Prediction of reactivity to noninherited maternal antigen in MHC-mismatched, minor histocompatibility antigen-matched stem cell transplantation in a mouse model

Maternal microchimerism and cell-mediated immune-modulation enhance engraftment following semi-allogenic intrauterine transplantation

Successful intrauterine hematopoietic cell transplantation (IUT) for congenital hemoglobinopathies is hampered by maternal alloresponsiveness. We investigate these interactions in semi-allogenic murine IUT. E14 fetuses (B6 females × BALB/c males) were each treated with 5E+6 maternal (B6) or paternal (BALB/c) bone marrow cells and serially monitored for chimerism (>1% engraftment), trafficked maternal immune cells, and immune responsiveness to donor cells. A total of 41.0% of maternal IUT recipients (mIUT) were chimeras (mean donor chimerism 3.0 ± 1.3%) versus 75.0% of paternal IUT recipients (pIUT, 3.6 ± 1.1%). Chimeras showed higher maternal microchimerism of CD4, CD8, and CD19 than non-chimeras. These maternal cells showed minimal responsiveness to B6 or BALB/c stimulation. To interrogate tolerance, mIUT were injected postnatally with 5E+6 B6 cells/pup; pIUT received BALB/c cells. IUT-treated pups showed no changes in trafficked maternal or fetal immune cell levels compared to controls. Donor-specific IgM and IgG were expressed by 1%-3% of recipients. mIUT splenocytes showed greater proliferation of regulatory T cells (Treg) upon BALB/c stimulation, while B6 stimulation upregulated the pro-inflammatory cytokines more than BALB/c. pIUT splenocytes produced identical Treg and cytokine responses to BALB/c and B6 cells, with higher Treg activity and lower pro-inflammatory cytokine expression upon exposure to BALB/c. In contrast, naïve fetal splenocytes demonstrated greater alloresponsiveness to BALB/c compared to B6 cells. Thus pIUT, associated with increased maternal cell trafficking, modulates fetal Treg, and cytokine responsiveness to donor cells more efficiently than mIUT, resulting in improved engraftment. Paternal donor cells may be considered alternatively to maternal donor cells for intrauterine and postnatal transplantation to induce tolerance and maintain engraftment.

Immunological Consequences of In Utero Exposure to Foreign Antigens

Immunologic tolerance refers to a state of immune nonreactivity specific to particular antigens as an important issue in the field of transplantation and the management of autoimmune diseases. Tolerance conceptually originated from Owen’s observation of blood cell sharing in twin calves. Owen’s conceptual framework subsequently constituted the backbone of Medawar’s “actively acquired tolerance” as the major tenet of modern immunology. Based upon this knowledge, the delivery of genetically distinct hematopoietic stem cells into pre-immune fetuses represented a novel and unique approach to their engraftment without the requirement of myeloablation or immunosuppression. It might also make fetal recipients commit donor alloantigens to memory of their patterns as “self” so as to create a state of donor-specific tolerance. Over the years, the effort made experimentally or clinically toward in utero marrow transplantation could not reliably yield sufficient hematopoietic chimerism for curing candidate diseases as anticipated, nor did allogeneic graft tolerance universally develop as envisaged by Medawar following in utero exposure to various forms of alloantigens from exosomes, lymphocytes or marrow cells. Enduring graft tolerance was only conditional on a state of significant hematopoietic chimerism conferred by marrow inocula. Notably, fetal exposure to ovalbumin, oncoprotein and microbial antigens did not elicit immune tolerance, but instead triggered an event of sensitization to the antigens inoculated. These fetal immunogenic events might be clinically relevant to prenatal imprinting of atopy, immune surveillance against developmental tumorigenesis, and prenatal immunization against infectious diseases. Briefly, the immunological consequences of fetal exposure to foreign antigens could be tolerogenic or immunogenic, relying upon the type or nature of antigens introduced. Thus, the classical school of “actively acquired tolerance” might oversimplify the interactions between developing fetal immune system and antigens. Such interactions might rely upon fetal macrophages, which showed up earlier than lymphocytes and were competent to phagocytose foreign antigens so as to bridge toward antigen-specific adaptive immunity later on in life. Thus, innate fetal macrophages may be the potential basis for exploring how the immunological outcome of fetal exposure to foreign antigens is determined to improve the likelihood and reliability of manipulating fetal immune system toward tolerization or immunization to antigens.

Chimeric maternal cells in offspring do not respond to renal injury, inflammatory or repair signals

Maternal microchimerism (MMc) can persist for years in a child, and has been implicated in the pathogenesis of chronic inflammatory autoimmune diseases. Chimeric cells may either contribute to disease by acting as immune targets or expand in response to signals of injury, inflammation or repair. We investigated the role of maternal cells in tissue injury in the absence of autoimmunity by quantifying MMc by quantitative PCR in acute and chronic models of renal injury: (1) reversible acute renal injury, inflammation and regeneration induced by rhabdomyolysis and (2) chronic injury leading to fibrosis after unilateral ureteral obstruction. We found that MMc is common in the mouse kidney. In mice congenic with their mothers neither acute nor chronic renal injury with fibrosis influenced the levels or prevalence of MMc. Maternal cells expressing MHC antigens not shared by offspring (H2(b/d)) were detected at lower levels in all groups of homozygous H2(b/b) or H2(d/d) offspring, with or without renal injury, suggesting that partial tolerance to low levels of alloantigens may regulate the homeostatic levels of maternal cells within tissues. Maternal cells homozygous for H2(b) were lost in H2(b/d) offspring only after acute renal failure, suggesting that an inflammatory stimulus led to loss of tolerance to homozygous maternal cells. The study suggests that elevated MMc previously found in association with human autoimmune diseases may not be a response to non-specific injury or inflammatory signals, but rather a primary event integral to the pathogenesis of autoimmunity.

Major and minor histocompatibility antigens to NIMA: Prediction of a tolerogenic NIMA effect

The immunologic effects of developmental exposure to non-inherited maternal antigens (NIMA) are heterogeneous, either tolerogenic or immunogenic. The role of minor histocompatibility antigens (MiHA) in NIMA effects is unknown. We have recently reported that the NIMA effect can be classified into two distinct reactivities, low and high responder, to NIMA in utero and during nursing depending on the degree of maternal microchimerism (MMc) and Foxp3 expression of peripheral blood CD4(+)CD25(+) cells after graft-versus-host disease (GVHD) induction. These reactivities were predictable before transplantation, using an MLR-ELISPOT (mixed lymphocyte reaction; enzyme-linked immunospot) assay by comparing the number of IFNγ-producing cells stimulated with NIMA. Moreover, this assay was also applicable in both major and minor NIMA-mismatched setting. These observations are clinically relevant and suggest that it is possible to predict the immunological tolerance to NIMA.

How do we choose the best donor for T-cell-replete, HLA-haploidentical transplantation?

In haploidentical stem cell transplantations (haplo-SCT), nearly all patients have more than one donor. A key issue in the haplo-SCT setting is the search for the best donor, because donor selection can significantly impact the incidences of acute and chronic graft-versus-host disease, transplant-related mortality, and relapse, in addition to overall survival. In this review, we focused on factors associated with transplant outcomes following unmanipulated haplo-SCT with anti-thymocyte globulin (ATG) or after T-cell-replete haplo-SCT with post-transplantation cyclophosphamide (PT/Cy). We summarized the effects of the primary factors, including donor-specific antibodies against human leukocyte antigens (HLA); donor age and gender; killer immunoglobulin-like receptor-ligand mismatches; and non-inherited maternal antigen mismatches. We also offered some expert recommendations and proposed an algorithm for selecting donors for unmanipulated haplo-SCT with ATG and for T-cell-replete haplo-SCT with PT/Cy.

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.

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.

Neutrophils induced licensing of natural killer cells

Natural killer (NK) cells acquire effector function through a licensing process and exert anti-leukemia/tumor effect. However, there is no means to promote a licensing effect of allogeneic NK cells other than cytomegalovirus reactivation-induced licensing in allogeneic hematopoietic stem cell transplantation in human. In mice, a licensing process is mediated by Ly49 receptors which recognize self-major histocompatibility complex class I. The distribution of four Ly49 receptors showed similar pattern in congenic mice, B10, B10.BR, and B10.D2, which have B10 background. Forty Gy-irradiated 2 × 10⁶ B10.D2 cells including splenocytes, peripheral blood mononuclear cells in untreated mice, or granulocyte colony-stimulating factor treated mice were injected intraperitoneally into B10 mice. We found that murine NK cells were effectively licensed by intraperitoneal injection of donor neutrophils with its corresponding NK receptor ligand in B10 mice as a recipient and B10.D2 as a donor. Mechanistic studies revealed that NK cells showed the upregulation of intracellular interferon-γ and CD107a expression as markers of NK cell activation. Moreover, enriched neutrophils enhanced licensing effect of NK cells; meanwhile, licensing effect was diminished by depletion of neutrophils. Collectively, injection of neutrophils induced NK cell licensing (activation) via NK receptor ligand interaction.

Maternal-fetal cellular trafficking: clinical implications and consequences

PURPOSE OF REVIEW

Maternal-fetal cellular trafficking (MFCT) is the bidirectional passage of cells between mother and fetus during pregnancy. This results in the presence of fetal cells in the maternal circulation, known as fetal microchimerism, and maternal cells in the fetal circulation, known as maternal microchimerism. The biologic role of this transplacental cellular trafficking during pregnancy is not known, although it has been implicated in development of the fetal immune system, tolerance mechanisms during pregnancy, tissue repair in autoimmune disease and cancer, and immune surveillance.

RECENT FINDINGS

Clinical utility of MFCT has been identified in prenatal testing for aneuploidies and prediction of pregnancy complications. Additionally, this transplacental passage of cells has been implicated in the delicate balance between immunologic priming and tolerance, which can influence the occurrence of autoimmune disease and transplantation outcomes. Ongoing studies are evaluating the utility of microchimerism in predicting the risk of graft rejection in transplantation.

SUMMARY

In this review, we will discuss the clinical implications of MFCT in pregnancy, fetal surgery, autoimmune disease, transplantation, and cancer.

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.

Pretransplant immune-regulation predicts allograft tolerance

CD4⁺ Tregs specific for noninherited maternal antigens (NIMA(d) ) are detectable in some but not all B6 × BDF₁ backcross, H-2(b) homozygous offspring, and their presence is strongly correlated with extent of maternal (BDF₁) microchimerism. We hypothesized that the level of pretransplant donor antigen-specific Tregs could predict allograft tolerance. To test this idea, mice were screened for bystander suppression in a DTH assay, followed 1 week later by DBA/2 heterotopic heart transplantation. NIMA(d) -exposed, H-2(b) offspring that failed to suppress DTH uniformly rejected heart allografts (12/12) by d15. In contrast, 5/6 NIMA(d) -exposed DTH 'regulators' accepted their allografts >100 days. The defect in 'nonregulator" offspring could be corrected by transfer of CD4⁺CD25⁺, but not CD4⁺ CD25(neg) or CD8⁺ T cells from transplant acceptor mice. In conclusion, donor-specific T reg screening of F1 backcross offspring correctly predicted which recipients would accept a heart allograft. If translated to the clinic, similar pretransplant Treg screening could greatly enhance the effectiveness of tolerance as a clinical strategy in transplantation between family members.