Crystallization, sequence, and preliminary crystallographic data for an antipeptide Fab 50.1 and peptide complexes with the principal neutralizing determinant of HIV-1 gp120

Prediction of the secondary structure of HIV-1 gp120

The secondary structure of HIV-1 gp120 was predicted using multiple alignment and a combination of two independent methods based on neural network and nearest-neighbor algorithms. The methods agreed on the secondary structure for 80% of the residues in BH10 gp120. Six helices were predicted in HIV strain BH10 gp120, as well as in 27 other HIV-1 strains examined. Two helical segments were predicted in regions displaying profound sequence variation, one in a region suggested to be critical for CD4 binding. The predicted content of helix, beta-strand, and coil was consistent with estimates from Fourier transform infrared spectroscopy. The predicted secondary structure of gp120 compared well with data from NMR analysis of synthetic peptides from the V3 loop and the C4 region. As a first step towards modeling the tertiary structure of gp120, the predicted secondary structure may guide the design of future HIV subunit vaccine candidates.

Secondary structural elements as a basis for antibody recognition in the immunodominant region of human immunodeficiency viruses 1 and 2

Synthetic peptide antigens corresponding to the entire third variable region V3, the principal neutralizing determinant of the human immunodeficiency virus (HIV) envelope glycoprotein of HIV-1 subtype B (1), HIV-2 subtype A (5), and HIV-2 subtype B (7) were synthesized by solid-phase peptide synthesis (Table 1). 1 and 5 were also prepared as their GlcNAc-glycosylated forms at the natural N-glycosylation site NXT (positions 6-8; peptides 4 and 6). Additionally, the proposed beta-turn region of 1 (GPGR; positions 15-18) was altered by introducing D-Ala17 (2) and D-Pro16 (3). All compounds have been studied by two-dimensional NMR techniques. Interproton distances and 3JNH/H alpha coupling constants derived from NMR data are used as restraints in distance geometry and ENSEMBLE-Distance and angle-bound driven dynamics calculations. The stimulations led to disordered conformations except for a high propensity of a beta II-turn in the region GPXR (positions 15-18) in 1, 2, and 4. In 3 (G-D-ProGR, positions 15-18), a type beta I'-turn was mainly found instead. For peptide 7, the consensus sequence of HIV-2 subtype B, a type beta II-turn was also found although the primary structure (VSGL; positions 15-18) differs grossly from the HIV-1 peptide 1. With the exception of 2, all beta II-turns were able to form a canonically opened beta-turn by a 180 degree rotation of phi(G17). Surprisingly, compounds 5 and 6 that are highly similar to 7 showed no beta II-type turn within MSGL (positions 15-18). They form a type beta VIII-turn across the tetrapeptide SGLV (positions 16-19) together with a non-canonical turn conformation across LMSG (positions 14-17) leading to an S-conformation. The reaction of the peptides with HIV-positive sera from patients infected with different subtypes of HIV-1 and HIV-2 was tested in enzyme-linked immunosorbent assays (ELISA reactions). No HIV-2 sera reacted peptide 1 and no HIV-1 sera showed reactivity to peptide 5. We propose that certain amino acid exchanges within the V3 domain lead to altered conformations of the V3 loop resulting in antibodies that show altered binding properties to the peptide antigens used in the ELISA reactions.

Cloning and sequencing of V genes from anti-osteosarcoma monoclonal antibodies TP-1 and TP-3: location of lysine residues and implications for radiolabeling

Monoclonal antibodies TP-1 and TP-3 are of potential utility for the radioimmunodiagnosis of osteosarcoma in both human and canine patients. The V genes of these antibodies were cloned and sequenced and to facilitate radiolabeling of these proteins, the location of the lysine residues within these sequences have been determined. The V-domains of TP-1 contain a total of 12 lysines, 10 in the framework region and 2 in the CDR region, while the V-domains of TP-3 contain a total of 14 lysines, 11 in the framework region and 3 in the CDR regions. Using space-filling models, the availability of each lysine residue for radiolabeling, and potential interference with antigen binding was predicted.

Structural and immunological reactivity of the principal neutralizing determinant V3 of glycoprotein gp120 of HIV-1

The variable domain V3 in the outer glycoprotein gp120 of HIV-1 is a highly important region with respect to immune response during the course of viral infection. Neutralizing antibodies are produced against this domain: in addition, it has been shown to be a functionally active epitope for T helper and cytotoxic T cells. The high degree of amino acid variability in individual HIV-isolates, however, limits the use of the V3-domain in approaches to vaccine development. In order to characterize the residues important for antibody interaction and binding to MHC class I proteins, we constructed a consensus sequence of the V3-domain with broad reactivity [1] and used synthetic peptides derived from this consensus sequence with individual residues altered to alanine. These peptides were used as antigens in ELISA tests to define the amino acids which are important for binding to human and rabbit/anti-peptide immunoglobulins. In addition, we used these alanine-derived peptides in interaction studies with human HLA-A2.1 and mouse H-2Dd by testing their capacity to stabilize the respective MHC class I protein complexes on the surface of mutant cell lines T2 and RMA-S transfected with Dd gene. The experimental tests allowed us to define individual residues involved in antibody and MHC-protein interaction, respectively. In a further approach, we used those results to design interaction models with HLA-A2.1 and H-2Dd. Therefore, a structural model for H-2Dd was built that exhibits an overall similar conformation to the parental crystal structure of HLA-A2.1. The resulting interaction models show V3-peptide bound in an extended beta-conformation with a bulge in its centre for both H-2Dd and HLA-A2.1 complexes. The N- and C-termini of V3 peptide reside in conserved pockets within both MHC-proteins. Anchoring residues could be determined that are crucial for the binding of the respective MHC class I haplotype. The cross-reactivity of V3-peptide in enhancing the expression of two different MHC class I molecules (H-2Dd and HLA-A2.1) is shown to be based on similar peptide binding that induces an almost identical peptide conformation.

Identification and characterization of a TNF alpha antagonist derived from a monoclonal antibody

Peptides derived from the CDRs of the anti-TNF alpha monoclonal antibody Di62 were tested for inhibition of binding of Di62 to TNF alpha as well as of TNF alpha to its 55 and 75 kDa receptor. A peptide derived from the CDR1 of the light chain was shown to specifically inhibit Di62 binding to TNF alpha with markedly higher activity (Ki = 4 microM) than all other CDR-derived peptides. This peptide also significantly inhibited binding of TNF alpha to its 55 and 75 kDa receptor and protected L929 cells from the cytotoxic effect of TNF alpha (IC50 = 6 microM). The C-terminal region of this peptide, which is homologous to the 55 and 75 kDa TNF receptor, was found to be essential for activity.

Major antigen-induced domain rearrangements in an antibody

BACKGROUND

Recent structural results have shown that antibodies use an induced fit mechanism to recognize and bind their antigens. Here we present the crystallographically determined structure of an Fab directed against an HIV-1 peptide (Fab 50.1) in the unliganded state and compare it with the peptide-bound structure. We perform a detailed analysis of the components that contribute to enhanced antigen binding and recognition.

RESULTS

Induced fit of Fab 50.1 to its peptide antigen involves a substantial rearrangement of the third complementarity determining region loop of the heavy chain (H3), as well as a large rotation of the variable heavy (VH) chain relative to the variable light (VL) chain. Analysis of other Fab structures suggests that the extent of the surface area buried at the VL-VH interface correlates with the ability to alter antibody quaternary structure by reorientation of the VL-VH domains.

CONCLUSION

Fab 50.1 exhibits the largest conformational changes yet observed in a single antibody. These can be attributed to the flexibility of the variable region. Comparisons of new data with previous examples lend to the general conclusion that a small VL-VH interface, due in part to a short H3 loop, permits substantial alterations to the antigen-binding pocket. This has major implications for the prediction, engineering and design of antibody-combining sites.

Crystal structure of a human immunodeficiency virus type 1 neutralizing antibody, 50.1, in complex with its V3 loop peptide antigen

The crystal structure of the Fab fragment of a human immunodeficiency virus type 1 (HIV-1) neutralizing monoclonal antibody Fab has been determined at 2.8 A resolution in complex with a linear 16-residue peptide from the third hypervariable region (V3) of gp120. The first 9 residues of the peptide are ordered in the electron density maps, and their conformation is in partial agreement with the beta-strand-type II beta-turn structure predicted for this portion of the V3 loop. Notably, several of the peptide residues that are well conserved among different HIV-1 isolates contact a nonpolar 25-A-long groove in the antibody-combining site. The largely extended structure of the peptide differs from the beta-turns seen as the primary determinants in other published anti-peptide Fab structures. Analysis of the specific Fab-peptide interactions only partially explains the MN isolate specificity shown by this antibody.