Concurrent lipopolysaccharide enhances chemotactic response of human polymorphonuclear leukocytes to bacterial chemotaxin

Isomerase-catalyzed binding of interleukin-1 receptor-associated kinase 1 to the EVH1 domain of vasodilator-stimulated phosphoprotein

Interleukin-1 receptor-associated kinase 1 (IRAK1) is a crucial signaling kinase in the immune system, involved in Toll-like receptor signaling. Vasodilator-stimulated phosphoprotein (VASP) is a central player in cell migration that regulates actin polymerization and connects signaling events to cytoskeletal remodeling. A VASP–IRAK1 interaction is thought to be important in controlling macrophage migration in response to protein kinase C-ε activation. We show that the monomeric VASP EVH1 domain directly binds to the 168WPPPP172 motif in the IRAK1 undefined domain (IRAK1-UD) with moderate affinity (KDApp = 203 ± 3 μM). We further show that this motif adopts distinct cis and trans isomers for the Trp168–Pro169 peptide bond with nearly equal populations, and that binding to the VASP EVH1 domain is specific for the trans isomer, coupling binding to isomerization. Nuclear magnetic resonance line shape analysis and tryptophan fluorescence experiments reveal the complete kinetics and thermodynamics of the binding reaction, showing diffusion-limited binding to the trans isomer followed by slow, isomerization-dependent binding. We further demonstrate that the peptidyl-prolyl isomerase cyclophilin A (CypA) catalyzes isomerization of the Trp168–Pro169 peptide bond and accelerates binding of the IRAK1-UD to the VASP EVH1 domain. We propose that binding of IRAK1 to tetrameric VASP is regulated by avidity through the assembly of IRAK1 onto receptor-anchored signaling complexes and that an isomerase such as CypA may modulate IRAK1 signaling in vivo. These studies demonstrate a direct interaction between IRAK1 and VASP and suggest a potential mechanism for how this interaction might be regulated by both assembly of IRAK1 onto an activated signaling complex and PPIase enzymes.

TLR signaling paralyzes monocyte chemotaxis through synergized effects of p38 MAPK and global Rap-1 activation

Toll-like receptors (TLRs) that recognize pathogen associated molecular patterns and chemoattractant receptors (CKRs) that orchestrate leukocyte migration to infected tissue are two arms of host innate immunity. Although TLR signaling induces synthesis and secretion of proinflammatory cytokines and chemokines, which recruit leukocytes, many studies have reported the paradoxical observation that TLR stimulation inhibits leukocyte chemotaxis in vitro and impairs their recruitment to tissues during sepsis. There is consensus that physical loss of chemokine receptor (CKR) at the RNA or protein level or receptor usage switching are the mechanisms underlying this effect. We show here that a brief (<15 min) stimulation with LPS (lipopolysaccharide) at ~0.2 ng/ml inhibited chemotactic response from CCR2, CXCR4 and FPR receptors in monocytes without downmodulation of receptors. A 3 min LPS pre-treatment abolished the polarized accumulation of F-actin, integrins and PIP(3) (phosphatidylinositol-3,4,5-trisphosphate) in response to chemokines in monocytes, but not in polymorphonuclear neutrophils (PMNs). If chemoattractants were added before or simultaneously with LPS, chemotactic polarization was preserved. LPS did not alter the initial G-protein signaling, or endocytosis kinetics of agonist-occupied chemoattractant receptors (CKRs). The chemotaxis arrest did not result from downmodulation of receptors or from inordinate increase in adhesion. LPS induced rapid p38 MAPK activation, global redistribution of activated Rap1 (Ras-proximate-1 or Ras-related protein 1) GTPase and Rap1GEF (guanylate exchange factor) Epac1 (exchange proteins activated by cyclic AMP) and disruption of intracellular gradient. Co-inhibition of p38 MAPK and Rap1 GTPase reversed the LPS induced breakdown of chemotaxis suggesting that LPS effect requires the combined function of p38 MAPK and Rap1 GTPase.

Neutrophils play a critical role in development of LPS-induced airway disease

We investigated the role of neutrophils in the development of endotoxin-induced airway disease via systemic neutrophil depletion of C3H/HeBFeJ mice and coincident inhalation challenge with lipopolysaccharide (LPS) over a 4-wk period. Mice were made neutropenic with intraperitoneal injections of neutrophil antiserum before and throughout the exposure period. Experimental conditions included LPS-exposed, antiserum-treated; LPS-exposed, control serum-treated; air-exposed, antiserum-treated; and air-exposed, control serum-treated groups. Physiological, biological, and morphological assessments were performed after a 4-wk exposure and again after a 4-wk recovery period. After the 4-wk exposure, LPS-induced inflammation of the lower airways was significantly attenuated in the neutropenic mice, although airway responsiveness (AR) to methacholine (MCh) remained unchanged. After the recovery period, LPS-exposed neutrophil-replete mice had increased AR to MCh when compared with the LPS-exposed neutropenic animals. Morphometric data indicate that the 4-wk exposure to LPS leads to a substantial expansion of the subepithelial area of the medium-sized airways (90-129 microm diameter) in nonneutropenic mice but not neutropenic mice, and this difference persisted even after the recovery period. Expression of bronchial epithelial and subepithelial transforming growth factor-beta1 (TGF-beta1) was diminished in the challenged neutropenic mice compared with the neutrophil-sufficient mice. These studies demonstrate that neutrophils play a critical role in the development of chronic LPS-induced airway disease.

Coelomocyte locomotion in the sipunculan Themiste petricola induced by exogenous and endogenous chemoattractants: role of a CD44-like antigen--HA interaction

Cell migration is a key event in the invertebrate immuno-defense system. Microbial products like lipopolysacharide (LPS) and formyl-methyl-leucyl-phenylalanine (fMLP) promote cell recruitment to sites of infection. In mammals, complement activation by factors such as zymosan induces C5a production, which influences leukocyte migration. The endogenous factor hyaluronic acid (HA), an extracellular matrix component, also promotes cell migration through its receptor CD44. We evaluated whether coelomocytes from the sipunculan worm T. petricola migrated towards LPS, fMLP, or zymosan treated plasma (ZTP) and if HA was involved in coelomocyte migration and adhesion. We also evaluated if antibodies specific for mouse HA receptor CD44 inhibited any of the effects induced by HA. Using microchemotaxis chambers we found that coelomocytes migrated towards exogenously and endogenously derived chemoattractants. We also observed that HA was a potent chemotactic signal and that coelomocytes adhered strongly to plates coated with LMW-HA but not with HMW-HA. In addition we found that these HA mediated effects were blocked by the monoclonal antibody IM7 directed to mouse CD44, suggesting that a CD44-like cross-reactive antigen might play a role in HA mediated coelomocyte locomotion.

Inhibition of LPS-induced airway hyperresponsiveness and airway inflammation by LPS antagonists

To determine whether the inflammatory effects of inhaled endotoxin could be prevented, we pretreated mice with synthetic competitive antagonists (975, 1044, and 1287) for lipopolysaccharide (LPS) before a LPS inhalation challenge. In preliminary studies, we found that these LPS antagonists did not act as agonists in vitro (THP-1 cells) or in vivo (after intratracheal instillation of 10 microg) and that these compounds (at least 1 microg/ml) effectively antagonized the release of tumor necrosis factor-alpha by LPS-stimulated THP-1 cells. Pretreatment of mice with 10 microg of either 1044 or 1287 resulted in a decrease in the LPS-induced airway hyperreactivity. Moreover, pretreatment of mice with 10 microg of 975, 1044, or 1287 resulted in significant reductions in LPS-induced lung lavage fluid concentrations of total cells, neutrophils, and specific proinflammatory cytokines compared with mice pretreated with sterile saline. Using residual oil fly ash to induce airway inflammation, we found that the action of the LPS antagonists was specific to LPS-induced airway disease. These results suggest that LPS antagonists may be an effective and potentially safe treatment for endotoxin-induced airway disease.

Chitin and chitosan stimulate canine polymorphonuclear cells to release leukotriene B4 and prostaglandin E2

The effects of chitin and chitosan on the release of arachidonic acid products were investigated in this study. Supernatants of canine polymorphonuclear cell (PMN) suspensions incubated with chitin and chitosan contained a leukotriene B4 (LTB4) concentration high enough to induce canine PMN migration in vitro. The supernatants also contained the same concentration of prostaglandin E2 (PGE2) as that normally found in the peripheral blood of dogs. Intraperitoneal administration of chitosan to dogs induced peritoneal exudative fluid (PEF), but chitin did not. The PEF contained numerous PMNs and macrophages. The supernatant of PEF contained both heat-stable and heat-labile chemotactic factors for canine PMNs. It also contained enough LTB4 to attract the canine PMNs in vitro.

Chemosensory responses of Acanthamoeba castellanii: visual analysis of random movement and responses to chemical signals

A visual assay slide chamber was used in conjunction with time-lapse videomicroscopy to analyze chemotactic behavior of axenically grown Acanthamoeba castellanii. Data were collected and analyzed as vector scatter diagrams and cell tracks. Amebas responded to a variety of bacterial products or potential bacterial products by moving actively toward the attractant. Responses to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP), lipopolysaccharide, and lipid A were statistically significant (P < or = 0.03), as was the response to fMLP benzylamide (P < or = 0.05). Significant responses to cyclic AMP, lipoteichoic acid, and N-acetyl glucosamine were also found. Chemotactic peptide antagonists, mannose, mannosylated bovine serum albumin, and N-acetyl muramic acid all yielded nonsignificant responses (P > 0.05). There was no single optimal concentration for response to any of the attractants tested, and amebas responded equally over the range of concentrations tested. Pretreatment of amebas with chemotactic peptides, bacterial products, and bacteria reduced the directional response to attractants. Amebas that had been grown in the presence of bacteria appeared more responsive to chemotactic peptides. Treatment of amebas with trypsin reduced the response of cells to chemotactic peptides, though sensitivity was restored within a couple of hours. This suggests the ameba membrane may have receptors, sensitive to these bacterial substances, which are different from the mannose receptors involved in binding bacteria to the membrane during phagocytosis. The rate of movement was relatively constant (ca. 0.40 microns/s), indicating that the locomotor response to these signals is a taxis, or possibly a klinokinesis, but not an orthokinesis. Studies of the population diffusion rate in the absence of signals indicate that the basic population motility follows the pattern of a Levy walk, rather than the more familiar Gaussian diffusion. This suggests that the usual mathematical models of ameboid dispersion may need to be modified.

Analysis of the attraction of haemocytes from Mytilus edulis by molecules of bacterial origin

Boyden chamber assays were performed to test the stimulatory effect of different bacterial products on the migratory activity of Mytilus haemocytes. The results indicate that these blood cells exhibit chemotactic as well as chemokinetic reactions. Lipopolysaccharides (LPS) from both Serratia marcescens and Escherichia coli stimulated the migration of cells through the membrane of the Boyden chamber when LPS was present in the lower compartment only. In contrast, addition of LPS to the lower as well as to the upper chamber did not increase the rate of migrating cells. Thus, LPS seemed to act as a chemotaxis-stimulating substance. Further analysis indicated that complete LPS molecules are required for cell stimulation because this did not occur when either the lipid or polysaccharide moieties of LPS were tested alone. Unlike LPS, the formylated tripeptide N-FMLP stimulated random cell migration. The peptide, which is released by bacteria, induced a higher haemocyte motility when present in both wells of the Boyden chamber than in tests where it was added to the lower compartment only. This chemokinetic response was not stimulated by the tetrapeptide N-FMLPLys. These findings demonstrate that bacterial products may elicit chemotactic and/or chemokinetic reactions in haemocytes from an invertebrate, and that the type of reaction that occurs is dependent upon the nature of the molecules presented.