Detection of a TLR2 agonist by hematopoietic stem and progenitor cells impacts the function of the macrophages they produce

GM-CSF receptor expression determines opposing innate memory phenotypes at different stages of myelopoiesis

ABSTRACT

Inflammatory responses must be tightly coordinated with the activation of emergency myelopoiesis to produce potent myeloid cells that fight infection without causing excessive host damage. Here, we show that granulocyte-macrophage colony-stimulating factor (GM-CSF) programs myeloid-committed progenitors to produce trained macrophages (increased cytokine response), but programs the upstream noncommitted LKS+ progenitors (defined as Lin- c-Kit+ Sca-1+ cells) to produce tolerized macrophages (decreased cytokine response). In myeloid progenitors, GM-CSF strongly activates signal transducer and activator of transcription 5 (STAT5), Ras-Raf-extracellular signal regulated kinase (ERK), and Akt-mTOR signaling pathways, which are essential to establish a training program, whereas in LKS+ progenitors, GM-CSF induces NF-κB translocation to the nucleus to establish a tolerization program. These differences arise from higher GM-CSF receptor expression in myeloid progenitors compared with LKS+ cells. We demonstrate that β-catenin regulation of NF-κB nuclear translocation is central in this process. In myeloid progenitors, glycogen synthase kinase 3 (GSK3) inactivation by strong ERK and phosphatidylinositol 3 kinase (PI3K)-Akt signaling increases cytoplasmic β-catenin levels to block NF-κB nuclear translocation. In contrast, when ERK and PI3K-Akt signaling are weak, active GSK3 causes a decrease in β-catenin, allowing NF-κB nuclear translocation in LKS+ progenitors. Finally, GM-CSF-induced LKS+ tolerization takes place in several murine models of trained immunity and in human CD34+ CD38- progenitors. Our study reveals that in addition to activating myelopoiesis, GM-CSF also programs early and immediate myeloid progenitors to produce opposing immune memory phenotypes. We propose that the inflammatory response from immediate myeloid progenitors may be balanced by the tolerized phenotype of early progenitors, thus providing a mechanism for appropriate resolution of inflammation and protection against a prolonged cytokine storm.

Macrophage memory: Types, mechanisms, and its role in health and disease

On the basis of the mechanisms of action and characteristics of immune effects, immunity is commonly categorized into innate and adaptive immunity. Adaptive immunity is associated with the response to non-self-entities and is characterized by high specificity and memory properties. In contrast, innate immunity has traditionally been considered devoid of memory characteristics. However, an increasing number of studies have sought to challenge this conventional immunological dogma and shown that innate immune cells exhibit a more robust and rapid response to secondary stimulation, thus providing evidence of the immunological memory in innate immunity. Macrophages, which are among the most important innate immune cells, can also acquire memory phenotype that facilitates the mediation of recall responses. Macrophage memory is a relatively new concept that is revolutionizing our understanding of macrophage biology and immunological memory and could lead to a new class of vaccines and immunotherapies. In this review, we describe the characteristics and mechanisms of macrophage memory, as well as its essential roles in various diseases.

Restricting genomic actions of innate immune mediators on fetal hematopoietic progenitor cells

Innate immune signaling protects against pathogens, controls hematopoietic development, and functions in oncogenesis, yet the relationship between these mechanisms is undefined. Downregulating the GATA2 transcription factor in fetal hematopoietic progenitor cells upregulates genes encoding innate immune regulators, increases Interferon-γ (IFNγ) signaling, and disrupts differentiation. We demonstrate that deletion of an enhancer that confers GATA2 expression in fetal progenitors elevated Toll-like receptor (TLR) TLR1/2 and TLR2/6 expression and signaling. Rescue by expressing GATA2 downregulated elevated TLR signaling. IFNγ amplified TLR1/2 and TLR2/6 signaling in GATA2-deficient progenitors, synergistically activating cytokine/chemokine genes and elevating cytokine/chemokine production in myeloid cell progeny. Genomic analysis of how innate immune signaling remodels the GATA2-deficient progenitor transcriptome revealed hypersensitive responses at innate immune genes harboring motifs for signal-dependent transcription factors and factors not linked to these mechanisms. As GATA2 establishes a transcriptome that constrains innate immune signaling, insufficient GATA2 renders fetal progenitor cells hypersensitive to innate immune signaling.

Immunomodulatory role of oral microbiota in inflammatory diseases and allergic conditions

In recent years, the interplay between oral microbiota and systemic disease has gained attention as poor oral health is associated with several pathologies. The oral microbiota plays a role in the maintenance of overall health, and its dysbiosis influences chronic inflammation and the pathogenesis of gum diseases. Periodontitis has also been associated with other diseases and health complications such as cancer, neurogenerative and autoimmune disorders, chronic kidney disease, cardiovascular diseases, rheumatic arthritis, respiratory health, and adverse pregnancy outcomes. The host microbiota can influence immune cell development and immune responses, and recent evidence suggests that changes in oral microbiota composition may also contribute to sensitization and the development of allergic reactions, including asthma and peanut allergies. Conversely, there is also evidence that allergic reactions within the gut may contribute to alterations in oral microbiota composition. Here we review the current evidence of the role of the oral microbiota in inflammatory diseases and health complications, as well as its future relevance in improving health and ameliorating allergic disease.

Emergency Hematopoiesis in the Pathobiology of COVID-19: The Dark Side of an Early Innate Protective Mechanism

The recognition of pathogens to which we are constantly exposed induces the immediate replenishment of innate immune cells from the most primitive stages of their development through emergency hematopoiesis, a central mechanism contributing to early infection control. However, as with other protective mechanisms, its functional success is at risk when the excess of inducing signals accelerates immunological catastrophes. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection exhibits a clinical spectrum that ranges from completely asymptomatic states to fatal outcomes, with the amplification of inflammatory components being the critical point that determine the progress, complication, and severity of the disease. This review focuses on the most relevant findings that entail emergency hematopoiesis to SARS-CoV-2 infection response and revolutionize our understanding of the mechanisms governing the clinical prognosis of COVID-19. Of special interest are the metabolic or hyperinflammatory conditions in aging that exacerbate the phenomenon and favor the uncontrolled emergency myelopoiesis leading to the evolution of severe disease.

Role of the hematopoietic stem cells in immunological memory

Purpose of review

Immunological memory is an important evolutionary adaptation of the immune system. Previously restricted to the adaptive immune system, the concept of memory has recently been broadened to the innate immune system. This review summarizes recent studies that highlight the contribution of the hematopoietic stem cells (HSCs) in supporting immunological memory.

Recent findings

Short-lived innate immune cells can build a long-lasting memory of infection to improve their response to secondary challenges. Studies show that these unexpected properties of the innate immune system are sustained by epigenetic and metabolic changes in the HSC compartment.

Summary

HSCs are durably altered in response to pathogens and serve as long-term support for innate immune memory. Many questions remain regarding the mechanisms contributing to the induction and the maintenance of this immune memory in HSCs. Answering these questions will open new perspectives to understand how environmental factors shape the HSC activity over time.

Microbial Sensing by Hematopoietic Stem and Progenitor Cells

Balanced production of immune cells is critical for the maintenance of steady-state immune surveillance, and increased production of myeloid cells is sometimes necessary to eliminate pathogens. Hematopoietic stem and progenitor cell (HSPC) sensing of commensal microbes and invading pathogens has a notable impact on hematopoiesis. In this review, we examine how commensal microbes regulate bone marrow HSPC activity to maintain balanced hematopoiesis in the steady state, and how HSPCs proliferate and differentiate during emergency myelopoiesis in response to infection. HSPCs express a variety of pattern recognition receptors and cytokine receptors that they use to sense the presence of microbes, either directly via detection of microbial components and metabolites, or indirectly by responding to cytokines produced by other host cells. We describe direct and indirect mechanisms of microbial sensing by HSPCs and highlight evidence demonstrating long-term effects of acute and chronic microbial stimuli on HSPCs. We also discuss a possible connection between myeloid-biased hematopoiesis and elevated levels of circulating microbiome-derived components in the context of aging and metabolic stress. Finally, we highlight the prospect of trained immunity-based vaccines that could exploit microbial stimulation of HSPCs.

Abnormal monocyte differentiation and function in chronic myelomonocytic leukemia

PURPOSE OF REVIEW

Monocytes serve as the phagocytic defense surveillance system of the human body. Although there is comprehensive evidence regarding monocyte development, characterization and function under steady state hematopoietic continuum, the deviations and complexities in the monocyte secretome during myeloid malignancies have not been comprehensively examined and delineated.

RECENT FINDINGS

This review summarizes the aspects of development, functions, transcriptional and cytokine-mediated regulation of monocytes during steady state hematopoiesis and also contrasts the aberrations observed in myelomonocytic leukemias like chronic myelomonocytic leukemia (CMML). It presents the findings from the major studies highlighting the novel markers for identifying CMML monocytes, altered signaling cascades, roles in disease progression and potential therapeutic interventions to reduce the monocyte mediated inflammatory milieu for disease amelioration.

SUMMARY

Recent findings provide rationale for the development of therapeutic strategies aimed at disrupting the leukemic initiating cells and malignant monocyte axis.

Train the Trainer: Hematopoietic Stem Cell Control of Trained Immunity

Recent evidence shows that innate immune cells, in addition to B and T cells, can retain immunological memory of their encounters and afford long-term resistance against infections in a process known as 'trained immunity'. However, the duration of the unspecific protection observed in vivo is poorly compatible with the average lifespan of innate immune cells, suggesting the involvement of long-lived cells. Accordingly, recent studies demonstrate that hematopoietic stem and progenitor cells (HSPCs) lay at the foundation of trained immunity, retaining immunological memory of infections and giving rise to a "trained" myeloid progeny for a long time. In this review, we discuss the research demonstrating the involvement of HSPCs in the onset of long-lasting trained immunity. We highlight the roles of specific cytokines and Toll-like receptor ligands in influencing HSPC memory phenotypes and the molecular mechanisms underlying trained immunity HSPCs. Finally, we discuss the potential benefits and drawbacks of the long-lasting trained immune responses, and describe the challenges that the field is facing.

Hematopoietic and Mesenchymal Stromal Cells: New Immunological Roles During Fungal Infections

Adult stem cells are characterized not only by their regenerative and immunomodulatory capacity but also by their therapeutic potential in various pathologies that include hematological malignancies, cancer, and autoimmune and inflammatory diseases, among others. However, these cells seem to play a paradoxical role during the development of the immune response in some infectious diseases. As an example, Candida albicans can induce the proliferation and differentiation of hematopoietic stem cells (HSCs) and their progenitors, a process known as emergency hematopoiesis. Moreover, Aspergillus fumigatus and C. albicans, once recognized by mesenchymal stromal cells (MSCs), can induce an anti-inflammatory or proinflammatory profile, respectively, and, in turn, these cells can inhibit the growth of these fungal pathogens. Additionally, the transplantation of MSCs, in an experimental pulmonary model of paracoccidioidomycosis, has been shown to exacerbate the inflammatory response. More recently, in vitro studies have shown that MSCs recognize Paracoccidioides brasiliensis through a mechanism mediated by toll-like receptor (TLR)2, TLR4, and Dectin-1, which, in turn, induces a proinflammatory profile. This review describes the main mechanisms and immunomodulatory properties of HSCs and MSCs during infections caused by some medically important fungal pathogens described so far in literature.

Signals governing monocyte differentiation during inflammation

Monocytes are innate immune cells that develop in the bone marrow and are continually released into circulation, where they are poised to enter tissues in response to homeostatic or inflammatory cues. Monocytes are highly plastic cells that can differentiate in tissues into a variety of monocyte-derived cells to replace resident tissue macrophages, promote inflammatory responses, or resolution of inflammation. As such, monocytes can support tissue homeostasis as well as productive and pathogenic immune responses. Recent work shows previously unappreciated heterogeneity in monocyte development and differentiation in the steady state and during infectious, autoimmune, and inflammatory diseases. Monocyte-derived cells can differentiate via signals from cytokines, pattern recognition receptors or other factors, which can influence development in the bone marrow or in tissues. An improved understanding of these monocyte-derived cells and the signals that drive their differentiation in distinct inflammatory settings could allow for targeting these pathways in pathological inflammation.

Ex vivo mRNA expression of toll-like receptors during latent tuberculosis infection

Background

Understanding immune mechanisms, particularly the role of innate immune markers during latent TB infection remains elusive. The main objective of this study was to evaluate mRNA gene expression patterns of toll-like receptors (TLRs) as correlates of immunity during latent TB infection and further infer their roles as potential diagnostic biomarkers.

Methods

Messenger RNA (mRNA) levels were analysed in a total of 64 samples collected from apparently healthy children and adolescents latently infected with tuberculosis (n = 32) or non-infected (n = 32). Relative expression in peripheral blood of selected genes encoding TLRs (TLR-1, TLR-2, TLR-4, TLR-6 and TLR-9) was determined with a quantitative real-time polymerase chain reaction (qRT-PCR) using specific primers and florescent labelled probes and a comparative threshold cycle method to define fold change. Data were analysed using Graph-Pad Prism 7.01 for Windows and a p-value less than 0.05 was considered statistically significant.

Results

An increased mean fold change in the relative expression of TLR-2 and TLR-6 mRNA was observed in LTBI groups relative to non-LTBI groups (p < 0.05), whereas a slight fold decrease was observed for TLR-1 gene.

Conclusions

An increased mRNA expression of TLR-2 and TLR-6 was observed in latently infected individuals relative to those non-infected, possibly indicating the roles these biomarkers play in sustenance of the steady state interaction between the dormant TB bacilli and host immunity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00400-4.

Trained innate immunity

The innate immune system acts rapidly in an identical and nonspecific way every time the body is exposed to pathogens. As such, it cannot build and maintain immunological memory to help prevent reinfection. Researchers contend that trained immunity is influenced by intracellular metabolic pathways and epigenetic remodeling. The purpose of this review was to explore the topic of trained innate immunity based on the results of relevant previous studies. This systematic review entailed identifying articles related to trained innate immunity. The sources were obtained from PubMed using different search terms that included "trained innate immunity," "trained immunity," "trained," "innate," "immunity," and "immune system." Boolean operators were used to combine terms and phrases. A review of previous study results revealed that little is currently known about the molecular and cellular processes that mediate or induce a trained immune response in animals. However, it is believed that alterations in the phenotypes of cell populations and the numbers of specific cells may play a critical role in mediating the trained immune response. Increasing evidence shows that the protective processes and actions that occur during a secondary infection are not entirely linked to the adaptive immune system. Instead, these events also involve heightened activation of innate immune cells. While trained innate immune cells may have a shorter memory, they assist in the fight against pathogens and provide cross-protection. Identification of the mechanisms and molecules that underlie trained innate immunity has highlighted important features of the human immune response. Such advances continue to open doors for future research on how the body responds to disease-causing pathogens.

Immuno-Modulation of Hematopoietic Stem and Progenitor Cells in Inflammation

Lifelong blood production is maintained by bone marrow (BM)-residing hematopoietic stem cells (HSCs) that are defined by two special properties: multipotency and self-renewal. Since dysregulation of either may lead to a differentiation block or extensive proliferation causing dysplasia or neoplasia, the genomic integrity and cellular function of HSCs must be tightly controlled and preserved by cell-intrinsic programs and cell-extrinsic environmental factors of the BM. The BM had been long regarded an immune-privileged organ shielded from immune insults and inflammation, and was thereby assumed to provide HSCs and immune cells with a protective environment to ensure blood and immune homeostasis. Recently, accumulating evidence suggests that hemato-immune challenges such as autoimmunity, inflammation or infection elicit a broad spectrum of immunological reactions in the BM, and in turn, influence the function of HSCs and BM environmental cells. Moreover, in analogy with the emerging concept of “trained immunity”, certain infection-associated stimuli are able to train HSCs and progenitors to produce mature immune cells with enhanced responsiveness to subsequent challenges, and in some cases, form an inflammatory or infectious memory in HSCs themselves. In this review, we will introduce recent findings on HSC and hematopoietic regulation upon exposure to various hemato-immune stimuli and discuss how these challenges can elicit either beneficial or detrimental outcomes on HSCs and the hemato-immune system, as well as their relevance to aging and hematologic malignancies.

Exposome and Immunity Training: How Pathogen Exposure Order Influences Innate Immune Cell Lineage Commitment and Function

Immune memory is a defining characteristic of adaptive immunity, but recent work has shown that the activation of innate immunity can also improve responsiveness in subsequent exposures. This has been coined “trained immunity” and diverges with the perception that the innate immune system is primitive, non-specific, and reacts to novel and recurrent antigen exposures similarly. The “exposome” is the cumulative exposures (diet, exercise, environmental exposure, vaccination, genetics, etc.) an individual has experienced and provides a mechanism for the establishment of immune training or immunotolerance. It is becoming increasingly clear that trained immunity constitutes a delicate balance between the dose, duration, and order of exposures. Upon innate stimuli, trained immunity or tolerance is shaped by epigenetic and metabolic changes that alter hematopoietic stem cell lineage commitment and responses to infection. Due to the immunomodulatory role of the exposome, understanding innate immune training is critical for understanding why some individuals exhibit protective phenotypes while closely related individuals may experience immunotolerant effects (e.g., the order of exposure can result in completely divergent immune responses). Research on the exposome and trained immunity may be leveraged to identify key factors for improving vaccination development, altering inflammatory disease development, and introducing potential new prophylactic treatments, especially for diseases such as COVID-19, which is currently a major health issue for the world. Furthermore, continued exposome research may prevent many deleterious effects caused by immunotolerance that frequently result in host morbidity or mortality.

Tuberculosis Treatment Outcomes and Associated Factors Among Patients Treated at Woldia General Hospital in Northeast Ethiopia: An Institution-Based Cross-Sectional Study

Background

Tuberculosis remains a major global health problem. It causes ill-health among millions of people each year and ranks alongside the human immunodeficiency virus (HIV) as a leading cause of death worldwide. For effective tuberculosis control, it is a prerequisite to detect the cases as early as possible and to ensure that the tuberculosis patients complete their treatment and get cured. However, the burden of the problem is still a national issue, and there is a scarcity of research to show treatment outcomes and associated factors of tuberculosis at the North Wollo Zone, specifically Woldia.

Methods

Institution-based, retrospective register-based data were collected from medical records of tuberculosis patients from 2015 up to 2018 at Woldia General Hospital. The data were analyzed using SPSS version 24, and multiple logistic regression methods were used to investigate the association between independent and dependent variables. A P-value of less than 5% was considered statistically significant in the final model.

Results

The prevalence of successful tuberculosis treatment outcomes was 80.7%. Among all patients, 73% were pulmonary tuberculosis cases. This study results show that age less than 24 years old [AOR: 4.7; 95% CI (1.3–10.1)], male sex [AOR: 2.8; 95% CI (2.1–4.8)], year of registration in 2018 [AOR: 4.8; 95% CI (3.9–7.4)], and HIV negative status [AOR: 3.9; 95% CI (1.4–10.7)]  were found to be significantly associated factors with the treatment outcomes of tuberculosis.

Conclusion

The study showed that nearly 20% of tuberculosis patients had an unsuccessful treatment outcome. Older age, female sex, year of registration in 2015, and being HIV positive were found significantly associated with poor tuberculosis treatment outcomes. Therefore, targeted measures should be considered to decrease poor TB treatment outcomes among high-risk patients through careful monitoring, making the DOTs program more accessible, counseling, and linking HIV patients.

Dectin-1 Stimulation of Hematopoietic Stem and Progenitor Cells Occurs In Vivo and Promotes Differentiation Toward Trained Macrophages via an Indirect Cell-Autonomous Mechanism

Invasive candidiasis is an increasingly frequent cause of serious and often fatal infections. Understanding host defense is essential to design novel therapeutic strategies to boost immune protection against Candida albicans. In this article, we delve into two new concepts that have arisen over the last years: (i) the delivery of myelopoiesis-inducing signals by microbial components directly sensed by hematopoietic stem and progenitor cells (HSPCs) and (ii) the concept of “trained innate immunity” that may also apply to HSPCs. We demonstrate that dectin-1 ligation in vivo activates HSPCs and induces their differentiation to trained macrophages by a cell-autonomous indirect mechanism. This points to new mechanisms by which pathogen detection by HSPCs may modulate hematopoiesis in real time to generate myeloid cells better prepared to deal with the infection. Manipulation of this process may help to boost the innate immune response during candidiasis.

Toll-like receptor (TLR) agonists drive hematopoietic stem and progenitor cells (HSPCs) to differentiate along the myeloid lineage. In this study, we used an HSPC transplantation model to investigate the possible direct interaction of β-glucan and its receptor (dectin-1) on HSPCs in vivo. Purified HSPCs from bone marrow of B6Ly5.1 mice (CD45.1 alloantigen) were transplanted into dectin-1^−/− mice (CD45.2 alloantigen), which were then injected with β-glucan (depleted zymosan). As recipient mouse cells do not recognize the dectin-1 agonist injected, interference by soluble mediators secreted by recipient cells is negligible. Transplanted HSPCs differentiated into macrophages in response to depleted zymosan in the spleens and bone marrow of recipient mice. Functionally, macrophages derived from HSPCs exposed to depleted zymosan in vivo produced higher levels of inflammatory cytokines (tumor necrosis factor alpha [TNF-α] and interleukin 6 [IL-6]). These results demonstrate that trained immune responses, already described for monocytes and macrophages, also take place in HSPCs. Using a similar in vivo model of HSPC transplantation, we demonstrated that inactivated yeasts of Candida albicans induce differentiation of HSPCs through a dectin-1- and MyD88-dependent pathway. Soluble factors produced following exposure of HSPCs to dectin-1 agonists acted in a paracrine manner to induce myeloid differentiation and to influence the function of macrophages derived from dectin-1-unresponsive or β-glucan-unexposed HSPCs. Finally, we demonstrated that an in vitro transient exposure of HSPCs to live C. albicans cells, prior to differentiation, is sufficient to induce a trained phenotype of the macrophages they produce in a dectin-1- and Toll-like receptor 2 (TLR2)-dependent manner.

TLR2 and Dectin-1 Signaling in Mouse Hematopoietic Stem and Progenitor Cells Impacts the Ability of the Antigen Presenting Cells They Produce to Activate CD4 T Cells

Microbial recognition by pattern recognition receptors (PRRs) expressed on hematopoietic stem and progenitor cells (HSPCs) not only activates myelopoiesis but also programs the function of the monocytes and macrophages they produce. For instance, changes in HSPC programming modify the ability of macrophages derived from them to produce inflammatory cytokines. While HSPCs exposed to a TLR2 agonist give rise to tolerized macrophages (lower proinflammatory cytokine production), HSPCs treated with Dectin-1 ligands produce trained macrophages (higher proinflammatory cytokine production). However, nothing is known about the impact of HSPC exposure to microbes on the function of antigen presenting cells (APCs). In this study we evaluated whether treatment of murine bone marrow HSPCs with a TLR2 or Dectin-1 ligand impacts the antigen presenting capacity of APCs derived from them in vitro. Following activation with microbial ligands or Candida albicans yeasts, APCs derived from TLR2/Dectin-1-programed HSPCs exhibit altered expression of MHCII (signal 1), co-stimulatory molecules (CD40, CD80 and CD86; signal 2) and cytokines (TNF-α, IL-6, IL-12 p40 and IL-2; signal 3). Moreover, APCs derived from TLR2/Dectin-1-programed HSPCs prime enhanced Th1 and Th17 responses, which are important for antifungal defense, in CD4 T cell cocultures. Overall, these results demonstrate for the first time that microbial detection by bone marrow HSPCs can modulate the adaptive immune response by inducing the production of APCs with an altered phenotype.

Urate-induced immune programming: Consequences for gouty arthritis and hyperuricemia

Trained immunity is a process in which innate immune cells undergo functional reprogramming in response to pathogens or damage-associated molecules leading to an enhanced non-specific immune response to subsequent stimulation. While this capacity to respond more strongly to stimuli is beneficial for host defense, in some circumstances it can lead to maladaptive programming and chronic inflammation. Gout is characterized by persistent low-grade inflammation and is associated with an increased number of comorbidities. Hyperuricemia is the main risk factor for gout and is linked to the development of comorbidities. Several experimental studies have shown that urate can mechanistically alter the inflammatory capacity of myeloid cells, while observational studies have indicated an association of hyperuricemia to a wide spectrum of common adult inflammatory diseases. In this review, we argue that hyperuricemia is a main culprit in the development of the long-term systemic inflammation seen in gout. We revisit existing evidence for urate-induced transcriptional and epigenetic reprogramming that could lead to an altered functional state of circulating monocytes consisting in enhanced responsiveness and maladaptive immune responses. By discussing specific functional adaptations of monocytes and macrophages induced by soluble urate or monosodium urate crystals and their contribution to inflammation in vitro and in vivo, we further enforce that urate is a metabolite that can induce innate immune memory and we discuss future research and possible new therapeutic approaches for gout and its comorbidities.

Monocytes and the Host Response to Fungal Pathogens

Monocytes and their derivatives, including macrophages and dendritic cells, play diverse roles in the response to fungal pathogens. Sensing of fungi by monocytes triggers signaling pathways that mediate direct effects like phagocytosis and cytokine production. Monocytes can also present fungal antigens to elicit adaptive immune responses. These monocyte-mediated pathways may be either beneficial or harmful to the host. In some instances, fungi have developed mechanisms to evade the consequences of monocyte activation and subvert these cells to promote disease. Thus, monocytes are critically involved in mediating the outcomes of these often highly fatal infections. This review will highlight the roles of monocytes in the immune response to some of the major fungi that cause invasive human disease, including Aspergillus, Cryptococcus, Candida, Histoplasma, Blastomyces, and Coccidioides, and discuss potential strategies to manipulate monocyte responses in order to enhance anti-fungal immunity in susceptible hosts.

Potential Role of Gut Microbiota in Induction and Regulation of Innate Immune Memory

The gut microbiota significantly regulates the development and function of the innate and adaptive immune system. The attribute of immunological memory has long been linked only with adaptive immunity. Recent evidence indicates that memory is also present in the innate immune cells such as monocytes/macrophages and natural killer cells. These cells exhibit pattern recognition receptors (PRRs) that recognize microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) expressed by the microbes. Interaction between PRRs and MAMPs is quite crucial since it triggers the sequence of signaling events and epigenetic rewiring that not only play a cardinal role in modulating the activation and function of the innate cells but also impart a sense of memory response. We discuss here how gut microbiota can influence the generation of innate memory and functional reprogramming of bone marrow progenitors that helps in protection against infections. This article will broaden our current perspective of association between the gut microbiome and innate memory. In the future, this knowledge may pave avenues for development and designing of novel immunotherapies and vaccination strategies.

The Ontogeny of Monocyte Subsets

Classical and non-classical monocytes, and the macrophages and monocyte-derived dendritic cells they produce, play key roles in host defense against pathogens, immune regulation, tissue repair and many other processes throughout the body. Recent studies have revealed previously unappreciated heterogeneity among monocytes that may explain this functional diversity, but our understanding of mechanisms controlling the functional programming of distinct monocyte subsets remains incomplete. Resolving monocyte heterogeneity and understanding how their functional identity is determined holds great promise for therapeutic immune modulation. In this review, we examine how monocyte origins and developmental influences shape the phenotypic and functional characteristics of monocyte subsets during homeostasis and in the context of infection, inflammation, and cancer. We consider how extrinsic signals and transcriptional regulators impact monocyte production and functional programming, as well as the influence of epigenetic and metabolic mechanisms. We also examine the evidence that functionally distinct monocyte subsets are produced via different developmental pathways during homeostasis and that inflammatory stimuli differentially target progenitors during an emergency response. We highlight the need for a more comprehensive understanding of the relationship between monocyte ontogeny and heterogeneity, including multiparametric single-cell profiling and functional analyses. Studies defining mechanisms of monocyte subset production and maintenance of unique monocyte identities have the potential to facilitate the design of therapeutic interventions to target specific monocyte subsets in a variety of disease contexts, including infectious and inflammatory diseases, cancer, and aging.

Microbes, metabolites, and the gut-lung axis

The microbiota plays an essential role in the education, development, and function of the immune system, both locally and systemically. Emerging experimental and epidemiological evidence highlights a crucial cross-talk between the intestinal microbiota and the lungs, termed the 'gut-lung axis'. Changes in the constituents of the gut microbiome, through either diet, disease or medical interventions (such as antibiotics) is linked with altered immune responses and homeostasis in the airways. The importance of the gut-lung axis has become more evident following the identification of several gut microbe-derived components and metabolites, such as short-chain fatty acids (SCFAs), as key mediators for setting the tone of the immune system. Recent studies have supported a role for SCFAs in influencing hematopoietic precursors in the bone marrow-a major site of innate and adaptive immune cell development. Here, we review the current understanding of host-microbe cross-talk along the gut-lung axis. We highlight the importance of SCFAs in shaping and promoting bone marrow hematopoiesis to resolve airway inflammation and to support a healthy homeostasis.

Opposite effects of Vaccinia and modified Vaccinia Ankara on trained immunity

Vaccines such as Vaccinia or BCG have non-specific effects conferring protection against other diseases than their target infection, which are likely partly mediated through induction of innate immune memory (trained immunity). MVA85A, a recombinant strain of modified Vaccinia Ankara (MVA), has been suggested as an alternative vaccine against tuberculosis, but its capacity to induce positive or negative non-specific immune effects has not been studied. This study assesses whether Vaccinia and MVA are able to induce trained innate immunity in monocytes. Human primary monocytes were primed in an in vitro model with Vaccinia or MVA for 1 day, after which the stimulus was washed off and the cells were rechallenged with unrelated microbial ligands after 1 week. Heterologous cytokine responses were assessed and the capacity of MVA to induce epigenetic changes at the level of cytokine genes was investigated using chromatin immunoprecipitation and pharmacological inhibitors. Monocytes trained with Vaccinia showed significantly increased IL-6 and TNF-α production to stimulation with non-related stimuli, compared to non-trained monocytes. In contrast, monocytes primed with MVA showed significant decreased heterologous IL-6 and TNF-α responses, an effect which was abrogated by the addition of a histone methyltransferase inhibitor. No effects on H3K4me3 were observed after priming with MVA. It can be thus concluded that Vaccinia induces trained immunity in vitro, whereas MVA induces innate immune tolerance. This suggests the induction of trained immunity as an immunological mechanism involved in the non-specific effects of Vaccinia vaccination and points to a possible explanation for the lack of effect of MVA85A against tuberculosis.

Epigenetics and Trained Immunity

SIGNIFICANCE

A growing body of clinical and experimental evidence has challenged the traditional understanding that only the adaptive immune system can mount immunological memory. Recent findings describe the adaptive characteristics of the innate immune system, underscored by its ability to remember antecedent foreign encounters and respond in a nonspecific sensitized manner to reinfection. This has been termed trained innate immunity. Although beneficial in the context of recurrent infections, this might actually contribute to chronic immune-mediated diseases, such as atherosclerosis. Recent Advances: In line with its proposed role in sustaining cellular memories, epigenetic reprogramming has emerged as a critical determinant of trained immunity. Recent technological and computational advances that improve unbiased acquisition of epigenomic profiles have significantly enhanced our appreciation for the complexities of chromatin architecture in the contexts of diverse immunological challenges.

CRITICAL ISSUES

Key to resolving the distinct chromatin signatures of innate immune memory is a comprehensive understanding of the precise physiological targets of regulatory proteins that recognize, deposit, and remove chemical modifications from chromatin as well as other gene-regulating factors. Drawing from a rapidly expanding compendium of experimental and clinical studies, this review details a current perspective of the epigenetic pathways that support the adapted phenotypes of monocytes and macrophages.

FUTURE DIRECTIONS

We explore future strategies that are aimed at exploiting the mechanism of trained immunity to improve the prevention and treatment of infections and immune-mediated chronic disorders.

Systemic Candidiasis and TLR2 Agonist Exposure Impact the Antifungal Response of Hematopoietic Stem and Progenitor Cells

We have previously demonstrated that Candida albicans induces differentiation of hematopoietic stem and progenitor cells (HSPCs) toward the myeloid lineage both in vitro and in vivo in a TLR2- and Dectin-1-dependent manner, giving rise to functional macrophages. In this work, we used an ex vivo model to investigate the functional consequences for macrophages derived from HSPCs in vivo-exposed to Pam₃CSK₄ (a TLR2 agonist) or C. albicans infection. Short in vivo treatment of mice with Pam₃CSK₄ results in a tolerized phenotype of ex vivo HSPC-derived macrophages, whereas an extended Pam₃CSK₄ treatment confers a trained phenotype. Early during candidiasis, HSPCs give rise to macrophages trained in their response to Pam₃CSK₄ and with an increased fungicidal activity; however, as the infection progresses to higher fungal burden, HSPC-derived macrophages become tolerized, while their fungicidal capacity is maintained. These results demonstrate that memory-like innate immune responses, already described for monocytes and macrophages, also take place in HSPCs. Interestingly, extended Pam₃CSK₄ treatment leads to an expansion of spleen HSPCs and myeloid cells, and drastically reduces the fungal burden in the kidney and spleen during systemic C. albicans infection. This protection against tissue invasion is abrogated by immunodepletion of HSPCs, suggesting their protective role against infection in this model. In addition, HSPCs produce in vitro cytokines and chemokines in response to C. albicans and Pam₃CSK₄, and these secretomes are capable of inducing myeloid differentiation of HSPCs and modulating peritoneal macrophage cytokine responses. Taken together, these data assign an active role for HSPCs in sensing pathogens during infection and in contributing to host protection by diverse mechanisms.

Microbiome-Gut-Brain Axis and Toll-Like Receptors in Parkinson's Disease

Parkinson’s disease (PD) is a progressively debilitating neurodegenerative disease characterized by α-synucleinopathy, which involves all districts of the brain-gut axis, including the central, autonomic and enteric nervous systems. The highly bidirectional communication between the brain and the gut is markedly influenced by the microbiome through integrated immunological, neuroendocrine and neurological processes. The gut microbiota and its relevant metabolites interact with the host via a series of biochemical and functional inputs, thereby affecting host homeostasis and health. Indeed, a dysregulated microbiota-gut-brain axis in PD might lie at the basis of gastrointestinal dysfunctions which predominantly emerge many years prior to the diagnosis, corroborating the theory that the pathological process is spread from the gut to the brain. Toll-like receptors (TLRs) play a crucial role in innate immunity by recognizing conserved motifs primarily found in microorganisms and a dysregulation in their signaling may be implicated in α-synucleinopathy, such as PD. An overstimulation of the innate immune system due to gut dysbiosis and/or small intestinal bacterial overgrowth, together with higher intestinal barrier permeability, may provoke local and systemic inflammation as well as enteric neuroglial activation, ultimately triggering the development of alpha-synuclein pathology. In this review, we provide the current knowledge regarding the relationship between the microbiota-gut⁻brain axis and TLRs in PD. A better understanding of the dialogue sustained by the microbiota-gut-brain axis and innate immunity via TLR signaling should bring interesting insights in the pathophysiology of PD and provide novel dietary and/or therapeutic measures aimed at shaping the gut microbiota composition, improving the intestinal epithelial barrier function and balancing the innate immune response in PD patients, in order to influence the early phases of the following neurodegenerative cascade.

Infectious Agents as Stimuli of Trained Innate Immunity

The discoveries made over the past few years have modified the current immunological paradigm. It turns out that innate immunity cells can mount some kind of immunological memory, similar to that observed in the acquired immunity and corresponding to the defense mechanisms of lower organisms, which increases their resistance to reinfection. This phenomenon is termed trained innate immunity. It is based on epigenetic changes in innate immune cells (monocytes/macrophages, NK cells) after their stimulation with various infectious or non-infectious agents. Many infectious stimuli, including bacterial or fungal cells and their components (LPS, β-glucan, chitin) as well as viruses or even parasites are considered potent inducers of innate immune memory. Epigenetic cell reprogramming occurring at the heart of the phenomenon may provide a useful basis for designing novel prophylactic and therapeutic strategies to prevent and protect against multiple diseases. In this article, we present the current state of art on trained innate immunity occurring as a result of infectious agent induction. Additionally, we discuss the mechanisms of cell reprogramming and the implications for immune response stimulation/manipulation.

UV Irradiation of Skin Enhances Glycolytic Flux and Reduces Migration Capabilities in Bone Marrow-Differentiated Dendritic Cells

A systemic immunosuppression follows UV irradiation of the skin of humans and mice. In this study, dendritic cells (DCs) differentiating from the bone marrow (BM) of UV-irradiated mice had a reduced ability to migrate toward the chemokine (C-C motif) ligand 21. Fewer DCs also accumulated in the peritoneal cavity of UV-chimeric mice (ie, mice transplanted with BM from UV-irradiated mice) after injection of an inflammatory stimulus into that site. We hypothesized that different metabolic states underpin altered DC motility. Compared with DCs from the BM of nonirradiated mice, those from UV-irradiated mice produced more lactate, consumed more glucose, and had greater glycolytic flux in a bioenergetics stress test. Greater expression of 3-hydroxyanthranilate 3,4-dioxygenase was identified as a potential contributor to increased glycolysis. Inhibition of 3-hydroxyanthranilate 3,4-dioxygenase by 6-chloro-dl-tryptophan prevented both increased lactate production and reduced migration toward chemokine (C-C motif) ligand 21 by DCs differentiated from BM of UV-irradiated mice. UV-induced prostaglandin E₂ has been implicated as an intermediary in the effects of UV radiation on BM cells. DCs differentiating from BM cells pulsed in vitro for 2 hours with dimethyl prostaglandin E₂ were functionally similar to those from the BM of UV-irradiated mice. Reduced migration of DCs to lymph nodes associated with increased glycolytic flux may contribute to their reduced ability to initiate new immune responses in UV-irradiated mice.

GM-CSF and IL-3 Modulate Human Monocyte TNF-α Production and Renewal in In Vitro Models of Trained Immunity

GM-CSF and IL-3 are hematopoietic cytokines that also modulate the effector functions of several immune cell subsets. In particular, GM-CSF and IL-3 exert a significant control on monocyte and macrophage effector functions, as assessed in experimental models of inflammatory and autoimmune diseases and also in human studies. Here, we sought to investigate the mechanisms and the extent to which GM-CSF and IL-3 modulate the pro-inflammatory, LPS-mediated, activation of human CD14⁺ monocytes taking into account the new concept of trained immunity (i.e., the priming stimulus modulates the response to subsequent stimuli mainly by inducing chromatin remodeling and increased transcription at relevant genetic loci). We demonstrate that GM-CSF and IL-3 priming enhances TNF-α production upon subsequent LPS stimulation (short-term model of trained immunity) in a p38- and SIRT2-dependent manner without increasing TNF primary transcript levels (a more direct measure of transcription), thus supporting a posttranscriptional regulation of TNF-α in primed monocytes. GM-CSF and IL-3 priming followed by 6 days of resting also results in increased TNF-α production upon LPS stimulation (long-term model of trained immunity). In this case, however, GM-CSF and IL-3 priming induces a c-Myc-dependent monocyte renewal and increase in cell number that is in turn responsible for heightened TNF-α production. Overall, our results provide insights to understand the biology of monocytes in health and disease conditions in which the hematopoietic cytokines GM-CSF and IL-3 play a role and also extend our knowledge of the cellular and molecular mechanisms of trained immunity.

Emerging Roles for Epigenetic Programming in the Control of Inflammatory Signaling Integration in Heath and Disease

Macrophages and dendritic cells initiate the innate immune response to infection and injury and contribute to inflammatory signaling to maintain the homeostasis of various tissues, which includes resident macrophages for the elimination of invading microorganisms and tissue damage. Inappropriate inflammatory signaling can lead to persistent inflammation and further develop into autoimmune and inflammation-associated diseases. Inflammatory signaling pathways have been well characterized, but how these signaling pathways are converted into sustained and diverse patterns of expression of cytokines, chemokines, and other genes in response to environmental challenges is unclear. Emerging evidence suggests the important role of epigenetic mechanisms in finely tuning the outcome of the host innate immune response. An understanding of epigenetic regulation of innate immune cell identity and function will enable the identification of the mechanism between gene-specific host defenses and inflammatory disease and will also allow for exploration of the program of innate immune memory in health and disease. This information could be used to develop therapeutic agents to enhance the host response, preventing chronic inflammation through preserving tissues and signaling integrity.

The therapeutic potential of epigenetic manipulation during infectious diseases

Epigenetic modifications are increasingly recognized as playing an important role in the pathogenesis of infectious diseases. They represent a critical mechanism regulating transcriptional profiles in the immune system that contributes to the cell-type and stimulus specificity of the transcriptional response. Recent data highlight how epigenetic changes impact macrophage functional responses and polarization, influencing the innate immune system through macrophage tolerance and training. In this review we will explore how post-translational modifications of histone tails influence immune function to specific infectious diseases. We will describe how these may influence outcome, highlighting examples derived from responses to acute bacterial pathogens, models of sepsis, maintenance of viral latency and HIV infection. We will discuss how emerging classes of pharmacological agents, developed for use in oncology and other settings, have been applied to models of infectious diseases and their potential to modulate key aspects of the immune response to bacterial infection and HIV therapy.

Innate immune memory: Implications for host responses to damage-associated molecular patterns

Cells of the innate immune system build immunological memory via epigenetic reprogramming after stimulations with microbial ligands. This functional readjustment allows for enhanced nonspecific inflammatory responses upon secondary challenges, a process termed "trained immunity." The epigenomic blueprint of trained monocytes has been recently reported, which revealed several important immunologic and metabolic mechanisms that underlie these changes. Interestingly, similar long-term reprogramming of cytokine production has also been described to be induced by endogenous damage-associated molecular patterns (DAMPs). Here, we present an overview of the novel data showing that endogenous alarm signals associated with tissue damage and sterile inflammation can induce trained immunity through epigenetic regulation of transcriptional programs. We describe new and old evidence of persistent effects of DAMPs in driving inflammation and enforce the concept that the influence of tissue-derived signals is critical in adjusting the magnitude and type of immune response built by the host. The better characterization of trained immunity for the persistence of inflammation induced by DAMPs would provide new possibilities for intervention in aging and autoinflammatory disorders.

Mycobacterium leprae Activates Toll-Like Receptor-4 Signaling and Expression on Macrophages Depending on Previous Bacillus Calmette-Guerin Vaccination

Toll-like receptor (TLR)-1 and TLR2 have been shown to be receptors for Mycobacterium leprae (M. leprae), yet it is unclear whether M. leprae can signal through alternative TLRs. Other mycobacterial species possess ligands for TLR4 and genetic association studies in human populations suggest that people with TLR4 polymorphisms may be protected against leprosy. Using human embryonic kidney (HEK)-293 cells co-transfected with TLR4, we demonstrate that M. leprae activates TLR4. We used human macrophages to show that M. leprae stimulation of cytokine production is diminished if pre-treated with TLR4 neutralizing antibody. TLR4 protein expression was up-regulated on macrophages derived from non-bacillus Calmette-Guerin (BCG) vaccinated healthy volunteers after incubation with M. leprae, whereas it was down-regulated in macrophages derived from BCG-vaccinated donors. Finally, pre-treatment of macrophages derived from BCG-naive donors with BCG reversed the effect of M. leprae on TLR4 expression. This may be a newly described phenomenon by which BCG vaccination stimulates “non-specific” protection to the human immune system.

Heterologous vaccine effects

The heterologous or non-specific effects (NSEs) of vaccines, at times defined as "off-target effects" suggest that they can affect the immune response to organisms other than their pathogen-specific intended purpose. These NSEs have been the subject of clinical, immunological and epidemiological studies and are increasingly recognized as an important biological process by a growing group of immunologists and epidemiologists. Much remain to be learned about the extent and underlying mechanisms for these effects. The conference "Off-target effects of vaccination" held in Annecy-France (June 8-10 2015) intended to take a holistic approach drawing from the fields of immunology, systems biology, epidemiology, bioinformatics, public health and regulatory science to address fundamental questions of immunological mechanisms, as well as translational questions about vaccines NSEs. NSE observations were examined using case-studies on live attenuated vaccines and non-live vaccines followed by discussion of studies of possible biological mechanisms. Some possible pathways forward in the study of vaccines NSE were identified and discussed by the expert group.

Harnessing the beneficial heterologous effects of vaccination

Clinical evidence strongly suggests that certain live vaccines, in particular bacille Calmette-Guérin (BCG) and measles vaccines, can reduce all-cause mortality, most probably through protection against non-targeted pathogens in addition to the targeted pathogen. The underlying mechanisms are currently unknown. We discuss how heterologous lymphocyte activation and innate immune memory could promote protection beyond the intended target pathogen and consider how vaccinologists could leverage heterologous immunity to improve outcomes in vulnerable populations, in particular the very young and the elderly.

Trained immunity: A program of innate immune memory in health and disease

The general view that only adaptive immunity can build immunological memory has recently been challenged. In organisms lacking adaptive immunity, as well as in mammals, the innate immune system can mount resistance to reinfection, a phenomenon termed "trained immunity" or "innate immune memory." Trained immunity is orchestrated by epigenetic reprogramming, broadly defined as sustained changes in gene expression and cell physiology that do not involve permanent genetic changes such as mutations and recombination, which are essential for adaptive immunity. The discovery of trained immunity may open the door for novel vaccine approaches, new therapeutic strategies for the treatment of immune deficiency states, and modulation of exaggerated inflammation in autoinflammatory diseases.

TLR2, TLR4 and Dectin-1 signalling in hematopoietic stem and progenitor cells determines the antifungal phenotype of the macrophages they produce

TLRs represent an attractive target for the stimulation of myeloid cell production by HSPCs. We have previously demonstrated that HSPCs use TLR2 to sense Candida albicans in vivo and induce the production of macrophages. In this work, we used an in vitro model of HSPCs differentiation to investigate the functional consequences for macrophages of exposure of HSPCs to various PAMPs and C. albicans cells. Mouse HSPCs (Lin(-) cells) were cultured with M-CSF to induce macrophage differentiation, in the presence or absence of the following PRR agonists: Pam3CSK4 (TLR2 ligand), LPS (TLR4 ligand), depleted zymosan (which only activates Dectin-1), or C. albicans yeasts (which activate several PRRs, but principally TLR2 and Dectin-1). Our data show that these PAMPs differentially impact the anti-microbial function of the macrophages produced by the exposed HSPCs. Pure TLR2 and TLR4 ligands generate macrophages with a diminished ability to produce inflammatory cytokines. In contrast, HSPCs activation in response to C. albicans leads to the generation of macrophages that are better prepared to deal with the infection, as they produce higher amounts of inflammatory cytokines and have higher fungicidal capacity than control macrophages. Therefore, the tailored manipulation of the differentiation process may help to boost the innate immune response to infection.

Inflammatory cytokines provide both infection-responsive and developmental signals for blood development: Lessons from the zebrafish

Hematopoietic stem cells (HSCs) are rare, largely dormant, long-lived cells that are capable of establishing and regenerating all mature blood cell lineages throughout the life of the host. Given their therapeutic importance, understanding factors that regulate HSC development and influence HSC proliferation and differentiation is of great interest. Exploring HSC biology through the lens of infection has altered our traditional view of the HSC. The HSC can now be considered a component of the immune response to infection. In response to inflammatory cytokine signaling, HSCs enhance their proliferative state and contribute to the production of in-demand blood cell lineages. Similar cytokine signaling pathways also participate during embryonic HSC production. With its highly conserved hematopoietic system and experimental tractability, the zebrafish model has made significant contributions to the hematopoietic field. In particular, the zebrafish system has been ideally suited to help reveal the molecular and cellular mechanisms underlying HSC development. This review highlights recent zebrafish studies that have uncovered new mechanistic insights into how inflammatory signaling pathways influence HSC behavior during infection and HSC production within the embryo.

Trained immunity: consequences for the heterologous effects of BCG vaccination

A growing body of evidence from epidemiologic and immunologic studies have shown that in addition to target disease-specific effects, vaccines have heterologous effects towards unrelated pathogens. Like some other vaccines, bacille Calmette-Guerin (BCG) has shown in observational studies and randomized clinical trials to increase survival beyond the disease burden of the target disease. The immunologic substrate for these non-specific protective effects have been ascertained to heterologous T cell effects on the one hand, and to priming of innate immunity on the other hand. The term 'trained immunity' has been proposed to describe these potentiating effects of vaccines on innate immune responses. This process can explain the rapid effects of BCG vaccination and has been suggested to be mediated by epigenetic programming of monocytes or macrophages. This novel concept has important implications for the possible use of vaccines, for vaccination policy and even for the design of novel immunotherapeutic approaches.

Innate Immune Memory: The Latest Frontier of Adjuvanticity

Recent findings in the field of immune memory have demonstrated that B and T cell mediated immunity following infections are enhanced by the so-called trained immunity. This effect has been most extensively investigated for the tuberculosis vaccine strain Bacillus Calmette-Guérin (BCG). Epidemiological studies suggest that this vaccine is associated with a substantial reduction in overall child mortality that cannot be solely explained by prevention of the target disease but that it seems to rely on inducing resistance to other infections. Upon infection or vaccination, monocytes/macrophages can be functionally reprogrammed so as to display an enhanced defensive response against unrelated infections. Epigenetic modifications seem to play a key role in the induction of this “innate memory.” These findings are revolutionising our knowledge of the immune system, introducing the concept of memory also for mammalian innate immunity. Thus, vaccines are likely to nonspecifically affect the overall immunological status of individuals in a clinically relevant manner. As a consequence, future vaccine strategies ought to take into account the contribution of innate memory through appropriate design of formulations and administration scheduling.

Reduced immune responses in chimeric mice engrafted with bone marrow cells from mice with airways inflammation

OBJECTIVE

During respiratory inflammation, it is generally assumed that dendritic cells differentiating from the bone marrow are immunogenic rather than immunoregulatory. Using chimeric mice, the outcomes of airways inflammation on bone marrow progenitor cells were studied.

METHODS

Immune responses were analyzed in chimeric mice engrafted for >16 weeks with bone marrow cells from mice with experimental allergic airways disease (EAAD).

RESULTS

Responses to sensitization and challenge with the allergen causing inflammation in the bone marrow-donor mice were significantly reduced in the chimeric mice engrafted with bone marrow cells from mice with EAAD (EAAD-chimeric). Responses to intranasal LPS and topical fluorescein isothiocyanate (non-specific challenges) were significantly attenuated. Fewer activated dendritic cells from the airways and skin of the EAAD-chimeric mice could be tracked to the draining lymph nodes, and may contribute to the significantly reduced antigen/chemical-induced hypertrophy in the draining nodes, and the reduced immune responses to sensitizing allergens. Dendritic cells differentiating in vitro from the bone marrow of >16 weeks reconstituted EAAD-chimeric mice retained an ability to poorly prime immune responses when transferred into naïve mice.

CONCLUSIONS

Dendritic cells developing from bone marrow progenitors during airways inflammation are altered such that daughter cells have reduced antigen priming capabilities.