A gammaherpesvirus provides protection against allergic asthma by inducing the replacement of resident alveolar macrophages with regulatory monocytes

Niche signals and transcription factors involved in tissue-resident macrophage development

Tissue-resident macrophages form an essential part of the first line of defense in all tissues of the body. Next to their immunological role, they play an important role in maintaining tissue homeostasis. Recently, it was shown that they are primarily of embryonic origin. During embryogenesis, precursors originating in the yolk sac and fetal liver colonize the embryonal tissues where they develop into mature tissue-resident macrophages. Their development is governed by two distinct sets of transcription factors. First, in the pre-macrophage stage, a core macrophage program is established by lineage-determining transcription factors. Under the influence of tissue-specific signals, this core program is refined by signal-dependent transcription factors. This nurturing by the niche allows the macrophages to perform tissue-specific functions. In the last 15 years, some of these niche signals and transcription factors have been identified. However, detailed insight in the exact mechanism of development is still lacking.

Alveolar Macrophages in the Resolution of Inflammation, Tissue Repair, and Tolerance to Infection

Pathogen persistence in the respiratory tract is an important preoccupation, and of particular relevance to infectious diseases such as tuberculosis. The equilibrium between elimination of pathogens and the magnitude of the host response is a sword of Damocles for susceptible patients. The alveolar macrophage is the first sentinel of the respiratory tree and constitutes the dominant immune cell in the steady state. This immune cell is a key player in the balance between defense against pathogens and tolerance toward innocuous stimuli. This review focuses on the role of alveolar macrophages in limiting lung tissue damage from potentially innocuous stimuli and from infections, processes that are relevant to appropriate tolerance of potential causes of lung disease. Notably, the different anti-inflammatory strategies employed by alveolar macrophages and lung tissue damage control are explored. These two properties, in addition to macrophage manipulation by pathogens, are discussed to explain how alveolar macrophages may drive pathogen persistence in the airways.

Mouse Yolk Sac Hematopoiesis

The yolk sac is the first observed site of hematopoiesis during mouse ontogeny. Primitive erythroid cells are the most well-recognized cell lineages produced from this tissue. In addition to primitive erythroid cells, several types of hematopoietic cells are present, including multipotent hematopoietic progenitors. Yolk sac-derived blood cells constitute a transient wave of embryonic and fetal hematopoiesis. However, recent studies have demonstrated that some macrophage and B cell lineages derived from the early yolk sac may persist to adulthood. This review discusses the cellular basis of mouse yolk sac hematopoiesis and its contributions to embryonic and adult hematopoietic systems.

The hygiene hypothesis: immunological mechanisms of airway tolerance

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Microbial and environmental signals set tonic activation status of barrier tissues.

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Signaling from barrier tissues licenses dendritic cells to induce T helper 2 cells.

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Pulmonary immune system in early-life prone to asthma development.

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Mechanistic understanding needed to translate epidemiological findings in therapies.

The hygiene hypothesis was initially proposed as an explanation for the alarming rise in allergy prevalence in the last century. The immunological idea behind this hypothesis was a lack of infections associated with a Western lifestyle and a consequential reduction in type 1 immune responses. It is now understood that the development of tolerance to allergens depends on microbial colonization and immunostimulatory environmental signals during early-life or passed on by the mother. These environmental cues are sensed and integrated by barrier epithelial cells of the lungs and possibly skin, which in turn instruct dendritic cells to regulate or impede adaptive T cell responses. Recent reports also implicate immunoregulatory macrophages as powerful suppressors of allergy by the microbiome. We propose that loss of adequate microbial stimulation due to a Western lifestyle may result in hypersensitive barrier tissues and the observed rise in type 2 allergic disease.

Effects of Influenza on Alveolar Macrophage Viability Are Dependent on Mouse Genetic Strain

Secondary bacterial coinfections following influenza virus pose a serious threat to human health. Therefore, it is of significant clinical relevance to understand the immunological causes of this increased susceptibility. Influenza-induced alterations in alveolar macrophages (AMs) have been shown to be a major underlying cause of the increased susceptibility to bacterial superinfection. However, the mechanisms responsible for this remain under debate, specifically in terms of whether AMs are depleted in response to influenza infection or are maintained postinfection, but with disrupted phagocytic activity. The data presented in this article resolves this issue by showing that either mechanism can differentially occur in individual mouse strains. BALB/c mice exhibited a dramatic IFN-γ-dependent reduction in levels of AMs following infection with influenza A, whereas AM levels in C57BL/6 mice were maintained throughout the course of influenza infection, although the cells displayed an altered phenotype, namely an upregulation in CD11b expression. These strain differences were observed regardless of whether infection was performed with low or high doses of influenza virus. Furthermore, infection with either the H1N1 A/California/04/2009 (CA04) or H1N1 A/PR8/1934 (PR8) virus strain yielded similar results. Regardless of AM viability, both BALB/c and C57BL/6 mice showed a high level of susceptibility to postinfluenza bacterial infection. These findings resolve the apparent inconsistencies in the literature, identify mouse strain-dependent differences in the AM response to influenza infection, and ultimately may facilitate translation of the mouse model to clinical application.

Respiratory virus-induced heterologous immunity: Part of the problem or part of the solution?

Purpose

To provide current knowledge on respiratory virus-induced heterologous immunity (HI) with a focus on humoral and cellular cross-reactivity. Adaptive heterologous immune responses have broad implications on infection, autoimmunity, allergy and transplant immunology. A better understanding of the mechanisms involved might ultimately open up possibilities for disease prevention, for example by vaccination.

Methods

A structured literature search was performed using Medline and PubMed to provide an overview of the current knowledge on respiratory-virus induced adaptive HI.

Results

In HI the immune response towards one antigen results in an alteration of the immune response towards a second antigen. We provide an overview of respiratory virus-induced HI, including viruses such as respiratory syncytial virus (RSV), rhinovirus (RV), coronavirus (CoV) and influenza virus (IV). We discuss T cell receptor (TCR) and humoral cross-reactivity as mechanisms of HI involving those respiratory viruses. Topics covered include HI between respiratory viruses as well as between respiratory viruses and other pathogens. Newly developed vaccines, which have the potential to provide protection against multiple virus strains are also discussed. Furthermore, respiratory viruses have been implicated in the development of autoimmune diseases, such as narcolepsy, Guillain-Barré syndrome, type 1 diabetes or myocarditis. Finally, we discuss the role of respiratory viruses in asthma and the hygiene hypothesis, and review our recent findings on HI between IV and allergens, which leads to protection from experimental asthma.

Conclusion

Respiratory-virus induced HI may have protective but also detrimental effects on the host. Respiratory viral infections contribute to asthma or autoimmune disease development, but on the other hand, a lack of microbial encounter is associated with an increasing number of allergic as well as autoimmune diseases. Future research might help identify the elements which determine a protective or detrimental outcome in HI-based mechanisms.

Respiratory virus-induced heterologous immunity: Part of the problem or part of the solution?

Purpose

To provide current knowledge on respiratory virus-induced heterologous immunity (HI) with a focus on humoral and cellular cross-reactivity. Adaptive heterologous immune responses have broad implications on infection, autoimmunity, allergy and transplant immunology. A better understanding of the mechanisms involved might ultimately open up possibilities for disease prevention, for example by vaccination.

Methods

A structured literature search was performed using Medline and PubMed to provide an overview of the current knowledge on respiratory-virus induced adaptive HI.

Results

In HI the immune response towards one antigen results in an alteration of the immune response towards a second antigen. We provide an overview of respiratory virus-induced HI, including viruses such as respiratory syncytial virus (RSV), rhinovirus (RV), coronavirus (CoV) and influenza virus (IV). We discuss T cell receptor (TCR) and humoral cross-reactivity as mechanisms of HI involving those respiratory viruses. Topics covered include HI between respiratory viruses as well as between respiratory viruses and other pathogens. Newly developed vaccines which have the potential to provide protection against multiple virus strains are also discussed. Furthermore, respiratory viruses have been implicated in the development of autoimmune diseases, such as narcolepsy, Guillain–Barré syndrome, type 1 diabetes or myocarditis. Finally, we discuss the role of respiratory viruses in asthma and the hygiene hypothesis, and review our recent findings on HI between IV and allergens, which leads to protection from experimental asthma.

Conclusion

Respiratory-virus induced HI may have protective but also detrimental effects on the host. Respiratory viral infections contribute to asthma or autoimmune disease development, but on the other hand, a lack of microbial encounter is associated with an increasing number of allergic as well as autoimmune diseases. Future research might help identify the elements which determine a protective or detrimental outcome in HI-based mechanisms.

Developmental control of macrophage function

The combination between novel fate-mapping tools and single-cell RNA-sequencing technology has revealed the presence of multiple macrophage progenitors. This raises the fascinating possibility that what was once perceived as immense functional plasticity of macrophages could in fact come down to separate macrophage subsets performing distinct functions because of their differential cellular origin. The question of macrophage plasticity versus macrophage heterogeneity is broader than the difference between macrophages of embryonic or adult hematopoietic origin and is particularly relevant in the context of inflammation. In this manuscript, we review the potential impact of cellular origin on the function of macrophages. We also highlight the need for novel 'functional fate-mapping' tools that would reveal the history of the functional state of macrophages, rather than their cellular origin, in order to finally study their true plasticity in vivo.

Gammaherpesvirus is nothing to sneeze at

Gammaherpesvirus protects against allergic asthma by inducing the replacement of alveolar macrophages.