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Polymorphonuclear neutrophils (PMNs) play a critical role in host defense against infection and in the resolution of inflammation. However, immune responses mediated by PMN must be tightly regulated to facilitate elimination of invading pathogens without inducing detrimental inflammation and host tissue damage. Specific engagement of cell surface immunoreceptors by a diverse range of extracellular signals regulates PMN effector functions through differential activation of intracellular signaling cascades. Although mechanisms of PMN activation mediated via cell signaling pathways have been well described, less is known about negative regulation of PMN function by immune signaling cascades. Here, we provide an overview of immunoreceptor-mediated negative regulation of key PMN effector functions including maturation, migration, phagocytosis, reactive oxygen species release, degranulation, apoptosis, and NET formation. Increased understanding of mechanisms of suppression of PMN effector functions may point to possible future therapeutic targets for the amelioration of PMN-mediated autoimmune and inflammatory diseases., (©2017 Society for Leukocyte Biology.)

CD47 is known to play an important role in CD4 + T cell homeostasis. We recently reported a reduction in mice deficient in the Cd47 gene (Cd47 -/- ) CD4 + T cell adhesion and transendothelial migration (TEM) in vivo and in vitro as a result of impaired expression of high-affinity forms of LFA-1 and VLA-4 integrins. A prior study concluded that Cd47 -/- mice were resistant to experimental autoimmune encephalomyelitis (EAE) as a result of complete failure in CD4 + T cell activation after myelin oligodendrocyte glycoprotein peptide 35-55 aa (MOG 35-55 ) immunization. As the prior EAE study was published before our report, authors could not have accounted for defects in T cell integrin function as a mechanism to protect Cd47 -/- in EAE. Thus, we hypothesized that failure of T cell activation involved defects in LFA-1 and VLA-4 integrins. We confirmed that Cd47 -/- mice were resistant to MOG 35-55 -induced EAE. Our data, however, supported a different mechanism that was not a result of failure of CD4 + T cell activation. Instead, we found that CD4 + T cells in MOG 35-55 -immunized Cd47 -/- mice were activated, but clonal expansion contracted within 72 h after immunization. We used TCR crosslinking and mitogen activation in vitro to investigate the underlying mechanism. We found that naïve Cd47 -/- CD4 + T cells exhibited a premature block in proliferation and survival because of impaired activation of LFA-1, despite effective TCR-induced activation. These results identify CD47 as an important regulator of LFA-1 and VLA-4 integrin-adhesive functions in T cell proliferation, as well as recruitment, and clarify the roles played by CD47 in MOG 35-55 -induced EAE., (© Society for Leukocyte Biology.)

Neutrophils play significant roles in immune homeostasis and as neutralizers of microbial infections. Recent evidence further suggests heterogeneity of neutrophil developmental and activation states that exert specialized effector functions during inflammatory disease conditions. Neutrophils can play multiple roles during viral infections, secreting inflammatory mediators and cytokines that contribute significantly to host defense and pathogenicity. However, their roles in viral immunity are not well understood. In this review, we present an overview of neutrophil heterogeneity and its impact on the course and severity of viral respiratory infectious diseases. We focus on the evidence demonstrating the crucial roles neutrophils play in the immune response toward respiratory infections, using influenza as a model. We further extend the understanding of neutrophil function with the studies pertaining to COVID‐19 disease and its neutrophil‐associated pathologies. Finally, we discuss the relevance of these results for future therapeutic options through targeting and regulating neutrophil‐specific responses. [ABSTRACT FROM AUTHOR]

CXCL8 is the principal human neutrophil‐attracting chemokine and a major mediator of inflammation. The chemokine exerts its neutrophil‐chemotactic and neutrophil‐activating activities via interaction with glycosaminoglycans (GAGs) and activation of the G protein‐coupled receptors (GPCRs) CXCR1 and CXCR2. Natural CXCL8 displays an exceptional degree of amino (NH2)‐terminal heterogeneity. Most CXCL8 forms result from proteolytic processing of authentic CXCL8(1‐77). Here, we compared the potencies to activate and recruit neutrophils of the 3 most abundant natural CXCL8 forms: full‐length 77 amino acid CXCL8 and the 2 major natural truncated forms lacking 5 or 8 NH2‐terminal amino acids. NH2‐terminal truncation hardly affected the capacity of CXCL8 to induce shedding of CD62L or to up‐regulate the expression of the adhesion molecules CD11a, CD11b, or CD15 on human neutrophils. In addition, the potency of CXCL8 to induce neutrophil degranulation and its effect on phagocytosis remained unaltered upon removal of 5 or 8 NH2‐terminal residues. However, NH2‐terminal truncation strongly potentiated CXCL8‐induced actin polymerization. CXCL8(6‐77) and CXCL8(9‐77) showed a comparable capacity to induce Ca2+ signaling in human neutrophils and to direct in vitro neutrophil migration. Strikingly, the ability of CXCL8(9‐77) to recruit neutrophils into the peritoneal cavity of mice was significantly enhanced compared to CXCL8(6‐77). These results suggest that NH2‐terminal truncation influences specific biological activities of CXCL8 and indicate that CXCL8(9‐77) may be the most potent neutrophil‐attracting CXCL8 form in vivo. [ABSTRACT FROM AUTHOR]

Neutrophils are the most abundant leukocytes in innate immunity where they elicit powerful effector functions to eliminate invading pathogens and modulate the adaptive as well as the innate immune response. Neutrophil function must be tightly regulated during inflammation and infection to avoid additional tissue damage. Increasing evidence suggests that transcription factors (TFs) function as key regulators to modulate transcriptional output, thereby controlling cell fate decision and the inflammatory responses. However, the molecular mechanisms underlying neutrophil differentiation and function during inflammation remain largely uncharacterized. Here, we provide a comprehensive overview of TFs known to be crucial for neutrophil maturation and in the signaling pathways that control neutrophil differentiation and activation. We also outline how emerging genomic and single‐cell technologies may facilitate further discovery of neutrophil transcriptional regulators. [ABSTRACT FROM AUTHOR]

Glucocorticoid‐induced leucine zipper (GILZ) exerts anti‐inflammatory effects on the immune cells. However, less is known about GILZ function in neutrophils. We aimed to define the specific role of GILZ in basal neutrophil activity during an inflammatory response. GILZ knockdown resulted in a persistent activation state of neutrophils, as evidenced by increased phagocytosis, killing activity, and oxidative burst in GILZ‐knockout (KO) neutrophils. This enhanced response caused severe disease in a dinitrobenzene sulfonic acid (DNBS)‐induced colitis model, where GILZ‐KO mice had prominent granulocytic infiltrate and excessive inflammatory state. We used a Candida albicans intraperitoneal infection model to unravel the intracellular pathways affected by GILZ expression in activated neutrophils. GILZ‐KO neutrophils had stronger ability to clear the infectious agent than the wild‐type (WT) neutrophils, and there was more activation of the NOX2 (NADPH oxidase 2) and p47phox proteins, which are directly involved in oxidative burst. Similarly, the MAPK pathway components, that is, ERK and p38, which are involved in the oxidative burst pathway, were highly phosphorylated in GILZ‐KO neutrophils. Evaluation of GILZ expression kinetics during C. albicans infection revealed down‐regulation that correlated inversely with the state of neutrophil activation, which was evaluated as oxidative burst. Overall, our findings define GILZ as a regulator of neutrophil functions, as its expression contributes to limiting neutrophil activation by reducing the activation of the signaling pathways that control the basal neutrophil functions. Controlling GILZ expression could help regulate a continuous inflammatory state that can result in chronic inflammatory and autoimmune diseases. GILZ expression contributes to the inhibition of neutrophil activation by reducing MAPK pathway protein and NOX2 activity that control basal neutrophil functions. [ABSTRACT FROM AUTHOR]

Abstract: Leukocyte recruitment plays a critical role during both normal inflammation and chronic inflammatory diseases, and ongoing studies endeavor to better understand the complexities of this process. Focal adhesion kinase (FAK) is well known for its role in cancer, yet it also has been shown to regulate aspects of neutrophil and B16 melanoma cell recruitment by rapidly influencing endothelial cell focal adhesion dynamics and junctional opening. Recently, we found that FAK related non‐kinase (FRNK), a protein that is often used as a FAK dominant negative, blocked eosinophil transmigration by preventing the transcription of vascular cell adhesion molecule‐1 (VCAM‐1) and eotaxin‐3 (CCL26). Surprisingly, the blocking occurred even in the absence of endogenous FAK. To better understand the role of FAK in leukocyte recruitment, we used a FAK‐specific inhibitor (PF‐573228) and determined the effect on IL‐4 induced eosinophil recruitment in vitro and in vivo. PF‐573228 prevented the expression of VCAM‐1 and CCL26 expression in IL‐4‐stimulated human endothelial cells in vitro. As a result, eosinophil adhesion and transmigration were blocked. PF‐572338 also prevented IL‐4‐induced VCAM‐1 expression in vivo. Using brightfield intravital microscopy, we found that PF‐573228 decreased leukocyte rolling flux, adhesion, and emigration. We specifically examined eosinophil recruitment in vivo by using an eosinophil‐GFP reporter mouse and found PF‐573228 attenuated eosinophil emigration. This study reveals that a FAK inhibitor influences inflammation through its action on eosinophil recruitment. [ABSTRACT FROM AUTHOR]

Review of how cAMP modulates TCR signaling, and how T cells generate local cAMP pools to spatially restrict signaling during immune synapse assembly. T Lymphocyte activation involves a substantial reorganization of the membranous and intracellular compartments. Signaling complexes assemble and dismantle in a highly ordered fashion in both compartments and orchestrate the activation of T cells with high sensitivity and specificity. TCR ligation leads to a short burst of cAMP production, which is centrally required for T cell activation; however, sustained elevations in intracellular cAMP concentrations are immunosuppressive. Emerging evidence of the existence of local cAMP pools gleaned from studies on other cell types suggests that cAMP compartmentalization may account, in part, for these opposing effects. Whereas cAMP compartmentalization has been identified as a central factor in the control of the cAMP‐dependent processes in other cell types, this has, as yet, not been addressed in T lymphocytes. In this review, we discuss the role of cAMP in T cell activation and differentiation, with an emphasis on the effects mediated by the cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC)1, and on the regulatory proteins that may control the generation of local cAMP pools in T cells. We also present an overview of the available tools to image cAMP production at the subcellular level and discuss how bacterial adenylate cyclase (AC) toxins that are known to generate local cAMP pools can be exploited to address the role of cAMP compartmentalization in T cell activation. [ABSTRACT FROM AUTHOR]