HLA class I noninherited maternal antigens in cord blood and breast milk

Current Understanding of an Emerging Role of HLA-DRB1 Gene in Rheumatoid Arthritis-From Research to Clinical Practice

Rheumatoid arthritis (RA) is an autoimmune disease with an unclear pathogenic mechanism. However, it has been proven that the key underlying risk factor is a genetic predisposition. Association studies of the HLA-DRB1 gene clearly indicate its importance in RA morbidity. This review presents the current state of knowledge on the impact of HLA-DRB1 gene, functioning both as a component of the patient’s genome and as an environmental risk factor. The impact of known HLA-DRB1 risk variants on the specific structure of the polymorphic HLA-DR molecule, and epitope binding affinity, is presented. The issues of the potential influence of HLA-DRB1 on the occurrence of non-articular disease manifestations and response to treatment are also discussed. A deeper understanding of the role of the HLA-DRB1 gene is essential to explore the complex nature of RA, which is a result of multiple contributing factors, including genetic, epigenetic and environmental factors. It also creates new opportunities to develop modern and personalized forms of therapy.

Exposure to non-inherited maternal antigens by breastfeeding affects antibody responsiveness

The observation, by Ray Owen and colleagues in 1954, that D-negative women were less likely to form anti-D antibodies against their D-positive fetus if their mother possessed the D-antigen, was not found in all later studies. We hypothesized that breastfeeding, received by the mother, may affect her immunity against non-inherited maternal red blood cell antigens. We studied a cohort of 125 grandmother-mother-child combinations, from a follow-up study of mothers after intrauterine transfusion of the fetus for alloimmune hemolytic disease. For mismatched red blood cell antigens the mother was exposed to, whether or not antibodies were formed, we determined whether her mother, the grandmother, carried these antigens. The duration for which the mothers were breastfed was estimated by way of a questionnaire. Using multivariate logistic regression analyses, the interaction term (non-inherited maternal antigen exposure by categorized breastfeeding period) showed that a longer breastfeeding period was associated with decreased alloimmunization against non-inherited maternal antigens (adjusted odds ratio 0.66; 95% confidence interval 0.48–0.93). Sensitivity analysis with dichotomized (shorter versus longer) breastfeeding periods showed that this lower risk was reached after two months (aOR 0.22; 95% CI 0.07–0.71) and longer duration of breastfeeding did not seem to provide additional protection. These data suggest that oral neonatal exposure to non-inherited maternal red blood cell antigens through breastfeeding for at least two months diminishes the risk of alloimmunization against these antigens when encountered later in life.

Breast milk and transplantation tolerance

Recent experimental and clinical studies suggest that exposure of the fetus to noninherited maternal antigens (NIMAs) during pregnancy has an impact on allogeneic transplantations performed later in life. We have reported that NIMA exposure by breastfeeding further potentiates the tolerogenic NIMA effect mediated by in utero NIMA exposure during pregnancy in mice of allogeneic hematopoietic stem cell transplantation (HSCT). Breastfeeding generates Foxp3(+) regulatory T cells that suppress anti-maternal immunity and persist until adulthood. These results reveal a previously unknown impact of breastfeeding on the outcome of allogeneic HSCT.

The pendulum swings: Tolerance versus priming to NIMA

Fetal and/or perinatal exposure to noninherited maternal antigens (NIMA) has been reported to induce NIMA-specific tolerance. This tolerant state is highly beneficial in transplantation settings; enhanced graft acceptance has been observed when transplanted tissues express NIMA. Reduction in severe graft-vs-host disease has also been noted when bone marrow grafts originate from donors exposed to NIMA in early life. However, there is emerging evidence that exposure to NIMA can alternatively lead to specific priming. The processes regulating tolerance versus priming to NIMA are poorly understood and probably multifactorial. Based on studies in both humans and mice, we propose that both the quality and the quantity of NIMA exposure will be found to be key determinants of these opposing outcomes.

Stem cell microchimerism and tolerance to non-inherited maternal antigens

Exposure to non-inherited maternal antigens (NIMA) in fetal and neonatal life of an F(1) backcross (BDF(1) female × B6 male) mouse can result in lifelong tolerance to allografts expressing the NIMA (H2(d)). We have recently shown that the NIMA-specific regulatory T cells were directly correlated with level of maternal microchimerism (MMc) in adult mice, indicating a causative link between the two, and that both Tregs and multi-lineage MMc were dependent on ingestion of milk from a NIMA(+) mother during nursing. Yet how maternal cells obtained in fetal and neonatal life are maintained in adult life remains unclear. Since stem cells are deficient in MHC class I & II expression, we hypothesized that maternally derived stem cells that replenish MMc remain throughout life without eliciting immunity, but differentiated maternal cells can either be deleted by alloreactive T and B effector cells or persist, inducing NIMA-specific tolerance. Consistent with this hypothesis, we found maternally-derived lineage(neg) c-kit(+) cells in the bone marrow of most of adult offspring by quantitative PCR; however, only 50% had detectable MMc in lineage(+) bone marrow cells. Mesenchymal stem cells (lineage(neg) and plate-adherent cells) propagated from the bone marrow also contained maternally-derived cells, albeit in 10-fold lower frequency compared with MMc in myeloid lineage (CD11b(+) and CD11c(+)) cells. Maternally-derived cardiac stem cells were also detected in lineage(neg) c-kit(+) cells purified from heart tissue of NIMA-exposed mice, indicating a local pool of stem cells sustaining MMc in a non-lymphoid tissue. Cardiac stem cell MMc correlated with the presence of maternally derived cardiomyocytes. Lastly, liver MMc increased after nursing suggesting a seeding of maternal cells into the liver via breast milk. Whether orally-derived liver MMc also included maternal stem cells, was not determined. Maternal stem cells in bone marrow and tissues of NIMA-exposed mice are likely responsible for sustaining MMc in adult mice, but their presence alone does not guarantee multi-lineage MMc and tolerance.

Fetal microchimerism persists at high levels in c-kit stem cells in sensitized mothers

We previously showed that fetal and maternal exposure to non-inherited maternal antigens (NIMA) during gestation and nursing resulted in lifelong tolerance to NIMA in some offspring. This NIMA-specific tolerance was mediated by regulatory T cells (Tregs) and was correlated with the level of multi-lineage maternal microchimerism (Mc) indicating a causative link between Mc and Treg development. To determine if transfer of fetal cells into mothers resulted in a similar tolerance to fetal cells, we used qPCR to detect rare fetal derived cells and a delayed type hypersensitivity (DTH) assay to detect fetal alloantigen-specific effector and regulatory T cells in mothers. We found that 5/8 B6 mothers of H2(b/d) offspring were sensitized to the alloantigens H2(d) and HY, indicating a dominance of alloantigen-specific effector T cells. Though these sensitized mothers did not have detectable fetal Mc (FMc) in any of the organs tested, they had very high levels of fetus-derived c-kit(+) stem cells in their bone marrow. The remaining 3/8 B6 mothers that were not sensitized to the fetal antigens had detectable FMc found mostly in heart, lungs and liver, and in 2/3, we could detect alloantigen-specific regulatory T cells. This data indicates that, as in NIMA-specific tolerance, tolerance in multiparous females to inherited paternal antigens (IPA) expressed by the fetus is associated with the presence of fetal Mc in differentiated cell subsets. Surprisingly, robust lin(-)c-kit(+) bone marrow cell fetal Mc can occur in sensitized mothers. This suggests a continuous source of allospecific priming, coupled with active elimination of mature IPA-expressing lin(+) cells by effector T cells of the maternal host.

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.

Frequency of fetal-maternal microchimerism: an analysis of the HLA-DRB1 gene in cord blood and maternal sample pairs

PURPOSE

We aimed to investigate the frequency of fetal-maternal microchimerism among cord blood (CB) from a Korean population.

MATERIALS AND METHODS

We previously developed a nested polymerase chain reaction-single-strand conformation polymorphism method for microchimerism detection that is highly sensitive (0.01-0.001%) and specific. We used this method to investigate the frequency of fetal-maternal HLA-DRB1 microchimerism among 153 maternal and 152 CB samples.

RESULTS

Among the tested pairs, 41.1% exhibited at least one direction of microchimerism, 32.0% of the mothers possessed fetal microchimerism, and 23.4% of the newborns possessed maternal microchimerism. The overall microchimerism frequency was 28.2%.

CONCLUSIONS

We hypothesize that the different microchimerism frequencies among population and methods are due to differences in detection specificities and subject characteristics. This study provides basic data on fetal-maternal microchimerism that may be useful for future studies on autoimmune disorder and virtual phenotyping in transplantation.

Tolerance to noninherited maternal antigens, reproductive microchimerism and regulatory T cell memory: 60 years after 'Evidence for actively acquired tolerance to Rh antigens'

Compulsory exposure to genetically foreign maternal tissue imprints in offspring sustained tolerance to noninherited maternal antigens (NIMA). Immunological tolerance to NIMA was first described by Dr. Ray D. Owen for women genetically negative for erythrocyte rhesus (Rh) antigen with reduced sensitization from developmental Rh exposure by their mothers. Extending this analysis to HLA haplotypes has uncovered the exciting potential for therapeutically exploiting NIMA-specific tolerance naturally engrained in mammalian reproduction for improved clinical outcomes after allogeneic transplantation. Herein, we summarize emerging scientific concepts stemming from tolerance to NIMA that includes postnatal maintenance of microchimeric maternal origin cells in offspring, expanded accumulation of immune suppressive regulatory T cells with NIMA-specificity, along with teleological benefits and immunological consequences of NIMA-specific tolerance conserved across mammalian species.

Maternal microchimerism in health and disease

Circulating maternal cells transfer to the fetus during pregnancy, where they may integrate with the fetal immune and organ systems, creating a state of maternal microchimerism (MMc). MMc can persist throughout the child's life, and it has been implicated in the triggering or perpetuation of chronic inflammatory autoimmune diseases, in the context of specific major histocompatibility genes. Correlative data in humans have now been tested in animal model systems. Results suggest that maternal-fetal tolerance may have health implications far beyond the time of pregnancy and into the child's life.

Cross-Generational Reproductive Fitness Enforced by Microchimeric Maternal Cells

Exposure to maternal tissue during in utero development imprints tolerance to immunologically foreign non-inherited maternal antigens (NIMA) that persists into adulthood. The biological advantage of this tolerance, conserved across mammalian species, remains unclear. Here, we show maternal cells that establish microchimerism in female offspring during development promote systemic accumulation of immune suppressive regulatory T cells (Tregs) with NIMA specificity. NIMA-specific Tregs expand during pregnancies sired by males expressing alloantigens with overlapping NIMA specificity, thereby averting fetal wastage triggered by prenatal infection and non-infectious disruptions of fetal tolerance. Therefore, exposure to NIMA selectively enhances reproductive success in second-generation females carrying embryos with overlapping paternally inherited antigens. These findings demonstrate that genetic fitness, canonically thought to be restricted to Mendelian inheritance, is enhanced in female placental mammals through vertically transferred maternal cells that promote conservation of NIMA and enforce cross-generational reproductive benefits.

A feasibility study on the prediction of acute graft-vs.-host disease before hematopoietic stem cell transplantation based on fetomaternal tolerance

The contact between the immune systems of mother and child during pregnancy affects an immune response of the child against noninherited maternal antigens (NIMA) and the mother against inherited paternal antigens (IPA). However, the immunologic effects of developmental exposure to NIMA or IPA are heterogeneous, and can be either tolerogenic or immunogenic. Although we have reported that prediction of acute graft-vs.-host disease (GVHD) is feasible in a murine model, there has been no literature in human. We devised a novel method for predicting a tolerogenic effect by using mixed lymphocyte reaction combined with enzyme-linked immunospot (MLR-ELISPOT) assay. The assay can evaluate reactivity of interferon-γ spot-forming cells of donor against the recipient. Although we have shown only two examples of mother to child reactivity so far, our preliminary results suggest that this pre-screened assay may be used to predict acute GVHD. The clinical trial is in progress to evaluate MLR-ELISPOT assay as a predicting measure of acute GVHD in haploidentical transplantation from NIMA or IPA-mismatched family donor.

Tolerogenic effect of non-inherited maternal antigens in hematopoietic stem cell transplantation

Major histocompatibility complex antigens that provoke severe transplant reactions are referred to as the human leukocyte antigen (HLA) in human and as the H-2 in mice. Even if the donor and recipient are HLA-identical siblings, graft-versus-host reactions have been linked to differences in the minor histocompatibility antigen. As the chance of finding an HLA-identical sibling donor is only 25%, attention has been focused on using alternative donors. An HLA-mismatched donor with non-inherited maternal antigens (NIMA) is less immunogenic than that with non-inherited paternal antigens, because the contact between the immune systems of the mother and child during pregnancy affects the immune response of the child against NIMA. However, the immunologic effects of developmental exposure to NIMA are heterogeneous, and can be either tolerogenic or immunogenic. We recently have devised a novel method for predicting the tolerogenic effect of NIMA. In this review, we overview the evidence for the existence of the NIMA tolerogenic effect, the possible cellular and molecular basis of the phenomenon, and its utilization in hematopoietic stem cell transplantation. We suggest a future direction for the safe clinical use of this phenomenon, fetomaternal tolerance, in the transplantation field.

Microchimerism: tolerance vs. sensitization

PURPOSE OF REVIEW

The bidirectional exchange of cells, both mature and progenitor types, at the maternal-fetal interface is a common feature of mammalian reproduction. The presence of semiallogeneic cells in a host can have significant immunological effects on transplantation tolerance and rejection. Here, we review recent advances in this area.

RECENT FINDINGS

Maternal microchimerism (MMc) in blood and various organs was found to be directly correlated with noninherited maternal antigen (NIMA)-specific CD4(+) regulatory T cells (Tregs), in F(1) backcross mice. In humans, MMc induced NIMA-specific FoxP3(+) CD4 Tregs in lymph nodes and spleen of fetuses. Tolerance to NIMA(+) allografts could be predicted in mice by measuring levels of the NIMA-specific Tregs in offspring before transplantation. On the contrary, fetal microchimerism (FMc) in multiparous female mice was largely confined to CD34(+) hematopoietic stem cells (HSCs) and was associated with sensitization rather than Treg induction. The recent discovery of a 'layered' T-cell development in humans whereby fetal HSCs are more likely to produce Tregs than adult HSCs, which may explain why MMc often induces tolerance, whereas FMc tends to induce sensitization.

SUMMARY

Microchimerism may cause tolerance resulting in acceptance of an allograft bearing antigens shared by the microchimeric cells. However, microchimerism may also cause sensitization resulting in rejection. Distinguishing these effects prior to the transplant may revolutionize the field of living-related renal transplantation wherein MMc and FMc can exert a powerful influence on graft outcome.

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.

Microchimerism is strongly correlated with tolerance to noninherited maternal antigens in mice

In mice and humans, the immunologic effects of developmental exposure to noninherited maternal antigens (NIMAs) are quite variable. This heterogeneity likely reflects differences in the relative levels of NIMA-specific T regulatory (T(R)) versus T effector (T(E)) cells. We hypothesized that maintenance of NIMA-specific T(R) cells in the adult requires continuous exposure to maternal cells and antigens (eg, maternal microchimerism [MMc]). To test this idea, we used 2 sensitive quantitative polymerase chain reaction (qPCR) tests to detect MMc in different organs of NIMA(d)-exposed H2(b) mice. MMc was detected in 100% of neonates and a majority (61%) of adults; nursing by a NIMA+ mother was essential for preserving MMc into adulthood. MMc was most prevalent in heart, lungs, liver, and blood, but was rarely detected in unfractionated lymphoid tissues. However, MMc was detectable in isolated CD4+, CD11b+, and CD11c+ cell subsets of spleen, and in lineage-positive cells in heart. Suppression of delayed type hypersensitivity (DTH) and in vivo lymphoproliferation correlated with MMc levels, suggesting a link between T(R) and maternal cell engraftment. In the absence of neonatal exposure to NIMA via breastfeeding, MMc was lost, which was accompanied by sensitization to NIMA in some offspring, indicating a role of oral exposure in maintaining a favorable T(R) > T(E) balance.

Tolerance to noninherited maternal antigens in mice and humans

PURPOSE OF REVIEW

Exposure to noninherited maternal antigens (NIMAs) in fetal and neonatal life has life-long immunological consequences. Although there is a plethora of evidence of effects of mother on the immune responses of her offspring, there is very little knowledge available on how exposure to NIMA can result in either tolerance or sensitization. Understanding the mechanism of NIMA effects will impact different fields of immunology including transplantation, autoimmunity, and tumor immunotherapy.

RECENT FINDINGS

Following the discoveries of beneficial effects of NIMA exposure on clinical outcomes in solid organ and bone marrow transplantation, it has now been shown that the exposure to NIMA induces various types of T regulatory (T(R)) cells in fetus and adult, which may partially account for tolerance to allografts bearing the NIMA. Although all offspring are exposed to the maternal antigens, they exhibit a great variability in the NIMA effects, which can be explained by the variability in the extent of maternal microchimerism (MMc).

SUMMARY

Exposure to NIMA can have tolerogenic or sensitizing effects on the offspring, resulting in acceptance or rejection of allografts expressing the NIMA. This variability may be partly explained by the level and distribution of maternal cells persisting in the offspring.

Improved outcome of allogeneic bone marrow transplantation due to breastfeeding-induced tolerance to maternal antigens

Exposure of offspring to noninherited maternal antigens (NIMAs) during pregnancy may have an impact on transplantations performed later in life. Using a mouse model, we recently showed that bone marrow transplantation (BMT) from NIMA-exposed offspring to the mother led to a reduction of graft-versus-host disease (GVHD). Since offspring can also be exposed to NIMAs by breastfeeding after birth, we tested whether breast milk could mediate the tolerogenic NIMA effect. We found that oral exposure to NIMAs by breastfeeding alone was sufficient to reduce GVHD, and that in utero exposure to NIMAs is required for maximum reduction of GVHD. The tolerogenic milk effects disappeared when donor mice were injected with CD25 monoclonal antibodies during the lactation period, suggesting a CD4+CD25+ regulatory T cell–dependent mechanism. Our results suggest a previously unknown impact of breastfeeding on the outcome of transplantation.

Chimeric maternal cells with tissue-specific antigen expression and morphology are common in infant tissues

Maternal microchimerism (MMc) has been purported to play a role in the pathogenesis of autoimmunity, but how a small number of foreign cells could contribute to chronic, systemic inflammation has not been explained. Reports of peripheral blood cells differentiating into tissue-specific cell types may shed light on the problem in that chimeric maternal cells could act as target cells within tissues. We investigated MMc in tissues from 7 male infants. Female cells, presumed maternal, were characterized by simultaneous immunohistochemistry and fluorescence in situ hybridization for X- and Y-chromosomes. Maternal cells constituted 0.017% to 1.9% of parenchymal cells and were found in all infants in liver, pancreas, lung, kidney, bladder, skin, and spleen. Maternal cells were differentiated: maternal hepatocytes in liver, renal tubular cells in kidney, and beta-islet cells in pancreas. Maternal cells were not found in areas of tissue injury or inflammatory infiltrate. Maternal hematopoietic cells were found only in hearts from patients with neonatal lupus. Thus, differentiated maternal cells are present in multiple tissue types and occur independently of inflammation or tissue injury. Loss of tolerance to maternal parenchymal cells could lead to organ-specific "auto" inflammatory disease and elimination of maternal cells in areas of inflammation.

The non-inherited maternal HLA haplotype affects the risk for type 1 diabetes

The aim was to test the hypothesis that the human leucocyte antigen (HLA) haplotype that is not inherited from the mother, that is, the non-inherited maternal antigen (NIMA) affects the risk for type 1 diabetes (T1D). A total of 563 children with T1D and 286 non-diabetic control children from Sweden were genotyped for DRB1, DQA1 and DQB1 alleles. The frequency of positively (DR4-DQA1*0301-B1*0302 and DR3-DQA1*0501-B1*0201), negatively (DR15-DQ A1*0102-B1*0602) or neutrally (all other) T1D associated HLA haplotypes were compared between NIMA and non-inherited paternal antigen (NIPA). All comparisons were carried out between HLA-matched patients and controls. The frequency of positively associated NIMA was higher among both DR4/X-positive healthy individuals compared wit DR4/X-positive patients (P < 0.00003) and DR3/X-positive healthy individuals compared with DR3/X-positive patients (P < 0.009). No such difference was observed for NIPA. High-risk NIMA was increased compared to NIPA among healthy DR3/X- and DR4/X-positive children (P < 0.05). There was no difference in frequency of positively associated haplotypes between patient NIMA and NIPA. The NIMA but not the NIPA affects the risk for T1D, suggesting that not only the inherited but also non-inherited maternal HLA haplotypes, perhaps through microchimerism or other mechanisms, may influence the risk for the disease.

Are pediatric autoimmune diseases primarily genetic diseases?

PURPOSE OF REVIEW

Powerful new methods are allowing identification of genetic risk factors in large populations of adults with autoimmune diseases. In this review, we describe the advantages and limitations of genetic methodologies, and how these methods have been used to discover candidate genes in smaller populations of pediatric patients. We also introduce novel concepts for nontraditional modes of genetic inheritance that may be important in the pathogenesis of autoimmunity.

RECENT FINDINGS

Candidate genes identified by linkage analyses and genome-wide association scans in adult populations have led to focused genetic studies in pediatric populations. Some genes are associated with subsets of both adult and pediatric patients; others appear to be age specific. Novel concepts in genetics have uncovered potential contributions of maternal compared with paternal transmission, noninherited maternal alleles that may work through maternal microchimerism, and sex-specific epigenetic mechanisms of immunoregulation.

SUMMARY

Advancing methods are leading to the discovery of genes associated with childhood autoimmune diseases. However, the genetic contribution to disease risk for any one gene remains less than 30% for most diseases, suggesting that pediatric autoimmunity is not primarily genetic in a classical sense. A combinatorial approach considering the contributions of multiple genes, mode of inheritance, and environmental influences will be required to fully understand the mechanisms of pathogenesis in pediatric autoimmune disease.

Murine neonates develop vigorous in vivo cytotoxic and Th1/Th2 responses upon exposure to low doses of NIMA-like alloantigens

Early life exposure to noninherited maternal antigens (NIMAs) may occur via transplacental transfer and/or breast milk. There are indications that early life exposure to NIMAs may lead to lifelong tolerance. However, there is mounting evidence that exposure to NIMAs may also lead to immunologic priming. Understanding how these different responses arise could be critical in transplantation with donor cells expressing NIMAs. We recently reported that murine neonates that received a transplant of low doses of NIMA-like alloantigens develop vigorous memory cytotoxic responses, as assessed by in vitro assays. Here, we demonstrate that robust allospecific cytotoxicity is also manifest in vivo. Importantly, at low doses, NIMA-expressing cells induced the development of in vivo cytotoxicity during the neonatal period. NIMA-exposed neonates also developed vigorous primary and memory allospecific Th1/Th2 responses that exceeded the responses of adults. Overall, we conclude that exposure to low doses of NIMA-like alloantigens induces robust in vivo cytotoxic and Th1/Th2 responses in neonates. These findings suggest that early exposure to low levels of NIMA may lead to long-term immunologic priming of all arms of T-cell adaptive immunity, rather than tolerance.

Developmental exposure to noninherited maternal antigens induces CD4+ T regulatory cells: relevance to mechanism of heart allograft tolerance

We hypothesize that developmental exposure to noninherited maternal Ags (NIMA) results in alloantigen-specific natural and adaptive T regulatory (T(R)) cells. We compared offspring exposed to maternal H-2(d) (NIMA(d)) with nonexposed controls. In vitro assays did not reveal any differences in T cell responses pretransplant. Adoptive transfer assays revealed lower lymphoproliferation and greater cell surface TGF-beta expression on CD4(+) T cells of NIMA(d)-exposed vs control splenocytes. NIMA(d)-exposed splenocytes exhibited bystander suppression of tetanus-specific delayed-type hypersensitivity responses, which was reversed with Abs to TGF-beta and IL-10. Allospecific T effector cells were induced in all mice upon i.v. challenge with B6D2F1 splenocytes or a DBA/2 heart transplant, but were controlled in NIMA(d)-exposed mice by T(R) cells to varying degrees. Some (40%) NIMA(d)-exposed mice accepted a DBA/2 allograft while others (60%) rejected in delayed fashion. Rejector and acceptor NIMA(d)-exposed mice had reduced T effector responses and increased Foxp3(+) T(R) cells (CD4(+)CD25(+)Foxp3(+) T(R)) in spleen and lymph nodes compared with controls. The key features distinguishing NIMA(d)-exposed acceptors from all other mice were: 1) higher frequency of IL-10- and TGF-beta-producing cells primarily in the CD4(+)CD25(+) T cell subset within lymph nodes and allografts, 2) a suppressed delayed-type hypersensitivity response to B6D2F1 Ags, and 3) allografts enriched in LAP(+), Foxp3(+), and CD4(+) T cells, with few CD8(+) T cells. We conclude that the beneficial NIMA effect is due to induction of NIMA-specific T(R) cells during ontogeny. Their persistence in the adult, and the ability of the host to mobilize them to the graft, may determine whether NIMA-specific tolerance is achieved.

Impact of Donor and Recipient Sex and Parity on Outcomes of HLA-Identical Sibling Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (SCT) may cure patients with hematologic malignancies, but it carries significant risks. Careful donor selection is an important component of the clinical transplantation decision-making process and includes evaluation of HLA typing and other criteria, the most controversial of which is parity. We examined the effect of donor sex and parity on outcomes of HLA-identical sibling SCT. Because the effect of recipient sex/parity has never been explicitly evaluated, we also analyzed the effect of recipient sex/parity on outcomes of transplantation. We found that (1) parous female donors result in an increased risk of chronic graft-versus-host disease (GVHD) in all recipients, (2) the magnitude of this increased risk is similar in male and female recipients, and (3) nulliparous female donors increase the risk of chronic GVHD in male recipients to a degree comparable to that from parous donors. A decrease in the risk of relapse was not observed, and there was no effect on overall survival, acute GVHD, or transplant-related mortality. Recipient parity had no independent effect on any endpoint. Until the effects of pregnancy on the maternal immune system are better understood, it is appropriate whenever possible to avoid parous female donors and to choose male donors for male recipients in HLA-identical related donor SCT.

Fetal tolerance to maternal antigens improves the outcome of allogeneic bone marrow transplantation by a CD4+CD25+ T-cell–dependent mechanism

The lack of donor availability is a major limitation to the widespread use of allogeneic hematopoietic stem cell transplantation, and therefore it would be beneficial to identify less immunogenic HLA mismatches. The maternal and fetal antigens that are transmitted through the bidirectional transplacental passage during pregnancy may induce tolerance to noninherited maternal antigens (NIMAs) in offspring and to inherited paternal antigens (IPAs) in the mother. Using mouse models of bone marrow transplantation (BMT), we found that a “child-to-mother” BMT from a NIMA-exposed donor reduced the morbidity and mortality of graft-versus-host disease in an antigen-specific manner; however, a “mother-to-child” BMT from an IPA-exposed donor did not. The NIMA-complementary BMT preserved the graft-versus-leukemia effects and favored the immune reconstitution, thus resulting in a marked improvement of the outcome after BMT. These tolerogenic NIMA effects were completely abolished by the depletion of CD4+CD25+ cells from the donor inocula, thus suggesting the involvement of CD4+CD25+ regulatory T cells in the tolerogenic NIMA effects. Our findings may therefore have profound implications on the performance of clinical BMT while also potentially helping to develop new strategies for using a NIMA-mismatched donor in the absence of an HLA-identical donor.

No in Vitro Evidence for a Decreased Alloreactivity toward Noninherited Maternal HLA Antigens in Healthy Individuals

Pre- and/or perinatal exposure to noninherited maternal HLA antigens (NIMA) is associated with a decreased HLA antibody formation against the NIMA and a significantly better graft survival of kidney grafts from siblings or those from unrelated donors who were mismatched for the NIMA haplotype compared with the NIPA (noninherited paternal HLA antigens) haplotype later in life. These observations suggest that some form of immunological tolerance against NIMA is induced. We analyzed the in vitro T cell reactivity of healthy individuals toward their parents and/or siblings expressing the NIMA or NIPA haplotype to explore whether the alloimmune response to NIMA has distinct characteristics compared with NIPA. No differences were detected by mixed lymphocyte reactions (MLR) and supernatants taken from the MLR showed no differences in IFN-gamma and IL-10 production. Additionally, no differences were found with IFN-gamma and IL-10 Elispot analyses. Phenotypic analysis revealed no selective increase in the number of CD3-CD8dim cells (thought to be a NK-like regulator cell) and the number of CD4+CD25+CD152+ cells (naturally occurring regulatory T cells) after stimulation with NIMA-expressing cells when compared with NIPA-expressing cells. In conclusion, no evidence of an influence of a NIMA effect on the cellular level was found in healthy individuals with "standard" immunological techniques.

Fetal-maternal microchimerism: impact on hematopoietic stem cell transplantation

Reciprocal cell traffic between mother and fetus during pregnancy gives rise to postpartum fetal-maternal lymphohematopoietic microchimerism, which is frequently detected in blood or tissue from healthy individuals. Although such microchimerism has been implicated in the pathogenesis of autoimmune diseases and tissue repair, recent clinical experiences have suggested the association of microchimerism with acquired immunologic hyporesponsiveness to non-inherited maternal HLA antigens (NIMAs) or inherited paternal HLA antigens (IPAs); T cell-replete HLA-haploidentical hematopoietic stem cell transplantation from a microchimeric IPA/NIMA-mismatched donor confers relatively lower incidence of severe graft-versus-host disease. The underlying mechanisms by which fetal-maternal microchimerism contributes to IPA/NIMA-specific tolerance are still elusive, although emerging experimental evidence suggests an involvement of the central deletion of IPA/NIMA-reactive T cells, the induction of peripheral regulatory T cells, and affinity-dependent modulation of NIMA-reactive B cells.