Neutrophil Swarming Research Articles

In the eye of the neutrophil swarm-navigation signals that bring neutrophils together in inflamed and infected tissues.

Neutrophils are sentinel cells that express in higher vertebrates >30 chemokine and chemoattractant receptors to sense and quickly react to tissue damage signals. Intravital microscopy studies in mouse models of wounding, inflammation, and infection have revealed that neutrophils form cell swarms at local sites of tissue injury and cell death. This swarming response is choreographed by chemokines, lipids, and other chemoattractants, controlling sequential phases of highly coordinated chemotaxis, intercellular signal relay, and cluster formation among neutrophils. This review will give a brief overview about the basic principles and key molecules that have led to the refined multistep model of how neutrophils come together to isolate sites of tissue injury and microbial invasion from healthy tissue. Whereas auto- and paracrine signaling among neutrophils during later phases of swarming can provide a level of self-organization for robust navigation in diverse inflammatory settings, guidance factors from primary tissue lesions, resident bystander cells, and dying cells regulate the initial phases of the swarming response. This review will discuss how the specific environmental context and mixture of attractants at the locally inflamed site can lead to variants of the multistep attraction model and influence the extent of neutrophil swarming, ranging from accumulations of only few individual cells to the aggregation of several hundreds of neutrophils, as found in abscesses. Given the critical roles of neutrophils in both host protection and tissue destruction, novel insights on neutrophil swarming might provide useful for the therapeutic modulation of neutrophil-dependent inflammatory processes.

Microbiota-derived compounds drive steady-state granulopoiesis via MyD88/TICAM signaling.

Neutropenia is probably the strongest known predisposition to infection with otherwise harmless environmental or microbiota-derived species. Because initial swarming of neutrophils at the site of infection occurs within minutes, rather than the hours required to induce "emergency granulopoiesis," the relevance of having high numbers of these cells available at any one time is obvious. We observed that germ-free (GF) animals show delayed clearance of an apathogenic bacterium after systemic challenge. In this article, we show that the size of the bone marrow myeloid cell pool correlates strongly with the complexity of the intestinal microbiota. The effect of colonization can be recapitulated by transferring sterile heat-treated serum from colonized mice into GF wild-type mice. TLR signaling was essential for microbiota-driven myelopoiesis, as microbiota colonization or transferring serum from colonized animals had no effect in GF MyD88(-/-)TICAM1(-/-) mice. Amplification of myelopoiesis occurred in the absence of microbiota-specific IgG production. Thus, very low concentrations of microbial Ags and TLR ligands, well below the threshold required for induction of adaptive immunity, sets the bone marrow myeloid cell pool size. Coevolution of mammals with their microbiota has probably led to a reliance on microbiota-derived signals to provide tonic stimulation to the systemic innate immune system and to maintain vigilance to infection. This suggests that microbiota changes observed in dysbiosis, obesity, or antibiotic therapy may affect the cross talk between hematopoiesis and the microbiota, potentially exacerbating inflammatory or infectious states in the host.

Clearance of Apoptotic Bodies, NETs, and Biofilm DNA: Implications for Autoimmunity.

Apoptosis and NETosis, two important pathways of programed cell death, differ in their morphologic features and their effects on the immune system. In apoptosis, nuclear chromatin compacts as it is packaged into nuclear fragments and apoptotic blebs (1), and uptake of apoptotic cells by phagocytes generally suppresses the immune response (2). In NETosis, named after neutrophil extracellular traps (NETs), nuclear chromatin relaxes and forms a fibrous meshwork upon release from the cell (3). In general, NETosis is induced by infection, inflammation, or trauma and represents a mechanism of innate immune activation (4). Neutrophils, the most abundant type of white blood cells, migrate toward a stimulus in coordinated fashion, and NETs may synchronize such neutrophil swarms (5). Despite the structural and functional differences between apoptosis and NETosis, significant aspects of their clearance pathways likely overlap, as specific serum proteins participate in the recognition and uptake of remnants from either cell death pathway. In vivo, it is likely that both cell death pathways are concurrently present and that apoptotic bodies and NETs entangle (6). Yet, a third type of DNA may intertwine with DNA from apoptotic and NETotic cells, as certain bacteria and fungi release extracellular DNA that is used to construct biofilms (7). How apoptotic bodies, NETs, and biofilm DNA (Figure ​(Figure1)1) are safely cleared is of great interest, because incomplete clearance leads to systemic inflammation and autoantibody production. Figure 1 Self and foreign antigens that may induce autoantibodies in autoimmunity. The potential contribution of apoptotic bodies, NETs, and bacterial biofilms to immune tolerance versus stimulation is indicated. The distribution and content of self (green) and ... Systemic Autoimmune Diseases and Autoantibodies to Nuclear Antigens Molecular structures associated with dying cells are targets of autoantibodies in autoimmune diseases such as systemic lupus (SLE) (8), antiphospholipid syndrome (APS) (9), as well as other musculoskeletal/rheumatoid disorders (10). The resulting autoreactivities are idiosyncratic for each condition and thus are useful for clinical diagnosis. However, the antigens recognized by the autoantibodies are also involved in pathogenesis, as they accumulate at the sites of tissue damage and contribute to immune complex deposition (11). Tissue damage may worsen in the absence of serum nucleases such as DNAse I (12). Furthermore, the interactions between dying cells and the adaptive immune system strengthen over time, as somatic mutations and antigen selection optimize antibodies for improved binding (13). In SLE, antibodies to nuclear or plasma membrane antigens arise in the course of disease (14, 15). These antibodies avidly bind to apoptotic cells (16). Classical studies recognized that apoptotic cells are far better substrates for autoantibody binding than viable cells (17). However, monoclonal antibodies from mouse lupus models that bind to apoptotic blebs (16) also tightly bind to NETs released in response to bacterial pathogens (18). Our laboratory showed that NETotic cells provide suitable targets for autoantibodies from diverse human autoimmune disorders (19). Whether apoptotic or NETotic cell death, or both, provide antigens that induce autoantibody production is essential information for understanding the etiopathogenesis of autoimmune diseases (20).

Immunohistochemical detection of interleukin-8 in inflamed porcine tissues

The objective of this study was to identify the specific localization of interleukin-8 (IL-8) in cells in situ in a variety of inflammatory processes in different tissues from pigs. Our hypothesis was that IL-8 primarily is a neutrophil related cytokine present in all extravascular neutrophils while expression also occurs in other cell types in response to an inflammatory stimulus. Using IL-8 immunohistochemistry we discovered that neutrophils were the predominant IL-8 positive cell population while epithelial cell types and endothelium of postcapillary venules could be positive when situated in close vicinity of an inflammatory lesion. Furthermore, endothelial cells of newly formed vessels in granulation tissue were positive in some specimens. Some sub-populations of inflammatory neutrophils were, however, IL-8 negative which could reflect some phase of neutrophil swarming.

Transcranial amelioration of inflammation and cell death after brain injury

Traumatic brain injury (TBI) is increasingly appreciated to be highly prevalent and deleterious to neurological function 1, 2. At present no effective treatment options are available, and little is known about the complex cellular response to TBI during its acute phase. To gain novel insights into TBI pathogenesis, we developed a novel closed-skull brain injury model that mirrors some pathological features associated with mild TBI in humans and used long-term intravital microscopy to study the dynamics of the injury response from its inception. Here we demonstrate that acute brain injury induces vascular damage, meningeal cell death, and the generation of reactive oxygen species (ROS) that ultimately breach the glial limitans and promote spread of the injury into the parenchyma. In response, the brain elicits a neuroprotective, purinergic receptor dependent inflammatory response characterized by meningeal neutrophil swarming and microglial reconstitution of the damaged glial limitans. We additionally show that the skull bone is permeable to small molecular weight compounds and use this delivery route to modulate inflammation and therapeutically ameliorate brain injury through transcranial administration of the ROS scavenger, glutathione. Our results provide novel insights into the acute cellular response to TBI and a means to locally deliver therapeutic compounds to the site of injury.

Neutrophil swarms require LTB4 and integrins at sites of cell death in vivo

Neutrophil recruitment from blood to extravascular sites of sterile or infectious tissue damage is a hallmark of early innate immune responses, and the molecular events leading to cell exit from the bloodstream have been well defined. Once outside the vessel, individual neutrophils often show extremely coordinated chemotaxis and cluster formation reminiscent of the swarming behaviour of insects. The molecular players that direct this response at the single-cell and population levels within the complexity of an inflamed tissue are unknown. Using two-photon intravital microscopy in mouse models of sterile injury and infection, we show a critical role for intercellular signal relay among neutrophils mediated by the lipid leukotriene B4, which acutely amplifies local cell death signals to enhance the radius of highly directed interstitial neutrophil recruitment. Integrin receptors are dispensable for long-distance migration, but have a previously unappreciated role in maintaining dense cellular clusters when congregating neutrophils rearrange the collagenous fibre network of the dermis to form a collagen-free zone at the wound centre. In this newly formed environment, integrins, in concert with neutrophil-derived leukotriene B4 and other chemoattractants, promote local neutrophil interaction while forming a tight wound seal. This wound seal has borders that cease to grow in kinetic concert with late recruitment of monocytes and macrophages at the edge of the displaced collagen fibres. Together, these data provide an initial molecular map of the factors that contribute to neutrophil swarming in the extravascular space of a damaged tissue. They reveal how local events are propagated over large-range distances, and how auto-signalling produces coordinated, self-organized neutrophil-swarming behaviour that isolates the wound or infectious site from surrounding viable tissue.

Dynamics of Neutrophil Migration in Lymph Nodes during Infection

Although the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning-laser microscopy to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We found that neutrophils formed both small, transient and large, persistent swarms via a coordinated migration pattern. We provided evidence that cooperative action of neutrophils and parasite egress from host cells could trigger swarm formation. Neutrophil swarm formation coincided in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.