The glycoprotein Ib-IX complex-specific monoclonal antibody SZ1 binds to a conformation-sensitive epitope on glycoprotein IX: implications for the target antigen of quinine/quinidine-dependent autoantibodies

The monoclonal antibody SZ1 is of interest for two reasons: it was used to define complex formation between glycoprotein (GP) Ib and GP IX, and its epitope is likely to be identical to that recognized by most quinine- and quinidine-dependent autoantibodies that cause thrombocytopenia. To determine the location of the epitope for SZ1 within the GP Ib-IX complex (which consists of three subunits: GP Ib alpha, GP Ib beta, and GP IX), we tested the ability of the antibody to bind transfected cells that expressed different combinations of complex subunits, and compared this binding to the binding of antibodies of known specificity. SZ1 bound to cells that expressed the entire GP Ib-IX complex in the same pattern as did AN51 (an antibody specific for GP Ib alpha). However, unlike AN51, SZ1 did not bind alpha beta cells (ie, cells that express GP Ib alpha and GP Ib beta, but not GP IX), but did bind to beta IX and alpha IX cells. We then compared the binding patterns of SZ1 and FMC25, an antibody specific for GP IX. Both bound virtually identically to cell lines that expressed every combination of two of the three GP Ib-IX complex subunits. However, the epitopes of the two antibodies were not identical, because fixation with 4% paraformaldehyde of cells that expressed GP IX destroyed the SZ1 epitope while maintaining the FMC25 epitope. Because of the ability of SZ1 to block the binding of many quinine- and quinidine-dependent antibodies, these data strongly suggest that GP IX is the component of the GP Ib-IX complex recognized by those antibodies.

HBsAg subtype distribution among different populations of HBsAg carriers in Spain

Data concerning the HBsAg subtype distribution in Spain are out-of-date and confined to a restricted geographical area. Furthermore, the complex distribution observed in the countries surrounding Spain prevents any prediction. To obtain further data on HBsAg subtype distribution among Spanish HBsAg carriers, subtyping analysis (d and y determinants) was performed in 670 samples from subjects belonging to various epidemiological risk groups and coming from different geographical areas of the country. Similar frequencies were found for both mutually exclusive d/y subtype determinants among non-risk, normal HBsAg carriers from almost all geographical areas studied. In contrast, the ay subtype was clearly predominant (79-87%) among intravenous drug users, irrespective of their geographical origin. Thirteen different institutions for mentally retarded patients behaved as closed communities for HBV circulation, showing independent subtype distributions. Thus, no significant geographical variations were found for HBsAg subtype distribution in Spain. The prevalence of each particular subtype is mainly dependent on the epidemiological characteristics of the carriers studied. Subtype distribution was independent of the presence of HBeAg or HDV infection serum markers when homogeneous groups were considered separately. Atypical HBsAg phenotypes, either with coexistence or absence of both subtype determinants, were found in some cases.

Tyrosine sulfation of the glycoprotein Ib-IX complex: identification of sulfated residues and effect on ligand binding

Here, we present evidence that glycoprotein (GP) Ib alpha, one of three polypeptides that make up the GP Ib--IX complex--the receptor for von Willebrand factor (vWf) on the surface of unactivated platelets--is modified by sulfation of tyrosine residues. Only GP Ib alpha was found to incorporate 35S when the GP Ib--IX complex was immunoprecipitated from [35S]sulfate metabolically labeled L and CHO cells that express the recombinant complex. The occurrence of sulfation on tyrosine residues of the polypeptide backbone was determined by removing O- and N-linked oligosaccharides. Limited proteolytic digestion of metabolically labeled GP Ib alpha revealed that sulfated tyrosine residues are located in the 45-kDa globular region containing the vWf binding site. By mutating potentially sulfated tyrosine residues to phenylalanine and comparing the stoichiometry of sulfate incorporation of these mutants to the incorporation in wild-type GP Ib alpha, three clustered tyrosine residues--Tyr-276, Tyr-278, and Tyr-279-were identified that undergo the modification. Culturing cells in sulfate-depleted medium containing sodium chlorate and guaiacol completely inhibited GP Ib alpha sulfation but did not decrease GP Ib-IX expression on the cell surface. Similarly, transiently transfected CHO cells expressed the mutant GP Ib alpha polypeptide on their surfaces at the same levels as they expressed wild-type GP Ib alpha. These results suggest that tyrosine sulfation of GP Ib alpha has little or no effect on the synthesis, assembly, and surface expression of the GP Ib-IX complex. Nevertheless, inhibiting sulfation of GP Ib alpha reduced the binding of 125I-labeled vWf in the presence of ristocetin by up to 37%.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycoprotein (GP) Ib beta is the critical subunit linking GP Ib alpha and GP IX in the GP Ib-IX complex. Analysis of partial complexes

The glycoprotein (GP) Ib-IX complex is the receptor on platelet surfaces that mediates their adhesion to subendothelium. It comprises three polypeptides (GP Ib alpha, GP Ib beta, GP IX), each of which belongs to a superfamily of proteins containing conserved leucine-rich motifs. In this study, we used Chinese hamster ovary (CHO) cells expressing every combination of two GP Ib-IX complex subunits to demonstrate that GP Ib beta plays an essential role in the synthesis of the heterotrimer by associating with both of the other two subunits. Confocal microscopy demonstrated that GP Ib beta was present in the same cellular locations as GP Ib alpha in CHO alpha beta cells (cells expressing only GP Ib alpha and GP Ib beta) and as GP IX in CHO beta IX cells. The two polypeptides expressed in CHO alpha IX cells did not co-localize. Association between GP Ib alpha and GP Ib beta was demonstrated biochemically on immunoblots of detergent lysates of CHO alpha beta cells; electrophoresis under nonreducing conditions revealed the two subunits to be covalently linked through a disulfide bond. Association of GP Ib alpha and GP Ib beta was further demonstrated by the finding that immunoprecipitations with antibodies against either polypeptide precipitated both. Similarly, immunoprecipitations of lysates of CHO beta IX cells with antibodies against GP Ib beta or GP IX precipitated both polypeptides. In contrast, co-immunoprecipitation of the two polypeptides expressed in CHO alpha IX cells could not be demonstrated. Transient expression in CHO cells of GP Ib beta with GP IX yielded higher GP IX levels on the cell membrane than did expression of GP IX alone; supertransfection of CHO alpha IX cells with GP Ib beta also increased GP IX levels on the cell surface.

Association of GP Ib with actin-binding protein does not require GP IX

GP IX is necessary for optimal expression of the GP Ib-IX complex on the surface of transfected cells, and presumably also on the surface of the platelet. The authors investigated whether increasing complex association with the cytoskeleton is one mechanism by which GP IX exerts its effect. CHO and L cell lines that express high levels of GP Ib were used to determine whether GP Ib (GPIb alpha and GPIb beta) associated with the cytoskeleton. GP Ib in these cells was found in the insoluble cytoskeletal fraction from Triton X-100 lysates in a proportion similar to that found in cells expressing the full complex. As in platelets and cells expressing the full complex, the association of GP Ib with the cytoskeleton was shown to be mediated by actin-binding protein (ABP). This was demonstrated by the observation that a monoclonal antibody against GPIb alpha precipitated ABP from GP Ib-expressing cells, and polyclonal anti-ABP antibodies specifically coprecipitated GP Ib. In addition, colocalization of the two components in intact cells was demonstrated by confocal microscopy. These data indicate that the association of GP Ib with the cytoskeleton is independent of GP IX, which therefore must increase surface expression of the complex by another mechanism.

Severe spondylocostal dysostosis associated with other congenital anomalies: a clinical/epidemiologic analysis and description of ten cases from the Spanish registry

We describe 10 cases born to nondiabetic mothers who presented with severe spondylocostal dysostosis (SCD) associated with other anomalies, identified among 20,526 malformed liveborn infants from the Spanish Collaborative Study of Congenital Malformations (ECEMC). We analyze the associated malformations in the 10 cases with severe SCD, as well as in all cases with less severe SCD among children with MCA patterns of unknown cause. Cases with SCD were preferentially associated with caudal dysgenesis, diaphragmatic hernia, and central nervous system anomalies.

The platelet glycoprotein Ib-IX complex

The GP Ib-IX complex is part of a conglomerate of polypeptides on the platelet surface that perform several key roles of central importance to the haemostatic function of platelets. When deranged, these interactions can also lead to pathological thrombosis, with potentially disastrous consequences for the organism. In this manuscript, several aspects of the structure and biology of the complex are reviewed, including the structures of its polypeptides and their relationships to other members of a phylogenetically widespread protein family, its topology on the platelet membrane and relationship with cytoskeletal components, peptide sequences involved in binding its ligands, von Willebrand factor and thrombin, its polymorphisms, its biosynthesis, and the organizations of the genes that encode its subunits.

Conformational constraints of conserved neutralizing epitopes from a major antigenic area of human respiratory syncytial virus fusion glycoprotein

To study the conformational requirements of epitopes from a conserved antigenic area (area II) of respiratory syncytial (RS) virus fusion (F) glycoprotein, peptides of increasing length containing amino acids essential for these epitopes were synthesized. The synthetic peptides were tested for binding to a panel of neutralizing monoclonal antibodies (MAbs) for this area as well as to rabbit hyperimmune and human convalescent antisera. Antibody binding was dependent on peptide length; thus, a 61-residue peptide spanning amino acids 215 to 275 of the F1 subunit (peptide F215-275) reacted with more antibodies than a shorter (41-residue) peptide F235-275, and this one with more than the (21-residue) peptide F255-275. Most human convalescent sera contained antibodies that reacted with peptides F215-275 and F235-275 but failed to react with F255-275. The results of antibody binding could be related to the structure adopted by the peptides in solution, as determined by circular dichroism spectroscopy and susceptibility of peptides to trypsin digestion. Pretreatment of peptide F215-275 with SDS abolished reactivity with certain MAbs, supporting the notion that higher order structures were needed for antibody binding. High titre anti-peptide antisera were induced in rabbits inoculated with the peptides; however, these sera failed to react with the native F molecule. In mice, only the largest F215-275 peptide induced an anti-peptide response, but their sera reacted poorly with the native F protein and the animals were not protected against an RS virus challenge. These results illustrate the potential use of synthetic peptides in studies of the F protein physical and antigenic structures as well as the problems in designing synthetic RS virus vaccines.

Evaluation of laboratory assays for screening antibody to hepatitis C virus

An evaluation study, involving 11 screening methods for the detection of antibody to hepatitis C virus (anti-HCV) and a panel of 500 serum samples, was performed. Samples were tested by all 11 methods, and those showing reactivity in at least one method were studied by a combination of supplemental assays (recombinant immunoblot assays, first and second generation; neutralization test for anti-c100; synthetic peptide immunoblot assay; recombinant multi-dot immunoassay) and classified as positive (110 samples), indeterminate (4 samples), or negative (386 samples) on the basis of the results obtained. Second-generation recombinant methods performed better on positive samples than first-generation assays or synthetic peptide-based methods (99.1-100% correlation vs. 64.5-85.5% and 93.6-99.1%, respectively), whereas the latter showed higher correlations on negative samples than recombinant assays (97.4-99.7% vs. 82.4-93%). Further investigations, using broad panels of indeterminate samples from blood donors, should be done, however, before synthetic peptide-based methods are recommended for blood bank screening. Reactivity of samples must be confirmed by one supplemental test in all cases before the donor is informed. In some cases, it may require the use of two or more different tests to obtain definite conclusions.

Perinatal outcome of very low birthweight infants by mode of delivery

In order to evaluate the influence of mode of delivery on perinatal morbidity and mortality in vertex infants weighing less than 1500 g (VLBW), we made a retrospective study of 152 singleton newborns, in vertex presentation, with a birthweight of less than 1500 g, delivered in the Cruces Hospital (Vizcaya, Spain), a major perinatal referral center, between 1 January 1987 and 31 December 1989. Twins and infants with lethal congenital anomalies or gross intrauterine growth deviations were excluded from the study (n = 71). Of the infants studied (n = 81), 37 were delivered by cesarean section (mean weight 1120 +/- 206 g, range: 680-1495 g) and 44 were delivered vaginally (mean weight 1029 +/- 283 g, range: 530-1475 g). The patients were divided into four groups: Group A: 500-749 g (n = 10); Group B: 750-999 g (n = 21); Group C: 1000-1249 g (n = 27); and Group D: 1250-1499 g (n = 23). The percentages of cesarean sections in each group were 10%, 42%, 66% and 39%, respectively. A comparison within each group of immediate perinatal outcome (Apgar score and umbilical vein cord pH), as well as mortality and sequelae up to 1 year of age did not yield any significant differences between cesarean and vaginal birth. We conclude that cesarean delivery does not appear to offer improved outcome over vaginal delivery in live births without congenital anomalies. For this reason, we believe that fetal weight should not be the only obstetrical variable considered when deciding whether or not to perform a cesarean section in these circumstances.

Leishmania mexicana promastigotes induce cytotoxic T lymphocytes in vivo that do not recognize infected macrophages

The question is addressed whether antigens of Leishmania, a parasite residing in the endosomal compartment of macrophages, can be presented in the context of major histocompatibility complex class I molecules. We used E. coli beta-galactosidase as a model antigen which can be expressed in high levels in L. mexicana promastigotes (L. mexicana-gal). Infection of BALB/c mice with L. mexicana-gal induces beta-galactosidase-specific cytotoxic T cells (CTL), which can be isolated using a beta-galactosidase-expressing mastocytoma line as an antigen-presenting cell. These CTL recognize epitopes of beta-galactosidase in the context of H-2Kd; however, they do not recognize L. mexicana-gal-infected macrophages even after killing of the intracellular amastigotes by drug treatment or macrophage activation by lymphokines, although class I-peptide interaction and the presentation of endogenously produced antigens is normal. It is concluded that parasite antigens can induce a CTL response in vivo but that these CTL cannot recognize infected macrophages because the relevant epitopes cannot gain access to class I molecules. The effect of priming in vivo may be explained by the well-known but ill-understood phenomenon of cross-priming.

Direct binding of peptides to MHC class I molecules on living cells. Analysis at the single cell level

To directly assess the binding of exogenous peptides to cell surface-associated MHC class I molecules at the single cell level, we examined the possibility of combining the use of biotinylated peptide derivatives with an immunofluorescence detection system based on flow cytometry. Various biotinylated derivatives of the adenovirus 5 early region 1A peptide 234-243, an antigenic peptide recognized by CTL in the context of H-2Db, were first screened in functional assays for their ability to bind efficiently to Db molecules on living cells. Suitable peptide derivatives were then tested for their ability to generate positive fluorescence signals upon addition of phycoerythrin-labeled streptavidin to peptide derivative-bearing cells. Strong fluorescent staining of Db-expressing cells was achieved after incubation with a peptide derivative containing a biotin group at the C-terminus. Competition experiments using the unmodified parental peptide as well as unrelated peptides known to bind to Kd, Kb, or Db, respectively, established that binding of the biotinylated peptide to living cells was Db-specific. By using Con A blasts derived from different H-2 congenic mouse strains, it could be shown that the biotinylated peptide bound only to Db among > 20 class I alleles tested. Moreover, binding of the biotinylated peptide to cells expressing the Dbm13 and Dbm14 mutant molecules was drastically reduced compared to Db. Binding of the biotinylated peptide to freshly isolated Db+ cells was readily detectable, allowing direct assessment of the relative amount of peptide bound to distinct lymphocyte subpopulations by three-color flow cytometry. While minor differences between peripheral T and B cells could be documented, thymocytes were found to differ widely in their peptide binding activity. In all cases, these differences correlated positively with the differential expression of Db at the cell surface. Finally, kinetic studies at different temperatures strongly suggested that the biotinylated peptide first associated with Db molecules available constitutively at the cell surface and then with newly arrived Db molecules.

Direct binding of peptides to MHC class I molecules on living cells. Analysis at the single cell level

To directly assess the binding of exogenous peptides to cell surface-associated MHC class I molecules at the single cell level, we examined the possibility of combining the use of biotinylated peptide derivatives with an immunofluorescence detection system based on flow cytometry. Various biotinylated derivatives of the adenovirus 5 early region 1A peptide 234-243, an antigenic peptide recognized by CTL in the context of H-2Db, were first screened in functional assays for their ability to bind efficiently to Db molecules on living cells. Suitable peptide derivatives were then tested for their ability to generate positive fluorescence signals upon addition of phycoerythrin-labeled streptavidin to peptide derivative-bearing cells. Strong fluorescent staining of Db-expressing cells was achieved after incubation with a peptide derivative containing a biotin group at the C-terminus. Competition experiments using the unmodified parental peptide as well as unrelated peptides known to bind to Kd, Kb, or Db, respectively, established that binding of the biotinylated peptide to living cells was Db-specific. By using Con A blasts derived from different H-2 congenic mouse strains, it could be shown that the biotinylated peptide bound only to Db among > 20 class I alleles tested. Moreover, binding of the biotinylated peptide to cells expressing the Dbm13 and Dbm14 mutant molecules was drastically reduced compared to Db. Binding of the biotinylated peptide to freshly isolated Db+ cells was readily detectable, allowing direct assessment of the relative amount of peptide bound to distinct lymphocyte subpopulations by three-color flow cytometry. While minor differences between peripheral T and B cells could be documented, thymocytes were found to differ widely in their peptide binding activity. In all cases, these differences correlated positively with the differential expression of Db at the cell surface. Finally, kinetic studies at different temperatures strongly suggested that the biotinylated peptide first associated with Db molecules available constitutively at the cell surface and then with newly arrived Db molecules.

[Comparative sensitivity of different solid-phase immunoassays for detecting HBsAg and anti-HCV in blood]

The sensitivities of seven methods of enzyme-immunoassay for HBsAg detection, as well as of twelve immunoassays for detecting antibody to HCV, were compared, in an attempt to evaluate the need for an official technical validation of such methods prior to its commercialization in Spain. Significant differences were seen for the sensitivities of these methods, which may influence the proficiency of blood bank screening for HBsAg and anti-HCV for preventing cases of post-transfusional viral hepatitis. As a conclusion, it is recommended to establish legal regulations for pre-marketing validation of such assays in Spain, as well as to take the results obtained in these evaluation studies as the basis for selecting those tests which may be more convenient for screening purposes at the blood banks.

Characterization of two antigenic sites recognized by neutralizing monoclonal antibodies directed against the fusion glycoprotein of human respiratory syncytial virus

Two antigenic sites recognized by neutralizing monoclonal antibodies (MAbs) directed against the fusion (F) glycoprotein of human respiratory syncytial virus were mapped on the primary structure of the protein by (i) the identification of amino acid substitutions selected in antibody-escape mutants and (ii) the reactivity of synthetic peptides with MAbs. The first site contained several overlapping epitopes which were located within the trypsin-resistant amino-terminal third of the large F1 subunit. Only one of these epitopes was faithfully reproduced by a short synthetic peptide; the others might require specific local conformations to react with MAbs. The second antigenic site was located in a trypsin-sensitive domain of the F1 subunit towards the carboxy-terminal end of the cysteine-rich region. One of these epitopes was reproduced by synthetic peptides. In addition, mutagenized F protein with a substitution of serine for arginine at position 429 did not bind MAbs to the second site. These results are discussed in terms of F protein structure and the mechanisms of virus neutralization.

Sensitivity of seven commercial assays in the detection of hepatitis B virus type 2-like infection

Two hundred serum samples taken from patients with hepatitis B virus type 2-like infections were tested with seven commercial enzyme immunoassay methods for detection of HBsAg. Positive results were obtained with all methods in some samples, the rate of detection of HBsAg ranging from 9% to 90% for the different methods. A direct correlation was found between the analytical sensitivity of a method and its ability to yield positive results. It is suggested that these findings should be taken into consideration when selecting HBsAg detection methods in blood banks in order to avoid transmission of the HBV2 agent to recipients of blood or blood products.

Efficient plasma membrane expression of a functional platelet glycoprotein Ib-IX complex requires the presence of its three subunits

The glycoprotein (GP) Ib-IX complex of the platelet plasma membrane mediates the adhesion of platelets to damaged blood vessel wall. The complex is composed of three membrane-spanning polypeptides, GP Ib alpha, GP Ib beta, and GP IX, all of which are absent from the platelets of patients with the hereditary bleeding disorder Bernard-Soulier syndrome. In this study we report stable expression of the recombinant receptor in three cell lines and demonstrate that the three subunits of the complex are necessary for its efficient expression on the plasma membrane. The expressed complex associates with the cytoskeleton of the transfected cells through an interaction with actin-binding protein and binds its ligand, von Willebrand factor. These data suggest that the lack of plasma membrane GP Ib-IX complex in the Bernard-Soulier syndrome could potentially arise from mutations affecting any one of its three subunits.

Polymorphism of human glycoprotein Ib alpha results from a variable number of tandem repeats of a 13-amino acid sequence in the mucin-like macroglycopeptide region. Structure/function implications

Four polymorphic variants of the platelet receptor for von Willebrand factor, glycoprotein Ib, have been described that differ in molecular weight on sodium dodecyl sulfate-polyacrylamide gels (Moroi, M., Jung, S. M., and Yoshida, N. (1984) Blood 64, 622-629). A recent report localized the polymorphic site to the heavily O-glycosylated region of the glycoprotein Ib alpha-chain known as the macroglycopeptide (Meyer, M., and Schellenberg, I. (1990) Thromb. Res. 58, 233-242). This region contains several tandem repeats of a mucin-like sequence, which appeared to be a likely site for polymorphic variation. We amplified genomic DNA corresponding to the macroglycopeptide from 206 individuals from four ethnic groups and identified three length variants based on the migration of the amplified DNA on denaturing polyacrylamide gels. DNA sequencing revealed that the three variants represented four alleles, two of which varied by only one base pair, a difference that did not result in an amino acid change. The three length variants differed in the number of tandem repeats of a 39-base pair sequence that results in perfect duplication of a 13-amino acid sequence that originated within a region flanked by Glu-396 and Thr-411. The smallest isoform contained one such sequence; the next largest, two repeats; and the largest, three repeats. The DNA sequence containing the tandem repeats was flanked by direct repeats typical of the target site duplications found flanking transposed DNA, suggesting a mechanism for acquisition of this region by the primordial glycoprotein Ib alpha precursor. The amino acid sequence of the repeated element that accounts for the polymorphism contained five sites for potential O-glycosylation, which together with the repeated amino acids would result in incremental differences in molecular weight of approximately 6,000 between the different isoforms. The addition of repeats to the macroglycopeptide is predicted to increase the length of this elongated glycosylated region and extend the distance between the ligand-binding domain of glycoprotein Ib and the platelet plasma membrane, an effect that would project the ligand-binding domain farther into the bloodstream. Such a change may alter the susceptibility of platelets to shear-induced activation, a process that requires an interaction between glycoprotein Ib and von Willebrand factor.

The digestive and metabolic utilization of the dietary protein: effect of clenbuterol and protein level

The influence of a beta-agonist, clenbuterol, added to the diet at 0, 30 and 50 mg/Kg was studied on the digestive and metabolic use of the dietary protein in Wistar rats of two different weights (70 and 200 g). The rats had been fed diets containing different protein levels (4, 10 and 15%). Treatment with clenbuterol did not have any effects on the absorption of the dietary protein in any of the studies released. The beta-agonist did not alter fecal nitrogen excretion nor did it change the digestion coefficient of the protein at the doses used. Clenbuterol significantly increased (P less than 0.01) urinary nitrogen excretion in younger animals fed diets containing low levels of protein (4%). On the other hand, in animals of greater age/weight that had been fed diets with a sufficient protein content (15%), the beta-agonist significantly reduced urinary nitrogen excretion (P less than 0.001). From the results of this study, it can be concluded that in order that clenbuterol can significantly increase nitrogen balance and protein retention, the animals must be given a sufficient exogenous protein intake and they must not be at a stage of maximum growth potential.