Innate Immunity in Heart, Lung and Blood Disease

See OMIM.

Responses of inflammatory cells to subpicomolar concentrations of bacterial pathogens require the cooperation between CD14 and LPS-binding protein (LBP), a lipid transfer protein that recognizes lipid A, the biologically active moiety of endotoxin. LBP facilitates the binding of LPS to soluble CD14 or membrane CD14 and plays a critical role in the CD14-dependent clearance of bacteria from the circulation. However, LBP does not seem to participate directly in initiating signal transduction. The bactericidal/permeability-increasing protein (BPI) is a 456-residue cationic protein produced only by precursors of neutrophils and is stored in the primary granules of these cells. The potent cytotoxicity of BPI is limited to gram-negative bacteria, reflecting the high affinity (<10 nM) of BPI for bacterial LPS. The biological effects of isolated BPI are linked to complex formation with LPS. Binding of BPI to live bacteria via LPS causes immediate growth arrest. Actual killing coincides with later damage to the inner membrane. Complex formation of BPI with cell-associated or cell-free LPS inhibits all LPS-induced host cell responses. BPI and LBP are members of a family of structurally and functionally related proteins. The members of this family, including cholesteryl ester transfer protein and phospholipid transfer protein, are highly conserved at the mRNA and protein levels, suggesting biological importance of the transfer lipid-like structures in aqueous environments, which has been identified as their main function. The BPI and LBP genes were both localized to human chromosome 20, and their genomic structures are almost identical.

Common nucleotide exchanges with a frequency of more than 1%, have been identified in both BPI and LBP. The respective genotype frequencies were analyzed in a group of patients with sepsis (n=204), compared with control probands (n=250). The association to unfavorable outcome was also assessed. Common polymorphisms in the gene for LBP in combination with male gender were found to be associated with an increased risk for the development of sepsis and, furthermore, may be linked to an unfavorable outcome. These data support the important immunomodulatory role of LBP in Gram-negative sepsis and suggest that genetic testing may be helpful for the identification of patients with an unfavorable response to Gram-negative infection. By contrast, the same study did not show any association of the polymorphisms of BPI that were analyzed with either the incidence or outcome of sepsis.

See OMIM.

The innate immune system regulates the immediate response to microbial pathogens and is initiated by recognition of specific pathogen components by receptors in host immune cells. NOD1 and NOD2 seem to function as intracellular receptors for LPS. NOD2 has structural homology with both the apoptosis regulators Apaf-1/Ced-4 and a class of plant disease resistant gene products. Like the latter gene products, NOD2 comprises an amino-terminal effector domain, a nucleotide-binding domain and leucine-rich repeats. NOD2 is a cytosolic protein whose expression is restricted to monocytes. The gene has been mapped to chromosome 16q12. It has recently been shown that truncation of the tenth leucin-rich repeat of NOD2 is associated with Crohn's disease. Functional analyses indicate that the disease-associated NOD2 variant is significantly less active than the wild-type protein in conferring responsiveness to bacterial LPS. Other leucine-rich repeat variants have been found to be associated with Chron's disease.

CARD4 (aka NOD1) is a member of a growing family of intracellular proteins with structural homology to apoptosis regulators Apaf-1/Ced-4 and a class of plant disease-resistant gene products. Bacterial LPS, but not other pathogen components tested, has been found to induced TLR4- and MyD88-independent NF-kB activation in human embryonic kidney 293T cells expressing trace amounts of Nod1. Nod2, another Nod family member, also conferred responsiveness to bacterial components but with a response pattern different from that observed with Nod1. The leucine-rich repeats of Nod1 and Nod2 were required for LPS-induced NF-kB activation. An LPS-binding activity could be specifically coimmunopurified with Nod1 from cytosolic extracts. These observations suggest that Nod1 and Nod2 are mammalian gene products that may function as cytosolic receptors for pathogen components derived from invading bacteria.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

CD14 was the first pattern recognition receptor to be identified. CD14 is expressed on, and secreted by, myeloid cells. CD14-negative cells, such as epithelial and endothelial cells, become responsive to bacterial pathogens in the presence of soluble CD14 (sCD14), a protein present in the serum in microgram amounts and secreted by monocytes and the liver. Membrane-bound and sCD14 bind a variety of bacterial products, e.g., LPS from Gram-negative bacteria, lipoteichoic acids from Gram-positive bacteria, mycobacterial glycolipids, and mannans from yeast s. Responses of inflammatory cells to subpicomolar concentrations of bacterial ligands require LPS-binding protein (LBP), a lipid transfer protein that recognizes the lipid A moiety of LPS and facilitates the binding of LPS to sCD14 or membrane CD14. At the molecular level, CD14 acts by transferring LPS and other bacterial ligands from circulating LPS-binding protein to the Toll-like receptor 4/MD-2 signaling complex. Engagement of this complex results in the activation of innate host defense mechanisms, such as release of inflammatory cytokines, and in upregulation of costimulatory molecules, thus providing cues that are essential to directing adaptive immune responses. A single nucleotide polymorphism in the CD14 promoter that is accompanied by increased levels of sCD14 has been found to be associated with decreased total serum IgE levels. Functional genomics studies have shown that the same polymorphism enhances CD14 transcriptional activity by decreasing the affinity of the promoter for Sp proteins..

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

ß-catenin a transcriptional coactivator, is part of the intracellular signal transduction pathways triggered by LPS in human macrophages, and appears to be involved in both LPS-induction of gene transcription and LPS-dependent regulation of vascular permeability. Exposure of alveolar macrophages to bacterial LPS results in the activation of a number of signal transduction pathways. An early event after the alveolar macrophage comes in contact with LPS is activation of PI 3-kinase and phosphorylation of Akt. In turn, Akt activation results in the phosphorylation-dependent inactivation of glycogen synthase kinase (GSK-3). Inactivation of GSK-3 reduces the ubiquitination of ß-catenin, resulting in nuclear accumulation and expression of genes that require nuclear ß-catenin for their activation. On the other hand, LPS induces actin reorganization, increased paracellular permeability, and endothelial cell detachment from the underlying extracellular matrix. Components of cell-cell (γ- and ß-catenin) and cell-matrix adherens junctions are cleaved by caspases activated during apoptosis. Cleavage of focal adhesion kinase leads to its dissociation from the focal adhesion-associated signaling protein, paxillin, resulting in reduced paxillin tyrosine phosphorylation. Therefore, endotoxin-induced disruption of endothelial monolayer integrity and survival signaling events is mediated, in part, through caspase cleavage of adherens junction proteins.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

IL10 is a regulatory cytokine produced by antigen-presenting cells stimulated by bacterial pathogens, as well as human T cells of both Th1 and Th2 phenotype. IL-10 is the only cytokine that is known to exert a differential effect on IL-4-dependent isotype switching to IgE and IgG4. Indeed, IL-10 has been shown to suppress IgE synthesis while enhancing IgG4 expression. Peripheral tolerance has been shown to be critically mediated by IL-10. Single nucleotide polymorphisms in the IL-10 gene have been associated with dysregulated IgE synthesis.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

Responses of inflammatory cells to subpicomolar concentrations of bacterial pathogens require the cooperation between CD14 and LBP, a lipid transfer protein that recognizes lipid A, the biologically active moiety of endotoxin. LBP facilitates the binding of LPS to soluble CD14 or membrane CD14 and plays a critical role in the CD14-dependent clearance of bacteria from the circulation. However, LBP does not seem to participate directly in initiating signal transduction. BPI is a 456-residue cationic protein produced only by precursors of neutrophils and is stored in the primary granules of these cells. The potent cytotoxicity of BPI is limited to gram-negative bacteria, reflecting the high affinity (<10 nM) of BPI for bacterial LPS. The biological effects of isolated BPI are linked to complex formation with LPS. Binding of BPI to live bacteria via LPS causes immediate growth arrest. Actual killing coincides with later damage to the inner membrane. Complex formation of BPI with cell-associated or cell-free LPS inhibits all LPS-induced host cell responses. BPI and LBP are members of a family of structurally and functionally related proteins. The members of this family, including cholesteryl ester transfer protein and phospholipid transfer protein, are highly conserved at the mRNA and protein levels, suggesting biological importance of the transfer lipid-like structures in aqueous environments, which has been identified as their main function. The BPI and LBP genes were both localized to human chromosome 20, and their genomic structures are almost identical.

Common nucleotide exchanges with a frequency of more than 1%, have been identified in both BPI and LBP. The respective genotype frequencies were analyzed in a group of patients with sepsis (n=204), compared with control probands (n=250). The association to unfavorable outcome was also assessed. Common polymorphisms in the gene for LBP in combination with male gender were found to be associated with an increased risk for the development of sepsis and, furthermore, may be linked to an unfavorable outcome. These data support the important immunomodulatory role of LBP in Gram-negative sepsis and suggest that genetic testing may be helpful for the identification of patients with an unfavorable response to Gram-negative infection. By contrast, the same study did not show any association of the polymorphisms of BPI that were analyzed with either the incidence or outcome of sepsis.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

In mammals, LPS initiates its biological activities through a heteromeric receptor complex containing CD14, TLR4, and at least one other protein, MD-2. LPS binds directly to CD14 and is cross-linked specifically to TLR4 and MD-2 only when co-expressed with CD14. Thus LPS is in close proximity to the three known proteins of its membrane receptor complex, and binds directly to each of the members of the tripartite LPS receptor complex. MD-2 is a required component of the LPS signaling complex and (similar to CD14) can function as a soluble receptor for cells that do not otherwise express it. Molecular genetic analysis of an LPS-nonresponder mutant cell line has recently shown that a point mutation in a conserved region of MD-2 abolishes LPS-induced signaling. Of note, decreased expression of MD-2 and TLR4 correlates with protection of intestinal epithelial cells against dysregulated expression of proinflammatory genes in response to bacterial LPS..

See OMIM.

See OMIM.

See OMIM.

See OMIM.

Because the TLRs [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ] share sequence similarity with the IL-1R family in their cytoplasmic regions, it is not unexpected that downstream events are mediated by common components. MyD88 is an adaptor protein that links the IL-1 (α, β) receptor to IRAK, a serine-threonine kinase that is related to the Pelle kinase of Drosophila. Upon binding of ligand to IL-1R, IRAK is phosphorylated, subsequently dissociated from the receptor complex and associates with tumor necrosis factor receptor activated factor 6 (TRAF6). This process results in the activation of two different pathways that involve the c-Jun NH2-terminal kinase (Jnk) and p38 mitogen-activated protein kinase (MAPK) family and the Rel family transcription factor NF-kB. These pathways are evolutionally conserved. MyD88-/- mice do not respond to IL-1, IL-18, LPS or other microbial cell wall components, such as peptidoglycan and lipopeptides, which shows that this molecule is indispensable for responses to these stimuli. However, studies in MyD88-/- macrophages have suggested differences between TLR2 and TLR4 signaling. In MyD88-/- macrophages, production of inflammatory cytokines such as IL-1, TNF-alpha and IL-6 in response to LPS or mycoplasma lipopeptides is completely impaired. Mycoplasma-dependent activation of NF-kB and MAPK, which is mediated by TLR2, is completely abolished in both TLR2-/- and MyD88-/- macrophages. However, LPS activates NF-kB, Jnk or p38 in MyD88-/- macrophages, although this activation is delayed compared to wild-type macrophages. This suggests that there is a MyD88-independent pathway(s) that mediates NF-kB, Jnk or p38 activation after TLR4 signaling.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

TLR2 plays a pivotal role in the recognition of Gram-positive bacteria and Mycobacteria. The immunostimulatory properties of bacterial lipoproteins for TLR2 are attributed to the presence of a lipoylated N-terminus. Pathogen recognition by TLR2 is strongly enhanced by CD14. TLR2 knockout mice are selectively impaired in their ability to recognize Gram-positive bacteria.

Toll-like receptor 3 is the pattern recognition receptor that recognizes double-stranded RNA (dsRNA). This molecular pattern is associated with viral infection, because it is produced by most viruses at some point during their replication. Activation of TLR3 induces the activation of NF-kB and the production of type I interferons. TLR3-deficient mice showed reduced responses to polyinosine?polycytidylic acid (poly(I:C)), resistance to the lethal effect of poly(I:C) when sensitized with D-galactosamine, and reduced production of inflammatory cytokines. MyD88 is an adaptor protein that is shared by all the known TLRs. When activated by poly(I:C), TLR3 induces cytokine production through a signalling pathway dependent on MyD88. Moreover, poly(I:C) can induce activation of NF-kB and mitogen-activated protein kinases independently of MyD88, and cause dendritic cells to mature.

TLR4 is a critical component of the heteromeric receptor complex that transduces signals delivered by lipopolysaccharide (LPS) of Gram-negative bacteria. CD14, a molecule selectively expressed by monocytes and granulocytes, an d MD-2 are also involved in LPS-mediated signaling. C3H/HeJ mice which bear mutations in the Tlr4 gene exhibit defective LPS signaling, and TLR4 knockout mice are selectively impaired in their ability to recognize Gram-negative bacteria. In humans, TLR4 mutations have been found to be associated with endotoxin hyporesponsiveness. Recognition of viral products has also been proposed to be mediated by TLR4, and studies in animal models point to a major role of TLR4 in the response to respiratory sync ytial virus infection.

TLR5 is the Toll-like molecule that recognizes flagellin from both Gram-positive and Gram-negative bacteria. Activation of the receptor mobilizes NF-kB and stimulates TNF-a production. Murine TLR5 lies within a locus that is associated with susceptibility to Salmonella. Flagellin is critical for bacterial motility and is therefore conserved. Thus it fits well the profile of a relatively invariant PAMP (pathogen-associated molecular pattern) that serves as a ligand for a pattern recognition receptor, TLR5

TLR6 is not as well understood as other members of the TLR family. Studies with TLR6- and TLR2-deficient mice have shown that TLR2 is essential for the response to bacterial lipoproteins triacylated at the N-terminus cysteine residue. By contrast, mycoplasma macrophage-activating lipopeptide-2 kD (MALP-2) is only diacylated and recognized cooperatively by TLR6 and TLR2. Thus TLR6 appears to discriminate between the N-terminal lipoylated structures of MALP-2 and lipopeptides derived from other bacteria.

See OMIM.

See OMIM.

TLR9 was recently identified as the TLR family member responsible for mediating the recognition of bacterial DNA. Of note, TLR9-deficient mice failed to respond to synthetic, unmethylated oligonucleotides containing CpG motif s, but retained responsiveness to both Gram-negative and Gram-positive bacteria. The molecular mechanisms and topology of TLR9-mediated signaling remain incompletely defined.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.

See OMIM.