Bivalency and epitope specificity of a high-affinity IgG3 monoclonal antibody to the Streptococcus group A carbohydrate antigen. Molecular modeling of a Fv fragment.

The binding of Strep 9, a mouse monoclonal antibody (mAb) of the IgG3 subclass directed against the cell-wall polysaccharide of Group A Streptococcus (GAS), has been characterized. The intact antibody and proteolytic fragments of Strep 9 bind differently to GAS: the intact mAb and F(ab)2' have greater affinity for the carbohydrate epitope than the monomeric Fab or F(ab)'. A mode of binding in which Strep 9 binds bivalently to portions of the polysaccharide on adjacent chains on GAS is proposed. A competitive ELISA protocol using a panel of carbohydrate inhibitors shows that the branched trisaccharide, beta-D-GlcpNAc-(1-->3)-[alpha-L-Rhap-(1-->2)]-alpha-L-Rhap, and an extended surface are key components of the epitope recognized by Strep 9. Microcalorimetry measurements with the mAb and two synthetic haptens, a tetrasaccharide and a hexasaccharide, show enthalpy-entropy compensation as seen in other oligosaccharide-protein interactions. Molecular modeling of the antibody variable region by homology modeling techniques indicates a groove-shaped combining site that can readily accommodate extended surfaces. Visual docking of an oligosaccharide corresponding to the cell-wall polysaccharide into the site provides a putative model for the complex, in which a heptasaccharide unit occupies the site and the GlcpNAc residues of two adjacent branched trisaccharide units occupy binding pockets within the groove-shaped binding site.

Potent suppression of HIV-1 replication in humans by T-20, a peptide inhibitor of gp41-mediated virus entry.

T-20, a synthetic peptide corresponding to a region of the transmembrane subunit of the HIV-1 envelope protein, blocks cell fusion and viral entry at concentrations of less than 2 ng/ml in vitro. We administered intravenous T-20 (monotherapy) for 14 days to sixteen HIV-infected adults in four dose groups (3, 10, 30 and 100 mg twice daily). There were significant, dose-related declines in plasma HIV RNA in all subjects who received higher dose levels. All four subjects receiving 100 mg twice daily had a decline in plasma HIV RNA to less than 500 copies/ml, by bDNA assay. A sensitive RT-PCR assay (detection threshold 40 copies/ml) demonstrated that, although undetectable levels were not achieved in the 14-day dosing period, there was a 1.96 log10 median decline in plasma HIV RNA in these subjects. This study provides proof-of-concept that viral entry can be successfully blocked in vivo. Short-term administration of T-20 seems safe and provides potent inhibition of HIV replication comparable to anti-retroviral regimens approved at present.

Quantification of lipoprotein(a) particles containing various apolipoprotein(a) isoforms by a monoclonal anti-apo(a) capture antibody and a polyclonal anti-apolipoprotein B detection antibody sandwich enzyme immunoassay.

A quantitative sandwich ELISA for lipoprotein(a) [Lp(a)], utilizing a monoclonal capture antibody that recognizes human and rhesus monkey apolipoprotein(a) [apo(a)] isoforms in combination with a polyclonal anti-apolipoprotein B-peroxidase conjugate was developed. This assay generates a linear calibration curve from 31.2 to 1000 mg/L, is highly reproducible (intra- and interassay CV of < 5% and < or = 12%, respectively), and shows no interference from plasminogen (1 g/L), low-density lipoprotein (6.00 g/L), triglycerides (27.00 g/L from chylomicrons and 10.00 g/L from very-low-density lipoprotein), hemoglobin (5 g/L), or bilirubin (30 mg/L). This assay format quantifies the concentration of Lp(a) on an equal molar basis regardless of apo(a) isoform. In contrast, a commercially available ELISA [Macra Lp(a)] method with a monoclonal anti-apo(a) capture antibody and a polyclonal anti-apo(a) conjugate was found to underestimate the Lp(a) concentrations of individuals with lower-M(r) apo(a) isoforms--whether quantifying the Lp(a) in plasma or the purified lipoprotein. This demonstrates the importance of assay format selection in quantifying Lp(a).

Tromantadine inhibits a late step in herpes simplex virus type 1 replication and syncytium formation.

Addition of tromantadine after virus penetration inhibited HSV-1 induced syncytium formation and virus production in HEp-2 and VERO cells and acted additively with neutralizing antibody in blocking virus spread and cytopathology. Inhibition of syncytium formation in VERO cells infected with 0.01 pfu/cell of HSV-1 GC+ was observed at a concentration greater than 25 micrograms/ml. The extent of inhibition was dependent upon the multiplicity of infection and cell type. Tromantadine inhibited a late event in HSV-1 replication which appeared to be sensitive to cycloheximide. Reversal of the inhibitory effect of tromantadine on syncytium formation required new protein synthesis. HSV-1 gB, gC, and gD were synthesized in the presence of tromantadine and could be detected on the cell surface by immunofluorescence. Tromantadine most likely inhibits a cellular process that is required for syncytium formation, such as glycoprotein processing, which occurs after the synthesis of the fusion protein but before its expression on the cell surface.

Mild acidic pH inhibition of the major pathway of herpes simplex virus entry into HEp-2 cells.

Penetration of the KOS strain of herpes simplex virus type 1 (HSV-1) and the MS and 333 strains of herpes simplex virus type 2 (HSV-2) into HEp-2 cells at pH 6.3 was at least 100-fold less efficient than at pH 7.4. Penetration of two low passage clinical isolates was completely blocked at pH 6.3. The syncytium-forming HSV-1 strains GC and MP were less sensitive than KOS to the mild acidic conditions. The inhibition was completely reversed upon neutralization of the medium. Penetration was assayed by plaque production following protection from acid inactivation upon virus entry. Penetration of HSV-1 KOS into Vero and HEL diploid fibroblast cells was similarly inhibited. HSV-1 KOS grown in 2-deoxy-D-glucose and monensin was also extensively inhibited at pH 6.3 but virus grown in 2-deoxy-D-glucose penetrated more slowly than normal virus at pH 7.4. Electron microscopy of HSV-1 KOS infection indicated that fusion and endocytosis occur at both pH 7.4 and 6.3 but that fusion predominates at pH 7.4 and endocytosis predominates at pH 6.3. These results indicate that fusion at the plasma membrane is the major route of productive entry for HSV, that strains of HSV can differ in their pH dependence for penetration and this may determine whether virus infection can occur following endocytic uptake.