Interferon-γ impairs proliferation of hematopoietic stem cells in mice

Inflammatory responses to infection: the Dutch contribution

At any given moment, our body is under attack by a large variety of pathogens, which aim to enter and use our body to propagate and disseminate. The extensive cellular and molecular complexity of our immune system enables us to efficiently eliminate invading pathogens or at least develop a condition in which propagation of the microorganism is reduced to a minimum. Yet, the evolutionary pressure on pathogens to circumvent our immune defense mechanisms is immense, which continuously leads to the development of novel pathogenic strains that challenge the health of mankind. Understanding this battle between pathogen and the immune system has been a fruitful area of immunological research over the last century and will continue to do so for many years. In this review, which has been written on the occasion of the 50th anniversary of the Dutch Society for Immunology, we provide an overview of the major contributions that Dutch immunologists and infection biologists have made in the last decades on the inflammatory response to viral, bacterial, fungal or parasitic infections. We focus on those studies that have addressed both the host and the pathogen, as these are most interesting from an immunological point of view. Although it is not possible to completely cover this comprehensive research field, this review does provide an interesting overview of Dutch research on inflammatory responses to infection.

IFN-γ causes aplastic anemia by altering hematopoietic stem/progenitor cell composition and disrupting lineage differentiation

Aplastic anemia (AA) is characterized by hypocellular marrow and peripheral pancytopenia. Because interferon gamma (IFN-γ) can be detected in peripheral blood mononuclear cells of AA patients, it has been hypothesized that autoreactive T lymphocytes may be involved in destroying the hematopoietic stem cells. We have observed AA-like symptoms in our IFN-γ adenylate-uridylate-rich element (ARE)-deleted (del) mice, which constitutively express a low level of IFN-γ under normal physiologic conditions. Because no T-cell autoimmunity was observed, we hypothesized that IFN-γ may be directly involved in the pathophysiology of AA. In these mice, we did not detect infiltration of T cells in bone marrow (BM), and the existing T cells seemed to be hyporesponsive. We observed inhibition in myeloid progenitor differentiation despite an increase in serum levels of cytokines involved in hematopoietic differentiation and maturation. Furthermore, there was a disruption in erythropoiesis and B-cell differentiation. The same phenomena were also observed in wild-type recipients of IFN-γ ARE-del BM. The data suggest that AA occurs when IFN-γ inhibits the generation of myeloid progenitors and prevents lineage differentiation, as opposed to infiltration of activated T cells. These results may be useful in improving treatment as well as maintaining a disease-free status.

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

Signal, transduction, and the hematopoietic stem cell

The hematopoietic stem cell (HSC) is a unique cell positioned highest in the hematopoietic hierarchical system. The HSC has the ability to stay in quiescence, to self-renew, or to differentiate and generate all lineages of blood cells. The path to be actualized is influenced by signals that derive from the cell's microenvironment, which activate molecular pathways inside the cell. Signaling pathways are commonly organized through inducible protein-protein interactions, mediated by adaptor proteins that link activated receptors to cytoplasmic effectors. This review will focus on the signaling molecules and how they work in concert to determine the HSC's fate.

Parallels between immune driven-hematopoiesis and T cell activation: 3 signals that relay inflammatory stress to the bone marrow

Quiescence, self-renewal, lineage commitment and differentiation of hematopoietic stem cells (HSCs) towards fully mature blood cells are a complex process that involves both intrinsic and extrinsic signals. During steady-state conditions, most hematopoietic signals are provided by various resident cells inside the bone marrow (BM), which establish the HSC micro-environment. However, upon infection, the hematopoietic process is also affected by pathogens and activated immune cells, which illustrates an effective feedback mechanism to hematopoietic stem and progenitor cells (HSPCs) via immune-mediated signals. Here, we review the impact of pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), costimulatory molecules and pro-inflammatory cytokines on the quiescence, proliferation and differentiation of HSCs and more committed progenitors. As modulation of HSPC function via these immune-mediated signals holds an interesting parallel with the "three-signal-model" described for the activation and differentiation of naïve T-cells, we propose a novel "three-signal" concept for immune-driven hematopoiesis. In this model, the recognition of PAMPs and DAMPs will activate HSCs and induce proliferation, while costimulatory molecules and pro-inflammatory cytokines confer a second and third signal, respectively, which further regulate expansion, lineage commitment and differentiation of HSPCs. We review the impact of inflammatory stress on hematopoiesis along these three signals and we discuss whether they act independently from each other or that concurrence of these signals is important for an adequate response of HSPCs upon infection.

Production and differentiation of myeloid cells driven by proinflammatory cytokines in response to acute pneumovirus infection in mice

Respiratory virus infections are often pathogenic, driving severe inflammatory responses. Most research has focused on localized effects of virus infection and inflammation. However, infection can induce broad-reaching, systemic changes that are only beginning to be characterized. In this study, we assessed the impact of acute pneumovirus infection in C57BL/6 mice on bone marrow hematopoiesis. We hypothesized that inflammatory cytokine production in the lung upregulates myeloid cell production in response to infection. We demonstrate a dramatic increase in the percentages of circulating myeloid cells, which is associated with pronounced elevations in inflammatory cytokines in serum (IFN-γ, IL-6, CCL2), bone (TNF-α), and lung tissue (TNF-α, IFN-γ, IL-6, CCL2, CCL3, G-CSF, osteopontin). Increased hematopoietic stem/progenitor cell percentages (Lineage(-)Sca-I(+)c-kit(+)) were also detected in the bone marrow. This increase was accompanied by an increase in the proportions of committed myeloid progenitors, as determined by colony-forming unit assays. However, no functional changes in hematopoietic stem cells occurred, as assessed by competitive bone marrow reconstitution. Systemic administration of neutralizing Abs to either TNF-α or IFN-γ blocked expansion of myeloid progenitors in the bone marrow and also limited virus clearance from the lung. These findings suggest that acute inflammatory cytokines drive production and differentiation of myeloid cells in the bone marrow by inducing differentiation of committed myeloid progenitors. Our findings provide insight into the mechanisms via which innate immune responses regulate myeloid cell progenitor numbers in response to acute respiratory virus infection.

Impact of interferon-γ on hematopoiesis

The proinflammatory cytokine interferon-γ (IFN-γ) is well known for its important role in innate and adaptive immunity against intracellular infections and for tumor control. Yet, it has become clear that IFN-γ also has a strong impact on bone marrow (BM) output during inflammation, as it affects the differentiation of most hematopoietic progenitor cells. Here, we review the impact of IFN-γ on hematopoiesis, including the function of hematopoietic stem cells (HSCs) and more downstream progenitors. We discuss which hematopoietic lineages are functionally modulated by IFN-γ and through which underlying molecular mechanism(s). We propose the novel concept that IFN-γ acts through upregulation of suppressor of cytokine signaling molecules, which impairs signaling of several cytokine receptors. IFN-γ has also gained clinical interest from different angles, and we discuss how chronic IFN-γ production can lead to the development of anemia and BM failure and how it is involved in malignant hematopoiesis. Overall, this review illustrates the wide-ranging effect of IFN-γ on the (patho-)physiological processes in the BM.

Systemic juvenile idiopathic arthritis-like syndrome in mice following stimulation of the immune system with Freund's complete adjuvant: regulation by interferon-γ

OBJECTIVE

Systemic juvenile idiopathic arthritis (JIA) is unique among the rheumatic diseases of childhood, given its distinctive systemic inflammatory character. Inappropriate control of innate immune responses following an initially harmless trigger is thought to account for the excessive inflammatory reaction. The aim of this study was to generate a similar systemic inflammatory syndrome in mice by injecting a relatively innocuous, yet persistent, immune system trigger: Freund's complete adjuvant (CFA), containing heat-killed mycobacteria.

METHODS

Given the central role of interferon-γ (IFNγ) in immune regulation, we challenged wild-type (WT) and IFNγ-knockout (KO) BALB/c mice with CFA, and analyzed their clinical symptoms and biologic characteristics. The production of cytokines and the effects of anticytokine antibodies were investigated.

RESULTS

In WT mice, CFA injection resulted in splenomegaly, lymphadenopathy, neutrophilia, thrombocytosis, and increased cytokine expression. In the absence of IFNγ, these symptoms were more pronounced and were accompanied by weight loss, arthritis, anemia, hemophagocytosis, abundance of immature blood cells, and increased levels of interleukin-6 (IL-6), all of which are reminiscent of the symptoms of systemic JIA. CFA-challenged IFNγ-KO mice showed increased expression of IL-17 by CD4+ T cells and by innate γ/δ T cells. Inflammatory and hematologic changes were prevented by treatment with anti-IL-12/IL-23p40 and anti-IL-17 antibodies.

CONCLUSION

Immune stimulation of IFNγ-KO mice with CFA produces a systemic inflammatory syndrome reflecting the clinical, biologic, and histopathologic picture of systemic JIA. The protective function of IFNγ in preventing anemia and overall systemic inflammation is a striking observation. The finding that both adaptive and innate T cells are important sources of IL-17 may be of relevance in the pathogenesis of systemic JIA.

Pro-inflammatory cytokines: emerging players regulating HSC function in normal and diseased hematopoiesis

Hematopoiesis is the hierarchical process in which all lineages of blood cells are produced by self-renewing hematopoietic stem cells (HSCs) in the bone marrow (BM). While the regulatory factors that maintain proper HSC function and lineage output under normal conditions are well understood, significantly less is known about how HSC fate is regulated in response to inflammation or disease. As many blood disorders are associated with overproduction of pro-inflammatory cytokines, significant interest has emerged in understanding the impact of these factors on HSC function. In this review we highlight key advances demonstrating the impact of pro-inflammatory cytokines on the biology of HSCs and the BM niche, and address ongoing questions regarding their role in normal and pathogenic hematopoiesis.

Activated MHC-mismatched T helper-1 lymphocyte infusion enhances GvL with limited GvHD

DLI is traditionally used to provide graft-versus-leukemia (GvL) effects when given to patients relapsing post-hematopoietic cell transplantation (HCT). However, it is often associated with significant GvHD and has only modest efficacy against acute leukemias. Therefore, novel cellular therapies are needed to improve the outcome of high-risk or relapsed leukemia patients following HCT. Activated T helper-1 (aTh-1) lymphocytes are CD4(+)CD25(+)CD40L(+)CD62L(lo) effector memory cells that produce large amounts of IFN-γ and TNF-α. We demonstrate that post-transplant adoptive aTh-1 cell therapy enhances GvL with limited GvHD in an MHC-mismatched murine BMT model. aTh-1 infusions result in superior leukemia-free survival when compared with unstimulated splenocytes (SC), purified CD4(+) T-cells and T-cell-enriched SC. aTh-1 cells display cytotoxicity against A20 leukemia cells in vitro and persist in vivo for at least 2 months following adoptive transfer. Furthermore, in contrast to unstimulated SC, aTh-1 cell infusion is associated with only transient, mild suppression of donor-derived hematopoiesis. aTh-1 cell therapy is safe, effective and warrants further investigation as an alternative to DLI.

Emergency granulopoiesis

Neutrophils are a key cell type of the innate immune system. They are short-lived and need to be continuously generated in steady-state conditions from haematopoietic stem and progenitor cells in the bone marrow to ensure their immediate availability for the containment of invading pathogens. However, if microbial infection cannot be controlled locally, and consequently develops into a life-threatening condition, neutrophils are used up in large quantities and the haematopoietic system has to rapidly adapt to the increased demand by switching from steady-state to emergency granulopoiesis. This involves the markedly increased de novo production of neutrophils, which results from enhanced myeloid precursor cell proliferation in the bone marrow. In this Review, we discuss the molecular and cellular events that regulate emergency granulopoiesis, a process that is crucial for host survival.

Hematopoietic stem cells, infection, and the niche

The immune response to infection is a rapid and multifaceted process. Infection affects homeostasis within the hematopoietic stem cell (HSC) niche, as lost immune cells must be replaced by HSCs. During the immune response, interferon is produced. Surprisingly, HSCs respond directly to interferon, entering the cell cycle from even the most dormant state. The complex response of both the HSCs and the niche to infection is a unique platform on which to consider HSC-niche interactions. Here, we comment on the contribution of the immune system to the niche and on the direct and indirect effect that infection has on HSCs in the niche.

Infection-induced changes in hematopoiesis

The bone marrow (BM) is an important site for the interrelated processes of hematopoiesis, granulopoiesis, erythropoiesis, and lymphopoiesis. A wide variety of microbial challenges are associated with profound changes in this compartment that impact on hematopoietic differentiation and mobilization of a variety of cell types. This article reviews some of the key pathways that control BM homeostasis, the infectious and inflammatory processes that affect the BM, and how addressing the knowledge gaps in this area has the potential to widen our comprehension of immune homeostasis.

Nov/CCN3 regulates long-term repopulating activity of murine hematopoietic stem cells via integrin αvβ3

Throughout life, hematopoietic stem cells (HSCs) sustain the blood cell supply through their capacities for self-renewal and multilineage differentiation. These processes are regulated within a specialized microenvironment termed the 'niche'. Here, we show a novel mechanism for regulating HSC function that is mediated by nephroblastoma overexpressed (Nov/CCN3), a matricellular protein member of the CCN family. We found that Nov contributes to the maintenance of long-term repopulating (LTR) activity through association with integrin αvβ3 on HSCs. The resultant β3 integrin outside-in signaling is dependent on thrombopoietin (TPO), a crucial cytokine involved in HSC maintenance. TPO was required for Nov binding to integrin αvβ3, and stimulated Nov expression in HSCs. However, in the presence of IFNγ, a cytokine known to impair HSC function, not only was TPO-induced expression of Nov suppressed, but the LTR activity was conversely impaired by TPO-mediated ligation of integrin αvβ3 with exogenous ligands, including Nov, as well. Thus, Nov/integrin αvβ3-mediated maintenance of HSCs appears to be modulated by simultaneous stimulation by other cytokines. Our finding suggests that this system contributes to the regulation of HSCs within the bone marrow niche.

Re-entry into quiescence protects hematopoietic stem cells from the killing effect of chronic exposure to type I interferons

Quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure

Type I interferons (IFN-1s) are antiviral cytokines that suppress blood production while paradoxically inducing hematopoietic stem cell (HSC) proliferation. Here, we clarify the relationship between the proliferative and suppressive effects of IFN-1s on HSC function during acute and chronic IFN-1 exposure. We show that IFN-1–driven HSC proliferation is a transient event resulting from a brief relaxation of quiescence-enforcing mechanisms in response to acute IFN-1 exposure, which occurs exclusively in vivo. We find that this proliferative burst fails to exhaust the HSC pool, which rapidly returns to quiescence in response to chronic IFN-1 exposure. Moreover, we demonstrate that IFN-1–exposed HSCs with reestablished quiescence are largely protected from the killing effects of IFNs unless forced back into the cell cycle due to culture, transplantation, or myeloablative treatment, at which point they activate a p53-dependent proapoptotic gene program. Collectively, our results demonstrate that quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure. We show that IFN-1s can poise HSCs for apoptosis but induce direct cell killing only upon active proliferation, thereby establishing a mechanism for the suppressive effects of IFN-1s on HSC function.

Interferon-γ suppresses activin A/NF-E2 induction of erythroid gene expression through the NF-κB/c-Jun pathway

Interferon (IFN)-γ is a proinflammatory cytokine that is linked to erythropoiesis inhibition and may contribute to anemia. However, the mechanism of IFN-γ-inhibited erythropoiesis is unknown. Activin A, a member of the transforming growth factor (TGF)-β superfamily, induces the erythropoiesis of hematopoietic progenitor cells. In this study, a luciferase reporter assay showed that IFN-γ suppressed activin A-induced ζ-globin promoter activation in K562 erythroblast cells in a dose-dependent manner. Activin A reversed the suppressive effect of IFN-γ on the luciferase activity of ζ-globin promoter in a dose-dependent manner. IFN-γ also suppressed the activation of activin A-induced α-globin promoter. IFN-γ reduced the mRNA expression of α-globin, ζ-globin, NF-E2p45, and GATA-1 induced by activin A. The results also showed that IFN-γ induced c-Jun expression when NF-κBp65 and c-Jun bound to two AP-1-binding sites on the c-Jun promoter. The luciferase activity of α-globin and ζ-globin promoters were enhanced by wild-type c-Jun and eliminated by dominant-negative (DN) c-Jun. The suppressive effects of IFN-γ on the mRNA expression of α-globin and ζ-globin were absent in cells expressing DN c-Jun. The ability of NF-E2 to enhance activin A-induced ζ-globin promoter activation decreased when c-Jun was present, and IFN-γ treatment further enhanced the decreasing effect of c-Jun. Chromatin immunoprecipitation revealed that NF-E2p45 bound to the upstream regulatory element (HS-40) of the α-globin gene cluster in response to activin A, whereas c-Jun eliminated this binding. These results suggest that IFN-γ modulates NF-κB/c-Jun to antagonize activin A-mediated NF-E2 transcriptional activity on globin gene expression.

TLRs control hematopoiesis during infection

Recent research has shown that (i) Toll-like receptor (TLR) agonists drive hematopoietic stem and progenitor cells (HSPCs) to proliferate and differentiate along the myeloid lineage in vitro, and (ii) direct TLR-mediated stimulation of HSPCs also promotes macrophage differentiation in vivo following infection. These new insights demonstrate that TLR signaling in HSPCs, in addition to other TLR-dependent mechanisms, can contribute to HSPC expansion and myeloid differentiation after infection. Evidence is, therefore, mounting that direct TLR-induced programming of hematopoiesis plays a key role in host defense by rapidly replenishing the innate immune system with the cells needed to deal with pathogens.

JAK2V617F expression in mice amplifies early hematopoietic cells and gives them a competitive advantage that is hampered by IFNα

The acquired gain-of-function V617F mutation in the Janus Kinase 2 (JAK2(V617F)) is the main mutation involved in BCR/ABL-negative myeloproliferative neoplasms (MPNs), but its effect on hematopoietic stem cells as a driver of disease emergence has been questioned. Therefore, we reinvestigated the role of endogenous expression of JAK2(V617F) on early steps of hematopoiesis as well as the effect of interferon-α (IFNα), which may target the JAK2(V617F) clone in humans by using knock-in mice with conditional expression of JAK2(V617F) in hematopoietic cells. These mice develop a MPN mimicking polycythemia vera with large amplification of myeloid mature and precursor cells, displaying erythroid endogenous growth and progressing to myelofibrosis. Interestingly, early hematopoietic compartments [Lin-, LSK, and SLAM (LSK/CD48-/CD150+)] increased with the age. Competitive repopulation assays demonstrated disease appearance and progressive overgrowth of myeloid, Lin-, LSK, and SLAM cells, but not lymphocytes, from a low number of engrafted JAK2(V617F) SLAM cells. Finally, IFNα treatment prevented disease development by specifically inhibiting JAK2(V617F) cells at an early stage of differentiation and eradicating disease-initiating cells. This study shows that JAK2(V617F) in mice amplifies not only late but also early hematopoietic cells, giving them a proliferative advantage through high cell cycling and low apoptosis that may sustain MPN emergence but is lost upon IFNα treatment.

Interferons in hematopoiesis and leukemia

Interferons not only exert a fundamental role during inflammation and immune responses but also modulate the activity of hematopoietic stem cells during homeostatic and demand-adapted hematopoiesis. Identical mechanisms regulate the homeostasis and proliferation of leukemic stem cells (LSCs). Understanding these mechanisms may lead to novel therapeutic approaches against leukemia.

Current concepts in the pathophysiology and treatment of aplastic anemia

Historically viewed in isolation as an odd, rare, and invariably fatal blood disease, aplastic anemia is now of substantial interest for its immune pathophysiology, its relationship to constitutional BM failure syndromes and leukemia, and the success of both stem cell transplantation and immunosuppressive therapies in dramatically improving survival of patients. Once relegated to a few presentations in the red cell and anemia sessions of the ASH, the Society now sponsors multiple simultaneous sessions and plenary and scientific committee presentations on these topics. This update emphasizes developments in our understanding of immune mechanisms and hematopoietic stem cell biology and new clinical approaches to stem cell stimulation as a therapy, alone and in combination with conventional suppression of the aberrant immune system.