Selective induction of phospholipase D1 in pathogen-activated human monocytes

Targeting of Phospholipase D1 Ameliorates Collagen-Induced Arthritis via Modulation of Treg and Th17 Cell Imbalance and Suppression of Osteoclastogenesis

Phospholipase D1 (PLD1) plays a crucial role in various inflammatory and autoimmune diseases. Rheumatoid arthritis (RA) is a chronic and systemic autoimmune disease. However, the role of PLD1 in the pathogenesis of RA remains unknown. Here, we first investigated the role and effects of PLD1 in collagen-induced arthritis (CIA) and found that genetic and pharmacological inhibition of PLD1 in DBA1/J mice with CIA reduced the incidence of CIA, decreased the clinical score, and abrogated disease symptoms including infiltration of leukocytes, synovial inflammation, bone erosion, and cartilage destruction. Moreover, ablation and inhibition of PLD1 suppressed the production of type II collagen-specific IgG2a autoantibody and proinflammatory cytokines, accompanied by an increase in the regulatory T (Treg) cell population and a decrease in the Th17 cell population in CIA mice. The PLD1 inhibitor also promoted differentiation of Treg cells and suppressed differentiation of Th17 cells in vitro. Furthermore, the PLD1 inhibitor attenuated pathologic bone destruction in CIA mice by suppressing osteoclastogenesis and bone resorption. Thus, our findings indicate that the targeting of PLD1 can ameliorate CIA by modulating the imbalance of Treg and Th17 cells and suppressing osteoclastogenesis, which might be a novel strategy to treat autoimmune diseases, such as RA.

Functional regulation of phospholipase D expression in cancer and inflammation

Phospholipase D (PLD) regulates downstream effectors by generating phosphatidic acid. Growing links of dysregulation of PLD to human disease have spurred interest in therapeutics that target its function. Aberrant PLD expression has been identified in multiple facets of complex pathological states, including cancer and inflammatory diseases. Thus, it is important to understand how the signaling network of PLD expression is regulated and contributes to progression of these diseases. Interestingly, small molecule PLD inhibitors can suppress PLD expression as well as enzymatic activity of PLD and have been shown to be effective in pathological mice models, suggesting the potential for use of PLD inhibitors as therapeutics against cancer and inflammation. Here, we summarize recent scientific developments regarding the regulation of PLD expression and its role in cancer and inflammatory processes.

LysoPC and PAF Trigger Arachidonic Acid Release by Divergent Signaling Mechanisms in Monocytes

Oxidized low-density lipoproteins (LDLs) play an important role during the development of atherosclerosis characterized by intimal inflammation and macrophage accumulation. A key component of LDL is lysophosphatidylcholine (lysoPC). LysoPC is a strong proinflammatory mediator, and its mechanism is uncertain, but it has been suggested to be mediated via the platelet activating factor (PAF) receptor. Here, we report that PAF triggers a pertussis toxin- (PTX-) sensitive intracellular signaling pathway leading to sequential activation of sPLA(2), PLD, cPLA(2), and AA release in human-derived monocytes. In contrast, lysoPC initiates two signaling pathways, one sequentially activating PLD and cPLA(2), and a second parallel PTX-sensitive pathway activating cPLA(2) with concomitant activation of sPLA(2), all leading to AA release. In conclusion, lysoPC and PAF stimulate AA release by divergent pathways suggesting involvement of independent receptors. Elucidation of monocyte lysoPC-specific signaling mechanisms will aid in the development of novel strategies for atherosclerosis prevention, diagnosis, and therapy.

Lack of alpha-synuclein modulates microglial phenotype in vitro

Alpha (α)-synuclein neuronal effects are continually being defined although its role in regulating glial phenotypes remains unclear. An ability to regulate microglial activation was investigated using primary cultures from wild type and α-synuclein deficient mice (Snca-/-). Snca-/- microglia demonstrated increased secretion of the cytokine tumor necrosis factor-alpha (TNF-α), impaired phagocytic ability, elevated prostaglandin levels, and increased protein levels of key enzymes in lipid-mediated signaling events, cytosolic phospholipase (cPLA(2)), cyclooxygenase-2 (Cox-2) and phospholipase D2 (PLD2) when compared to wild type cells. Increased cytokine secretion and cPLA(2) and Cox-2 levels in Snca-/- microglia were partially attenuated by inhibiting PLD-dependent signaling with n-butanol treatment.

CTLA-4 is expressed by human monocyte-derived dendritic cells and regulates their functions

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is the major negative regulator of T-cell responses, although growing evidence supports its wider role as an immune attenuator that may also act in other cell lineages. Here, we have analyzed the expression of CTLA-4 in human monocytes and monocyte-derived dendritic cells (DCs), and the effect of its engagement on cytokine production and T-cell stimulatory activity by mature DCs. CTLA-4 was highly expressed on freshly isolated monocytes, then down-modulated upon differentiation toward immature DCs (iDCs) and it was markedly upregulated on mature DCs obtained with different stimulations (lipopolysaccharides [LPS], Poly:IC, cytokines). In line with the functional role of CTLA-4 in T cells, treatment of mDCs with an agonistic anti-CTLA-4 mAb significantly enhanced secretion of regulatory interleukin (IL)-10 but reduced secretion of IL-8/IL-12 pro-inflammatory cytokines, as well as autologous CD4+ T-cell proliferation in response to stimulation with recall antigen purified protein derivative (PPD) loaded-DCs. Neutralization of IL-10 with an anti-IL-10 antibody during the mDCs-CD4+ T-cell co-culture partially restored the ability of anti-CTLA-4-treated mDCs to stimulate T-cell proliferation in response to PPD. Taken together, our data provide the first evidence that CTLA-4 receptor is expressed by human monocyte-derived mDCs upon their full activation and that it exerts immune modulatory effects.

Phospholipase D1 mediates TNFalpha-induced inflammation in a murine model of TNFalpha-induced peritonitis

Background

Tumor Necrosis Factor alpha (TNFα) is a pleiotropic cytokine extensively studied for its role in the pathogenesis of a variety of disease conditions, including in inflammatory diseases. We have recently shown that, in vitro, that TNFα utilizes PLD1 to mediate the activation of NFκB and ERK1/2 in human monocytes. The aim of this study was to investigate the role(s) played by phospholipase D1 (PLD1) in TNFα-mediated inflammatory responses in vivo.

Methodology/Findings

Studies were performed in vivo using a mouse model of TNFα-induced peritonitis. The role of PLD1 was investigated by functional genomics, utilizing a specific siRNA to silence the expression of PLD1. Administration of the siRNA against PLD1 significantly reduced PLD1 levels in vivo. TNFα triggers a rapid pyrogenic response, but the in vivo silencing of PLD1 protects mice from the TNFα-induced rise in temperature. Similarly TNFα caused an increase in the serum levels of IL-6, MIP-1α and MIP-1β: this increase in cytokine/chemokine levels was inhibited in mice where PLD1 had been silenced. We then induced acute peritonitis with TNFα. Intraperitoneal injection of TNFα triggered a rapid increase in vascular permeability, and the influx of neutrophils and monocytes into the peritoneal cavity. By contrast, in mice where PLD1 had been silenced, the TNFα-triggered increase in vascular permeability and phagocyte influx was substantially reduced. Furthermore, we also show that the TNFα-mediated upregulation of the cell adhesion molecules VCAM and ICAM1, in the vascular endothelium, were dependent on PLD1.

Conclusions

These novel data demonstrate a critical role for PLD1 in TNFα-induced inflammation in vivo and warrant further investigation. Indeed, our results suggest PLD1 as a novel target for treating inflammatory diseases, where TNFα play key roles: these include diseases ranging from sepsis to respiratory and autoimmune diseases; all diseases with considerable unmet medical need.

Mycobacteria attenuate nociceptive responses by formyl peptide receptor triggered opioid peptide release from neutrophils

In inflammation, pain is regulated by a balance of pro- and analgesic mediators. Analgesic mediators include opioid peptides which are secreted by neutrophils at the site of inflammation, leading to activation of opioid receptors on peripheral sensory neurons. In humans, local opioids and opioid peptides significantly downregulate postoperative as well as arthritic pain. In rats, inflammatory pain is induced by intraplantar injection of heat inactivated Mycobacterium butyricum, a component of complete Freund's adjuvant. We hypothesized that mycobacterially derived formyl peptide receptor (FPR) and/or toll like receptor (TLR) agonists could activate neutrophils, leading to opioid peptide release and inhibition of inflammatory pain. In complete Freund's adjuvant-induced inflammation, thermal and mechanical nociceptive thresholds of the paw were quantified (Hargreaves and Randall-Selitto methods, respectively). Withdrawal time to heat was decreased following systemic neutrophil depletion as well as local injection of opioid receptor antagonists or anti-opioid peptide (i.e. Met-enkephalin, beta-endorphin) antibodies indicating an increase in pain. In vitro, opioid peptide release from human and rat neutrophils was measured by radioimmunoassay. Met-enkephalin release was triggered by Mycobacterium butyricum and formyl peptides but not by TLR-2 or TLR-4 agonists. Mycobacterium butyricum induced a rise in intracellular calcium as determined by FURA loading and calcium imaging. Opioid peptide release was blocked by intracellular calcium chelation as well as phosphoinositol-3-kinase inhibition. The FPR antagonists Boc-FLFLF and cyclosporine H reduced opioid peptide release in vitro and increased inflammatory pain in vivo while TLR 2/4 did not appear to be involved. In summary, mycobacteria activate FPR on neutrophils, resulting in tonic secretion of opioid peptides from neutrophils and in a decrease in inflammatory pain. Future therapeutic strategies may aim at selective FPR agonists to boost endogenous analgesia.

Role of macrophage phospholipase D in natural and CpG-induced antimycobacterial activity

The present study addresses the differential ability of macrophages to control intracellular growth of non-pathogenic Mycobacterium smegmatis (Msm) and pathogenic M. tuberculosis (MTB). Results reported herein show that 3 h post infection, intracellular Msm, but not MTB, was significantly killed by macrophages. As the role of human macrophage phospholipase D (PLD) in the activation of antimicrobial mechanisms has been documented, we hypothesised the role of such enzyme in antimycobacterial mechanisms. To this aim, macrophage PLD activity was analysed at different times after exposure with either pathogenic MTB or non-pathogenic Msm. Results showed that, starting from 15 min after mycobacterial exposure, MTB did not induce macrophage PLD activity, whereas the environmental non-pathogenic Msm stably increased it. The direct contribution of PLD in intracellular mycobacterial killing was also analysed by inhibiting enzymatic activity with ethanol or calphostin C. Results show that PLD inhibition significantly increases intracellular Msm replication. In order to see whether the innate PLD-mediated antimicrobial mechanisms against MTB are also induced after CpG ODN stimulation, the role of PLD has been analysed in the course of CpG-mediated intracellular MTB killing. CpG DNA increased PLD activity in both uninfected and MTB-infected macrophages, and the inhibition of PLD activity resulted in a significant reduction of CpG-induced MTB killing. Taken together, our data suggest a relationship between host PLD activation and the macrophage ability to control intracellular mycobacterial growth.

Benzydamine inhibits monocyte migration and MAPK activation induced by chemotactic agonists

1. The present study was aimed to investigate the effect of benzydamine, an anti-inflammatory drug devoid of activity on arachidonic acid metabolism, on monocyte chemotaxis and to define the possible biochemical correlates of activity. 2. Benzydamine inhibited monocyte chemotaxis in response to three classes of chemoattractants: the prototypic CC-chemokine CCL2 (MCP-1), the microbial product fMLP and the complement cascade component C5a. The effect was dose-dependent with IC50's of 100, 50 and 45 microm for MCP-1/CCL2, fMLP and C5a, respectively. At the dose of 100 microm, the effect resulted in a 50+/-10% inhibition of MCP-1/CCL2-induced chemotaxis and 53+/-6 and 54+/-5% inhibitions of chemotaxis in response of fMLP and C5a, respectively (n=3). 3. Receptor expression as well as calcium fluxes in response to chemoattractants were not affected by benzydamine. 4. Benzydamine strongly inhibited chemoattractant-induced activation of the mitogen-activated protein kinase (MAPK) ERK1/2, and of its upstream activator kinase MEK1/2. ERK1/12 activation in response to chemoattractants was 89-98% inhibited by a 100 microm concentration of benzydamine with an IC50 of 30 microm. 5. Under the same experimental conditions, pretreatment with 100 microm benzydamine caused a 75-89% inhibition of p38 activation (IC50 25 microm). 6. These results indicate that the anti-inflammatory activity of benzydamine is exerted at multiple levels, including monocyte migration to chemotactic factors associated to a blockage of ERK and p38 MAPK pathways.