Pro-inflammatory Responses In Monocytes Research Articles

ProBDNF contributed to patrolling monocyte infiltration and renal damage in systemic lupus erythematosus

Monocyte aberrations have been increasingly recognized as contributors to renal damage in systemic lupus erythematosus (SLE), however, recognition of the underlying mechanisms and modulating strategies is at an early stage. Our studies have demonstrated that brain-derived neurotrophic factor precursor (proBDNF) drives the progress of SLE by perturbing antibody-secreting B cells, and proBDNF facilitates pro-inflammatory responses in monocytes. By utilizing peripheral blood from patients with SLE, GEO database and spontaneous MRL/lpr lupus mice, we demonstrated in the present study that CX3CR1+ patrolling monocytes (PMo) numbers were decreased in SLE. ProBDNF was specifically expressed in CX3CR1+ PMo and was closely correlated with disease activity and the degree of renal injury in SLE patients. In MRL/lpr mice, elevated proBDNF was found in circulating PMo and the kidney, and blockade of proBDNF restored the balance of circulating and kidney-infiltrating PMo. This blockade also led to the reversal of pro-inflammatory responses in monocytes and a noticeable improvement in renal damage in lupus mice. Overall, the results indicate that the upregulation of proBDNF in PMo plays a crucial role in their infiltration into the kidney, thereby contributing to nephritis in SLE. Targeting of proBDNF offers a potential therapeutic role in modulating monocyte-driven renal damage in SLE.

Chlamydia psittaci hypothetical inclusion membrane protein CPSIT_0842 evokes a pro-inflammatory response in monocytes via TLR2/TLR4 signaling pathways

Chlamydia psittaci (C. psittaci) is an obligate intracellular pathogen that resides within a membrane-bound compartment known as the inclusion. Upon entering the host cell, Chlamydiae secrete numerous proteins to modify the inclusion membrane. Inclusion membrane (Inc) proteins are important pathogenic factors in Chlamydia and play crucial roles in the growth and development of Chlamydia. In the present study, the C. psittaci protein, CPSIT_0842, was identified and shown to localize to the inclusion membrane. Temporal analysis revealed that CPSIT_0842 is an early expression protein of Chlamydia. Moreover, this protein was shown to induce the expression of pro-inflammatory cytokines IL-6 and IL-8 in human monocytes (THP-1 cells) via the TLR2/TLR4 signaling pathway. CPSIT_0842 increases the expression of TLR2, TLR4, and adaptor MyD88. Suppression of TLR2, TLR4, and MyD88 markedly attenuated CPSIT_0842-induced production of IL-6 and IL-8. MAP kinases and NF-κB, important downstream molecules of TLR receptors in inflammatory signaling pathways, were also confirmed to be activated by CPSIT_0842. CPSIT_0842-induced production of IL-6 was reliant on activation of the ERK, p38, and NF-κB signaling pathways while IL-8 expression was regulated by the ERK, JNK, and NF-κB signaling pathways. Specific inhibitors of these signaling pathways significantly decreased CPSIT_0842-mediated expression of IL-6 and IL-8. Together these findings demonstrate that CPSIT_0842 upregulates the expression of IL-6 and IL-8 via TLR-2/TLR4-mediated MAPK and NF-κB signaling pathways in THP-1 cells. Exploring these molecular mechanisms enhances our understanding of C. psittaci pathogenesis.

Human Bone Marrow Mesenchymal Stromal/Stem Cells Regulate the Proinflammatory Response of Monocytes and Myeloid Dendritic Cells from Patients with Rheumatoid Arthritis.

Rheumatoid arthritis (RA) is a disabling autoimmune disease whose treatment is ineffective for one-third of patients. Thus, the immunomodulatory potential of mesenchymal stromal/stem cells (MSCs) makes MSC-based therapy a promising approach to RA. This study aimed to explore the immunomodulatory action of human bone marrow (BM)-MSCs on myeloid dendritic cells (mDCs) and monocytes, especially on cytokines/chemokines involved in RA physiopathology. For that, LPS plus IFNγ-stimulated peripheral blood mononuclear cells from RA patients (n = 12) and healthy individuals (n = 6) were co-cultured with allogeneic BM-MSCs. TNF-α, CD83, CCR7 and MIP-1β protein levels were assessed in mDCs, classical, intermediate, and non-classical monocytes. mRNA expression of other cytokines/chemokines was also evaluated. BM-MSCs effectively reduced TNF-α, CD83, CCR7 and MIP-1β protein levels in mDCs and all monocyte subsets, in RA patients. The inhibition of TNF-α production was mainly achieved by the reduction of the percentage of cellsproducing this cytokine. BM-MSCs exhibited a remarkable suppressive action over antigen-presenting cells from RA patients, potentially affecting their ability to stimulate the immune adaptive response at different levels, by hampering their migration to the lymph node and the production of proinflammatory cytokines and chemokines. Accordingly, MSC-based therapies can be a valuable approach for RA treatment, especially for non-responder patients.

Role of mitochondria in pro-inflammatory response of monocytes and macrophages.

Background and Aims: Mitochondria is believed to be one of important for inflammation cellular organelles (in addition to its main function as a cell power plant). To create cybrid lines carrying mitochondrial mutations, patients were selected with a relatively neutral level of mtDNA mutational load, but with a predominance of certain mutant variants.

Thioredoxin-1 distinctly promotes NF-κB target DNA binding and NLRP3 inflammasome activation independently of Txnip.

Antioxidant systems, such as the thioredoxin-1 (Trx1) pathway, ensure cellular redox homeostasis. However, how such systems regulate development and function of myeloid cells is barely understood. Here we show that in contrast to its critical role in T cells, the murine Trx1 system is dispensable for steady-state myeloid-cell hematopoiesis due to their capacity to tap the glutathione/glutaredoxin pathway for DNA biosynthesis. However, the Trx1 pathway instrumentally enables nuclear NF-κB DNA-binding and thereby pro-inflammatory responses in monocytes and dendritic cells. Moreover, independent of this activity, Trx1 is critical for NLRP3 inflammasome activation and IL-1β production in macrophages by detoxifying excessive ROS levels. Notably, we exclude the involvement of the Trx1 inhibitor Txnip as a redox-sensitive ligand of NLRP3 as previously proposed. Together, this study suggests that targeting Trx1 may be exploited to treat inflammatory diseases.

Mtor-Dependent Oxidative Stress Regulates Oxldl-Induced Trained Innate Immunity In Human Monocytes

Background and Aims: Macrophages have been recognized as pivotal players during atherosclerosis formation. Recently it has been shown that oxLDL induces a long-term pro-inflammatory response in monocytes, an emerging new concept called trained innate immunity. Here we have analyzed the role of reactive oxygen species (ROS) in regulating this proinflammatory priming.

Diet Rich in Animal Protein Promotes Pro-inflammatory Macrophage Response and Exacerbates Colitis in Mice.

Diet is a major factor determining gut microbiota composition and perturbances in this complex ecosystem are associated with the inflammatory bowel disease (IBD). Here, we used gnotobiotic approach to analyze, how interaction between diet rich in proteins and gut microbiota influences the sensitivity to intestinal inflammation in murine model of ulcerative colitis. We found that diet rich in animal protein (aHPD) exacerbates acute dextran sulfate sodium (DSS)-induced colitis while diet rich in plant protein (pHPD) does not. The deleterious effect of aHPD was also apparent in chronic DSS colitis and was associated with distinct changes in gut bacteria and fungi. Therefore, we induced acute DSS-colitis in germ-free mice and transferred gut microbiota from aCD or aHPD fed mice to find that this effect requires presence of microbes and aHPD at the same time. The aHPD did not change the number of regulatory T cells or Th17 cells and still worsened the colitis in immuno-deficient RAG2 knock-out mice suggesting that this effect was not dependent on adaptive immunity. The pro-inflammatory effect of aHPD was, however, abrogated when splenic macrophages were depleted with clodronate liposomes. This treatment prevented aHPD induced increase in colonic Ly-6Chigh pro-inflammatory monocytes, but the ratio of resident Ly-6C−/low macrophages was not changed. These data show that the interactions between dietary protein of animal origin and gut microbiota increase sensitivity to intestinal inflammation by promoting pro-inflammatory response of monocytes.

Homogenized mouse neural retina extracts stimulate an immune response in the mouse monocyte RAW 264.7 cell line

There are two main protein products of the Basigin gene. One protein, known as Basiginvariant‐1 is expressed in the neural retina, specifically by the photoreceptor neurons. The other protein, known as Basigin‐variant‐2 is expressed throughout the body, including monocytes, as well as Müller glial cells and the retinal pigmented epithelium of the eye. Studies by this laboratory using recombinant proteins indicate that the two Basigin gene products interact via their extracellular domains and that the region of the Ig0 domain of Basigin‐variant‐1 used to interact with Basigin‐variant‐2 stimulates a proinflammatory response in monocytes. The purpose of the present study was to determine the biological relevance of this proinflammatory response using mouse neural retina samples rather than recombinantly‐generated proteins. It was hypothesized that mouse neural retina extracts would induce an immune response in mouse monocytes. Mouse neural retinas were obtained using an approved protocol and homogenized in phosphate buffered saline (PBS). The mouse neural retina extracts were then incubated with mouse monocytic RAW 264.7 cells. After 24 hours, the cell culture medium was collected and assayed for the expression of the pro‐inflammatory cytokine interleukin‐6 (IL‐6) via an ELISA and the cells were collected for RNA isolation and subsequent quantitative reverse transcription PCR (q‐RT‐PCR). Cells treated with a similar volume of PBS served as the controls. It was determined that incubation of the neural retina extracts with the RAW 264.7 cells elicits a proinflammatory response in the monocytes and that the expression of the innate immune molecule TLR4 significantly increases in expression in those cells, when compared to the response observed with PBS treatment. The data suggest a biological relevance for the observation that recombinant Basigin‐variant‐1 interacts with Basigin‐variant‐2 to stimulate a proinflammatory response in monocytes. This study is important for a better understanding of immune response aspects of diabetic retinopathy and other diseases in which the retinal pigmented epithelium is compromised and can allow the infiltration of monocytes into the neural retina.Support or Funding InformationUNF Office of Academic AffairsThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The Basigin‐variant‐2 binding domain in the Ig0 domain of Basigin‐variant‐1 stimulates an immune response in the mouse monocyte RAW 264.7 cell line

There are two main protein products of the Basigin gene. One protein, known as Basigin‐variant‐1 is expressed in the neural retina, specifically by the photoreceptor neurons. The other protein, known as Basigin‐variant‐2 is expressed throughout the body, including monocytes, as well as Müller glial cells and the retinal pigmented epithelium of the eye. A study by this laboratory indicates that the two Basigin gene products interact via their extracellular domains. A different study by another research group indicates that the Ig0 domain of Basigin‐variant‐1 can elicit an immune response in several cell lines. The purpose of the present study was to determine if the region of the Ig0 domain of Basigin‐variant‐1 thought to interact with Basigin‐variant‐2 is the same region that elicits an immune response. Recombinant versions of the Basigin‐variant‐1 Ig0 domain were incubated with mouse monocytic RAW 264.7 cells. After 24 hours, the cell culture medium was collected and assayed for the expression of the proinflammatory cytokine interleukin‐6 (IL‐6) via an ELISA. Cells treated with a control protein generated from the expression vector used to make the Basigin‐variant‐1 recombinant proteins served as the controls. The data indicate that the region of the Ig0 domain used to interact with Basigin‐variant‐2 does indeed stimulate production of a significantly greater amount of IL‐6 than the control protein. The data suggest that Basigin‐variant‐1 interacts with Basigin‐variant‐2 to stimulate a proinflammatory response in monocytes. This study is important for a better understanding of immune response aspects of diabetic retinopathy and other diseases in which the retinal pigmented epithelium is compromised.Support or Funding InformationUNF Office of Academic AffairsThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

MTOR-Dependent Oxidative Stress Regulates oxLDL-Induced Trained Innate Immunity in Human Monocytes.

Introduction: Cells of the innate immune system particularly monocytes and macrophages have been recognized as pivotal players both during the initial insult as well as the chronic phase of atherosclerosis. It has recently been shown that oxidized low-density lipoprotein (oxLDL) induces a long-term pro-inflammatory response in monocytes due to epigenetic and metabolic reprogramming, an emerging new concept called trained innate immunity. Changes in the cellular redox state are crucial events in the regulation of many physiologic functions in macrophages including transcription, differentiation and inflammatory response. Here we have analyzed the role of reactive oxygen species (ROS) in regulating this proinflammatory monocyte priming in response to oxLDL-treatment.Methods and Results: Human monocytes were isolated and incubated with oxLDL for 24 h. After 5 days of resting, oxLDL treated cells produced significantly more inflammatory cytokines upon restimulation with the TLR2-agonist Pam3cys. Furthermore, oxLDL incubation induced persistent mTOR activation, ROS formation, HIF1α accumulation and HIF1α target gene expression, while pharmacologic mTOR inhibition or siRNA mediated inhibition of the mTORC1 subunit Raptor prevented ROS formation and proinflammatory priming. mTOR dependent ROS formation was associated with increased expression of NAPDH oxidases and necessary for the emergence of the primed phenotype as antioxidant treatment blocked oxLDL priming. Inhibition of cytosolic ROS formation could also block mTOR activation and HIF1α accumulation suggesting a positive feedback loop between mTOR and cytosolic ROS. Although mitochondrial ROS scavenging did not block HIF1α-accumulation at an early time point (24 h), it was persistently reduced on day 6. Therefore, mitochondrial ROS formation appears to occur initially downstream of the mTOR-cytoROS-HIF1α feedback loop but seems to be a crucial factor that controls the long-term activation of the mTOR-HIF1α-axis.Conclusion: In summary, our data demonstrate that mTOR dependent ROS production controls the oxLDL-induced trained innate immunity phenotype in human monocyte derived macrophages. Pharmacologic modulation of these pathways might provide a potential approach to modulate inflammation, associated with aberrant monocyte activation, during atherosclerosis development.

CD14 Is a Co-Receptor for TLR4 in the S100A9-Induced Pro-Inflammatory Response in Monocytes.

The cytosolic Ca2+-binding S100A9 and S100A8 proteins form heterodimers that are primarily expressed in human neutrophils and monocytes. We have recently shown that S100A9 binds to TLR4 in vitro and induces TLR4-dependent NF-κB activation and a pro-inflammatory cytokine response in monocytes. In the present report we have further investigated the S100A9-mediated stimulation of TLR4 in monocytes. Using transmission immunoelectron microscopy, we detected focal binding of S100A9 to monocyte membrane subdomains containing the caveolin-1 protein and TLR4. Furthermore, the S100A9 protein was detected in early endosomes of the stimulated cells, indicating that the protein could be internalized by endocytosis. Although stimulation of monocytes with S100A9 was strictly TLR4-dependent, binding of S100A9 to the plasma membrane and endocytosis of S100A9 was still detectable and coincided with CD14 expression in TLR4-deficient cells. We therefore investigated whether CD14 would be involved in the TLR4-dependent stimulation and could show that the S100A9-induced cytokine response was inhibited both in CD14-deficient cells and in cells exposed to CD14 blocking antibodies. Further, S100A9 was not internalized into CD14-deficient cells suggesting a direct role of CD14 in endocytosis of S100A9. Finally, we could detect satiable binding of S100A9 to CD14 in surface plasmon resonance experiments. Taken together, these results indicate that CD14 is a co-receptor of TLR4 in the S100A9-induced cytokine response.

Pleiotropic benefits of metformin: macrophage targeting its anti-inflammatory mechanisms.

Metformin is a biguanide family member used in the treatment of type 2 diabetes and one of the most widely prescribed antidiabetes drugs. This drug increases the peripheral uptake of glucose, decreases hepatic glucose production, and reduces insulin resistance in liver and skeletal muscle. The exact molecular mechanisms responsible for its effect on glucose homeostasis are still not completely understood but may include the triggering of the AMPK pathway (1), although it has been indicated that the acute inhibitory effects of high doses of metformin on hepatic gluconeogenesis are independent of AMPK activation (2). Interestingly, experimental and clinical studies have shed new light on an array of potential benefits of metformin, not only in the treatment of diabetes. Notably, accumulating evidence suggests that the cardiovascular protective role of metformin is largely beyond its hypoglycemic action and is ascribed to pleiotropic effects (3–5). Previous studies have described the anti-inflammatory effects of metformin on different types of cells, including human vascular endothelial cells and smooth muscle cells (6,7). Recent reports have also demonstrated that metformin can attenuate lipopolysaccharide (LPS)-induced or oxidized LDL–induced proinflammatory responses in monocytes and macrophages (8,9). Although the crucial mechanisms underlying the anti-inflammatory effects of metformin remain to be fully elucidated, the current concept of atherosclerosis as an inflammatory disorder may imply that such anti-inflammatory properties could contribute, at least in part, to the anti-atherosclerotic …

Transmembrane oligomeric form of Vibrio cholerae cytolysin triggers TLR2/TLR6-dependent proinflammatory responses in monocytes and macrophages.

Vibrio cholerae cytolysin (VCC) kills target eukaryotic cells by forming transmembrane oligomeric β-barrel pores. Once irreversibly converted into the transmembrane oligomeric form, VCC acquires an unusual structural stability and loses its cytotoxic property. It is therefore possible that, on exertion of its cytotoxic activity, the oligomeric form of VCC retained in the disintegrated membrane fractions of the lysed cells would survive within the host cellular milieu for a long period, without causing any further cytotoxicity. Under such circumstances, VCC oligomers may potentially be recognized by the host immune cells. Based on such a hypothesis, in the present study we explored the interaction of the transmembrane oligomeric form of VCC with the monocytes and macrophages of the innate immune system. Our study shows that the VCC oligomers assembled in the liposome membranes elicit potent proinflammatory responses in monocytes and macrophages, via stimulation of the toll-like receptor (TLR)2/TLR6-dependent signalling cascades that involve myeloid differentiation factor 88 (MyD88)/interleukin-1-receptor-associated kinase (IRAK)1/tumour-necrosis-factor-receptor-associated factor (TRAF)6. VCC oligomer-mediated proinflammatory responses critically depend on the activation of the transcription factor nuclear factor-κB. Proinflammatory responses induced by the VCC oligomers also require activation of the mitogen-activated protein kinase (MAPK) family member c-Jun N-terminal kinase, which presumably acts via stimulation of the transcription factor activator protein-1. Notably, the role of the MAPK p38 could not be documented in the process.

OP0252 Modulation of pro-inflammatory response of monocytes by glucocorticoids

BackgroundActive resolution of inflammation is a previously unrecognized process essential for tissue homeostasis1. Monocytes and macrophages are the central component of the immune system critical not only for initiation and...

Angiopoietin-1 elicits pro-inflammatory responses in monocytes and differentiating macrophages.

The angiopoietin/Tie2 system is an important regulator of angiogenesis and inflammation. In addition to its functions in endothelial cells, Tie2 expression on non-endothelial cells allows for angiopoietin ligands to stimulate the cells. Although Ang1 is a strong Tie2 receptor agonist, little is known regarding the effect of Ang1 on non-endothelial cells, such as monocytes and macrophages. In this study, we found that Ang1 functionally binds to and stimulates monocytes via p38 and Erk1/2 phosphorylation. Ang1-mediated monocyte stimulation is associated with proinflammatory cytokine TNF-α expression. We also determined that Ang1 switched macrophage differentiation toward a pro-inflammatory phenotype, even in the presence of an anti-inflammatory mediator. These findings suggest that Ang1 plays a role in stimulating pro-inflammatory responses and could provide a new strategy by which to manage inflammatory responses.

Abstract 473: Fluid Shear Stress Specifically Exacerbates Chlamydia pneumoniae-mediated Pro-inflammatory Responses in Monocytes

Background and Significance The precise role of Chlamydia pneumoniae infection in various stages of the atherosclerosis is not well understood. During the transit from lung to the atherosclerotic sites, C. pneumoniae-infected monocytes travel through circulation and are subjected to shear stress due to blood flow. Elucidating the effects of these mechanical stimuli on infected monocytes is critical in our understanding of the link between C. pneumoniae infection and atherosclerosis. Objectives We hypothesized that fluid shear stress alters the inflammatory response of C. pneumoniae-infected monocytes. Using an in vitro model of blood flow, we determined the effect of physiological levels of shear stress on monocyte responses pertinent to atherosclerosis including cytokine secretion, adhesion molecule expression, intracellular signaling, and endothelial adhesion. Methods Primary human monocytes and THP-1 cells were infected with C. pneumoniae at an MOI-2 for 2 h at 35 °C with intermittent rocking, and cultured for 36 h. The infected cells were then subjected to a shear stress of 7.5 dyn/cm2 for 1 h, and the supernatants were analyzed for a panel of 17 cytokines, and the cells were analyzed for signaling proteins, surface receptors, and endothelial adhesion. Uninfected cells and static conditions were used as controls, and a P<0.05 (Student’s t-test) was considered significant. Results We observed that shear stress on infected cells resulted in a caspase-independent upregulation of pro-inflammatory IL-1β (3-fold), a modest increase in MIP-1α and MIP-1β, a decrease in anti-inflammatory IL-10, and no change in other cytokines including TNFα and IL-12. These changes were manifested as an increase in the adhesion of monocytes to endothelium activated with the infected supernatants. We also observed differences in the chemotaxis of monocytes exposed to sheared vs. static supernatants. Conclusions We observe that physiological shear stress drives Chlamydia infected monocytes towards a pro-inflammatory state in circulation suggesting a critical synergy between mechanical and chemical factors in atherogenesis.

Tenofovir Selectively Regulates Production of Inflammatory Cytokines and Shifts the IL-12/IL-10 Balance in Human Primary Cells

In this study, we aimed to investigate the possible immune modulatory effects of HIV nucleoside reverse transcriptase inhibitors during secondary infections and inflammation, focusing on inflammatory cytokine responses and the interleukin (IL)-12/IL-10 balance. We investigated the in vitro effect of tenofovir and zidovudine (AZT) on production of proinflammatory cytokines in monocytes and human peripheral blood mononuclear cells (PBMCs). Stimulation panels included Toll-Like receptor (TLR) ligands; the inflammation mediator tumor necrosis factor-α; and the pathogens cytomegalovirus, Neisseria meningitides, Escherichia coli, and Streptococcus pneumoniae. Cytokine levels were measured using enzyme-linked immunosorbent assay and luminex technology. RNA levels were assessed using real-time polymerase chain reaction. Activity of mitogen-activated protein kinase and NF-κB signaling was evaluated using flow cytometry and multispectral imaging cytometry, respectively. Tenofovir decreased and AZT increased both IL-8 and CCL3 production from monocytes after stimulation with TLR ligands, tumor necrosis factor-α, or live pathogens. Similarly, tenofovir decreased CCL3 levels in human PBMCs. Furthermore, tenofovir strongly decreased induction of IL-10 but increased levels of IL-12. AZT did not affect IL-12 or IL-10 levels. The observed drug-induced changes in cytokine production were independent from transcriptional regulation through the mitogen-activated protein kinase and nuclear factor kappa B pathways. Our data suggest divergent effects of tenofovir and AZT on proinflammatory responses in monocytes (CCL3 and IL-8) and PBMCs (CCL3). Moreover, tenofovir shifts the IL-10/IL-12 balance after cell stimulation with TLR ligands or infection with live bacteria, thus suggesting that the choice of nucleoside reverse transcriptase inhibitor affects overall inflammation and early immune responses against secondary pathogens.

Structural Basis of Rnd1 Binding to Plexin Rho GTPase Binding Domains (RBDs)

Plexin receptors regulate cell adhesion, migration, and guidance. The Rho GTPase binding domain (RBD) of plexin-A1 and -B1 can bind GTPases, including Rnd1. By contrast, plexin-C1 and -D1 reportedly bind Rnd2 but associate with Rnd1 only weakly. The structural basis of this differential Rnd1 GTPase binding to plexin RBDs remains unclear. Here, we solved the structure of the plexin-A2 RBD in complex with Rnd1 and the structures of the plexin-C1 and plexin-D1 RBDs alone, also compared with the previously determined plexin-B1 RBD.Rnd1 complex structure. The plexin-A2 RBD·Rnd1 complex is a heterodimer, whereas plexin-B1 and -A2 RBDs homodimerize at high concentration in solution, consistent with a proposed model for plexin activation. Plexin-C1 and -D1 RBDs are monomeric, consistent with major residue changes in the homodimerization loop. In plexin-A2 and -B1, the RBD β3-β4 loop adjusts its conformation to allow Rnd1 binding, whereas minimal structural changes occur in Rnd1. The plexin-C1 and -D1 RBDs lack several key non-polar residues at the corresponding GTPase binding surface and do not significantly interact with Rnd1. Isothermal titration calorimetry measurements on plexin-C1 and -D1 mutants reveal that the introduction of non-polar residues in this loop generates affinity for Rnd1. Structure and sequence comparisons suggest a similar mode of Rnd1 binding to the RBDs, whereas mutagenesis suggests that the interface with the highly homologous Rnd2 GTPase is different in detail. Our results confirm, from a structural perspective, that Rnd1 does not play a role in the activation of plexin-C1 and -D1. Plexin functions appear to be regulated by subfamily-specific mechanisms, some of which involve different Rho family GTPases.

Type 2 Diabetes and Lipoprotein Metabolism Affect LPS-Induced Cytokine and Chemokine Release in Primary Human Monocytes

Obesity and insulin resistance are characterized by a chronic and low grade state of inflammation and the pro-inflammatory response of monocytes is affected in type 2 diabetes mellitus (T2D). We aimed to investigate whether LPS-induced monocytic cytokine and chemokine release depends on serum lipoprotein parameters in T2D patients. Primary human monocytes were isolated from 29 patients with known T2D and from 20 healthy volunteers. Anthropometric and disease-related parameters such as age, gender, BMI, WHR, diabetes duration, diabetes complications, and diabetes control (HbA1c) were documented. Monocytes were stimulated for 18 h with LPS (1 μg/ml). Unstimulated monocytes served as control. The supernatant concentrations of CCL2, CCL3, CCL4, CCL5, MIF and resistin were measured by ELISA. LPS-stimulation significantly (p<0.001) increased CCL chemokine and resistin concentrations in healthy controls and in patients with T2D, whereas MIF release was not affected in both groups. LPS-induced CCL2 and resistin concentrations were significantly higher in T2D patients when compared to healthy controls. In T2D patients, LPS-induced CCL3 concentration was higher in males when compared to females (p=0.039) and supernatant resistin concentration upon stimulation with LPS showed a significant and positive correlation with age (r=0.6; p=0.001). LPS-induced CCL2 concentration was significantly and positively correlated with serum triglyceride concentration (r=0.4; p=0.009) in T2D patients. Furthermore, LPS-induced CCL4 concentration was significantly and positively correlated with total (r=0.4; p=0.035) and LDL cholesterol (r=0.4; p=0.033) concentration. LPS responsiveness of monocytes is altered in T2D and is affected by the respective serum lipoprotein metabolism.

Endothelial‐derived thrombospondin‐1 promotes macrophage recruitment and apoptotic cell clearance

Rapid apoptotic cell engulfment is crucial for prevention of inflammation and autoimmune diseases and is conducted by special immunocompetent cells like macrophages or immature dendritic cells. We recently demonstrated that endothelial cells (ECs) also participate in apoptotic cell clearance. However, in contrast to conventional phagocytes they respond with an inflammatory phenotype. To further confirm these pro-inflammatory responses human ECs were exposed to apoptotic murine ECs and changes in thrombospondin-1 (TSP-1) expression and in activation of intracellular signalling cascades were determined by real-time qPCR, immunoblotting and immunocytochemistry. Human primary macrophages or monocytic lymphoma cells (U937) were incubated with conditioned supernatant of human ECs exposed to apoptotic cells and changes in activation, migration and phagocytosis were monitored. Finally, plasma levels of TSP-1 in patients with anti-neutrophil cytoplasmic antibody(ANCA)-associated vasculitis (AAV) were determined by ELISA. We provided evidence that apoptotic cells induce enhanced expression of TSP-1 in human ECs and that this increase in TSP-1 is mediated by the mitogen-activated protein kinases (MAPK) ERK1 and 2 and their upstream regulators MEK and B-Raf. We also showed that plasma TSP-1 levels are increased in patients with AAV. Finally, we showed that conditioned supernatant of ECs exposed to apoptotic cells induces pro-inflammatory responses in monocytes or U937 cells and demonstrated that increased TSP-1 expression enhances migration and facilitates engulfment of apoptotic cells by monocyte-derived macrophages or U937 cells. These findings suggest that under pathological conditions with high numbers of uncleared dying cells in the circulation endothelial-derived elevated TSP-1 level may serve as an attraction signal for phagocytes promoting enhanced recognition and clearance of apoptotic cells.