Endotoxin inhibits apoptosis but induces primary necrosis in neutrophils

Delayed neutrophil apoptosis may enhance NET formation in ARDS

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a neutrophil-associated disease. Delayed neutrophil apoptosis and increased levels of neutrophil extracellular traps (NETs) have been described in ARDS. We aimed to investigate the relationship between these phenomena and their potential as inflammation drivers. We hypothesized that delayed neutrophil apoptosis might enhance NET formation in ARDS.

METHOD

Our research was carried out in three aspects: clinical research, animal experiments, and in vitro experiments. First, we compared the difference between neutrophil apoptosis and NET levels in healthy controls and patients with ARDS and analyzed the correlation between neutrophil apoptosis and NET levels in ARDS. Then, we conducted animal experiments to verify the effect of neutrophil apoptosis on NET formation in Lipopolysaccharide-induced acute lung injury (LPS-ALI) mice. Furthermore, this study explored the relationship between neutrophil apoptosis and NETs at the cellular level. Apoptosis was assessed using morphological analysis, flow cytometry, and western blotting. NET formation was determined using immunofluorescence, PicoGreen assay, SYTOX Green staining, and western blotting.

RESULTS

ARDS neutrophils lived longer because of delayed apoptosis, and the cyclin-dependent kinase inhibitor, AT7519, reversed this phenomenon both in ARDS neutrophils and neutrophils in bronchoalveolar lavage fluid (BALF) of LPS-ALI mice. Neutrophils in a medium containing pro-survival factors (LPS or GM-CSF) form more NETs, which can also be reversed by AT7519. Tissue damage can be reduced by promoting neutrophil apoptosis.

CONCLUSIONS

Neutrophils with extended lifespan in ARDS usually enhance NET formation, which aggravates inflammation. Enhancing neutrophil apoptosis in ARDS can reduce the formation of NETs, inhibit inflammation, and consequently alleviate ARDS.

Octyl gallate decrease lymphocyte activation and regulates neutrophil extracellular traps release

BACKGROUND

Inflammation is a complex mechanism with an objective to destroy and eliminate the invading microorganisms. During acute inflammation, the neutrophils are the major cells involved in this process and, although they defend the organism, must die to not generate damage. The two major mechanisms that drive neutrophils to death are: apoptosis and a novel mechanism recently discovered denominated NETosis. This process is a "suicidal mechanism", in which the cells release "neutrophil extracellular traps" (NETs) during the inflammatory response. Octyl gallate (OG) is one of the gallic acid derivates, with several protective effects, such as antioxidant and anti-inflammatory in cancer models. Thus, this study aimed to investigate the action of OG on the proliferation of lymphocytes, neutrophils activation, and its effectiveness in an experimental sepsis model.

METHODS

Lymphocytes and neutrophils were obtained from healthy donors. Cell viability, apoptosis, NETs release and antioxidant capacity of OG were observed. In addition, survival was evaluated in an experimental model of sepsis in C57BL/6 mice.

RESULTS

Our study demonstrated, for the first time, that the OG can act as an inhibitor of reactive oxygen species (ROS) release, NETs formation in primary human neutrophils and, modulates the lipopolysaccharide (LPS) effect in neutrophil apoptosis. The OG also inhibited peripheral blood mononuclear cells (PBMCs) proliferation in vitro. Despite the positive results, we did not observe an increase in the survival of septic animals.

CONCLUSIONS

The pharmacological potential of OG, modulating activation of neutrophils and lymphocytes, suggests the use as an adjuvant therapeutic strategy in inflammatory diseases.

IgG-aggregates rapidly upregulate FcgRI expression at the surface of human neutrophils in a FcgRII-dependent fashion: A crucial role for FcgRI in the generation of reactive oxygen species

Autoimmune complexes are an important feature of several autoimmune diseases such as lupus, as they contribute to tissue damage through the activation of immune cells. Neutrophils, key players in lupus, interact with immune complexes through Fc gamma receptors (FcgR). Incubation of neutrophils with aggregated-IgGs caused degranulation and increased the surface expression of FcgRI within minutes in a concentration-dependent fashion. After 30 minutes, IgG aggregates (1 mg/mL) upregulated FcgRI by 4.95 ± 0.45-fold. FcgRI-positive neutrophils reached 67.24% ± 6.88% on HA-IgGs stimulated neutrophils, from 3.12% ± 1.62% in non-stimulated cells, ranking IgG-aggregates among the most potent known agonists. FcgRIIa, and possibly FcgRIIIa, appeared to mediate this upregulation. Also, FcgRI-dependent signaling proved necessary for reactive oxygen species (ROS) production in response to IgG-aggregates. Finally, combinations of bacterial materials with aggregates dramatically boosted ROS production. This work suggests FcgRI as an essential component in the response of human neutrophils to immune complexes leading to the production of ROS, which may help explain how neutrophils contribute to tissue damage associated with immune complex-associated diseases, such as lupus.

Game of neutrophils: modeling the balance between apoptosis and necrosis

BACKGROUND

Neutrophils are one of the key players in the human innate immune system (HIIS). In the event of an insult where the body is exposed to inflammation triggering moieties (ITMs), neutrophils are mobilized towards the site of insult and antagonize the inflammation. If the inflammation is cleared, neutrophils go into a programmed death called apoptosis. However, if the insult is intense or persistent, neutrophils take on a violent death pathway called necrosis, which involves the rupture of their cytoplasmic content into the surrounding tissue that causes local tissue damage, thus further aggravating inflammation. This seemingly paradoxical phenomenon fuels the inflammatory process by triggering the recruitment of additional neutrophils to the site of inflammation, aimed to contribute to the complete neutralization of severe inflammation. This delicate balance between the cost and benefit of the neutrophils' choice of death pathway has been optimized during the evolution of the innate immune system. The goal of our work is to understand how the tradeoff between the cost and benefit of the different death pathways of neutrophils, in response to various levels of insults, has been optimized over evolutionary time by using the concepts of evolutionary game theory.

RESULTS

We show that by using evolutionary game theory, we are able to formulate a game that predicts the percentage of necrosis and apoptosis when exposed to various levels of insults.

CONCLUSION

By adopting an evolutionary perspective, we identify the driving mechanisms leading to the delicate balance between apoptosis and necrosis in neutrophils' cell death in response to different insults. Using our simple model, we verify that indeed, the global cost of remaining ITMs is the driving mechanism that reproduces the percentage of necrosis and apoptosis observed in data and neutrophils need sufficient information of the overall inflammation to be able to pick a death pathway that presumably increases the survival of the organism.

Effects of Pasteurella multocida lipopolysaccharides on bovine leukocytes

Lipopolysaccharide (LPS) is a major virulence factor of Gram-negative bacteria playing a major role in stimulating protective immune response in mammalian host. However, in many gram-negative bacterial infections, LPS also elicits immunopathology by inducing excessive inflammatory changes. P. multocida (Pm), a gram-negative bacterium, causes acute lung inflammation and fatal septicemic disease in animals. However, the effects of Pm LPS on host cells are little known. In this study, LPS isolated from three different serotypes (B:2, A:1 and A:3) of Pm were individually tested in vitro to assess the response of bovine leukocytes. Pm LPS induced cell proliferation and cell death of leukocytes, in a dose- and time-dependent manner. In these cells, mitochondrial dysfunction and caspase activation mediate cell death.

Impaired polymorphonuclear neutrophils in the oral cavity of edentulous individuals

Oral health is characterized by functional oral polymorphonuclear neutrophils (oPMNs). Edentulism might be associated with a loss of oPMNs because these cells enter the oral cavity primarily through the gingival crevices. The main aim of this study was to investigate the numbers of oPMNs in rinse samples obtained from edentulous (n = 21) and dentate (n = 20) subjects. A second study aim was to investigate possible differences between oPMNs and peripheral blood polymorphonuclear neutrophils (cPMNs). Apoptosis/necrosis and cell-activation markers (CD11b, CD63 and CD66b) were analyzed using flow cytometry. Reactive oxygen species (ROS) production was determined either without stimulation (constitutive) or in response to 10 μM phorbol myristate acetate or Fusobacterium nucleatum. The edentulous subjects presented with lower oPMN counts and higher percentages of apoptotic/necrotic oPMNs compared with dentate subjects. Furthermore, oPMNs from edentulous donors expressed low levels of all three activation markers and low constitutive ROS. In contrast, oPMNs from dentate subjects expressed high levels of all three activation markers and a higher level of constitutive ROS than cPMNs. When challenged, oPMNs from edentulous subjects showed no upregulation in ROS production, whereas oPMNs from dentate subjects retained their ability to respond to stimulation. The functional characteristics of cPMNs were comparable between edentulous and dentate subjects. This study demonstrates that despite having functional cPMNs, edentulous subjects have low oPMN numbers that are functionally impaired.

Nicotine can modulate the effects of the mesenchymal stem cells on neutrophils

PURPOSE

It has been revealed that mesenchymal stem cells (MSCs) express some of the nicotinic receptor subunits. Moreover, the crosstalk between MSCs and neutrophils is not far-fetched. Therefore, the aim of the present study is to determine the role of nicotine on the effects of MSCs on neutrophils.

METHODS

After the isolation of mesenchymal stem cells from the bone marrow of rats, these cells were pulsed with different concentrations of nicotine (0, 0.1, 0.5, and 1μM) for different periods (24, 48, and 72h). Then, the neutrophils were co-cultured with MSCs for 4h and the functions of neutrophils were evaluated.

RESULTS

The obtained findings showed that MSCs pulsed with nicotine significantly enhanced the viability and the phagocytic activity of co-cultured neutrophils and simultaneously, decreased the production of reactive oxygen substances (ROS), induced by f-MLP in neutrophils, more profound than MSCs without treatment. Moreover, MSCs, pulsed with nicotine at low to moderate concentrations, preserved the respiratory burst, triggered by opsonized yeast in neutrophils. Nevertheless, a high concentration of nicotine can interfere with the latter aspect of the crosstalk between MSCs and neutrophils.

CONCLUSION

The obtained data can offer a new insight into the potential mechanisms, underlying the immunomodulatory effects of nicotine.

Pathogenic bacterial species associated with endodontic infection evade innate immune control by disabling neutrophils

Endodontic infections, in which oral bacteria access the tooth pulp chamber, are common and do not resolve once established. To investigate the effects of these infections on the innate immune response, we established a mouse subcutaneous chamber model, where a mixture of four oral pathogens commonly associated with these infections (endodontic pathogens [EP]), i.e., Fusobacterium nucleatum, Streptococcus intermedius, Parvimonas micra, and Prevotella intermedia, was inoculated into subcutaneously implanted titanium chambers. Cells that infiltrated the chamber after these infections were primarily neutrophils; however, these neutrophils were unable to control the infection. Infection with a nonpathogenic oral bacterial species, Streptococcus mitis, resulted in well-controlled infection, with bacterial numbers reduced by 4 to 5 log units after 7 days. Propidium iodide (PI) staining of the chamber neutrophils identified three distinct populations: neutrophils from EP-infected chambers were intermediate in PI staining, while cells in chambers from mice infected with S. mitis were PI positive (apoptotic) or negative (live). Strikingly, neutrophils from EP-infected chambers were severely impaired in their ability to phagocytose and to generate reactive oxygen species in vitro after removal from the chamber compared to cells from S. mitis-infected chambers. The mechanism of neutrophil impairment was necrotic cell death as determined by morphological analyses. P. intermedia alone could induce a similar neutrophil phenotype. We conclude that the endodontic pathogens, particularly P. intermedia, can efficiently disable and kill infiltrating neutrophils, allowing these infections to become established. These results can help explain the persistence of endodontic infections and demonstrate a new virulence mechanism associated with P. intermedia.

Evaluation of neutrophil apoptosis in horses with acute abdominal disease

OBJECTIVE

To quantify peripheral blood neutrophil apoptosis in equine patients with acute abdominal disease (ie, colic) caused by strangulating or nonstrangulating intestinal lesions and compare these values with values for horses undergoing elective arthroscopic surgery.

ANIMALS

20 client-owned adult horses.

PROCEDURES

Peripheral blood was collected from horses immediately prior to and 24 hours after surgery for treatment of colic (n = 10) or elective arthroscopic surgery (10), and neutrophils were counted. Following isolation by means of a bilayer colloidal silica particle gradient and culture for 24 hours, the proportion of neutrophils in apoptosis was detected by flow cytometric evaluation of cells stained with annexin V and 7-aminoactinomycin D. Values were compared between the colic and arthroscopy groups; among horses with colic, values were further compared between horses with and without strangulating intestinal lesions.

RESULTS

Percentage recovery of neutrophils was significantly smaller in preoperative samples (median, 32.5%) and in all samples combined (35.5%) for the colic group, compared with the arthroscopy group (median, 66.5% and 58.0%, respectively). No significant differences in the percentages of apoptotic neutrophils were detected between these groups. Among horses with colic, those with strangulating intestinal lesions had a significantly lower proportion of circulating apoptotic neutrophils in postoperative samples (median, 18.0%) than did those with nonstrangulating lesions (66.3%).

CONCLUSIONS AND CLINICAL RELEVANCE

The smaller proportion of apoptotic neutrophils in horses with intestinal strangulation suggested that the inflammatory response could be greater or prolonged, compared with that of horses with nonstrangulating intestinal lesions. Further investigations are needed to better understand the relationship between neutrophil apoptosis and inflammation during intestinal injury.

Trehalose does not affect the functions of human neutrophils in vitro

PURPOSE

Trehalose, naturally occurring disaccharide, has been reported to prevent postoperative abdominal adhesions in animal models. We investigated whether trehalose affects the function of human polymorphonuclear neutrophils (PMNs) in vitro to assess the feasibility of its clinical application as an anti-adhesive barrier.

METHODS

Human PMNs were obtained from 17 healthy volunteers. Escherichia coli and Staphylococcus aureus were used for the bacterial infection model, whereas lipopolysaccharide (LPS) and interleukin (IL)-1β were used for inflammation induction model. The PMN phagocytosis rates of bacteria and apoptosis/necrosis were assessed on trehalose, maltose, and control media. Cytokines; namely, tumor necrosis factor-α, IL-1α, IL-1Ra, IL-6, and IL-8; and PMN-elastase were measured on each medium in both models.

RESULTS

There were no significant differences in the phagocytosis rates, apoptosis/necrosis rates, or levels of all cytokines or PMN-elastase among the three media in the bacterial infection model. There were also no significant differences in the levels of all cytokines and PMN-elastase among the three media in the IL-1β inflammation induction model. PMN-elastase was lower in trehalose and maltose medium after LPS stimulation, at 3 and 24 h.

CONCLUSIONS

Our results suggest that trehalose does not affect the cellular function, cytokine production, or release of PMN-elastase of human PMNs in an in vitro bacterial infection model.

Viability and cell death of synovial fluid neutrophils as diagnostic biomarkers in equine infectious joint disease: a pilot study

Synovial fluid samples from culture-confirmed infected joints (n=13), joints with pronounced non-infectious synovitis (n=11) and healthy joints (n=14) were collected from 24 equine patients and seven slaughterhouse horses. The samples from the joints with non-infectious synovitis and healthy joints served as negative controls. After isolation, counting and identification of neutrophils, the percentage viability, and the proportion apoptotic and necrotic neutrophils were determined by flow cytometry. Viability was significantly higher in infected samples compared to the controls. A significant difference in cell death type was observed, with apoptosis predominating in infected joints, and necrosis being more present in joints with pronounced non-infectious synovitis and healthy joints. The results of this pilot study suggest that flow cytometric analysis of neutrophil viability and cell death dynamics may assist the discrimination between infected and non-infected joints.

Acute lung injury: apoptosis in effector and target cells of the upper and lower airway compartment

Apoptotic cell death has been considered an underlying mechanism in acute lung injury. To evaluate the evidence of this process, apoptosis rate was determined in effector cells (alveolar macrophages, neutrophils) and target cells (tracheobronchial and alveolar epithelial cells) of the respiratory compartment upon exposure to hypoxia and endotoxin stimulation in vitro. Cells were exposed to 5% oxygen or incubated with lipopolysaccharide (LPS) for 4, 8 and 24 h, and activity of caspase-3, -8 and -9 was determined. Caspase-3 of alveolar macrophages was increased at all three time-points upon LPS stimulation, while hypoxia did not affect apoptosis rate at early time-points. In neutrophils, apoptosis was decreased in an early phase of hypoxia at 4 h. However, enhanced expression of caspase-3 activity was seen at 8 and 24 h. In the presence of LPS a decreased apoptosis rate was observed at 8 h compared to controls, while it was increased at 24 h. Tracheobronchial as well as alveolar epithelial cells experienced an enhanced caspase-3 activity upon LPS stimulation with no change of apoptosis rate under hypoxia. While increased apoptosis rate is triggered through an intrinsic and extrinsic pathway in alveolar macrophages, intrinsic signalling is activated in tracheobronchial epithelial cells. The exact pathway pattern in neutrophils and alveolar epithelial cells could not be determined. These data clearly demonstrate that upon injury each cell type experiences its own apoptosis pattern. Further experiments need to be performed to determine the functional role of these apoptotic processes in acute lung injury.

The human cationic host defense peptide LL-37 mediates contrasting effects on apoptotic pathways in different primary cells of the innate immune system

The human cathelicidin LL-37 is a cationic host defense peptide (antimicrobial peptide) expressed primarily by neutrophils and epithelial cells. This peptide, up-regulated under conditions of inflammation, has immunomodulatory and antimicrobial functions. We demonstrate that LL-37 is a potent inhibitor of human neutrophil apoptosis, signaling through P2X(7) receptors and G-protein-coupled receptors other than the formyl peptide receptor-like-1 molecule. This process involved modulation of Mcl-1 expression, inhibition of BID and procaspase-3 cleavage, and the activation of phosphatidylinositol-3 kinase but not the extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase pathway. In contrast to the inhibition of neutrophil apoptosis, LL-37 induced apoptosis in primary airway epithelial cells, demonstrating alternate consequences of LL-37-mediated modulation of apoptotic pathways in different human primary cells. We propose that these novel immunomodulatory properties of LL-37 contribute to peptide-mediated enhancement of innate host defenses against acute infection and are of considerable significance in the development of such peptides and their synthetic analogs as potential therapeutics for use against multiple antibiotic-resistant infectious diseases.