Nikolaus Rieber

Neutrophils: Between host defence, immune modulation, and tissue injury.

Neutrophils, the most abundant human immune cells, are rapidly recruited to sites of infection, where they fulfill their life-saving antimicrobial functions. While traditionally regarded as short-lived phagocytes, recent findings on long-term survival, neutrophil extracellular trap (NET) formation, heterogeneity and plasticity, suppressive functions, and tissue injury have expanded our understanding of their diverse role in infection and inflammation. This review summarises our current understanding of neutrophils in host-pathogen interactions and disease involvement, illustrating the versatility and plasticity of the neutrophil, moving between host defence, immune modulation, and tissue damage.

CXCR2 mediates NADPH oxidase-independent neutrophil extracellular trap formation in cystic fibrosis airway inflammation

Upon activation, neutrophils release DNA fibers decorated with antimicrobial proteins, forming neutrophil extracellular traps (NETs). Although NETs are bactericidal and contribute to innate host defense, excessive NET formation has been linked to the pathogenesis of autoinflammatory diseases. However, the mechanisms regulating NET formation, particularly during chronic inflammation, are poorly understood. Here we show that the G protein–coupled receptor (GPCR) CXCR2 mediates NET formation. Downstream analyses showed that CXCR2-mediated NET formation was independent of NADPH oxidase and involved Src family kinases. We show the pathophysiological relevance of this mechanism in cystic fibrosis lung disease, characterized by chronic neutrophilic inflammation. We found abundant NETs in airway fluids of individuals with cystic fibrosis and mouse cystic fibrosis lung disease, and NET amounts correlated with impaired obstructive lung function. Pulmonary blockade of CXCR2 by intra-airway delivery of small-molecule antagonists inhibited NET formation and improved lung function in vivo without affecting neutrophil recruitment, proteolytic activity or antibacterial host defense. These studies establish CXCR2 as a receptor mediating NADPH oxidase–independent NET formation and provide evidence that this GPCR pathway is operative and druggable in cystic fibrosis lung disease.

X-linked lymphoproliferative disease due to SAP/SH2D1A deficiency: a multicenter study on the manifestations, management and outcome of the disease

X-linked lymphoproliferative disease (XLP1) is a rare immunodeficiency characterized by severe immune dysregulation and caused by mutations in the SH2D1A/SAP gene. Clinical manifestations are varied and include hemophagocytic lymphohistiocytosis (HLH), lymphoma and dysgammaglobulinemia, often triggered by Epstein-Barr virus infection. Historical data published before improved treatment regimens shows very poor outcome. We describe a large cohort of 91 genetically defined XLP1 patients collected from centers worldwide and report characteristics and outcome data for 43 patients receiving hematopoietic stem cell transplant (HSCT) and 48 untransplanted patients. The advent of better treatment strategies for HLH and malignancy has greatly reduced mortality for these patients, but HLH still remains the most severe feature of XLP1. Survival after allogeneic HSCT is 81.4% with good immune reconstitution in the large majority of patients and little evidence of posttransplant lymphoproliferative disease. However, survival falls to 50% in patients with HLH as a feature of disease. Untransplanted patients have an overall survival of 62.5% with the majority on immunoglobulin replacement therapy, but the outcome for those untransplanted after HLH is extremely poor (18.8%). HSCT should be undertaken in all patients with HLH, because outcome without transplant is extremely poor. The outcome of HSCT for other manifestations of XLP1 is very good, and if HSCT is not undertaken immediately, patients must be monitored closely for evidence of disease progression.

Notch Signaling Induces Multilineage Myeloid Differentiation and Up-Regulates PU.1 Expression

Hemopoietic commitment is initiated by and depends on activation of transcription factors. However, it is unclear whether activation of lineage-affiliated transcription factors is extrinsically regulated by to date unknown agents or is the result of a cell autonomous program. Here we show that signaling by the Notch1 transmembrane receptor instructively induces myeloid differentiation of multipotent hemopoietic progenitor cells and concomitantly up-regulates the expression of the transcription factor PU.1. Transient activation of Notch1 signaling is sufficient to irreversibly reduce self-renewal of multipotent progenitor cells accompanied by increased and accelerated differentiation along the granulocyte, macrophage, and dendritic cell lineages. Activated Notch1 has no direct influence on apoptosis of multipotent progenitor cells, shows a weak inhibition of proliferation, and does not substitute for survival and proliferation signals provided by cytokines. Activated Notch1 directly increases PU.1 RNA levels, leading to a high concentration of PU.1 protein, which has been shown to direct myeloid differentiation. These findings identify Notch as an extrinsic regulator of myeloid commitment, and the lineage-affiliated transcription factor PU.1 as a specific direct target gene of Notch.

Diagnostic approach to the hyper-IgE syndromes: Immunologic and clinical key findings to differentiate hyper-IgE syndromes from atopic dermatitis

BACKGROUND Hyper-IgE syndromes (HIES) are primary immunodeficiency disorders characterized by Staphylococcus aureus abscesses, recurrent pneumonia, increased serum IgE levels, and eczema. The association of heterozygous signal transducer and activator of transcription 3 (STAT3) mutations with autosomal dominant (AD)-HIES allows the differentiation of AD-HIES from disorders associated with eczema and increased serum IgE levels, such as other primary immunodeficiencies and atopic dermatitis. OBJECTIVE To facilitate early diagnosis of AD-HIES to initiate appropriate therapy. METHODS The clinical phenotype (suggested by a National Institutes of Health [NIH] score of >or=40 points), STAT3 genotype, and T(H)17 cell counts were compared in a cohort of 78 patients suspected of having HIES. RESULTS Heterozygous STAT3 missense mutations and in-frame deletions were identified in 48 patients, all but 2 with an NIH score >or=40 points. Patients with STAT3 mutations with HIES showed significantly lower T(H)17 cell counts compared with patients with wild-type STAT3 and control subjects. Only 1 patient with wild-type STAT3 had both an NIH score >or=40 points and abnormal T(H)17 cell counts (<or=0.2% of CD4(+) cells), with this exception being identified with a homozygous dedicator of cytogenesis 8 protein (DOCK8) mutation. Pathologic shedding of primary teeth was present in 3 patients with wild-type STAT3 and 33 patients with STAT3 mutations. Internal abscesses and severe infections were exclusively seen in patients with STAT3 mutations, who also had increased pneumatocele formation and skeletal or connective tissue manifestations compared with patients with wild-type STAT3. CONCLUSION We expanded the number of STAT3 mutations and validated that the NIH score sensitively identifies patients with HIES. Based on our patient cohort, we propose key findings that, when combined with T(H)17 cell numbers, predict patients with AD-HIES with STAT3 mutations, supporting early diagnosis of AD-HIES.

Current Concepts of Hyperinflammation in Chronic Granulomatous Disease

Chronic granulomatous disease (CGD) is the most common inherited disorder of phagocytic functions, caused by genetic defects in the leukocyte nicotinamide dinucleotide phosphate (NADPH) oxidase. Consequently, CGD phagocytes are impaired in destroying phagocytosed microorganisms, rendering the patients susceptible to bacterial and fungal infections. Besides this immunodeficiency, CGD patients suffer from various autoinflammatory symptoms, such as granuloma formation in the skin or urinary tract and Crohn-like colitis. Owing to improved antimicrobial treatment strategies, the majority of CGD patients reaches adulthood, yet the autoinflammatory manifestations become more prominent by lack of causative treatment options. The underlying pathomechanisms driving hyperinflammatory reactions in CGD are poorly understood, but recent studies implicate reduced neutrophil apoptosis and efferocytosis, dysbalanced innate immune receptors, altered T-cell surface redox levels, induction of Th17 cells, the enzyme indolamine-2,3-dioxygenase (IDO), impaired Nrf2 activity, and inflammasome activation. Here we discuss immunological mechanisms of hyperinflammation and their potential therapeutic implications in CGD.

Flagellin Induces Myeloid-Derived Suppressor Cells: Implications for Pseudomonas aeruginosa Infection in Cystic Fibrosis Lung Disease

Pseudomonas aeruginosa persists in patients with cystic fibrosis (CF) and drives CF lung disease progression. P. aeruginosa potently activates the innate immune system, mainly mediated through pathogen-associated molecular patterns, such as flagellin. However, the host is unable to eradicate this flagellated bacterium efficiently. The underlying immunological mechanisms are incompletely understood. Myeloid-derived suppressor cells (MDSCs) are innate immune cells generated in cancer and proinflammatory microenvironments and are capable of suppressing T cell responses. We hypothesized that P. aeruginosa induces MDSCs to escape T cell immunity. In this article, we demonstrate that granulocytic MDSCs accumulate in CF patients chronically infected with P. aeruginosa and correlate with CF lung disease activity. Flagellated P. aeruginosa culture supernatants induced the generation of MDSCs, an effect that was 1) dose-dependently mimicked by purified flagellin protein, 2) significantly reduced using flagellin-deficient P. aeruginosa bacteria, and 3) corresponded to TLR5 expression on MDSCs in vitro and in vivo. Both purified flagellin and flagellated P. aeruginosa induced an MDSC phenotype distinct from that of the previously described MDSC-inducing cytokine GM-CSF, characterized by an upregulation of the chemokine receptor CXCR4 on the surface of MDSCs. Functionally, P. aeruginosa–infected CF patient ex vivo–isolated as well as flagellin or P. aeruginosa in vitro–generated MDSCs efficiently suppressed polyclonal T cell proliferation in a dose-dependent manner and modulated Th17 responses. These studies demonstrate that flagellin induces the generation of MDSCs and suggest that P. aeruginosa uses this mechanism to undermine T cell–mediated host defense in CF and other P. aeruginosa–associated chronic lung diseases.

Localization and Functionality of the Inflammasome in Neutrophils

Background: The inflammasome generates IL-1 family proteins, but its role in neutrophils is poorly understood. Results: Neutrophils store key inflammasome components in distinct intracellular compartments and release IL-1β and IL-18, but not IL-1α or IL-33. Conclusion: Neutrophils store inflammasome components in intracellular compartments. Significance: Targeting the inflammasome in neutrophils represents a future anti-inflammatory strategy. Neutrophils represent the major fraction of circulating immune cells and are rapidly recruited to sites of infection and inflammation. The inflammasome is a multiprotein complex that regulates the generation of IL-1 family proteins. The precise subcellular localization and functionality of the inflammasome in human neutrophils are poorly defined. Here we demonstrate that highly purified human neutrophils express key components of the NOD-like receptor family, pyrin domain containing 3 (NLRP3), and absent in melanoma 2 (AIM2) inflammasomes, particularly apoptosis-associated speck-like protein containing a CARD (ASC), AIM2, and caspase-1. Subcellular fractionation and microscopic analyses further showed that inflammasome components were localized in the cytoplasm and also noncanonically in secretory vesicle and tertiary granule compartments. Whereas IL-1β and IL-18 were expressed at the mRNA level and released as protein, highly purified neutrophils neither expressed nor released IL-1α at baseline or upon stimulation. Upon inflammasome activation, highly purified neutrophils released substantially lower levels of IL-1β protein compared with partially purified neutrophils. Serine proteases and caspases were differentially involved in IL-1β release, depending on the stimulus. Spontaneous activation of the NLRP3 inflammasome in neutrophils in vivo affected IL-1β, but not IL-18 release. In summary, these studies show that human neutrophils express key components of the inflammasome machinery in distinct intracellular compartments and release IL-1β and IL-18, but not IL-1α or IL-33 protein. Targeting the neutrophil inflammasome may represent a future therapeutic strategy to modulate neutrophilic inflammatory diseases, such as cystic fibrosis, rheumatoid arthritis, or sepsis.

Neutrophilic myeloid-derived suppressor cells in cord blood modulate innate and adaptive immune responses

Neonates show an impaired anti‐microbial host defence, but the underlying immune mechanisms are not understood fully. Myeloid‐derived suppressor cells (MDSCs) represent an innate immune cell subset characterized by their capacity to suppress T cell immunity. In this study we demonstrate that a distinct MDSC subset with a neutrophilic/granulocytic phenotype (Gr‐MDSCs) is highly increased in cord blood compared to peripheral blood of children and adults. Functionally, cord blood isolated Gr‐MDSCs suppressed T cell proliferation efficiently as well as T helper type 1 (Th1), Th2 and Th17 cytokine secretion. Beyond T cells, cord blood Gr‐MDSCs controlled natural killer (NK) cell cytotoxicity in a cell contact‐dependent manner. These studies establish neutrophilic Gr‐MDSCs as a novel immunosuppressive cell subset that controls innate (NK) and adaptive (T cell) immune responses in neonates. Increased MDSC activity in cord blood might serve as key fetomaternal immunosuppressive mechanism impairing neonatal host defence. Gr‐MDSCs in cord blood might therefore represent a therapeutic target in neonatal infections.

Granulocytic myeloid derived suppressor cells expand in human pregnancy and modulate T-cell responses

Immune tolerance toward the semiallogeneic fetus plays a crucial role in the maintenance of pregnancy. Myeloid‐derived suppressor cells (MDSCs) are innate immune cells characterized by their ability to modulate T‐cell responses. Recently, we showed that MDSCs accumulate in cord blood of healthy newborns, yet their role in materno–fetal tolerance remained elusive. In the present study, we demonstrate that MDSCs with a granulocytic phenotype (GR‐MDSCs) are highly increased in the peripheral blood of healthy pregnant women during all stages of pregnancy compared with nonpregnant controls, whereas numbers of monocytic MDSCs were unchanged. GR‐MDSCs expressed the effector enzymes arginase‐I and iNOS, produced high amounts of ROS and efficiently suppressed T‐cell proliferation. After parturition, GR‐MDSCs decreased within a few days. In combination, our results show that GR‐MDSCs expand in normal human pregnancy and may indicate a role for MDSCs in materno–fetal tolerance.