The bacterial peptide N-formyl-Met-Leu-Phe inhibits killing of Staphylococcus epidermidis by human neutrophils in fibrin gels

On the origin of low-density neutrophils

Here we elaborate on the origin of low(er)-density neutrophils (LDNs) to better understand the variation found in literature. Supplemented with original data, we test the hypothesis that buoyant density of neutrophils is characterized by a spectrum that as a whole shifts to a lower density after activation. Both the 20% highest density (HDNs) and 20% lowest density (LDNs) neutrophils from healthy donors were isolated by Percoll of different densities. Using this method we found that LDNs were significantly better in T-cell suppression and bacterial containment than their 20% highest density counterparts. We found no statistically relevant differences in neutrophil survival or bacterial phagocytosis. Stimulation of healthy donor neutrophils with N-formyl-methionyl-leucyl-phenylalanine induced LDNs co-segregating with peripheral blood mononuclear cells after Ficoll separation. These in vitro induced LDNs showed increased activation markers compared to HDNs and were comparable to the activation markers found on the LDN fraction seen in patients with chronic inflammatory conditions such as present in cancer patients. This all fits with the hypothesis that the density of neutrophils is distributed in a spectrum partially coupled to maturation. Additionally a shift in this spectrum can be induced by in vitro stimulation or by activation in disease.

A comprehensive three-dimensional assay to assess neutrophil defense against bacteria

Neutrophil antibacterial capacity is measured in animal models and in vitro as an important indicator of neutrophil function. To be able to extrapolate their conclusions, in vitro experiments should mimic the in vivo situation. In vivo, antibacterial capacity depends on multiple steps of bacterial sensing, priming, chemotaxis, phagocytosis and intracellular killing. Therefore, we developed a simply executed assay that involves multiple steps in one assay. The neutrophils were incorporated into a three-dimensional matrix of fibrin fibers, in which they could freely migrate. The fibrin matrix provided a more physiological representation of tissue structure than a shaken suspension and extended ex vivo survival of neutrophils. Staphylococci endogenously producing GFP (Green Fluorescent Protein) provided a real-time quantification of the bacterial load without the need for lysing the fibrin matrix or counting of colony forming units on agar plates. The delay in bacterial outgrowth serves as a measure for the relative antibacterial capacity of the neutrophils. Additionally, neutrophil capacity could easily be measured high-throughput in a 96-wells format. In this new assay we study neutrophil behavior in a physiologically relevant setting and explore many functions of the neutrophil in a single test. The functional capacity of neutrophils from different in vitro treatments or different donors can directly be compared.

Co-Activation of Th17 and Antibody Responses Provides Efficient Protection against Mucosal Infection by Group A Streptococcus

Conserved protein antigens among serotypes of group A Streptococcus pyogenes (GAS) have been focused for vaccine development because of the diversity of GAS serotypes and risks of autoimmunity post-GAS infection. Precise delineation of protective immune response to each of GAS antigens is critical for vaccine efficacy and safety. We recently reported that immunization with SrtA of GAS provides Th17-dependent clearance of heterologous serotypes of GAS in NALT. SCPA is a surface virulence molecule of GAS and known to induce antibody-mediated protection against GAS. We hypothesized that co-immunization with SrtA and SCPA would provide more efficient protection by eliciting combined Th17 and antibody responses. The present study showed that mice that were intranasally co-immunized with SrtA/SCPA cleared GAS more efficiently than the mice that were immunized with either SrtA or SCPA individually, and as efficient as the mice that experienced repeated GAS infections. The co-immunization induced Th17 and robust SCPA antibody responses, accompanied by a rapid influx of neutrophils and high myeloperoxidase activity in NALT, suggesting that simultaneous induction of mucosal Th17 and neutralizing antibody responses offers more effective GAS elimination through rapid infiltration and activation of neutrophils. Moreover, Th17 response was strongly induced in mice that experienced repeated GAS-infection and maintained at a high level even after the bacteria were cleared; whereas, it was moderately induced and promptly returned to baseline following bacterial elimination in SrtA/SCPA co-immunized mice. Additional results showed that the survival rate of systemic challenge was significantly higher in infection experienced than in co-immunized mice, indicating that more immune elements are required for protection against systemic than mucosal GAS infection.

The Vi capsular polysaccharide enables Salmonella enterica serovar typhi to evade microbe-guided neutrophil chemotaxis

Salmonella enterica serovar Typhi (S. Typhi) causes typhoid fever, a disseminated infection, while the closely related pathogen S. enterica serovar Typhimurium (S. Typhimurium) is associated with a localized gastroenteritis in humans. Here we investigated whether both pathogens differ in the chemotactic response they induce in neutrophils using a single-cell experimental approach. Surprisingly, neutrophils extended chemotactic pseudopodia toward Escherichia coli and S. Typhimurium, but not toward S. Typhi. Bacterial-guided chemotaxis was dependent on the presence of complement component 5a (C5a) and C5a receptor (C5aR). Deletion of S. Typhi capsule biosynthesis genes markedly enhanced the chemotactic response of neutrophils in vitro. Furthermore, deletion of capsule biosynthesis genes heightened the association of S. Typhi with neutrophils in vivo through a C5aR-dependent mechanism. Collectively, these data suggest that expression of the virulence-associated (Vi) capsular polysaccharide of S. Typhi obstructs bacterial-guided neutrophil chemotaxis.

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.

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.

Clocking the Lyme spirochete

In order to clear the body of infecting spirochetes, phagocytic cells must be able to get hold of them. In real-time phase-contrast videomicroscopy we were able to measure the speed of Borrelia burgdorferi (Bb), the Lyme spirochete, moving back and forth across a platelet to which it was tethered. Its mean crossing speed was 1,636 µm/min (N = 28), maximum, 2800 µm/min (N = 3). This is the fastest speed recorded for a spirochete, and upward of two orders of magnitude above the speed of a human neutrophil, the fastest cell in the body. This alacrity and its interpretation, in an organism with bidirectional motor capacity, may well contribute to difficulties in spirochete clearance by the host.

The lupus-susceptibility locus, Sle3, mediates enhanced resistance to bacterial infections

The genetic predisposition to many autoimmune diseases is inherited as a polygenic trait. It is conceivable that some of the causative alleles in these diseases became prevalent in the population by conferring a survival benefit against environmental assaults, such as infections. We used mice cogenic for genetic loci predisposing to systemic lupus erythomatosus to test the hypothesis that some of these genetic loci protect the host from bacterial infections. Mice with the Sle3 lupus-susceptibility locus on a wild-type background were found to have enhanced antibacterial responses in the context of pneumonia and intra-abdominal sepsis than wild-type animals. This was associated with markedly augmented accumulation of neutrophils in infected tissues, and was bone marrow transferable and dependent on the presence of neutrophils, but not lymphocytes. There was no difference in in vitro leukocyte killing of bacteria nor influx of phagocytes between lupus-susceptible and wild-type animals, but neutrophils from lupus-susceptible mice displayed markedly reduced rate of apoptosis, associated with altered expression of Bcl-2 family proteins, contributing to their greater accumulation. Importantly, deliberate inhibition of apoptosis in wild-type animals significantly boosted the accumulation of neutrophils at the site of infection and resulted in an enhanced antimicrobial response. These observations support the concept that some of the genetic loci that mediate autoimmunity may also confer augmented antimicrobial innate immunity.

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

We showed previously that the competition between bacterial killing by neutrophils and bacterial growth in stirred serum-containing suspensions could be modeled as the competition between a first-order reaction (bacterial growth) and a second-order reaction (bacterial killing by neutrophils). The model provided a useful parameter, the critical neutrophil concentration (CNC), below which bacterial concentration increased and above which it decreased, independent of the initial bacterial concentration. We report here that this model applies to neutrophil killing of bacteria in three-dimensional fibrin matrices and in rabbit dermis. We measured killing of 10(3)-10(8) colony forming units/ml Staphylococcus epidermidis by 10(5)-10(8) human neutrophils/ml in fibrin gels. The CNC was approximately 4 x 10(6) neutrophils/ml gel in the presence of normal serum and approximately 1.6 x 10(7) neutrophils/ml gel in the presence of C5-deficient serum. Application of our model to published data of others on killing of approximately 5 x 10(7) to 2 x 10(8) E. coli/ml rabbit dermis yielded CNCs from approximately 4 x 10(6) to approximately 8 x 10(6) neutrophils/ml dermis. Thus, in disparate tissues and tissuelike environments, our model fits the kinetics of bacterial killing and gives similar lower limits (CNCs) to the neutrophil concentration required to control bacterial growth.

Histamine releasing peptide (HRP) has proinflammatory effects and is present at sites of inflammation

Albumin, the most abundant plasma protein, readily enters sites of inflammation during the period of increased vascular permeability. There it encounters proteases released from mast cells and invading leukocytes which earlier work has shown can act on albumin to liberate the peptide, histamine releasing peptide (HRP), first identified and named by its ability to stimulate histamine release from isolated mast cells. In this report we show that HRP releases histamine from cutaneous mast cells in vivo resulting in increased vascular permeability and persistent edema while in vitro, HRP promotes chemotaxis of leukocytes and enhances macrophage phagocytosis. Moreover, we show that the level of HRP is increased with the induction of an acute cutaneous inflammatory response in rats, that HRP is present at sites of acute and chronic inflammation in humans and that HRP is rapidly degraded by proteases thereby limiting its action to the area of its generation. We suggest that HRP is a pro-inflammatory peptide that helps amplify and perpetuate the inflammatory response. Inhibitors of inflammatory proteases or antagonists that block the action of peptides like HRP may, therefore, be useful in breaking the cycle of inflammation.

Unique structural features that influence neutrophil emigration into the lung

Neutrophil emigration in the lung differs substantially from that in systemic vascular beds where extravasation occurs primarily through postcapillary venules. Migration into the alveolus occurs directly from alveolar capillaries and appears to progress through a sequence of steps uniquely influenced by the cellular anatomy and organization of the alveolar wall. The cascade of adhesive and stimulatory events so critical to the extravasation of neutrophils from postcapillary venules in many tissues is not evident in this setting. Compelling evidence exists for unique cascades of biophysical, adhesive, stimulatory, and guidance factors that arrest neutrophils in the alveolar capillary bed and direct their movement through the endothelium, interstitial space, and alveolar epithelium. A prominent path accessible to the neutrophil appears to be determined by the structural interactions of endothelial cells, interstitial fibroblasts, as well as type I and type II alveolar epithelial cells.

Measuring chemotaxis and chemokinesis: the under-agarose cell migration assay

Chemotaxis is the primary mechanism by which cell movements are directed within multicellular organisms, and it is a major component of embryonic development, wound healing, and immune responses. Chemotaxis involves a complex cascade of events--formation of signaling complexes, receptor polarization, adhesion molecule activation, and cytoskeletal reorganization. Previous assay methods were limited in several ways that reduced users' abilities to obtain quantitative data or to control conditions precisely. We describe a unique chemotactic assay that can incorporate multiple chemotactic gradients in different spatial and temporal combinations. In addition, this assay is easily adapted for live-cell imaging and fluorescent microscopy. With its relative simplicity, flexibility, and precision, this method is a key tool for the study of cellular chemotactic responses and the signaling processes underlying them.

A critical concentration of neutrophils is required for effective bacterial killing in suspension

We have examined the effect of neutrophil concentration on killing of a clinical isolate of Staphylococcus epidermidis. Human neutrophils at concentrations varying from 10(5) to 10(7) per ml were mixed in suspension with S. epidermidis at concentrations varying from 10(3) to 10(8) colony-forming units/ml, and the concentration of viable bacteria was assayed after various times at 37 degrees C. The rate of bacterial killing depended on the concentration of neutrophils and not on the ratio of neutrophils to bacteria. Below a critical concentration of neutrophils, bacteria growth was greater than neutrophil killing of bacteria even when the ratio of neutrophils to bacteria was 100:1. We fitted the time course of bacterial concentration and its dependence on neutrophil concentration with an exponential function, the exponent of which is (-kp + g)t, where k is the second-order rate constant for bacterial killing, p is the neutrophil concentration, g is the first-order rate constant for bacterial growth, and t is time. We found that k approximately 2 x 10(-8) ml per neutrophil per min, and g approximately 8 x 10(-3)/min. Only when p is greater than g/k, which we call the critical neutrophil concentration, does the bacterial concentration fall. Under optimal assay conditions, the critical neutrophil concentration was 3-4 x 10(5) per ml, a value very close to that (< or =5 x 10(5) per ml) known to predispose humans to bacterial and fungal infections.