Extramedullary myelopoiesis in malaria depends on mobilization of myeloid-restricted progenitors by IFN-γ induced chemokines

Plasmodium yoelii Infection Enhances the Expansion of Myeloid-Derived Suppressor Cells via JAK/STAT3 Pathway

Myeloid-derived suppressor cells (MDSCs), the negative immune regulators, have been demonstrated to be involved in immune responses to a variety of pathological conditions, such as tumors, chronic inflammation, and infectious diseases. However, the roles and mechanisms underlying the expansion of MDSCs in malaria remain unclear. In this study, the phenotypic and functional characteristics of splenic MDSCs during Plasmodium yoelii NSM infection are described. Furthermore, we provide compelling evidence that the sera from P. yoelii-infected C57BL/6 mice containing excess IL-6 and granulocyte-macrophage colony-stimulating factor promote the accumulation of MDSCs by inducing Bcl2 expression. Serum-induced MDSCs exert more potent suppressive effects on T cell responses than control MDSCs within both in vivo P. yoelii infection and in vitro serum-treated bone marrow cells experiments. Serum treatment increases the MDSC inhibitory effect, which is dependent on Arg1 expression. Moreover, mechanistic studies reveal that the serum effects are mediated by JAK/STAT3 signaling. By inhibiting STAT3 phosphorylation with the JAK inhibitor JSI-124, effects of serum on MDSCs are almost eliminated. In vivo depletion of MDSCs with anti-Gr-1 or 5-fluorouracil significantly reduces the parasitemia and promotes Th1 immune response in P. yoelii-infected C57BL/6 mice by upregulating IFN-γ expression. In summary, this study indicates that P. yoelii infection facilitates the accumulation and function of MDSCs by upregulating the expression of Bcl2 and Arg1 via JAK/STAT3 signaling pathway in vivo and in vitro. Manipulating the JAK/STAT3 signaling pathway or depleting MDSCs could be promising therapeutic interventions to treat malaria.

Host Genetic Impact on Infectious Diseases among Different Ethnic Groups

Infectious diseases such as malaria, tuberculosis (TB), human immunodeficiency virus (HIV), and the coronavirus disease of 2019 (COVID-19) are problematic globally, with high prevalence particularly in Africa, attributing to most of the death rates. There have been immense efforts toward developing effective preventative and therapeutic strategies for these pathogens globally, however, some remain uncured. Disease susceptibility and progression for malaria, TB, HIV, and COVID-19 vary among individuals and are attributed to precautionary measures, environment, host, and pathogen genetics. While studying individuals with similar attributes, it is suggested that host genetics contributes to most of an individual's susceptibility to disease. Several host genes are identified to associate with these pathogens. Interestingly, many of these genes and polymorphisms are common across diseases. This paper analyzes genes and genetic variations within host genes associated with HIV, TB, malaria, and COVID-19 among different ethnic groups. The differences in host-pathogen interaction among these groups, particularly of Caucasian and African descent, and which gene polymorphisms are prevalent in an African population that possesses protection or risk to disease are reviewed. The information in this review could potentially help develop personalized treatment that could effectively combat the high disease burden in Africa.

Modulation of haematopoiesis by protozoal and helminth parasites

During inflammation, haematopoietic stem cells (HSCs) in the bone marrow (BM) and periphery rapidly expand and preferentially differentiate into myeloid cells that mediate innate immune responses. HSCs can be directed into quiescence or differentiation by sensing alterations to the haematopoietic niche, including cytokines, chemokines, and pathogen-derived products. Most studies attempting to identify the mechanisms of haematopoiesis have focused on bacterial and viral infections. From intracellular protozoan infections to large multicellular worms, parasites are a global health burden and represent major immunological challenges that remain poorly defined in the context of haematopoiesis. Immune responses to parasites vary drastically, and parasites have developed sophisticated immunomodulatory mechanisms that allow development of chronic infections. Recent advances in imaging, genomic sequencing, and mouse models have shed new light on how parasites induce unique forms of emergency haematopoiesis. In addition, parasites can modify the haematopoiesis in the BM and periphery to improve their survival in the host. Parasites can also induce long-lasting modifications to HSCs, altering future immune responses to infection, inflammation or transplantation, a term sometimes referred to as central trained immunity. In this review, we highlight the current understanding of parasite-induced haematopoiesis and how parasites target this process to promote chronic infections.

African Plasmodium vivax malaria improbably rare or benign

The overwhelming dominance of Duffy blood group negativity among most people living in sub-Saharan Africa has been considered the basis of their protection from endemic Plasmodium vivax malaria. New evidence demonstrates widespread transmission of P. vivax in Duffy-negative Africa, though currently of unknown distribution, magnitude, or consequences. Other new evidence from outside of Africa demonstrates marked tropisms of P. vivax for extravascular tissues of bone marrow and spleen. Those establish states of proliferative infection with low-grade or undetectable parasitemia of peripheral blood causing acute and chronic disease. This review examines the plausibility of those infectious processes also operating in Duffy-negative Africans and causing harm of unrecognized origin.

Oral Trypanosoma cruzi Acute Infection in Mice Targets Primary Lymphoid Organs and Triggers Extramedullary Hematopoiesis

During the acute phase of Chagas disease, Trypanosoma cruzi circulation through the bloodstream leads to high tissue parasitism in the host. In primary lymphoid organs, progenitor cell reduction paralleled transient immunosuppression. Herein we showed that acute oral infection in mice promotes diffuse parasitism in bone marrow cells at 14 and 21 days post-infection (dpi), with perivascular regions, intravascular regions, and regions near the bone being target sites of parasite replication. Phenotypic analysis of hematopoietic differentiation in the bone marrow of infected mice showed that the cell number in the tissue is decreased (lineage-negative and lineage-positive cells). Interestingly, analysis of hematopoietic branching points showed that hematopoietic stem and progenitor cells (HSPCs) were significantly increased at 14 dpi. In addition, the pool of progenitors with stem plasticity (HSC-MPP3), as well as multipotent progenitors (MPPs) such as MPP4, also showed this pattern of increase. In contrast, subsequent progenitors that arise from MPPs, such as common lymphoid progenitors (CLPs), lymphoid-primed MPPs (LMPPs), and myeloid progenitors, were not enhanced; conversely, all presented numeric decline. Annexin V staining revealed that cell death increase in the initial hematopoietic branching point probably is not linked to CLPs and that myeloid progenitors decreased at 14 and 21 dpi. In parallel, our investigation provided clues that myeloid progenitor decrease could be associated with an atypical expression of Sca-1 in this population leading to a remarkable increase on LSK-like cells at 14 dpi within the HSPC compartment. Finally, these results led us to investigate HSPC presence in the spleen as a phenomenon triggered during emergency hematopoiesis due to mobilization or expansion of these cells in extramedullary sites. Splenocyte analysis showed a progressive increase in HSPCs between 14 and 21 dpi. Altogether, our study shows that the bone marrow is a target tissue in T. cruzi orally infected mice, leading to a hematopoietic disturbance with LSK-like cell bias accounting on HSPCs possibly affecting myeloid progenitor numbers. The LMPP and CLP reduction converges with defective thymocyte development. Lastly, it is tempting to speculate that the extramedullary hematopoiesis seen in the spleen is a mechanism involved in the hematological maintenance reported during the acute phase of oral T. cruzi infection.

Comparative transcriptomic analysis reveals translationally relevant processes in mouse models of malaria

Recent initiatives to improve translation of findings from animal models to human disease have focussed on reproducibility but quantifying the relevance of animal models remains a challenge. Here, we use comparative transcriptomics of blood to evaluate the systemic host response and its concordance between humans with different clinical manifestations of malaria and five commonly used mouse models. Plasmodium yoelii 17XL infection of mice most closely reproduces the profile of gene expression changes seen in the major human severe malaria syndromes, accompanied by high parasite biomass, severe anemia, hyperlactatemia, and cerebral microvascular pathology. However, there is also considerable discordance of changes in gene expression between the different host species and across all models, indicating that the relevance of biological mechanisms of interest in each model should be assessed before conducting experiments. These data will aid the selection of appropriate models for translational malaria research, and the approach is generalizable to other disease models.

Remodeling of the Bone Marrow Stromal Microenvironment During Pathogenic Infections

The bone marrow (BM) is the primary hematopoietic organ and a hub in which organismal demands for blood cellular output are systematically monitored. BM tissues are additionally home to a plethora of mature immune cell types, providing functional environments for the activation of immune responses and acting as preferred anatomical reservoirs for cells involved in immunological memory. Stromal cells of the BM microenvironment crucially govern different aspects of organ function, by structuring tissue microanatomy and by directly providing essential regulatory cues to hematopoietic and immune components in distinct niches. Emerging evidence demonstrates that stromal networks are endowed with remarkable functional and structural plasticity. Stress-induced adaptations of stromal cells translate into demand-driven hematopoiesis. Furthermore, aberrations of stromal integrity arising from pathological conditions critically contribute to the dysregulation of BM function. Here, we summarize our current understanding of the alterations that pathogenic infections and ensuing inflammatory conditions elicit on the global topography of the BM microenvironment, the integrity of anatomical niches and cellular interactions, and ultimately, on the regulatory function of diverse stromal subsets.

Interleukin-10 induces interferon-γ-dependent emergency myelopoiesis

In emergency myelopoiesis (EM), expansion of the myeloid progenitor compartment and increased myeloid cell production are observed and often mediated by the pro-inflammatory cytokine interferon gamma (IFN-γ). Interleukin-10 (IL-10) inhibits IFN-γ secretion, but paradoxically, its therapeutic administration to humans causes hematologic changes similar to those observed in EM. In this work, we use different in vivo systems, including a humanized immune system mouse model, to show that IL-10 triggers EM, with a significant expansion of the myeloid progenitor compartment and production of myeloid cells. Hematopoietic progenitors display a prominent IFN-γ transcriptional signature, and we show that IFN-γ mediates IL-10-driven EM. We also find that IL-10, unexpectedly, reprograms CD4 and CD8 T cells toward an activation state that includes IFN-γ production by these T cell subsets in vivo. Therefore, in addition to its established anti-inflammatory properties, IL-10 can induce IFN-γ production and EM, opening additional perspectives for the design of IL-10-based immunotherapies.

Immunosuppression in Malaria: Do Plasmodium falciparum Parasites Hijack the Host?

Malaria reflects not only a state of immune activation, but also a state of general immune defect or immunosuppression, of complex etiology that can last longer than the actual episode. Inhabitants of malaria-endemic regions with lifelong exposure to the parasite show an exhausted or immune regulatory profile compared to non- or minimally exposed subjects. Several studies and experiments to identify and characterize the cause of this malaria-related immunosuppression have shown that malaria suppresses humoral and cellular responses to both homologous (Plasmodium) and heterologous antigens (e.g., vaccines). However, neither the underlying mechanisms nor the relative involvement of different types of immune cells in immunosuppression during malaria is well understood. Moreover, the implication of the parasite during the different stages of the modulation of immunity has not been addressed in detail. There is growing evidence of a role of immune regulators and cellular components in malaria that may lead to immunosuppression that needs further research. In this review, we summarize the current evidence on how malaria parasites may directly and indirectly induce immunosuppression and investigate the potential role of specific cell types, effector molecules and other immunoregulatory factors.

Systemic bacterial infections affect dendritic cell development and function

Dendritic cells (DCs) are critical in host defense against infection. DC depletion is an early event in the course of sepsis that may impair the host defense mechanisms. Here, we addressed whether DC depletion and dysfunction are pathogen-independent, mediated via pattern recognition receptors, and are due to impaired DC development upon systemic infection with the Gram-negative bacterium Escherichia coli and the Gram-positive pathogen Staphylococcus aureus. Infection with E. coli and S. aureus led to reduced numbers of splenic DC subsets and of DC progenitors in the bone marrow (BM) with this effect persisting significantly longer in mice infected with S. aureus than with E. coli. The reduction of DC subsets and their progenitors was mainly TLR-independent as was the infection-induced monopoiesis. Moreover, de novo DC development was impaired in mice infected with S. aureus, and BM cells from E. coli or S. aureus infected mice favored macrophage differentiation in vitro. As a consequence of reduced DC numbers and their reduced expression of MHC II less CD4⁺ and CD8⁺ T cells, especially Th1 and IFN-γ producing CD8⁺ T cells, could be detected in S. aureus compared to E. coli infected mice. These differences are reflected in the rapid killing of E. coli as opposed to an increase in bacterial load in S. aureus. In summary, our study supports the idea that systemic bacterial infections generally affect the number and development of DCs and thereby the T cell responses, but the magnitude is pathogen-dependent.

Yin and Yang: The dual effects of interferons on hematopoiesis

Interferons are an ancient and well-conserved group of inflammatory cytokines most famous for their role in viral immunity. A decade ago, we discovered that interferons also play an important role in the biology of hematopoietic stem cells (HSCs), which are responsible for lifelong blood production. Though we have learned a great deal about the role of interferons on HSC quiescence, differentiation, and self-renewal, there remains some controversy regarding how interferons impact these stem cells, with differing conclusions depending on experimental models and clinical context. Here, we review the contradictory roles of Type 1 and 2 interferons in hematopoiesis. Specifically, we highlight the roles of interferons in embryonic and adult hematopoiesis, along with short-term and long-term adaptive and maladaptive responses to inflammation. We discuss experimental challenges in the study of these powerful yet short-lived cytokines and strategies to address those challenges. We further review the contribution by interferons to disease states including bone marrow failure and aplastic anemia as well as their therapeutic use to treat myeloproliferative neoplasms and viral infections, including SARS-CoV2. Understanding the opposing effects of interferons on hematopoiesis will elucidate immune responses and bone marrow failure syndromes, and future therapeutic approaches for patients undergoing HSC transplantation or fighting infectious diseases and cancer.

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

Integrative analysis of microRNA and mRNA expression profiles of monocyte-derived dendritic cells differentiation during experimental cerebral malaria

Heterogeneity and high plasticity are common features of cells from the mononuclear phagocyte system: monocytes (MOs), macrophages, and dendritic cells (DCs). Upon activation by microbial agents, MO can differentiate into MO-derived DCs (MODCs). In previous work, we have shown that during acute infection with Plasmodium berghei ANKA (PbA), MODCs become, transiently, the main CD11b+ myeloid population in the spleen (SP) and once recruited to the brain play an important role in the development of experimental cerebral malaria (ECM). Here, we isolated 4 cell populations: bone marrow (BM) MOs (BM-MOs) and SP-MOs from uninfected mice; BM inflammatory MOs (BM-iMOs) and SP-MODCs from PbA-infected mice and used a system biology approach to a holistic transcriptomic comparison and provide an interactome analysis by integrating differentially expressed miRNAs (DEMs) and their differentially expressed gene targets (DEGs) data. The Jaccard index (JI) was used for gauging the similarity and diversity among these cell populations. Whereas BM-MOs, BM-iMOs, and SP-MOs presented high similarity of DEGs, SP-MODCs distinguished by showing a greater number of DEGs. Moreover, functional analysis identified an enrichment in canonical pathways, such as DC maturation, neuroinflammation, and IFN signaling. Upstream regulator analysis identified IFNγ as the potential upstream molecule that can explain the observed DEMs-Target DEGs intersections in SP-MODCs. Finally, directed target analysis and in vivo/ex vivo assays indicate that SP-MODCs differentiate in the SP and IFNγ is a main driver of this process.

The Innate Immune Response to Infection Induces Erythropoietin-Dependent Replenishment of the Dendritic Cell Compartment

Dendritic cells (DC) play a key role in the adaptive immune response due to their ability to present antigens and stimulate naïve T cells. Many bacteria and viruses can efficiently target DC, resulting in impairment of their immunostimulatory function or elimination. Hence, the DC compartment requires replenishment following infection to ensure continued operational readiness of the adaptive immune system. Here, we investigated the molecular and cellular mechanisms of inflammation-induced DC generation. We found that infection with viral and bacterial pathogens as well as Toll-like receptor 9 (TLR9) ligation with CpG-oligodeoxynucleotide (CpG-ODN) expanded an erythropoietin (EPO)-dependent TER119⁺CD11a⁺ cell population in the spleen that had the capacity to differentiate into TER119⁺CD11c^high and TER119^-CD11c^high cells both in vitro and in vivo. TER119⁺CD11c^high cells contributed to the conventional DC pool in the spleen and specifically increased in lymph nodes draining the site of local inflammation. Our results reveal a so far undescribed inflammatory EPO-dependent pathway of DC differentiation and establish a mechanistic link between innate immune recognition of potential immunosuppressive pathogens and the maintenance of the DC pool during and after infection.

Host-pathogen interaction in the tissue environment during Plasmodium blood-stage infection

Human malarial infection occurs after an infectious Anopheles mosquito bites. Following the initial liver-stage infection, parasites transform into merozoites, infecting red blood cells (RBCs). Repeated RBC infection then occurs during the blood-stage infection, while patients experience various malarial symptoms. Protective immune responses are elicited by this systemic infection, but excessive responses are sometimes harmful for hosts. As parasites infect only RBCs and their immediate precursors during this stage, direct parasite-host interactions occur primarily in the environment surrounded by endothelial lining of blood vessels. The spleen is the major organ where the immune system encounters infected RBCs, causing immunological responses. Its tissue structure is markedly altered during malarial infection in mice and humans. Plasmodium falciparum parasites inside RBCs express proteins, such as PfEMP-1 and RIFIN, transported to the RBC surfaces in order to evade immunological attack by sequestering themselves in the peripheral vasculature avoiding spleen or by direct immune cell inhibition through inhibitory receptors. Host cell production of regulatory cytokines IL-10 and IL-27 limits excessive immune responses, avoiding tissue damage. The regulation of the protective and inhibitory immune responses through host-parasite interactions allows chronic Plasmodium infection. In this review, we discuss underlying interaction mechanisms relevant for developing effective strategies against malaria.

Acute Plasmodium berghei Mouse Infection Elicits Perturbed Erythropoiesis With Features That Overlap With Anemia of Chronic Disease

Severe malaria anemia is one of the most common causes of morbidity and mortality arising from infection with Plasmodium falciparum. The pathogenesis of malarial anemia is complex, involving both parasite and host factors. As mouse models of malaria also develop anemia, they can provide a useful resource to study the impact of Plasmodium infections and the resulting host innate immune response on erythropoiesis. In this study, we have characterized the bone marrow and splenic responses of the erythroid as well as other hematopoietic lineages after an acute infection of Balb/c mice with Plasmodium berghei. Such characterization of the hematopoietic changes is critical to underpin future studies, using knockout mice and transgenic parasites, to tease out the interplay between host genes and parasite modulators implicated in susceptibility to malaria anemia. P. berghei infection led to a clear perturbation of steady-state erythropoiesis, with the most profound defects in polychromatic and orthochromatic erythroblasts as well as erythroid colony- and burst-forming units (CFU-E and BFU-E), resulting in an inability to compensate for anemia. The perturbation in erythropoiesis was not attributable to parasites infecting erythroblasts and affecting differentiation, nor to insufficient erythropoietin (EPO) production or impaired activation of the Signal transducer and activator of transcription 5 (STAT5) downstream of the EPO receptor, indicating EPO-signaling remained functional in anemia. Instead, the results point to acute anemia in P. berghei-infected mice arising from increased myeloid cell production in order to clear the infection, and the concomitant release of pro-inflammatory cytokines and chemokines from myeloid cells that inhibit erythroid development, in a manner that resembles the pathophysiology of anemia of chronic disease.

Dendritic cell development in infection

The immune system protects from infections primarily by detecting and eliminating invading pathogens. This is predominantly mediated by innate immune cells like neutrophils, monocytes and dendritic cells (DCs) expressing specific receptors recognizing pathogen-associated molecular patterns. DC activation by pathogens leads to the initiation of antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. However, various pathogens have evolved immune evasion strategies to ensure their survival. In this review, we highlight recent findings on how various microorganisms or their structural features affect or modulate DC development and whether this has any consequences for a protective immune response.

B-cell memory in malaria: Myths and realities

B-cell and antibody responses to Plasmodium spp., the parasite that causes malaria, are critical for control of parasitemia and associated immunopathology. Antibodies also provide protection to reinfection. Long-lasting B-cell memory has been shown to occur in response to Plasmodium spp. in experimental model infections, and in human malaria. However, there are reports that antibody responses to several malaria antigens in young children living with malaria are not similarly long-lived, suggesting a dysfunction in the maintenance of circulating antibodies. Some studies attribute this to the expansion of atypical memory B cells (AMB), which express multiple inhibitory receptors and activation markers, and are hyporesponsive to B-cell receptor (BCR) restimulation in vitro. AMB are also expanded in other chronic infections such as tuberculosis, hepatitis B and C, and HIV, as well as in autoimmunity and old age, highlighting the importance of understanding their role in immunity. Whether AMB are dysfunctional remains controversial, as there are also studies in other infections showing that AMB can produce isotype-switched antibodies and in mouse can contribute to protection against infection. In light of these controversies, we review the most recent literature on either side of the debate and challenge some of the currently held views regarding B-cell responses to Plasmodium infections.

Interferon-γ exerts dual functions on human erythropoiesis via interferon regulatory factor 1 signal pathway

Hematopoiesis is systematically regulated by microenvironmental factors. The positive and negative factors coordinated together to yield a complicated blood system. Interferon-γ (IFNγ) has been identified as a common cause of various hematopoietic abnormalities, such as aplastic anemia. However, its impact on monolineage development, especially erythropoiesis, has not been fully elucidated from the cellular angle. In this study, we investigated the behavior of IFNγ and found that IFNγ plays dual functions on erythropoiesis; it not only blocks the erythroid lineage commitment but also accelerates the erythroid differentiation process, ultimately leading to the erythropoietic window clearance. IFNγ can even powerfully initiate early differentiation without the existence of erythropoietin (EPO). Interferon regulatory factor 1 (IRF1) was confirmed as the essential downstream effector, and its ectopic overexpression can also have the same effect as that of IFNγ. These results reveal that the IFNγ-IRF1 axis plays a bidirectional role on erythropoiesis, impeding the access to erythroid lineage and driving the coming cells toward the differentiation endpoint. This model may place an innovative implication for IFNγ-IRF1 axis to understand its in-depth mechanism on normal hematopoiesis and abnormal blood disorders, especially aplastic anemia.

Monocyte-derived dendritic cells in malaria

The pathogenesis of malaria is a multifactorial syndrome associated with a deleterious inflammatory response that is responsible for many of the clinical manifestations. While dendritic cells (DCs) play a critical role in initiating acquired immunity and host resistance to infection, they also play a pathogenic role in inflammatory diseases. In our recent studies, we found in different rodent malaria models that the monocyte-derived DCs (MO-DCs) become, transiently, a main DC population in spleens and inflamed non-lymphoid organs. These studies suggest that acute infection with Plasmodium berghei promotes the differentiation of splenic monocytes into inflammatory monocytes (iMOs) and thereafter into MO-DCs that play a pathogenic role by promoting inflammation and tissue damage. The recruitment of MO-DCs to the lungs and brain are dependent on expression of CCR4 and CCR5, respectively, and expression of respective chemokine ligands in each organ. Once they reach the target organ the MO-DCs produce the CXCR3 ligands (CXCL9 and CXCL10), recruit CD8⁺ T cells, and produce toxic metabolites that play an important role in the development of experimental cerebral malaria (ECM) and acute respiratory distress syndrome (ARDS).

Transcriptional profiling unveils type I and II interferon networks in blood and tissues across diseases

Understanding how immune challenges elicit different responses is critical for diagnosing and deciphering immune regulation. Using a modular strategy to interpret the complex transcriptional host response in mouse models of infection and inflammation, we show a breadth of immune responses in the lung. Lung immune signatures are dominated by either IFN-γ and IFN-inducible, IL-17-induced neutrophil- or allergy-associated gene expression. Type I IFN and IFN-γ-inducible, but not IL-17- or allergy-associated signatures, are preserved in the blood. While IL-17-associated genes identified in lung are detected in blood, the allergy signature is only detectable in blood CD4+ effector cells. Type I IFN-inducible genes are abrogated in the absence of IFN-γ signaling and decrease in the absence of IFNAR signaling, both independently contributing to the regulation of granulocyte responses and pathology during Toxoplasma gondii infection. Our framework provides an ideal tool for comparative analyses of transcriptional signatures contributing to protection or pathogenesis in disease.

The Regulatory Role of IFN-γ on the Proliferation and Differentiation of Hematopoietic Stem and Progenitor Cells

The replenishment of all blood cell lineages is hierarchically organized by the process of hematopoiesis, which is based on the differentiation pathways of hematopoietic stem and progenitor cells (HSPCs). Due to the ability to balance between self-renewal and differentiation, hematopoietic stem cells (HSCs) can generate the appropriate cell type that is required by the immune system and peripheral blood in response to physiological or pathological conditions. Numerous studies have shown that some proinflammatory cytokines contribute to the regulation of the various hematopoietic compartments. Of these, IFN-γ is a type II interferon primarily produced by T cells and natural killer cells, and plays a major role in the defense against invading pathogens and transformed cancer cells; moreover, a growing amount of research indicates that it exerts negative or positive regulatory effect on hematopoiesis. Although IFN-γ is a widely regarded negative regulator of HSC proliferation, it also participates in some chronic infections or hematological malignancies that induce bone marrow failure. Recent studies have demonstrated unexpected effects of IFN-γ, including the promotion of HSC formation and the stimulation of myelopoiesis. Here, we review the direct and indirect effects of IFN-γ on hematopoiesis, as well as the underlying signaling mechanisms of how IFN-γ modulates the self-renewal, cell cycle entry, and proliferation of HSCs. Next, we describe how IFN-γ affects different stages of the lineage differentiation from HSCs. Finally, we discuss the relationship between IFN-γ and compensatory extramedullary hematopoiesis, as well as some related clinical diseases.

Regulation of myelopoiesis by proinflammatory cytokines in infectious diseases

Hematopoiesis is hierarchically orchestrated by a very small population of hematopoietic stem cells (HSCs) that reside in the bone-marrow niche and are tightly regulated to maintain homeostatic blood production. HSCs are predominantly quiescent, but they enter the cell cycle in response to inflammatory signals evoked by severe systemic infection or injury. Thus, hematopoietic stem and progenitor cells (HSPCs) can be activated by pathogen recognition receptors and proinflammatory cytokines to induce emergency myelopoiesis during infection. This emergency myelopoiesis counterbalances the loss of cells and generates lineage-restricted hematopoietic progenitors, eventually replenishing mature myeloid cells to control the infection. Controlled generation of such signals effectively augments host defense, but dysregulated stimulation by these signals is harmful to HSPCs. Such hematopoietic failure often results in blood disorders including chronic inflammatory diseases and hematological malignancies. Recently, we found that interleukin (IL)-27, one of the IL-6/IL-12 family cytokines, has a unique ability to directly act on HSCs and promote their expansion and differentiation into myeloid progenitors. This process resulted in enhanced production of neutrophils by emergency myelopoiesis during the blood-stage mouse malaria infection. In this review, we summarize recent advances in the regulation of myelopoiesis by proinflammatory cytokines including type I and II interferons, IL-6, IL-27, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, and IL-1 in infectious diseases.

Mechanism of splenic cell death and host mortality in a Plasmodium yoelii malaria model

Malaria is a fatal disease that displays a spectrum of symptoms and severity, which are determined by complex host-parasite interactions. It has been difficult to study the effects of parasite strains on disease severity in human infections, but the mechanisms leading to specific disease phenotypes can be investigated using strains of rodent malaria parasites that cause different disease symptoms in inbred mice. Using a unique mouse malaria model, here we investigated the mechanisms of splenic cell death and their relationship to control of parasitemia and host mortality. C57BL/6 mice infected with Plasmodium yoelii nigeriensis N67C display high levels of pro-inflammatory cytokines and chemokines (IL-6, IFN-γ, TNF-α, CXCL1, and CCL2) and extensive splenic damage with dramatic reduction of splenic cell populations. These disease phenotypes were rescued in RAG2^−/−, IFN-γ^−/−, or T cell depleted mice, suggesting IFN-γ and T cell mediated disease mechanisms. Additionally, apoptosis was one of the major pathways involved in splenic cell death, which coincides with the peaks of pro-inflammatory cytokines. Our results demonstrate the critical roles of T cells and IFN-γ in mediating splenic cell apoptosis, parasitemia control, and host lethality and thus may provide important insights for preventing/reducing morbidity associated with severe malaria in humans.

IL-17 Promotes Differentiation of Splenic LSK- Lymphoid Progenitors into B Cells following Plasmodium yoelii Infection

Lineage^-Sca-1⁺c-Kit^- (LSK^-) cells are a lymphoid progenitor population that expands in the spleen and preferentially differentiates into mature B cells in response to Plasmodium yoelii infection in mice. Furthermore, LSK^- derived B cells can subsequently contribute to the ongoing immune response through the generation of parasite-specific Ab-secreting cells, as well as germinal center and memory B cells. However, the factors that promote their differentiation into B cells in the spleen postinfection are not defined. In this article, we show that LSK^- cells produce the cytokine IL-17 in response to Plasmodium infection. Using Il-17ra^-/- mice, IL-17R signaling in cells other than LSK^- cells was found to support their differentiation into B cells. Moreover, primary splenic stromal cells grown in the presence of IL-17 enhanced the production of CXCL12, a chemokine associated with B cell development in the bone marrow, by a population of IL-17RA-expressing podoplanin⁺CD31^- stromal cells, a profile associated with fibroblastic reticular cells. Subsequent blockade of CXCL12 in vitro reduced differentiation of LSK^- cells into B cells, supporting a direct role for this chemokine in this process. Immunofluorescence indicated that podoplanin⁺ stromal cells in the red pulp were the primary producers of CXCL12 after P. yoelii infection. Furthermore, podoplanin staining on stromal cells was more diffuse, and CXCL12 staining was dramatically reduced in Il-17ra^-/- mice postinfection. Together, these results identify a distinct pathway that supports lymphoid development in the spleen during acute Plasmodium infection.

The Role of the Bone Marrow Stromal Compartment in the Hematopoietic Response to Microbial Infections

Continuous production of blood cells unfolds within a complex three-dimensional tissue scaffold established by highly organized stromal cell networks of mesenchymal, neural, and vascular origin inside bone marrow (BM) cavities. Collectively, stromal cells have been shown to serve two principal roles; first as primary participants of bone remodeling and metabolism and second as master regulators of different stages of blood cell development and production. Indeed, ample evidence demonstrates that stromal cells can sense and integrate systemic signals to shape hematopoietic responses and that these regulatory mechanisms are subverted in multiple pathologic conditions. Microbial infections are stressors that elicit potent inflammatory reactions and induce substantial alterations of hematopoietic output. Whether the cellular components of the BM stromal microenvironment are targeted by infections and participate in infection-induced hematopoiesis has not been investigated in sufficient detail to date. In this manuscript, we provide a succinct updated overview of the different cell populations that are currently known to form BM stroma. We discuss experimental evidence demonstrating that different stromal components are actively damaged or functionally altered by pathogens and/or ensuing inflammatory signals and review how these effects are known to contribute to the hematologic manifestations observed during infections.

Macrophage Colony Stimulating Factor Derived from CD4+ T Cells Contributes to Control of a Blood-Borne Infection

Dynamic regulation of leukocyte population size and activation state is crucial for an effective immune response. In malaria, Plasmodium parasites elicit robust host expansion of macrophages and monocytes, but the underlying mechanisms remain unclear. Here we show that myeloid expansion during P. chabaudi infection is dependent upon both CD4+ T cells and the cytokine Macrophage Colony Stimulating Factor (MCSF). Single-cell RNA-Seq analysis on antigen-experienced T cells revealed robust expression of Csf1, the gene encoding MCSF, in a sub-population of CD4+ T cells with distinct transcriptional and surface phenotypes. Selective deletion of Csf1 in CD4+ cells during P. chabaudi infection diminished proliferation and activation of certain myeloid subsets, most notably lymph node-resident CD169+ macrophages, and resulted in increased parasite burden and impaired recovery of infected mice. Depletion of CD169+ macrophages during infection also led to increased parasitemia and significant host mortality, confirming a previously unappreciated role for these cells in control of P. chabaudi. This work establishes the CD4+ T cell as a physiologically relevant source of MCSF in vivo; probes the complexity of the CD4+ T cell response during type 1 infection; and delineates a novel mechanism by which T helper cells regulate myeloid cells to limit growth of a blood-borne intracellular pathogen.

STING-Licensed Macrophages Prime Type I IFN Production by Plasmacytoid Dendritic Cells in the Bone Marrow during Severe Plasmodium yoelii Malaria

Malaria remains a global health burden causing significant morbidity, yet the mechanisms underlying disease outcomes and protection are poorly understood. Herein, we analyzed the peripheral blood of a unique cohort of Malawian children with severe malaria, and performed a comprehensive overview of blood leukocytes and inflammatory mediators present in these patients. We reveal robust immune cell activation, notably of CD14⁺ inflammatory monocytes, NK cells and plasmacytoid dendritic cells (pDCs) that is associated with very high inflammation. Using the Plasmodium yoelii 17X YM surrogate mouse model of lethal malaria, we report a comparable pattern of immune cell activation and inflammation and found that type I IFN represents a key checkpoint for disease outcomes. Compared to wild type mice, mice lacking the type I interferon (IFN) receptor exhibited a significant decrease in immune cell activation and inflammatory response, ultimately surviving the infection. We demonstrate that pDCs were the major producers of systemic type I IFN in the bone marrow and the blood of infected mice, via TLR7/MyD88-mediated recognition of Plasmodium parasites. This robust type I IFN production required priming of pDCs by CD169⁺ macrophages undergoing activation upon STING-mediated sensing of parasites in the bone marrow. pDCs and macrophages displayed prolonged interactions in this compartment in infected mice as visualized by intravital microscopy. Altogether our findings describe a novel mechanism of pDC activation in vivo and precise stepwise cell/cell interactions taking place during severe malaria that contribute to immune cell activation and inflammation, and subsequent disease outcomes.

The Plasmodium parasite is the number one killer among human parasitic diseases worldwide. Protection is associated with length of exposure for people living in endemic areas, with severe disease primarily affecting young children. Inflammation is a key component in the pathophysiology in malaria, and disease severity has been linked to the degree of activation of the immune system. However, the underlying mechanisms of protection and disease outcomes remain poorly understood. We provide a comprehensive analysis of peripheral blood immune cells obtained from a cohort of children with severe malaria. Our results show heightened inflammation and immune cell activation, in particular for monocytes, natural killer cells, and plasmacytoid dendritic cells (pDCs). We have also utilized a mouse model of lethal malaria that recapitulates many features identified in this cohort of severe malaria patients to examine drivers of immune cell activation and inflammation. Our studies provide evidence that type I interferon (IFN) acts as an early switch in inducing a potent inflammatory response in the infected host. Type I IFN production is massively produced in the bone marrow and the blood of infected mice by plasmacytoid dendritic cells (pDCs), a subset of DCs. We also demonstrate that resident macrophages in the bone marrow, control type I IFN production by the pDCs. We define how both myeloid cells “sense” the parasite to initiate the host immune response and report a previously uncharacterized physical interaction between pDCs and macrophages in the bone marrow as visualized by intravital microscopy in vivo. Our results define cellular processes underlying the marked inflammation of severe malaria and could open novel therapeutic opportunities to improve outcomes in this important human infectious disease.

Cadmium modulates hematopoietic stem and progenitor cells and skews toward myelopoiesis in mice

The heavy metal cadmium (Cd) is known to modulate immunity and cause osteoporosis. However, how Cd influences on hematopoiesis remain largely unknown. Herein, we show that wild-type C57BL/6 (B6) mice exposed to Cd for 3months had expanded bone marrow (BM) populations of long-term hematopoietic stem cells (LT-HSCs), common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs), while having reduced populations of multipotent progenitors (MPPs) and common lymphoid progenitors (CLPs). A competitive mixed BM transplantation assay indicates that BM from Cd-treated mice had impaired LT-HSC ability to differentiate into mature cells. In accordance with increased myeloid progenitors and decreased lymphoid progenitors, the BM and spleens of Cd-treated mice had more monocytes and/or neutrophils and fewer B cells and T cells. Cd impaired the ability of the non-hematopoietic system to support LT-HSCs, in that lethally irradiated Cd-treated recipients transplanted with normal BM cells had reduced LT-HSCs after the hematopoietic system was fully reconstituted. This is consistent with reduced osteoblasts, a known critical component for HSC niche, observed in Cd-treated mice. Conversely, lethally irradiated control recipients transplanted with BM cells from Cd-treated mice had normal LT-HSC reconstitution. Furthermore, both control mice and Cd-treated mice that received Alendronate, a clinical drug used for treating osteoporosis, had BM increases of LT-HSCs. Thus, the results suggest Cd increase of LT-HSCs is due to effects on HSCs and not on osteoblasts, although, Cd causes osteoblast reduction and impaired niche function for maintaining HSCs. Furthermore, Cd skews HSCs toward myelopoiesis.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

Systematic tracking of altered haematopoiesis during sporozoite-mediated malaria development reveals multiple response points

Haematopoiesis is the complex developmental process that maintains the turnover of all blood cell lineages. It critically depends on the correct functioning of rare, quiescent haematopoietic stem cells (HSCs) and more numerous, HSC-derived, highly proliferative and differentiating haematopoietic progenitor cells (HPCs). Infection is known to affect HSCs, with severe and chronic inflammatory stimuli leading to stem cell pool depletion, while acute, non-lethal infections exert transient and even potentiating effects. Both whether this paradigm applies to all infections and whether the HSC response is the dominant driver of the changes observed during stressed haematopoiesis remain open questions. We use a mouse model of malaria, based on natural, sporozoite-driven Plasmodium berghei infection, as an experimental platform to gain a global view of haematopoietic perturbations during infection progression. We observe coordinated responses by the most primitive HSCs and multiple HPCs, some starting before blood parasitaemia is detected. We show that, despite highly variable inter-host responses, primitive HSCs become highly proliferative, but mathematical modelling suggests that this alone is not sufficient to significantly impact the whole haematopoietic cascade. We observe that the dramatic expansion of Sca-1(+) progenitors results from combined proliferation of direct HSC progeny and phenotypic changes in downstream populations. We observe that the simultaneous perturbation of HSC/HPC population dynamics is coupled with early signs of anaemia onset. Our data uncover a complex relationship between Plasmodium and its host's haematopoiesis and raise the question whether the variable responses observed may affect the outcome of the infection itself and its long-term consequences on the host.

Promotion of Expansion and Differentiation of Hematopoietic Stem Cells by Interleukin-27 into Myeloid Progenitors to Control Infection in Emergency Myelopoiesis

Emergency myelopoiesis is inflammation-induced hematopoiesis to replenish myeloid cells in the periphery, which is critical to control the infection with pathogens. Previously, pro-inflammatory cytokines such as interferon (IFN)-α and IFN-γ were demonstrated to play a critical role in the expansion of hematopoietic stem cells (HSCs) and myeloid progenitors, leading to production of mature myeloid cells, although their inhibitory effects on hematopoiesis were also reported. Therefore, the molecular mechanism of emergency myelopoiesis during infection remains incompletely understood. Here, we clarify that one of the interleukin (IL)-6/IL-12 family cytokines, IL-27, plays an important role in the emergency myelopoiesis. Among various types of hematopoietic cells in bone marrow, IL-27 predominantly and continuously promoted the expansion of only Lineage⁻Sca-1⁺c-Kit⁺ (LSK) cells, especially long-term repopulating HSCs and myeloid-restricted progenitor cells with long-term repopulating activity, and the differentiation into myeloid progenitors in synergy with stem cell factor. These progenitors expressed myeloid transcription factors such as Spi1, Gfi1, and Cebpa/b through activation of signal transducer and activator of transcription 1 and 3, and had enhanced potential to differentiate into migratory dendritic cells (DCs), neutrophils, and mast cells, and less so into macrophages, and basophils, but not into plasmacytoid DCs, conventional DCs, T cells, and B cells. Among various cytokines, IL-27 in synergy with the stem cell factor had the strongest ability to augment the expansion of LSK cells and their differentiation into myeloid progenitors retaining the LSK phenotype over a long period of time. The experiments using mice deficient for one of IL-27 receptor subunits, WSX-1, and IFN-γ revealed that the blood stage of malaria infection enhanced IL-27 expression through IFN-γ production, and the IL-27 then promoted the expansion of LSK cells, differentiating and mobilizing them into spleen, resulting in enhanced production of neutrophils to control the infection. Thus, IL-27 is one of the limited unique cytokines directly acting on HSCs to promote differentiation into myeloid progenitors during emergency myelopoiesis.

Emergency myelopoiesis is inflammation-induced hematopoiesis that is critical for controlling infection with pathogens, but the molecular mechanism remains incompletely understood. Here, we clarify that one of the interleukin (IL)-6/IL-12 family cytokines, IL-27, plays an important role in emergency myelopoiesis. Among various types of hematopoietic cells in bone marrow, IL-27 predominantly and continuously promoted expansion of only Lineage⁻Sca-1⁺c-Kit⁺ (LSK) cells, especially long-term repopulating hematopoietic stem cells, and differentiation into myeloid progenitors in synergy with stem cell factor. These progenitors expressed myeloid transcription factors such as Spi1, Gfi1, and Cebpa/b through activation of signal transducer and activator of transcription 1 and 3, and had enhanced potential to differentiate into neutrophils, but not into plasmacytoid dendritic cells. Among various cytokines, IL-27 in synergy with stem cell factor had the strongest ability to augment the expansion of LSK cells and their differentiation into myeloid progenitors. The blood stage of malaria infection was revealed to enhance IL-27 expression through interferon-γ production, and IL-27 then promoted the expansion of LSK cells, differentiating and mobilizing them into the spleen, resulting in enhanced production of neutrophils to control the infection. Thus, IL-27 is one of the limited unique cytokines directly acting on hematopoietic stem cells to promote differentiation into myeloid progenitors during emergency myelopoiesis.

One Episode of Self-Resolving Plasmodium yoelii Infection Transiently Exacerbates Chronic Mycobacterium tuberculosis Infection

Malaria and tuberculosis (Tb) are two of the main causes of death from infectious diseases globally. The pathogenic agents, Plasmodium parasites and Mycobacterium tuberculosis, are co-endemic in many regions in the world, however, compared to other co-infections like HIV/Tb or helminth/Tb, malaria/Tb has been given less attention both in clinical and immunological studies. Due to the lack of sufficient human data, the impact of malaria on Tb and vice versa is difficult to estimate but co-infections are likely to occur very frequently. Due to its immunomodulatory properties malaria might be an underestimated risk factor for latent or active Tb patients particularly in high-endemic malaria settings were people experience reinfections very frequently. In the present study, we used the non-lethal strain of Plasmodium yoelii to investigate, how one episode of self-resolving malaria impact on a chronic M. tuberculosis infection. P. yoelii co-infection resulted in exacerbation of Tb disease as demonstrated by increased pathology and cellular infiltration of the lungs which coincided with elevated levels of pro- and anti-inflammatory mediators. T cell responses were not impaired in co-infected mice but enhanced and likely contributed to increased cytokine production. We found a slight but statistically significant increase in M. tuberculosis burden in co-infected animals and increased lung CFU was positively correlated with elevated levels of TNFα but not IL-10. Infection with P. yoelii induced the recruitment of a CD11c⁺ population into lungs and spleens of M. tuberculosis infected mice. CD11c⁺ cells isolated from P. yoelii infected spleens promoted survival and growth of M. tuberculosis in vitro. 170 days after P. yoelii infection changes in immunopathology and cellular immune responses were no longer apparent while M. tuberculosis numbers were still slightly higher in lungs, but not in spleens of co-infected mice. In conclusion, one episode of P. yoelii co-infection transiently exacerbated disease severity but had no long-term consequences on disease progression and survival of M. tuberculosis infected mice.

Macrophages: Key regulators of steady-state and demand-adapted hematopoiesis

Hematopoietic stem cell (HSC) function is required for balanced blood production throughout life; it is thus essential to understand the mechanisms regulating this highly dynamic process. Bone marrow-resident macrophages (Mϕs) have recently emerged as an important component of the HSC niche, where they contribute to regulating HSC and progenitor cell (HSPC) mobilization and function. Here we review the role of macrophages (Mϕs) on immune cell production, HSPC pool size, and mobilization at steady state and under inflammatory conditions. Inflammation induces marked changes in hematopoiesis to restrict or promote generation of specific cell lineages, and this often has a negative impact on HSC function. Cytokines and growth factors induced during inflammation influence hematopoiesis by acting directly on HSPCs and/or by modulating niche cell function. We focus particular attention on the opposing effects of two key inflammatory proteins, interferon-γ and granulocyte-colony stimulating factor, in regulating bone marrow-resident macrophages (Mϕs) and HSPCs. Macrophages (Mϕs) are essential for tissue homeostasis, and here we highlight their emerging role as a central regulator of both steady-state and demand-adapted hematopoiesis.

Osteoclasts Are Required for Hematopoietic Stem and Progenitor Cell Mobilization but Not for Stress Erythropoiesis in Plasmodium chabaudi adami Murine Malaria

The anemia and inflammation concurrent with blood stage malaria trigger stress haematopoiesis and erythropoiesis. The activity of osteoclasts seems required for the mobilization of hematopoietic stem and progenitor cells (HSPC) from the bone marrow to the periphery. Knowing that BALB/c mice with acute Plasmodium chabaudi adami malaria have profound alterations in bone remodelling cells, we evaluated the extent to which osteoclasts influence their hematopoietic response to infection. For this, mice were treated with osteoclast inhibiting hormone calcitonin prior to parasite inoculation, and infection as well as hematological parameters was studied. In agreement with osteoclast-dependent HSPC mobilization, administration of calcitonin led to milder splenomegaly, reduced numbers of HSPC in the spleen, and their retention in the bone marrow. Although C-terminal telopeptide (CTX) levels, indicative of bone resorption, were lower in calcitonin-treated infected mice, they remained comparable in naive and control infected mice. Calcitonin-treated infected mice conveniently responded to anemia but generated less numbers of splenic macrophages and suffered from exacerbated infection; interestingly, calcitonin also decreased the number of macrophages generated in vitro. Globally, our results indicate that although osteoclast-dependent HSC mobilization from bone marrow to spleen is triggered in murine blood stage malaria, this activity is not essential for stress erythropoiesis.

IFN-γ-mediated hematopoietic cell destruction in murine models of immune-mediated bone marrow failure

Interferon gamma (IFN-γ) has been reported to have both negative and positive activity on hematopoietic cells, adding complexity to the interpretation of its pleiotropic functions. We examined the effects of IFN-γ on murine hematopoietic stem cells (HSCs) and progenitors in vitro and in vivo by using mouse models. IFN-γ treatment expanded bone marrow (BM) c-Kit(+)Sca1(+)Lin(-) (KSL) cell number but reduced BM KLCD150(+) and KLCD150(+)CD48(-) cells. IFN-γ-expanded KSL cells engrafted poorly when tested by competitive repopulation in vivo. KSL, KLCD150(+), and KLCD150(+)CD48(-) cells from IFN-γ-treated animals all showed significant upregulation in Fas expression. When cocultured with activated T cells in vitro, KSL and KLCD150(+) cells from IFN-γ-treated donors showed increased apoptosis relative to those from untreated animals, and infusion of activated CD8 T cells into IFN-γ-injected animals in vivo led to partial elimination of KSL cells. Exposure of BM cells or KSL cells to IFN-γ increased expression of Fas, caspases, and related proapoptotic genes and decreased expression of Ets-1 and other hematopoietic genes. In mouse models of BM failure, mice genetically deficient in IFN-γ receptor expression showed attenuation of immune-mediated marrow destruction, whereas effector lymphocytes from IFN-γ-deficient donors were much less potent in initiating BM damage. We conclude that the activity of IFN-γ on murine hematopoiesis is context dependent. IFN-γ-augmented apoptotic gene expression facilitates destruction of HSCs and progenitors in the presence of activated cytotoxic T cells, as occurs in human BM failure.

The immunological balance between host and parasite in malaria

Coevolution of humans and malaria parasites has generated an intricate balance between the immune system of the host and virulence factors of the parasite, equilibrating maximal parasite transmission with limited host damage. Focusing on the blood stage of the disease, we discuss how the balance between anti-parasite immunity versus immunomodulatory and evasion mechanisms of the parasite may result in parasite clearance or chronic infection without major symptoms, whereas imbalances characterized by excessive parasite growth, exaggerated immune reactions or a combination of both cause severe pathology and death, which is detrimental for both parasite and host. A thorough understanding of the immunological balance of malaria and its relation to other physiological balances in the body is of crucial importance for developing effective interventions to reduce malaria-related morbidity and to diminish fatal outcomes due to severe complications. Therefore, we discuss in this review the detailed mechanisms of anti-malarial immunity, parasite virulence factors including immune evasion mechanisms and pathogenesis. Furthermore, we propose a comprehensive classification of malaria complications according to the different types of imbalances.

The Spleen Plays No Role in Nephrotoxic Serum Nephritis, but Constitutes a Place of Compensatory Haematopoiesis

Background

The spleen has been implicated in the pathogenesis of immune-complex glomerulonephritis by initiating and resolving adaptive immune responses. Thus, we aimed to evaluate the role of the spleen in experimental nephrotoxic serum nephritis (NTS).

Methods

In order to accelerate the disease, animals were subjected to NTS by preimmunizing male C57BL/6J mice with rabbit IgG three days before injecting the rabbit anti-glomerular basement antiserum, or were immunized only. A group underwent splenectomy before NTS induction.

Results

We observed enlargement of the spleen with a maximum at 14 days after NTS induction or immunization only. Splenectomized mice were found to develop albuminuria and renal histological changes comparable to sham-operated controls. Nevertheless, anaemia was aggravated in mice after splenectomy. During the course of NTS, we detected CD41⁺ megakaryocytes and Ter119⁺ erythroid precursor cells in the spleen of mice with NTS and of immunized mice. Ter119⁺Cxcr4⁺ cells and the binding partner Cxcl12 increased in the spleen, and decreased in the bone marrow. This was accompanied by a significant systemic increase of interferon-gamma in the serum.

Conclusions

In summary, splenectomy does not influence the course of NTS per se, but is involved in concomitant anaemia. Extramedullary haematopoiesis in the spleen is probably facilitated through the migration of Cxcr4⁺ erythroid precursor cells from the bone marrow to the spleen via a Cxcl12 gradient and likely arises from the suppressive capacity of chronic inflammation on the bone marrow.

Extramedullary hematopoiesis (EMH) in laboratory animals: offering an insight into stem cell research

Extramedullary hematopoiesis (EMH) is a pathological process secondary to underlying bone marrow (BM) insufficiency in adults. It is characterized by the emergence of multipotent hematopoietic progenitors scattered around the affected tissue, most likely in the spleen, liver, and lymph node, etc. EMH in patients frequently receives less medical attention and is neglected unless a compressive or obstructive hematopoietic mass appears to endanger the patient's life. However, on a biological basis, EMH reflects the alteration of relationships among hematopoietic stem and progenitor cells (HSPCs) and their original and new microenvironments. The ability of hematopoietic stem cells (HSCs) to mobilize from the bone marrow and to accommodate and function in extramedullary tissues is rather complicated and far from our current understanding. Fortunately, many reports from the studies of drugs and genetics using animals have incidentally found EMH to be involved. Thereby, the molecular basis of EMH could further be elucidated from those animals after cross-comparison. A deeper understanding of the extramedullary hematopoietic niche could help expand stem cells in vitro and establish a better treatment in patients for stem cell transplantation.

A novel model for IFN-γ-mediated autoinflammatory syndromes

Autoinflammatory disease and hyperinflammatory syndromes represent a growing number of diseases associated with inappropriately controlled inflammation in multiple organs. Systemic inflammation commonly results from dysregulated activation of innate immune cells, and therapeutic targeting of the IL-1β pathway has been used to ameliorate some of these diseases. Some hyperinflammatory syndromes, however, such as hemophagocytic lymphohistiocytosis and the newly classified proteasome disability syndromes, are refractory to such treatments, suggesting that other factors or environmental stressors may be contributing. In comparing two cytokine reporter mouse strains, we identify IFN-γ as a mediator of systemic autoinflammatory disease. Chronically elevated levels of IFN-γ resulted in progressive multiorgan inflammation and two copies of the mutant allele resulted in increased mortality accompanied by myeloproliferative disease. Disease was alleviated by genetic deletion of T-bet. These studies raise the possibility that therapeutics targeting the IFN-γ pathway might be effective in hyperinflammatory conditions refractory to IL-1β-targeted therapies.

Insights how monocytes and dendritic cells contribute and regulate immune defense against microbial pathogens

The immune system protects from infections primarily by detecting and eliminating invading pathogens. Beside neutrophils, monocytes and dendritic cells (DCs) have been recently identified as important sentinels and effectors in combating microbial pathogens. In the steady state mononuclear phagocytes like monocytes and DCs patrol the blood and the tissues. Mammalian monocytes contribute to antimicrobial defense by supplying tissues with macrophage and DC precursors. DCs recognize pathogens and are essential in presenting antigens to initiate antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. Both, monocytes and DCs play distinct roles in the shaping of immune response. In this review we will focus on the contributions of monocytes and lymphoid organ DCs to immune defense against microbial pathogens in the mouse and their dynamic regulation from steady state to infection.

Role of Chemokines and Trafficking of Immune Cells in Parasitic Infections

Parasites are diverse eukaryotic pathogens that can have complex life cycles. Their clearance, or control within a mammalian host requires the coordinated effort of the immune system. The cell types recruited to areas of infection can combat the disease, promote parasite replication and survival, or contribute to disease pathology. Location and timing of cell recruitment can be crucial. In this review, we explore the role chemokines play in orchestrating and balancing the immune response to achieve optimal control of parasite replication without promoting pathology.