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      Myeloid Cells during Viral Infections and Inflammation

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          Abstract

          Myeloid cells represent a diverse range of innate leukocytes that are crucial for mounting successful immune responses against viruses. These cells are responsible for detecting pathogen-associated molecular patterns, thereby initiating a signaling cascade that results in the production of cytokines such as interferons to mitigate infections. The aim of this review is to outline recent advances in our knowledge of the roles that neutrophils and inflammatory monocytes play in initiating and coordinating host responses against viral infections. A focus is placed on myeloid cell development, trafficking and antiviral mechanisms. Although known for promoting inflammation, there is a growing body of literature which demonstrates that myeloid cells can also play critical regulatory or immunosuppressive roles, especially following the elimination of viruses. Additionally, the ability of myeloid cells to control other innate and adaptive leukocytes during viral infections situates these cells as key, yet under-appreciated mediators of pathogenic inflammation that can sometimes trigger cytokine storms. The information presented here should assist researchers in integrating myeloid cell biology into the design of novel and more effective virus-targeted therapies.

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          Most cited references122

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          Antiviral actions of interferons.

          C Samuel (2001)
          Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.
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            Modulation of Myelopoiesis Progenitors Is an Integral Component of Trained Immunity

            Summary Trained innate immunity fosters a sustained favorable response of myeloid cells to a secondary challenge, despite their short lifespan in circulation. We thus hypothesized that trained immunity acts via modulation of hematopoietic stem and progenitor cells (HSPCs). Administration of β-glucan (prototypical trained-immunity-inducing agonist) to mice induced expansion of progenitors of the myeloid lineage, which was associated with elevated signaling by innate immune mediators, such as IL-1β and granulocyte-macrophage colony-stimulating factor (GM-CSF), and with adaptations in glucose metabolism and cholesterol biosynthesis. The trained-immunity-related increase in myelopoiesis resulted in a beneficial response to secondary LPS challenge and protection from chemotherapy-induced myelosuppression in mice. Therefore, modulation of myeloid progenitors in the bone marrow is an integral component of trained immunity, which to date, was considered to involve functional changes of mature myeloid cells in the periphery.
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              PAMP s and DAMP s: signal 0s that spur autophagy and immunity

              Summary Pathogen‐associated molecular pattern molecules (PAMPs) are derived from microorganisms and recognized by pattern recognition receptor (PRR)‐bearing cells of the innate immune system as well as many epithelial cells. In contrast, damage‐associated molecular pattern molecules (DAMPs) are cell‐derived and initiate and perpetuate immunity in response to trauma, ischemia, and tissue damage, either in the absence or presence of pathogenic infection. Most PAMPs and DAMPs serve as so‐called ‘Signal 0s’ that bind specific receptors [Toll‐like receptors, NOD‐like receptors, RIG‐I‐like receptors, AIM2‐like receptors, and the receptor for advanced glycation end products (RAGE)] to promote autophagy. Autophagy, a conserved lysosomal degradation pathway, is a cell survival mechanism invoked in response to environmental and cellular stress. Autophagy is inferred to have been present in the last common eukaryotic ancestor and only to have been lost by some obligatory intracellular parasites. As such, autophagy represents a unifying biology, subserving survival and the earliest host defense strategies, predating apoptosis, within eukaryotes. Here, we review recent advances in our understanding of autophagic molecular mechanisms and functions in emergent immunity.
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                Author and article information

                Journal
                Viruses
                Viruses
                viruses
                Viruses
                MDPI
                1999-4915
                19 February 2019
                February 2019
                : 11
                : 2
                : 168
                Affiliations
                Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada; aross14@ 123456uoguelph.ca (A.A.S.); jvanvlot@ 123456uoguelph.ca (J.P.v.V.); rmould@ 123456uoguelph.ca (R.C.M.); eklafuri@ 123456uoguelph.ca (E.M.K.); minott@ 123456uoguelph.ca (J.A.M.); kwootton@ 123456uoguelph.ca (S.K.W.); bbridle@ 123456uoguelph.ca (B.W.B.)
                Author notes
                [* ]Correspondence: kkarimi@ 123456uoguelph.ca ; Tel.: +1-(519)-824-4120 (ext. 54668)
                [†]

                These authors contributed equally to this work.

                [‡]

                These authors contributed equally to this work.

                Author information
                https://orcid.org/0000-0002-2296-8288
                https://orcid.org/0000-0002-5985-2406
                https://orcid.org/0000-0002-8335-1118
                Article
                viruses-11-00168
                10.3390/v11020168
                6410039
                30791481
                89ab8daa-0c1d-4ea8-a8ec-61eba826865a
                © 2019 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 01 February 2019
                : 16 February 2019
                Categories
                Review

                Microbiology & Virology
                neutrophils,inflammatory monocytes,inflammation,viral infection,myeloid cells,type i interferon

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