Serum amyloid A induces G-CSF expression and neutrophilia via Toll-like receptor 2

Characterization of the oligomerization and aggregation of human Serum Amyloid A

The fibrillation of Serum Amyloid A (SAA) – a major acute phase protein – is believed to play a role in the disease Amyloid A (AA) Amyloidosis. To better understand the amyloid formation pathway of SAA, we characterized the oligomerization, misfolding, and aggregation of a disease-associated isoform of human SAA – human SAA1.1 (hSAA1.1) – using techniques ranging from circular dichroism spectroscopy to atomic force microscopy, fluorescence spectroscopy, immunoblot studies, solubility measurements, and seeding experiments. We found that hSAA1.1 formed alpha helix-rich, marginally stable oligomers in vitro on refolding and cross-beta-rich aggregates following incubation at 37°C. Strikingly, while hSAA1.1 was not highly amyloidogenic in vitro, the addition of a single N-terminal methionine residue significantly enhanced the fibrillation propensity of hSAA1.1 and modulated its fibrillation pathway. A deeper understanding of the oligomerization and fibrillation pathway of hSAA1.1 may help elucidate its pathological role.

S100A12 suppresses pro-inflammatory, but not pro-thrombotic functions of serum amyloid A

S100A12 is elevated in the circulation in patients with chronic inflammatory diseases and recent studies indicate pleiotropic functions. Serum amyloid A induces monocyte cytokines and tissue factor. S100A12 did not stimulate IL-6, IL-8, IL-1β or TNF-α production by human peripheral blood mononuclear cells but low amounts consistently reduced cytokine mRNA and protein levels induced by serum amyloid A, by ∼49% and ∼46%, respectively. However, S100A12 did not affect serum amyloid A-induced monocyte tissue factor. In marked contrast, LPS-induced cytokines or tissue factor were not suppressed by S100A12. S100A12 did not alter cytokine mRNA stability or the cytokine secretory pathway. S100A12 and serum amyloid A did not appear to form complexes and although they may have common receptors, suppression was unlikely via receptor competition. Serum amyloid A induces cytokines via activation of NF-κB and the MAPK pathways. S100A12 reduced serum amyloid A-, but not LPS-induced ERK1/2 phosphorylation to baseline. It did not affect JNK or p38 phosphorylation or the NF-κB pathway. Reduction in ERK1/2 phosphorylation by S100A12 was unlikely due to changes in intracellular reactive oxygen species, Ca²⁺ flux or to recruitment of phosphatases. We suggest that S100A12 may modulate sterile inflammation by blunting pro-inflammatory properties of lipid-poor serum amyloid A deposited in chronic lesions where both proteins are elevated as a consequence of macrophage activation.

Endogenous Acute Phase Serum Amyloid A Lacks Pro-Inflammatory Activity, Contrasting the Two Recombinant Variants That Activate Human Neutrophils through Different Receptors

Most notable among the acute phase proteins is serum amyloid A (SAA), levels of which can increase 1000-fold during infections, aseptic inflammation, and/or trauma. Chronically elevated SAA levels are associated with a wide variety of pathological conditions, including obesity and rheumatic diseases. Using a recombinant hybrid of the two human SAA isoforms (SAA1 and 2) that does not exist in vivo, numerous in vitro studies have given rise to the notion that acute phase SAA is a pro-inflammatory molecule with cytokine-like properties. It is however unclear whether endogenous acute phase SAA per se mediates pro-inflammatory effects. We tested this in samples from patients with inflammatory arthritis and in a transgenic mouse model that expresses human SAA1. Endogenous human SAA did not drive production of pro-inflammatory IL-8/KC in either of these settings. Human neutrophils derived from arthritis patients displayed no signs of activation, despite being exposed to severely elevated SAA levels in circulation, and SAA-rich sera also failed to activate cells in vitro. In contrast, two recombinant SAA variants (the hybrid SAA and SAA1) both activated human neutrophils, inducing L-selectin shedding, production of reactive oxygen species, and production of IL-8. The hybrid SAA was approximately 100-fold more potent than recombinant SAA1. Recombinant hybrid SAA and SAA1 activated neutrophils through different receptors, with recombinant SAA1 being a ligand for formyl peptide receptor 2 (FPR2). We conclude that even though recombinant SAAs can be valuable tools for studying neutrophil activation, they do not reflect the nature of the endogenous protein.

Regulation of thrombospondin-1 expression in alternatively activated macrophages and adipocytes: role of cellular cross talk and omega-3 fatty acids

Thrombospondin-1 (TSP-1) expression in human adipose positively correlates with body mass index and may contribute to adipose dysfunction by activating transforming growth factor-β and/or inhibiting angiogenesis. Our objective was to determine how TSP-1 is regulated in adipocytes and polarized macrophages using a coculture system and to determine whether fatty acids, including the ω-3 fatty acid docosahexaenoic acid (DHA), regulate TSP-1 expression. Coculture of M1, M2a or M2c macrophages with adipocytes induced TSP-1 gene expression in adipocytes (from 2.4- to 4.2-fold, P<.05), and adipocyte coculture induced TSP-1 gene expression in M1 and M2c macrophages (M1: 8.6-fold, M2c: 26-fold; P<.05). TSP-1 protein levels in the shared media of adipocytes and M2c cells were also strongly induced by coculture (>10-fold, P<.05). DHA treatment during the coculture of adipocytes and M2c macrophages potently inhibited the M2c macrophage TSP-1 mRNA level (97% inhibition, P<.05). Adipocyte coculture induced interleukin (IL)-10 expression in M2c macrophages (10.1-fold, P<.05), and this increase in IL-10 mRNA expression was almost completely blocked with DHA treatment (96% inhibition, P<.05); thus, IL-10 expression closely paralleled TSP-1 expression. Since IL-10 has been shown to regulate TSP-1 in other cell types, we reduced IL-10 expression with siRNA in the M2c cells and found that this caused TSP-1 to be reduced in response to adipocyte coculture by 60% (P<.05), suggesting that IL-10 regulates TSP-1 expression in M2c macrophages. These results suggest that supplementation with dietary ω-3 fatty acids could potentially be beneficial to adipose tissue in obesity by reducing TSP-1 and fibrosis.

Interleukin-6 is essential for primary resistance to Francisella tularensis live vaccine strain infection

We employed Francisella tularensis live vaccine strain (LVS) to study mechanisms of protective immunity against intracellular pathogens and, specifically, to understand protective correlates. One potential molecular correlate identified previously was interleukin-6 (IL-6), a cytokine with pleotropic roles in immunity, including influences on T and B cell functions. Given its role as an immune modulator and the correlation with successful anti-LVS vaccination, we examined the role IL-6 plays in the host response to LVS. IL-6-deficient (IL-6 knockout [KO]) mice infected with LVS intradermally or intranasally or anti-IL-6-treated mice, showed greatly reduced 50% lethal doses compared to wild-type (WT) mice. Increased susceptibility was not due to altered splenic immune cell populations during infection or decreased serum antibody production, as IL-6 KO mice had similar compositions of each compared to WT mice. Although LVS-infected IL-6 KO mice produced much less serum amyloid A and haptoglobin (two acute-phase proteins) than WT mice, there were no other obvious pathophysiological differences between LVS-infected WT and IL-6 KO mice. IL-6 KO or WT mice that survived primary LVS infection also survived a high-dose LVS secondary challenge. Using an in vitro overlay assay that measured T cell activation, cytokine production, and abilities of primed splenocytes to control intracellular LVS growth, we found that IL-6 KO total splenocytes or purified T cells were slightly defective in controlling intracellular LVS growth but were equivalent in cytokine production. Taken together, IL-6 is an integral part of a successful immune response to primary LVS infection, but its exact role in precipitating adaptive immunity remains elusive.

Epithelial cells augment barrier function via activation of the Toll-like receptor 2/phosphatidylinositol 3-kinase pathway upon recognition of Salmonella enterica serovar Typhimurium curli fibrils in the gut

Curli fibrils, the best-characterized functional bacterial amyloids, are an important component of enterobacterial biofilms. We have previously shown that curli fibrils are recognized by the Toll-like receptor 2 (TLR2)/TLR1 heterodimer complex. Utilizing polarized T-84 cells, an intestinal epithelial cell line derived from colon carcinoma grown on semipermeable tissue culture inserts, we determined that infection with a Salmonella enterica serovar Typhimurium csgBA mutant, which does not express curli, resulted in an increase in intestinal barrier permeability and an increase in bacterial translocation compared to infection with curliated wild-type S. Typhimurium. When the TLR2 downstream signaling molecule phosphatidylinositol 3-kinase (PI3K) was blocked using wortmannin or LY294002, the difference in disruption of the intestinal epithelium and bacterial translocation was no longer observed. Additionally, disruption of polarized T-84 cells treated basolaterally with the TLR5 ligand flagellin was prevented when the polarized cells were simultaneously treated with the synthetic TLR2/TLR1 ligand Pam(3)CSK(4) or with purified curli fibrils in the apical compartment. Similar to in vitro observations, C57BL/6 mice infected with the csgBA mutant suffered increased disruption of the intestinal epithelium and therefore greater dissemination of the bacteria to the mesenteric lymph nodes than mice infected with wild-type S. Typhimurium. The differences in disruption of the intestinal epithelium and bacterial dissemination in the mice infected with csgBA mutant or wild-type S. Typhimurium were not apparent in TLR2-deficient mice. Overall, these studies report for the first time that activation of the TLR2/PI3K pathway by microbial amyloids plays a critical role in regulating the intestinal epithelial barrier as well as monitoring bacterial translocation during infection.

Microbial amyloids induce interleukin 17A (IL-17A) and IL-22 responses via Toll-like receptor 2 activation in the intestinal mucosa

The Toll-like receptor 2 (TLR2)/TLR1 receptor complex responds to amyloid fibrils, a common component of biofilm material produced by members of the phyla Firmicutes, Bacteroidetes, and Proteobacteria. To determine whether this TLR2/TLR1 ligand stimulates inflammatory responses when bacteria enter intestinal tissue, we investigated whether expression of curli amyloid fibrils by the invasive enteric pathogen Salmonella enterica serotype Typhimurium contributes to T helper 1 and T helper 17 responses by measuring cytokine production in the mouse colitis model. A csgBA mutant, deficient in curli production, elicited decreased expression of interleukin 17A (IL-17A) and IL-22 in the cecal mucosa compared to the S. Typhimurium wild type. In TLR2-deficient mice, IL-17A and IL-22 expression was blunted during S. Typhimurium infection, suggesting that activation of the TLR2 signaling pathway contributes to the expression of these cytokines. T cells incubated with supernatants from bone marrow-derived dendritic cells (BMDCs) treated with curli fibrils released IL-17A in a TLR2-dependent manner in vitro. Lower levels of IL-6 and IL-23 production were detected in the supernatants of the TLR2-deficient BMDCs treated with curli fibrils. Consistent with this, three distinct T-cell populations-CD4(+) T helper cells, cytotoxic CD8(+) T cells, and γδ T cells-produced IL-17A in response to curli fibrils in the intestinal mucosa during S. Typhimurium infection. Notably, decreased IL-6 expression by the dendritic cells and decreased IL-23 expression by the dendritic cells and macrophages were observed in the cecal mucosa of mice infected with the curli mutant. We conclude that TLR2 recognition of bacterial amyloid fibrils in the intestinal mucosa represents a novel mechanism of immunoregulation, which contributes to the generation of inflammatory responses, including production of IL-17A and IL-22, in response to bacterial entry into the intestinal mucosa.

SAA does not induce cytokine production in physiological conditions

SAA has been shown to have potential proinflammatory properties in inflammatory diseases such as atherosclerosis. These include induction of tumor necrosis factor α, interleukin-6, and monocyte chemoattractant protein 1 in vitro. However, concern has been raised that these effects might be due to use of recombinant SAA with low level of endotoxin contaminants or its non-native forms. Therefore, physiological relevance has not been fully elucidated. In this study, we investigated the role of SAA in the production of inflammatory cytokines. Stimulation of mouse monocyte J774 cells with lipid-poor recombinant human SAA and purified SAA derived from cardiac surgery patients, but not ApoA-I and ApoA-II, elicited pro-inflammatory cytokines like granulocyte colony stimulating factor (G-CSF). However, HDL-associated SAA failed to stimulate production of these cytokines. Using neutralizing antibodies against toll like receptor (TLR) 2 and 4, we could evaluate that TLR 2 is responsible for G-CSF production by lipid-poor SAA. To confirm these data in vivo, we expressed mouse SAA in SAA deficient C57BL/6 mice using an adenoviral vector. G-CSF was identically expressed in SAA-Adenoviral infected mice as well as in control null-Adenoviral mice at the early time points (4-8h) and could not be detected in plasma 24h after infection when plasma SAA levels were maximally elevated, indicating that adenoviral vector rather than SAA affected G-CSF levels. Taken together, our findings suggest that lipid-poor SAA, but not HDL-associated SAA, stimulates G-CSF production and this stimulation is mediated through TLR 2 in J774 cells. However, its physiological role in vivo remains ambiguous.

Microbial manipulation of the amyloid fold

Microbial biofilms are encased in a protein, DNA, and polysaccharide matrix that protects the community, promotes interactions with the environment, and helps cells adhere together. The protein component of these matrices is often a remarkably stable, β-sheet-rich polymer called amyloid. Amyloids form ordered, self-templating fibers that are highly aggregative, making them a valuable biofilm component. Some eukaryotic proteins inappropriately adopt the amyloid fold, and these misfolded protein aggregates disrupt normal cellular proteostasis, which can cause significant cytotoxicity. Indeed, until recently amyloids were considered solely the result of protein misfolding. However, research over the past decade has revealed how various organisms have capitalized on the amyloid fold by developing sophisticated biogenesis pathways that coordinate gene expression, protein folding, and secretion so that amyloid-related toxicities are minimized. How microbes manipulate amyloids, by augmenting their advantageous properties and by reducing their undesirable properties, will be the subject of this review.

Herpes simplex virus type 1 induces simultaneous activation of Toll-like receptors 2 and 4 and expression of the endogenous ligand serum amyloid A in astrocytes

Herpes simplex virus type 1 (HSV-1) is the most common pathogenic cause of sporadic acute encephalitis and it produces latent persistent infection lifelong in infected individuals. Brain inflammation is associated with activation of glial cells, which can detect pathogen-associated molecular patterns (PAMPs) through a variety of pattern-recognition receptors (PRR), including Toll-like receptors (TLRs). In this study, we evaluated the expression and activation of TLR2, TLR3, and TLR4 in HSV-1-infected astrocyte and neuronal primary cultures. Our results showed a clear induction in TLR2 and TLR4 expression in astrocytes as early as 1 h after HSV-1 infection, whereas no significant change was observed in neurons. In addition, infected astrocytes showed increased levels of interferon regulatory factors IRF3 and IRF7, interferon β (INFβ), interleukin 6 (IL6), and serum amyloid A (SAA3) transcripts, as well as phospho-IRF3 protein. These effects seemed to be dependent on viral replication since previous treatment of the cells with acyclovir resulted in low levels of TLRs expression and activation even after 4 h post-infection. These results suggest that reactivation of HSV-1 at the central nervous system (CNS) would likely induce and activate TLR2 and TLR4 receptors directly through interaction of astrocytes with the pathogen and also indirectly by endogenous ligands produced locally, such as serum amyloid protein, potentiating the neuroinflammatory response.

Airway epithelial regulation of allergic sensitization in asthma

While many of the contributing cell types and mediators of allergic asthma are known, less well understood are the factors that influence the development of allergic responses that lead to the development of allergic asthma. As the first airway cell type to respond to inhaled factors, the epithelium orchestrates downstream interactions between dendritic cells (DCs) and CD4⁺ T cells that quantitatively and qualitatively dictate the degree and type of the allergic asthma phenotype, making the epithelium of critical importance for the genesis of allergies that later manifest in allergic asthma. Amongst the molecular processes of critical importance in airway epithelium is the transcription factor, nuclear factor-kappaB (NF-κB). This review will focus primarily on the genesis of pulmonary allergies and the participation of airway epithelial NF-κB activation therein, using examples from our own work on nitrogen dioxide (NO₂) exposure and genetic modulation of airway epithelial NF-κB activation. In addition, the mechanisms through which Serum Amyloid A (SAA), an NF-κB-regulated, epithelial-derived mediator, influences allergic sensitization and asthma severity will be presented. Knowledge of the molecular and cellular processes regulating allergic sensitization in the airways has the potential to provide powerful insight into the pathogenesis of allergy, as well as targets for the prevention and treatment of asthma.

Shifting of immune responsiveness to house dust mite by influenza A infection: genomic insights

Respiratory viral infections have been associated with an increased incidence of allergic asthma. However, the mechanisms by which respiratory infections facilitate allergic airway disease are incompletely understood. We previously showed that exposure to a low dose of house dust mite (HDM) resulted in enhanced HDM-mediated allergic airway inflammation, and, importantly, marked airway hyperreactivity only when allergen exposure occurred during an acute influenza A infection. In this study, we evaluated the impact of concurrent influenza infection and allergen exposure at the genomic level, using whole-genome microarray. Our data showed that, in contrast to exposure to a low dose of HDM, influenza A infection led to a dramatic increase in gene expression, particularly of TLRs, C-type lectin receptors, several complement components, as well as FcεR1. Additionally, we observed increased expression of a number of genes encoding chemokines and cytokines associated with the recruitment of proinflammatory cells. Moreover, HDM exposure in the context of an influenza A infection resulted in the induction of unique genes, including calgranulin A (S100a8), an endogenous damage-associated molecular pattern and TLR4 agonist. In addition, we observed significantly increased expression of serum amyloid A (Saa3) and serine protease inhibitor 3n (Serpina3n). This study showed that influenza infection markedly increased the expression of multiple gene classes capable of sensing allergens and amplifying the ensuing immune-inflammatory response. We propose that influenza A infection primes the lung environment in such a way as to lower the threshold of allergen responsiveness, thus facilitating the emergence of a clinically significant allergic phenotype.

Intrinsic danger: activation of Toll-like receptors in rheumatoid arthritis

RA is a debilitating disorder that manifests as chronic localized synovial and systemic inflammation leading to progressive joint destruction. Recent advances in the molecular basis of RA highlight the role of both the innate and adaptive immune system in disease pathogenesis. Specifically, data obtained from in vivo animal models and ex vivo human tissue explants models has confirmed the central role of Toll-like receptors (TLRs) in RA. TLRs are pattern recognition receptors (PRRs) that constitute one of the primary host defence mechanisms against infectious and non-infectious insult. This receptor family is activated by pathogen-associated molecular patterns (PAMPs) and by damage-associated molecular patterns (DAMPs). DAMPs are host-encoded proteins released during tissue injury and cell death that activate TLRs during sterile inflammation. DAMPs are also proposed to drive aberrant stimulation of TLRs in the RA joint resulting in increased expression of cytokines, chemokines and proteases, perpetuating a vicious inflammatory cycle that constitutes the hallmark chronic inflammation of RA. In this review, we discuss the signalling mechanisms of TLRs, the central function of TLRs in the pathogenesis of RA, the role of endogenous danger signals in driving TLR activation within the context of RA and the current preclinical and clinical strategies available to date in therapeutic targeting of TLRs in RA.

Antisense oligonucleotide suppression of serum amyloid A reduces amyloid deposition in mice with AA amyloidosis

AA amyloid patients who experience disease progression and develop renal failure have not received sufficient benefit from agents that treat inflammation or infection. We have begun to explore the potential application of antisense oligonucleotides (ASOs) to specifically suppress SAA production and thereby reduce amyloid deposition. Proof-of-concept experiments conducted in mice initially examined ASO ability to reduce serum levels of SAA during an acute inflammatory response. Peak SAA levels in ASO-treated mice were reduced as much as 65% relative to levels in saline-treated mice. The extent of suppression was dose-dependent and influenced by the time interval between ASO administration and inflammatory stimulation. Subsequent experiments tested whether ASO suppression of SAA was sufficient to mitigate amyloid deposition. Amyloidosis was induced by amyloid-enhancing factor and silver nitrate injection; ASO treatment was initiated 1 week later and continued 1× or 3× per week; inflammation was re-triggered by subsequent injection(s) of silver nitrate; mice were sacrificed after 4-5 weeks. Examination of tissues by Congo red staining and SAA/AA immunohistochemistry revealed consistently less amyloid in the organs of ASO-treated mice compared to saline-treated counterparts. These findings provide rationale for further investigation of SAA-specific ASOs as a potential therapy for AA amyloidosis.

Lipopolysaccharide-induced anti-inflammatory acute phase response is enhanced in spermidine/spermine N1-acetyltransferase (SSAT) overexpressing mice

Bacterial lipopolysaccharide (LPS) is an effective activator of the components of innate immunity. It has been shown that polyamines and their metabolic enzymes affect the LPS-induced immune response by modulating both pro- and anti-inflammatory actions. On the other hand, LPS causes changes in cellular polyamine metabolism. In this study, the LPS-induced inflammatory response in spermidine/spermine N(1)-acetyltransferase overexpressing transgenic mice (SSAT mice) was analyzed. In liver and kidneys, LPS enhanced the activity of the polyamine biosynthetic enzyme ornithine decarboxylase and increased the intracellular putrescine content in both SSAT overexpressing and wild-type mice. In survival studies, the enhanced polyamine catabolism and concomitantly altered cellular polyamine pools in SSAT mice did not affect the LPS-induced mortality of these animals. However, in the acute phase of LPS-induced inflammatory response, the serum levels of proinflammatory cytokines interleukin-1β and interferon-γ were significantly reduced and, on the contrary, anti-inflammatory cytokine interleukin-10 was significantly increased in the sera of SSAT mice compared with the wild-type animals. In addition, hepatic acute-phase proteins C-reactive protein, haptoglobin and α(1)-acid glycoprotein were expressed in higher amounts in SSAT mice than in the wild-type animals. In summary, the study suggests that SSAT overexpression obtained in SSAT mice enhances the anti-inflammatory actions in the acute phase of LPS-induced immune response.

Serum amyloid A in atherosclerosis

PURPOSE OF REVIEW

Serum amyloid A (SAA) is a family of acute-phase proteins which are shown to correlate with cardiovascular disease, but whether this SAA contributes causally to atherosclerosis development or reflects underlying disease or risk factors remains unclear.

RECENT FINDINGS

SAA has been detected within atherosclerotic lesions and within adipose tissue where it is hypothesized that it may play a contributory role in disease development. In the acute-phase response SAA is synthesized by the liver and transported primarily in association with HDL. However, there is a growing literature suggesting that localized synthesis of SAA within the vasculature, or adipose tissue, may play a distinct role in disease development. Furthermore, SAA can be found in association with apoB-containing lipoproteins, in which its biological activity may be different.

SUMMARY

This review will discuss recent experimental evidence supporting a causal role of SAA with atherosclerosis.

Serum amyloid A activates the NLRP3 inflammasome and promotes Th17 allergic asthma in mice

IL-1β is a cytokine critical to several inflammatory diseases in which pathogenic Th17 responses are implicated. Activation of the NLRP3 inflammasome by microbial and environmental stimuli can enable the caspase-1-dependent processing and secretion of IL-1β. The acute-phase protein serum amyloid A (SAA) is highly induced during inflammatory responses, wherein it participates in systemic modulation of innate and adaptive immune responses. Elevated levels of IL-1β, SAA, and IL-17 are present in subjects with severe allergic asthma, yet the mechanistic relationship among these mediators has yet to be identified. In this study, we demonstrate that Saa3 is expressed in the lungs of mice exposed to several mixed Th2/Th17-polarizing allergic sensitization regimens. SAA instillation into the lungs elicits robust TLR2-, MyD88-, and IL-1-dependent pulmonary neutrophilic inflammation. Furthermore, SAA drives production of IL-1α, IL-1β, IL-6, IL-23, and PGE(2), causes dendritic cell (DC) maturation, and requires TLR2, MyD88, and the NLRP3 inflammasome for secretion of IL-1β by DCs and macrophages. CD4(+) T cells polyclonally stimulated in the presence of conditioned media from SAA-exposed DCs produced IL-17, and the capacity of polyclonally stimulated splenocytes to secrete IL-17 is dependent upon IL-1, TLR2, and the NLRP3 inflammasome. Additionally, in a model of allergic airway inflammation, administration of SAA to the lungs functions as an adjuvant to sensitize mice to inhaled OVA, resulting in leukocyte influx after Ag challenge and a predominance of IL-17 production from restimulated splenocytes that is dependent upon IL-1R signaling.

Toll-like receptors as targets for immune disorders

Since the identification of the first Toll-like receptor (TLR) in humans in 1997, understanding of the molecular basis for innate immunity has increased significantly. The TLR family and downstream signalling pathways have been extensively characterised, There is now significant evidence suggesting a role for TLRs in human inflammatory and immune diseases such as rheumatoid arthritis, diabetes, allergy/asthma and atherosclerosis. Various approaches have been taken to identify novel therapeutic agents targeting TLRs including biologics, small molecules and nucleic acid-based drugs. Several are now being evaluated in the clinic and showing promise against various diseases. This review paper outlines the recent advances in the understanding of TLR biology and highlights novel TLR agonists and antagonists in development for the treatment of immune diseases.

Serum amyloid A activates the NLRP3 inflammasome via P2X7 receptor and a cathepsin B-sensitive pathway

Serum amyloid A (SAA) is an acute-phase protein, the serum levels of which can increase up to 1000-fold during inflammation. SAA has a pathogenic role in amyloid A-type amyloidosis, and increased serum levels of SAA correlate with the risk for cardiovascular diseases. IL-1β is a key proinflammatory cytokine, and its secretion is strictly controlled by the inflammasomes. We studied the role of SAA in the regulation of IL-1β production and activation of the inflammasome cascade in human and mouse macrophages, as well as in THP-1 cells. SAA could provide a signal for the induction of pro-IL-1β expression and for inflammasome activation, resulting in secretion of mature IL-1β. Blocking TLR2 and TLR4 attenuated SAA-induced expression of IL1B, whereas inhibition of caspase-1 and the ATP receptor P2X(7) abrogated the release of mature IL-1β. NLRP3 inflammasome consists of the NLRP3 receptor and the adaptor protein apoptosis-associated speck-like protein containing CARD (a caspase-recruitment domain) (ASC). SAA-mediated IL-1β secretion was markedly reduced in ASC(-/-) macrophages, and silencing NLRP3 decreased IL-1β secretion, confirming NLRP3 as the SAA-responsive inflammasome. Inflammasome activation was dependent on cathepsin B activity, but it was not associated with lysosomal destabilization. SAA also induced secretion of cathepsin B and ASC. In conclusion, SAA can induce the expression of pro-IL-1β and activation of the NLRP3 inflammasome via P2X(7) receptor and a cathepsin B-sensitive pathway. Thus, during systemic inflammation, SAA may promote the production of IL-1β in tissues. Furthermore, the SAA-induced secretion of active cathepsin B may lead to extracellular processing of SAA and, thus, potentially to the development of amyloid A amyloidosis.

Hematopoietic cytokine-induced transcriptional regulation and Notch signaling as modulators of MDSC expansion

Hematopoietic stem cells (HSCs) differentiate into mature lineage restricted blood cells under the influence of a complex network of hematopoietic cytokines, cytokine-mediated transcriptional regulators, and manifold intercellular signaling pathways. The classical model of hematopoiesis proposes that progenitor cells undergo a dichotomous branching into myelo-erythroid and lymphoid lineages. Nonetheless, erythroid and lymphoid restricted progenitors retain their myeloid potential, supporting the existence of an alternative 'myeloid-based' mechanism of hematopoiesis. In this case, abnormal pathology is capable of dysregulating hematopoiesis in favor of myelopoiesis. The accumulation of immature CD11b+Gr-1+ myeloid-derived suppressor cells (MDSCs) has been shown to correlate with the presence of several hematopoietic cytokines, transcription factors and signaling pathways, lending support to this hypothesis. Although the negative role of MDSCs in cancer development is firmly established, it is now understood that MDSCs can exert a paradoxical, positive effect on transplantation, autoimmunity, and sepsis. Our conflicted understanding of MDSC function and the complexity of hematopoietic cytokine signaling underscores the need to elucidate molecular pathways of MDSC expansion for the development of novel MDSC-based therapeutics.

Serum amyloid A induces reactive oxygen species (ROS) production and proliferation of fibroblast

Serum amyloid A (SAA) levels are elevated highly in acute phase response and elevated slightly and persistently in chronic diseases such as rheumatoid arthritis and diabetes. Given that fibroblasts exert profound effects on progression of inflammatory chronic diseases, the aim of this study was to investigate the response of fibroblasts to SAA. A dose-dependent increase in O(2) (-) levels was observed by treatment of fibroblasts with SAA (r = 0·99 and P ≤ 0·001). In addition, the expression of p47-phox was up-regulated by SAA (P < 0·001) and diphenyliodonium (DPI), a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor, reduced the release of O(2) (-) by 50%. Also, SAA raised fibroblast proliferation (P < 0·001) and this effect was completely abolished by the addition of anti-oxidants (P < 0·001). These findings support the notion that, in chronic inflammatory sites, SAA activated fibroblast proliferation and ROS production.

Toll-like receptors 1 and 2 cooperatively mediate immune responses to curli, a common amyloid from enterobacterial biofilms

Responses to host amyloids and curli amyloid fibrils of Escherichia coli and Salmonella enterica serotype Typhimurium are mediated through Toll-like receptor (TLR) 2. Here we show that TLR2 alone was not sufficient for mediating responses to curli. Instead, transfection experiments with human cervical cancer (HeLa) cells and antibody-mediated inhibition of TLR signalling in human macrophage-like (THP-1) cells suggested that TLR2 interacts with TLR1 to recognize curli amyloid fibrils. TLR1/TLR2 also serves as a receptor for tri-acylated lipoproteins, which are produced by E. coli and other Gram-negative bacteria. Despite the presence of multiple TLR1/TLR2 ligands on intact bacterial cells, an inability to produce curli amyloid fibrils markedly reduced the ability of E. coli to induce TLR2-dependent responses in vitro and in vivo. Collectively, our data suggest that curli amyloid fibrils from enterobacterial biofilms significantly contribute to TLR1/TLR2-mediated host responses against intact bacterial cells.

Interferon-α suppressed granulocyte colony stimulating factor production is reversed by CL097, a TLR7/8 agonist

BACKGROUND AND AIM

Neutropenia, a major side-effect of interferon-α (IFN-α) therapy can be effectively treated by the recombinant form of granulocyte colony stimulating factor (G-CSF), an important growth factor for neutrophils. We hypothesized that IFN-α might suppress G-CSF production by peripheral blood mononuclear cells (PBMCs), contributing to the development of neutropenia, and that a toll-like receptor (TLR) agonist might overcome this suppression.

METHODS

Fifty-five patients who were receiving IFN-α/ribavirin combination therapy for chronic hepatitis C virus (HCV) infection were recruited. Absolute neutrophil counts (ANC), monocyte counts and treatment outcome data were recorded. G-CSF levels in the supernatants of PBMCs isolated from the patients and healthy controls were assessed by enzyme-linked immunosorbent assay following 18 h of culture in the absence or presence of IFN- α or the TLR7/8 agonist, CL097.

RESULTS

Therapeutic IFN-α caused a significant reduction in neutrophil counts in all patients, with 15 patients requiring therapeutic G-CSF. The reduction in ANC over the course of IFN-α treatment was paralleled by a decrease in the ability of PBMCs to produce G-CSF. In vitro G-CSF production by PBMCs was suppressed in the presence of IFN-α; however, co-incubation with a TLR7/8 agonist significantly enhanced G-CSF secretion by cells obtained both from HCV patients and healthy controls.

CONCLUSIONS

Suppressed G-CSF production in the presence of IFN-α may contribute to IFN-α-induced neutropenia. However, a TLR7/8 agonist elicits G-CSF secretion even in the presence of IFN-α, suggesting a possible therapeutic role for TLR agonists in treatment of IFN-α-induced neutropenia.

The multifaceted effects of granulocyte colony-stimulating factor in immunomodulation and potential roles in intestinal immune homeostasis

The three colony-stimulating factors, granulocyte/macrophage colony-stimulating factor (GM-CSF), macrophage colony-stimulating factor (M-CSF), and granulocyte colony-stimulating factor (G-CSF), have been regarded as immunostimulators because of their role in granulocyte and myeloid hematopoiesis and immune function. However, unlike GM-CSF and M-CSF, G-CSF possesses immunosuppressive effects on other immune cells including monocytes/macrophages, dendritic cells, and T lymphocytes when exogenously administered. Given the immunomodulatory effects of exogenous G-CSF, endogenous G-CSF may also play an important role in maintaining local immune homeostasis in tissue in which it is highly and constitutively produced. This review highlights the potential role of G-CSF in immunomodulation and intestinal immune homeostasis.

Invariant NKT cells modulate the suppressive activity of IL-10-secreting neutrophils differentiated with serum amyloid A

Neutrophils are the primary effector cells during inflammation, but can also control excessive inflammatory responses by secreting anti-inflammatory cytokines. However, the mechanisms modulating their plasticity remain unclear. We now show that systemic serum amyloid A-1 (SAA-1) controls the plasticity of neutrophil differentiation. SAA-1 not only induced anti-inflammatory IL-10-secreting neutrophils but also promoted invariant NKT (iNKT) cell interaction with these neutrophils, a process that limits their suppressive activity by reducing IL-10 and enhancing IL-12 production. Because SAA-1-producing melanomas promote differentiation of IL-10-secreting neutrophils, harnessing iNKT cells could be useful therapeutically by reducing the frequency of immunosuppressive neutrophils and restoring tumor specific immune responses.

A pertussis toxin sensitive G-protein-independent pathway is involved in serum amyloid A-induced formyl peptide receptor 2-mediated CCL2 production

Serum amyloid A (SAA) induced CCL2 production via a pertussis toxin (PTX)-insensitive pathway in human umbilical vein endothelial cells (HUVECs). SAA induced the activation of three MAPKs (ERK, p38 MAPK, and JNK), which were completely inhibited by knock-down of formyl peptide receptor 2 (FPR2). Inhibition of p38 MAPK and JNK by their specific inhibitors (SB203580 and SP600125), or inhibition by a dominant negative mutant of p38 MAPK dramatically decreased SAA-induced CCL2 production. Inactivation of G((i)) protein(s) by PTX inhibited the activation of SAA-induced ERK, but not p38 MAPK or JNK. The results indicate that SAA stimulates FPR2-mediated activation of p38 MAPK and JNK, which are independent of a PTX-sensitive G-protein and are essential for SAA-induced CCL2 production.

DAMPening inflammation by modulating TLR signalling

Damage-associated molecular patterns (DAMPs) include endogenous intracellular molecules released by activated or necrotic cells and extracellular matrix (ECM) molecules that are upregulated upon injury or degraded following tissue damage. DAMPs are vital danger signals that alert our immune system to tissue damage upon both infectious and sterile insult. DAMP activation of Toll-like receptors (TLRs) induces inflammatory gene expression to mediate tissue repair. However, DAMPs have also been implicated in diseases where excessive inflammation plays a key role in pathogenesis, including rheumatoid arthritis (RA), cancer, and atherosclerosis. TLR activation by DAMPs may initiate positive feedback loops where increasing tissue damage perpetuates pro-inflammatory responses leading to chronic inflammation. Here we explore the current knowledge about distinct signalling cascades resulting from self TLR activation. We also discuss the involvement of endogenous TLR activators in disease and highlight how specifically targeting DAMPs may yield therapies that do not globally suppress the immune system.

Airway epithelial NF-κB activation promotes allergic sensitization to an innocuous inhaled antigen

Activation of NF-κB in airway epithelium is observed in allergic asthma and is induced by inhalation of numerous infectious and reactive substances. Many of the substances that activate NF-κB in the airway epithelium are also capable of acting as adjuvants to elicit antigen-specific sensitization to concomitantly inhaled protein, thereby circumventing the inherent bias of the lung to promote tolerance to innocuous antigens. We have used a transgenic mouse inducibly expressing a constitutively active mutant of the inhibitor of nuclear factor κB (IκB) kinase β ((CA)IKKβ) that activates NF-κB only in nonciliated airway epithelial cells to test whether activation of this intracellular signaling pathway in this specific cell type is sufficient to establish a pulmonary environment permissive to the development of allergic sensitization to inhaled protein. When airway epithelial (CA)IKKβ was transiently expressed in antigen-naive mice only during initial inhalation of ovalbumin, the mice became allergically sensitized to the antigen. As a consequence, subsequent inhalation of ovalbumin alone led to an allergic asthma-like response that included airway hyperresponsiveness to methacholine, eosinophilia, mucus expression, elevated serum levels of antigen-specific IgE and IgG1, and splenic CD4(+) T cells that secreted T helper type 2 and type 17 cytokines in response to in vitro antigen restimulation. Furthermore, CD11c(+) cells in the mediastinal lymph nodes (MLN) of (CA)IKKβ-expressing mice displayed significantly elevated levels of activation markers. These data implicate airway epithelial NF-κB activation as a critical modulator of the adaptive immune response to inhaled antigens via the secretion of soluble mediators that affect the capacity of CD11c(+) cells to undergo maturation and promote antigen-specific allergic responses.

Therapeutic anti-inflammatory potential of formyl-peptide receptor agonists

The need for novel anti-inflammatory drugs justifies the search for innovative targets that could satisfy this goal. For quite some time now, we have proposed the study of endogenous anti-inflammation as a distinctive approach to the discovery of new drugs. This approach requires development of new compounds that activate specific receptor targets to downregulate the cellular and tissue pathways operative in the host during inflammation. Here we dwell on a family of G-protein coupled receptors (GPCR) termed FPRs, acronym for formyl-peptide receptors. With three and seven members in man and mouse, respectively, these receptors harness many biological functions, spanning odour perception and hair growth, to the control of multiple facets (pain; cell migration; oxidative burst; xenobiotic engulfment) of the inflammatory reaction. We focus on FPR biology with particular attention to molecules able to produce pharmacological effects by interacting with these GPCRs, describing endogenous agonists of FPRs and, more relevantly, the current development of synthetic agonists. Besides being potential leads for the development of the anti-inflammatory therapeutics of the future, these compounds could also help clarify the properties and roles that each FPR might play in the complex network of pathways that is inflammation. We conclude that FPR2 agonists could be valid warhorses for defining a novel philosophy for anti-inflammatory drug discovery programmes: mimicking - with new compounds - the way our body disposes of inflammation could be a viable approach to regulate aberrant inflammatory responses as in the case of several chronic rheumatic and cardiovascular pathologies.

An integrated view of humoral innate immunity: pentraxins as a paradigm

The innate immune system consists of a cellular and a humoral arm. Pentraxins (e.g., the short pentraxin C reactive protein and the long pentraxin PTX3) are key components of the humoral arm of innate immunity which also includes complement components, collectins, and ficolins. In response to microorganisms and tissue damage, neutrophils, macrophages, and dendritic cells are major sources of fluid-phase pattern-recognition molecules (PRMs) belonging to different molecular classes. Humoral PRMs in turn interact with and regulate cellular effectors. Effector mechanisms of the humoral innate immune system include activation and regulation of the complement cascade; agglutination and neutralization; facilitation of recognition via cellular receptors (opsonization); and regulation of inflammation. Thus, the humoral arm of innate immunity is an integrated system consisting of different molecules and sharing functional outputs with antibodies.

Endogenous ligands of TLR2 and TLR4: agonists or assistants?

The mammalian TLRs serve as key sensors of PAMPs, such as bacterial LPS, lipopeptides, and flagellins, which are present in microbial cells but not host cells. TLRs have therefore been considered to play a central role in the discrimination between "self" and "non-self". However, since the discovery of their microbial ligands, many studies have provided evidence that host-derived molecules may also stimulate TLR2- or TLR4-dependent signaling. To date, more than 20 of these endogenous TLR ligands have been proposed, which have tended to fall into the categories of released intracellular proteins, ECM components, oxidatively modified lipids, and other soluble mediators. This review aims to summarize the evidence supporting the intrinsic TLR-stimulating capacity of each of these proposed endogenous ligands with a particular emphasis on the measures taken to exclude contaminating LPS and lipopeptides from experimental systems. The emerging evidence that many of these molecules may be more accurately described as PAMP-binding molecules (PBMs) or PAMP-sensitizing molecules (PSMs), rather than genuine ligands of TLR2 or TLR4, is also summarized. The relevance of this possibility to the pathogenesis of chronic inflammatory diseases, tumor surveillance, and autoimmunity is discussed.

Anti-inflammatory role of the murine formyl-peptide receptor 2: ligand-specific effects on leukocyte responses and experimental inflammation

The human formyl-peptide receptor (FPR)-2 is a G protein-coupled receptor that transduces signals from lipoxin A(4), annexin A1, and serum amyloid A (SAA) to regulate inflammation. In this study, we report the creation of a novel mouse colony in which the murine FprL1 FPR2 homologue, Fpr2, has been deleted and describe its use to explore the biology of this receptor. Deletion of murine fpr2 was verified by Southern blot analysis and PCR, and the functional absence of the G protein-coupled receptor was confirmed by radioligand binding assays. In vitro, Fpr2(-/-) macrophages had a diminished response to formyl-Met-Leu-Phe itself and did not respond to SAA-induced chemotaxis. ERK phosphorylation triggered by SAA was unchanged, but that induced by the annexin A1-derived peptide Ac2-26 or other Fpr2 ligands, such as W-peptide and compound 43, was attenuated markedly. In vivo, the antimigratory properties of compound 43, lipoxin A(4), annexin A1, and dexamethasone were reduced notably in Fpr2(-/-) mice compared with those in wild-type littermates. In contrast, SAA stimulated neutrophil recruitment, but the promigratory effect was lost following Fpr2 deletion. Inflammation was more marked in Fpr2(-/-) mice, with a pronounced increase in cell adherence and emigration in the mesenteric microcirculation after an ischemia-reperfusion insult and an augmented acute response to carrageenan-induced paw edema, compared with that in wild-type controls. Finally, Fpr2(-/-) mice exhibited higher sensitivity to arthrogenic serum and were completely unable to resolve this chronic pathology. We conclude that Fpr2 is an anti-inflammatory receptor that serves varied regulatory functions during the host defense response. These data support the development of Fpr2 agonists as novel anti-inflammatory therapeutics.

Serum amyloid A regulates granulomatous inflammation in sarcoidosis through Toll-like receptor-2

RATIONALE

The critical innate immune mechanisms that regulate granulomatous inflammation in sarcoidosis are unknown. Because the granuloma-inducing component of sarcoidosis tissues has physicochemical properties similar to those of amyloid fibrils, we hypothesized that host proteins capable of forming poorly soluble aggregates or amyloid regulate inflammation in sarcoidosis.

OBJECTIVES

To determine the role of the amyloid precursor protein, serum amyloid A, as an innate regulator of granulomatous inflammation in sarcoidosis.

METHODS

Serum amyloid A expression was determined by immunohistochemistry in sarcoidosis and control tissues and by ELISA. The effect of serum amyloid A on nuclear factor (NF)-kappaB induction, cytokine expression, and Toll-like receptor-2 stimulation was determined with transformed human cell lines and bronchoalveolar lavage cells from patients with sarcoidosis. The effects of serum amyloid A on regulating helper T cell type 1 (Th1) granulomatous inflammation were determined in experimental models of sarcoidosis, using Mycobacterium tuberculosis catalase-peroxidase.

MEASUREMENTS AND MAIN RESULTS

We found that the intensity of expression and distribution of serum amyloid A within sarcoidosis granulomas was unlike that in many other granulomatous diseases. Serum amyloid A localized to macrophages and giant cells within sarcoidosis granulomas but correlated with CD3(+) lymphocytes, linking expression to local Th1 responses. Serum amyloid A activated NF-kappaB in Toll-like receptor-2-expressing human cell lines; regulated experimental Th1-mediated granulomatous inflammation through IFN-gamma, tumor necrosis factor, IL-10, and Toll-like receptor-2; and stimulated production of tumor necrosis factor, IL-10, and IL-18 in lung cells from patients with sarcoidosis, effects inhibited by blocking Toll-like receptor-2.

CONCLUSIONS

Serum amyloid A is a constituent and innate regulator of granulomatous inflammation in sarcoidosis through Toll-like receptor-2, providing a mechanism for chronic disease and new therapeutic targets.

Responses to amyloids of microbial and host origin are mediated through toll-like receptor 2

Curli fibrils are proteinaceous bacterial structures formed by amyloid fibrils composed of the major curli subunit CsgA. Like beta-amyloid 1-42, which is associated with brain inflammation and Alzheimer's disease, curli fibrils have been implicated in the induction of host inflammatory responses. However, the underlying mechanisms of amyloid-induced inflammation are not fully understood. In a mouse sepsis model, we show that curli fibrils contributed to Nos2 expression, a hallmark of inflammation, by stimulating Toll-like receptor (TLR) 2. The TLR2 agonist activity was reduced by an amyloidogenicity-lowering amino acid substitution (N122A) in CsgA. Amyloid-forming synthetic peptides corresponding to beta-amyloid 1-42 or CsgA 111-151 stimulated Nos2 production in macrophages and microglia cells through a TLR2-dependent mechanism. This activity was abrogated when an N122A substitution was introduced into the synthetic CsgA peptide. The induction of TLR2-mediated responses by bacterial and eukaryotic amyloids may explain the inflammation associated with amyloids and the resulting pathologies.

Is Porphyromonas gingivalis cell invasion required for atherogenesis? Pharmacotherapeutic implications

Various studies have demonstrated an association between chronic bacterial infections and atherosclerotic cardiovascular disease. Porphyromonas gingivalis, which can invade endothelial cells, is one pathogen that may link these disorders. If so, antibiotics that block its invasiveness may ameliorate atherosclerotic plaque progression. To explore the role of invasion of P. gingivalis in inflammation- and infection-associated atherosclerosis, 10-wk-old ApoE(+/-) mice were fed either a high fat diet or a regular chow diet. All mice were inoculated i.v., once per week for 24 consecutive wk, with either 50 microl of live P. gingivalis (strain 381) (10(7) CFU); a fimbria-deficient P. gingivalis; or metronidazole before P. gingivalis. Mice were euthanized and evaluated 24 wk after the first inoculation. ApoE(+/-) mice injected with DPG3 or metronidazole showed significantly fewer atheromatous lesions in the proximal aorta and the aortic tree compared with ApoE(+/-) mice injected with wild-type P. gingivalis for either diet condition. Serum amyloid A levels were significantly lower in ApoE(+/-) mice that received either DPG3 or metronidazole before P. gingivalis than from ApoE(+/-) mice that received P. gingivalis alone. Serum cytokine analysis revealed decreased levels of proinflammatory cytokines in both DPG3-injected and metronidazole/P. gingivalis-treated ApoE(+/-) mice compared with mice receiving only P. gingivalis, irrespective of diet. P. gingivalis invasion is a critical phenomenon in the progression of atherosclerosis. The present data offer new insights into the pathophysiological pathways involved in atherosclerosis and pave the way for new pharmacological interventions aimed at reducing atherosclerosis.