Regulation of CD18 expression in human neutrophils as related to shape changes

Intracellular β2 integrin (CD11/CD18) interacting partners in neutrophil trafficking

BACKGROUND

Neutrophil recruitment during acute inflammation critically depends on the spatial and temporal regulation of β₂ integrins (CD11/CD18). This regulation occurs by inside-out and outside-in signalling via interaction of cytoplasmic proteins with the intracellular domains of the integrin α- and β-subunits. The underlying molecular mechanisms regulating β₂ integrins in neutrophils are still incompletely understood.

AIM

This review provides a comprehensive overview of our current knowledge on proteins interacting with the cytoplasmic tail of CD18, the conserved β-subunit of β₂ integrins, their regulation and their functional importance for neutrophil trafficking during acute inflammation.

RESULTS

A total of 22 proteins including Talin, Kindlin 3 and Coronin 1A have been reported to interact with the CD18 cytoplasmic tail. Here, proteins binding to the cytoplasmic domain of CD18 in experiments using purified, recombinant proteins or peptides in, for example, pull-down assays, are defined as direct interactors. Proteins that have been shown to interact with the cytoplasmic domain of CD18 using whole cell lysates in, for example, pull-down experiments are claimed as interacting proteins without evidence for direct interaction. In summary, β₂ integrin activation and signalling depend on a specific subset of proteins interacting with CD18 and their precise regulation. If disturbed, profound defects of neutrophil recruitment and activation become evident compromising the innate immune response.

CONCLUSIONS

The knowledge of proteins interacting with β₂ integrins and their regulation during neutrophil trafficking does not only improve our basic understanding of innate immunity but may pave the way to novel therapeutic strategies in the treatment of inflammatory diseases.

Salmonella detoxifying enzymes are sufficient to cope with the host oxidative burst

The oxidative burst produced by the NADPH oxidase (Phox) is an essential weapon used by host cells to eradicate engulfed pathogens. In Salmonella typhimurium, oxidative stress resistance has been previously proposed to be mediated by the pathogenicity island 2 type III secretion system (T3SS-2), periplasmic superoxide dismutases and cytoplasmic catalases/peroxidases. Here, we fused an OxyR-dependent promoter to the gfp to build the ahpC-gfp transcriptional fusion. This reporter was used to monitor hydrogen peroxide levels as sensed by Salmonella during the course of an infection. We showed that the expression of this fusion was under the exclusive control of reactive oxygen species produced by the host. The ahpC-gfp expression was noticeably modified in the absence of bacterial periplasmic superoxide dismutases or cytoplasmic catalases/peroxidases. Surprisingly, inactivation of the T3SS-2 had no effect on the ahpC-gfp expression. All together, these results led to a model in which Salmonella resistance relies on its arsenal of detoxifying enzymes to cope with Phox-mediated oxidative stress.

Generation of inflammatory stimuli: how bacteria set up inflammatory responses in the gingiva

OBJECTIVES

The primary aetiologic factor of periodontal disease is the bacterial biofilm. Gram-positive and gram-negative bacteria possess a plethora of structural or secreted components that may cause direct destruction to periodontal tissues or stimulate host cells to activate a wide range of inflammatory responses. These responses are intended to eliminate the microbial challenge, but may often cause further tissue damage.

METHODS

This review has been divided into three parts: (a) bacterial virulence factors, which includes basic information on bacterial virulence factors, and the principle inflammatory responses that host cells elicit against these factors, (b) main receptors and signalling pathways, which includes basic information about the main receptors that interact with the bacterial virulence factors, the nature of these interactions, and the activated signalling pathways that lead to inflammatory responses, and (c) initiation of inflammation, which includes a model by which the virulence factors may interact with host cells and lead to inflammatory responses in the gingiva.

FINDINGS AND CONCLUSIONS

Bacterial components/virulence factors may be involved in modulating inflammatory responses and include: lipopolysaccharides (LPS), peptidoglycans, lipotechoic acids, fimbriae, proteases, heat-shock proteins, formyl-methionyl peptides, and toxins. Potential host cell receptors involved in recognizing bacterial components and initiating signalling pathways that lead to inflammatory responses include: Toll-like receptors (TLRs), CD14, nucleotide-binding oligomerization domain proteins (Nod) and G-protein-coupled receptors, including formyl-methionyl peptide receptors and protease-activated receptors. Of the above bacterial and host molecules, evidence from experimental animal studies implicate LPS, fimbriae, proteases, TLRs, and CD14 in periodontal tissue or alveolar bone destruction. However, evidence verifying the involvement of any of the above molecules in periodontal tissue destruction in humans does not exist.

Expression and function of formyl peptide receptors on human fibroblast cells

The migration of polymorphonuclear leukocytes from the blood to sites of infection in tissues is a hallmark of the innate immune response. Formylated peptides produced as a byproduct of bacterial protein synthesis are powerful chemoattractants for leukocytes. Formyl peptides bind to two different G protein-coupled receptors (formyl peptide receptor (FPR) and the low affinity formyl peptide receptor-like-1 (FPRL1)) to initiate a signal transduction cascade leading to cell activation and migration. Our analysis of expressed sequences from many cDNA libraries draws attention to the fact that FPRs are widely expressed in nonlymphoid tissues. Here we demonstrate that FPRs are expressed by normal human lung and skin fibroblasts and the human fibrosarcoma cell line HT-1080. The expression on fibroblasts of receptors for bacteria-derived peptides raises questions about the possible function of these receptors in nonleukocyte cells. We studied the function of FPRs on fibroblasts and find that stimulation with fMLP triggers dose-dependent migration of these cells. Furthermore, fMLP induces signal transduction including intracellular calcium flux and a transient increase in F-actin. The fMLP-induced adhesion and motility of fibroblasts on fibronectin require functional protein kinase C and phosphatidylinositol 3-kinase. This first report of a functional formyl peptide receptor in cells of fibroblast origin opens new possibilities for the role of fibroblasts in innate immune responses.

Ceramide generation in situ alters leukocyte cytoskeletal organization and beta 2-integrin function and causes complete degranulation

Ceramide levels increase in activated polymorphonuclear neutrophils, and here we show that endogenous ceramide induced degranulation and superoxide generation and increased surface beta(2)-integrin expression. Ceramide accumulation reveals a bifurcation in integrin function, as it abolished agonist-induced adhesion to planar surfaces, yet had little effect on homotypic aggregation. We increased cellular ceramide content by treating polymorphonuclear neutrophils with sphingomyelinase C and controlled for loss of sphingomyelin by pretreatment with sphingomyelinase D to generate ceramide phosphate, which is not a substrate for sphingomyelinase C. Pretreatment with the latter enzyme blocked all the effects of sphingomyelinase C. Ceramide generation caused a Ca(2+) flux and complete degranulation of both primary and secondary granules and increased surface beta(2)-integrin expression. These integrins were in a nonfunctional state, and subsequent activation with platelet-activating factor or formyl-methionyl-leucyl-phenylalanine induced beta(2)-integrin-dependent homotypic aggregation. However, these cells were completely unable to adhere to surfaces via beta(2)-integrins. This was not due to a defect in the integrins themselves because the active conformation could be achieved by cation switching. Rather, ceramide affected cytoskeletal organization and inside-out signaling, leading to affinity maturation. Cytochalasin D induced the same disparity between aggregation and surface adhesion. We conclude that ceramide affects F-actin rearrangement, leading to massive degranulation, and reveals differences in beta(2)-integrin-mediated adhesive events.

Cellular responses to FMLP challenging: a mini-review

FMLP (N-formyl-methionyl-leucyl-phenylalanine) and other N-formylpeptides are powerful "activators" of polymorphonuclear and mononuclear phagocytes, but they are also active on other cell types. Present knowledge about formylpeptide receptors and the relevant tools for their imaging and the study of their dynamics are briefly discussed. The main responses elicited by FMLP in granulocytes are cell polarisation, the generation of reactive oxygen species, the production of arachidonic acid metabolites, and the release of lysosomal enzymes. The transduction cascades involved and the agents able to modulate these responses are reviewed. Homologous desensitization and heterologous desensitization of the FMLP-receptor following ligation of other chemokine receptors are also outlined. Finally, the receptor expression and the pharmacological and toxic actions of FMLP upon other tissues and organs, and its actions on the developing embryo, are illustrated.

Modeling of molecular mechanisms of cell adhesion

Cellular adhesion is a process of great importance in biology. We present a simple model of the adhesion process in which the molecular mechanisms involve a receptor, a ligand, and the cytoskeleton of the cell. Based on the energetic consideration of the process, we propose a molecular interpretation of the existing experimental data. The model suggests that the interaction of the receptor and (or) receptor-ligand with the cytoskeleton can have important influence on the formation and strength of the adhesion complex as well as on the subsequent interaction with different ligands. When conformational changes take place during the adhesion process, the characterization of the adhesion bonds based on chemical kinetics alone seems to be incomplete and must be supplemented by parameters, describing the functionality of the complex, i.e., change of the affinity for different ligands, as in the signal transduction, or the strength of the bond, as in the adhesion process.