Upregulation of interleukin-8 receptor in human polymorphonuclear neutrophils by formyl peptide and lipopolysaccharide

Regulation of human neutrophil chemokine receptor expression and function by activation of Toll-like receptors 2 and 4

Neutrophil chemokine receptor expression can be altered by exposure to Toll-like receptor (TLR) agonists, a process that is thought to have the potential to localize neutrophils to sites of infection. In order to investigate this process in more detail, we examined the regulation of highly pure neutrophil CXCR1 and CXCR2 expression and function by selective agonists of TLR2 (Pam(3)CSK(4)) and TLR4 (lipopolysaccharide, LPS). CXCR1 and CXCR2 were down-regulated by TLR engagement. CXCR2 loss was more rapid and showed a dependence upon soluble helper molecules (LPS binding protein and CD14) that was not evident for CXCR1, suggesting differential coupling of LPS signalling to CXCR1 and CXCR2 loss. However, TLR engagement in highly pure neutrophils did not result in complete loss of chemokine receptors, and LPS-treated neutrophils remained able to mount a respiratory burst to CXCL8 and CXCL1, and were able to migrate towards CXCL8 in assays of under-agarose chemotaxis. Thus, although treatment of purified human neutrophils with TLR2 and TLR4 agonists modifies chemokine receptor expression, remaining receptors remain functionally competent.

α-Melanocyte-stimulating hormone inhibits lipopolysaccharide-induced biological responses by downregulating CD14 from macrophages

Monocytes/macrophages are the first cells involved in inflammation. alpha-Melanocyte-stimulating hormone (alpha-MSH) is known to possess an anti-inflammatory role induced by a variety of stimuli; however, the molecular mechanisms underlying these effects are not clearly defined. In this report we provide evidence that alpha-MSH inhibited serum-activated lipopolysaccharide (SA-LPS)-induced proteolytic enzyme release, oxidative burst response, reactive oxygen intermediate generation, nitric oxide production, and adhesion molecule expression in monocyte-derived macrophages. alpha-MSH also inhibited SA-LPS-induced nuclear transcription factor kappaB activation not only in macrophages, but also in a T-cell line and human neutrophils isolated from fresh blood. alpha-MSH downregulated CD14, but not interleukin-1 receptor, tumor necrosis factor receptor 1 or 2 from the surface of macrophages. Anti-CD14 antibody was unable to protect alpha-MSH-mediated downregulation of CD14. Overall, our results suggest that alpha-MSH exerts its anti-inflammatory effect by a novel mechanism in macrophages through downregulating of the endotoxin receptor CD14.

Selective roles for Toll-like receptor (TLR)2 and TLR4 in the regulation of neutrophil activation and life span

Neutrophil responses to commercial LPS, a dual Toll-like receptor (TLR)2 and TLR4 activator, are regulated by TLR expression, but are amplified by contaminating monocytes in routine cell preparations. Therefore, we investigated the individual roles of TLR2 and TLR4 in highly purified, monocyte-depleted neutrophil preparations, using selective ligands (TLR2, Pam(3)CysSerLys(4) and Staphylococcus aureus peptidoglycan; TLR4, purified LPS). Activation of either TLR2 or TLR4 caused changes in adhesion molecule expression, respiratory burst (alone, and synergistically with fMLP), and IL-8 generation, which was, in part, dependent upon p38 mitogen-activated protein kinase signaling. Neutrophils also responded to Pam(3)CysSerLys(4) and purified LPS with down-regulation of the chemokine receptor CXCR2 and, to a lesser extent, down-regulation of CXCR1. TLR4 was the principal regulator of neutrophil survival, and TLR2 signals showed relatively less efficacy in preventing constitutive apoptosis over short time courses. TLR4-mediated neutrophil survival depended upon signaling via NF-kappa B and mitogen-activated protein kinase cascades. Prolonged neutrophil survival required both TLR4 activation and the presence of monocytes. TLR4 activation of monocytes was associated with the release of neutrophil survival factors, which was not evident with TLR2 activation, and TLR2 activation in monocyte/neutrophil cocultures did not prevent late neutrophil apoptosis. Thus, TLRs are important regulators of neutrophil activation and survival, with distinct and separate roles for TLR2 and TLR4 in neutrophil responses. TLR4 signaling presents itself as a pharmacological target that may allow therapeutic modulation of neutrophil survival by direct and indirect mechanisms at sites of inflammation.

Priming effects of substance P on calcium changes evoked by interleukin-8 in human neutrophils

The neurokinin (NK) substance P (SP), which is a mediator of neurogenic inflammation, has been reported to prime human polymorphonuclear neutrophils (PMNs). The priming effects of SP on PMNs activated by recombinant interleukin-8 (rIL-8) were investigated. SP enhanced, in a dose- and time-dependent way, the rise in cytosolic free-calcium concentration, [Ca(2+)]i, evoked by the chemokine. The priming effects of SP were abolished by exposing PMNs to a calcium-free medium supplemented with EGTA. The C-terminal peptides SP(4-11) and SP(6-11) but not the N-terminal peptide SP(1-7) shared the priming effects of SP. The selective NK-1 receptor agonist [Sar-9, MetO2-11]SP mimicked the effects of SP, which were not reproduced by the selective NK-2 receptor agonist [betaAla-8]-NKA(4-10) or the selective NK-3 agonist senktide. Two selective NK-1 antagonists, CP96,345 and L703,606, dose dependently inhibited SP priming effects. These results demonstrated that SP primes PMNs exposed to rIL-8 and suggested that SP priming effects are receptor mediated.

Identification of distinct surface-expressed and intracellular CXC-chemokine receptor 2 glycoforms in neutrophils: N-glycosylation is essential for maintenance of receptor surface expression

The G protein-coupled CXC-chemokine receptor CXCR-2 mediates activation of neutrophil effector functions in response to multiple ligands, including IL-8 and neutrophil-activating peptide 2 (NAP-2). Although CXCR-2 has been successfully cloned and expressed in several cell lines, the molecular properties of the native neutrophil-expressed receptor have remained largely undefined. Here we report on the identification and characterization of distinct CXCR-2 glycoforms and their subcellular distribution in neutrophils. Immunoprecipitation and Western blot analyses of surface-expressed receptors covalently linked to IL-8 or NAP-2 as well as in their unloaded state revealed the occurrence of a single CXCR-2 variant with an apparent size of 56 kDa. According to deglycosylation experiments surface-expressed CXCR-2 carries two N-linked 9-kDa carbohydrate moieties that are both of complex structure. In addition, two other CXCR-2 variants of 38 and 40 kDa were found to occur exclusively intracellular and to carry N-glycosylations of high mannose or hybrid type. These receptors did not participate in ligand-induced receptor trafficking, while surface-expressed CXCR-2 was internalized and re-expressed following stimulation with NAP-2. By enzymatic removal of one 9-kDa carbohydrate moiety in surface-expressed CXCR-2 we can show that neither NAP-2-induced trafficking nor signaling of the receptor is dependent on its full glycosylation. Instead, glycosylation was found to protect CXCR-2 from proteolytic attack, as even partial deglycosylation is associated with serine protease-mediated disappearance of the receptor from the neutrophil surface. Thus, although not directly involved in signaling, glycosylation appears to be required to maintain neutrophil responsiveness to CXC-chemokines during inflammation.

Chemokine receptors and their role in inflammation and infectious diseases

Chemokines are small peptides that are potent activators and chemoattractants for leukocyte subpopulations and some nonhemopoietic cells. Their actions are mediated by a family of 7-transmembrane G-protein–coupled receptors, the size of which has grown considerably in recent years and now includes 18 members. Chemokine receptor expression on different cell types and their binding and response to specific chemokines are highly variable. Significant advances have been made in understanding the regulation of chemokine receptor expression and the intracellular signaling mechanisms used in bringing about cell activation. Chemokine receptors have also recently been implicated in several disease states including allergy, psoriasis, atherosclerosis, and malaria. However, most fascinating has been the observation that some of these receptors are used by human immunodeficiency virus type 1 in gaining entry into permissive cells. This review will discuss structural and functional aspects of chemokine receptor biology and will consider the roles these receptors play in inflammation and in infectious diseases.

Chemokine receptors and their role in inflammation and infectious diseases

Chemokines are small peptides that are potent activators and chemoattractants for leukocyte subpopulations and some nonhemopoietic cells. Their actions are mediated by a family of 7-transmembrane G-protein–coupled receptors, the size of which has grown considerably in recent years and now includes 18 members. Chemokine receptor expression on different cell types and their binding and response to specific chemokines are highly variable. Significant advances have been made in understanding the regulation of chemokine receptor expression and the intracellular signaling mechanisms used in bringing about cell activation. Chemokine receptors have also recently been implicated in several disease states including allergy, psoriasis, atherosclerosis, and malaria. However, most fascinating has been the observation that some of these receptors are used by human immunodeficiency virus type 1 in gaining entry into permissive cells. This review will discuss structural and functional aspects of chemokine receptor biology and will consider the roles these receptors play in inflammation and in infectious diseases.

The role of chemokines in inflammation

Chemokines, together with adhesion molecules, cytokines, and proteases, are essential for the directional migration of leukocytes during normal and inflammatory processes. Interleukin-8 and monocyte chemotactic protein-1 are the best-characterized members of the C-X-C and C-C chemokine subfamilies, respectively. However, more than 20 human chemokines have been identified but are only partially characterized at the biological level. Chemokines are involved in chemotaxis of monocytes, lymphocytes, neutrophils, eosinophils, basophils, natural killer cells, dendritic cells, and endothelial cells. This review describes the chemokine subfamilies, the chemokine producer and target cells, their receptors, signal transduction mechanisms, and the role of chemokines during physiological and pathological conditions. More and more evidence points to a role for chemokines in chemotaxis-related phenomena, such as the expression of adhesion molecules, the secretion of proteinases, inhibition of apoptosis, hematopoiesis, and angiogenesis. Chemokines are also involved in diseases such as cancer (tumor regression and tumor metastasis), autoimmune diseases, and bacterial or viral infection.