Mast cell responses to danger signals

Detecting and responding to danger is a paramount function of the immune system. Compounds heralding danger can be divided into two groups: exogenous and endogenous danger signals. The former group consists of conserved microbial structures such as lipopolysaccharide (LPS), while the latter consists of host compounds released or exposed by dead or dying cells as a consequence of trauma, stress or infection. Mast cells are long-lived immune cells present in almost all tissues, and are especially numerous at sites facing the external environment, making them ideal responders to danger signals. The aim of the work presented in this thesis was to investigate mast cell responses to danger signals of exogenous and endogenous origin. In Paper I, we investigated mast cell responses to the exogenous danger signal M-TriDAP, a bacterial peptidoglycan degradation product. We found that cord bloodderived mast cells (CBMCs) express NOD1, the receptor for M-TriDAP. Furthermore, M-TriDAP-treatment of CBMCs resulted in degranulation-independent release of cytokines and chemokines such as TNF, IL-8/CXCL8, MIP-1α/CCL3 and MIP-1β/CCL4. Importantly, we observed an augmented response when M-TriDAP was combined with the TLR4 agonist LPS, indicating cooperation between intracellular and extracellular pattern recognition receptors. In Paper II, we investigated mast cell responses to cell injury by subjecting murine mast cells to the supernatant of fibroblasts rendered necrotic by freeze-thawing. We found that mast cells respond to cell injury in this model by initiating a proinflammatory response, characterized by degranulation-independent release of cytokines and leukotrienes. By using genetically modified mice and molecular inhibitors, we found that the recognition of cell injury was MyD88-, T1/ST2- and p38- dependent. Finally, by using RNA-interference, we could pinpoint IL-33 as the necrotic cell compound that was responsible for the mast cell activation. In Paper III, we investigated responses to IL-33 administration in vivo. Here we found that wild-type C57BL/6 mice respond to intraperitoneal IL-33 administration with neutrophil infiltration. This response was not observed in mast cell-deficient mice but could be restored upon mast cell reconstitution, thus demonstrating a mast cell dependent mechanism. In Paper IV, we investigated the hypothesis that mast cells might function as sensors of damaged epithelia by responding to IL-33 during chronic inflammations of the airways, for instance in asthma. We found that IL-33 is released from necrotic airway epithelial cells and that CBMCs respond to the necrotic supernatant of these cells by secreting IL-5, IL-8/CXCL8, TNF and GM-CSF. However, no release of histamine, LTB4 or PGD2 could be detected. Interestingly, the exact same mediator release pattern was observed when CBMCs were treated with recombinant IL-33, suggesting that IL-33 might be an important factor released by injured airway epithelial cells that activates mast cells. In conclusion, the work presented in this thesis provides further evidence for important roles of mast cells in innate immune responses. The function of mast cells as sensors of cell injury is highlighted; a role that potentially can be either beneficial or detrimental. Finally, novel evidence is provided for the notion that IL-33 is an important danger signal capable of mast cell activation

Organ-specific features of natural killer cells.

Natural killer (NK) cells can be swiftly mobilized by danger signals and are among the earliest arrivals at target organs of disease. However, the role of NK cells in mounting inflammatory responses is often complex and sometimes paradoxical. Here, we examine the divergent phenotypic and functional features of NK cells, as deduced largely from experimental mouse models of pathophysiological responses in the liver, mucosal tissues, uterus, pancreas, joints and brain. Moreover, we discuss how organ-specific factors, the local microenvironment and unique cellular interactions may influence the organ-specific properties of NK cells

DAMPening COVID-19 Severity by Attenuating Danger Signals

COVID-19 might lead to multi-organ failure and, in some cases, to death. The COVID-19 severity is associated with a “cytokine storm.” Danger-associated molecular patterns (DAMPs) are proinflammatory molecules that can activate pattern recognition receptors, such as toll-like receptors (TLRs). DAMPs and TLRs have not received much attention in COVID-19 but can explain some of the gender-, weight- and age-dependent effects. In females and males, TLRs are differentially expressed, likely contributing to higher COVID-19 severity in males. DAMPs and cytokines associated with COVID-19 mortality are elevated in obese and elderly individuals, which might explain the higher risk for severer COVID-19 in these groups. Adenosine signaling inhibits the TLR/NF-κB pathway and, through this, decreases inflammation and DAMPs’ effects. As vaccines will not be effective in all susceptible individuals and as new vaccine-resistant SARS-CoV-2 mutants might develop, it remains mandatory to find means to dampen COVID-19 disease severity, especially in high-risk groups. We propose that the regulation of DAMPs via adenosine signaling enhancement might be an effective way to lower the severity of COVID-19 and prevent multiple organ failure in the absence of severe side effects

Heat Shock Proteins as Danger Signals for Cancer Detection

First discovered in 1962, heat shock proteins (HSPs) are highly studied with about 35,500 publications on the subject to date. HSPs are highly conserved, function as molecular chaperones for a large panel of “client” proteins and have strong cytoprotective properties. Induced by many different stress signals, they promote cell survival in adverse conditions. Therefore, their roles have been investigated in several conditions and pathologies where HSPs accumulate, such as in cancer. Among the diverse mammalian HSPs, some members share several features that may qualify them as cancer biomarkers. This review focuses mainly on three inducible HSPs: HSP27, HPS70, and HSP90. Our survey of recent literature highlights some recurring weaknesses in studies of the HSPs, but also identifies findings that indicate that some HSPs have potential as cancer biomarkers for successful clinical applications

Divergence of canonical danger signals: The genome-level expression patterns of human mononuclear cells subjected to heat shock or lipopolysaccharide

<p>Abstract</p> <p>Background</p> <p>Peripheral blood mononuclear cells (PBMC) serve a sentinel role allowing the host to efficiently sense and adapt to the presence of danger signals. Herein we have directly compared the genome-level expression patterns (microarray) of a human PBMC model (THP-1 cells) subjected to one of two canonical danger signals, heat shock or lipopolysaccharide (LPS).</p> <p>Results and Discussion</p> <p>Based on sequential expression and statistical filters, and in comparison to control cells, we found that 3,988 genes were differentially regulated in THP-1 cells subjected to LPS stress, and 2,921 genes were differentially regulated in THP-1 cells subjected to heat shock stress. Venn analyses demonstrated that the majority of differentially regulated genes (≥ 70%) were uniquely expressed in response to one of the two danger signals. Functional analyses demonstrated that the two danger signals induced expression or repression of genes corresponding to unique pathways, molecular functions, biological processes, and gene networks. In contrast, there were 184 genes that were commonly upregulated by both stress signals, and 430 genes that were commonly downregulated by both stress signals. Interestingly, the 184 commonly upregulated genes corresponded to a gene network broadly related to inflammation, and more specifically to chemokine signaling.</p> <p>Conclusion</p> <p>These data demonstrate that the mononuclear cell responses to the canonical stress signals, heat shock and LPS, are highly divergent. However, there is a heretofore unrecognized common pattern of gene network expression corresponding to chemokine-related biology. The data also serve as a reference database for investigators in the field of stress signaling.</p

Alarmins in frozen shoulder: a molecular association between inflammation and pain

Background: The pathophysiological mechanisms behind proliferation of fibroblasts and deposition of dense collagen matrix in idiopathic frozen shoulder remain unclear. Alarmins (also known as danger signals) are endogenous molecules that are released into the extracellular milieu after infection or tissue injury and that signal cell and tissue damage. Purpose: To investigate whether the presence of alarmins is higher in patients with idiopathic frozen shoulder than in control subjects. Study Design: Controlled laboratory study. Methods: Shoulder capsule samples were collected from 10 patients with idiopathic frozen shoulder and 10 patients with unstable shoulders (control). The samples were stained with hematoxylin and eosin (H&#38;E) and analyzed by immunohistochemistry using antibodies against alarmin molecules including high-mobility group protein B1 (HMGB1), interleukin 33, S100A8, S100A9, and the peripheral nerve marker PGP9.5. Immunoreactivities were rated in a blinded fashion from “none” to “strong.” Immunohistochemical distribution within the capsule was noted. Before surgery, patient-ranked pain frequency, severity, stiffness, and the range of passive shoulder motion were recorded and statistically analyzed. Results: Compared with control patients, patients with frozen shoulder had greater frequency and severity of self-reported pain (P = .02) and more restricted range of motion in all planes (P &lt; .05). H&#38;E-stained capsular tissue from frozen shoulder showed fibroblastic hypercellularity and increased subsynovial vascularity. Immunoreactivity of alarmins was significantly stronger in frozen shoulder capsules compared with control capsules (P &lt; .05). Furthermore, the expression of the alarmin molecule HMGB1 significantly correlated (r &gt; 0.9, P &lt; .05) with the severity of patient-reported pain. Conclusion: This study demonstrates a potential role for key molecular danger signals in frozen shoulder and suggests an association between the expression of danger molecules and the pain experienced by patients