Determination of the critical concentration of neutrophils required to block bacterial growth in tissues

Model-based assessment of neutrophil-mediated phagocytosis and digestion of bacteria across in vitro and in vivo studies

Neutrophil granulocytes are key components of the host response against pathogens, and severe neutropenia, with neutrophil counts below 0.5 × 106 cells/mL, renders patients increasingly vulnerable to infections. Published in vitro (n = 7) and in vivo (n = 5) studies with time-course information on bacterial and neutrophil counts were digitized to characterize the kinetics of neutrophil-mediated bacterial killing and inform on the immune systems' contribution to the clearance of bacterial infections. A mathematical model for the in vitro dynamics of bacteria and the kinetics of neutrophil-mediated phagocytosis and digestion was developed, which was extended to in vivo studies in immune-competent and immune-compromised mice. Neutrophil-mediated bacterial killing was described by two first-order processes-phagocytosis and digestion-scaled by neutrophil concentration, where 50% of the maximum was achieved at neutrophil counts of 1.19 × 106 cells/mL (phagocytosis) and 6.55 × 106 cells/mL (digestion). The process efficiencies diminished as the phagocytosed bacteria to total neutrophils ratio increased (with 50% reduction at a ratio of 3.41). Neutrophil in vivo dynamics were captured through the characterization of myelosuppressive drug effects and postinoculation neutrophil influx into lungs and by system differences (27% bacterial growth and 9.3% maximum capacity, compared with in vitro estimates). Predictions showed how the therapeutically induced reduction of neutrophil counts enabled bacterial growth, especially when falling below 0.5 × 106 cells/mL, whereas control individuals could deal with all tested bacterial burdens (up to 109  colony forming units/g lung). The model-based characterization of neutrophil-mediated bacterial killing simultaneously predicted data across in vitro and in vivo studies and may be used to inform the capacity of host-response at the individual level.

Innate Immune Mechanism of Neutrophil Extracellular Trap Formation is Impaired in at-Risk Term Low Birth Weight Newborns

Low birth weight (LBW) is a leading cause of newborn's mortality however the underlying defects in cellular immunity and immune mechanisms leading to severe neonatal infections in term LBW (tLBW) newborns are not well understood. Neutrophil extracellular traps (NETs), or NETosis, is an innate immune defense mechanism of neutrophils involved in trapping and killing of microbes. The efficiency of NET formation in cord blood derived neutrophils of tLBW and normal birth weight (NBW) newborns in the presence of toll like receptor (TLR) agonist inductions was evaluated. The NET formation was observed to be substantially impaired in tLBW newborns along with NET proteins expression, extracellular deoxyribonucleic acid (DNA) release and reactive oxygen species generation. The placental tissues from tLBW newborns delivery also showed minimal NETosis. These findings suggest impaired NET formation to be an important factor underlying the deficient immune status of tLBW newborns making them susceptible to life- threatening infections.

Human neutrophil IL1β directs intestinal epithelial cell extrusion during Salmonella infection

Infection of the human gut by Salmonella enterica Typhimurium (STM) results in a localized inflammatory disease that is not mimicked in murine infections. To determine mechanisms by which neutrophils, as early responders to bacterial challenge, direct inflammatory programming of human intestinal epithelium, we established a multi-component human intestinal organoid (HIO) model of STM infection. HIOs were micro-injected with STM and seeded with primary human polymorphonuclear leukocytes (PMN-HIOs). PMNs did not significantly alter luminal colonization of Salmonella, but their presence reduced intraepithelial bacterial burden. Adding PMNs to infected HIOs resulted in substantial accumulation of shed TUNEL⁺ epithelial cells that was driven by PMN Caspase-1 activity. Inhibition of Caspases-1, -3 or -4 abrogated epithelial cell death and extrusion in the infected PMN-HIOs but only Caspase-1 inhibition significantly increased bacterial burden in the PMN-HIO epithelium. Thus, PMNs promote cell death in human intestinal epithelial cells through multiple caspases as a protective response to infection. IL-1β was necessary and sufficient to induce cell shedding in the infected HIOs. These data support a critical innate immune function for human neutrophils in amplifying cell death and extrusion of human epithelial cells from the Salmonella-infected intestinal monolayer.

Neutrophils are early responders to Salmonella intestinal infection, but how they influence infection progression and outcome is unknown. Here we use a co-culture model of human intestinal organoids and human primary neutrophils to study the contribution of human neutrophils to Salmonella infection of the intestinal epithelium. We found that neutrophils markedly enhanced epithelial defenses, including enhancing cell extrusion to reduce intraepithelial burden of Salmonella and close association with the epithelium. These findings reveal an early role for neutrophils in the gut in shaping the gut environment to control epithelial infection.

Neutrophils Orchestrate the Periodontal Pocket

The subgingival biofilm attached to tooth surfaces triggers and maintains periodontitis. Previously, late-onset periodontitis has been considered a consequence of dysbiosis and a resultant polymicrobial disruption of host homeostasis. However, a multitude of studies did not show "healthy" oral microbiota pattern, but a high diversity depending on culture, diets, regional differences, age, social state etc. These findings relativise the aetiological role of the dysbiosis in periodontitis. Furthermore, many late-onset periodontitis traits cannot be explained by dysbiosis; e.g. age-relatedness, attenuation by anti-ageing therapy, neutrophil hyper-responsiveness, and microbiota shifting by dysregulated immunity, yet point to the crucial role of dysregulated immunity and neutrophils in particular. Furthermore, patients with neutropenia and neutrophil defects inevitably develop early-onset periodontitis. Intra-gingivally injecting lipopolysaccharide (LPS) alone causes an exaggerated neutrophil response sufficient to precipitate experimental periodontitis. Vice versa to the surplus of LPS, the increased neutrophil responsiveness characteristic for late-onset periodontitis can effectuate gingiva damage likewise. The exaggerated neutrophil extracellular trap (NET) response in late-onset periodontitis is blameable for damage of gingival barrier, its penetration by bacteria and pathogen-associated molecular patterns (PAMPs) as well as stimulation of Th17 cells, resulting in further neutrophil activation. This identifies the dysregulated immunity as the main contributor to periodontal disease.

Dichotomous roles of neutrophils in modulating pathogenic and repair processes of inflammatory bowel diseases

Neutrophils are considered as complex innate immune cells and play a critical role in maintaining intestinal mucosal homeostasis. They exert robust pro-inflammatory effects and recruit other immune cells in the acute phase of pathogen infection and intestinal inflammation, but paradoxically, they also limit exogenous microbial invasion and facilitate mucosal restoration. Hyperactivation or dysfunction of neutrophils results in abnormal immune responses, leading to multiple autoimmune and inflammatory diseases including systemic lupus erythematosus, rheumatoid arthritis, and inflammatory bowel diseases (IBD). As a refractory intestinal inflammatory disease, the pathogenesis and progression of IBD are associated with complicated immune response processes in which neutrophils are profoundly involved. However, the consensus on potential roles of neutrophils in modulating pathogenic and repair processes of IBD remains not fully understood. Accumulated infiltrating neutrophils cross the epithelial barrier and contribute to microbial dysbiosis, aggravated intestinal architectural damage, compromised resolution of intestinal inflammation and increased risk of thrombosis during IBD. Paradoxically, activated neutrophils are also associated with effective elimination of invaded microbiota, promoted angiogenesis and tissue restoration of gut mucosa in IBD. Here, we discuss the beneficial and detrimental roles of neutrophils in the onset and resolution of intestinal mucosal inflammation, hoping to provide a precise overview of neutrophil functions in the pathogenesis of IBD.

Blocking ADAM17 Function with a Monoclonal Antibody Improves Sepsis Survival in a Murine Model of Polymicrobial Sepsis

Sepsis is the culmination of hyperinflammation and immune suppression in response to severe infection. Neutrophils are critical early responders to bacterial infection but can become highly dysfunctional during sepsis and other inflammatory disorders. The transmembrane protease ADAM17 (a disintegrin and metalloproteinase 17) is expressed by leukocytes and most other cells and has many substrates that regulate inflammation. We have reported that conditional knockout mice lacking ADAM17 in all leukocytes had a survival advantage during sepsis, which was associated with improved neutrophil effector functions. These and other findings indicate aberrant ADAM17 activity during sepsis. For this study, we evaluated for the first time the effects of an ADAM17 function blocking monoclonal antibody (mAb) on the pathogenesis of polymicrobial sepsis. Mice treated with the ADAM17 mAb MEDI3622 prior to sepsis induction exhibited significantly decreased mortality. When the ADAM17 mAb was combined with antibiotic administration, sepsis survival was markedly enhanced compared to either intervention alone, which was associated with a significant reduction in plasma levels of various inflammation-related factors. MEDI3622 and antibiotic administration after sepsis induction also significantly improved survival. Our results indicate that the combination of blocking ADAM17 as an immune modulator and appropriate antibiotics may provide a new therapeutic avenue for sepsis treatment.

New automated analysis to monitor neutrophil function point-of-care in the intensive care unit after trauma

BACKGROUND

Patients often develop infectious complications after severe trauma. No biomarkers exist that enable early identification of patients who are at risk. Neutrophils are important immune cells that combat these infections by phagocytosis and killing of pathogens. Analysis of neutrophil function used to be laborious and was therefore not applicable in routine diagnostics. Hence, we developed a quick and point-of-care method to assess a critical part of neutrophil function, neutrophil phagosomal acidification. The aim of this study was to investigate whether this method was able to analyze neutrophil functionality in severely injured patients and whether a relation with the development of infectious complications was present.

RESULTS

Fifteen severely injured patients (median ISS of 33) were included, of whom 6 developed an infection between day 4 and day 9 after trauma. The injury severity score did not significantly differ between patients who developed an infection and patients who did not (p = 0.529). Patients who developed an infection showed increased acidification immediately after trauma (p = 0.006) and after 3 days (p = 0.026) and a decrease in the days thereafter to levels in the lower normal range. In contrast, patients who did not develop infectious complications showed high-normal acidification within the first days and increased tasset to identify patients at risk for infections after trauma and to monitor the inflammatory state of these trauma patients.

CONCLUSION

Neutrophil function can be measured in the ICU setting by rapid point-of-care analysis of phagosomal acidification. This analysis differed between trauma patients who developed infectious complications and trauma patients who did not. Therefore, this assay might prove a valuable asset to identify patients at risk for infections after trauma and to monitor the inflammatory state of these trauma patients.

TRIAL REGISTRATION

Central Committee on Research Involving Human Subjects, NL43279.041.13. Registered 14 February 2014. https://www.toetsingonline.nl/to/ccmo_search.nsf/Searchform?OpenForm.

Differential antibacterial control by neutrophil subsets

Neutrophils comprise a heterogeneous population of cells essential for bacterial eradication, and defects in neutrophil function are associated with increased susceptibility to infection. In this study, neutrophils from healthy controls were shown to prevent bacterial proliferation for at least 48 hours when cocultured with methicillin-resistant Staphylococcus aureus (MRSA) in tissue-like scaffolds by establishing a bacteriostatic environment inside their phagolysosome. This intracellular bacterial containment is independent of reactive oxygen species because neutrophils that lack a functional nicotinamide adenine dinucleotide phosphate-oxidase complex displayed no defect in intracellular bacterial containment, whereas killing of the pathogen was impaired. During acute inflammation, a subset of CD16^bright/CD62L^dim hypersegmented neutrophils displayed normal phagocytosis associated with a remarkably poor capacity to contain bacteria intracellularly. Conversely, CD16^dim-banded neutrophils were the only neutrophil subset that adequately contained MRSA. These findings demonstrate a clear neutrophil heterogeneity in their antimicrobial capacity and the appearance of neutrophil subsets with a clear differentiation in functionality during acute inflammation. Furthermore, this study provides an evolutionary basis for the rapid release of banded neutrophils into the circulation during acute inflammation.

The role of neutrophils in the pathogenesis of Crohn's disease

Crohn's disease (CD) is caused by a trigger, almost certainly enteric infection by one of a multitude of organisms that allows faeces access to the tissues, at which stage the response of individuals predisposed to CD is abnormal. In CD the failure of acute inflammation results in the failure to recruit neutrophils to the inflammatory site, as a consequence of which the clearance of bacteria from the tissues is defective. The retained faecal products result in the characteristic chronic granulomatous inflammation and adaptive immune response. Impaired of digestion of bacteria and fungi by CGD neutrophils can result in a similar pathological and clinical picture. The neutrophils in CD are normal and their inadequate accumulation at sites of inflammation generally results from diminished secretion of proinflammatory cytokines by macrophages consequent upon disordered vesicle trafficking.

Neutrophils use superoxide to control bacterial infection at a distance

Understanding the roles of neutrophils and macrophages in fighting bacterial infections is a critical issue in human pathologies. Although phagocytic killing has been extensively studied, little is known about how bacteria are eliminated extracellularly in live vertebrates. We have recently developed an infection model in the zebrafish embryo in which leukocytes cannot reach the injected bacteria. When Escherichia coli bacteria are injected within the notochord, both neutrophils and macrophages are massively recruited during several days, but do not infiltrate the infected tissue presumably because of its tough collagen sheath. Nevertheless, the bacteria are killed during the first 24 hours, and we report here that neutrophils, but not macrophages are involved in the control of the infection. Using genetic and chemical approaches, we show that even in absence of phagocytosis, the bactericidal action relies on NADPH oxidase-dependent production of superoxide in neutrophils. We thus reveal a host effector mechanism mediated by neutrophils that eliminates bacteria that cannot be reached by phagocytes and that is independent of macrophages, NO synthase or myeloperoxidase.

Deciphering the defence mechanisms of leukocytes remains a challenge for public health. Although phagocytic killing has been extensively studied, little is known about how bacteria are eliminated extracellularly in live vertebrates. Herein we use the notochord infection model in the zebrafish embryo to describe how leukocytes eliminate distant bacteria that are inaccessible for phagocytosis. In this context neutrophils but not macrophages are instrumental for bacterial clearance and larva survival. We then found that neutrophil bactericidal action relies on the NADPH oxidase dependent production of superoxide and is independent of NO synthase or myeloperoxidase.

Pharmacologic inhibition of MEK1/2 reduces lung inflammation without impairing bacterial clearance in experimental Pseudomonas aeruginosa pneumonia

This study was designed to test the therapeutic potential of a MEK1/2 inhibitor (MEKi) in an experimental model of Pseudomonas aeruginosa pneumonia. The study found that treatment with MEKi reduced alveolar neutrophilic inflammation and led to faster recovery of weight compared to carrier-treated mice, without impairing bacterial clearance. Alveolar macrophages isolated from MEKi-treated mice also had increased M2 gene and protein expression, supporting the concept that MEKi modulates in vivo macrophage inflammatory responses. In summary, this report demonstrates the potential of MEKi to promote the resolution of inflammation in vivo during a primary lung infection without impairing bacterial clearance.

Efficiency of riboflavin and ultraviolet light treatment against high levels of biofilm-derived Staphylococcus epidermidis in buffy coat platelet concentrates

BACKGROUND AND OBJECTIVES

Staphylococcus epidermidis forms surface-attached aggregates (biofilms) in platelet concentrates (PCs), which are linked to missed detection during PC screening. This study was aimed at evaluating the efficacy of riboflavin-UV treatment to inactivate S. epidermidis biofilms in buffy coat (BC) PCs.

MATERIALS AND METHODS

Biofilm and non-biofilm cells from S. epidermidis ST-10002 and S. epidermidis AZ-66 were individually inoculated into whole blood (WB) units (~10⁶ colony-forming units (CFU)/ml) (N = 4-5). One spiked and three unspiked WB units were processed to produce a BC-PC pool. Riboflavin was added to the pool which was then split into two bags: one for UV treatment and the second was untreated. Bacterial counts were determined before and after treatment. In vitro PC quality was assessed by flow cytometry and dynamic light scattering.

RESULTS

Bacterial counts were reduced during BC-PC production from ~10⁶ CFU/ml in WB to 10³ -10⁴ CFU/ml in PCs (P < 0·0001). Riboflavin-UV treatment resulted in significantly higher reduction of S. epidermidis AZ-66 than strain ST-10002 (≥3·5 log reduction and 2·6-2·8 log reduction, respectively, P < 0·0001). Remaining bacteria post-treatment were able to proliferate in PCs. No differences in S. epidermidis inactivation were observed in PCs produced from WB inoculated with biofilm or non-biofilm cells (P > 0·05). Platelet activation was enhanced in PCs produced with WB inoculated with biofilms compared to non-biofilm cells (P < 0·05).

CONCLUSION

Riboflavin-UV treatment was similarly efficacious in PCs produced from WB inoculated with S. epidermidis biofilm or non-biofilm cells. Levels of biofilm-derived S. epidermidis ≥10³ CFU/ml were not completely inactivated; however, further testing is necessary with lower (real-life) bacterial levels.

Mathematical Modeling of Streptococcus pneumoniae Colonization, Invasive Infection and Treatment

Streptococcus pneumoniae (Sp) is a commensal bacterium that normally resides on the upper airway epithelium without causing infection. However, factors such as co-infection with influenza virus can impair the complex Sp-host interactions and the subsequent development of many life-threatening infectious and inflammatory diseases, including pneumonia, meningitis or even sepsis. With the increased threat of Sp infection due to the emergence of new antibiotic resistant Sp strains, there is an urgent need for better treatment strategies that effectively prevent progression of disease triggered by Sp infection, minimizing the use of antibiotics. The complexity of the host-pathogen interactions has left the full understanding of underlying mechanisms of Sp-triggered pathogenesis as a challenge, despite its critical importance in the identification of effective treatments. To achieve a systems-level and quantitative understanding of the complex and dynamically-changing host-Sp interactions, here we developed a mechanistic mathematical model describing dynamic interplays between Sp, immune cells, and epithelial tissues, where the host-pathogen interactions initiate. The model serves as a mathematical framework that coherently explains various in vitro and in vitro studies, to which the model parameters were fitted. Our model simulations reproduced the robust homeostatic Sp-host interaction, as well as three qualitatively different pathogenic behaviors: immunological scarring, invasive infection and their combination. Parameter sensitivity and bifurcation analyses of the model identified the processes that are responsible for qualitative transitions from healthy to such pathological behaviors. Our model also predicted that the onset of invasive infection occurs within less than 2 days from transient Sp challenges. This prediction provides arguments in favor of the use of vaccinations, since adaptive immune responses cannot be developed de novo in such a short time. We further designed optimal treatment strategies, with minimal strengths and minimal durations of antibiotics, for each of the three pathogenic behaviors distinguished by our model. The proposed mathematical framework will help to design better disease management strategies and new diagnostic markers that can be used to inform the most appropriate patient-specific treatment options.

CFTR targeting during activation of human neutrophils

Cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel, plays critical roles in phagocytic host defense. However, how activated neutrophils regulate CFTR channel distribution subcellularly is not well defined. To investigate, we tested multiple Abs against different CFTR domains, to examine CFTR expression in human peripheral blood neutrophils by flow cytometry. The data confirmed that resting neutrophils had pronounced CFTR expression. Activation of neutrophils with soluble or particulate agonists did not significantly increase CFTR expression level, but induced CFTR redistribution to cell surface. Such CFTR mobilization correlated with cell-surface recruitment of formyl-peptide receptor during secretory vesicle exocytosis. Intriguingly, neutrophils from patients with ΔF508-CF, despite expression of the mutant CFTR, showed little cell-surface mobilization upon stimulation. Although normal neutrophils effectively targeted CFTR to their phagosomes, ΔF508-CF neutrophils had impairment in that process, resulting in deficient hypochlorous acid production. Taken together, activated neutrophils regulate CFTR distribution by targeting this chloride channel to the subcellular sites of activation, and ΔF508-CF neutrophils fail to achieve such targeting, thus undermining their host defense function.

Hypoxic regulation of neutrophil function and consequences for Staphylococcus aureus infection

Staphylococcal infection and neutrophilic inflammation can act in concert to establish a profoundly hypoxic environment. In this review we summarise how neutrophils and Staphylococcus aureus are adapted to function under hypoxic conditions, with a particular focus on the impaired ability of hypoxic neutrophils to effect Staphylococcus aureus killing.

Making sense of the cause of Crohn's - a new look at an old disease

The cause of Crohn’s disease (CD) has posed a conundrum for at least a century. A large body of work coupled with recent technological advances in genome research have at last started to provide some of the answers. Initially this review seeks to explain and to differentiate between bowel inflammation in the primary immunodeficiencies that generally lead to very early onset diffuse bowel inflammation in humans and in animal models, and the real syndrome of CD. In the latter, a trigger, almost certainly enteric infection by one of a multitude of organisms, allows the faeces access to the tissues, at which stage the response of individuals predisposed to CD is abnormal. Direct investigation of patients’ inflammatory response together with genome-wide association studies (GWAS) and DNA sequencing indicate that in CD the failure of acute inflammation and the clearance of bacteria from the tissues, and from within cells, is defective. The retained faecal products result in the characteristic chronic granulomatous inflammation and adaptive immune response. In this review I will examine the contemporary evidence that has led to this understanding, and look for explanations for the recent dramatic increase in the incidence of this disease.

Making sense of the cause of Crohn's - a new look at an old disease

The cause of Crohn's disease (CD) has posed a conundrum for at least a century. A large body of work coupled with recent technological advances in genome research have at last started to provide some of the answers. Initially this review seeks to explain and to differentiate between bowel inflammation in the primary immunodeficiencies that generally lead to very early onset diffuse bowel inflammation in humans and in animal models, and the real syndrome of CD. In the latter, a trigger, almost certainly enteric infection by one of a multitude of organisms, allows the faeces access to the tissues, at which stage the response of individuals predisposed to CD is abnormal. Direct investigation of patients' inflammatory response together with genome-wide association studies (GWAS) and DNA sequencing indicate that in CD the failure of acute inflammation and the clearance of bacteria from the tissues, and from within cells, is defective. The retained faecal products result in the characteristic chronic granulomatous inflammation and adaptive immune response. In this review I will examine the contemporary evidence that has led to this understanding, and look for explanations for the recent dramatic increase in the incidence of this disease.

Comparison of capillary and venous blood in the analysis of concentration and function of leucocyte sub-populations

PURPOSE

Compare capillary and venous blood in the analysis of concentration and function of leucocyte sub-populations. This study hypothesised that capillary samples may be used in a site-specific manner as an alternative source of blood samples for assays of leucocyte concentration and neutrophilic phagocytic function and reactive oxygen species (ROS) production, allowing acquisition of multiple samples to better monitor transient but significant post-exercise immune modulation.

METHODS

Resting blood samples were simultaneously obtained from vein, finger and earlobe of healthy subjects (n = 10, age: 25.1 ± 3.1 years). Leucocyte concentrations were measured using a five-part differential haematological analyser. Leucocyte sub-populations (CD3, CD4, CD8, CD19, CD56, CD14) and granulocytic functional-related (CD11b, CD18, CD16b, CD66b) surface antigen markers, neutrophil phagocytosis (FITC-labelled Escherichia coli) and stimulated ROS production (DHR) were quantified utilizing flow cytometry. A MANOVA (α < 0.05 significance) analysed the effects of the different sampling sites in the concentrations of leucocyte populations, their surface antigen expression and granulocytic functions.

RESULTS

Leucocyte concentration and neutrophilic ROS production yielded non-significant differences between sampling sites. Expression of granulocytic surface antigens was increased in both capillary sites compared to venous site (p = 0.008), particularly for adhesion markers CD11b/CD18. The percentage of neutrophils performing phagocytosis was higher in venous samples compared to finger (p = 0.025). Increased number of E. coli ingested was observed in venous sample compared to finger (p = 0.001) and to earlobe (p = 0.006).

CONCLUSION

Whilst attention must be paid for varying neutrophilic surface antigen expression and further studies are needed to establish appropriate reference ranges, this study supports the use of capillary blood samples in a site-specific manner to enhance sampling capabilities field-based research.

Neutrophils and inflammatory metabolism in antimicrobial functions of the mucosa

In this mini-review, we will discuss recent findings that implicate neutrophil infiltration and function in establishing a metabolic environment to facilitate efficient pathogen clearance. For decades, neutrophils have been regarded as short lived, nonspecific granulocytes, equipped with toxic antimicrobial factors and a respiratory burst generating ROS. Recent findings demonstrate the importance of HIF signaling in leukocytes and surrounding tissues during inflammation. Here, we will review the potential mechanisms and outcomes of HIF stabilization within the intestinal mucosa.

Characterization of a novel mouse model with genetic deletion of CD177

Neutrophils play an essential role in the innate immune response to infection. Neutrophils migrate from the vasculature into the tissue in response to infection. Recently, a neutrophil cell surface receptor, CD177, was shown to help mediate neutrophil migration across the endothelium through interactions with PECAM1. We examined a publicly available gene array dataset of CD177 expression from human neutrophils following pulmonary endotoxin instillation. Among all 22,214 genes examined, CD177 mRNA was the most upregulated following endotoxin exposure. The high level of CD177 expression is also maintained in airspace neutrophils, suggesting a potential involvement of CD177 in neutrophil infiltration under infectious diseases. To determine the role of CD177 in neutrophils in vivo, we constructed a CD177-genetic knockout mouse model. The mice with homozygous deletion of CD177 have no discernible phenotype and no significant change in immune cells, other than decreased neutrophil counts in peripheral blood. We examined the role of CD177 in neutrophil accumulation using a skin infection model with Staphylococcus aureus. CD177 deletion reduced neutrophil counts in inflammatory skin caused by S. aureus. Mechanistically we found that CD177 deletion in mouse neutrophils has no significant impact in CXCL1/KC- or fMLP-induced migration, but led to significant cell death. Herein we established a novel genetic mouse model to study the role of CD177 and found that CD177 plays an important role in neutrophils.

M1 and M2 macrophages: the chicken and the egg of immunity

The purpose of this perspective is to describe a critical advance in understanding how immune responses work. Macrophages are required for all animal life: 'Inhibit' type macrophages in all animals (called M1) can rapidly kill pathogens, and are thus the primary host defense, and 'Heal' type macrophages (M2) routinely repair and maintain tissue integrity. Macrophages perform these activities in all animals without T cells, and also in T cell-deficient vertebrates. Although adaptive immunity can amplify macrophage polarization, the long-held notion that macrophages need to be 'activated' or 'alternatively activated' by T cells is incorrect; indeed, immunology has had it backward. M1/M2-type macrophages necessarily direct T cells toward Th1- or Th2-like activities, respectively. That such macrophage-innate activities are the central directing element in immune responses is a dramatic change in understanding how immune systems operate. Most important, this revelation is opening up whole new approaches to immunotherapy. For example, many modern diseases, such as cancer and atherosclerosis, may not display 'foreign' antigens. However, there are clear imbalances in M1/M2-type responses. Correcting such innate imbalances can result in better health. Macrophages are the chicken and the egg of immunity.

Pathogen-related differences in the abundance of presented antigen are reflected in CD4+ T cell dynamic behavior and effector function in the lung

Exposure to pathogens in the periphery elicits effector T cell differentiation in local lymph nodes followed by migration of activated T cells to and within the infected site. However, the relationships among pathogen abundance, Ag display on MHC molecules, effector T cell dynamics, and functional responses at the infected sites are incompletely characterized. In this study, we compared CD4(+) T cell effector dynamics and responses during pulmonary mycobacterial infection versus acute influenza infection. Two-photon imaging together with in situ as well as ex vivo analysis of cytokine production revealed that the proportion of migration-arrested, cytokine-producing effector T cells was dramatically higher in the influenza-infected lungs due to substantial differences in Ag abundance in the two infectious states. Despite the marked inflammatory conditions associated with influenza infection, histocytometric analysis showed that cytokine production was focal, with a restriction to areas of significant Ag burden. Optimal effector function is thus constrained by the availability of TCR ligands, pointing to the value of increasing Ag stimulation rather than effector numbers in harnessing CD4(+) T cells for therapeutic purposes in such conditions.

What is wrong with granulocytes in inflammatory bowel diseases?

The neutrophil plays a central role in the acute inflammatory response, a crucial mechanism required for the efficient clearance of invading microorganisms and antigenic material. Patients with primary immunodeficiencies of neutrophil function, particularly chronic granulomatous disease, are predisposed to develop bowel inflammation that is indistinguishable from Crohn's disease (CD) on the basis of clinical, endoscopic and histopathological features. The intrinsic function of the neutrophil is normal in the vast majority of patients with CD; however, there is clear evidence of an impairment of neutrophil recruitment to sites of trauma and bacterial infection. This is associated with an inability to adequately clear bacteria that have penetrated the tissues, resulting in the formation of granulomata, the histological hallmark of the disease, and the subsequent initiation of a chronic adaptive immune response. The reduced secretion of proinflammatory cytokines by macrophages, most notably TNF-α, may account for the attenuated neutrophil recruitment observed in CD. Stimulation of the innate immune system in CD, particularly in patients in remission, may be an alternative therapeutic strategy that could reduce the risk of future disease relapses.

BIIL 284 reduces neutrophil numbers but increases P. aeruginosa bacteremia and inflammation in mouse lungs

BACKGROUND

A clinical study to investigate the leukotriene B(4) (LTB(4))-receptor antagonist BIIL 284 in cystic fibrosis (CF) patients was prematurely terminated due to a significantly increased risk of adverse pulmonary events. We aimed to establish the effect of BIIL284 in models of Pseudomonas aeruginosa lung infection, thereby contributing to a better understanding of what could have led to adverse pulmonary events in CF patients.

METHODS

P. aeruginosa DNA in the blood of CF patients during and after acute pulmonary exacerbations and in stable patients with non-CF bronchiectasis (NCFB) and healthy individuals was assessed by PCR. The effect of BIIL 284 treatment was tested in an agar bead murine model of P. aeruginosa lung infection. Bacterial count and inflammation were evaluated in lung and other organs.

RESULTS

Most CF patients (98%) and all patients with NCFB and healthy individuals had negative P. aeruginosa DNA in their blood. Similarly, the P. aeruginosa-infected mice showed bacterial counts in the lung but not in the blood or spleen. BIIL 284 treatment decreased pulmonary neutrophils and increased P. aeruginosa numbers in mouse lungs leading to significantly higher bacteremia rates and lung inflammation compared to placebo treated animals.

CONCLUSIONS

Decreased airway neutrophils induced lung proliferation and severe bacteremia in a murine model of P. aeruginosa lung infection. These data suggest that caution should be taken when administering anti-inflammatory compounds to patients with bacterial infections.

Surface acoustic waves enhance neutrophil killing of bacteria

Biofilms are structured communities of bacteria that play a major role in the pathogenicity of bacteria and are the leading cause of antibiotic resistant bacterial infections on indwelling catheters and medical prosthetic devices. Failure to resolve these biofilm infections may necessitate the surgical removal of the prosthetic device which can be debilitating and costly. Recent studies have shown that application of surface acoustic waves to catheter surfaces can reduce the incidence of infections by a mechanism that has not yet been clarified. We report here the effects of surface acoustic waves (SAW) on the capacity of human neutrophils to eradicate S. epidermidis bacteria in a planktonic state and within biofilms. Utilizing a novel fibrin gel system that mimics a tissue-like environment, we show that SAW, at an intensity of 0.3 mW/cm², significantly enhances human neutrophil killing of S. epidermidis in a planktonic state and within biofilms by enhancing human neutrophil chemotaxis in response to chemoattractants. In addition, we show that the integrin CD18 plays a significant role in the killing enhancement observed in applying SAW. We propose from out data that this integrin may serve as mechanoreceptor for surface acoustic waves enhancing neutrophil chemotaxis and killing of bacteria.

On the computational modeling of the innate immune system

In recent years, there has been an increasing interest in the mathematical and computational modeling of the human immune system (HIS). Computational models of HIS dynamics may contribute to a better understanding of the relationship between complex phenomena and immune response; in addition, computational models will support the development of new drugs and therapies for different diseases. However, modeling the HIS is an extremely difficult task that demands a huge amount of work to be performed by multidisciplinary teams. In this study, our objective is to model the spatio-temporal dynamics of representative cells and molecules of the HIS during an immune response after the injection of lipopolysaccharide (LPS) into a section of tissue. LPS constitutes the cellular wall of Gram-negative bacteria, and it is a highly immunogenic molecule, which means that it has a remarkable capacity to elicit strong immune responses. We present a descriptive, mechanistic and deterministic model that is based on partial differential equations (PDE). Therefore, this model enables the understanding of how the different complex phenomena interact with structures and elements during an immune response. In addition, the model's parameters reflect physiological features of the system, which makes the model appropriate for general use.

Relapse or eradication of cancer is predicted by peptide-major histocompatibility complex affinity

Cancers often relapse after adoptive therapy, even though specific T cells kill cells from the same cancer efficiently in vitro. We found that tumor eradication by T cells required high affinities of the targeted peptides for major histocompatibility complex (MHC) class I. Affinities of at least 10 nM were required for relapse-free regression. Only high-affinity peptide-MHC interactions led to efficient cross-presentation of antigen, thereby stimulating cognate T cells to secrete cytokines. These findings highlight the importance of targeting peptides with high affinity for MHC class I when designing T cell-based immunotherapy.

Generalized Lévy walks and the role of chemokines in migration of effector CD8+ T cells

Chemokines play a central role in regulating processes essential to the immune function of T cells^1-3, such as their migration within lymphoid tissues and targeting of pathogens in sites of inflammation. Here we track T cells using multi-photon microscopy to demonstrate that the chemokine CXCL10 enhances the ability of CD8⁺ T cells to control the pathogen T. gondii in the brains of chronically infected mice. This chemokine boosts T cell function in two different ways: it maintains the effector T cell population in the brain and speeds up the average migration speed without changing the nature of the walk statistics. Remarkably, these statistics are not Brownian; rather, CD8⁺ T cell motility in the brain is well described by a generalized Lévy walk. According to our model, this surprising feature enables T cells to find rare targets with more than an order of magnitude more efficiency than Brownian random walkers. Thus, CD8⁺ T cell behavior is similar to Lévy strategies reported in organisms ranging from mussels to marine predators and monkeys^4-10, and CXCL10 aids T cells in shortening the average time to find rare targets.

How many neutrophils are enough (redux, redux)?

Many chemotherapeutic regimens produce neutropenia, which predisposes to microbial infection. However, not all neutropenic individuals develop infections, so the ability to predict this outcome would be a powerful clinical tool. In this issue of the JCI, Malka et al. describe a dynamic system model of neutrophil bactericidal activity that confirms and extends the concept of critical neutrophil concentration. The authors demonstrate that when the neutrophil concentration approaches the critical concentration, bacterial populations in contact with them exhibit bistability. Their experimental findings raise the intriguing possibility of greater variability in bactericidal activity of neutrophils from healthy adults than heretofore recognized; their model predicts that this could have life-and-death consequences.

Evidence for bistable bacteria-neutrophil interaction and its clinical implications

Neutropenia, which may develop as a consequence of chemotherapy, increases the risk of bacterial infection. Similarly, increased risk of bacterial infection appears in disorders of phagocytic functions, such as the genetic disorder chronic granulomatous disease. To elucidate the organizing principles behind these distinct immunodeficiency conditions, we investigated the interaction between in vitro bacteria and human neutrophils by experiments and mathematical modeling. The model and the experiments showed that the in vitro bacterial dynamics exhibit bistability for a certain range of neutrophil concentration and function. Thus, there is a critical bacterial concentration above which infection develops, and below which neutrophils defeat the bacteria. Whereas with normal neutrophil concentration and function, an infection may develop when the initial bacterial concentration is very high, under neutropenic conditions or when there is neutrophil dysfunction, the critical bacterial concentration can be lower, within the clinically relevant range. We conclude that critical bacterial concentration has clinically relevant implications. The individual maximum bearable bacterial concentration depended on neutrophil concentration, phagocytic activity, and patient barrier integrity; thus, the resulting maximal bearable bacterial concentration may vary by orders of magnitude between patients. Understanding the interplay between neutrophils and bacteria may enhance the development of new therapeutic approaches to bacterial infections.

The role of neutrophils during intestinal inflammation

Polymorphonuclear leukocytes or neutrophils play a critical role in the maintenance of intestinal homeostasis. They have elegant defense mechanisms to eliminate microbes that have translocated across a single layer of mucosal epithelial cells that form a critical barrier between the gut lumen and the underlying tissue. During the inflammatory response, neutrophils also contribute to the recruitment of other immune cells and facilitate mucosal healing by releasing mediators necessary for the resolution of inflammation. Although the above responses are clearly beneficial, excessive recruitment and accumulation of activated neutrophils in the intestine under pathological conditions such as inflammatory bowel disease is associated with mucosal injury and debilitating disease symptoms. Thus, depending on the circumstances, neutrophils can be viewed as either good or bad. In this article, we summarize the beneficial and deleterious roles of neutrophils in the intestine during health and disease and provide an overview of what is known about neutrophil function in the gut.

The immunobiology of IL-27

Like many cytokines, IL-27 has pleiotropic properties that can limit or enhance ongoing immune responses depending on context. Thus, under certain circumstances, IL-27 can promote TH1 differentiation and has been linked to the activation of CD8(+) T cells and enhanced humoral responses. However, IL-27 also has potent inhibitory properties and mice that lack IL-27 mediated signaling develop exaggerated inflammatory responses in the context of infection or autoimmunity. This chapter reviews in depth the biology of IL-27, including the initial discovery, characterization, and signaling mediated by IL-27 as well as more recent insights into the molecular and cellular basis for its pleiotropic effects. Many of these advances are relevant to human diseases and highlight the potential of therapies that harness the regulatory properties of IL-27.

Protein ultrastructure and the nanoscience of complement activation

The complement system constitutes an important barrier to infection of the human body. Over more than four decades structural properties of the proteins of the complement system have been investigated with X-ray crystallography, electron microscopy, small-angle scattering, and atomic force microscopy. Here, we review the accumulated evidence that the nm-scaled dimensions and conformational changes of these proteins support functions of the complement system with regard to tissue distribution, molecular crowding effects, avidity binding, and conformational regulation of complement activation. In the targeting of complement activation to the surfaces of nanoparticulate material, such as engineered nanoparticles or fragments of the microbial cell wall, these processes play intimately together. This way the complement system is an excellent example where nanoscience may serve to unravel the molecular biology of the immune response.

Congenital neutropenia: diagnosis, molecular bases and patient management

The term congenital neutropenia encompasses a family of neutropenic disorders, both permanent and intermittent, severe (<0.5 G/l) or mild (between 0.5-1.5 G/l), which may also affect other organ systems such as the pancreas, central nervous system, heart, muscle and skin. Neutropenia can lead to life-threatening pyogenic infections, acute gingivostomatitis and chronic parodontal disease, and each successive infection may leave permanent sequelae. The risk of infection is roughly inversely proportional to the circulating polymorphonuclear neutrophil count and is particularly high at counts below 0.2 G/l.When neutropenia is detected, an attempt should be made to establish the etiology, distinguishing between acquired forms (the most frequent, including post viral neutropenia and auto immune neutropenia) and congenital forms that may either be isolated or part of a complex genetic disease.Except for ethnic neutropenia, which is a frequent but mild congenital form, probably with polygenic inheritance, all other forms of congenital neutropenia are extremely rare and have monogenic inheritance, which may be X-linked or autosomal, recessive or dominant.About half the forms of congenital neutropenia with no extra-hematopoietic manifestations and normal adaptive immunity are due to neutrophil elastase (ELANE) mutations. Some patients have severe permanent neutropenia and frequent infections early in life, while others have mild intermittent neutropenia.Congenital neutropenia may also be associated with a wide range of organ dysfunctions, as for example in Shwachman-Diamond syndrome (associated with pancreatic insufficiency) and glycogen storage disease type Ib (associated with a glycogen storage syndrome). So far, the molecular bases of 12 neutropenic disorders have been identified.Treatment of severe chronic neutropenia should focus on prevention of infections. It includes antimicrobial prophylaxis, generally with trimethoprim-sulfamethoxazole, and also granulocyte-colony-stimulating factor (G-CSF). G-CSF has considerably improved these patients' outlook. It is usually well tolerated, but potential adverse effects include thrombocytopenia, glomerulonephritis, vasculitis and osteoporosis. Long-term treatment with G-CSF, especially at high doses, augments the spontaneous risk of leukemia in patients with congenital neutropenia.

Bacteria-phagocyte dynamics, axiomatic modelling and mass-action kinetics

Axiomatic modeling is ensued to provide a family of models that describe bacterial growth in the presence of phagocytes, or, more generally, prey dynamics in a large spatially homogenous eco-system. A classification of the possible bifurcation diagrams that arise in such models is presented. It is shown that other commonly used models that do not belong to this class may miss important features that are associated with the limited growth curve of the bacteria (prey) and the saturation associated with the phagocytosis (predator kill) term. Notably, these features appear at relatively low concentrations, much below the saturation range. Finally, combining this model with a model of neutrophil dynamics in the blood after chemotherapy treatments we obtain new insights regarding the development of infections under neutropenic conditions.

Investigational agents for Crohn's disease

IMPORTANCE OF THE FIELD

Increased understanding of the biological mechanisms of Crohn's disease has opened the door to a large number of new molecules; some of these are approved for clinical use, while others remain under evaluation. In this review, we examine the clinical efficacy of all the new drugs that have been evaluated in controlled trials in the last 12 years.

AREAS COVERED IN THIS REVIEW

Anti-TNF therapy has been reviewed briefly, given the many comprehensive reviews on this topic; attention is focused mainly on the other biological therapies. In assessing the clinical efficacy of these molecules, we consider only the remission rate, as this is considered the most meaningful end point in clinical practice.

WHAT THE READER WILL GAIN

We analyzed the main biological mechanisms of Crohn's disease and the new drugs whose use is based on insights into these mechanisms. We reviewed the following new drugs: probiotics, GM-CSF, IL-10, IL-11, anti-IL-6, anti-IL-12/-23, everolimus, anti-IFN-γ, IFN-β-I, co-stimulators, anti-integrins, anti-intercellular adhesion molecule 1, small molecules and mitogen-activated protein kinase inhibitors.

TAKE HOME MESSAGE

Anti-TNF therapies remain the best options, followed by anti-integrin drugs. The most promising new therapies are anti-IL-23, but further data are necessary. The disappointing results with other molecules may depend on the quality of trials and possibly on inadequate dosage of the drug.

Crohn's disease as an immunodeficiency

The pathogenesis of Crohn's disease (CD) has widely been regarded as the consequence of a dysregulated T-cell-mediated response to intestinal microbes, and the majority of the worldwide research effort has focused on characterizing and treating the chronic inflammatory phase of the disease. However, recent molecular biological and clinical investigations indicate that CD is actually a primary immunodeficiency. At first counter-intuitive, the apparent paradox of a pathogenic innate immune defect can be linked mechanistically to the granulomatous chronic inflammation characteristic of the disease. Genome-wide association studies have corroborated the involvement of innate immune dysfunction in the pathogenesis of CD, but less than 20% of the heritable risk is accounted for. By contrast, in vitro and in vivo stimulation of the immune system has highlighted novel areas of interest that may lead to the development of targeted therapeutic and diagnostic tools.

Bistability and bacterial infections

Bacterial infections occur when the natural host defenses are overwhelmed by invading bacteria. The main component of the host defense is impaired when neutrophil count or function is too low, putting the host at great risk of developing an acute infection. In people with intact immune systems, neutrophil count increases during bacterial infection. However, there are two important clinical cases in which they remain constant: a) in patients with neutropenic-associated conditions, such as those undergoing chemotherapy at the nadir (the minimum clinically observable neutrophil level); b) in ex vivo examination of the patient's neutrophil bactericidal activity. Here we study bacterial population dynamics under fixed neutrophil levels by mathematical modelling. We show that under reasonable biological assumptions, there are only two possible scenarios: 1) Bacterial behavior is monostable: it always converges to a stable equilibrium of bacterial concentration which only depends, in a gradual manner, on the neutrophil level (and not on the initial bacterial level). We call such a behavior type I dynamics. 2) The bacterial dynamics is bistable for some range of neutrophil levels. We call such a behavior type II dynamics. In the bistable case (type II), one equilibrium corresponds to a healthy state whereas the other corresponds to a fulminant bacterial infection. We demonstrate that published data of in vitro Staphylococcus epidermidis bactericidal experiments are inconsistent with both the type I dynamics and the commonly used linear model and are consistent with type II dynamics. We argue that type II dynamics is a plausible mechanism for the development of a fulminant infection.

CD8+ T cell concentration determines their efficiency in killing cognate antigen-expressing syngeneic mammalian cells in vitro and in mouse tissues

We describe a quantitative model for assessing the cytolytic activity of antigen-specific CD8⁺ T cells in vitro and in vivo in which the concentration of antigen-specific CD8⁺ T cells determines the efficiency with which these cells kill cognate antigen–expressing melanoma cells in packed cell pellets, in three-dimensional collagen-fibrin gels in vitro, and in established melanomas in vivo. In combination with a clonogenic assay for melanoma cells, collagen-fibrin gels are 4,500–5,500-fold more sensitive than the packed cell pellet–type assays generally used to measure CD8⁺ T cell cytolytic activity. An equation previously used to describe neutrophil bactericidal activity in vitro and in vivo also describes antigen-specific CD8⁺ T cell–mediated cytolysis of cognate antigen-expressing melanoma cells in collagen-fibrin gels in vitro and in transplanted tumors in vivo. We have used this equation to calculate the critical concentration of antigen-specific CD8⁺ T cells, which is the concentration of these cells required to hold constant the concentration of a growing population of cognate antigen-expressing melanoma cells. It is ∼3.5 × 10⁵/ml collagen-fibrin gel in vitro and ∼3 × 10⁶/ml or /g melanoma for previously published studies of ex vivo–activated adoptively transferred tumor antigen–specific CD8⁺ T cell killing of cognate antigen–expressing melanoma cells in established tumors in vivo. The antigen-specific CD8⁺ T cell concentration required to kill 100% of 2 × 10⁷/ml cognate antigen-expressing melanoma cells in collagen fibrin gels is ≥10⁷/ml of gel.

Disordered macrophage cytokine secretion underlies impaired acute inflammation and bacterial clearance in Crohn's disease

The cause of Crohn's disease (CD) remains poorly understood. Counterintuitively, these patients possess an impaired acute inflammatory response, which could result in delayed clearance of bacteria penetrating the lining of the bowel and predispose to granuloma formation and chronicity. We tested this hypothesis in human subjects by monitoring responses to killed Escherichia coli injected subcutaneously into the forearm. Accumulation of (111)In-labeled neutrophils at these sites and clearance of (32)P-labeled bacteria from them were markedly impaired in CD. Locally increased blood flow and bacterial clearance were dependent on the numbers of bacteria injected. Secretion of proinflammatory cytokines by CD macrophages was grossly impaired in response to E. coli or specific Toll-like receptor agonists. Despite normal levels and stability of cytokine messenger RNA, intracellular levels of tumor necrosis factor (TNF) were abnormally low in CD macrophages. Coupled with reduced secretion, these findings indicate accelerated intracellular breakdown. Differential transcription profiles identified disease-specific genes, notably including those encoding proteins involved in vesicle trafficking. Intracellular destruction of TNF was decreased by inhibitors of lysosomal function. Together, our findings suggest that in CD macrophages, an abnormal proportion of cytokines are routed to lysosomes and degraded rather than being released through the normal secretory pathway.

Human peritoneal fibroblasts are a potent source of neutrophil-targeting cytokines: a key role of IL-1beta stimulation

Polymorphonuclear leukocyte (PMN) infiltration is a cardinal feature of peritonitis. CXC chemokine ligands 1 and 8 (CXCL1 and CXCL8), and the cytokine granulocyte colony-stimulating factor (G-CSF) are the key mediators of PMN accumulation. Increasing evidence points to an important role of human peritoneal fibroblasts (HPFB) in the response of the peritoneum to infection. We have examined the synthesis of PMN-targeting cytokines by HPFB exposed to intraperitoneal milieu as represented by peritoneal dialysate effluent (PDE) from patients undergoing peritoneal dialysis. PDE obtained during peritonitis, but not during infection-free periods, significantly increased production of CXCL1, CXCL8, and G-CSF by HPFB. The effect was largely blocked by antibodies to interleukin-1beta (IL-1beta), whereas neutralization of tumor necrosis factor-alpha (TNFalpha) had no major effect. Similar pattern of inhibition was observed when HPFB were exposed to conditioned media from endotoxin-stimulated peritoneal macrophages. Significance of IL-1beta stimulation was further shown in experiments with recombinant cytokines. Compared with TNFalpha, exposure of HPFB to recombinant IL-1beta resulted in a much higher release of PMN-targeting cytokines. The assessment of mRNA degradation revealed that the IL-1beta-induced transcripts of CXCL1, CXCL8, and G-CSF were more stable compared with those induced by TNFalpha. These data indicate that HPFB can be a significant source of PMN-targeting cytokines when stimulated with IL-1beta in the inflamed peritoneum.

ITAM signaling by Vav family Rho guanine nucleotide exchange factors regulates interstitial transit rates of neutrophils in vivo

Background

In response to infection, neutrophils are quickly recruited from the blood into inflamed tissues. The interstitial migration of neutrophils is crucial for the efficient capture and control of rapidly proliferating microbes before microbial growth can overwhelm the host's defenses. However, the molecular mechanisms that regulate interstitial migration are incompletely understood.

Methodology/Principal Findings

Here, we use two-photon microscopy (2PM) to study discrete steps of neutrophil responses during subcutaneous infection with bacteria. Our study demonstrates that signals emanating from ITAM-containing receptors mediated by Vav family Rho GEFs control the velocity, but not the directionality, of neutrophil migration towards sites of bacterial infection.

Conclusions/Significance

Here we show that during neutrophil migration towards sites of bacterial infection, signals emanating from ITAM-containing receptors specifically control interstitial neutrophil velocity.

Impaired neutrophil extracellular trap (NET) formation: a novel innate immune deficiency of human neonates

Neutrophils are highly specialized innate effector cells that have evolved for killing of pathogens. Human neonates have a common multifactorial syndrome of neutrophil dysfunction that is incompletely characterized and contributes to sepsis and other severe infectious complications. We identified a novel defect in the antibacterial defenses of neonates: inability to form neutrophil extracellular traps (NETs). NETs are lattices of extracellular DNA, chromatin, and antibacterial proteins that mediate extracellular killing of microorganisms and are thought to form via a unique death pathway signaled by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-generated reactive oxygen species (ROS). We found that neutrophils from term and preterm infants fail to form NETs when activated by inflammatory agonists-in contrast to leukocytes from healthy adults. The deficiency in NET formation is paralleled by a previously unrecognized deficit in extracellular bacterial killing. Generation of ROSs did not complement the defect in NET formation by neonatal neutrophils, as it did in adult cells with inactivated NADPH oxidase, demonstrating that ROSs are necessary but not sufficient signaling intermediaries and identifying a deficiency in linked or downstream pathways in neonatal leukocytes. Impaired NET formation may be a critical facet of a common developmental immunodeficiency that predisposes newborn infants to infection.

Exploring the role of the immune response in preventing antibiotic resistance

For many bacterial infections, drug resistant mutants are likely present by the time antibiotic treatment starts. Nevertheless, such infections are often successfully cleared. It is commonly assumed that this is due to the combined action of drug and immune response, the latter facilitating clearance of the resistant population. However, most studies of drug resistance emergence during antibiotic treatment focus almost exclusively on the dynamics of bacteria and the drug and neglect the contribution of immune defenses. Here, we develop and analyze several mathematical models that explicitly include an immune response. We consider different types of immune responses and investigate how each impacts the emergence of resistance. We show that an immune response that retains its strength despite a strong drug-induced decline of bacteria numbers considerably reduces the emergence of resistance, narrows the mutant selection window, and mitigates the effects of non-adherence to treatment. Additionally, we show that compared to an immune response that kills bacteria at a constant rate, one that trades reduced killing at high bacterial load for increased killing at low bacterial load is sometimes preferable. We discuss the predictions and hypotheses derived from this study and how they can be tested experimentally.

Listeria monocytogenes: epidemiology, human disease, and mechanisms of brain invasion

Listeria monocytogenes is a facultative intracellular bacterium that has predilection for causing central nervous systemic infections in humans and domesticated animals. This pathogen can be found worldwide in the food supply and most L. monocytogenes infections are acquired through ingestion of contaminated food. The main clinical syndromes caused by L. monocytogenes include febrile gastroenteritis, perinatal infection, and systemic infections marked by central nervous system infections with or without bacteremia. Experimental infection of mice has been used for over 50 years as a model system to study the pathogenesis of this organism including the mechanisms by which it invades the brain. Data from this model indicate that a specific subset of monocytes, distinguished in part by high expression of the Ly-6C antigen, become parasitized in the bone marrow and have a key role in transporting intracellular bacteria across the blood-brain barriers and into the central nervous system. This Minireview will summarize recent epidemiologic and clinical information regarding L. monocytogenes as a human pathogen and will discuss current in vitro and in vivo data relevant to the role of parasitized monocytes and the pathogenetic mechanisms that underlie its formidable ability to invade the central nervous system.