In vivo imaging of Treg cells providing immune privilege to the haematopoietic stem-cell niche

Severe Developmental B Lymphopoietic Defects in Foxp3-Deficient Mice are Refractory to Adoptive Regulatory T Cell Therapy

The role of Foxp3-expressing regulatory T (T_reg) cells in tolerance and autoimmunity is well-established. However, although of considerable clinical interest, the role of T_reg cells in the regulation of hematopoietic homeostasis remains poorly understood. Thus, we analysed B and T lymphopoiesis in the scurfy (Sf) mouse model of T_reg cell deficiency. In these experiments, the near-complete block of B lymphopoiesis in the BM of adolescent Sf mice was attributed to autoimmune T cells. We could exclude a constitutive lympho-hematopoietic defect or a B cell-intrinsic function of Foxp3. Efficient B cell development in the BM early in ontogeny and pronounced extramedullary B lymphopoietic activity resulted in a peripheral pool of mature B cells in adolescent Sf mice. However, marginal zone B and B-1a cells were absent throughout ontogeny. Developmental B lymphopoietic defects largely correlated with defective thymopoiesis. Importantly, neonatal adoptive T_reg cell therapy suppressed exacerbated production of inflammatory cytokines and restored thymopoiesis but was ineffective in recovering defective B lymphopoiesis, probably due to a failure to compensate production of stroma cell-derived IL-7 and CXCL12. Our observations on autoimmune-mediated incapacitation of the BM environment in Foxp3-deficient mice will have direct implications for the rational design of BM transplantation protocols for patients with severe genetic deficiencies in functional Foxp3⁺ T_reg cells.

Reactive oxygen species in cancer stem cells

SIGNIFICANCE

Reactive oxygen species (ROS), byproducts of aerobic metabolism, are increased in many types of cancer cells. Increased endogenous ROS lead to adaptive changes and may play pivotal roles in tumorigenesis, metastasis, and resistance to radiation and chemotherapy. In contrast, the ROS generated by xenobiotics disturb the redox balance and may selectively kill cancer cells but spare normal cells.

RECENT ADVANCES

Cancer stem cells (CSCs) are integral parts of pathophysiological mechanisms of tumor progression, metastasis, and chemo/radio resistance. Currently, intracellular ROS in CSCs is an active field of research.

CRITICAL ISSUES

Normal stem cells such as hematopoietic stem cells reside in niches characterized by hypoxia and low ROS, both of which are critical for maintaining the potential for self-renewal and stemness. However, the roles of ROS in CSCs remain poorly understood.

FUTURE DIRECTIONS

Based on the regulation of ROS levels in normal stem cells and CSCs, future research may evaluate the potential therapeutic application of ROS elevation by exogenous xenobiotics to eliminate CSCs.

CD86 is expressed on murine hematopoietic stem cells and denotes lymphopoietic potential

A unique subset of CD86(-) HSCs was previously discovered in mice that were old or chronically stimulated with lipopolysaccharide. Functionally defective HSCs were also present in those animals, and we now show that CD86(-) CD150(+) CD48(-) HSCs from normal adult mice are particularly poor at restoring the adaptive immune system. Levels of the marker are high on all progenitors with lymphopoietic potential, and progressive loss helps to establish relations between progenitors corresponding to myeloid and erythroid lineages. CD86 represents an important tool for subdividing HSCs in several circumstances, identifying those unlikely to generate a full spectrum of hematopoietic cells.

How I treat acquired aplastic anemia

Survival in severe aplastic anemia (SAA) has markedly improved in the past 4 decades because of advances in hematopoietic stem cell transplantation, immunosuppressive biologics and drugs, and supportive care. However, management of SAA patients remains challenging, both acutely in addressing the immediate consequences of pancytopenia and in the long term because of the disease's natural history and the consequences of therapy. Recent insights into pathophysiology have practical implications. We review key aspects of differential diagnosis, considerations in the choice of first- and second-line therapies, and the management of patients after immunosuppression, based on both a critical review of the recent literature and our large personal and research protocol experience of bone marrow failure in the Hematology Branch of the National Heart, Lung, and Blood Institute.

Ex vivo expansion of cord blood progenitors impairs their short-term and long-term repopulating activity associated with transcriptional dysregulation of signalling networks

OBJECTIVES

Cord blood (CB) has been established to be an alternative source of haematopoietic stem/progenitor cells (HPC) for transplantation. The number of HPC per CB unit is limited, which results in engraftment delay. Ex vivo expansion of HPC improvement must overcome this.

MATERIALS AND METHODS

Flow cytometry was used to extensively phenotype HPC pre- and post-expansion and CFDA-SE staining was used to track cell divisions. The NSG mouse model was employed in transplantation studies to determine long and short term repopulation in human cells. Gene array analysis was used to evaluate signalling pathways regulated following ex vivo expansion of HPC.

RESULTS

expansion of CD34(+) HPC impaired their regenerative function. In this xenograft transplantation model we showed that repopulating activity of CB cells declined following expansion. Expanded HPC had delayed engraftment at early and late stages post-transplant. High resolution division tracking revealed that the cultured HPC had reduced expansion and self-renewal probability and increased differentiation rate compared to non-expanded cells. Gene expression analysis exposed significant modulation of a complex network of genes and pathways that normally maintain HPC proliferation and limit their differentiation.

CONCLUSIONS

The decline in short-term engraftment is consistent with the loss of rapid SCID repopulating ability r(SRA) by expanded CD34(+) CD38(+) cells recently reported. Our data raise concerns for future clinical applications of expanded HPC alone in transplantation.

Tolerance induction by allogeneic hematopoietic stem cells

In this issue of Cell Stem Cell, Zheng et al. (2011) report that HSCs expressing PD-L1 display enhanced engraftment in irradiated allogeneic recipients. Independently in Nature, Fujisaki et al. (2011) observe allogeneic HSCs persisting in proximity to regulatory T cells in nonirradiated recipients, further connecting HSCs and immune tolerance.

A family business: stem cell progeny join the niche to regulate homeostasis

Stem cell niches, the discrete microenvironments in which the stem cells reside, play a dominant part in regulating stem cell activity and behaviours. Recent studies suggest that committed stem cell progeny become indispensable components of the niche in a wide range of stem cell systems. These unexpected niche inhabitants provide versatile feedback signals to their stem cell parents. Together with other heterologous cell types that constitute the niche, they contribute to the dynamics of the microenvironment. As progeny are often located in close proximity to stem cell niches, similar feedback regulations may be the underlying principles shared by different stem cell systems.

Prolonged renal allograft survival by donor interleukin-6 deficiency: association with decreased alloantibodies and increased intragraft T regulatory cells

Both humoral and cellular immune responses are involved in renal allograft rejection. Interleukin (IL)-6 is a regulatory cytokine for both B and Foxp3 (forkhead box P3)-expressing regulatory T (Treg) cells. This study was designed to investigate the impact of donor IL-6 production on renal allograft survival. Donor kidneys from IL-6 knockout (KO) vs. wild-type (WT) C57BL/6 mice (H-2b) were orthotopically transplanted to nephrotomized BALB/c mice (H-2d). Alloantibodies and Treg cells were examined by fluorescence-activated cell sorting analysis. Graft survival was determined by the time to graft failure. Here, we showed that a deficiency in IL-6 expression in donor kidneys significantly prolonged renal allograft survival compared with WT controls. IL-6 protein was upregulated in renal tubules and endothelium of renal allografts following rejection, which correlated with an increase in serum IL-6 compared with that in those receiving KO grafts or naive controls. The absence of graft-producing IL-6 or lower levels of serum IL-6 in the recipients receiving IL-6 KO allografts was associated with decreased circulating anti-graft alloantibodies and increased the percentage of intragraft CD4+CD25+Foxp3+ Treg cells compared with those with WT allografts. In conclusion, the lack of graft-producing IL-6 significantly prolongs renal allograft survival, which is associated with reduced alloantibody production and/or increased intragraft Treg cell population, implying that targeting donor IL-6 may effectively prevent both humoral and cellular rejection of kidney transplants.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

The bone marrow at the crossroads of blood and immunity

Progenitor cells that are the basis for all blood cell production share the bone marrow with more mature elements of the adaptive immune system. Specialized niches within the bone marrow guide and, at times, constrain the development of haematopoietic stem and progenitor cells (HSPCs) and lineage-restricted immune progenitor cells. Specific niche components are organized into distinct domains to create a diversified landscape in which specialized cell differentiation or population expansion programmes proceed. Local cues that reflect the tissue and organismal state affect cellular interactions to alter the production of a range of cell types. Here, we review the organization of regulatory elements in the bone marrow and discuss how these elements provide a dynamic means for the host to modulate stem cell and adaptive immune cell responses to physiological challenges.

The bone marrow at the crossroads of blood and immunity

Progenitor cells that are the basis for all blood cell production share the bone marrow with more mature elements of the adaptive immune system. Specialized niches within the bone marrow guide and, at times, constrain the development of haematopoietic stem and progenitor cells (HSPCs) and lineage-restricted immune progenitor cells. Specific niche components are organized into distinct domains to create a diversified landscape in which specialized cell differentiation or population expansion programmes proceed. Local cues that reflect the tissue and organismal state affect cellular interactions to alter the production of a range of cell types. Here, we review the organization of regulatory elements in the bone marrow and discuss how these elements provide a dynamic means for the host to modulate stem cell and adaptive immune cell responses to physiological challenges.

Tolerance and lymphoid organ structure and function

This issue of Frontiers in Immunologic Tolerance explores barriers to tolerance from a variety of views of cells, molecules, and processes of the immune system. Our laboratory has spent over a decade focused on the migration of the cells of the immune system, and dissecting the signals that determine how and where effector and suppressive regulatory T cells traffic from one site to another in order to reject or protect allografts. These studies have led us to a greater appreciation of the anatomic structure of the immune system, and the realization that the path taken by lymphocytes during the course of the immune response to implanted organs determines the final outcome. In particular, the structures, microanatomic domains, and the cells and molecules that lymphocytes encounter during their transit through blood, tissues, lymphatics, and secondary lymphoid organs are powerful determinants for whether tolerance is achieved. Thus, the understanding of complex cellular and molecular processes of tolerance will not come from “96-well plate immunology,” but from an integrated understanding of the temporal and spatial changes that occur during the response to the allograft. The study of the precise positioning and movement of cells in lymphoid organs has been difficult since it is hard to visualize cells within their three-dimensional setting; instead techniques have tended to be dominated by two-dimensional renderings, although advanced confocal and two-photon systems are changing this view. It is difficult to precisely modify key molecules and events in lymphoid organs, so that existing knockouts, transgenics, inhibitors, and activators have global and pleiotropic effects, rather than precise anatomically restricted influences. Lastly, there are no well-defined postal codes or tracking systems for leukocytes, so that while we can usually track cells from point A to point B, it is exponentially more difficult or even impossible to track them to point C and beyond. We believe this represents one of the fundamental barriers to understanding the immune system and devising therapeutic approaches that take into account anatomy and structure as major controlling principles of tolerance.

Lineage-committed osteoclast precursors circulate in blood and settle down into bone

Osteoclasts are derived from the monocyte/macrophage lineage, but little is known about osteoclast precursors in circulation. We previously showed that cell cycle-arrested quiescent osteoclast precursors (QOPs) were detected along bone surfaces as direct osteoclast precursors. Here we show that receptor activator of NF-κB (RANK)-positive cells isolated from bone marrow and peripheral blood possess characteristics of QOPs in mice. RANK-positive cells expressed c-Fms (receptors of macrophage colony-stimulating factor) at various levels, but scarcely expressed other monocyte/granulocyte markers. RANK-positive cells failed to exert phagocytic and proliferating activities, and differentiated into osteoclasts but not into dendritic cells. To identify circulating QOPs, collagen disks containing bone morphogenetic protein-2 (BMP disks) were implanted into mice, which were administered bromodeoxyuridine daily. Most nuclei of osteoclasts detected in BMP-2-induced ectopic bone were bromodeoxyuridine-negative. RANK-positive cells in peripheral blood proliferated more slowly and had a much longer lifespan than F4/80 (a macrophage marker)-positive macrophages. When BMP disks and control disks were implanted in RANK ligand-deficient mice, RANK-positive cells were observed in the BMP disks but not in the controls. F4/80-positive cells were distributed in both disks. Administration of FYT720, a sphingosine 1-phosphate agonist, promoted the egress of RANK-positive cells from hematopoietic tissues into bloodstream. These results suggest that lineage-determined QOPs circulate in the blood and settle in the bone.

Survival time of cardiac allografts prolonged by isogeneic BMT in mice

To find an approach to prolong the survival time of cardiac allografts in a BALB/c-to-C57/BL6 heterotopic heart transplant model and to try to figure out related chemokines and cytokines, isogeneic and allogeneic BM cells were obtained from pregnant C57/BL6 (♀C57/BL6 × ♂BALB/c) and regular BALB/c mice and injected to the half lethally irradiated C57/BL6 mice 1 day before heart transplantation. Recipients were treated with CsA or phosphate-buffered saline for 7 days. Isogeneic BMT (iBMT) from pregnant C57/BL6 mice was observed to significantly prolong the survival of BALB/c allografts and reduce the lymphocyte infiltration. Allogeneic BMT (aBMT) and iBMT both exhibited signicantly less T-cell proliferation reactivity and the similar degree of chimerism. There was no significant difference in these groups of IFN-γ and IL-4 production. The level of chemokine MIG (CXCL9) dramatically decreased in aBMT and iBMT groups compared with the control group. But there were no significant differences between aBMT and iBMT group. IL-17 and RORγ(t) (receptor-related orphan receptor) production were downregulated in iBMT recipients. These results indicate that iBMT can prolong the survival of cardiac allografts. IL-17 production downregulated in iBMT recipients. This means that iBMT may have important therapeutic implications.

A new chapter: hematopoietic stem cells are direct players in immunity

Several lines of evidence support the hypothesis that hematopoietic stem cells (HSCs) directly interact with the immune system and have potential for immune privilege. Although the microenvironment or niche provides protection for HSCs from immune attack, HSCs are also capable of interacting with the immune system as signal "providers" and signal "receivers". On the one hand, HSCs display surface immune inhibitory molecules to evade the attack from the innate and adaptive immune systems; on the other hand, HSCs are capable of directly sensing the signals from the immune system through their surface receptors. Thus, HSCs are important direct players in the immune system.

Targeting glioma stem cells: a novel framework for brain tumors

The past decade has seen a dramatic increase in stem cell research that focuses on glioma stem cells (GSC) and their mechanisms of action, revealing multiple potential targets for primary malignant brain tumors. Herein, we present a novel framework for considering GSC targets based on direct and indirect strategies. Direct strategies target GSC molecular pathways to overcome their resistance to radiation and chemotherapy, block their function or induce their differentiation. Indirect strategies target the microenvironment of the GSC, namely the perivascular, hypoxic and immune niches. Progress made on GSC targets is reviewed in detail and specific pathways are identified in context of the proposed framework. The potential barriers for translation to the clinical setting are also discussed. Overall, targeting GSC provides an unprecedented opportunity for revolutionary approaches to treat high-grade gliomas that continue to have a poor patient prognosis.

Pregnancy, microchimerism, and the maternal grandmother

Background

A woman of reproductive age often harbors a small number of foreign cells, referred to as microchimerism: a preexisting population of cells acquired during fetal life from her own mother, and newly acquired populations from her pregnancies. An intriguing question is whether the population of cells from her own mother can influence either maternal health during pregnancy and/or the next generation (grandchildren).

Methodology/Principal Findings

Microchimerism from a woman's (i.e. proband's) own mother (mother-of-the-proband, MP) was studied in peripheral blood samples from women followed longitudinally during pregnancy who were confirmed to have uncomplicated obstetric outcomes. Women with preeclampsia were studied at the time of diagnosis and comparison made to women with healthy pregnancies matched for parity and gestational age. Participants and family members were HLA-genotyped for DRB1, DQA1, and DQB1 loci. An HLA polymorphism unique to the woman's mother was identified, and a panel of HLA-specific quantitative PCR assays was employed to identify and quantify microchimerism. Microchimerism from the MP was identified during normal, uncomplicated pregnancy, with a peak concentration in the third trimester. The likelihood of detection increased with advancing gestational age. For each advancing trimester, there was a 12.7-fold increase in the probability of detecting microchimerism relative to the prior trimester, 95% confidence intervals 3.2, 50.3, p<0.001. None of the women with preeclampsia, compared with 30% of matched healthy women, had microchimerism (p = 0.03).

Conclusions/Significance

These results show that microchimerism from a woman's own mother is detectable in normal pregnancy and diminished in preeclampsia, supporting the previously unexplored hypothesis that MP microchimerism may be a marker reflecting healthy maternal adaptation to pregnancy.