Out of the phagocyte or into its phagosome: signalling to secretion

[Nontuberculous mycobacterial infections]

Nontuberculous mycobacterial (NTM) are found ubiquitously in the environment and are usually of low pathogenicity. Infection occurs via inhalation of aerosols, and some species may cause severe infections. The incidence of NTM infections is rising worldwide. The risk of developing NTM disease depends on the susceptibility of the host as well as the frequency and duration of exposure. In addition to congenital immune deficiencies and immunosuppressive therapy, structural lung and systemic diseases, including rheumatoid arthritis (RA), are associated with an increased risk for NTM infections. The immune response to NTM is complex and relies on the interplay between professional phagocytes and lymphoid cells. This interplay is concerted by three key cytokines: interleukin-12 (IL-12), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ). Targeted immunotherapies, e. g., treatment with TNF inhibitors, interfere with these essential pathways and increase the risk of NTM infection significantly. This review focuses on the relationship between the immune response to NTM and intrinsic and iatrogenic dispositions for NTM infection, with an emphasis on RA.

Autophagy mediates neutrophil responses to bacterial infection

Neutrophils constitute the first line of cellular defense against pathogens and autophagy is a fundamental cellular homeostasis pathway that operates with the intracellular degradation/recycling system. Induction of the autophagic process in neutrophils, in response to invading pathogens, constitutes a crucial mechanism in innate immunity. Exploration of autophagy has greatly progressed and diverse strategies have been reported for studying this molecular process in different biological systems; especially in infectious and inflammatory diseases. Furthermore, the role of autophagy in neutrophils, during pathogenic infection, continues to be of interest, due to the role of the cell in immunity function, its recruitment to the site of infection and its implication in inflammatory diseases. This review focuses on the known role of autophagy in neutrophils defence against pathogenic infections. A more detailed discussion will concern the recent findings highlighting the role of autophagy in inflammation and cell death in infected neutrophils.

Pathogen recognition and activation of the innate immune response in zebrafish

The zebrafish has proven itself as an excellent model to study vertebrate innate immunity. It presents us with possibilities for in vivo imaging of host-pathogen interactions which are unparalleled in mammalian model systems. In addition, its suitability for genetic approaches is providing new insights on the mechanisms underlying the innate immune response. Here, we review the pattern recognition receptors that identify invading microbes, as well as the innate immune effector mechanisms that they activate in zebrafish embryos. We compare the current knowledge about these processes in mammalian models and zebrafish and discuss recent studies using zebrafish infection models that have advanced our general understanding of the innate immune system. Furthermore, we use transcriptome analysis of zebrafish infected with E. tarda, S. typhimurium, and M. marinum to visualize the gene expression profiles resulting from these infections. Our data illustrate that the two acute disease-causing pathogens, E. tarda and S. typhimurium, elicit a highly similar proinflammatory gene induction profile, while the chronic disease-causing pathogen, M. marinum, induces a weaker and delayed innate immune response.

Secretion of heparin-binding protein from human neutrophils is determined by its localization in azurophilic granules and secretory vesicles

Human neutrophils have an important role in host defense against microbial infection. At different stages of an infectious process, neutrophils progressively up-regulate receptors and release various effector molecules. These are stored in several distinct types of granules with varying propensity to be secreted. Heparin-binding protein (HBP), also known as CAP37 or azurocidin, is a multifunctional, inactive serine-protease homologue. The present work shows that HBP is released from neutrophils on stimulation with secretagogues that do not trigger the secretion of azurophilic granule content. Therefore, the subcellular localization of HBP was investigated in more detail. Immunofluorescence microscopy revealed that HBP was localized close to the plasma membrane. Further analysis by fractionation of postnuclear supernatants from cavitated neutrophils showed that HBP is stored in azurophilic granules and secretory vesicles but that it is also detected to a minor extent in the plasma membrane. These findings were confirmed by immunoelectron microscopy showing that HBP colocalized with marker proteins of azurophilic granules and secretory vesicles. The presence of HBP in secretory vesicles possibly depends on the stage of cell differentiation, since the promyelocytic cell line HL-60 contains less HBP than mature neutrophils, stored exclusively in the less easily mobilized azurophilic granules. Our findings suggest that HBP can be synthesized or targeted to easily mobilized compartments at a late stage of neutrophil maturation. The ability of neutrophils to secrete HBP from secretory vesicles may be important for proinflammatory functions of this protein, such as the alteration of vascular permeability.

Regulation of tyrosine kinase activation and granule release through beta-arrestin by CXCRI

Chemoattractant-stimulated granule release from neutrophils, basophils and eosinophils is critical for the innate immune response against infectious bacteria. Interleukin 8 (IL-8) activation of the chemokine receptor CXCRI was found to stimulate rapid formation of beta-arrestin complexes with Hck or c-Fgr. Formation of beta-arrestin-Hck complexes led to Hck activation and trafficking of the complexes to granule-rich regions. Granulocytes expressing a dominant-negative beta-arrestin-mutant did not release granules or activate tyrosine kinases after IL-8 stimulation. Thus, beta-arrestins regulate chemokine-induced granule exocytosis, indicating a broader role for beta-arrestins in the regulation of cellular functions than was previously suspected.

Induction of necrosis in human neutrophils by Shigella flexneri requires type III secretion, IpaB and IpaC invasins, and actin polymerization

Infection by Shigella flexneri is characterized by infiltration of neutrophils in the intestinal mucosa and by a strong inflammatory reaction. Although neutrophils are constitutively programmed to die by apoptosis, we show that isolated human neutrophils undergo necrosis 2 h after infection with virulent S. flexneri strain M90T but not with the virulence plasmid-cured strain BS176. This was demonstrated by the release of azurophil granule proteins concomitant with the release of lactate dehydrogenase (LDH), disruption of the plasma membrane, and absence of DNA fragmentation. Mutants with the mxiD1 gene, coding for an essential component of the secretion type III machinery, or the genes coding for IpaB or IpaC invasins deleted were not cytotoxic. Neutrophil necrosis occurred independently of the bacterial ability to leave phagosomes, and it involved actin polymerization, as the addition of cytochalasin D after phagocytosis of Shigella inhibited the release of LDH. In conclusion, Shigella kills neutrophils by necrosis, a process characterized by the release of tissue-injurious granular proteins. This probably contributes to disruption of the epithelial barrier, leading to the dysentery observed in shigellosis and allowing Shigella to enter its host cells.