Toll-like receptor-4 (TLR4) signaling augments chemokine-induced neutrophil migration by modulating cell surface expression of chemokine receptors

Activation of RAW264.7 macrophages by bacterial DNA and lipopolysaccharide increases cell surface DNA binding and internalization

Bacterial DNA containing unmethylated CpG motifs is a pathogen-associated molecular pattern (PAMP) that interacts with host immune cells via a toll-like receptor (TLR) to induce immune responses. DNA binding and internalization into cells is independent of TLR expression, receptor-mediated, and required for cell activation. The objective of this study was to determine whether exposure of immune cells to bacterial DNA affects DNA binding and internalization. Treatment of RAW264.7 cells with CpG oligodeoxynucleotide (ODN) for both 18 and 42 h resulted in a significant increase in DNA binding, whereas non-CpG ODN had no effect on DNA binding. Enhanced DNA binding was non-sequence-specific, inhibited by unlabeled DNA, showed saturation, was consistent with increased cell surface DNA receptors, and resulted in enhanced internalization of DNA. Treatment with Escherichia coli DNA or lipopolysaccharide (LPS) also resulted in a significant increase in DNA binding, but treatment with interleukin-1alpha, tumor necrosis factor-alpha, or phorbol 12-myristate 13-acetate had no effect on DNA binding. Soluble factors produced in response to treatment with CpG ODN or LPS did not affect DNA binding. These studies demonstrate that one consequence of activating the host innate immune response by bacterial infection is enhanced binding and internalization of DNA. During this period of increased DNA internalization, RAW264.7 cells were hypo-responsive to continued stimulation by CpG ODN, as assessed by tumor necrosis factor-alpha activity. We speculate the biological significance of increasing DNA binding and internalization following interaction with bacterial PAMPs may provide a mechanism to limit an ongoing immune inflammatory response by enhancing clearance of bacterial DNA from the extracellular environment.

Toll-like receptors and the host defense against microbial pathogens: bringing specificity to the innate-immune system

Toll-like receptors (TLRs) have been identified as a major class of pattern-recognition receptors. Recognition of pathogen-associated molecular patterns (PAMPs) by TLRs, alone or in heterodimerization with other TLR or non-TLR receptors, induces signals responsible for the activation of genes important for an effective host defense, especially proinflammatory cytokines. Although a certain degree of redundancy exists between signals induced by the various TLRs, recent studies have identified intracellular pathways specific for individual TLRs. This leads to the release of cytokine profiles specific for particular PAMPs, and thus, TLRs confer a certain degree of specificity to the innate-immune response. In addition to the activation of the innate-immune response, TLR-mediated recognition represents a link between the innate- and acquired-immune systems, by inducing the maturation of dendritic cells and directing the T helper responses. Alternatively, recent data have also suggested TLR-mediated escape mechanisms used by certain pathogenic microorganisms, especially through TLR2 induction of anti-inflammatory cytokines. Finally, the crucial role of TLRs for the host defense against infections has been strengthened recently by the description of patients partially defective in the TLR-activation pathways.

Reduced stem cell mobilization in mice receiving antibiotic modulation of the intestinal flora: involvement of endotoxins as cofactors in mobilization

Since endotoxins are potent inducers of stem cell mobilization, we hypothesized that their presence in the gut may play a role in cytokine-induced mobilization. To address this possibility we added ciprofloxacin and polymyxin B to the drinking water of Balb/c mice mobilized with either interleukin-8 (IL-8), granulocyte colony-stimulating factor (G-CSF), or flt3 ligand (FL). The yield of colony-forming units (CFUs) was significantly reduced in all mice treated with these antibiotics when compared with controls (IL-8: 192 +/- 61 vs 290 +/- 64, P <.05; G-CSF: 1925 +/- 1216 vs 3371 +/- 1214, P <.05; FL: 562 +/- 213 vs 1068 +/- 528, P <.05). Treatment with ciprofloxacin eliminated only aerobic Gram-negative bacteria from the feces without effect on mobilization. Polymyxin B treatment did not result in decontamination but significantly reduced the number of mobilized hematopoietic progenitor cells (HPCs) most likely due to the endotoxin binding capacity of polymyxin B. More than 90% of the gastrointestinal flora consists of anaerobic bacteria. Elimination of the anaerobic flora by metronidazol led to a significantly reduced number of mobilized HPCs when compared with controls (IL-8: 55 +/- 66 vs 538 +/- 216, P <.05). Germ-free OF1 mice showed a significantly reduced mobilization compared with their wild-type controls (IL-8 controls: 378 +/- 182, IL-8 germ free: 157 +/- 53, P <.05). Finally, we performed reconstitution experiments adding Escherichia coli-derived endotoxins to the drinking water of decontaminated mice. This resulted in partial restoration of the IL-8-induced mobilization (67 +/- 28 vs 190 +/- 98.1, P <.01). Our results indicate that endotoxins serve as cofactors in cytokine-induced mobilization. Modification of the endotoxin content by antibiotic treatment may affect the yield of cytokine-induced mobilization.

TLR4 signaling induces TLR2 expression in endothelial cells via neutrophil NADPH oxidase

Interactions of polymorphonuclear neutrophils (PMNs) with endothelial cells may contribute to the activation of endothelial cell responses involved in innate immunity. We explored a novel function of PMN NADPH oxidase in the mechanism of Toll-like receptor-2 (TLR2) upregulation induced by LPS-TLR4 signaling in endothelial cells. We showed that LPS induced TLR2 up-regulation through TLR4- and MyD88-dependent signaling. In neutropenic mice, the LPS-induced NF-kB activation and TLR2 expression were significantly reduced, and both responses were restored upon repletion by PMN obtained from WT mice but not by PMNs from NADPH oxidase gp91pho(-/-) mice. These findings were recapitulated in mouse lung vascular endothelial cells cocultured with PMNs, indicating that the augmented NF-kB activation and the resultant TLR2 upregulation in endothelial cells were secondary to oxidant signaling generated by PMN NADPH oxidase. The functional relevance of NADPH oxidase in mediating TLR4-induced TLR2 expression in endothelial cells was evident by markedly elevated and stable ICAM-1 expression as well as augmented PMN migration in response to sequential challenge with LPS and peptidoglycan. Thus, PMN NADPH oxidase-derived oxidant signaling is an important determinant of the cross talk between TLR4 and TLR2 and the control of endothelial cell activation.

Neutrophil cell signaling in infection: role of phosphatidylinositide 3-kinase

Neutrophils play a pivotal role in the innate immune response to microbial pathogens. They are uniquely suited to this role by virtue of specialized antimicrobial capabilities that include the capacity to sense minute amounts of microbial products and inflammatory mediators, to move to the site of infection, and finally to bind, internalize and kill the pathogens. To optimize host defense capabilities while minimizing damage to host tissues ('collateral damage'), these microbicidal responses must be tightly regulated. Additionally, neutrophils clear inflammatory debris, a process that is necessary for restoration of the native architecture and function of the tissue. This review highlights some recent advances in our knowledge of cell signaling as it pertains to neutrophil function, with specific emphasis on the role of the phosphatidylinositide 3-kinase in antimicrobial function.

Toll-like receptors stimulate human neutrophil function

The first immune cell to arrive at the site of infection is the neutrophil. Upon arrival, neutrophils quickly initiate microbicidal functions, including the production of antimicrobial products and proinflammatory cytokines that serve to contain infection. This allows the acquired immune system enough time to generate sterilizing immunity and memory. Neutrophils detect the presence of a pathogen through germ line-encoded receptors that recognize microbe-associated molecular patterns. In vertebrates, the best characterized of these receptors are Toll-like receptors (TLRs). We have determined the expression and function of TLRs in freshly isolated human neutrophils. Neutrophils expressed TLR1, 2, 4, 5, 6, 7, 8, 9, and 10-all the TLRs except TLR3. Granulocyte-macrophage colony-stimulating factor (GM-CSF) treatment increased TLR2 and TLR9 expression levels. The agonists of all TLRs expressed in neutrophils triggered or primed cytokine release, superoxide generation, and L-selectin shedding, while inhibiting chemotaxis to interleukin-8 (IL-8) and increasing phagocytosis of opsonized latex beads. The response to the TLR9 agonist nonmethylated CpG-motif-containing DNA (CpG DNA) required GM-CSF pretreatment, which also enhanced the response to the other TLR agonists. Finally, using quantitative polymerase chain reaction (QPCR), we demonstrate a chemokine expression profile that suggests that TLR-stimulated neutrophils recruit innate, but not acquired, immune cells to sites of infection.

Chemokines: roles in leukocyte development, trafficking, and effector function

Chemokines, representing a large superfamily of 8- to 15-kd proteins, were originally discovered through their ability to recruit various cell types into sites of inflammation. It is now clear that these molecules play a much wider role in immune homeostasis, playing key roles in driving the maturation, homing, and activation of leukocytes. In this review we analyze the roles chemokines play in the development, recruitment, and activation of leukocytes. Because signaling from the receptors drives these processes, signal transduction from chemokine receptors will also be reviewed. Taken together, we highlight the various points at which chemokines contribute to allergic inflammation and at which their targeting might contribute to new therapies for type I hypersensitivity reactions.