Monocyte exposure to LPS induces a transient state in which these cells are refractory to further endotoxin stimulation. This phenomenon, termed endotoxin tolerance (ET), is characterized by a decreased production of cytokines in response to the proinflammatory stimulus. We have established a robust model of ET and have determined the time frame and features of LPS unresponsiveness in cultured human monocytes. A large number of genes transcribed in tolerant monocytes were classified as either "tolerizable" or "nontolerizable" depending on their expression levels during the ET phase. Tolerant monocytes exhibit rapid IL-1R-associated kinase-M (IRAK-M) overexpression, high levels of triggering receptor expressed on myeloid cells-1 (TREM-1) and CD64, and a marked down-regulation of MHC molecules and NF-B2. These cells combine potent phagocytic activity with impaired capability for Ag presentation. We also show that circulating monocytes isolated from cystic fibrosis patients share all the determinants that characterize cells locked in an ET state. These findings identify a new mechanism that contributes to impaired inflammation in cystic fibrosis patients despite a high frequency of infections. Our results indicate that a tolerant phenotype interferes with timing, efficiency, and outcome of the innate immune responses against bacterial infections.

Triggering receptors expressed on myeloid cell (TREM) proteins are a family of cell surface receptors that participate in diverse cellular processes such as inflammation, coagulation, and bone homeostasis. TREM-1, in particular, is expressed on neutrophils and monocytes and is a potent amplifier of inflammatory responses. LPS and other microbial products induce up-regulation of cell surface-localized TREM-1 and the release of its soluble form, sTREM-1. Two hypotheses have been advanced to explain the origin of sTREM-1: alternative splicing of TREM-1 mRNA and proteolytic cleavage(s) of mature, membrane-anchored TREM-1. In this report, we present conclusive evidence in favor of the proteolytic mechanism of sTREM-1 generation. No alternative splicing forms of TREM-1 were detected in monocytes/macrophages. Besides, metalloproteinase inhibitors increased the stability of TREM-1 at the cell surface while significantly reducing sTREM-1 release in cultures of LPS-challenged human monocytes and neutrophils. We conclude that metalloproteinases are responsible for shedding of the TREM-1 ectodomain through proteolytic cleavage of its long juxtamembrane linker.

Cystic Fibrosis (CF) is an inherited pleiotropic disease that results from abnormalities in the gene that codes for the chloride channel, Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). CF patients are frequently colonized by several pathogens, but the mechanisms that allow colonization in spite of apparently functional immune systems are incompletely understood. In this paper we show that blood peripheral monocytes isolated from CF patients are found in an endotoxin tolerance state, yet this is not due to a deficient TLR activation. On the other hand, levels of the amplifier of inflammatory responses, TREM-1 (Triggering Receptor Expressed on Myeloid cells), are notably down-regulated in monocytes from patients, in comparison to those extracted from healthy volunteers. Furthermore, the soluble form of TREM-1 (sTREM-1) was not detected in the sera of patients. Additionally, and in strict contrast to patients who suffer from Chronic Obstructive Pulmonary Disease (COPD), CF monocytes challenged ex vivo with LPS neither up-regulated membrane-anchored TREM-1 nor sTREM-1. Finally, similar levels of PGE2 expression and p65 translocation into the nucleus were found in both patients and healthy volunteers, thus suggesting that TREM-1 regulation is neither controlled by PGE2 levels nor by p65 activation in this case. However, PU.1 translocation into the nucleus was significantly higher in CF monocytes than in controls, suggesting a role for this transcription factor in the control of TREM-1 expression. We conclude that down-regulation of TREM-1 expression in cystic fibrosis patients is at least partly responsible for the endotoxin tolerance state in which their monocytes are locked.

Acute coronary syndrome (ACS) groups different cardiac diseases whose development is associated with inflammation. Here we have analyzed the levels of inflammatory cytokines and of members of the TLR/IRAK pathway including IRAK-M in monocytes from ACS patients classified as either UA (unstable angina), STEMI (ST-elevation myocardial infarction) or NSTEMI (non-ST-elevation myocardial infarction). Circulating monocytes from all patients, but not from healthy individuals, showed high levels of pro-inflammatory cytokines, TNF-alpha and IL-6, as well as of IRAK-M and IL-10. TLR4 was also up-regulated, but IRAK-1, IRAK-4 and MyD88 levels were similar in patients and controls. Further, we investigated the consequences of cytokines/IRAK-M expression on the innate immune response to endotoxin. Ex vivo responses to LPS were markedly attenuated in patient monocytes compared to controls. Control monocytes cultured for 6 h in supplemented medium (10% serum from ACS patients) expressed IRAK-M, and LPS stimulation failed to induce TNF-alpha and IL-6 in these cultures. Pre-incubation of the serum with a blocking anti-TNF-alpha antibody reduced this endotoxin tolerance effect, suggesting that TNF-alpha controls this phenomenon, at least partially. We show for the first time that inflammatory responses associated with ACS induce an unresponsiveness state to endotoxin challenge in circulating monocytes, which correlates with expression of IRAK-M, TLR4 and IL-10. The magnitude of this response varies according to the clinical condition (UA, STEMI or NSTEMI), and is regulated by TNF-alpha.

Cystic Fibrosis (CF) is an inherited pleiotropic disease that results from abnormalities in the gene codes of a chloride channel. The lungs of CF patients are chronically infected by several pathogens but bacteraemia have rarely been reported in this pathology. Besides that, circulating monocytes in CF patients exhibit a patent Endotoxin Tolerance (ET) state since they show a significant reduction of the inflammatory response to bacterial stimulus. Despite a previous description of this phenomenon, the direct cause of ET in CF patients remains unknown. In this study we have researched the possible role of microbial/endotoxin translocation from a localized infection to the bloodstream as a potential cause of ET induction in CF patients. Plasma analysis of fourteen CF patients revealed high levels of LPS compared to healthy volunteers and patients who suffer from Chronic Obstructive Pulmonary Disease. Experiments in vitro showed that endotoxin concentrations found in plasma of CF patients were enough to induce an ET phenotype in monocytes from healthy controls. In agreement with clinical data, we failed to detect bacterial DNA in CF plasma. Our results suggest that soluble endotoxin present in bloodstream of CF patients causes endotoxin tolerance in their circulating monocytes.

Blood monocytes recognize Gram-negative bacteria through the TLR4, which signal via MyD88- and TRIF-dependent pathway to trigger an immune-inflammatory response. However, a dysregulated inflammatory response by these cells often leads to severe pathologies such as sepsis. We investigated the role of CD16 in the regulation of human monocyte response to Gram-negative endotoxin and sepsis. Blood monocytes from sepsis patients demonstrated an upregulation of several TRIF-dependent genes as well as a selective expansion of CD16-expressing (CD16+) monocytes. Gene expression and biochemical studies revealed CD16 to regulate the TRIF-dependent TLR4 pathway in monocytes by activating Syk, IFN regulatory factor 3, and STAT1, which resulted in enhanced expression of IFNB, CCL5, and CXCL10. CD16 also upregulated the expression of IL-1R–associated kinase M and IL-1 receptor antagonist, which are negative regulators of the MyD88-dependent pathway. CD16 overexpression or small interfering RNA knockdown in monocytes confirmed the above findings. Interestingly, these results were mirrored in the CD16+ monocyte subset isolated from sepsis patients, providing an in vivo confirmation to our findings. Collectively, the results from the current study demonstrate CD16 as a key regulator of the TRIF-dependent TLR4 pathway in human monocytes and their CD16-expressing subset, with implications in sepsis.

Endotoxin tolerance (ET) is a state of reduced responsiveness to endotoxin stimulation after a primary bacterial insult. This phenomenon has been described in several pathologies, including sepsis, in which an endotoxin challenge results in reduced cytokine production. In this study, we show that the NFκ L chain enhancer of activated B cells 2 (NFκB2)/p100 was overexpressed and accumulated in a well-established in vitro human monocyte model of ET. The p100 accumulation in these cells inversely correlated with the inflammatory response after LPS stimulation. Knocking down NFκB2/p100 using small interfering RNA in human monocytes further indicated that p100 expression is a crucial factor in the progression of ET. The monocytes derived from patients with sepsis had high levels of p100, and a downregulation of NFκB2/p100 in these septic monocytes reversed their ET status.

The MMPs constitute a family of endopeptidases that can cleavage extracellular proteins. They are involved in a number of events; some of these include inflammatory processes. One of its targets is the TREM-1, which has emerged as an important modulator of innate immune responses in mammals. This transmembrane glycoprotein possesses an Ig-like ectodomain readily shed by MMPs to generate sTREM-1. Whereas membrane-anchored TREM-1 amplifies inflammatory responses, sTREM-1 exhibits anti-inflammatory properties. Here we show that sustained cell surface expression of TREM-1 in human monocytes, through metalloproteinase inhibition, counteracts the well-characterized down-regulation of several proinflammatory cytokines during the ET time-frame, also known as M2 or alternative activation. In addition to the cytokines profile, other features of the ET phenotype were underdeveloped when TREM-1 was stabilized at the cell surface. These events were mediated by the signal transducers PI3Ks and Syk. We also show that sTREM-1 counteracts the proinflammatory response obtained by membrane TREM-1 stabilization but failed to induce ET on naïve human monocytes. As the sustained TREM-1 expression at the cell surface suffices to block the progress of a refractory state in human monocytes, our data indicate that TREM-1 and MMPs orchestrate an “adaptive” form of innate immunity by modulating the human monocytes response to endotoxin.