Structure and Biological Activity of the Short-chain Lipopolysaccharide from Bartonella henselae ATCC 49882T

The facultative intracellular pathogen Bartonella henselae is responsible for a broad range of clinical manifestations, including the formation of vascular tumors as a result of increased proliferation and survival of colonized endothelial cells. This remarkable interaction with endotoxin-sensitive endothelial cells and the apparent lack of septic shock are considered to be due to a reduced endotoxic activity of the B. henselae lipopolysaccharide. Here, we show that B. henselae ATCC 49882(T) produces a deep-rough-type lipopolysaccharide devoid of O-chain and report on its complete structure and Toll-like receptor-dependent biological activity. The major short-chain lipopolysaccharide was studied by chemical analyses, electrospray ionization, and matrix-assisted laser desorption/ionization mass spectrometry, as well as by NMR spectroscopy after alkaline deacylation. The carbohydrate portion of the lipopolysaccharide consists of a branched trisaccharide containing a glucose residue attached to position 5 of an alpha-(2-->4)-linked 3-deoxy-d-manno-oct-2-ulosonic acid disaccharide. Lipid A is a pentaacylated beta-(1'-->6)-linked 2,3-diamino-2,3-dideoxy-glucose disaccharide 1,4'-bisphosphate with two amide-linked residues each of 3-hydroxydodecanoic and 3-hydroxyhexadecanoic acids and one residue of either 25-hydroxyhexacosanoic or 27-hydroxyoctacosanoic acid that is O-linked to the acyl group at position 2'. The lipopolysaccharide studied activated Toll-like receptor 4 signaling only to a low extent (1,000-10,000-fold lower compared with that of Salmonella enterica sv. Friedenau) and did not activate Toll-like receptor 2. Some unusual structural features of the B. henselae lipopolysaccharide, including the presence of a long-chain fatty acid, which are shared by the lipopolysaccharides of other bacteria causing chronic intracellular infections (e.g. Legionella and Chlamydia), may provide the molecular basis for low endotoxic potency.

Induction of various immune modulatory molecules in CD34+hematopoietic cells

Lipopolysaccharide (LPS) has been shown to induce proliferation of human T-lymphocytes only in the presence of monocytes and CD34(+) hematopoietic cells (HCs) from peripheral blood. This finding provided evidence of an active role of CD34(+) HCs during inflammation and immunological events. To investigate mechanisms by which CD34(+) HCs become activated and exert their immune-modulatory function, we used the human CD34(+) acute myeloid leukemia cell line KG-1a and CD34(+) bone marrow cells (BMCs). We showed that culture supernatants of LPS-stimulated mononuclear cells (SUP(LPS)) as well as tumor necrosis factor alpha (TauNF-alpha), but not LPS alone, can activate nuclear factor-kappaB in KG-1a cells. By cDNA subtraction and multiplex polymerase chain reaction, we revealed differential expression of cellular inhibitor of apoptosis protein-1, inhibitor of kappaB (IkappaB)/IkappaBalpha (MAD-3), and intercellular adhesion molecule-1 (ICAM-1) in SUP(LPS)-stimulated KG-1a cells and up-regulation of interferon (IFN)-inducible T cell-chemoattractant, interleukin (IL)-8, macrophage-inflammatory protein-1alpha (MIP-1alpha), MIP-1beta, RANTES, CD70, granulocyte macrophage-colony stimulating factor, and IL-1beta in stimulated KG-1a cells and CD34(+) BMCs. Although monokine induced by IFN-gamma, IFN-inducible protein 10, and IFN-gamma were exclusively up-regulated in KG-1a cells, differential expression of monocyte chemoattractant protein-1 (MCP-1), macrophage-derived chemokine, myeloid progenitor inhibitory factor-2, and IL-18 receptor was only detectable in CD34(+) BMCs. More importantly, CD34(+) BMCs stimulated by TNF-alpha also showed enhanced secretion of MCP-1, MIP-1alpha, MIP-1beta, and IL-8, and increased ICAM-1 protein expression could be detected in stimulated KG-1a cells and CD34(+) BMCs. Furthermore, we revealed that T cell proliferation can be induced by TNF-alpha-stimulated KG-1a cells, which is preventable by blocking anti-ICAM-1 monoclonal antibodies. Our results demonstrate that CD34(+) HCs have the potential to express a variety of immune-regulatory mediators upon stimulation by inflammatory cytokines including TNF-alpha, which may contribute to innate- and adaptive-immune processes.

Acylated cholesteryl galactoside as a novel immunogenic motif in Borrelia burgdorferi sensu stricto

Borrelia burgdorferi sensu lato is the causing agent of Lyme disease, an infectious disease frequently occurring in the United States, Europe, and Northern Asia. Currently, diagnosis of and vaccination strategies against this pathogen are exclusively based on proteinaceous structures. Here we report on a novel class of immunogenic glycolipids purified from B. burgdorferi sensu stricto B31. Employing a butanol/water extraction procedure with subsequent Bligh/Dyer extraction of the organic phase, thin layer chromatography analysis revealed the presence of three distinct glycolipids, which were chemically analyzed employing combined gas-liquid chromatography/mass spectroscopy, matrix-assisted laser desorption/ionization mass spectrometry, and NMR. We identified acylated cholesteryl galactoside (ACG) next to cholesteryl galactoside and alpha-monogalactosyl-diacylglycerol. After extensive purification, the glycolipids investigated failed to cause proinflammatory responses in human cells transfected with human toll-like receptor (TLR)-2 or -4. However, we observed a marked recognition of ACG by sera derived from patients suffering from Lyme disease. These data indicate that newly described ACG is involved in developing host immunity during Lyme disease and thus may be useful for diagnosis and vaccination.

Physicochemical characterization and biological activity of a glycoglycerolipid from Mycoplasma fermentans

We report a comprehensive physicochemical characterization of a glycoglycerolipid from Mycoplasma fermentans, MfGl-II, in relation to its bioactivity and compared this with the respective behaviors of phosphatidylcholine (PC) and a bacterial glycolipid, lipopolysaccharide (LPS) from deep rough mutant Salmonella minnesota strain R595. The beta left arrow over right arrow alpha gel-to-liquid crystalline phase transition behavior of the hydrocarbon chains with Tc = 30 degrees C for MfGl-II as well as for LPS exhibits high similarity between the two glycolipids. A lipopolysaccharide-binding protein (LBP)-mediated incorporation into negatively charged liposomes is observed for both glycolipids. The determination of the supramolecular aggregate structure confirms the existence of a mixed unilamellar/cubic structure for MfGl-II, similar to that observed for the lipid A moiety of LPS. The biological data clearly show that MfGl-II is able to induce cytokines such as tumor necrosis factor-alpha (TNF-alpha) in human mononuclear cells, although to a significantly lower degree than LPS. In contrast, in the Limulus amebocyte lysate test, MfGl-II is completely inactive, and in the CHO reporter cell line it does not indicate any reactivity with the Toll-like receptors TLR-2 and -4, in contrast to control lipopeptides and LPS. These data confirm the applicability of our conformational concept of endotoxicity to nonlipid A structures: an amphiphilic molecule with a nonlamellar cubic aggregate structure corresponding to a conical conformation of the single molecules and a sufficiently high negative charge density in the backbone.

The generalized endotoxic principle

Bacterial lipopolysaccharides (endotoxins, LPS) belong to the most potent immunostimulators in mammals. The endotoxic principle of LPS is located in its lipid A moiety, which for Escherichia coli-type LPS consists of a hexaacylated diphosphoryl diglucosamine backbone. This lipid A adopts a cubic inverted aggregate structure from which a conical shape of the molecule can be deduced, whereas the tetraacyl lipid A precursor IVa adopts a cylindrical shape and is endotoxically inactive, but antagonizes active LPS. We hypothesize that non-lipid A amphiphiles with similar physicochemical properties of amphiphilicity, charge, and shape, might mimic the respective lipid A. To test this hypothesis, phospholipid-like amphiphiles with six acyl chains attached to a bisphosphorylated serine-like backbone of varying length replacing the diglucosamine backbone were synthesized. The compound with a short backbone fulfills all criteria of an endotoxic agonist, and that with long backbone fulfills those of an antagonist. This holds true for the human as well as for the murine system. Interestingly, these compounds are inactive in the Limulus amebocyte lysate test which is specific for LPS diglucosamine backbone. These results define a general endotoxic principle and, furthermore, provide new insights into an understanding of early steps of endotoxin action.

CD55/decay accelerating factor is part of the lipopolysaccharide-induced receptor complex

Recently, we described an 80-kDa lipopolysaccharide (LPS)-binding membrane protein to be identical to CD55 [decay accelerating factor (DAF)]. Here, we demonstrate that CD55 is able to contribute to lipopolysaccharide (LPS) signaling. Transfection of Chinese hamster ovary (CHO) cells with human CD55 resulted in a translocation of NF-kappa B after stimulation with LPS as well as with free lipid A. In addition, interaction of lipid A and CD55 was shown by co-immuno-precipitation of these molecules from CHO-CD55 cells after incubation with lipid A and anti-lipid A monoclonal antibody, as well as by fluorescence resonance energy transfer (FRET) analysis in human monocytes. The comparison of LPS-induced signaling pathways in CHO-CD55 and CHO-CD14 cells revealed that p38, JNK and ERK MAP kinases are activated upon LPS stimulation in both cell lines, and that the activation by LPS can be blocked at the level of Toll-like receptor 4. Finally, through FRET analysis we could demonstrate LPS-induced clustering of CD55 and CD11/CD18 in human monocytes. Our results imply a new functional role of CD55 as a member of a multimeric LPS receptor complex.

Toll-like receptors and their function in innate and adaptive immunity

Over the past 3 years our knowledge about how we sense the microbial world has been fundamentally changed. It has been known for decades that microbial products, such as lipopolysaccharide, lipoproteins, or peptidoglycan, have a profound activity on human cells. Whereas the structure of many different pathogenic microbial compounds has been extensively studied and characterized, the molecular basis of their recognition by the cells of the innate immune system remained elusive for a long time. It was Charles Janeway [Cold Spring Harb Symp Quant Biol 1989;54/1:1-13] who developed the idea of microbial structures forming pathogen-associated molecular patterns that would be recognized by pattern recognition receptors. The discovery of the family of Toll receptors in species as diverse as DROSOPHILA and humans, and the recognition of their role in distinguishing molecular patterns that are common to microorganisms have led to a renewed appreciation of the innate immune system. Moreover, it is now clear that the activation of the innate immune system through mammalian Toll-like receptors has also an instructive role for the responses of the adaptive immune response and, thus, may influence allergic diseases such as asthma.

Endotoxic activity and chemical structure of lipopolysaccharides from Chlamydia trachomatis serotypes E and L2 and Chlamydophila psittaci 6BC

The lipopolysaccharide (LPS) of Chlamydia trachomatis serotype E was isolated from tissue culture-grown elementary bodies and analyzed structurally by mass spectrometry and 1H, 13C and 31P nuclear magnetic resonance. The LPS is composed of the same pentasaccharide bisphosphate alphaKdo-(2-8)-alphaKdo-(2-4)-alphaKdo-(2-6)-betaGlcN-4P-(1-6)-alphaGlcN-1P (Kdo is 3-deoxy-alpha-d-manno-oct-2-ulosonic acid) as reported for C. trachomatis serotype L2[Rund, S., Lindner, B., Brade, H. and Holst, O. (1999) J. Biol. Chem. 274, 16819-16824]. The glucosamine disaccharide backbone is substituted with a complex mixture of fatty acids with ester or amide linkage whereby no ester-linked hydroxy fatty acids were found. The LPS was purified carefully (with contaminations by protein or nucleic acids below 0.3%) and tested for its ability to induce proinflammatory cytokines in several readout systems in comparison to LPS from C. trachomatis serotype L2 and Chlamydophila psittaci strain 6BC as well as enterobacterial smooth and rough LPS and synthetic hexaacyl lipid A. The chlamydial LPS were at least 10 times less active than typical endotoxins; specificity of the activities was confirmed by inhibition with the LPS antagonist, B1233, or with monoclonal antibodies against chlamydial LPS. Like other LPS, the chlamydial LPS used toll-like receptor TLR4 for signalling, but unlike other LPS activation was strictly CD14-dependent.

TLR4-mediated inflammatory activation of human coronary artery endothelial cells by LPS

OBJECTIVE

Blood levels of cytokines are commonly elevated in severe congestive heart failure (CHF) and in coronary artery disease (CAD). While the adverse effects of cytokines on contractile function and myocardial cell integrity are well studied, little is known on whether cardiac cells are only targets or active players in these inflammatory reactions.

METHODS AND RESULTS

We tested if human coronary artery endothelial cells (HCAEC) may become a source of cytokine and adhesion molecule expression when stimulated with bacterial lipopolysaccharide (LPS). Analysis of HCAEC supernatants by ELISA identified enhanced secretion of IL-6, IL-8, and MCP-1 while endothelin-1 was not increased. IL-1beta, IL-10, or TNF-alpha were not detectable by ELISA while RT-PCR revealed enhanced mRNA expression of IL-1beta and TNF-alpha but not IL-10. FACS analysis showed an LPS-induced upregulation of ICAM-1, VCAM, and ELAM-1. LFA-1 could not be detected. We further characterized receptors involved in LPS-induced signaling. Our results indicate that activation of HCAEC by LPS requires Toll-like receptor (TLR) 4. Pretreating the cells with the 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase inhibitor Cerivastatin reduced IL-6 release.

CONCLUSIONS

Taken together, our results indicate that activated HCAEC may act as inflammatory cells and thus directly contribute to the progression of CHF and CAD.

Platelet factor 4 inhibits proliferation and cytokine release of activated human T cells

Platelet factor 4 (PF-4), a platelet-derived CXC chemokine, has been shown to induce the differentiation of monocytes into a subset of macrophages that lack the expression of HLA-DR Ag. This suggests a potential role for PF-4 in the modulation of monocyte-dependent T cell activation. Using an Ag-specific stimulation model in which T cells were cocultured with monocytes in the presence of recall Ags, we could show that under these conditions PF-4-treatment caused a strong decrease of T cell proliferation as well as of IFN-gamma release. However, inhibition of T cell functions such as proliferation, IL-2 release, and IL-2 mRNA production did also occur when isolated T cells were activated in the absence of monocytes with immobilized Abs directed against CD3 in combination with cross-linked anti-CD28 Abs. The effect could be reversed when low concentrations of exogenous IL-2 instead of anti-CD28 were used as a costimulus in combination with anti-CD3 Abs. Further evidence for direct modulation of T cell function by PF-4 was obtained by the detection of specific binding sites for the chemokine on the surface of these cells. Taken together, our results show that specific binding of PF-4, resulting in the down-regulation of the IL-2-release correlates with the inhibition of functions in activated T cells.

Biophysical characterization of triacyl monosaccharide lipid a partial structures in relation to bioactivity

Synthetic triacyl glucosamine monosaccharide lipid A part structures corresponding to the non-reducing moiety of enterobacterial lipid A with an acyloxyacyl chain linked to position 3 of the glucosamine and an unbranched chain linked to position 2 (group 1) and vice versa (group 2) were analyzed biophysically: Fourier-transform infrared spectroscopy was performed to characterize the gel-to-liquid crystalline phase transition, the phosphate band contour, and the orientation of the glucosamine with respect to the membrane surface. Small-angle x-ray diffraction was applied for the elucidation of the supramolecular aggregate structure and, with that, of the molecular shape. With fluorescence resonance energy transfer the lipopolysaccharide-binding protein (LBP)-mediated intercalation of the lipid A partial structures into phospholipid liposomes was monitored. The physical data clearly exhibit a classification of the synthetic compounds into two groups: group 1 compounds have sharp phase transitions, indicating dense acyl chain packing and an inclination of the glucosamine backbone with respect to the membrane surface of 30 degrees with the phosphate buried in the membrane. Group 2 compounds have a very broad phase transition, indicating poorly packed acyl chains, and an inclination of -30 degrees with the phosphate group sticking outward. For the first group unilamellar phases are observed superimposed by a non-lamellar structure, and for the second one only multilamellar aggregate structures. The cytokine-inducing capacity in human mononuclear cells is relatively high for the first group and low or absent for the second group. Based on these data a model of the intra and intermolecular conformations is proposed which also extends the concept of "endotoxic conformation."

The biology of endotoxin

Endotoxin (lipopolysaccharide, LPS) is the major component of the outer leaflet of Gram-negative bacteria and has profound immunostimulatory and inflammatory capacity. The septic shock syndrome caused by endotoxin still has an unacceptably high mortality rate and, owing to increasing numbers of resistant strains, remains an ongoing threat throughout the world. However, the past years have provided new insights especially into the receptors of the innate immune system that are involved into the recognition of LPS and the initial signal transduction pathways that are engaged after the primary recognition on the cell surface. The knowledge about the molecular basis for the responses to endotoxin may eventually lead to the development of new drugs to fight the fatal effects of bacterial infections.

Lipopolysaccharide and ceramide docking to CD14 provokes ligand-specific receptor clustering in rafts

The glycosylphosphatidylinositol-anchored receptor CD14 plays a major role in the inflammatory response of monocytes to lipopolysaccharide. Here, we describe that ceramide, a constituent of atherogenic lipoproteins, binds to CD14 and induces clustering of CD14 to co-receptors in rafts. In resting cells, CD14 was associated with CD55, the Fcgamma-receptors CD32 and CD64 and the pentaspan CD47. Ceramide further recruited the complement receptor 3 (CD11b/CD18) and CD36 into proximity of CD14. Lipopolysaccharide, in addition, induced co-clustering with Toll-like receptor 4, Fcgamma-RIIIa (CD16a) and the tetraspanin CD81 while CD47 was dissociated. The different receptor complexes may be linked to ligand-specific cellular responses initiated by CD14.

Molecular genetic analysis of an endotoxin nonresponder mutant cell line: a point mutation in a conserved region of MD-2 abolishes endotoxin-induced signaling

Somatic cell mutagenesis is a powerful tool for characterizing receptor systems. We reported previously two complementation groups of mutant cell lines derived from CD14-transfected Chinese hamster ovary--K1 fibroblasts defective in responses to bacterial endotoxin. Both classes of mutants expressed a normal gene product for Toll-like receptor (TLR)4, and fully responded to stimulation by tumor necrosis factor (TNF)-alpha or interleukin (IL)-1 beta. We identified the lesion in one of the complementation groups in the gene for MD-2, a putative TLR4 coreceptor. The nonresponder phenotype of this mutant was reversed by transfection with MD-2. Cloning of MD-2 from the nonresponder cell line revealed a point mutation in a highly conserved region resulting in a C95Y amino acid exchange. Both forms of MD-2 colocalized with TLR4 on the cell surface after transfection, but only the wild-type cDNA reverted the lipopolysaccharide (LPS) nonresponder phenotype. Furthermore, soluble MD-2, but not soluble MD-2(C95Y), functioned to enable LPS responses in cells that expressed TLR4. Thus, MD-2 is a required component of the LPS signaling complex and can function as a soluble receptor for cells that do not otherwise express it. We hypothesize that MD-2 conformationally affects the extracellular domain of TLR4, perhaps resulting in a change in affinity for LPS or functioning as a portion of the true ligand for TLR4.

Decay-accelerating factor (DAF/CD55) is a functional active element of the LPS receptor complex

Previously, we identified an 80 kDa membrane protein (LMP80) that is capable of binding to LPS and lipid A in the presence of LBP and sCD14. LMP80 could also be detected after immuno-coprecipitation of cell membranes with LPS and lipid A, indicating a physical contact of LMP80 and LPS/lipid A. Further analysis and peptide sequencing revealed that LMP80 is identical to CD55 (decay accelerating factor, DAF), a regulatory molecule of the complement cascade. Transfection of LPS-hyporesponsive Chinese hamster ovary (CHO) cells with human CD55 resulted in the translocation of NF-B upon stimulation with LPS or lipid A. Our results demonstrate a new functional role of CD55 as a molecule able to mediate LPS-induced activation of cells that may be part of a multimeric LPS receptor complex.

Immunoregulation via 'bystander suppression' needs minute amounts of substances--a basis for homeopathic therapy?

One of the main characteristics of homeopathic drugs is the low concentration of substances they contain. In most discussions this serves as the predominant argument against homeopathic treatments. The small amount of ingredients is in most instances considered not to be able to induce significant changes in classical pharmacological models. A few years ago researchers at the Harvard Medical School in Boston observed that the auto-reactivity ofT-cells is managed by the immune system in at least two different ways that obviously were dependent upon the concentration of the antigen they encounter: If they see high concentrations of a self-antigen they are deleted (killed), but when given low doses they undergo a special kind of active inhibition (called 'bystander suppression'). We feel that this type of regulation induced by very low substance concentrations could serve as a model to explain the way in which at least some homeopathic pharmaceuticals mediate their therapeutic effects.

Toll-like receptor 4 imparts ligand-specific recognition of bacterial lipopolysaccharide

Lipopolysaccharide (LPS) is the main inducer of shock and death in Gram-negative sepsis. Recent evidence suggests that LPS-induced signal transduction begins with CD14-mediated activation of 1 or more Toll-like receptors (TLRs). The lipid A analogues lipid IVa and Rhodobacter sphaeroides lipid A (RSLA) exhibit an uncommon species-specific pharmacology. Both compounds inhibit the effects of LPS in human cells but display LPS-mimetic activity in hamster cells. We transfected human TLR4 or human TLR2 into hamster fibroblasts to determine if either of these LPS signal transducers is responsible for the species-specific pharmacology. RSLA and lipid IVa strongly induced NF-kappaB activity and IL-6 release in Chinese hamster ovary fibroblasts expressing CD14 (CHO/CD14), but these compounds antagonized LPS antagonists in CHO/CD14 fibroblasts that overexpressed human TLR4. No such antagonism occurred in cells overexpressing human TLR2. We cloned TLR4 from hamster macrophages and found that human THP-1 cells expressing the hamster TLR4 responded to lipid IVa as an LPS mimetic, as if they were hamster in origin. Hence, cells heterologously overexpressing TLR4 from different species acquired a pharmacological phenotype with respect to recognition of lipid A substructures that corresponded to the species from which the TLR4 transgene originated. These data suggest that TLR4 is the central lipid A-recognition protein in the LPS receptor complex.

Bacteraemia and antibiotic resistance of its pathogens reported in England and Wales between 1990 and 1998: trend analysis

OBJECTIVES

Determination of causes, trends, and antibiotic resistance in reports of bacterial pathogens isolated from blood in England and Wales from 1990 to 1998.

DESIGN

Description of bacterial isolates from blood, judged to be clinically significant by microbiology staff, reported to the Communicable Disease Surveillance Centre.

SETTING

Microbiology laboratories in England and Wales.

SUBJECTS

Patients yielding clinically significant isolates from blood.

MAIN OUTCOME MEASURES

Frequency and Poisson regression analyses for trend of reported causes of bacteraemia and proportions of antibiotic resistant isolates.

RESULTS

There was an upward trend in total numbers of reports of bacteraemia. The five most cited organisms accounted for over 60% of reports each year. There was a substantial increase in the proportion of reports of Staphylococcus aureus resistant to methicillin, Streptococcus pneumoniae resistance to penicillin and erythromycin, and Enterococcus faecalis and Enterococcus faecium resistance to vancomycin. No increase was seen in resistance of Escherichia coli to gentamicin.

CONCLUSIONS

Reports from laboratories provide valuable information on trends and antibiotic resistance in bacteraemia and show a worrying increase in resistance to important antibiotics.

Bacterial lipopolysaccharide induces expression of the stress response genes hop and H411

CD14-transfected Chinese hamster ovary K1 fibroblasts (CHO/CD14) respond to lipopolysaccharide (LPS) by metabolizing arachidonic acid and with translocation of NF-kappaB to the nucleus. Although previous experiments failed to identify the production of tumor necrosis factor-alpha and interleukin (IL)-1beta by CHO/CD14 cells, LPS did induce the expression of IL-6 mRNA and the subsequent release of the IL-6 protein. To identify additional LPS-inducible genes, a cDNA library derived from LPS-stimulated CHO/CD14 cells was screened by subtractive hybridization. Fourteen genes were found to be expressed differentially, and two were analyzed in detail: hop (Hsp70/Hsp90-organizing protein), which is the hamster homologue of the stress-inducible yeast gene, STI1, and clone H411, which encodes a novel LPS-inducible growth factor. In response to LPS, the expression of Hop mRNA was also increased in both the murine macrophage cell line, RAW 264.7, as well as in primary hamster macrophages. This suggested that the up-regulation of Hop expression is part of the macrophage stress response to LPS. Clone H411 encodes a protein in the epidermal growth factor-like repeat protein family. Overexpression of H411 cDNA in the RAW 264.7 macrophage cell line promoted an increased growth rate, suggesting that expression of H411 is part of the proliferative cell response to LPS. Both Hop and H411 represent novel gene products not previously recognized as part of the complex biological response to endotoxin.

Cutting edge: cells that carry A null allele for toll-like receptor 2 are capable of responding to endotoxin

Toll-like receptor (TLR) 2 and TLR4 have been implicated in the responses of cells to LPS (endotoxin). CD14-transfected Chinese hamster ovary (CHO)-K1 fibroblasts (CHO/CD14) are exquisitely sensitive to endotoxin. Sequence analysis of CHO-TLR2, compared with human and mouse TLR2, revealed a single base pair deletion. This frameshift mutation resulted in an alternative stop codon, encoding a protein devoid of transmembrane and intracellular domains. CHO-TLR2 cDNA failed to enable LPS signaling upon transient transfection into human epithelial kidney 293 cells. Site-directed mutagenesis of CHO-TLR2 enabled expression of a presumed full-length hamster TLR2 that conferred LPS responsiveness in human epithelial kidney 293 cells. Genomic TLR2 DNA from primary hamster macrophages also contained the frameshift mutation found in CHO fibroblasts. Nevertheless, hamster peritoneal macrophages were found to respond normally to LPS, as evidenced by the induction of cytokines. These results imply that expression of TLR2 is sufficient but not essential for mammalian responses to endotoxin.