Epitope-specific antibody response to murine hepatitis virus-4 (strain JHM)

Monoclonal hybridoma antibodies to the structural proteins of murine hepatitis virus-4, strain JHM (MHV-4) were used in a competition binding enzyme immunoassay to analyze at the epitope level the antibody response of mice after infection with MHV-4. Colonized mice often had pre-existing MHV antibodies directed against epitopes on the E2 glycoprotein, the E1 glycoprotein, and the nucleocapsid protein. These mice generated a secondary antibody response after virus inoculation, reaching peak levels 7 days after infection. In contrast, Nude/+ mice raised in a pathogen-free colony had no detectable circulating MHV antibodies and generated a primary antibody response which gradually increased to peak levels 14 to 28 days after infection. Kinetics of antibody responses against specific epitopes usually correlated well with measured total virus-specific antibody responses, but variation was observed. Mice injected with three antigenically distinct strains of MHV made antibody responses to conserved epitopes but not to an antigenic determinant absent in these strains. Measurement of epitope-specific responses in a polyclonal population of viral specific antibodies is feasible and a valuable adjunct in understanding viral immunity.     

Identification of a functional domain of human granulocyte colony-stimulating factor using neutralizing monoclonal antibodies.

Human granulocyte colony-stimulating factor (G-CSF) is a hemopoietic growth factor that is being used successfully to treat various forms of neutropenia. To define functionally important regions of G-CSF, we have prepared 37 monoclonal anti-G-CSF antibodies and mapped the regions of G-CSF recognized by different antibody groups. Antibodies recognizing similar epitopes were identified by competition assays, neutralization assays, conformation dependence and cross-reactivity with canine G-CSF. Seven of eight neutralizing antibodies fell into two related epitope groups and were conformation-dependent. The eighth was unrelated and conformation-independent. Peptides of G-CSF were generated by chemical or enzymatic digestion and tested for antibody reactivity. One of the neutralizing antibodies (LMM351) recognized a small, disulfide-bonded peptide from the V8 protease digest (residues 34-46). A synthetic peptide (residues 20-58) was recognized by all the neutralizing antibodies, implicating this disulfide-bonded loop in receptor binding. The epitopes recognized by nonneutralizing antibodies were found throughout G-CSF. Thus, regions of G-CSF that are not involved in receptor binding have also been defined. A CNBr peptide (residues 1-121) had greatly reduced biological activity, indicating that the COOH terminus is required for receptor binding. We predict that residues 20-46 and the COOH terminus bind to the G-CSF receptor.     

Relationship of the human immunodeficiency virus type 1 gp120 third variable loop to a component of the CD4 binding site in the fourth conserved region.

Neutralizing antibodies that recognize the human immunodeficiency virus gp120 exterior envelope glycoprotein and are directed against either the third variable (V3) loop or conserved, discontinuous epitopes overlapping the CD4 binding region have been described. Here we report several observations that suggest a structural relationship between the V3 loop and amino acids in the fourth conserved (C4) gp120 region that constitute part of the CD4 binding site and the conserved neutralization epitopes. Treatment of the gp120 glycoprotein with ionic detergents resulted in a V3 loop-dependent masking of both linear C4 epitopes and discontinuous neutralization epitopes overlapping the CD4 binding site. Increased recognition of the native gp120 glycoprotein by an anti-V3 loop monoclonal antibody, 9284, resulted from from single amino acid changes either in the base of the V3 loop or in the gp120 C4 region. These amino acid changes also resulted in increased exposure of conserved epitopes overlapping the CD4 binding region. The replication-competent subset of these mutants exhibited increased sensitivity to neutralization by antibody 9284 and anti-CD4 binding site antibodies. The implied relationship of the V3 loop, which mediates post-receptor binding steps in virus entry, and components of the CD4 binding region may be important for the interaction of these functional gp120 domains and for the observed cooperativity of neutralizing antibodies directed against these regions.     

Localization of epitopes of herpes simplex virus type 1 glycoprotein D.

We previously defined eight groups of monoclonal antibodies which react with distinct epitopes of herpes simplex virus glycoprotein D (gD). One of these, group VII antibody, was shown to react with a type-common continuous epitope within residues 11 to 19 of the mature glycoprotein (residues 36 to 44 of the predicted sequence of gD). In the current investigation, we have localized the sites of binding of two additional antibody groups which recognize continuous epitopes of gD. The use of truncated forms of gD as well as computer predictions of secondary structure and hydrophilicity were instrumental in locating these epitopes and choosing synthetic peptides to mimic their reactivity. Group II antibodies, which are type common, react with an epitope within residues 268 to 287 of the mature glycoprotein (residues 293 to 312 of the predicted sequence). Group V antibodies, which are gD-1 specific, react with an epitope within residues 340 to 356 of the mature protein (residues 365 to 381 of the predicted sequence). Four additional groups of monoclonal antibodies appear to react with discontinuous epitopes of gD-1, since the reactivity of these antibodies was lost when the glycoprotein was denatured by reduction and alkylation. Truncated forms of gD were used to localize these four epitopes to the first 260 amino acids of the mature protein. Competition experiments were used to assess the relative positions of binding of various pairs of monoclonal antibodies. In several cases, when one antibody was bound, there was no interference with the binding of an antibody from another group, indicating that the epitopes were distinct. However, in other cases, there was competition, indicating that these epitopes might share some common amino acids.     

Mapping the antigenic epitopes of human dihydrofolate reductase by systematic synthesis of peptides on solid supports.

All of the 181 possible overlapping hexapeptides as well as 179 octapeptides covering the amino acid sequence of human dihydrofolate reductase (hDHFR) were synthesized on polyethylene supports. The synthetic procedure of Geysen et al. (Geysen, H. M., Rodda, S. J., Mason, T. J., Tribbick, G., and Schoofs, P. G. (1987) J. Immunol. Methods 102, 259-274) was modified to obtain up to 100 nmol of peptide on each pin. Peptides constituting antigenic epitopes on hDHFR were identified by examining the binding of antibodies raised against both native and denatured hDHFR to these peptides by enzyme-linked immunosorbent assay. The peptides bound in a similar pattern to polyclonal antibodies against both native and denatured dihydrofolate reductase (DHFR). Six major epitopes were located corresponding to residues 27-33, 45-51, 67-74, 133-139, 153-158, and 176-181 using both hexapeptides and octapeptides. An additional epitope, constituting residues 14-21, was found by the use of octapeptides. Most of the epitopes are hydrophilic and reside largely in "loop" regions at the boundaries of secondary structural elements of hDHFR. This observation is consistent with our previous results which suggested that ligand binding at the active site of the enzyme can cause a dramatic reduction in antibody binding to DHFR due to conformational constraints in flexible loop regions in various parts of the molecule. The similarity of the immunogenic profiles of native versus denatured hDHFR indicates that the two forms of the antigen share the same amino acid sequence-specific epitopes. Competitive enzyme-linked immunosorbent assay showed that the binding of anti-hDHFR antiserum to both native and denatured hDHFR was inhibited by approximately 30% by the seven antigenic peptides, indicating that a significant proportion of the antibodies elicited by this enzyme is specific for short peptides. Besides revealing the antigenic structure of DHFR our results provide a rational basis for the design of mutant DHFRs to study the importance of loop residues in the conformational dynamics of the enzyme.     

Syngeneic anti-idiotypic monoclonal antibodies against an anti-human chorionic gonadotrophin antibody

Monoclonal antibody designated 1B10 (Mab 1B10) has been shown to be highly specific for the beta-chain of human chorionic gonadotrophin (HCG). We used this antibody to investigate its paratope using anti-idiotypic antibodies. Purified Mab 1B10 has been used to immunize syngeneic BALB/c mice to produce anti-idiotypic monoclonal antibodies. An enzyme immunoassay (ELISA) on Mab 1B10 coated plate was employed to screen the supernatants of growing hybridomas. The specificity of each antibody selected was assessed using an inhibition ELISA and immunoblotting. Monoclonal antibodies belonging to two categories were selected. (a) Those (designated Mab 4F8 and Mab 7G9) recognizing epitopes of the Ig molecule located in/or near the antigen-binding site of Mab 1B10. In ELISA these antibodies were shown to inhibit in a dose-dependent manner, the reaction of Mab 1B10 with its specific antigen; (b) those (Mab 2B8, Mab 3B8) reacting with epitopes located outside of the antigen binding site of the antiHCG antibody molecule and did not influence the reactions of Mab 1B10 and its antigen. Following immunization of syngeneic BALB/c mice monoclonal antibodies (Mab 4F8, Mab 7G9) were produced which recognized epitopes located on the variable region of Mab 1B10 since they did not react with other marine monoclonal antibodies of the same isotype. These antibodies inhibited the binding of Mab 1B10 to its corresponding epitope on the molecule of HCG and they can be defined as syngeneic anti-idiotypic antibodies.     

Identification of amino acids recognized by syncytium-inhibiting and neutralizing monoclonal antibodies to the human parainfluenza type 3 virus fusion protein.

Neutralizing monoclonal antibodies specific for the fusion (F) glycoprotein of human parainfluenza type 3 virus (PIV3) were used to select neutralization-resistant antigenic variants. Sequence analysis of the F genes of the variants indicated that their resistance to antibody binding, antibody-mediated neutralization or to both was a result of specific amino acid substitutions within the neutralization epitopes of the F1 and F2 subunits. Comparison of the locations of PIV3 neutralization epitopes with those of Newcastle disease and Sendai viruses indicated that the antigenic organization of the fusion proteins of paramyxoviruses is similar. Furthermore, some of the PIV3 epitopes recognized by syncytium-inhibiting monoclonal antibodies are located in an F1 cysteine cluster region which corresponds to an area of the measles virus F protein involved in fusion activity.     

Comparison of solution structures of dihydrofolate reductases and enzyme-ligand complexes using cross-reacting antibodies

Polyclonal antibodies against dihydrofolate reductase (DHFR) from the human lymphoblastoid cell line WIL-2/M4 were used as probes to compare the antigenic structures in solution of native DHFRs obtained from a broad range of species and their complexes with substrate, cofactor, and folate antagonist inhibitors. All these antibodies could bind to the denatured human DHFR, indicating that they were specific for the primary structure of this enzyme. Denatured chicken liver and L1210 murine leukemic DHFRs competed for all of the antibodies that bound to the human enzyme, although less effectively than the denatured human enzyme, showing the presence of similar epitopes among the vertebrate enzymes. However, both direct binding and competition experiments showed low antibody cross-reactivities with native chicken liver (8%) and murine (10%) DHFRs, suggesting differences in the disposition of similar epitopes in these enzymes. The lactobacillus casei DHFR showed a low amount (less than 2%) of cross-reactivity with the antibodies while the same antibodies did not cross-react with the Escherichia coli enzyme. DHFR from soybean seedlings competed for a large proportion (70%) of the anti-human DHFR antibodies, indicating a close similarity in the antigenic structures of plant and animal DHFRs. Binary complexes of the L. casei, avian, murine, and human DHFRs with dihydrofolate, methotrexate (MTX), trimethoprim (TMP), NADPH, and NADP+ all showed significantly lower antibody binding capacity as compared with the corresponding free enzymes. Further, these ligands inhibited antibody binding to the enzyme to varying degrees.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polyclonal B cell responses to conserved neutralization epitopes in a subset of HIV-1-infected individuals

A small proportion of HIV-infected individuals generate a neutralizing antibody (NAb) response of exceptional magnitude and breadth. A detailed analysis of the critical epitopes targeted by broadly neutralizing antibodies should help to define optimal targets for vaccine design. HIV-1-infected subjects with potent cross-reactive serum neutralizing antibodies were identified by assaying sera from 308 subjects against a multiclade panel of 12 "tier 2" viruses (4 each of subtypes A, B, and C). Various neutralizing epitope specificities were determined for the top 9 neutralizers, including clade A-, clade B-, clade C-, and clade A/C-infected donors, by using a comprehensive set of assays. In some subjects, neutralization breadth was mediated by two or more antibody specificities. Although antibodies to the gp41 membrane-proximal external region (MPER) were identified in some subjects, the subjects with the greatest neutralization breadth targeted gp120 epitopes, including the CD4 binding site, a glycan-containing quaternary epitope formed by the V2 and V3 loops, or an outer domain epitope containing a glycan at residue N332. The broadly reactive HIV-1 neutralization observed in some subjects is mediated by antibodies targeting several conserved regions on the HIV-1 envelope glycoprotein.     

The molecular nature of Fucus serratus sperm surface antigens recognised by monoclonal antibodies FS1 to FS12

Sperm of the brown alga Fucus serratus are highly differentiated, biflagellate, naked cells. Immunolocalisation studies, employing monoclonal antibodies (MAbs — designated FS1 to FS12) raised against antigens of these sperm cells, have revealed that some sperm surface components are distributed over the entire cell, whereas others are restricted to, or occur preferentially on, the surface of the anterior flagellum or cell body. This report describes the use of these MAbs in Western-blot procedures and antigen-modification binding assays to determine the nature of these sperm surface components. Monoclonal antibodies which bind to antigens found on the cell body and both flagella (FS3, FS4, FS6, FS8, FS10) recognise carbohydrate epitopes of a high-molecular-weight glycoprotein (M_r=205 kDa). These MAbs were initially chosen at random from a much larger number of antibodies which bound to sperm in a similar fashion, indicating that this glycoprotein is an immunodominant antigen. Though these MAbs compete under conditions of limited antigen availability, differences in the effects of periodate on antibody binding and differences in other binding data indicate that the MAbs recognise epitopes of this glycoprotein which are neighbouring or overlapping, rather than common. The MAb FS9, which has a similar binding pattern to the above antibodies, also seems to bind to carbohydrate epitopes, but the antigen recognised by this antibody could not be identified in Western-blotting procedures. The MAbs FS7 and FS12, which bind to the mastigonemes on the anterior flagellum and to the cell body and posterior flagellum, recognise a set of glycoproteins in the molecular-weight range 40–250 kDa. The evidence indicates that the antibodies are binding to N-linked carbohydrate side chains of these glycoproteins. Three MAbs that bind to the anterior flagellum (FS2, FS5 and FS11) recognise protein antigens in the molecular-weight range 90–250 kDa; it is not known whether these antigens are glycosylated. The MAb FS1, which binds primarily to the sperm cell body, could not be used in enzyme-linked immunosorbent assays or Western-blotting procedures and the antigen recognised by this antibody is so far uncharacterised.Abbreviations ELISA enzyme linked immunosorbent assay - HRP-RAMIG horseradish-peroxidase-labelled rabbit anti mouse immunoglobulin - Ig immunoglobulin - kDa kilodalton - MAb monoclonal antibody - M_r relative molecular mass - PBS phosphate-buffered saline - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis We are grateful to AFRC for financial support under the cell signalling initiative.     

Identification of two structurally and functionally distinct sites on human platelet membrane glycoprotein IIb-IIIa using monoclonal antibodies

Platelet membrane glycoprotein IIb-IIIa exists as a calcium-dependent complex of two large peptides (designated IIb and IIIa) in Triton X-100 solutions, but it remains unknown if these peptides are subunits of one glycoprotein or are actually two individual glycoproteins in the intact platelet membrane. We used crossed immunoelectrophoresis to define the epitopes of two monoclonal antibodies to IIb-IIIa, then used these antibodies to study the structural and functional organization of IIb and IIIa in the platelet membrane. Human platelets solubilized in Triton X-100 were electrophoresed through an intermediate gel containing 125I-monoclonal IgG, then into an upper gel containing rabbit anti-human platelet antibodies. Our previously characterized antibody. Tab, and a new monoclonal antibody, T10, both bound to the immunoprecipitate corresponding to the IIb-IIIa complex. When platelets were electrophoresed after solubilization in 5 mM EDTA, 125I-Tab bound to the dissociated IIb polypeptide, but not to IIIa. In contrast, 125-I-T10 did not react with either IIb or IIIa. Thus, Tab recognizes a determinant on IIb, while T10 recognizes a determinant created only after the association of IIb and IIIa. Gel-filtered platelets from six normal donors bound 50,600 +/- 5,600 125I-T10 molecules/platelet and 47,800 +/- 11,200 125I-Tab molecules/platelet, consistent with IIb-IIIa being a heterodimer. 125I-T10 binding was identical in unactivated platelets and platelets stimulated with 10 microM ADP. However, platelets did not aggregate or bind 125I-fibrinogen until ADP was added. T10, but not Tab or nonimmune mouse antibody, inhibited ADP-induced platelet aggregation and 125I-fibrinogen binding. Our findings suggest that IIb and IIIa exist as subunits of a single membrane glycoprotein in unstimulated platelets. Fibrinogen binding appears to require not only the interaction of IIb and IIIa, but also some additional change occurring after platelet activation.     

Topologic mapping of protective and nonprotective epitopes on the variant surface glycoprotein of the WRATat 1 clone of Trypanosoma brucei rhodesiense

Monoclonal antibodies were used in competitive antibody binding assays to define and map epitopes on the variant surface glycoprotein of the WRATat 1 clone of T. b. rhodesiense. By using a panel of 30 WRATat 1-specific monoclonal antibodies, 16 epitopes were defined that fall into four clusters, having 1, 1, 3, and 11 distinct epitopes respectively. All epitopes were easily classified as being 1) exposed uniformly on the surface of the trypanosome, 2) exposed only in the region of the flagellar pocket, or 3) "buried", based on the ability or inability of the monoclonal antibodies to bind living trypanosomes in a fluid phase immunofluorescence assay. Monoclonal antibodies that bind exposed surface epitopes are protective, whereas only three of seven that bind exclusively to flagellar pocket epitopes are protective. None of the nine monoclonal antibodies that recognize buried epitopes are protective. Also, antibody-mediated immunity to WRATat 1 trypanosomes is not associated with any particular subclass of antibody. The IgM, IgG1, IgG2a, IgG2b, IgG3, and IgA subclasses each contain examples of protective monoclonal antibodies.     

Analysis of the interaction of the human immunodeficiency virus type 1 gp120 envelope glycoprotein with the gp41 transmembrane glycoprotein.

The human immunodeficiency virus type 1 (HIV-1) gp120 exterior envelope glycoprotein interacts with the viral receptor (CD4) and with the gp41 transmembrane envelope glycoprotein. To study the interaction of the gp120 and gp41 envelope glycoproteins, we compared the abilities of anti-gp120 monoclonal antibodies to bind soluble gp120 and a soluble glycoprotein, sgp140, that contains gp120 and gp41 exterior domains. The occlusion or alteration of a subset of gp120 epitopes on the latter molecule allowed the definition of a gp41 "footprint" on the gp120 antibody competition map. The occlusion of these epitopes on the sgp140 glycoprotein was decreased by the binding of soluble CD4. The gp120 epitopes implicated in the interaction with the gp41 ectodomain were disrupted by deletions of the first (C1) and fifth (C5) conserved gp120 regions. These deletions did not affect the integrity of the discontinuous binding sites for CD4 and neutralizing monoclonal antibodies. Thus, the gp41 interface on the HIV-1 gp120 glycoprotein, which elicits nonneutralizing antibodies, can be removed while retaining immunologically desirable gp120 structures.     

Analysis of a highly flexible conformational immunogenic domain a in hepatitis C virus E2

Hepatitis C (HCV) E2 glycoprotein is involved in virus attachment and entry, and its structural organization is largely unknown. Characterization of a panel of human monoclonal antibodies (HMAbs) to HCV by competition studies has led to an immunogenic organization model of E2 with three domains designated A, B, and C and epitopes in each domain having similar structural and functional properties. Domain A contains nonneutralizing epitopes, and domains B and C contain neutralizing epitopes. The isolation and characterization of three new HMAbs within domain A for a total of six provide support for this model. All six domain A HMAbs do not neutralize HCV retroviral pseudotype particle (HCVpp) infection on Huh-7 cells, and all six HMAbs have similar binding affinity and maximum binding, B(max), a relative indicator of epitope density, as other neutralizing HMAbs, suggesting that neutralization is epitope specific and not by binding to any surface epitope. The dose-dependent neutralizing activity of CBH-7, an HMAb to a domain C epitope in spatial proximity to domain A, and of CBH-5, a domain B HMAb to a more distant epitope, were tested in the presence and absence of each domain A HMAb. No enhancement or reduction in CBH-7 or CBH-5 neutralizing activity was observed, indicating that the potential induction of nonneutralizing antibodies should not be a central issue for HCV vaccine design. To assess whether domain A is involved in the structural changes as part of a pH-dependent virus envelope fusion process, changes in antibody binding patterns to normal pH and acid pH-treated HCVpp were measured. Antibody binding affinity of HMAbs to HCVpp was not affected by low pH. However, the B(max) values for low-pH-treated HCVpp with antibodies to domain A increased 46%, for domain C (CBH-7) they increased 23%, and for domain B (CBH-5) there was a decrease of 12%. Collectively, the organization and function of HCV E2 antigenic domains are roughly analogous to the large envelope glycoprotein E organizational structure for other flaviviruses with three distinct structural and functional domains.     

Recognition of two Dermatophagoides pteronyssinus-specific epitopes on antigen P1 by using monoclonal antibodies: binding to each epitope can be inhibited by serum from dust mite-allergic patients

Monoclonal antibodies were raised against Antigen P1, the major allergen of the house dust mite (Dermatophagoides pteronyssinus). The majority were Antigen P1 specific, isotype IgG1, and did not react with a comparable D. farinae allergen. These antibodies bound 38 to 50% of 125I Antigen P1 in antigen-binding assays (titer greater than or equal to 1/1,000,000), and the quantities of IgG antibody in ascites were 2 to 4 logs greater than those in polyclonal mouse antiserum or in serum from a mite-allergic patient. Two IgM antibodies showed weak binding to Antigen P1 but reacted strongly with D. pteronyssinus in enzyme immunoassay (titer greater than or equal to 1/100,000). Assessments of the specificity of the IgG antibodies by using two inhibition radioimmunoassays suggested that they were directed against two different epitopes. Antibodies 10B9 F6 and 5H8 C12 were purified by preparative isoelectric focusing (isoelectric points of pI 6.25 and 7.4, respectively) and radiolabeled with 125I. Cross-inhibition experiments, using ascites dilutions to inhibit binding of each radiolabeled antibody to Antigen P1, confirmed that these antibodies recognized two distinct epitopes. Analysis of antibodies from 39 clones/hybrids showed that the majority were directed against the same epitopes as either 10B9 F6 or 5H8 C12 (3 out of 39 [8%] and 29 out of 39 [74%], respectively). None of the monoclonal antibodies significantly inhibited (greater than 10%) human IgE binding to Antigen P1 in the radioallergosorbent test. However, 12 of 14 sera from mite allergic patients inhibited binding by the monoclonal antibodies. One serum from a mite-allergic patient inhibited binding of both 10B9 F6 and 5H8 C12 by greater than 85% and showed parallel inhibition curves. The results suggest that these monoclonal antibodies could be used to assay Antigen P1 in both D. pteronyssinus and house dust extracts. It should also be possible to use monoclonal antibodies in inhibition assays to define the antigenic/allergenic determinants recognized by human IgG and IgE antibodies on this mite allergen.     

Neutralization epitopes of the F glycoprotein of respiratory syncytial virus: effect of mutation upon fusion function.

Eighteen neutralizing monoclonal antibodies (MAbs) specific for the fusion glycoprotein of the A2 strain of respiratory syncytial virus (RSV) were used to construct a detailed topological and operational map of epitopes involved in neutralization and fusion. Competitive binding assays identified three nonoverlapping antigenic sites (A, B, and C) and one bridge site (AB). Thirteen MAb-resistant mutants (MARMs) were selected, and the neutralization patterns of the MAbs with either MARMs or RSV clinical strains identified a minimum of 16 epitopes. MARMs selected with antibodies to six of the site A and AB epitopes displayed a small-plaque phenotype, which is consistent with an alteration in a biologically active region of the F molecule. Analysis of MARMs also indicated that these neutralization epitopes occupy topographically distinct but conformationally interdependent regions with unique biological and immunological properties. Antigenic variation in F epitopes was examined by using 23 clinical isolates (18 subgroup A and 5 subgroup B) in cross-neutralization assays with the 18 anti-F MAbs. This analysis identified constant, variable, and hypervariable regions on the molecule and indicated that antigenic variation in the neutralization epitopes of the RSV F glycoprotein is the result of a noncumulative genetic heterogeneity. Of the 16 eptiopes, 8 were conserved on all or all but 1 of 23 subgroup A or subgroup B clinical isolates.     

CD4-Induced Conformational Changes in the Human Immunodeficiency Virus Type 1 gp120 Glycoprotein: Consequences for Virus Entry and Neutralization

Human immunodeficiency virus type 1 (HIV-1) entry into target cells involves sequential binding of the gp120 exterior envelope glycoprotein to CD4 and to specific chemokine receptors. Soluble CD4 (sCD4) is thought to mimic membrane-anchored CD4, and its binding alters the conformation of the HIV-1 envelope glycoproteins. Two cross-competing monoclonal antibodies, 17b and CG10, that recognize CD4-inducible gp120 epitopes and that block gp120-chemokine receptor binding were used to investigate the nature and functional significance of gp120 conformational changes initiated by CD4 binding. Envelope glycoproteins derived from both T-cell line-adapted and primary HIV-1 isolates exhibited increased binding of the 17b antibody in the presence of sCD4. CD4-induced exposure of the 17b epitope on the oligomeric envelope glycoprotein complex occurred over a wide range of temperatures and involved movement of the gp120 V1/V2 variable loops. Amino acid changes that reduced the efficiency of 17b epitope exposure following CD4 binding invariably compromised the ability of the HIV-1 envelope glycoproteins to form syncytia or to support virus entry. Comparison of the CD4 dependence and neutralization efficiencies of the 17b and CG10 antibodies suggested that the epitopes for these antibodies are minimally accessible following attachment of gp120 to cell surface CD4. These results underscore the functional importance of these CD4-induced changes in gp120 conformation and illustrate viral strategies for sequestering chemokine receptor-binding regions from the humoral immune response.     

The biologic activity of anti-T cell receptor V region monoclonal antibodies is determined by the epitope recognized

We have used 10 independently isolated mAb reactive with the Ag R on a cloned Th cell line to map three distinct epitopes and three subepitopes on the R. One of these epitopes is clearly on the V beta 8 region, as it is defined by the antibodies KJ-16 and F23.1, known to react with the V beta 8 family of variable regions, and a functional rearranged V beta 8 gene has been cloned from this cell line. Antibodies directed at a second epitope, believed to be on V alpha because it is unaffected by anti-V beta antibodies, are completely inhibited from binding by monoclonal anti-CD3 epsilon-chain antibodies. Because the cloned Th cell line used, D10.G4.1, responds to soluble monoclonal anti-TCR antibodies, it has been possible to compare the binding of anti-R antibodies with their ability to activate this cloned T cell line. We find that for antibodies all specific for the same or a closely related epitope, activation is proportional to binding, by using antibodies that differ by greater than 100-fold in avidity for the R. By contrast, antibodies directed at different epitopes on the R differ markedly in their ability to activate the D10.G4.1 cell line. We have tested whether these differences reflect differences in the orientation of cross-linking the TCR or possible conformational changes induced in the R by the antibodies, and our data support the latter hypothesis as an explanation for the differences in activation potency between antibodies.     

The naive repertoire of human T helper cells specific for gp120, the envelope glycoprotein of HIV.

The envelope glycoprotein of HIV gp120 is a T cell Ag in experimental animals and in humans infected with HIV or deliberately immunized with gp120 in various forms. Inasmuch as T cell responses result from the interaction of Ag processed and presented by APC with the unprimed T cell repertoire, we have investigated the human T cell repertoire specific for gp120 in seronegative, normal individuals. T cell lines and clones specific for HIV gp120 were generated by repeated in vitro stimulation of peripheral blood T lymphocytes with gp120-pulsed APC, followed by IL-2 expansion. We observed that the T cell response to whole gp120 involved single restricted immunodominant epitopes in gp120 that differ between responding individuals. Focusing of the response to limited regions of gp120 when the whole Ag is used for priming suggests that one or more adjacent epitopes are immunodominant and mask responses to "immunorecessive" epitopes. We have been able to generate primary in vitro responses to recessive epitopes by stimulation in vitro with synthetic peptides of gp120. The results indicate that a much broader T repertoire can be detected when individual peptides are used for priming in vitro rather than gp120. This information has important implications for the development of vaccination protocols aimed at eliciting diverse immune responses to "immunorecessive" regions of envelope glycoprotein.     

Monoclonal antibodies that bind to distinct epitopes on Fc gamma RI are able to trigger receptor function

The human monocyte and macrophage Fc receptor that binds human IgG with high affinity is a surface glycoprotein with a relative molecular mass of approximately 70 kDa. This receptor (Fc gamma RI) has been partially characterized using mAb 32 which binds outside the Fc binding domain of the receptor, but nonetheless triggers Fc receptor-dependent functions. In this study, we describe the properties of four new antibodies with specificity for Fc gamma RI. Based on additivity and cross-blocking studies, we conclude that two of these antibodies (mAb 22 and 44) define a third epitope which is distinct from the binding sites for both mAb 32 and the Fc portion of human IgG. Each Fc gamma RI-specific hybridoma was selected for stable sublines expressing high levels of mAb on the cell surface, and then tested for the ability of this surface mAb to trigger antibody-dependent cell cytotoxicity. All sublines were killed by human monocytes when used as targets in a 51Cr-release assay, whereas hybridomas specific for myeloid Ag other than Fc gamma RI were not killed. We conclude that Fc receptor function is triggered through binding to each of the three epitopes of Fc gamma RI that we have defined. These mAb will be useful for additional characterization of Fc gamma RI, and may, when incorporated into tumor-directed heteroantibodies, enhance tumor cell killing by human monocytes and macrophages.