Structures of synthetic O-antigen fragments from serotype 2a Shigella flexneri in complex with a protective monoclonal antibody.

The anti-LPS IgG mAb F22-4, raised against Shigella flexneri serotype 2a bacteria, protects against homologous, but not heterologous, challenge in an experimental animal model. We report the crystal structures of complexes formed between Fab F22-4 and two synthetic oligosaccharides, a decasaccharide and a pentadecasaccharide that were previously shown to be both immunogenic and antigenic mimics of the S. flexneri serotype 2a O-antigen. F22-4 binds to an epitope contained within two consecutive 2a serotype pentasaccharide repeat units (RU). Six sugar residues from a contiguous nine-residue segment make direct contacts with the antibody, including the nonreducing rhamnose and both branching glucosyl residues from the two RUs. The glucosyl residue, whose position of attachment to the tetrasaccharide backbone of the RU defines the serotype 2a O-antigen, is critical for recognition by F22-4. Although the complete decasaccharide is visible in the electron density maps, the last four pentadecasaccharide residues from the reducing end, which do not contact the antibody, could not be traced. Although considerable mobility in the free oligosaccharides can thus be expected, the conformational similarity between the individual RUs, both within and between the two complexes, suggests that short-range transient ordering to a helical conformation might occur in solution. Although the observed epitope includes the terminal nonreducing residue, binding to internal epitopes within the polysaccharide chain is not precluded. Our results have implications for vaccine development because they suggest that a minimum of two RUs of synthetic serotype 2a oligosaccharide is required for optimal mimicry of O-Ag epitopes.

Structural and functional studies of FkpA from Escherichia coli, a cis/trans peptidyl-prolyl isomerase with chaperone activity.

The protein FkpA from the periplasm of Escherichia coli exhibits both cis/trans peptidyl-prolyl isomerase (PPIase) and chaperone activities. The crystal structure of the protein has been determined in three different forms: as the full-length native molecule, as a truncated form lacking the last 21 residues, and as the same truncated form in complex with the immunosuppressant ligand, FK506. FkpA is a dimeric molecule in which the 245-residue subunit is divided into two domains. The N-terminal domain includes three helices that are interlaced with those of the other subunit to provide all inter-subunit contacts maintaining the dimeric species. The C-terminal domain, which belongs to the FK506-binding protein (FKBP) family, binds the FK506 ligand. The overall form of the dimer is V-shaped, and the different crystal structures reveal a flexibility in the relative orientation of the two C-terminal domains located at the extremities of the V. The deletion mutant FkpNL, comprising the N-terminal domain only, exists in solution as a mixture of monomeric and dimeric species, and exhibits chaperone activity. By contrast, a deletion mutant comprising the C-terminal domain only is monomeric, and although it shows PPIase activity, it is devoid of chaperone function. These results suggest that the chaperone and catalytic activities reside in the N and C-terminal domains, respectively. Accordingly, the observed mobility of the C-terminal domains of the dimeric molecule could effectively adapt these two independent folding functions of FkpA to polypeptide substrates.

Structural and functional characterization of a monoclonal antibody specific for the preS1 region of hepatitis B virus.

The monoclonal antibody 5a19, raised against the ay serotype of hepatitis B virus, binds to the segment of the preS1 region comprising residues 37-43, which is implicated in attachment of the virus to hepatocytes. The dissociation constant, derived from kinetic studies using surface plasmon resonance techniques, is in the low nanomolar range. The nucleotide sequence of the variable domains has been determined and the corresponding germ-line genes have been identified. The three-dimensional structure of the Fab fragment has been determined by X-ray crystallography to 2.6 A resolution.

Structure of the Fab fragment from F124, a monoclonal antibody specific for hepatitis B surface antigen.

The crystal structure of the Fab fragment from the monoclonal anti-preS2 antibody F124 (IgG1,kappa) has been solved by molecular replacement and refined at 3.0 A resolution. The Fab crystallizes with two independent molecules in the asymmetric unit. F124 recognizes an epitope contained within the preS2 segment between residues 120 and 132 of the surface antigen of hepatitis B virus. The antibody shows a high affinity for the glycan N-linked to Asn123, but it also cross-reacts with the non-glycosylated peptide fragment 120-132. Although crystallization was performed in the presence of an eightfold excess of the cross-reactive peptide, no evidence for the ligand was found in the antigen-binding site, which is close to a neighbouring molecule in the crystal lattice. The antigen-binding site has a groove-like topology which is modulated with pocket-like cavities. It is characterized by a large number of tyrosine and aspartate residues. The importance of germ-line mutations at the binding site is discussed.

Crystal structure of a dominant B-cell epitope from the preS2 region of hepatitis B virus in the form of an inserted peptide segment in maltodextrin-binding protein.

We report here the crystal structure of MalE-B363, a recombinant construction of maltodextrin-binding protein bearing a dominant B-cell epitope sequence from the preS2 region of the hepatitis B surface antigen. The inserted peptide sequence, which replaces the seven carboxy-terminal residues of maltodextrin-binding protein, carries the 14 amino acid residue epitope contained between residues 132 and 145 from the preS2 region. The epitope sequence is flanked on either side by additional residues that result from the genetically engineered insertion, bringing the total length of the foreign peptide to 26 amino acid residues. The hybrid protein has been previously shown to be recognised by monoclonal antibodies elicited by the native viral antigen. Three independent molecules of MalE-B363 are present in the asymmetric unit of the crystal. All 14 epitope residues could be traced for one molecule, ten epitope residues had significant electron density for the second, but no density was visible for the epitope of the third. The conformation of the amino-terminal segment of the epitope from Gln132(e) to Gly138(e) is similar in the two molecules of MalE-B363 for which the foreign peptide could be traced. Moreover, the conformation of a smaller segment, comprising residues Asp133(e) to Arg137(e), is similar to that present in the previously determined crystal structure of MalE-B133, another insertion/deletion mutant of maltodextrin-binding protein bearing the preS2 epitope. The presence of a common structural motif for the same sequence in disparate molecular environments suggests that this conformation might be present also in the native viral antigen. This could provide a structural basis to explain the cross-reactivity of anti-preS2 monoclonal antibodies with these hybrid proteins.

Sequence analysis of a monoclonal antibody specific for the preS2 region of hepatitis B surface antigen, and the cloning, expression and characterisation of its single-chain Fv construction.

The nucleotide sequence of the monoclonal antibody F124, specific for the preS2 region of the surface antigen of hepatitis B virus, has been determined and an single-chain Fv fragment (scFv) recombinant construction has been cloned and expressed into the periplasmic region of Escherichia coli. The recombinant antibody fragment contains a (Gly4Ser)3 linker connecting the C-terminus of the heavy chain variable region (V(L)) domain to the N-terminus of the light chain variable region (V(L)) domain. A 23-residue peptide segment, containing a c-myc marker for immunochemical detection of the scFv and hexahistidine tag to facilitate its purification, was added C-terminal to the V(L) domain. The scFv mimics the antibody in binding to the native antigen in the form of recombinant hepatitis B surface antigen (HBsAg) particles as well as to peptide fragments carrying the viral epitope.

Maltodextrin-binding protein hybrids carrying epitopes from the preS2 region of hepatitis B virus: expression, antibody-binding and preliminary crystallographic studies.

Five hybrid constructions of maltodextrin-binding protein (MBP), each containing an inserted epitope(s) from the preS2 region of the envelope proteins of hepatitis B virus (HBV), have been expressed. The anti-preS2 monoclonal antibody S2.3 was shown to cross-react with the MBP hybrid constructions, demonstrating that the epitopes presented by these recombinant proteins mimic the antigenic behaviour of the native viral antigen. In addition, all five hybrid proteins have been crystallized. Preliminary structural solutions obtained by molecular replacement indicate that the native conformation of MBP is preserved in the hybrid constructions despite the significant length of the epitope insertions.

Crystal structure of a recombinant form of the maltodextrin-binding protein carrying an inserted sequence of a B-cell epitope from the preS2 region of hepatitis B virus.

We report the crystal structure of MalE-B133, a recombinant form of the maltodextrin-binding protein (MBP) of Escherichia coli carrying an inserted amino-acid sequence of a B-cell epitope from the preS2 region of the hepatitis B virus (HBV). The structure was determined by molecular replacement methods and refined to 2.7 A resolution. MalE-B133 is an insertion/deletion mutant of MBP in which residues from positions 134 to 142, an external alpha helix in the wild-type structure, are replaced by a foreign peptide segment of 19 amino acids. The inserted residues correspond to the preS2 sequence from positions 132 to 145 and five flanking residues that arise from the creation of restriction sites. The conformation of the recombinant protein, excluding the inserted segment, closely resembles that of wild-type MBP in the closed maltose-bound form. MalE-B133 was shown by previous studies to display certain immunogenic and antigenic properties of the hepatitis B surface antigen (HBsAg), which contains the preS2 region. The crystal structure reveals the conformation of the first nine epitope residues (preS2 positions 132 to 140) exposed on the surface of the molecule. The remaining five epitope residues (preS2 positions 141 to 145) are not visible in electron density maps. The path of the polypeptide chain in the visible portion of the insert differs from that of the deleted segment in the structure of wild-type MBP, displaying a helical conformation at positions 134 to 140 (preS2 sequence numbering). A tripeptide (Asp-Pro-Arg) at the N terminus of the helix forms a stable structural motif that may be implicated in the cross-reactivity of anti-HBsAg antibodies with the hybrid protein.

[Epitopes of the envelope of the hepatitis B virus: a structural approach]. / Les épitopes de l'enveloppe du virus de l'hépatite B: approche-structurale.

Hepatitis B is a major public health problem. More than 300 million people are chronically infected by the virus. During infection very large quantities of complete virions and empty envelopes, consisting of spherical or filamentous lipoprotein particles, are present in the blood. DNA genome coding for envelopes is divided into three domains, preS1, preS2 and S. All available data suggest that the preS1 and preS2 products are exposed at the surface of the virions. These proteins are more immunogenic than S in terms of in vivo antibody response and the number of epitopes identified. The three dimensional mapping of antigenic sites of the HBV will provide important strategic information for vaccine development and identification of targets for immunorecognition or immunoregulation of the disease.

Enzyme immunometric assay for leukotriene C4.

An enzyme immunometric assay of LTC4 named SPIE-IA is described. The assay involves different sequential steps: (1) immunocapture of LTC4 by monoclonal anti-LTC4 antibodies coated on 96-well microtiter plates; (2) cross-linking of LTC4 via its amino group to the wells using glutaraldehyde; (3) treatment with HCl; (4) measurement of linked LTC4 using the same monoclonal anti-LTC4 antibodies labeled with acetylcholinesterase. A minimal detectable concentration of 2 pg/ml after 60 min of enzymatic reaction was obtained. Cross-reactivity was less than 15% with LTD4 or LTE4. The coefficient of variation was less than 6% in the 20-1000 pg/ml range. Good correlation was observed between SPIE-IA and a competitive enzyme immunoassay for biological samples. The different sequential steps of the assay are investigated.

Determination of detergent critical micellar concentration by solubilization of a colored dye.

The critical micellar concentration of detergent was determined by the solubilization of a colored dye found in standard waterproof pen. The amount of solubilized dye is directly proportional to the concentration of micelles in the detergent solution. The method has been validated with a series of standard detergents.

Active metabolism of arachidonic acid by Kaposi sarcoma cells cultured from lung biopsies (KS-3); identification by HPLC and MS/MS of the predominant metabolite secreted as the 11,12-epoxy-eicosatrienoic acid.

The development of long-term culture of AIDS-KS cells has allowed us to investigate further a possible vascular origin of Kaposi sarcoma. Taking into account the relative specificity of arachidonic acid (AA) metabolism according to cell type, the AA 'cascade' was analyzed in cultured KS-3 cells established from lung biopsies and compared to human umbilical venous endothelial (H-UVE) cells and human myometrial smooth muscle (H-MSM) cells, under basal conditions and after stimulation with vasoactive agents such as histamine or thrombin. Considering strictly the 'prostaglandin' profile given by RIAs, the metabolism of AA was closer, whilst not identical, to H-UVE than to H-MSM cells. However, evaluation of all the eicosanoids released from [3H]AA labeled KS-3 cells revealed that the predominant metabolite was not prostacyclin (PGI2), as suggested from PG RIAs, but an epoxy-eicosatrienoic acid (EET), identified as the 11, 12 isomer by HPLC and MS/MS. The synthesis of this EET is probably cytochrome P-450 monooxygenase dependent. Its potential role in the development of the KS tumor cells is under investigation.

Effects of N-acetyl-aspartyl glutamic acid and sodium cromoglycate on leukotriene B4 secretion by human leukocytes.

Peripheral leukocytes from allergic subjects were treated for 30 min with sodium cromoglycate (SCG) or with N-acetyl-aspartyl glutamic acid (NAAGA) and challenged for leukotriene B4 (LTB4) production with calcium ionophore A 23187. NAAGA significantly inhibits LTB4 release at concentrations of 10(-2) M (-86%), 5 x 10(-3) M (-49%) and 10(-3) M (-34%), while SCG was not able to block LTB4 production within the range of 10(-2)-10(-4) M. In spite of the fact that SCG and NAAGA are chemically unrelated and that both show antiallergic properties, only NAAGA is able in this model to block production of LTB4, a chemical mediator strongly involved in inflammatory and hypersensitivity reactions.

12-Hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulate melatonin synthesis in rat pineals.

The role of arachidonic acid metabolites in norepinephrine (NE)-induced N-acetyltransferase (NAT) activity and melatonin release was examined from 6 h-incubations of rat pineal glands. A cyclooxygenase inhibitor, indomethacin (5 x 10(-8) - 5 x 10(-6) M) was ineffective on melatonin release, in the presence of absence of NE (5 x 10(-6) M) while a lipoxygenase inhibitor, nordihydroguaiaretic acid (5 x 10(-7) -5 x 10(-5) M) had an inhibitory effect. Among the lipoxygenase metabolites, 12-hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulated both NAT activity and melatonin release in a dose-dependent manner, with a maximal effect occurring at 10(-6) M, while 5-HPETE or hydroxy derivatives of these compounds (12-HETE, 15-HETE and 5-HETE) were ineffective. These results indicate that 12-HPETE and 15-HPETE can be involved in NE-induced melatonin release.

Structural identification of prostaglandin A1 biotransformation products from tumor cells.

Rat B104 neuroblastoma and C6 glioma cells are able to metabolize prostaglandin A1 (PGA1). Four metabolites were isolated by high performance liquid chromatography. Their structure was elucidated by fast atom bombardment mass spectrometry and 1H nuclear magnetic resonance. It appears that these biotransformation products are two sets of stereoisomers: the two isomers that eluted first are 9 alpha- and 9 beta-hydroxy-11 alpha-cysteinylglycyl adducts whereas the other two are 9 alpha- and 9 beta-hydroxy-11 alpha-cysteinyl derivatives. These compounds were compared with authentic samples prepared by Michael addition of the corresponding thiol onto PGA1, then by reduction with sodium borohydride.

Tumor cell biotransformation products of prostaglandin A1 with growth inhibitory activity.

The growth inhibitory effect and the fate of prostaglandin A1 (10(-6) M) were followed in cultures of rat B104 neuroblastoma and C6 glioma cells. More than 40% and 85% of the drug were neither recognized by a prostaglandin A1 antiserum nor extracted from the acidified medium with ethyl acetate, after 6 h and 24 h-incubation, respectively. When the supernatant of cells cultured in the presence of prostaglandin A1 during 24 hours was transferred to other cells and used as culture medium, the same growth inhibitory effect as with prostaglandin A1 was observed even when no prostaglandin A1 was added. After extensive purification and reverse phase HPLC of supernatant, four peaks more polar than prostaglandin A1 were shown; two of them were still active as growth inhibitors. This biotransformation was not observed with normal cells like L 929 or chick embryo fibroblasts, for which prostaglandin A1 had no inhibitory effect. The identification of these metabolites will allow the study of the structure-activity relationship.