Neutrophil apoptosis during viral infections

Phenotypic alteration by dengue virus serotype 2 delays neutrophil apoptosis and stimulates the release of prosurvival secretome with immunomodulatory functions

Neutrophils are the most abundant granuloytes, are phenotypically heterogeneous, and exert detrimental or protective roles during antiviral response. Dengue virus has been reported to activate neutrophils. However, the effect of the dengue virus on the neutrophil phenotypes, survival, and release of inflammatory secretome is yet to be understood. Herein, we investigated the effect of dengue virus serotype 2 (DV-2) on effector functions of naïve neutrophils and studied the impact of its secretome on different immune cells. We found that DV-2 activates purified human neutrophils and causes a significant shift toward the CD16bright/CD62Ldim subtype in a multiplicity of infection and time-dependent manner. These phenotypically altered neutrophils show delayed apoptosis through nuclear factor κB and PI3K pathways and have decreased phagocytic capacity. Treatment of neutrophils with myeloperoxidase and PAD4 inhibitor before DV-2 incubation significantly reduced DV-2-induced double-stranded DNA release, suggesting that myeloperoxidase and PAD4 were involved at early stages for the neutrophil activation and double-stranded DNA release. We also report that DV-2-stimulated neutrophil secretome had a significant effect on viral infection, platelet activation, and naïve neutrophil survival via binding of tumor necrosis factor α to tumor necrosis factor receptor 1/2 receptors. Furthermore, incubation of endothelial cells with the DV-2-stimulated neutrophil secretome potentially inhibits proliferation and wound healing capacity and induces endothelial cell death, which can contribute to endothelial barrier dysfunction. In conclusion, the neutrophil-DV-2 interaction modulates the phenotype of neutrophils and the release of prosurvival and antiviral secretome that may act as a double-edged sword during dengue pathogenesis.

The Effect of Centella asiatica Methanolic Extract on Expression of IL-1β Proinflammatory Cytokines in Severe Early Childhood Caries

OBJECTIVE

 This article analyzes the role of C. asiatica extract in reducing the proinflammatory cytokine interleukin (IL)-1β produced by salivary neutrophils.

MATERIAL AND METHODS

 Selected kindergartens in the Surabaya area provided samples. The sample was initially checked for dental caries by measuring its def-t index, and then the participants who satisfied the requirements for severe caries with a def-t of greater than 6 were chosen. At the time of sampling, all of the individuals were between the ages of 4 and 6. The sampling was performed by researchers and certified persons using well-known methodologies. For 60 minutes before to sampling, respondents were not allowed to eat, drink, chew gum, or brush their teeth. For analysis, the samples were collected and then frozen at -80°C.

RESULTS

 The administration of methanolic extract C. asiatica decreased the expression of the proinflammatory cytokine IL-1β on the surface of salivary neutrophils on S-ECC; The administration of C. asiatica methanol extract resulted in a decrease in the expression of the proinflammatory cytokine IL-1β on the surface of salivary neutrophils in S-ECC.

CONCLUSION

 C. asiatica extract has the effect of reducing the proinflammatory cytokine IL-1β produced by salivary neutrophils on S-ECC via inhibiting nuclear factor kappa B and mitogen-activated protein kinase signaling pathway activation and suggest that C. asiatica is a possible candidate for treating S-ECC.

B-cell lymphoma-2 family proteins-activated proteases as potential therapeutic targets for influenza A virus and severe acute respiratory syndrome coronavirus-2: Killing two birds with one stone?

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has led to a global health emergency. There are many similarities between SARS-CoV-2 and influenza A virus (IAV); both are single-stranded RNA viruses infecting airway epithelial cells and have similar modes of replication and transmission. Like IAVs, SARS-CoV-2 infections poses serious challenges due to the lack of effective therapeutic interventions, frequent appearances of new strains of the virus, and development of drug resistance. New approaches to control these infectious agents may stem from cellular factors or pathways that directly or indirectly interact with viral proteins to enhance or inhibit virus replication. One of the emerging concepts is that host cellular factors and pathways are required for maintaining viral genome integrity, which is essential for viral replication. Although IAVs have been studied for several years and many cellular proteins involved in their replication and pathogenesis have been identified, very little is known about how SARS-CoV-2 hijacks host cellular proteins to promote their replication. IAV induces apoptotic cell death, mediated by the B-cell lymphoma-2 (Bcl-2) family proteins in infected epithelia, and the pro-apoptotic members of this family promotes viral replication by activating host cell proteases. This review compares the life cycle and mode of replication of IAV and SARS-CoV-2 and examines the potential roles of host cellular proteins, belonging to the Bcl-2 family, in SARS-CoV-2 replication to provide future research directions.

Reactivity of selected markers of innate and adaptive immunity in rabbits experimentally infected with antigenic variants of RHD (Lagovirus europaeus/GI.1a)

Lagovirus europaeus/GI.1 causes a fatal viral condition in rabbits characterized by acute viral hepatitis and disseminated intravascular coagulation. Due to rapid viral and environmental changes variants (Lagovirus europaeus/GI.1a and GI.2) have appeared and few immunological studies were performed. The aim of the study was to determine innate and adaptive immunity parameters in rabbits infected with six Lagovirus europeus/GI.1a viruses. To achieve the goal several methods were used, i.e. cytometry, microscopy, biochemical and cytochemical tests, spectrophotometry. The results show that three immunotypes exists among the studied strains and they differ in innate (mainly) and adaptive immunity, partly depending on hemagglutination. The peak of changes is 24 h post infection in phagocytosis markers of polymorphonuclear cells and CD8⁺ T cells. Lagovirus europaeus/GI.1a strains differ from Lagovirus europaeus/GI.1 in terms of immunological response based on our previous work concerning the same parameters in immunological response against this disease.

On Deep Landscape Exploration of COVID-19 Patients Cells and Severity Markers

COVID-19 is a disease with a spectrum of clinical responses ranging from moderate to critical. To study and control its effects, a large number of researchers are focused on two substantial aims. On the one hand, the discovery of diverse biomarkers to classify and potentially anticipate the disease severity of patients. These biomarkers could serve as a medical criterion to prioritize attention to those patients with higher prone to severe responses. On the other hand, understanding how the immune system orchestrates its responses in this spectrum of disease severities is a fundamental issue required to design new and optimized therapeutic strategies. In this work, using single-cell RNAseq of bronchoalveolar lavage fluid of nine patients with COVID-19 and three healthy controls, we contribute to both aspects. First, we presented computational supervised machine-learning models with high accuracy in classifying the disease severity (moderate and severe) in patients with COVID-19 starting from single-cell data from bronchoalveolar lavage fluid. Second, we identified regulatory mechanisms from the heterogeneous cell populations in the lungs microenvironment that correlated with different clinical responses. Given the results, patients with moderate COVID-19 symptoms showed an activation/inactivation profile for their analyzed cells leading to a sequential and innocuous immune response. In comparison, severe patients might be promoting cytotoxic and pro-inflammatory responses in a systemic fashion involving epithelial and immune cells without the possibility to develop viral clearance and immune memory. Consequently, we present an in-depth landscape analysis of how transcriptional factors and pathways from these heterogeneous populations can regulate their expression to promote or restrain an effective immune response directly linked to the patients prognosis.

H3K4me3 Histone ChIP-Seq Analysis Reveals Molecular Mechanisms Responsible for Neutrophil Dysfunction in HIV-Infected Individuals

Peripheral neutrophils in HIV-infected individuals are characterized by impairment of chemotaxis, phagocytosis, bactericidal activity, and oxidative burst ability regardless of whether patients are receiving antiretroviral therapy or not. Neutrophil dysfunction leads not only to increased susceptibility to opportunistic infections but also to tissue damage through the release of reactive oxygen species (ROS), proteases, and other potentially harmful effector molecules contributing to AIDS progression. In this study, we demonstrated high levels of histone H3 lysine K4 trimethylated (H3K4me3) and dysregulation of DNA transcription in circulating neutrophils of HIV-infected subjects. This dysregulation was accompanied by a deficient response of neutrophils to LPS, impaired cytokine/chemokine/growth factor synthesis, and increased apoptosis. Chromatin immunoprecipitation sequencing (ChIPseq) H3K4me3 histone analysis revealed that the most spectacular abnormalities were observed in the exons, introns, and promoter-TSS regions. Bioinformatic analysis of Gene Ontology, including biological processes, molecular function, and cellular components, demonstrated that the main changes were related to the genes responsible for cell activation, cytokine production, adhesive molecule expression, histone remodeling via upregulation of methyltransferase process, and downregulation of NF-κB transcription factor in canonical pathways. Abnormalities within H3K4me3 implicated LPS-mediated NF-κB canonical activation pathway that was a result of low amounts of κB DNA sites within histone H3K4me3, low NF-κB (p65 RelA) and TLR4 mRNA expression, and reduced free NF-κB (p65 RelA) accumulation in the nucleus. Genome-wide survey of H3K4me3 provided evidence that chromatin modifications lead to an impairment within the canonical NF-κB cell activation pathway causing the neutrophil dysfunction observed in HIV-infected individuals.

Interactions of HIV and Antiretroviral Therapy With Neutrophils and Platelets

Neutrophils are important components of the innate immune system that mediate pathogen defense by multiple processes including phagocytosis, release of proteolytic enzymes, production of reactive oxygen species, and neutrophil extracellular trap formation. Abnormalities of neutrophil count and function have been described in the setting of HIV infection, with the majority of antiretroviral agents (ARVs), excluding zidovudine, having been reported to correct neutropenia. Questions still remain, however, about their impact on neutrophil function, particularly the possibility of persistent neutrophil activation, which could predispose people living with HIV to chronic inflammatory disorders, even in the presence of virally-suppressive treatment. In this context, the effects of protease inhibitors and integrase strand transfer inhibitors, in particular, on neutrophil function remain poorly understood and deserve further study. Besides mediating hemostatic functions, platelets are increasingly recognized as critical role players in the immune response against infection. In the setting of HIV, these cells have been found to harbor the virus, even in the presence of antiretroviral therapy (ART) potentially promoting viral dissemination. While HIV-infected individuals often present with thrombocytopenia, they have also been reported to have increased platelet activation, as measured by an upregulation of expression of CD62P (P-selectin), CD40 ligand, glycoprotein IV, and RANTES. Despite ART-mediated viral suppression, HIV-infected individuals reportedly have sustained platelet activation and dysfunction. This, in turn, contributes to persistent immune activation and an inflammatory vascular environment, seemingly involving neutrophil-platelet-endothelium interactions that increase the risk for development of comorbidities such as cardiovascular disease (CVD) that has become the leading cause of morbidity and mortality in HIV-infected individuals on treatment, clearly underscoring the importance of unraveling the possible etiologic roles of ARVs. In this context, abacavir and ritonavir-boosted lopinavir and darunavir have all been linked to an increased risk of CVD. This narrative review is therefore focused primarily on the role of neutrophils and platelets in HIV transmission and disease, as well as on the effect of HIV and the most common ARVs on the numbers and functions of these cells, including neutrophil-platelet-endothelial interactions.

15-Deoxy-△12,14-Prostaglandin J2 Promotes Resolution of Experimentally Induced Colitis

Uncontrolled macrophage functions cause failure to resolve gut inflammation and has been implicated in the pathogenesis of inflammatory bowel disease (IBD). 15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), one of endogenous lipid mediators formed from arachidonic acid during the inflammatory process, has been reported to terminate inflammation. However, the pro-resolving effect of 15d-PGJ₂ on intestinal inflammation and underlying molecular mechanisms remain largely unknown. In the present study, we examined the effects of 15d-PGJ₂ on the resolution of dextran sulfate sodium (DSS)-induced murine colitis that mimics human IBD. Pharmacologic inhibition of prostaglandin D synthase (PGDS) responsible for the synthesis of 15d-PGJ₂ hampered resolution of inflammation in the colonic mucosa of mice treated with DSS. Notably, intraperitoneal injection of 15d-PGJ₂ accelerated the resolution of experimentally induced colitis. 15d-PGJ₂ treatment reduced the number of neutrophils and M1 macrophages, while it increased the proportion of M2 macrophages. Moreover, 15d-PGJ₂ treated mice exhibited the significantly reduced proportion of macrophages expressing the pro-inflammatory cytokine, IL-6 with concomitant suppression of STAT3 phosphorylation in the colonic mucosa of mice administered 2.5% DSS in drinking water. Taken together, these findings clearly indicate that 15d-PGJ₂, endogenously generated from arachidonic acid by cyclooxygenase-2 and PGDS activities in inflamed tissue, promotes resolution of intestinal colitis.

CD89/CD35 Expression Ratio in Salivary Neutrophil as an Early Detection Marker for Severe Early Childhood Caries

Objectives  To analyze CD35/CD89 expression ratio on the surface of neutrophils as an early detection marker for S-ECC.

Materials and Methods  Saliva was collected from 4- to 6-year-old kindergarten students. Salivary neutrophils were obtained by instructing the subjects to rinse their mouth with 1 mL of sterile 1.5% NaCl for 30 seconds before expectorating it into a sterile glass. The expression of CFSE ⁺ CD35 ⁺ and CFSE ⁺ CD89 ⁺ was measured and analyzed using flow cytometry.

Results The expression of CFSE ⁺ CD89 ⁺ in the caries-free group (2.46 ± 0.39) was significantly lower than that in the S-ECC group (3.41 ± 1.11), with a p -value of 0.0001, while the expression of CFSE ⁺ CD35 ⁺ in the caries-free group was (2.35 ± 0.56) compared with (1.54 ± 0.35) ( p = 0.0001) in the S-ECC group.

Conclusions  The expression ratio of CFSE ⁺ CD89 ⁺ and CFSE ⁺ CD35 ⁺ constitutes a marker for S-ECC.

The roles of apoptosis, autophagy and unfolded protein response in arbovirus, influenza virus, and HIV infections

Virus infection induces different cellular responses in infected cells. These include cellular stress responses like autophagy and unfolded protein response (UPR). Both autophagy and UPR are connected to programed cell death I (apoptosis) in chronic stress conditions to regulate cellular homeostasis via Bcl2 family proteins, CHOP and Beclin-1. In this review article we first briefly discuss arboviruses, influenza virus, and HIV and then describe the concepts of apoptosis, autophagy, and UPR. Finally, we focus upon how apoptosis, autophagy, and UPR are involved in the regulation of cellular responses to arboviruses, influenza virus and HIV infections. Abbreviation: AIDS: Acquired Immunodeficiency Syndrome; ATF6: Activating Transcription Factor 6; ATG6: Autophagy-specific Gene 6; BAG3: BCL Associated Athanogene 3; Bak: BCL-2-Anatagonist/Killer1; Bax; BCL-2: Associated X protein; Bcl-2: B cell Lymphoma 2x; BiP: Chaperon immunoglobulin heavy chain binding Protein; CARD: Caspase Recruitment Domain; cART: combination Antiretroviral Therapy; CCR5: C-C Chemokine Receptor type 5; CD4: Cluster of Differentiation 4; CHOP: C/EBP homologous protein; CXCR4: C-X-C Chemokine Receptor Type 4; Cyto c: Cytochrome C; DCs: Dendritic Cells; EDEM1: ER-degradation enhancing-a-mannosidase-like protein 1; ENV: Envelope; ER: Endoplasmic Reticulum; FasR: Fas Receptor;G2: Gap 2; G2/M: Gap2/Mitosis; GFAP: Glial Fibrillary Acidic Protein; GP120: Glycoprotein120; GP41: Glycoprotein41; HAND: HIV Associated Neurodegenerative Disease; HEK: Human Embryonic Kidney; HeLa: Human Cervical Epithelial Carcinoma; HIV: Human Immunodeficiency Virus; IPS-1: IFN-β promoter stimulator 1; IRE-1: Inositol Requiring Enzyme 1; IRGM: Immunity Related GTPase Family M protein; LAMP2A: Lysosome Associated Membrane Protein 2A; LC3: Microtubule Associated Light Chain 3; MDA5: Melanoma Differentiation Associated gene 5; MEF: Mouse Embryonic Fibroblast; MMP: Mitochondrial Membrane Permeabilization; Nef: Negative Regulatory Factor; OASIS: Old Astrocyte Specifically Induced Substrate; PAMP: Pathogen-Associated Molecular Pattern; PERK: Pancreatic Endoplasmic Reticulum Kinase; PRR: Pattern Recognition Receptor; Puma: P53 Upregulated Modulator of Apoptosis; RIG-I: Retinoic acid-Inducible Gene-I; Tat: Transactivator Protein of HIV; TLR: Toll-like receptor; ULK1: Unc51 Like Autophagy Activating Kinase 1; UPR: Unfolded Protein Response; Vpr: Viral Protein Regulatory; XBP1: X-Box Binding Protein 1.

Complement Receptor 1 (CR1)/ CD35+ Expression Analysis of Salivary Neutrophils on Streptococcus mutans Phagocytosis

BACKGROUND

Severe early childhood caries (S-ECC) is a form of dental caries which is very destructive in early childhood since involving several teeth, include the maxillary anterior teeth. Streptococcus mutans (S. mutans) play an etiological integral role of ECC so that S. mutans are considered as the predictor of dental caries. The neutrophil is a key component of the rst line of defense against microbial invasion. The essential function of neutrophil is to kill pathogenic microbes through a phagocytosis process which is mediated by Complement Receptor 1 (CR1)/ (CD35+).

AIMS AND OBJECTIVES

To analyze the phagocytosis process of the salivary neutrophil which is mediated by innate immunity component, i.e., Complement Receptor 1/CR1 (CD35) on S-ECC.

MATERIALS AND METHODS

his study was an observational analysis with cross-sectional approach using t-test analysis. This study employed the isolation steps of neutrophils saliva of caries-free children and the S-ECC and then conducted phagocytosis of salivary neutrophils test on S. mutans mediated by CD35 using ow cytometry.

RESULTS

Phagocytosis of salivary neutrophils on S. mutans mediated by CD35 on caries-free (2.35 ± 0.56) is higher than that on the S-ECC (1.54 ± 0.35).

CONCLUSIONS

It is concluded that there is a decrease of phagocytic on S. mutans mediated by Complement Receptor 1/CR1 (CD35+) on S-ECC.

A Subset of Type I Conventional Dendritic Cells Controls Cutaneous Bacterial Infections through VEGFα-Mediated Recruitment of Neutrophils

Skin conventional dendritic cells (cDCs) exist as two distinct subsets, cDC1s and cDC2s, which maintain the balance of immunity to pathogens and tolerance to self and microbiota. Here, we examined the roles of dermal cDC1s and cDC2s during bacterial infection, notably Propionibacterium acnes (P. acnes). cDC1s, but not cDC2s, regulated the magnitude of the immune response to P. acnes in the murine dermis by controlling neutrophil recruitment to the inflamed site and survival and function therein. Single-cell mRNA sequencing revealed that this regulation relied on secretion of the cytokine vascular endothelial growth factor α (VEGF-α) by a minor subset of activated EpCAM+CD59+Ly-6D+ cDC1s. Neutrophil recruitment by dermal cDC1s was also observed during S. aureus, bacillus Calmette-Guérin (BCG), or E. coli infection, as well as in a model of bacterial insult in human skin. Thus, skin cDC1s are essential regulators of the innate response in cutaneous immunity and have roles beyond classical antigen presentation.

Innate immune cell responses in non pathogenic versus pathogenic SIV infections

HIV-1/SIVmac infections deeply disturb innate host responses. Most studies have focused on the impact on dendritic cells and NK cells. A few but insufficient data are available on other innate immune cell types, such as neutrophils. It has been shown that innate lymphoid cells are depleted early and irreversibly during SIVmac/HIV-1 infections. Studies in natural hosts of SIV have contributed to pinpoint that early control of inflammation is crucial. In natural hosts, plasmacytoid dendritic cells, myeloid dendritic cells and NK cells are depleted during acute infection but return to normal levels by the end of acute infection. We summarize here the similarities and differences of various types of innate immune responses in natural hosts compared to pathogenic HIV/SIV mac infections.

Dengue Virus Serotype-2 Interferes with the Formation of Neutrophil Extracellular Traps

OBJECTIVES

Neutrophils play an important role in the control of pathogens through several mechanisms, including phagocytosis and the formation of neutrophil extracellular traps (NETs). The latter consists of DNA as a backbone with embedded antimicrobial peptides, histones, and proteases, providing a matrix to entrap and in some cases to kill microbes. Some metabolic requirements for NET formation have recently been described. The virus-induced formation of NETs and the role of these traps in viral infections remain scarcely reported. Here, we analyzed whether dengue virus serotype-2 (DENV-2) induces NET formation and the DENV-2 effect on phorbol myristate acetate (PMA)-induced NETs.

METHODS

Peripheral blood-derived neutrophils were exposed in vitro to DENV-2 or exposed to DENV-2 and then stimulated with PMA. NET formation was assessed by fluorescence microscopy. Cell membrane Glut-1, glucose uptake, and reactive oxygen species (ROS) production were assessed.

RESULTS

DENV-2 does not induce the formation of NETs. Moreover, DENV-2 inhibits PMA-induced formation of NETs by about 80%. This effect is not related to the production of ROS. The mechanism seemingly accountable for this inhibitory effect is the DENV-2-mediated inhibition of PMA-induced glucose uptake by neutrophils.

CONCLUSION

Our results suggest that DENV-2 inhibits glucose uptake as a metabolism-based way to avoid the formation of NETs.

Targeting neutrophil apoptosis for enhancing the resolution of inflammation

Resolution of acute inflammation is an active process that requires inhibition of further leukocyte recruitment and removal of leukocytes from inflamed sites. Emigrated neutrophils undergo apoptosis before being removed by scavenger macrophages. Recent studies using a variety of gene knockout, transgenic and pharmacological strategies in diverse models of inflammation established neutrophil apoptosis as a critical control point in resolving inflammation. Analysis of death mechanisms revealed distinct features in executing the death program in neutrophils, which can be exploited as targets for controlling the lifespan of neutrophils. Indeed, anti-inflammatory and pro-resolution lipid mediators derived from essential fatty acids, such as lipoxin A₄ and resolvin E1, autacoids and proteins, such as annexin A1 and TRAIL, and cyclin-dependent kinase inhibitors, can enhance the resolution of inflammation through induction of neutrophil apoptosis and promoting their removal by efferocytosis. In this review, we discuss recent advances in understanding the molecular basis of these actions, highlighting the potential of therapeutic induction of neutrophil apoptosis for dampening neutrophil-mediated tissue injury and inflammation underlying a variety of diseases.

Modulation of Neutrophil Apoptosis and the Resolution of Inflammation through β2 Integrins

Precise control of the neutrophil death program provides a balance between their defense functions and safe clearance, whereas impaired regulation of neutrophil death is thought to contribute to a wide range of inflammatory pathologies. Apoptosis is essential for neutrophil functional shutdown, removal of emigrated neutrophils, and timely resolution of inflammation. Neutrophils receive survival and pro-apoptosis cues from the inflammatory microenvironment and integrate these signals through surface receptors and common downstream mechanisms. Among these receptors are the leukocyte-specific membrane receptors β2 integrins that are best known for regulating adhesion and phagocytosis. Accumulating evidence indicate that outside-in signaling through the β2 integrin Mac-1 can generate contrasting cues in neutrophils, leading to promotion of their survival or apoptosis. Binding of Mac-1 to its ligands ICAM-1, fibrinogen, or the azurophilic granule enzyme myeloperoxidase suppresses apoptosis, whereas Mac-1-mediated phagocytosis of bacteria evokes apoptotic cell death. Mac-1 signaling is also target for the anti-inflammatory, pro-resolving mediators, including lipoxin A4, aspirin-triggered lipoxin A4, and resolvin E1. This review focuses on molecular mechanisms underlying Mac-1 regulation of neutrophil apoptosis and highlights recent advances how hierarchy of survival and pro-apoptosis signals can be harnessed to facilitate neutrophil apoptosis and the resolution of inflammation.

Neutrophil in viral infections, friend or foe?

Polymorphonuclear leukocytes or neutrophils are the first immune cells to the site of injury and microbial infection. Neutrophils are crucial players in controlling bacterial and fungal infections, and in particular secondary infections, by phagocytosis, degranulation and neutrophil extracellular traps (NETs). While neutrophils have been shown to play important roles in viral pathogenesis, there is a lack of detailed investigation. In this article, we will review recent progresses toward understanding the role of neutrophils in viral pathogenesis.

Towards a four-dimensional view of neutrophils

Neutrophils are constitutively produced throughout adult life and are essential for host responses to many types of pathogen. Neutropenia has long been associated with poor prognosis in the clinic, yet we have an incomplete understanding of their life cycle, not only during homeostasis but also during infection and chronic inflammation. Here, we review recent advances that provide insight into the genetic and biochemical regulators of neutrophil production, function, and survival.

Regulation of neutrophil survival/apoptosis by Mcl-1

Neutrophil granulocytes have the shortest lifespan among leukocytes in the circulation and die via apoptosis. At sites of infection or tissue injury, prolongation of neutrophil lifespan is critical for effective host defense. Apoptosis of inflammatory neutrophils and their clearance are critical control points for termination of the inflammatory response. Evasion of neutrophil apoptosis aggravates local injury and leads to persistent tissue damage. The short-lived prosurvival Bcl-2 family protein, Mcl-1 (myeloid cell leukemia-1), is instrumental in controlling apoptosis and consequently neutrophil lifespan in response to rapidly changing environmental cues during inflammation. This paper will focus on multiple levels of control of Mcl-1 expression and function and will discuss targeting Mcl-1 as a potential therapeutic strategy to enhance the resolution of inflammation through accelerating neutrophil apoptosis.

Fas-mediated neutrophil apoptosis is accelerated by Bid, Bak, and Bax and inhibited by Bcl-2 and Mcl-1

During immune responses, neutrophils must integrate survival and death signals from multiple sources to regulate their lifespan. Signals that activate either the Bcl-2- or death receptor-regulated apoptosis pathways can provide powerful stimuli for neutrophils to undergo cell death, but whether they act cooperatively in parallel or directly cross-talk in neutrophils is not known. Previous studies suggested that Bcl-2 family proteins are not required for Fas-induced cell death in neutrophils, but did not examine whether they could modulate its rapid onset. By monitoring the rate of change in neutrophil viability associated with activation of the Fas-triggered death receptor pathway using real-time cell imaging, we show that the Bcl-2-related proteins Bid, Bax, and Bak accelerate neutrophil apoptosis but are not essential for cell death. Increased Bcl-2 or Mcl-1 expression prevents efficient induction of apoptosis by Fas stimulation indicating that the Bcl-2-regulated apoptosis pathway can directly interfere with Fas-triggered apoptosis. Fas has been shown to initiate NFκB activation and gene transcription in cell lines, however gene transcription is not altered in Fas-activated Bid(-/-) neutrophils, indicating that apoptosis occurs independently of gene transcription in neutrophils. The specification of kinetics of neutrophil apoptosis by Bid impacts on the magnitude of neutrophil IL-1β production, implicating a functional role for the Bcl-2-regulated pathway in controlling neutrophil responses to FasL. These data demonstrate that the intrinsic apoptosis pathway directly controls the kinetics of Fas-triggered apoptosis in neutrophils.

Neutrophil Apoptosis in Neutropenic Patients With Hepatitis C Infection: Role of Caspases 3, 10, and GM-CSF

Patients with chronic HCV infection are prone to increased susceptibility bacterial infection due to neutropenia complicating the course of this disease. Neutropenia in those patients may stem from enhanced neutrophil apoptosis. However, the molecular mechanism of neutrophil apoptosis has not been clearly defined. Neutrophils harvested from 26 neutropenic patients with hepatitis C infection and nine age and sex-matched healthy control subjects were examined for the degree of apoptosis. Neutrophil apoptosis was quantified by flow cytometry through determination of annexin-V expression at 0 time (fresh neutrophil), and 24 h culture. Neutrophils from healthy subjects were also incubated with either 10% heterologous normal or neutropenic sera, with and without 10 µg GM-CSF. Caspases 3, 10 were assessed colormetrically in neutrophils at 0 times and after 24 h culture. At 0 time culture the neutrophil apoptosis of the HCV patients was in significantly higher as compared to that of normal control (P = 0.059). At 24 h culture patients neutrophils cultured with neutropenic patients own sera showed neutrophil apoptosis significantly increased as compared to that at 0 time culture and this effect was significantly attenuated in similar culture with addition of GM-CSF (P < 0.001). On the other hand patient's neutrophil cultured with normal sera showed insignificantly increased neutrophil apoptosis at 24 h culture as compared to that at 0 time culture. Caspases 3 and 10 activities were significantly higher in patients neutrophil after 24 h cultured with patients own sera as compared to 0 time culture (P < 0.001 for both). Addition of GM-CSF to the neutrophil culture down regulates the caspases 3 and 10 activities. The correlation study between annexin-V expression and caspases activities revealed a borderline positive correlation between annexin-V and caspase 3 (r = 0.376, P = 0.058), and significant positive correlation with caspase 10 activity (r = 0.494, P = 0.01). In conclusion, these findings suggest that enhanced neutrophil apoptosis demonstrated in neutropenic patients with HCV infection might be induced through activation of caspase 10 and is attenuated by GM-CSF.

Protective role of Akt2 in Salmonella enterica serovar typhimurium-induced gastroenterocolitis

The Salmonella effector protein SopB has previously been shown to induce activation of Akt and protect epithelial cells from apoptosis in vitro. To characterize the role of Akt2 in host defense against Salmonella enterica serovar Typhimurium infection, wild-type (WT) mice and mice lacking Akt2 (Akt2 knockout [KO] mice) were infected using a Salmonella acute gastroenteritis model. Infected Akt2 KO mice showed a more pronounced morbidity and mortality associated with higher bacterial loads in the intestines and elevated levels of proinflammatory cytokines, including tumor necrosis factor alpha (TNF-α), gamma interferon (IFN-γ), and MCP-1, in the colons at 1 day postinfection compared to those shown in WT mice. Histopathological assessment and immunohistochemical analysis of cecal sections at 1 day postinfection revealed more severe inflammation and higher levels of neutrophil infiltration in the ceca of Akt2 KO mice. Flow cytometry analysis further confirmed an increase in the recruitment of Gr-1(+) CD11b(+) neutrophils and F4/80(+) CD11b(+) macrophages in the intestines of infected Akt2 KO mice. Additionally, enhanced levels of annexin V(+) and terminal transferase dUTP nick end labeling-positive (TUNEL(+)) apoptotic cells in the intestines of infected Akt2 KO mice were also observed, indicating that Akt2 plays an essential role in protection against apoptosis. Finally, the differences in bacterial loads and cecal inflammation in WT and Akt2 KO mice infected with WT Salmonella were abolished when these mice were infected with the sopB deletion mutant, indicating that SopB may play a role in protecting the mice from Salmonella infection through the activation of Akt2. These data demonstrate a definitive phenotypic abnormality in the innate response in mice lacking Akt2, underscoring the important protective role of Akt2 in Salmonella infection.

Neutrophil apoptosis: relevance to the innate immune response and inflammatory disease

Neutrophils are the most abundant cell type involved in the innate immune response. They are rapidly recruited to sites of injury or infection where they engulf and kill invading microorganisms. Neutrophil apoptosis, the process of programmed cell death that prevents the release of neutrophil histotoxic contents, is tightly regulated and limits the destructive capacity of neutrophil products to surrounding tissue. The subsequent recognition and phagocytosis of apoptotic cells by phagocytic cells such as macrophages is central to the successful resolution of an inflammatory response and it is increasingly apparent that the dying neutrophil itself exerts an anti-inflammatory effect through modulation of surrounding cell responses, particularly macrophage inflammatory cytokine release. Apoptosis may be delayed, induced or enhanced by micro-organisms dependent on their immune evasion strategies and the health of the host they encounter. There is now an established field of research aimed at understanding the regulation of apoptosis and its potential as a target for therapeutic intervention in inflammatory and infective diseases. This review focuses on the physiological regulation of neutrophil apoptosis with respect to the innate immune system and highlights recent advances in mechanistic understanding of apoptotic pathways and their therapeutic manipulation in appropriate and excessive innate immune responses.