Variation in the TLR10/TLR1/TLR6 locus is the major genetic determinant of interindividual difference in TLR1/2-mediated responses.

Toll-like receptor (TLR)-mediated innate immune responses are important in early host defense. Using a candidate gene approach, we previously identified genetic variation within TLR1 that is associated with hyper-responsiveness to a TLR1/2 agonist in vitro and with death and organ dysfunction in patients with sepsis. Here we report a genome-wide association study (GWAS) designed to identify genetic loci controlling whole blood cytokine responses to the TLR1/2 lipopeptide agonist, Pam(3)CSK(4) (N-palmitoyl-S-dipalmitoylglyceryl Cys-Ser-(Lys)(4)) ex vivo. We identified a very strong association (P<1 × 10(-27)) between genetic variation within the TLR10/1/6 locus on chromosome 4, and Pam(3)CSK(4)-induced cytokine responses. This was the predominant association explaining over 35% of the population variance for this phenotype. Notably, strong associations were observed within TLR10, suggesting that genetic variation in TLR10 may influence bacterial lipoprotein-induced responses. These findings establish the TLR10/1/6 locus as the dominant common genetic factor controlling interindividual variability in Pam(3)CSK(4)-induced whole blood responses in the healthy population.

Identification and characterization of a loss-of-function human MPYS variant.

MPYS, also known as STING and MITA, is an interferon (IFN)ß stimulator essential for host defense against RNA, DNA viruses and intracellular bacteria. MPYS also facilitates the adjuvant activity of DNA vaccines. Here, we report identification of a distinct human MPYS haplotype that contains three non-synonymous single nucleotide polymorphisms (SNPs), R71H-G230A-R293Q (thus, named the HAQ haplotype). We estimate, in two cohorts (1,074 individuals), that ∼3% of Americans are homozygous for this HAQ haplotype. HAQ MPYS exhibits a > 90% loss in the ability to stimulate IFNß production. Furthermore, fibroblasts and macrophage cells expressing HAQ are defective in Listeria monocytogenes infection-induced IFNß production. Lastly, we find that the loss of IFNß activity is due primarily to the R71H and R293Q SNPs in HAQ. We hypothesize that individuals carrying HAQ may exhibit heightened susceptibility to viral infection and respond poorly to DNA vaccines.

Genetic insights into sepsis: what have we learned and how will it help?

Sepsis and septic shock, are complex disorders that are a major cause of mortality in the intensive care unit. In spite of major advances in our understanding of the pathophysiology of sepsis, accurate prediction of susceptibility to sepsis, multi-organ dysfunction, and death, even in the setting of a seemingly similar burden of infection, continues to challenge critical care clinicians. Evidence from family-based studies and recent gene-association studies suggest that a significant portion of the apparent variability in susceptibility is due to genetic factors. Common sequence variations in genes coding for innate immune effectors, inflammatory mediators, and modulators of coagulation have received particular attention. This review will summarize and integrate the results of studies testing for associations between sequence variations in genes from these functional classes and susceptibility to sepsis and related clinical outcomes. The important insights on sepsis pathophysiology provided by these studies will be discussed along with the relevance of these findings to the design of future diagnostic approaches and therapeutic trials.

Lipopolysaccharide-binding protein and soluble CD14 transfer lipopolysaccharide to phospholipid bilayers: preferential interaction with particular classes of lipid.

LPS-binding protein (LBP) catalyzes the movement of LPS (endotoxin) from micelles directly to high density lipoprotein (HDL) particles, and this activity results in neutralization of the biologic activities of LPS. LBP also catalyzes the transfer of LPS to HDL by a two-step mechanism in which LPS is transferred to soluble CD14 (sCD14), and then from LPS-sCD14 complexes to HDL. In this work, we show that the phospholipid component of HDL, phosphatidylcholine (PC), is both necessary and sufficient for LBP-catalyzed neutralization of LPS through either mechanism. Our observation that LBP and sCD14 can transport LPS into phospholipid bilayers suggests that LBP and membrane CD14 may transport LPS into the phospholipid bilayer of cells such as monocytes and neutrophils. Studies with a variety of purified phospholipids showed that: 1) PC, phosphatidylserine, phosphatidylinositol, and sphingomyelin can neutralize LPS, while phosphatidylethanolamine, ceramide, and lactosylceramide cannot. 2) PC containing saturated long chain acyl groups (distearoyl-PC) does not neutralize LPS, but PC containing unsaturated long chain acyl groups (dioleoyl-PC) rapidly neutralizes LPS. 3) Inclusion of sCD14 is absolutely necessary to observe LBP-dependent neutralization of LPS by sphingomyelin, globoside, and phosphatidylserine. 4) Inclusion of sCD14 enhances movement to longer chain PC vesicles, but slows movement to certain short chain vesicles. These findings indicate that LBP and sCD14 will rapidly transfer LPS to certain membranes based on the kinetics of the movement of LPS into these membranes. This discrimination may target LPS to certain classes of lipoprotein, certain cell types, or even certain lipid domains at the cell surface.

Targeted deletion of the lipopolysaccharide (LPS)-binding protein gene leads to profound suppression of LPS responses ex vivo, whereas in vivo responses remain intact.

Gram-negative bacterial lipopolysaccharide (LPS) stimulates phagocytic leukocytes by interacting with the cell surface protein CD14. Cellular responses to LPS are markedly potentiated by the LPS-binding protein (LBP), a lipid-transfer protein that binds LPS aggregates and transfers LPS monomers to CD14. LBP also transfers LPS to lipoproteins, thereby promoting the neutralization of LPS. LBP present in normal plasma has been shown to enhance the LPS responsiveness of cells in vitro. The role of LBP in promoting LPS responsiveness in vivo was tested in LBP-deficient mice produced by gene targeting in embryonic stem cells. Whole blood from LBP-deficient animals was 1,000-fold less responsive to LPS as assessed by the release of tumor necrosis factor (TNF)-alpha. Blood from gene-targeted mice was devoid of immunoreactive LBP, essentially incapable of transferring LPS to CD14 in vitro, and failed to support cellular responses to LPS. These activities were restored by the addition of exogenous recombinant murine LBP to the plasma. Despite these striking in vitro findings, no significant differences in TNF-alpha levels were observed in plasma from wild-type and LBP-deficient mice injected with LPS. These data suggest the presence of an LBP-independent mechanism for responding to LPS. These LBP knockout mice may provide a tool for discovering the nature of the presumed second mechanism for transferring LPS to responsive cells.

Soluble CD14 acts as a shuttle in the neutralization of lipopolysaccharide (LPS) by LPS-binding protein and reconstituted high density lipoprotein.

We have recently shown that lipopolysaccharide (LPS)-binding protein (LBP) is a lipid transfer protein that catalyzes two distinct reactions: movement of bacterial LPS (endotoxin) from LPS micelles to soluble CD14 (sCD14) and movement of LPS from micelles to reconstituted high density lipoprotein (R-HDL) particles. Here we show that LBP facilitates a third lipid transfer reaction: movement of LPS from LPS-sCD14 complexes to R-HDL particles. This action of LBP is catalytic, with one molecule of LBP enabling the movement of multiple LPS molecules into R-HDL. LBP-catalyzed movement of LPS from LPS-sCD14 complexes to R-HDL neutralizes the capacity of LPS to stimulate polymorphonuclear leukocytes. Our findings show that LPS may be transferred to R-HDL either by the direct action of LBP or by a two-step reaction in which LPS is first transferred to sCD14 and subsequently to R-HDL. We have observed that the two-step pathway of LPS transfer to R-HDL is strongly favored over direct transfer. Neutralization of LPS by LBP and R-HDL was accelerated more than 30-fold by addition of sCD14. Several observations suggest that sCD14 accelerates this reaction by serving as a shuttle for LPS: addition of LBP and sCD14 to LPS micelles resulted in LPS-sCD14 complexes that could diffuse through a 100-kD cutoff filter; LPS-sCD14 complexes appeared transiently during movement of LPS to R-HDL facilitated by purified LBP; and sCD14 could facilitate transfer of LPS to R-HDL without becoming part of the final LPS-R-HDL complex. Complexes of LPS and sCD14 were formed transiently when LPS was incubated in plasma, suggesting that these complexes may play a role as intermediates in the neutralization of LPS under physiological conditions. These findings detail a new activity for sCD14 and suggest a novel mechanism for lipid transfer by LBP.

Lipopolysaccharide (LPS) binding protein catalyzes binding of LPS to lipoproteins.

Lipoproteins isolated from normal human plasma can bind and neutralize bacterial lipopolysaccharide (LPS) and may represent an important mechanism in host defense against gram-negative septic shock. We sought to define the components of high-density lipoproteins (HDL) that are required for neutralization of LPS. To accomplish this we have studied the functional neutralization of LPS by native and reconstituted HDL using a rapid assay that measures the CD14-dependent activation of leukocyte integrins on human neutrophils. Reconstituted HDL particles (R-HDL), prepared from purified apolipoprotein A-I (apoA-I) combined with phospholipid and free cholesterol, were not sufficient to neutralize the biologic activity of LPS. However, addition of recombinant LPS binding protein (LBP), a protein known to transfer LPS to CD14 and enhance responses of cells to LPS, enabled prompt binding and neutralization of LPS by R-HDL. Thus, LBP appears capable of transferring LPS not only to CD14 but also to lipoprotein particles. These results suggest that in addition to its ability to transfer LPS to CD14, LBP may also transfer LPS to lipoproteins. Additional studies demonstrated that LBP copurifies with native apoA-I containing lipoprotein (Lp(A-I)) particles. Since LBP appears to be physically associated with lipoproteins in plasma, it is positioned to play an important role in the neutralization of LPS.

Lipopolysaccharide (LPS)-binding protein is carried on lipoproteins and acts as a cofactor in the neutralization of LPS.

Lipoproteins isolated from normal human plasma can bind and neutralize bacterial lipopolysaccharide (LPS) and may represent an important mechanism in host defense against gram-negative septic shock. Recent studies have shown that experimentally elevating the levels of circulating high-density lipoproteins (HDL) provides protection against death in animal models of endotoxic shock. We sought to define the components of HDL that are required for neutralization of LPS. To accomplish this we have studied the functional neutralization of LPS by native and reconstituted HDL using a rapid assay that measures the CD14-dependent activation of leukocyte integrins on human neutrophils. We report here that reconstituted HDL particles (R-HDL), prepared from purified apolipoprotein A-I (apoA-I) combined with phospholipid and free cholesterol, are not sufficient to neutralize the biologic activity of LPS. However, addition of recombinant LPS binding protein (LBP), a protein known to transfer LPS to CD14 and enhance responses of cells to LPS, enabled prompt binding and neutralization of LPS by R-HDL. Thus, LBP appears capable of transferring LPS not only to CD14 but also to lipoprotein particles. In contrast with R-HDL, apoA-I containing lipoproteins (LpA-I) isolated from plasma by selected affinity immunosorption (SAIS) on an anti-apoA-I column, neutralized LPS without addition of exogenous LBP. Several lines of evidence demonstrated that LBP is a constituent of LpA-I in plasma. Passage of plasma over an anti-apoA-I column removed more than 99% of the LBP detectable by ELISA, whereas 31% of the LBP was recovered by elution of the column. Similarly, the ability of plasma to enable activation of neutrophils by LPS (LBP/Septin activity) was depleted and recovered by the same process. Furthermore, an immobilized anti-LBP monoclonal antibody coprecipitated apoA-I. The results described here suggest that in addition to its ability to transfer LPS to CD14, LBP may also transfer LPS to lipoproteins. Since LBP appears to be physically associated with lipoproteins in plasma, it is positioned to play an important role in the neutralization of LPS.

Afamin is a new member of the albumin, alpha-fetoprotein, and vitamin D-binding protein gene family.

A novel human serum protein with a molecular mass of 87,000 daltons was purified to homogeneity and subjected to amino acid sequence analyses. These sequences were used to design oligonucleotide primers and to isolate a full-length cDNA. The amino acid sequence encoded by the cDNA shares strong similarity to albumin family members and shares the characteristic pattern of Cys residues observed in this family. In addition, the gene maps to chromosome 4 as do other members of the albumin gene family. Based upon these observations, we conclude that the 87,000-dalton protein, which we designate afamin (AFM), is the fourth member of the albumin family of proteins. Afamin cDNA was stably transfected into Chinese hamster ovary cells and recombinant protein (rAFM) was purified from conditioned medium. Both rAFM and AFM purified from human serum react with a polyclonal antibody that was raised against a synthetic peptide derived from the deduced amino acid sequence of AFM.

Lipopolysaccharide (LPS)-binding protein accelerates the binding of LPS to CD14.

CD14 is a 55-kD protein found as a glycosylphosphatidylinositol (GPI)-anchored protein on the surface of monocytes, macrophages, and polymorphonuclear leukocytes, and as a soluble protein in the blood. Both forms of CD14 participate in the serum-dependent responses of cells to bacterial lipopolysaccharide (LPS). While CD14 has been described as a receptor for complexes of LPS with LPS-binding protein (LBP), there has been no direct evidence showing whether a ternary complex of LPS, LBP, and CD14 is formed, or whether CD14 binds LPS directly. Using nondenaturing polyacrylamide gel electrophoresis (native PAGE), we show that recombinant soluble CD14 (rsCD14) binds LPS in the absence of LBP or other proteins. Binding of LPS to CD14 is stable and of low stoichiometry (one or two molecules of LPS per rsCD14). Recombinant LBP (rLBP) does not form detectable ternary complexes with rsCD14 and LPS, but it does accelerate the binding of LPS to rsCD14. rLBP facilitates the interaction of LPS with rsCD14 at substoichiometric concentrations, suggesting that LBP functions catalytically, as a lipid transfer protein. Complexes of LPS and rsCD14 formed in the absence of LBP or other serum proteins strongly stimulate integrin function on PMN and expression of E-selectin on endothelial cells, demonstrating that LBP is not necessary for CD14-dependent stimulation of cells. These results suggest that CD14 acts as a soluble and cell surface receptor for LPS, and that LBP may function primarily to accelerate the binding of LPS to CD14.