Receptors for the chemoattractants C5a and IL-8 are clustered on the surface of human neutrophils

Chemokine receptor distribution on the surface of repolarizing T cells

T cells migrate constitutively with a polarized morphology, underpinned by signaling compartmentalization and discrete cytoskeletal organizations, giving rise to a dynamic and expansive leading edge, distinct from the stable and constricted uropod at the rear. In vivo, the motion and function of T cells at various stages of differentiation is highly directed by chemokine gradients. When cognate ligands bind chemokine receptors on their surface, T cells respond by reorientating their polarity axis and migrating toward the source of the chemokine signal. Despite the significance of such chemotactic repolarization to the accurate navigation and function of T cells, the precise signaling mechanisms that underlie it remain elusive. Notably, it remained unclear whether the distribution of chemokine receptors on the T cell surface is altered during repolarization. Here, we developed parallel cell-secreted and microfluidics-based chemokine gradient delivery methods and employed both fixed imaging and live lattice light-sheet microscopy to investigate the dynamics of chemokine receptor CCR5 on the surface of primary murine CD8+ T cells. Our findings show that, during constitutive migration, chemokine receptor distribution is largely isotropic on the T cell surface. However, upon exposure to a CCL3 gradient, surface chemokine receptor distributions exhibit a transient bias toward the uropod. The chemokine receptors then progressively redistribute from the uropod to cover the T cell surface uniformly. This study sheds new light on the dynamics of surface chemokine receptor distribution during T cell repolarization, advancing our understanding of the signaling of immune cells in the complex chemokine landscapes they navigate.

Staphylococcus aureus Staphopain A inhibits CXCR2-dependent neutrophil activation and chemotaxis

Neutrophil activation and recruitment to the site of infection are critical for host immunity. In humans, the cysteine protease Staphopain A of the pathogen S. aureus blocks this process by cleaving the chemokine receptor CXCR2.

The CXC chemokine receptor 2 (CXCR2) on neutrophils, which recognizes chemokines produced at the site of infection, plays an important role in antimicrobial host defenses such as neutrophil activation and chemotaxis. Staphylococcus aureus is a successful human pathogen secreting a number of proteolytic enzymes, but their influence on the host immune system is not well understood. Here, we identify the cysteine protease Staphopain A as a chemokine receptor blocker. Neutrophils treated with Staphopain A are unresponsive to activation by all unique CXCR2 chemokines due to cleavage of the N-terminal domain, which can be neutralized by specific protease inhibitors. Moreover, Staphopain A inhibits neutrophil migration towards CXCR2 chemokines. By comparing a methicillin-resistant S. aureus (MRSA) strain with an isogenic Staphopain A mutant, we demonstrate that Staphopain A is the only secreted protease with activity towards CXCR2. Although the inability to cleave murine CXCR2 limits in-vivo studies, our data indicate that Staphopain A is an important immunomodulatory protein that blocks neutrophil recruitment by specific cleavage of the N-terminal domain of human CXCR2.

Messenger RNA expression of multiple immune mediators in leukocytes from elective orthopaedic surgical patients

The aim of this longitudinal observational study was to investigate and describe the spectrum of messenger ribonucleic acid (mRNA) expression of multiple inflammatory markers in circulating leukocytes after major orthopaedic surgery. We studied ten elective arthroplasty patients perioperatively on the orthopaedic ward, and eight healthy volunteers for a comparison group. Venous blood specimens were collected preoperatively, and 6, and 24 hours postoperatively, together with 6- and 24-hour postoperative wound drain specimens. The mRNA of 21 different inflammatory mediators was measured by real-time reverse transcriptase Polymerase Chain Reaction. Comparisons were made with the venous blood of eight healthy comparison subjects. There were significant differences (P<0.01) between preoperative specimens and normal comparisons (i.e. higher MPO, PDGF, TREM and IRAKM; lower mtHSP) reflecting the effects of chronic inflammation associated with osteoarthritis. There were significant increases (P<0.01) in expression of IL-8, MPO, IL-1beta, TREM, MMP9, and C5aR in circulating blood at 24 hours postoperatively, but not at six hours. There was no significant decrease in expression of any inflammatory mediator. There was no statistical difference in inflammatory mediator expression between drain specimens and venous specimens taken at the same time. We conclude that, in uncomplicated orthopaedic surgical patients, there was up-regulation of some cytokine mRNAs at both the local and systemic levels during the first day after surgery. We observed no evidence of immune compartmentalization, and found no evidence for innate immune paresis within the first day after surgery.

The use of real time rtPCR to quantify inflammatory mediator expression in leukocytes from patients with severe sepsis

Real-time reverse transcriptase polymerase chain reaction (RT rtPCR) was used to quantify the pattern of inflammatory mediator mRNA expression in circulating leukocytes from adult patients diagnosed with severe sepsis. We analysed 29 blood samples from 26 severely septic patients with different septic sources and eight samples from eight healthy adult volunteers. RT rtPCR was used to quantify mRNA expression of 21 different inflammatory mediators in peripheral leukocytes. The median variability in gene expression in the sepsis patients was 10.5 times greater than the variability of the healthy comparison group. We found a significant change in the regulation for the following genes: C5aR (20-fold, P < 0.001), IL-8 (29-fold, P < 0.001), MMP9 (72-fold, P < 0.001), HSP70 (2.4-fold, P = 0.02), and RIP2 (1.8-fold, P < 0.04) were up-regulated. Conversely the median expression of IFNgamma, and IL-6 were zero (P < 0.001), and mtHSP (0.4-fold, P = 0.02) was significantly down-regulated. Using linear discriminant analysis, IFNgamma, IL-12, and TLR4 were correlated to a negative outcome. Different septic sources (peritonitis, burn, pneumonia and musculo-skeletal infections) resulted in significantly different mRNA patterns. The RT rtPCR is a useful tool to monitor the immune response in septic patients. We found a very high variability in inflammatory mediator expression among septic patients compared to healthy volunteers. This suggests that any future immune-modulatory therapy may need to be individualized to the patient's requirements as monitored by RT rtPCR. Different sources of sepsis may result in markedly different activation patterns.

A stochastic view of lymphocyte motility and trafficking within the lymph node

Two-photon microscopy is providing literal insight into the cellular dynamics of lymphoid organs and, guided by analysis of three-dimensional images, into mechanisms that underlie cell migration and antigen recognition in vivo. This review describes lymphocyte motility and antigen recognition in the native tissue environment and compares these results with a much more extensive literature on lymphocyte motility, signaling, and chemotaxis in vitro. We discuss the in vitro literature on dynamic aspects of lymphocyte motility, chemotaxis, and the response to antigen and present the view that random migration of lymphocytes may drive a stochastic mechanism of antigen recognition in lymphoid organs, rather than being guided by chemotaxis.

Development of a new method of analysing chemotactic deactivation of human neutrophil granulocytes

Measurement of chemotactic migration of human neutrophil granulocytes (PMN) induced by chemotaxins serves as a simple and reliable method for assessing the expression of chemotaxin receptors. Incubation of PMN with a certain chemotaxin leads to a diminished chemotactic migration towards this chemotaxin. This is called chemotactic deactivation. We developed a new deactivation chamber to determine chemotaxis and chemotactic deactivation of human PMN. This novel chamber is a modification of the commercially available acrylic 48-well microchemotaxis chamber consisting of an upper block with wells drilled all the way through the block and a blind-well lower block. Both blocks are separated by a polycarbonate membrane. PMN from the wells in the upper block migrate through the pores of the membrane into the wells of the lower block containing the chemoattractants. Migrated PMN on the lower side of the PC membrane were quantified after staining by measuring specific light absorbance. The chemotactic activity is quantified as a ratio of stimulated migration and random migration (chemotactic index=CI). For our novel chamber, only the upper blocks of this commercial chamber were connected like a sandwich, including a polyvinylpyrrolidone-free polycarbonate membrane with a pore size of 3 microm. The wells in the upper compartment were filled with 5 x 10(4) PMN and deactivating chemotaxin. The lower block was then filled with the chemotactic stimulus and the chamber was then incubated in humidified air with 5% CO2 atmosphere at 37 degrees C. The influence of cell concentration, incubation time, chemotactic factor concentration, pore size and alkaline treatment of polycarbonate membranes on migrational activity of PMN have been investigated. The technique was rigorously standardized in order to optimize the assay conditions. The method is relatively simple, sensitive and fast. The determination of chemotaxis and deactivation are performed in the same chamber, thus avoiding cell loss due to nonspecific adherence in other incubation tubes. The chamber can be used to characterize the chemotactic activity of chemoattractants of unknown structure via known and unknown receptors. This new chamber can be very helpful in detecting unknown chemotactic stimuli, which are not detectable by, for example, antibodies.

Elastase controls the binding of the vitamin D-binding protein (Gc-globulin) to neutrophils: a potential role in the regulation of C5a co-chemotactic activity

The vitamin D-binding protein (DBP) binds to the plasma membranes of numerous cell types and mediates a diverse array of cellular functions. DBP bound to the surface of leukocytes serves as a co-chemotactic factor for C5a, significantly enhancing the chemotactic activity of pM concentrations of C5a. This study investigated the regulation of DBP binding to neutrophils as a possible key step in the process of chemotaxis enhancement to C5a. Using radioiodinated DBP as a probe, neutrophils released 70% of previously bound DBP into the extracellular media during a 60-min incubation at 37 degrees C. This was suppressed by serine protease inhibitors (PMSF, Pefabloc SC), but not by metallo- or thiol-protease inhibitors. DBP shed from neutrophils had no detectable alteration in its m.w., suggesting that a serine protease probably cleaves the DBP binding site, releasing DBP in an unaltered form. Cells treated with PMSF accumulate DBP vs time with over 90% of the protein localized to the plasma membrane. Purified neutrophil plasma membranes were used to screen a panel of protease inhibitors for their ability to suppress shedding of the DBP binding site. Only inhibitors to neutrophil elastase prevented the loss of membrane DBP-binding capacity. Moreover, treatment of intact neutrophils with elastase inhibitors prevented the generation of C5a co-chemotactic activity from DBP. These results indicate that steady state binding of DBP is essential for co-chemotactic activity, and further suggest that neutrophil elastase may play a critical role in the C5a co-chemotactic mechanism.

Defective neutrophil degranulation induced by interleukin-8 and complement 5a and down-regulation of associated receptors in children vertically infected with human immunodeficiency virus type 1

The polymorphonuclear neutrophils (PMNs) of patients infected with human immunodeficiency virus type 1 (HIV-1) show impaired microbicidal responses. The present study assessed the functional integrity of PMN degranulation responses and the expression of specific receptors that mediate these responses in a group of children vertically infected with HIV-1. PMN degranulation in response to interleukin-8 (IL-8) and complement 5a (C5a) was measured in a group of HIV-1-infected children with mild and severe clinical disease and in an uninfected control group. In addition, the expression of CXCR1, CXCR2, and CD88 on whole-blood PMNs was quantified by flow cytometry. Although CXCR1 expression was found to be largely unaltered in the HIV-1-infected children relative to that in the control children, the intensity of CXCR2 expression was significantly reduced in those with severe disease. Furthermore, there was a significant reduction in the percentage of cells expressing CD88 and in the intensity of CD88 fluorescence in the HIV-1-infected children compared to that in control children, with CD88 fluorescence intensity more significantly reduced in the presence of severe disease. PMNs from a large proportion of the HIV-1-infected children either showed reciprocal degranulation responses or were unresponsive to IL-8 and C5a, whereas the PMNs from the uninfected children showed positive responses. Inefficient agonist-induced degranulation may contribute to the increased susceptibility of HIV-1-infected children to secondary microbial infections. Furthermore, reduced expression of CXCR2 and CD88 may be suggestive of defects in other functions of PMNs from HIV-1-infected children.

Neutrophil priming by cytokines and vitamin D binding protein (Gc-globulin): impact on C5a-mediated chemotaxis, degranulation and respiratory burst

At the site of acute inflammation, leukocytes are confronted with multiple mediators which are expected to modulate each other with respect to cell responses to the individual ligand. Previous contact of neutrophils with pro-inflammatory cytokines, such as TNF-alpha or GM-CSF, or with the vitamin D binding protein (Gc-globulin) leads to the alteration of either multiple or rather distinct C5a-mediated neutrophil functions. Gc-globulin, the transport protein for 25-(OH)-D3, serves selectively as a cochemotactic factor for C5a/Ca(des)Arg. In contrast, TNF-alpha and GM-CSF, previously shown to modulate FMLP-induced neutrophil responses, are able to reduce C5a-mediated neutrophil chemotaxis, but augment their degranulation and respiratory burst activity. Cytokine priming was shown to be accompanied by a down-regulation of C5a receptors (CD88) whereas vitamin D binding protein had no impact on the level of neutrophil C5a receptors. C5a itself diminishes chemotaxis as well as degranulation and oxidative burst in response to a second dose of the same ligand (homologous desensitization). A similar effect, termed heterologous desensitization, occurs, if cell responses to a given mediator (e.g. to C5a) are reduced or even abolished upon the activation of another receptor of the same G-protein coupled chemoattractant receptor subfamily (e.g. receptors for FMLP or IL-8). In concert with C5a, certain molecules may either augment chemotaxis or shift neutrophil effector functions from migration to exocytosis, an essential step within the sequence of events in a coordinated inflammatory response.

Dynamics of a chemoattractant receptor in living neutrophils during chemotaxis

Persistent directional movement of neutrophils in shallow chemotactic gradients raises the possibility that cells can increase their sensitivity to the chemotactic signal at the front, relative to the back. Redistribution of chemoattractant receptors to the anterior pole of a polarized neutrophil could impose asymmetric sensitivity by increasing the relative strength of detected signals at the cell's leading edge. Previous experiments have produced contradictory observations with respect to receptor location in moving neutrophils. To visualize a chemoattractant receptor directly during chemotaxis, we expressed a green fluorescent protein (GFP)-tagged receptor for a complement component, C5a, in a leukemia cell line, PLB-985. Differentiated PLB-985 cells, like neutrophils, adhere, spread, and polarize in response to a uniform concentration of chemoattractant, and orient and crawl toward a micropipette containing chemoattractant. Recorded in living cells, fluorescence of the tagged receptor, C5aR-GFP, shows no apparent increase anywhere on the plasma membrane of polarized and moving cells, even at the leading edge. During chemotaxis, however, some cells do exhibit increased amounts of highly folded plasma membrane at the leading edge, as detected by a fluorescent probe for membrane lipids; this is accompanied by an apparent increase of C5aR-GFP fluorescence, which is directly proportional to the accumulation of plasma membrane. Thus neutrophils do not actively concentrate chemoattractant receptors at the leading edge during chemotaxis, although asymmetrical distribution of membrane may enrich receptor number, relative to adjacent cytoplasmic volume, at the anterior pole of some polarized cells. This enrichment could help to maintain persistent migration in a shallow gradient of chemoattractant.

Responding to attraction: chemotaxis and chemotropism in Dictyostelium and yeast

Polarized growth in response to external signals is essential for both the internal organization of cells and generation of complex multicellular structures during development. Oriented growth or movement requires specific detection of an external cue, reorganization of the cytoskeleton and subsequent growth or movement. Genetic approaches in both the budding yeast Saccharomyces cerevisiae and the social amoeba Dictyostelium discoideum have shed light on the molecular and cellular aspects of growth or movement towards an external signal. This review discusses the mechanisms and signalling pathways that enable yeast and Dictyostelium cells to translate external signals into directed growth and movement, respectively.

A role for Cdc42 in macrophage chemotaxis

Three members of the Rho family, Cdc42, Rac, and Rho are known to regulate the organization of actin-based cytoskeletal structures. In Bac1.2F5 macrophages, we have shown that Rho regulates cell contraction, whereas Rac and Cdc42 regulate the formation of lamellipodia and filopodia, respectively. We have now tested the roles of Cdc42, Rac, and Rho in colony stimulating factor-1 (CSF-1)-induced macrophage migration and chemotaxis using the Dunn chemotaxis chamber. Microinjection of constitutively activated RhoA, Rac1, or Cdc42 inhibited cell migration, presumably because the cells were unable to polarize significantly in response to CSF-1. Both Rho and Rac were required for CSF-1-induced migration, since migration speed was reduced to background levels in cells injected with C3 transferase, an inhibitor of Rho, or with the dominant-negative Rac mutant, N17Rac1. In contrast, cells injected with the dominant-negative Cdc42 mutant, N17Cdc42, were able to migrate but did not polarize in the direction of the gradient, and chemotaxis towards CSF-1 was abolished. We conclude that Rho and Rac are required for the process of cell migration, whereas Cdc42 is required for cells to respond to a gradient of CSF-1 but is not essential for cell locomotion.