Presence of a generalist entomopathogenic fungus influences the oviposition behaviour of an aphid-specific predator

The predator Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae) and the generalist entomopathogenic fungus Metarhizium brunneum Petch (Hypocreales: Clavicipitaceae) are effective biological control agents that can co-occur in pest management programmes. We exposed larvae to M. brunneum, on leaves and in soil, to explore possible outcomes of combining the two natural enemies. The number of emerging adults and their longevity were negatively affected by fungus in the soil. When exposed on leaves, adult emergence was unaffected but longevity was reduced. In choice experiments, females chose to lay eggs on fungus-free leaves: more control leaves had eggs and there were more eggs per leaf in the control. In no-choice experiments, the same frequency of females laid eggs in the treatment and control, but significantly more eggs were laid on the control leaves. Gravid A. aphidimyza can perceive M. brunneum and respond by choosing fungus-free oviposition sites, thus reducing contact between them.     

Significant conformational changes in an antigenic carbohydrate epitope upon binding to a monoclonal antibody.

Transferred nuclear Overhauser enhancement spectroscopy (TRNOE) was used to observe changes in a ligand's conformation upon binding to its specific antibody. The ligands studied were methyl O-beta-D-galactopyranosyl(1----6)-4-deoxy-4-fluoro-beta-D-galactopyra nos ide (me4FGal2) and its selectively deuteriated analogue, methyl O-beta-D-galactopyranosyl(1----6)-4-deoxy-2-deuterio-4-fluoro-beta -D- galactopyranoside (me4F2dGal2). The monoclonal antibody was mouse IgA X24. The solution conformation of the free ligand me4F2dGal2 was inferred from measurements of vicinal 1H-1H coupling constants, long-range 1H-13C coupling constants, and NOE cross-peak intensities. For free ligand, both galactosyl residues adopt a regular chair conformation, but the NMR spectra are incompatible with a single unique conformation of the glycosidic linkage. Analysis of 1H-1H and 1H-13C constants indicates that the major conformer has an extended conformation: phi = -120 degrees; psi = 180 degrees; and omega = 75 degrees. TRNOE measurements on me4FGal2 and me4F2dGal2 in the presence of the specific antibody indicate that the pyranose ring pucker of each galactose ring remains unchanged, but rotations about the glycosidic linkage occur upon binding to X24. Computer calculations indicate that there are two sets of torsion angles that satisfy the observed NMR constraints, namely, phi = -152 +/- 9 degrees; psi = -128 +/- 7 degrees; and omega = -158 +/- 6 degrees; and a conformer with phi = -53 +/- 6 degrees; psi = 154 +/- 10 degrees; and omega = -173 +/- 6 degrees. Neither conformation is similar to any of the observed conformations of the free disaccharide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of a common enterobacterial flagellin epitope with a monoclonal antibody.

A monoclonal antibody (mAb), designated 15D8, was produced from BALB/c splenocytes of mice injected with Escherichia coli flagella. ELISA of motile cells, non-motile cells and partially purified flagellin proteins showed that the mAb reacted specifically with flagella of E. coli and with other members of the family Enterobacteriaceae. Western immunoblot analyses of enterobacterial flagella or cell extracts demonstrated that the antibody reacted with a single protein species in the extracts which was identical in size to purified flagellin. The antigenic determinant for this antibody appears to be surface exposed and linear in configuration, since the antibody reacted with native flagella and flagella which had been denatured. This antibody was also used to demonstrate that although the flagella proteins are heterogeneous in size, at least one epitope is highly conserved.     

Elucidation of the critical epitope of an anti-EGFR monoclonal antibody EMab-134

The epidermal growth factor receptor (EGFR) is a type-1 transmembrane receptor tyrosine kinase, which activates the downstream signaling cascades in many tumors, such as oral and lung cancers. We previously developed EMab-134, a novel anti-EGFR monoclonal antibody (mAb), which reacts with endogenous EGFR-expressing cancer cell lines and normal cells independent of glycosylation in Western blotting, flow cytometry, and immunohistochemical analysis. EMab-134 showed very high sensitivity (94.7%) to oral squamous cell carcinomas in immunohistochemical analysis. In this study, we performed enzyme-linked immunosorbent assay (ELISA), flow cytometry, and immunohistochemical analysis to determine the epitope of EMab-134. A blocking peptide (375–394 amino acids of EGFR) neutralized the EMab-134 reaction against oral cancer cells in flow cytometry and immunohistochemistry. The minimum epitope of EMab-134 was found to be the _377-RGDSFTHTPP_?386 sequence. Our findings can be applied for the production of more functional anti-EGFR mAbs that in turn can be used for antitumor treatments.     

Nonisotopic detection of RNA in an enzyme immunoassay using a monoclonal antibody against DNA-RNA hybrids

A sensitive nonisotopic solution hybridization assay for detection of RNA is described and characterized using a pSP65 plasmid model system. The assay procedure is based on a hybridization reaction in solution between a biotinylated DNA probe and a target RNA. The biotin-labeled hybrids are captured on a microtiter plate coated with an antibody to biotin. Bound DNA-RNA hybrids are detected by an immunoreaction with an enzyme-labeled monoclonal antibody specifically directed against DNA-RNA heteropolymers and the hybrids are quantitatively measured with the addition of a fluorogenic substrate. Optimal conditions under which to perform the assay were hybridization time, 1000 min; temperature, 75 degrees C; probe concentration, 0.2 microgram/ml; extent of probe biotinylation, 6.7%; buffer stringency, 2x SSC. A bisulfite-modified DNA probe was compared to nick-translated probes synthesized with reporter groups of different lengths (bio-11-dUTP or bio-19-dUTP). All probes could detect 10 pg/ml of target RNA. The presence of nonhomologous DNA or RNA sequences reduced the sensitivity of RNA detection by one half-log to 32 pg/ml (1.6 pg/assay).     

The aminoterminal domain of alpha-tubulin probed by monoclonal antibodies. Recognition of a rarely exposed epitope by the monoclonal antibody 111 B52 C2

The production and identification of a monoclonal antibody, 111 B52 C2, raised against fragments obtained after limited proteolysis of purified tubulin is described. The recognized epitope is located on the aminoterminal domain of the alpha-tubulin subunit and differs from the antigenic sites reacting with the presently existing panel of available monoclonal antibodies. This monoclonal antibody thus constitutes a potentially useful tool to explore interactions between tubulin and other specific ligands.     

Mapping the antigenic epitope for a monoclonal antibody against lysozyme

A monoclonal antibody (HyHEL-5), prepared to chicken lysozyme c by the method of K?hler and Milstein, identified an antigenic site (epitope) that was shared by the lysozymes of seven different species of galliform birds. The lysozymes of two galliform species, bobwhite quail and chachalaca, shared only partial antigenic identity with the epitope defined by this antibody. Duck lysozyme did not react with the antibody at all. Amino acids that determined the epitope structure were tentatively identified by comparing the amino acid sequences of these lysozymes and assuming the antigenic changes produced by evolutionary substitutions are not due to long-range conformational changes. Arg 68 was identified as a determining amino acid. Arg 68 is hydrogen-bonded to Arg 45, and together these two amino acids form a basic cluster that may be a subsite of the epitope. The antibody inhibited lysis of Micrococcus lysodeikticus by chicken lysozyme. Additionally, Biebrich Scarlet, a dye that binds to the catalytic site, inhibited antibody binding to this lysozyme, which indicates that the epitope extends into the cleft region between Arg 45 and Arg 114. The epitope was hypothesized to involve a region measuring at least 13 x 6 x 15 A including the Arg 68-Arg 45 complex that borders the enzymatic catalytic site. Four other monoclonal antibodies to lysozyme have been partially characterized; each had a distinct pattern of binding specificity for various species of bird lysozymes.     

Structural analysis of rod GTP-binding protein, Gt. Limited proteolytic digestion pattern of Gt with four proteases defines monoclonal antibody epitope.

The epitope of monoclonal antibody (mAb 4A), which recognizes the alpha subunit of the rod G protein, Gt, has been suggested to be both at the carboxyl terminus (Deretic, D., and Hamm, H.E. (1987) J. Biol. Chem. 262, 10839-10847) and the amino terminus (Navon, S.E., and Fung, B.K.-K. (1988) J. Biol. Chem. 263, 489-496) of the molecule. To characterize further the mAb 4A binding site on alpha t and to resolve the discrepancy between these results limited proteolytic digestion of Gt or alpha t using four proteases with different substrate specificities has been performed. Endoproteinase Arg-C, which cleaves the peptide bond at the carboxylic side of arginine residues, cleaved the majority of alpha t into two fragments of 34 and 5 kDa. The alpha t 34-kDa fragment in the holoprotein, but not alpha t-guanosine 5'-O-(3-thiotriphosphate), was converted further to a 23-kDa fragment. A small fraction of alpha t-GDP was cleaved into 23- and 15-kDa fragments. Endoproteinase Lys-C, which selectively cleaves at lysine residues, progressively removed 17 and then 8 residues from the amino terminus, forming 38- and 36-kDa fragments. Staphylococcus aureus V8 protease is known to remove 21 amino acid residues from the amino-terminal region of alpha t, with the formation of a 38-kDa fragment. L-1-Tosylamido-2-phenylethyl chloromethyl ketone-treated trypsin cleaved alpha t progressively into fragments of known amino acid sequences (38, then 32 and 5, then 21 and 12 kDa) and a transient 34 kDa fragment. The binding of mAb 4A to proteolytic fragments was analyzed by Western blot and immunoprecipitation. The major fragments recognized by mAb 4A on Western blots were the 34- and 23-kDa fragments obtained by endoproteinase Arg-C and tryptic digestion. Under conditions that allowed sequencing of the 15- and 5-kDa fragments neither the 34- nor the 23-kDa fragments could be sequenced by Edman degradation, indicating that they contained a blocked amino terminus. The smallest fragment that retained mAb 4A binding was the 23-kDa fragment containing Met1 to Arg204. Thus the main portion of the mAb 4A antigenic site was located within this fragment, indicating that the carboxyl-terminal residues from Lys205 to Phe350 were not required for recognition by the antibody. Additionally, the antibody did not bind the 38- and 36-kDa or other fragments containing the carboxyl terminus, showing that the amino-terminal residues from Met1 to Lys17 were essential for antibody binding to alpha t.     

Cryo-EM visualization of an exposed RGD epitope on adenovirus that escapes antibody neutralization.

Interaction of the adenovirus penton base protein with alpha v integrins promotes virus entry into host cells. The location of the integrin binding sequence Arg-Gly-Asp (RGD) on human type 2 adenovirus (Ad2) was visualized by cryo-electron microscopy (cryo-EM) and image reconstruction using a mAb (DAV-1) which recognizes a linear epitope, IRGDTFATR. The sites for DAV-1 binding corresponded to the weak density above each of the five 22 A protrusions on the adenovirus penton base protein. Modeling of a Fab fragment crystal structure into the adenovirus-Fab cryo-EM density indicated a large amplitude of motion for the Fab and the RGD epitope. An unexpected finding was that Fab fragments, but not IgG antibody molecules, inhibited adenovirus infection. Steric hindrance from the adenovirus fiber and a few bound IgG molecules, as well as epitope mobility, most likely prevent binding of IgG antibodies to all five RGD sites on the penton base protein within the intact virus. These studies indicate that the structure of the adenovirus particle facilitates interaction with cell integrins, whilst restricting binding of potentially neutralizing antibodies.     

Purification of RNA using an anti-RNA monoclonal antibody.

Studies of the synthesis and modification of RNA employ many types of in vitro reactions. Often, the RNA product must be concentrated or purified away from other reaction components such as salts, unincorporated nucleotides, protein, or DNA. Here I describe an immunological approach suitable for the isolation of RNA from in vitro reactions. A variety of RNAs of differing size and nucleotide sequence were immunoprecipitated with a monoclonal antibody specific for RNA. RNA binding took place in seconds with nearly quantitative recoveries. Immunoprecipitation was more efficient than ethanol precipitation in removing unincorporated nucleotides. Proteins which do not bind to RNA remained soluble. The immunoprecipitated RNA sample was solubilized directly with a buffered solution suitable for gel electrophoresis under denaturing conditions. Thus, RNAs can be rapidly concentrated for electrophoresis in a single step. Antibody-RNA binding was reversible under nondenaturing conditions in the presence of excess rRNA. This procedure serves as a novel means of purifying RNA and RNA-binding proteins from in vitro reactions.     

Localization on the mitochondrial F1 ATPase alpha subunit of an epitope masked in the membrane-bound enzyme using a monoclonal antibody and synthetic peptides.

The epitope of the monoclonal antibody 20D6 was localized by N-terminal sequencing of the smallest immunoreactive peptides obtained after CNBr and trypsin cleavage of the F1 alpha subunit of the mitochondrial ATPase/ATP synthase. Immunochemical analysis of overlapping synthetic octapeptides, covering the immunoreactive peptide sequence, has defined the seven-amino-acid sequence recognized by 20D6 as 84EGDIVKR90. The binding of 20D6 was lost after substituting either I87 by K or S, or R90 by C or A as it occurs in the alpha subunit sequence of Escherichia coli or chloroplast ATPase, respectively. This explained the lack of immunoreactivity of 20D6 to these species and indicated the importance of charged as well as hydrophobic residues in the epitope. Immunochemical analysis of synthetic peptides by polyclonal anti-F1 antisera showed that this region is highly immunodominant. In a competitive ELISA, the monoclonal antibody bound with similar affinity to F1 in the presence and absence of substrate as well as to cold dissociated F1, indicating that the epitope was located on the surface of the alpha subunit and not buried between F1 subunits. The lack of binding of 20D6 when F1 is bound to the membrane showed that the epitope exposed at the surface of purified soluble F1 became masked after binding to the membrane. This suggests that it is located at the interface between F1 and the membrane.     

Mapping of an epitope recognized by a neutralizing monoclonal antibody specific to toxin Cn2 from the scorpion Centruroides noxius, using discontinuous synthetic peptides.

The Na+-channel-affecting toxin Cn2 represents the major and one of the most toxic components of the venom of the Mexican scorpion Centruroides noxius Hoffmann. A monoclonal antibody BCF2 raised against Cn2 has been shown previously to be able to neutralize the toxic effect of Cn2 and of the whole venom of C. noxius. In the present study the epitope was mapped to a surface region comprising the N- and C-terminal segments of Cn2, using continuous and discontinuous synthetic peptides, designed on the basis of the sequence and a three-dimensional model of Cn2. The study of peptides of varying length resulted in the identification of segments 5-14 and 56-65 containing residues essential for recognition by BCF2. The peptide (abbreviated SP7) with the highest affinity to BCF2 (IC50 = 5.1 microM) was a synthetic heterodimer comprising the amino acid sequence from position 3-15 (amidated) of Cn2, bridged by disulfide to peptide from position 54-66, acetylated and amidated. Similar affinity was found with peptide SP1 [heterodimer comprising residues 1-14 (amidated) of Cn2, bridged with synthetic peptide 52-66 (acetylated)]. SP1 and SP7 were used to induce anti-peptide antibodies in mouse and rabbit. Both peptides were highly immunogenic. The sera obtained were able to recognize Cn2 and to neutralize Cn2 in vitro. The most efficient protection (8.3 microgram Cn2 neutralized per mL of serum) was induced by rabbit anti-SP1 serum.     

Derivation of a monoclonal anti-murine IL-4 antibody specific for an epitope expressed on denatured molecules.

Monoclonal antibodies to interleukin 4 (IL-4) were generated by immunization with recombinant IL-4 and screening by binding to IL-4 adsorbed to plastic surfaces. Three antibodies were obtained that scored well in this assay and one, 13E1, was studied in detail. It was very effective in detecting IL-4 by Western blotting whereas a neutralizing anti-IL-4 antibody, 11B11, was 50-100-fold less sensitive as a detecting agent. Sequential immunoprecipitation and biosynthetic labelling studies indicated that the 11B11 and 13E1 epitopes are largely expressed on different forms of IL-4. 13E1 was able to detect cytosolic IL-4 both by immunohistochemical and flow cytometric analysis of fixed cells. This was routinely successful in an insect cell line (Sf9) expressing large amounts of IL-4 as a result of infection with a recombinant 'IL-4 baculovirus'. Although stimulated D10 cells could also be shown to express IL-4 in their cytosol, positive results were not obtained in all such studies and we have failed to detect IL-4 production by normal T cells using this method. This antibody may have substantial value in detecting IL-4 by Western blots and as a tool to analyze the biosynthesis of IL-4. With suitable improvement in sensitivity, it also may prove valuable in the detection of IL-4 in the cytosol of individual cells.     

Immunocytochemical detection of bone marrow micrometastases in colorectal carcinoma patients, using a monoclonal antibody to villin.

The search continues to find methods to more effectively distinguish colorectal carcinoma patients who could be separated into high-risk and low-risk categories. Investigators have reported on the detection of occult micrometastases in bone marrow using antibodies to cytokeratin, which is a marker of epithelial cells but which has no tissue specificity, as opposed to villin, a cytoskeletal protein that is specifically involved in the formation of brush-border microvilli in the small intestine and colon epithelium. Specificity and sensitivity of antibody to villin (ID2C3) and antibody to cytokeratin (A45-B/B3) were first studied in normal bone marrow and in a test system in which cancer cell lines were mixed in normal bone marrow. In a preliminary study including 16 colorectal carcinoma patients, we compared the number of villin-positive cells with cytokeratin-presenting cells. As A45-B/B3, ID2C3 was determined to be sensitive enough to detect one cancer cell in 10(6) hematopoietic cells. Staining of hematopoietic cells with irrelevant antibody and a light staining of megakaryocytes with ID2C3 limited the specificity of the method. In colorectal carcinoma patients, correlation between ID2C3 and A45-B/B3 was 94%. Sensitivity and specificity of ID2C3 antibody to villin were satisfactory. Its clinical relevance must be investigated in further studies.     

Mechanism of cholesteryl ester transfer protein inhibition by a neutralizing monoclonal antibody and mapping of the monoclonal antibody epitope

The plasma cholesteryl ester transfer protein (CETP, Mr 74,000) has a binding site for neutral lipid which can readily equilibrate with lipoprotein cholesteryl esters or triglycerides. Recently, a monoclonal antibody (TP2) was obtained which neutralizes the cholesteryl ester (CE) and triglyceride (TG) transfer activities of the CETP. In this report, the epitope of the inhibitory monoclonal antibody has been localized to a hydrophobic 26-amino acid sequence at the COOH terminus of CETP. The Fab fragments of TP2 caused partial (50%) inhibition of CE transfer and complete inhibition of TG transfer by the CETP. Similarly, the Fab fragments inhibited (37%) the binding of CE to the CETP and abolished the binding of TG to the CETP. Surprisingly, the TP2 Fab was also found to enhance the binding of CETP to plasma lipoproteins and to phospholipid vesicles. In conclusion, the TP2 monoclonal antibody inhibits lipid transfer by blocking the uptake of lipid by CETP. The COOH-terminal epitope may be in or near the neutral lipid binding site. Occupancy of this site by TP2 Fab fragments or by neutral lipid may result in a conformational change of CETP causing enhanced binding to lipoproteins or vesicles.     

Sandwich capture ELISA by a murine monoclonal antibody against a genus-specific LPS epitope for the detection of different common serotypes of salmonellas.

A sandwich capture ELISA based on a murine monoclonal antibody against a genus-specific epitope in the outer core region of the Salmonella lipopolysaccharide is described for the detection of different common serotypes of salmonellas. Four h broth cultures of seven standard and 24 wild strains of salmonellas were all detected by the capture ELISA while overnight broth cultures of 21 non-salmonella standard strains were all negative. The capture ELISA detected 1 ng/ml of Ra lipopolysaccharide, 10(6)/ml of a smooth wild strain of Salm. typhimurium, and 1120 cells of Salm. heidelberg after enrichment culture for 4 h.     

Detection of dideoxyosone intermediates of glycation using a monoclonal antibody: characterization of major epitope structures

Glycation or the Maillard reaction in proteins forms advanced glycation end products (AGEs) that contribute to age- and diabetes-associated changes in tissues. Dideoxyosones, which are formed by the long-range carbonyl shift of the Amadori product, are newly discovered intermediates in the process of AGE formation in proteins. They react with o-phenylenediamine (OPD) to produce quinoxalines. We developed a monoclonal antibody against 2-methylquinoxaline-6-carboxylate coupled to keyhole limpet hemocyanin. The antibody reacted strongly with ribose and fructose (+OPD)-modified RNase A and weakly with glucose and ascorbate (+OPD)-modified RNase A. Reaction with substituted quinoxalines indicated that this antibody favored the 2-methyl group on the quinoxaline ring. We used high performance liquid chromatography to isolate and purify three antibody-reactive products from a reaction mixture of N alpha-hippuryl-L-lysine+ribose+OPD. The two most reactive products were identified as diastereoisomers of N1-benzoylglycyl-N6-(2-hydroxy-3-quinoxalin-2-ylpropyl)lysine and the other less reactive product as N1-benzoylglycyl-N6-[2-hydroxy-2-(3-methylquinoxalin-2-yl)ethyl]lysine. Our study confirms that dideoxyosone intermediates form during glycation and offers a new tool for the study of this important pathway in diabetes and aging.     

Differential carbohydrate epitope recognition of globotriaosyl ceramide by verotoxins and a monoclonal antibody.

The role of renal expression of the glycosphingolipid verotoxin receptor, globotriaosylceramide, in susceptibility to verotoxin-induced hemolytic uremic syndrome is unclear. We show that a single glycosphingolipid can discriminate multiple specific ligands. Antibody detection of globotriaosylceramide in renal sections does not necessarily predict verotoxin binding. The deoxyglobotriaosylceramide binding profile for verotoxin 1, verotoxin 2 and monoclonal anti-globotriaosylceramide are distinct. Anti-globotriaosylceramide had greater dependency on the intact alpha-galactose and reducing glucose of globotriaosylceramide than verotoxin 1, while verotoxin 2 was intermediate. These ligands differentially stained human kidney sections. Glomerulopathy is the primary verotoxin-associated pathology in hemolytic uremic syndrome. For most samples, verotoxin 1 immunostaining within adult glomeruli was observed (type A). Some samples, however, lacked glomerular binding (type B). Anti-globotriaosylceramide (and less effectively, verotoxin 2) stained all glomeruli. Verotoxin 1/anti-globotriaosylceramide tubular staining was comparable. Type B glomerular/tubular globotriaosylceramide showed minor, but significant, fatty acid compositional differences. Verotoxin 1 type B glomerular binding became evident following pretreatment with cold acetone, or methyl-beta-cyclodextrin, used to deplete cholesterol. Direct visualization, using fluorescein isothiocyanate-verotoxin 1B, showed paediatric, but no adult glomerular staining; this was confirmed by anti-fluorescein isothiocyanate immunostaining. Acetone induced fluorescein isothiocyanate-verotoxin 1B glomerular staining in type A, but poorly in type B samples. Comparison of fluorescein isothiocyanate-verotoxin 1B and native verotoxin 1B deoxyglobotriaosylceramide analogue binding showed an alteration in subspecificity. These studies indicate a marked heterogeneity of globotriaosylceramide expression within renal glomeruli and differential binding of verotoxin 1/verotoxin 2/anti-globotriaosylceramide to the same glycosphingolipid. Verotoxin 1 derivatization can induce subtle changes in globotriaosylceramide binding to significantly affect tissue binding. Heterogeneity in glomerular globotriaosylceramide expression may play a significant (cholesterol-dependent?) role in determining renal pathology following verotoxemia.     

A monoclonal antibody to aromatic amino acid hydroxylases. Identification of the epitope.

PH8 monoclonal antibody has previously been shown to react with all three aromatic amino acid hydroxylases, being particularly useful for immunohistochemical staining of brain tissue [Haan, Jennings, Cuello, Nakata, Chow, Kushinsky, Brittingham & Cotton (1987) Brain Res. 426, 19-27]. Western-blot analysis of liver extracts showed that PH8 reacted with phenylalanine hydroxylase from a wide range of vertebrate species. The epitope for antibody PH8 has been localized to the human phenylalanine hydroxylase sequence between amino acid residues 139 and 155. This highly conserved region of the aromatic amino acid hydroxylases has 11 out of 17 amino acids identical in phenylalanine hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase.     

Characterization of a human immunodeficiency virus neutralizing monoclonal antibody and mapping of the neutralizing epitope.

A monoclonal antibody was produced to the exterior envelope glycoprotein (gp120) of the human T-cell lymphotropic virus (HTLV)-IIIB isolate of the human immunodeficiency virus (HIV). This antibody binds to gp120 of HTLV-IIIB and lymphadenopathy-associated virus type 1 (LAV-1) and to the surface of HTLV-IIIB- and LAV-1-infected cells, neutralizes infection by cell-free virus, and prevents fusion of virus-infected cells. In contrast, it does not bind, or weakly binds, the envelope of four heterologous HIV isolates and does not neutralize heterologous isolates HTLV-IIIRF and HTLV-IIIMN. The antibody-binding site was mapped to a 24-amino-acid segment, using recombinant and synthetic segments of HTLV-IIIB gp120. This site is within a segment of amino acid variability known to contain the major neutralizing epitopes (S. D. Putney, T. J. Matthews, W. G. Robey, D. L. Lynn, M. Robert-Guroff, W. T. Mueller, A. J. Langlois, J. Ghrayeb, S. R. Petteway, K. J. Weinhold, P. J. Fischinger, F. Wong-Staal, R. C. Gallo, and D. P. Bolognesi, Science 234:1392-1395, 1986). These results localize an epitope of HIV type-specific neutralization and suggest that neutralizing antibodies may be effective in controlling cell-associated, as well as cell-free, virus infection.