Soluble CD14 mediates efflux of phospholipids from cells

Increased Serum CD14 Level Is Associated with Depletion of TNF-α in Monocytes in Migraine Patients during Interictal Period

The aim of the present study was to investigate the levels of circulating CD14 in relation to the expression of tumor necrosis factor alpha (TNF-α) in monocytes, and serum levels of TNF-α and macrophage inflammatory protein-1 (MIP-1) in migraine patients. Numerous studies revealed controversial changes in the components of the immune system during attacks and the interictal period in migraine patients. Our study included 40 migraineurs and 39 controls. The levels of TNF-α, MIP-1 and CD14 were measured in peripheral monocytes and in sera with the Enzyme-Linked Immunosorbent Assay (ELISA) method, and the monocyte expression of TNF-α was also analysed by immunostaining. Serum CD14 concentrations were higher and the expression of TNF-α in monocytes was decreased in migraineurs. The serum MIP-1 level correlated with Verbal Rating Scale (VRS); the MIP-1:CD14 ratio in monocytes correlated with Visual Analogue Scale (VAS); the MIP-1:CD14 ratio correlated with Migraine Severity (MIGSEV)-Pain scores; and serum CD14 concentration correlated with migraine duration in years. Increased serum CD14 and depletion of TNF-α in monocytes can orchestrate other components of the immune system during the interictal period.

CD14 but not MD2 transmit signals from DAMP

Both pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) activate antigen-presenting cells, often through the same pattern recognition receptors (PRR), such as Toll-like receptors (TLR). The TLR4-CD14-MD2 and TLR2-CD14 complexes have been shown to play a role in the recognition of lipopolysaccharide (LPS) and peptidoglycan (PG), respectively. Since many DAMPs have also been known to activate TLR2 or TLR4 pathways, we dissected the role of each molecule in the receptor complexes (TLR2-D14-MD2) responding to DAMP (necrotic cells) or PAMP (LPS and PG). CD14 played a significant role in the activation of NF-kappaB in response to necrotic cells in the presence or absence of TLR2. However, MD2 did not play a significant role in NF-kappaB activation by necrotic cells. Intriguingly, MD2 did play a significant role in activating NF-kappaB by PG in the presence of TLR2-CD14. Compared with CD14(pos) B6 mice, CD14(neg) B6 mice showed delayed production of IL12p40 in response to necrotic cells in vivo. Microarray analysis showed that various pro-inflammatory genes of peritoneal cells were regulated in response to necrotic cells, in a CD14-dependent manner. The CD14 appears to recognize necrotic cells in addition to LPS, PG, apoptotic cells, and lipids, suggesting that CD14 might be a universal adaptor for DAMP and PAMP. On the contrary, MD2 recognizes only exogenous PAMP, when complexed with TLR2-CD14 or TLR4-CD14. Taken together, MD2 appears to discriminate between DAMP and PAMP.

A phosphatidylserine species inhibits a range of TLR- but not IL-1beta-induced inflammatory responses by disruption of membrane microdomains

TLRs detect conserved molecular patterns that are unique to microbes, enabling tailored responses to invading pathogens and modulating a multitude of immunopathological conditions. We investigated the ability of a naturally occurring stearoyl-arachidonoyl form of phosphatidylserine (SAPS) to inhibit the proinflammatory effects of TLR agonists in models of inflammation investigating the interaction of leukocytes with epithelial and endothelial cells. The responses to LPS of both epithelial and endothelial cells were highly amplified in the presence of PBMCs. Coincubation with SAPS markedly inhibited activation of cocultures by LPS, principally through inhibition of the TLR4 signaling pathway in PBMCs; however, this was not through downmodulation of TLR4 or coreceptor expression, nor was IL-1beta-induced cytokine release affected. SAPS also impaired Pam(3)CSK(4) (TLR2/1), Gardiquimod (TLR7/8), and Streptococcus pneumoniae-induced cytokine release, but had only modest effects on poly(I:C) (TLR3)-induced responses. Fluorescence resonance energy transfer analysis of molecular associations revealed that SAPS disrupted the association of both TLR4 and TLR2 with their respective membrane partners that are required for signaling. Thus, our data reinforce the existence and importance of cooperative networks of TLRs, tissue cells, and leukocytes in mediating innate immunity, and identify a novel disrupter of membrane microdomains, revealing the dependence of TLR signaling on localization within these domains.

Phosphatidylinositol 4-phosphate accumulates extracellularly upon xylanase treatment in tomato cell suspensions

Various phosphoinositides have been implicated in plant defence signalling. Until now, such molecules have been exclusively related to intracellular signalling. Here, evidence is provided for the detection of extracellular phosphatidylinositol 4-phosphate (PI4P) in tomato cell suspensions. We have analysed and compared the intracellular and extracellular phospholipid profiles of [(32)P(i)]-prelabelled tomato cells, challenged with the fungal elicitor xylanase. These phospholipid patterns were found to be different, being phosphatidylinositol phosphate (PIP) the most abundant phospholipid in the extracellular medium. Moreover, while cells responded with a typical increase in phosphatidic acid and a decrease in intracellular PIP upon xylanase treatment, extracellular PIP level increased in a time- and dose-dependent manner. Using two experimental approaches, the extracellular PIP isoform was identified as PI4P. Addition of PI4P to tomato cell suspensions triggered the same defence responses as those induced by xylanase treatment. These include production of reactive oxygen species, accumulation of defence-related gene transcripts and induction of cell death. We demonstrate that extracellular PI4P is accumulated in xylanase-elicited cells and that exogenous application of PI4P mimics xylanase effects, suggesting its putative role as an intercellular signalling molecule.

Liver X Receptor: Crosstalk Node for the Signaling of Lipid Metabolism, Carbohydrate Metabolism, and Innate Immunity

Liver X Receptor-α (LXRα, also known as NR1H3) and LXRβ (NR1H2) are members of the nuclear receptor superfamily of ligand-activated transcription factors, a superfamily which includes the more widely known glucocorticoid receptor, estrogen receptor, thyroid receptor, and peroxisome proliferator-activated receptors. The LXRs are activated by physiologic sterol ligands (e.g., oxysterols) and by synthetic agonists. In recent years, our understanding of the importance of LXRs has expanded across several fields of (patho-)physiology. Perhaps best known from a sizeable literature as homeostatic 'cholesterol sensors' that drive transcriptional programs promoting cellular cholesterol efflux, 'reverse cholesterol transport,' and bile acid synthesis, more recent roles for LXRs in glucose homeostasis, atherosclerosis, and innate immunity have also been identified. These discoveries complement an emerging literature that continues to draw surprisingly intimate connections between host metabolism and host defense. The present review will discuss the roles of LXR in the signaling of metabolism and innate immunity, and the potential for synthetic LXR agonists as novel therapeutics in dyslipidemia, atherosclerosis, disordered glucose metabolism, and inflammation.

CD14: a soluble pattern recognition receptor in milk

An innate immune system capable of distinguishing among self, non-self, and danger is a prerequisite for health. Upon antigenic challenge, pattern recognition receptors (PRRs), such as the Toll-like receptor (TLR) family of proteins, enable this system to recognize and interact with a number of microbial components and endogenous host proteins. In the healthy host, such interactions culminate in tolerance to self-antigen, dietary antigen, and commensal microorganisms but in protection against pathogenic attack. This duality implies tightly regulated control mechanisms that are not expected of the inexperienced neonatal immune system. Indeed, the increased susceptibility of newborn infants to infection and to certain allergens suggests that the capacity to handle certain antigenic challenges is not inherent. The observation that breast-fed infants experience a lower incidence of infections, inflammation, and allergies than formula-fed infants suggests that exogenous factors in milk may play a regulatory role. There is increasing evidence to suggest that upon exposure to antigen, breast milk educates the neonatal immune system in the decision-making processes underlying the immune response to microbes. Breast milk contains a multitude of factors such as immunoglobulins, glycoproteins, glycolipids, and antimicrobial peptides that, qualitatively or quantitatively, may modulate how neonatal cells perceive and respond to microbial components. The specific role of several of these factors is highlighted in other chapters in this book. However, an emerging concept is that breast milk influences the neonatal immune system's perception of "danger." Here we discuss how CD14, a soluble PRR in milk, may contribute to this education.

A polymorphism in CD14 modifies the effect of farm milk consumption on allergic diseases and CD14 gene expression

BACKGROUND

Consumption of farm milk in early life is associated with less asthma and allergies.

OBJECTIVE

We hypothesized that genetic variation in the innate immunity receptor CD14 might modify the association between farm milk consumption and asthma and atopy.

METHODS

Questionnaire data, serum IgE levels, and genotypes for 4 single nucleotide polymorphisms in CD14 were assessed in farmers' and nonfarmers' children from 2 European populations (Allergy and Endotoxin study, n = 576; Prevention of Allergy Risk factors for Sensitization in children related to Farming and Anthroposophic Lifestyle study, n = 1539). In a subsample (n = 222) CD14 gene expression was measured in peripheral blood leukocytes. The effects of farm milk and CD14 genotypes on asthma, allergies, and CD14 expression and their interactions were investigated.

RESULTS

We found a significant interaction between genetic variation in CD14/-1721 and farm milk consumption. Adjusted odds ratios for the association between farm milk and asthma varied between the genotypes: AA, 0.18 (95% CI, 0.07-0.47); AG, 0.47 (95% CI, 0.26-0.86); and GG, 0.98 (95% CI, 0.46-2.08). Similar patterns were observed for symptoms of allergic rhinoconjunctivitis and pollen sensitization. CD14/-1721 also modified the association between farm milk and CD14 gene expression (adjusted geometric means ratios: AA, 1.61 (95% CI, 0.98-2.66); AG, 1.11 (95% CI, 0.71-1.72); and GG, 0.76 (95% CI, 0.39-1.48).

CONCLUSION

The protective effect of farm milk consumption on allergic diseases is stronger in children carrying the A allele in CD14/-1721 than in children homozygous for the G allele. This might be mediated through farm milk-induced upregulated CD14 gene expression.

CLINICAL IMPLICATIONS

Our results support the hypothesis that the inverse association between farm milk consumption and allergic diseases is mediated by CD14-activated innate immune mechanisms.

Genetics of the innate immune response in inflammatory bowel disease

The discovery of nucleotide-binding oligomerization domain 2/caspase recruitment domain-containing protein 15 (NOD2/CARD15) as the first susceptibility gene in Crohn's disease (CD) has shifted the focus of research into the pathogenesis of inflammatory bowel disease (IBD) firmly to the innate immune response and the integrity of the epithelial barrier. The subsequent implication in IBD of variant alleles of OCTN, DLG5, MDR1, and TLRs has provided further support for a new, more complex model of innate immunity function in the gastrointestinal tract. In this review, we examine the recent advances in our understanding of the influence of genetics of the innate immune response on IBD. We will focus on germline variation of genes encoding pathogen-recognition receptors, proteins involved in epithelial homeostasis and secreted antimicrobial proteins.

Breast milk fatty acids may link innate and adaptive immune regulation: analysis of soluble CD14, prostaglandin E2, and fatty acids

In addition to its role in sensing intraluminal microbial antigens, soluble (s)CD14 may regulate immune responses by its lesser known function as a lipid carrier with possible influences in the production of fatty acid-derived eicosanoids. We investigated the interrelations of fatty acids, prostaglandin E2 (PGE2), and sCD14 and their role in infant atopic eczema during the first year of life. Serum and breast milk samples from mothers and serum samples from their infants were collected at infant's age 3 mo and analyzed for sCD14 and PGE2 concentrations and for fatty acid compositions. The main correlation of sCD14 was with arachidonic acid (20:4n-6) (AA). Dihomo-gamma-linolenic acid (20:3n-6) (DHGLA) and the ratio of n-6 to n-3 fatty acids correlated positively and docosahexaenoic acid (22:6n-3) (DHA) and sum of n-3 fatty acid negatively with PGE2 in mother's serum and linoleic acid (LA) negatively with PGE2 in breast milk. Soluble CD14 tended to be higher and LA, total polyunsaturated fatty acid (PUFA), and sum of n-6 fatty acids were lower in breast milk received by infants with atopic eczema compared with those without. These results suggest that fatty acids contribute to the regulation of innate and adaptive immune responses and link intraluminal exposures, mother's diet, and microbes.

Genetic regulation of mouse glycosylphosphatidylinositol-phospholipase D

Glycosylphosphatidylinositol phospholipase D (GPI-PLD) has been proposed to be responsible for cleaving membrane-associated glycosylphosphatidyl inositol (GPI) molecules to generate inositol phosphoglycan (IPGs), which have growth factor-mimetic properties. We have cloned the mouse liver GPI-PLD cDNA, which has a sequence that differs from that previously isolated from a mouse glucagonoma cell library. Using a highly specific and very sensitive RNase protection assay, we found that the GPI-PLD expressed in adult/post-natal brain, antrum and insulin-producing cells is identical to that isolated from liver. The expression of mouse GPI-PLD in liver shows a complex genetic regulation with a mouse strain-specific variation. In addition, GPI-PLD mRNA levels were higher in 4-week old animals compared to older animals, and the GPI-PLD mRNA levels increased in mice that developed insulin dependent type 1 diabetes spontaneously. This suggests that the expression of liver GPI-PLD in mice is highly regulated.

Crystal structure of CD14 and its implications for lipopolysaccharide signaling

Lipopolysaccharide, the endotoxin of Gram-negative bacteria, induces extensive immune responses that can lead to fatal septic shock syndrome. The core receptors recognizing lipopolysaccharide are CD14, TLR4, and MD-2. CD14 binds to lipopolysaccharide and presents it to the TLR4/MD-2 complex, which initiates intracellular signaling. In addition to lipopolysaccharide, CD14 is capable of recognizing a few other microbial and cellular products. Here, we present the first crystal structure of CD14 to 2.5 angstroms resolution. A large hydrophobic pocket was found on the NH2-terminal side of the horseshoe-like structure. Previously identified regions involved in lipopolysaccharide binding map to the rim and bottom of the pocket indicating that the pocket is the main component of the lipopolysaccharide-binding site. Mutations that interfere with lipopolysaccharide signaling but not with lipopolysaccharide binding are also clustered in a separate area near the pocket. Ligand diversity of CD14 could be explained by the generous size of the pocket, the considerable flexibility of the rim of the pocket, and the multiplicity of grooves available for ligand binding.

Treponemal phospholipids inhibit innate immune responses induced by pathogen-associated molecular patterns

Host innate immune responses to microbial components, known as pathogen-associated molecular patterns (PAMPs), are regulated and modified by cellular receptors and serum proteins, including Toll-like receptors (TLRs), CD14, and LPS-binding protein (LBP). We demonstrated that a treponemal membrane lipid inhibited PAMPs-induced immune responses. The chemical structure of the lipid was elucidated as a phosphatidylglycerol (PG) derivative, which is scarce in most mammalian tissues, but relatively abundant in treponemal membrane lipids. Natural and synthetic PG counterparts as well as related natural anionic phospholipids, phosphatidylinositol, phosphatidylserine, and cardiolipin, also demonstrated an inhibitory effect. Further, we noted that PG inhibited PAMPs-induced immune responses by blocking the binding of PAMPs with LBP and CD14. In addition, PG decreased proinflammatory cytokine production in serum of LPS-injected mice and depressed abscess formation in mice infected with treponemes. These results suggest that treponemal phospholipid interfere the function of LBP/CD14 and act as a modulator of innate immune responses.

Oxidized low density lipoprotein and innate immune receptors

PURPOSE OF REVIEW

Atherosclerosis is now recognized as a chronic inflammatory disease. This review discusses recent literature reporting that innate immune receptors bind oxidatively modified LDL and its many oxidized moieties and consequently modulate the atherogenic process. These innate pattern recognition receptors are known to play a central role in pro-inflammatory responses to bacteria by binding pathogen-associated molecular patterns. It is hypothesized that oxidized LDL exposes similar molecular patterns recognized by receptors of innate immunity.

RECENT FINDINGS

Minimally modified LDL and its oxidized phospholipids have been found to bind to CD14 or activate Toll-like receptors on macrophages. In turn, various biological activities have been induced, including the stimulation of cytoskeletal rearrangements that alter phagocytic activity and the stimulation of cytokine secretion, such as IL-8. These findings link modified LDL with innate pattern recognition receptors, such as those involved in the lipopolysaccharide signaling pathway. Human epidemiological studies support the involvement of CD14 and TLR4 in cardiovascular diseases. Oxidized LDL has also been demonstrated to bind to C-reactive protein, an opsonic molecule activating classic complement pathway and Fcgamma receptor endocytosis. These data suggest that C-reactive protein may not only be a strong predictor of clinical disease, but may also play a role in atherogenesis. Recent data on other innate immune receptors are discussed in the context of their potential interactions with oxidized LDL and atherogenesis.

SUMMARY

Recent findings suggest that oxidized forms of LDL interact with innate immune receptors. Further studies are needed to identify the role of these interactions in inflammation and atherosclerosis.

CD14-dependent clearance of apoptotic cells by human macrophages: the role of phosphatidylserine

Apoptotic-cell clearance is dependent on several macrophage surface molecules, including CD14. Phosphatidylserine (PS) becomes externalised during apoptosis and participates in the clearance process through its ability to bind to a novel receptor, PS-R. CD14 has the proven ability to bind phospholipids and may function as an alternative receptor for the externalised PS of apoptotic cells. Here we demonstrate that CD14 does not function preferentially as a PS receptor in apoptotic-cell clearance. Compared with phosphatidylcholine and phosphatidylethanolamine, PS was the least active phospholipid binding to human monocyte-derived macrophages and showed no specificity for soluble or membrane-anchored CD14. Significantly, PS-containing liposomes failed to inhibit CD14-dependent uptake of apoptotic cells by macrophages. PS exposure was, however, found to be insufficient for either CD14-dependent or CD14-independent apoptotic-cell uptake by phagocytes. The additional features that enable apoptotic-cell clearance are derived from mechanisms that can be divorced temporally from those responsible for the morphological features of apoptosis.

Minimally modified LDL binds to CD14, induces macrophage spreading via TLR4/MD-2, and inhibits phagocytosis of apoptotic cells

Minimally modified low density lipoprotein (mmLDL) is a pro-inflammatory and pro-atherogenic lipoprotein that, unlike profoundly oxidized LDL (OxLDL), is not recognized by scavenger receptors and thus does not have enhanced uptake by macrophages. However, here we demonstrate that mmLDL (as well as OxLDL) induces actin polymerization and spreading of macrophages, which results in such pro-atherogenic consequences as inhibition of phagocytosis of apoptotic cells but enhancement of OxLDL uptake. We also demonstrate for the first time that the lipopolysaccharide receptor, CD14, and toll-like receptor-4/MD-2 are involved in these mmLDL effects. Macrophages of the J774 cell line exhibited higher mmLDL binding and F-actin response than its CD14-deficient mutant, LR-9 cells. Similarly, Chinese hamster ovary cells transfected with human CD14 specifically bound mmLDL and responded with higher F-actin compared with control cells. Macrophages from C3H/HeJ mice, which have a point mutation in the Tlr4 gene, responded with lower F-actin to mmLDL and did not spread as well as macrophages from control animals. A significantly higher F-actin response was also observed in Chinese hamster ovary cells transfected with human toll-like receptor-4/MD-2 but not with TLR4 alone or TLR2. Thus, in addition to inhibition of phagocytosis, the recognition of mmLDL by macrophage lipopolysaccharide receptors results in convergence of cellular immune responses to products of microorganisms and to oxidation-specific self-antigens, which could both influence macrophage function and atherogenesis.

CD14 signalling in lipid rafts: new ligands and co-receptors

PURPOSE OF REVIEW

Lipid rafts on monocytes/macrophages provide a dynamic microenvironment for an integrated lipopolysaccharide receptor (CD14)-dependent clustering of a set of receptors involved in innate immunity and clearance of atherogenic lipoproteins. The purpose of this review is to summarize the recent advances in our understanding of CD14-dependent receptor clustering and its relevance in atherogenesis.

RECENT FINDINGS

Upon binding of various ligands, CD14 as a multiligand pattern recognition receptor induces specific coassembly of additional receptors present on circulating monocytes.

SUMMARY

The composition of the receptor cluster and thus the associated signalling pathways defines a ligand specific cellular response, linking endogenous and exogenous host defense to a common recognition platform in rafts.

Protective role of phospholipid oxidation products in endotoxin-induced tissue damage

Lipopolysaccharide (LPS), an outer-membrane component of Gram-negative bacteria, interacts with LPS-binding protein and CD14, which present LPS to toll-like receptor 4 (refs 1, 2), which activates inflammatory gene expression through nuclear factor kappa B (NF kappa B) and mitogen-activated protein-kinase signalling. Antibacterial defence involves activation of neutrophils that generate reactive oxygen species capable of killing bacteria; therefore host lipid peroxidation occurs, initiated by enzymes such as NADPH oxidase and myeloperoxidase. Oxidized phospholipids are pro-inflammatory agonists promoting chronic inflammation in atherosclerosis; however, recent data suggest that they can inhibit expression of inflammatory adhesion molecules. Here we show that oxidized phospholipids inhibit LPS-induced but not tumour-necrosis factor-alpha-induced or interleukin-1 beta-induced NF kappa B-mediated upregulation of inflammatory genes, by blocking the interaction of LPS with LPS-binding protein and CD14. Moreover, in LPS-injected mice, oxidized phospholipids inhibited inflammation and protected mice from lethal endotoxin shock. Thus, in severe Gram-negative bacterial infection, endogenously formed oxidized phospholipids may function as a negative feedback to blunt innate immune responses. Furthermore, identified chemical structures capable of inhibiting the effects of endotoxins such as LPS could be used for the development of new drugs for treatment of sepsis.

Effect of CD14 promoter polymorphism and H. pylori infection and its clinical outcomes on circulating CD14

CD14 is a pattern recognition receptor on the membranes of monocytes and macrophages for several microbial products, of which lipopolysaccharide (LPS) is the best known. A shed form of CD14 is present in serum. As the CD14 gene promoter polymorphism -159C/T and some bacterial infections may affect the sCD14 levels, we compared the impact of both the CD14 promoter polymorphism and Helicobacter pylori infection on serum sCD14 levels in 201 dyspeptic patients (group 1) who had undergone gastroscopy, and 127 staff members (group 2) with no endoscopy. sCD14 was measured from the sera by a commercial enzyme immunoassay (EIA), and CD14 genotyping was carried out with PCR. Helicobacter pylori infection was detected by serology and/or culture or PCR. sCD14 levels were elevated in the subjects carrying the T allele (CT or TT genotype) in both groups when compared with subjects with the CC genotype. Overall, H. pylori-positive subjects tended to have higher sCD14 levels compared with H. pylori-negative subjects. In group 1 consisting of dyspeptic patients, those with gastric ulcer, gastric erosion or duodenal ulcer had significantly elevated levels of sCD14 compared with the patients with normal endoscopic findings or macroscopic gastritis. The recent use of NSAIDs was also associated with enhanced sCD14. Thus, we were able to show several factors, one genetic and the other environmental (H. pylori infection and mucosal lesion), to have an impact on sCD14.

Reduced soluble CD14 levels in amniotic fluid and breast milk are associated with the subsequent development of atopy, eczema, or both

BACKGROUND

Exposure to various microbial products in early life reduces the risk of atopy. Such exposure induces downregulation of T(H)2 allergy-biased responses by means of pattern recognition molecules, such as CD14, an LPS receptor.

OBJECTIVE

We sought to determine whether infant and maternal levels of soluble CD14 (sCD14) are associated with the atopic outcomes of infants.

METHODS

Levels of sCD14 in plasma, amniotic fluid, and breast milk were measured with a specific ELISA in different cohorts. Expression of toll-like receptors in the fetal gut was examined by using RT-PCR.

RESULTS

Soluble CD14 levels increased during fetal development and postnatally, attaining adult levels by around 4 months of age, with an overshoot of adult levels from 6 months of age. There was no difference in plasma sCD14 levels at birth of children with a high compared with those with a low risk of development of atopy. Amniotic fluid sCD14 levels at midgestation (16-17 weeks) were significantly lower when the child was subsequently atopic (P <.05). Soluble CD14 levels in breast milk collected 3 months postpartum were significantly lower in children with eczema at 6 months of age, irrespective of whether they were atopic (P =.003). Transcripts for toll-like receptor 4, which would enable transmembrane signaling for LPS/sCD14 complexes, were expressed within fetal gut and skin.

CONCLUSION

Exposure to reduced levels of sCD14 in the fetal and neonatal gastrointestinal tract is associated with the development of atopy, eczema, or both. Thus the exogenous supply of sCD14 might influence immunologic reactivity both locally and systemically in early life and thereby influence disease outcome.

Fetal immune responsiveness and routes of allergic sensitization

There is much interest in the role of early-life events in the subsequent development of atopy and/or atopic disease. Despite the ongoing debate about the intrauterine exposure of the fetus to environmental allergens and the establishment of T-cell memory, it is clear that the immunological response of the neonate at risk of atopy is more immature than that of the neonate likely to be non-atopic. The reasons for this remain unknown, but might reflect maternally transmitted signals that adapt the neonatal immune response. An inadvertent consequence of this might be an inappropriate host response to environmental signals such as those from microbial products during early post-natal life that result in an inability to dampen neonatal T helper 2-skewed responses. The developing gastrointestinal tract and theexogenous factors that impact on this, such as microbial flora and breast milk, should therefore be a focus of investigation.

Plasma CD14 decreases monocyte responses to LPS by transferring cell-bound LPS to plasma lipoproteins

CD14, a myeloid cell-surface receptor and soluble plasma protein, binds LPS and other microbial molecules and initiates the innate immune response to bacterial invasion. The blood concentration of soluble CD14 (sCD14) increases during the systemic response to infection. Although high sCD14 blood levels have correlated with increased risk of dying from severe sepsis, sCD14 can diminish cell responses to LPS. We show here that in human serum, sCD14 increases the rate at which cell-bound LPS is released from the monocyte surface and binds to plasma lipoproteins. This enhanced rate of LPS efflux is associated with a significant reduction in the ability of monocytes to produce cytokines in response to LPS. Serum from septic patients reduced the LPS-monocyte interaction by as much as tenfold, and depletion of sCD14 from the serum restored LPS-monocyte binding and release kinetics to near normal levels. In serum from septic patients, monocyte-bound LPS also moved more rapidly into lipoproteins, which completely neutralized the biologic activity of the LPS that bound to them. In human plasma, sCD14 thus diminishes monocyte responses to LPS by transferring cell-bound LPS to lipoproteins. Stress-related increases in plasma sCD14 levels may help prevent inflammatory responses within the blood.