male fetal DNA in maternal serum of gravid rhesus monkeys (Macaca mulatta)

The isolation of human fetal DNA from the maternal circulation has provided a source of fetal material for prenatal diagnosis. The objective of this study was to investigate whether a similar pattern could be observed in the maternal circulation of male-bearing gravid rhesus monkeys. A real-time PCR TaqMan system for the rhesus Y-chromosome sex determining region was used to determine fetal sex and to quantify fetal DNA concentrations. Results in 14 healthy pregnancies indicated that fetal male DNA could be routinely detected in maternal serum by 50 d of gestation (late first trimester; term 165 +/- 10 d). Fetal DNA concentrations increased with advancing gestation, reaching a mean of 341 genome equivalents/mL of serum (range 11-1570 copies/mL) in the last trimester of gestation, similar to findings in humans. The fetal DNA concentration corresponded to 2.7% of the total maternal serum DNA in the third trimester. Similar to findings in humans, male fetal DNA sequences were not detected postpartum (through 4 wk postpartum) or in animals with a previous history of delivering male offspring. These data indicate that fetal male DNA is present in the maternal circulation of gravid rhesus monkeys comparable to findings in humans and further support the use of this nonhuman primate species as a model to investigate fetomaternal cell trafficking and microchimerism.

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

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

Intrathyroidal fetal microchimerism in Graves' disease

During pregnancy, fetal cells are known to reach the maternal circulation and infiltrate a variety of tissues (fetal microchimerism). Although the presence of such cells has the potential to modulate the maternal immune response to both self antigens and fetal alloantigens, the degree of their influence remains unclear. The hyperthyroidism of Graves' disease frequently abates during pregnancy and exacerbates after childbearing. Thus, we have hypothesized that fetal cells in the maternal circulation and tissues may influence this decrescendo to crescendo pattern of autoimmune thyroid disease. Part of this hypothesis was tested using an ELISA-PCR for the detection of DNA for a male-specific gene, sex-determining region Y. The sensitivity of this assay was the equivalent of approximately 1 male cell among 10(5) female cells. We initially examined paraffin-embedded thyroid tissues and detected male cells in 4 of 20 female Graves' thyroid specimens, but not in 6 of 6 female adenoma specimens. Using frozen thyroid tissue specimens, an additional 6 of 7 Graves' disease samples demonstrated intrathyroidal fetal microchimerism, whereas 1 of 4 female samples with thyroid nodules showed male cells. The greater detection of the sex-determining region Y gene in frozen female thyroid tissues was probably due to DNA fragmentation in the paraffin-derived samples. In summary, we demonstrated that intrathyroidal fetal microchimerism was common and profound in female patients with Graves' disease. Thus, fetal male cells are valid candidates for modulating autoimmune thyroid disease in pregnancy and postpartum.

Presence of microchimerism in labial salivary glands in systemic sclerosis but not in Sjögren's syndrome

OBJECTIVE

To determine whether microchimerism can be implicated in Sjögren's syndrome (SS) by studying minor salivary glands, one of the targets of the disease.

METHODS

Labial salivary gland (LSG) biopsy specimens from 16 female patients with primary SS and 11 with systemic sclerosis (SSc) (a disease in which microchimerism is frequently detected) were analyzed. All 27 women had a history of pregnancy with a male baby. Specimens were microdissected, and polymerase chain reaction (PCR) was performed using the unique sex-determining region Y gene probe.

RESULTS

The sensitivity of PCR for detecting male cells in LSG was high; the presence of 3 male cells was consistently detected in DNA extracted from a normal female LSG specimen to which male DNA had been added, and 1 male cell was detected in 50% of specimens analyzed. Male DNA was not found in any of the specimens from the 16 SS patients but was detected in 5 (45%) of 11 SSc specimens (P = 0.006). No differences in the rate of detection were found between patients with diffuse and limited SSc (male DNA detected in 2 of 3 and 3 of 8, respectively; P = 0.55) or between patients with and those without secondary SS (1 of 6 and 4 of 5, respectively; P = 0.08).

CONCLUSION

The results of our study strengthen the possibility that microchimerism is implicated in SSc. This is the first study to demonstrate the presence of chimeric cells in LSG from 45% of SSc patients, independent of the presence of secondary SS. However, microchimerism was not detected in LSG from patients with primary SS, suggesting that the pathogenesis of the 2 diseases is different.

Human natural chimerism: an acquired character or a vestige of evolution?

Analysis on five common classes of human natural chimeras (cytomictical, whole body, fetal-maternal, germ cell, and tumor chimeras) reveals that (1) they initiate only during pregnancy, (2) the most common class are chimeras which contain maternal cells, and (3) the primary mechanisms that are involved in their formation and establishment are still elusive. These classes of natural chimerism, are involved only with maladaptive phenomena such as malignancy and autoimmune diseases and without any documented benefit. A recent review has challenged the accepted dogma that the evolution of immunity is pathogen-directed and asserted that preserving individuality from littering the soma and the germline by conspecific alien cells might have been the original function of the innate immunity. Following this tenet, I propose here that human natural chimerism is a by-product of the new role evolved from primitive components of immunity to "educate" the developing embryo with the armamentarium of effector mechanisms, dedicated to purge the individual from pervasive somatic and germline variants, and is not a vestige of evolution.

Genetic factors in the etiology of systemic sclerosis and Raynaud phenomenon

There is increasing evidence that genetic factors play important roles in susceptibility to and expression of systemic sclerosis (SSc), as well as primary Raynaud phenomenon. Familial aggregation for SSc, although infrequent (1.2%-1.5% of SSc families), has now been established, and when compared with population prevalence represents a significant risk factor for the disease and lays a firmer foundation for genetics in etiopathogenesis. Major histocompatibility complex class II alleles increase disease risk in some populations but are more strongly correlated with specific autoantibody profiles. Microchimerism influenced by human leukocyte antigen also remains an intriguing hypothesis. A variety of extracellular matrix genes, including fibrillin-1, have become additional candidates for contributing to what is likely a complex genetic disease. Reviewed here is evidence relating to these concepts, especially new data reported over the last year.

Fetal cells in the mother: from genetic diagnosis to diseases associated with fetal cell microchimerism

Fetal cells circulate in the blood of pregnant women. When the gestation is normal, fetal cells are low in number. Complications of pregnancy, such as pre-eclampsia, or fetal cytogenetic abnormalities, such as Down's syndrome, increase fetomaternal transfusion. The isolation of fetal cells from maternal blood is currently under active investigation as a non-invasive method for prenatal diagnosis. The fetal cells that are most commonly used for non-invasive genetic diagnosis, the nucleated erythrocyte and the trophoblast, are highly differentiated and do not persist post-partum. In the context of studying fetal cells in maternal blood it was discovered that fetal progenitor cells originating from a prior pregnancy could also be detected. This led to the appreciation that unlike fetal DNA in plasma, which is cleared almost immediately following delivery, fetal cells persist for decades post-partum. Following pregnancy, labor, and delivery, a woman becomes a chimera. Transfused fetal stem and progenitor cells appear to be capable of further differentiation and migration to maternal organs. A further research agenda is needed to explore the newly appreciated phenomenon of bi-directional fetomaternal cell trafficking. Any consideration of the fetus as a patient must also consider the fetus as a potential source of therapeutic stem cells for the mother.