TLR Signalling Pathways Diverge in Their Ability to Induce PGE2

PGE2 is a lipid mediator abundantly produced in inflamed tissues that exerts relevant immunoregulatory functions. Dendritic cells (DCs) are key players in the onset and shaping of the inflammatory and immune responses and, as such, are well known PGE2 targets. By contrast, the precise role of human DCs in the production of PGE2 is poorly characterized. Here, we asked whether different ligands of Toll-like receptors (TLRs), a relevant family of pathogen-sensing receptors, could induce PGE2 in human DCs. The only active ligands were LPS (TLR4 ligand) and R848 (TLR7-8 ligand) although all TLRs, but TLR9, were expressed and functional. While investigating the molecular mechanisms hindering the release of PGE2, our experiments highlighted so far oversight differences in TLR signalling pathways in terms of MAPK and NF-κB activation. In addition, we identified that the PGE2-limiting checkpoint downstream TLR3, TLR5, and TLR7 was a defect in COX2 induction, while TLR1/2 and TLR2/6 failed to mobilize arachidonic acid, the substrate for the COX2 enzyme. Finally, we demonstrated the in vivo expression of PGE2 by myeloid CD11c(+) cells, documenting a role for DCs in the production of PGE2 in human inflamed tissues

Selective activation of human dendritic cells by OM-85 through a NF-kB and MAPK dependent pathway.

OM-85 (Broncho-Vaxom®, Broncho-Munal®, Ommunal®, Paxoral®, Vaxoral®), a product made of the water soluble fractions of 21 inactivated bacterial strain patterns responsible for respiratory tract infections, is used for the prevention of recurrent upper respiratory tract infections and acute exacerbations in chronic obstructive pulmonary disease patients. OM-85 is able to potentiate both innate and adaptive immune responses. However, the molecular mechanisms responsible for OM-85 activation are still largely unknown. Purpose of this study was to investigate the impact of OM-85 stimulation on human dendritic cell functions. We show that OM-85 selectively induced NF-kB and MAPK activation in human DC with no detectable action on the interferon regulatory factor (IRF) pathway. As a consequence, chemokines (i.e. CXCL8, CXCL6, CCL3, CCL20, CCL22) and B-cell activating cytokines (i.e. IL-6, BAFF and IL-10) were strongly upregulated. OM-85 also synergized with the action of classical pro-inflammatory stimuli used at suboptimal concentrations. Peripheral blood mononuclear cells from patients with COPD, a pathological condition often associated with altered PRR expression pattern, fully retained the capability to respond to OM-85. These results provide new insights on the molecular mechanisms of OM-85 activation of the immune response and strengthen the rational for its use in clinical settings

Activation of PBMC and MoDC from COPD patients and healthy subjects by OM-85.

<p><b>A</b>) PBMC and <b>B</b>) MoDC (both 10⁶/ml) were stimulated as indicated in the presence or absence of 500 U/ml IFNγ or 100 ng/ml TNF-α. After 24 hours, supernatants were collected and analyzed by ELISA. Figure shows the results of healthy donors (open histograms) compared to COPD patients (black histograms). *P<0.05 by paired Student's <i>t</i> test.</p

Characteristic of COPD patients enrolled in the study.

<p>Table reports the main features (age, sex, age at diagnosis of COPD, disease stadium, therapy and notes) of selected patients <b>(P).</b></p

Synergism between OM-85 and pro-inflammatory agonists in MoDC.

<p>MoDC (10⁶/ml) were stimulated as indicated. After 24 hours, supernatants were collected and analyzed by ELISA. *P<0.05 and **P<0.005 by paired Student's <i>t</i> test.</p

Induction of selected cytokines and chemokines by OM-85 in MoDC.

<p><b>A</b>) MoDC (10⁶/ml) were stimulated with OM-85 as indicated and with 100 ng/ml LPS (IL-6, BAFF, CCL2, CXCL8 and CXCL6) or 10 ng/ml LPS (CCL3, CCL20 and CCL22) as a positive control. After 24 hours, supernatants were collected and analyzed by ELISA. Ut = untreated. *P<0.05 and **P<0.01 by Dunnett's Multiple Comparison Test. <b>B</b>) Supernatants of MoDC stimulated with OM-85 induce a G-protein-dependent migration of PMN. As a comparison, migration of PMN was elicited with unstimulated supernatants+CXCL8 and PMA. As expected, migration to CXCL8 was inhibited by both 10 nM M3 and 750 ng/ml, <i>Pertussis toxin</i> (P.Tox) while migration to PMA was not. Results are expressed as chemotactic index over migration to supernatants of unstimulated MoDC and represent means+/−SD of three independent Boyden chamber experiments. *P value<0.05 and ** P value<0.01 by Dunnett's Multiple Comparison Test.</p

Activation of the NF-kB and MAPK pathways by OM-85 in MoDC.

<p><b>A</b>) Immature human MoDC were stimulated with 100 µg/ml OM-85 for 30, 60, 90 and 120 minutes. 100 ng/ml LPS was used as a positive control. After cell lysis and protein fractionation, cytoplasmic (Cyto) and nuclear (Nuclei) extracts were blotted against NF-kB p65 and IkBα. β-actin and Lamin A/C represent loading controls for cytoplasmic and nuclear proteins respectively. The image depicts results obtained in one representative donor out of eight. <b>B</b>) EMSA (upper panel) and supershift (lower panel) showing the induction of NFkBp65-DNA binding activity by OM-85 in human moDC stimulated as in A). Signal specificity was assessed by competing each sample with a 125-fold excess unlabeled probe (lanes 2,4,6,8,10 upper panel). The image depicts results obtained in one representative donor out of four. <b>C</b>) OM-85 induces the production of luciferase in THP1 cells bearing a NF-kB-reporter plasmid (NF-kB pGL4, striped histograms). THP1 cells were stimulated with 1 µg/ml LPS and 1000 µg/ml OM-85. As expected, THP1 untransfected cells (untransfected, empty histograms) did not produce luciferase in response to stimulation. Similar results were obtained when cells were transfected with the pGL4 empty backbone (pGL4, black histograms). Results are expressed as mean+/−SD of three independent experiments. *P value<0.01 by Dunnett's Multiple Comparison Test. <b>D</b>) OM-85 activates the MAPK pathway. Cell extracts prepared as in A) were blotted with antibodies specific for phophorilated ERK1/2 (Cyto, upper panel) and total ATF2 and c-Jun (Nuclei, lower panel). β-actin and Lamin A/C represent loading controls for cytoplasmic and nuclear proteins respectively. The image depicts results obtained in one representative donor out of three. <b>E</b>) OM-85 induces NF-kB- and MAPK-dependent gene transcription. Immature MoDC were stimulated with 100 µg/ml OM-85 (open circles) and 100 ng/ml LPS (black circles) for 2, 4, 8 and 24 hours. After RNA extraction, reverse transcription and DNAse I digestion, samples were amplified by Q-PCR using gene-specific primers. Results represent means+/−SE of three independent donors and are expressed as fold induction (FI) over unstimulated samples (0).</p

Activation of primary DC subsets by OM-85.

<p><b>A</b>) MDC and <b>B</b>) PDC were isolated from buffy coats of three independent healthy donors and stimulated as indicated (10⁶/ml). After 24 hours, supernatants were collected and analyzed by ELISA. *P<0.05 by Dunnett's Multiple Comparison Test.</p