Production and characterization of highly specific anti-methotrexate monoclonal antibodies

Spleen cells of a Biozzi HR mouse immunized with a bovine serum albumin-methotrexate conjugate were fused with P3-X63-Ag8.653 mouse myeloma cells. Twenty-three monoclonal antibodies (MAbs), selected by indirect ELISA, were produced and partially characterized. Using methotrexate (MTX) and eight structurally related compounds, binding specificities of the MAbs were assessed by inhibition enzyme immunoassay. All the MAbs had very low affinity for folic acid and its analogs and for the major MTX metabolite 7-hydroxymethotrexate. Using a computer cluster analysis program based on the binding specificities, the MAbs were divided into three groups. The thirteen MAbs in group I recognized primarily the pteridine portion of the MTX molecule; the eight group II MAbs recognized the benzene ring as well as the pteridine structure. The two MAbs in group III poorly distinguished between the different parts of the MTX molecule. The apparent equilibrium association constants of the anti-MTX MAbs in groups I, II, and III ranged from 7 x 10(9) to 3 x 10(8) M-1 (except for 1 MAb), from 5 x 10(7) to 6 x 10(6) M-1 (except for 2 MAbs), and from 1 x 10(6) to 3.5 x 10(5) M-1, respectively.     

Monoclonal antibodies against bacterial glucosamine 6-phosphate synthase: production and use for structural studies.

Fifteen mouse x rat hybridoma cell lines producing rat monoclonal antibodies (MAbs) directed to Escherichia coli Glucosamine 6-P Synthase (GlmS) were established and characterized. Most of them (13/15) are IgG2a while 2 were typed as IgG1. Their Kaff ranged from 1.5 x 10(6) to 9.6 x 10(8) M-1 as determined by Beatty et al. (1). The epitopes recognized by these MAbs were assigned to one of the two catalytical domains of the enzyme (CT1 and CT2) as demonstrated both by ELISA and Western-blotting using purified GlmS proteolytic fragments. The binding of the MAbs on either the native or denatured forms of GlmS, CT1 and CT2 was further analyzed by competitive immunoassay and most of the MAbs were found to bind preferentially to the denatured proteins. The study of the antigenic topography of GlmS by competitive radioimmunoassay demonstrated the existence of at least 10 independent epitopes on GlmS, divided into three groups. The first one (3/15) includes MAbs whose binding was not inhibited by any of the other MAbs. The second group (9/15) is comprised of MAbs that exhibit reciprocal binding inhibitory activity while the third group includes MAbs (3/15) presenting asymmetric inhibitory activity. Finally, since most of the isolated antibodies (10/15) bind to the 27 kDa amino-terminal glutamine binding domain (CT2), the capacity of these MAb to interfere with the associated glutaminase activity was analyzed.     

Determination of intrinsic affinity constants of monoclonal antibodies against carcinoembryonic antigen

A 2-phase method is described for the determination of the intrinsic affinity constants (K-values) for the interaction between monoclonal antibodies (MAbs) against carcinoembryonic antigen (CEA) and CEA. The Mabs were coupled covalently to CNBr-activated paper discs. MAb coupled discs and a 2-fold dilution series of 125I-CEA were incubated at 20 degrees C until equilibrium was reached. Nonlinear curve-fitting was used for estimation of K-values and different calculation models were thereby tested. The K-values for 12 different anti-CEA MAbs were determined to be 0.3-52 X 10(6) M-1, which is 1-2 orders of magnitude lower than the values obtained in a previous study with some of these MAbs using the ammonium sulphate method to separate free from bound antigen. The K-values obtained by the paper disc method are probably better estimates of the intrinsic association constant than those obtained previously. There are two main reasons for this: (1) free and bound antigen are separated from each other under physiological conditions and (2) the opportunity for multipoint interaction between MAb and CEA is minimized when the MAb is coupled to a rigid carrier substance. Thus, even when the MAb reacts with greater than or equal to 2 epitopes in CEA, as seems to be the case with several of our anti-CEA MAbs, the intrinsic K-value should be obtained. The fundamental validity of 2-phase assays, sometimes questioned in the literature, is demonstrated.     

Novel anti-digoxin monoclonal antibodies with different binding specificities for digoxin metabolites and other glycosides

Spleen cells of BALB/c mice immunized with a digoxin-bovine serum albumin conjugate were fused with P3-X63-Ag8.653 mouse myeloma cells. Seven monoclonal antibodies (MAb) selected by indirect ELISA were produced, purified and characterized. All the MAb were IgG1 isotypes. The apparent equilibrium association constants (Ka) of four of the MAb, determined by Scatchard analysis of the RIA data, ranged from 1 x 10(9) M-1 to 5.9 x 10(9) M-1. The estimated Ka values of the three other MAb were found to be between 4.8 x 10(7) M-1 and 5.9 x 10(8) M-1. Using digoxin and eighteen structurally-related compounds, the seven MAb could be divided into five groups based on their binding specificities assessed by an inhibition immunoenzymatic test. The MAb in Groups I and II, in particular, showed very different specificity profiles: the two MAb in Group I had low cross-reactivity with cardioinactive digoxin metabolites, whereas the high affinity MAb in Group II recognized all the digoxin metabolites tested. The MAb in Group I might be useful in a digoxin immunoassay and the Group II MAb in therapeutic reversal of digoxin intoxication.     

Surface antigen detected by a Schistosoma mansoni monoclonal antibody in worm extracts and kidney deposits of infected mice and hamsters.

Four monoclonal antibodies (MAbs) derived from a Schistosoma mansoni-infected mouse reacted against the tegument or the cell layer of the digestive tract of the adult worm. They also showed similar patterns of immunofluorescence staining when schistosomula were used as antigens. Two of the MAbs (4A10 and 4D3) recognized immune complexes deposited in the kidneys of infected mice and hamsters as detected by indirect and direct immunofluorescence reactions. When adsorbed to polystyrene beads, both MAbs allowed the quantitative detection of antigen by an enzyme immunoassay. 125I-labeled 4A10 binding to live schistosomula and corresponding inhibition assay ruled out the possibility that this binding could be through its Fc fragment. The same MAb detected an antigen migrating as an 80-kilodalton protein by Western blot analysis of soluble worm antigen preparation after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Use of MAb 4A10 may help to elucidate mechanisms of renal pathology and also be of value in the development of immunodiagnostic assays.     

Production and characterization of monoclonal antibodies against human thyroglobulin

Spleen cells of Biozzi-HR mice immunized with human thyroglobulin (hTg) were fused with P3-X63-Ag8.653 mouse myeloma cells. Twenty monoclonal antibodies (MAbs) selected by an enzyme immunoassay (indirect ELISA) were produced, purified and characterized. The equilibrium association constant (Ka) of one of the MAbs, determined by Scatchard analysis of the ELISA data, was found to be 2 X 10(9) M-1; the Ka of the other MAb, estimated from titration curves by comparison with the aforementioned MAb, ranged from 8 X 10(9) M-1 to 6 X 10(7) M-1. The reaction between the MAb and hTg was not inhibited by thyroxin (T4), triiodothyronine (T3) and triiodothyropropionic acid (DT3). Species specificity of the MAb was studied using bovine and porcine Tgb. The topology of the MAb was investigated by competitive inhibition immunoassays. Seven distinct antigenic regions were identified.     

Production and characterization of two monoclonal antibodies to human pancreatic trypsin 1

Two monoclonal antibodies (MAb) G6 and A8 directed against human trypsin 1 have been produced by hybridization of myeloma cells with spleen cells of OF1 immunized mice. Antibodies were screened by radioimmunoassay. Monoclonal antibodies were purified by affinity chromatography on Protein A-Sepharose, and we found that MAb G6 was of the IgG2b class and MAb A8 of the IgG2a class. Both MAbs had a high affinity for trypsin 1 with the respective affinity constants equal to 1.3 x 10(8) l/mol for G6 and 3.2 x 10(7) l/mol for A8. Epitope specificity was studied by western blotting, using human trypsinogens 1 and 2. Both MAbs gave a positive reaction with native trypsinogen 1 and no reaction with the same protein after reduction. Only MAb G6 reacted with trypsinogen 2 in the native form. Its affinity for trypsin 2 was found similar to that for trypsin 1 with a constant equal to 2.7 x 10(7) l/mol. Both antibodies appeared directed against conformational and not sequential epitopes.     

Enhancement of bovine growth hormone activity in vivo by monoclonal antibodies

Monoclonal antibodies (MAbs) prepared against ovine growth hormone (oGH) and recognizing several distinct or related specificities on bovine growth hormone (bGH), have been shown to strikingly enhance the growth promoting properties of the hormone in vivo as determined by 35SO2-4 incorporation in cartilage. The phenomenon is dependent on both hormone and antibody dose and is saturable in the presence of excess antibody. Competition binding analysis between pairs of antibodies has shown that they define distinct antigenic regions on bGH of which one is represented by four MAbs. The most potent growth enhancement was associated with a group of three MAbs (OA11, OA12 and OA13) binding to topographically closely related sites. Another MAb (OA14) with a specificity similar to OA11 and OA13 as defined by competition assay failed to enhance bGH activity. Two antibodies binding to a further two sites (OA15 and OA16) demonstrated modest growth enhancement activity when in complex with bGH, whereas the binding of another antibody (OA17) did not significantly affect hormonal activity. Univalent antibody fragments (Fab) derived from MAb OA11 were equi-potent to the bivalent form of the antibody in the enhancement of bGH activity. Determination of the effects of the different MAbs on the binding of 125I-bGH to liver microsome receptors revealed substantial increase in specific binding (3.5-fold) and is associated with some growth enhancing MAbs. However, not all growth enhancing MAbs increased receptor binding and in the case of one, OA16, clear cut inhibition of receptor binding was observed. Tentative conclusions have been drawn on the possible underlying mechanisms of the MAb mediated enhancement phenomenon.     

Dissociation kinetics of antigen-antibody interactions: studies on a panel of anti-albumin monoclonal antibodies

Kinetic parameters and equilibrium association constants (K) are reported for a panel of anti-bovine serum albumin (BSA) monoclonal antibodies (MAb) immobilized onto agarose particles. For 12 covalently immobilized MAb of moderate affinity (K = 0.25 x 10(8)-1.2 x 10(8) M-1) measured dissociation time constants varied two orders of magnitude, from 2.1 to 410 min. Directly measured association rate parameters agree with values calculated from measured equilibrium and dissociation rate parameters. Dissociation time constants and equilibrium association constants were also determined for eight MAb immobilized biospecifically (via their Fc regions). A significantly lower K was observed with those MAb which were covalently immobilized as opposed to biospecifically immobilized. These decreases in K appear to reflect decreased association rates rather than increased dissociation rates. The data suggest that, for the MAb described herein, dissociation rates do not correlate with equilibrium association constants.     

Escherichia coli hemolysin may damage target cell membranes by generating transmembrane pores.

We used mouse monoclonal antibodies (MAbs) to characterize Neisseria gonorrhoeae lipooligosaccharide (LOS). LOSs that bound two or more MAbs in a solid-phase radioimmunoassay usually bound them to different LOS components, as separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE); strains with multiple LOS components on SDS-PAGE usually bound more than one MAb. However, the LOS of some strains bound the same MAb to two LOS components with different relative molecular weights, and some individual LOS components bound more than one MAb. LOSs from different strains bound different amounts of the same MAb at saturation, reflecting differences in the quantitative expression of individual LOS components. Not all components recognized by MAbs were stained by silver after periodate oxidation. Treatment with NaOH variously affected epitopes defined by different MAbs. MAb 3F11 completely inhibited and MAb 2-1-L8 partially inhibited the binding of 125I-labeled 06B4 MAb to WR220 LOS and WR220 outer membranes in competitive binding studies. Other MAbs did not compete with the binding of 125I-labeled 06B4 to either antigen. We conclude that a strain of N. gonorrhoeae elaborates multiple LOSs that can be separated by SDS-PAGE and that are antigenically distinct. Epitope expression within these glycolipids is complex.     

Active site contributions to fluorescein binding determined by in vitro heavy and light chain reassociations

In addition to prior crystallographic studies that determined antigen contact residues for high affinity murine monoclonal anti-fluorescein (Fl) antibody 4-4-20 (Ka = 1.7 x 10(10) M-1), primary structure comparisons revealed idiotypically cross-reactive monoclonal anti-Fl antibody 9-40 (Ka = 5.7 x 10(7) M-1) possessed identical Fl contact residues with the exception of L34 His for Arg. Mab 9-40 L34 His was germ-line encoded and 4-4-20 L34 Arg correlated with increased 4-4-20 affinity and enhanced Fl quenching. To better define L34 Arg and L96 Trp contributions to antigen binding, in vitro H and L chain reassociation experiments were performed. Following reassociations, affinity purified and homologous chain reassociated proteins 4-4-20 (H4-4-20 L4-4-20) and 9-40 (H9-40 L9-40) yielded identical idiotypes (greater than 91% related), Qmax values (91% and 44.7%), affinity constants (approximately 2.0 x 10(10) M-1 and 5.5 x 10(7) M-1) and iodide quenching values (1.2% and 2.1%), respectively. Although heterologous reassociated proteins were idiotypically related to prototypic proteins (greater than or equal to 87.1%), differences in Fl-binding characteristics were observed. Recombinant H4-4-20 L9-40 expressed Ka and Qmax values similar to 9-40 and implicated L34 Arg in increased 4-4-20 Ka and Qmax. Residue L34 Arg replacement in the 9-40 active site (H9-40 L4-4-20). however, exhibited low Qmax (44.4%) and slightly increased affinity (3.7 x 10(8) M-1). In addition, L96 Leu substitution into 4-4-20 (H4-4-20 L-04-01) and 9-40 (H9-40 L04-01) resulted in lower Qmax values (15.1% and 20.5%, respectively) and significantly reduced Fl affinity (approximately 10(3)-fold). These results demonstrated: (1) L34 Arg was responsible for increased 4-4-20 affinity, (2) L96 Trp was critical for an intermediate Fl affinity (approximately 10(7) M-1) and (3) active site Fl contact residue orientations were potentially affected by differences in H chain complementarity-determining region 3 and CH 1 residues.     

Generation of species-specific, rat monoclonal antibodies that bind to the kappa chain of mouse immunoglobulin.

Large numbers of mouse monoclonal antibodies (MAbs) with exquisite binding specificities have become available. However the study of these primary antibodies in biological fluids by second, anti-mouse antibody techniques, has been confounded by the large quantity of other animal immunoglobulin which is often present in these fluids. To overcome this problem, we have derived high affinity rat MAbs which bind specifically to the antigen binding fragments (Fab) of mouse antibody and display minimal or no crossreactivity with human or rabbit immunoglobulin at the concentrations which are usually found in plasma. Equilibrium binding studies demonstrated that these antibodies had binding affinity constants for mouse Fab that ranged from 4.3 x 10(8) to 4.1 x 10(9) M-1. All of these rat MAbs bound to the kappa chain of mouse immunoglobulin, though competition binding studies amongst these MAbs indicated that they probably recognized three different epitopes. Because of their specificity and affinity, these rat anti-mouse kappa MAbs may be useful in a wide variety of experimental biological systems both in vitro and in vivo.     

Two simple and rapid methods to detect monoclonal antibodies with identical epitope specificities in a large population of monoclonal antibodies.

Two novel and simple methods have been developed for detecting monoclonal antibodies (MAbs) with identical epitope specificities from a large population of MAbs against human chorionic gonadotropin (hCG). The first method was based on the observation that following radioiodination many molecules of an antigen are altered in such a manner that they cannot be recognized by MAbs. The proportion of radiolabelled antigen (Bmax) able to bind to a MAb was characteristic of that MAb and MAbs having identical specificities showed identical Bmax values. Using this principle it was possible to identify MAbs having identical epitope specificities within a large population of MAbs against hCG. In the second method one test MAb was immobilized on a plastic surface through an immunochemical bridge. This MAb was then incubated with 125I-hCG previously complexed with a second MAb. Such a complex could bind to the solid phase MAb only if the two MAbs were not identical. The results obtained with both methods were concordant. With such methods it is possible to identify MAbs with identical epitope specificities immediately after the initial screening of the fusion. These methods do not require subcloning, ascites production, or the purification and iodination of large number of MAbs.     

Epitope-mapped monoclonal antibodies as tools for functional and morphological analyses of the human urokinase receptor in tumor tissue.

uPAR (CD87), the receptor for the urokinase-type plasminogen activator (uPA) facilitates tumor cell invasion and metastasis by focusing uPA proteolytic activity to the cell surface. As uPAR exists in various molecular forms, it is desirable to use well defined antibodies for analyses of uPAR antigen expression in human malignant tumors by immunological methods. Therefore, twelve monoclonal antibodies (MAbs) directed against uPAR were generated by using nonglycosylated, recombinant human uPAR (spanning amino acids 1 to 284), expressed in Escherichia coli, as the immunogen. The reaction pattern of these MAbs with the immunogen and a series of carboxyl-terminally truncated versions of uPAR demonstrated that at least six different epitopes of uPAR are recognized. All MAbs reacted under reducing conditions in immunoblot analyses with E. coli-expressed uPA and also with highly glycosylated, functionally intact, recombinant human uPAR expressed in Chinese hamster ovary (CHO) cells. Seven of the MAbs recognized CHO uPAR under nonreducing conditions as well. By flow cytofluorometric analyses, three of these MAbs were shown to bind to native human uPAR present on the cell surface of monocytoid U937 cells with MAb IIIF10 being the best. Saturation of uPAR with uPA on U937 cells completely blocked interaction of MAb IIIF10 with uPAR (mapped epitope, amino acids 52 to 60 of domain I of uPAR). In turn, preincubation of U937 cells with MAb IIIF10 efficiently reduced binding of uPA to uPAR, indicating that the epitope detected by MAb IIIF10 is located within or closely to the uPA-binding site of uPAR, and thus, this site may be a target to influence uPA/uPAR-mediated proteolysis in tumors. Binding of MAbs IID7 or IIIB11 (mapped epitope, amino acids 125 to 132 of domain II of uPAR) to uPAR is not affected when uPAR is occupied by uPA. As these MAbs reacted strongly with cellular uPAR antigen in formalin-fixed paraffin-embedded tumor sections, the domain-II-specific antibodies IID7 and IIIB11 may be useful for immunohistochemical studies of uPAR expression in tissue remodeling processes in tumor invasion. In conclusion, we have devised well defined and epitope-mapped MAbs to uPAR that are highly specific tools for detection and targeting of uPAR in tumor tissue.     

Production of a monoclonal antibody (K 931) to a squamous cell carcinoma associated antigen identified as the 17-1A antigen

During our efforts to develop monoclonal antibodies (MAbs) to tumor associated surface antigens of squamous cell carcinomas of the head and neck, monoclonal antibody K 931 was produced. The high affinity antibody (Ka 5.0 x 10(10) M-1) showed reactivity with 58 out of 62 squamous cell carcinomas of the head and neck. In contrast normal squamous epithelium as found in epidermis, oral cavity, epiglottis, pharynx, larynx and esophagus did not express the antigen. All other tested epithelial (simple and transitional) tissues did express the antigen, but non-epithelial tissues were negative. Further characterization revealed that the antigen represents the 17-1A antigen. A not earlier reported, enhanced expression of the 17-1A antigen was observed among some primary and all metastatic SCC.     

Characterization of the D, c, E and G antigens of the Rh blood group system with human monoclonal antibodies

The human MAbs, anti-D, -c, -E and -G of the Rh blood group system, produced by Epstein-Barr virus transformed B-cell lines, were purified by protein A-Sepharose chromatography and used to characterize the Rh antigens of human red cells. Scatchard plot analyses performed with the radiolabelled MABs indicated that each R2R2 red cell carries 0.43, 0.32 and 0.38 x 10(5) D, c and E binding sites, respectively. About half this number of antigen sites are present on erythrocytes from heterozygote individuals using the appropriate antibody. We found, however, that only 0.18 x 10(5)G antigenic sites were present on each R1R1 red cell. The affinity constants of the anti-D, -E and -G were similar varying from 0.6 to 1.5 x 10(8) M-1 whereas that of the anti-c was much lower (0.035 x 10(8) M-1). The blood group specificity and binding properties indicate that the MAbs behave like the polyclonal anti-Rh reagents. Immunoprecipitation experiments carried out with membranes from R2R2 red cells show that a 30-32 kDa component can be identified whatever the antibody used. The immune complexes involving anti-c, -E or -G antibodies could be formed with the detergent lysates from red cell membranes. In contrast, membrane integrity was a prerequisite for the binding of the anti-D antibodies. Finally, from extraction studies of immunocomplexes with non-ionic detergents it was concluded that all the Rh-active components are bound to the membrane skeleton, suggesting that these molecules may have important function for maintaining red cell shape and viability.     

Decreased metabolism and viability of Mycoplasma hominis induced by monoclonal antibody-mediated agglutination.

Monoclonal antibodies (MAbs) were generated against lysates of clinical Mycoplasma hominis isolates. Three of these, designated BG2, BA10, and FE6, recognized an integral membrane protein of M. hominis with an apparent molecular weight of 50,000 (p50). Electron microscopy studies demonstrated that this protein is distributed evenly over the cell surface. These anti-p50 MAbs were species specific for M. hominis; they reacted with 42% of 126 tested clinical M. hominis isolates and showed no reactivity to heterologous mycoplasma species. Immunoblot analysis after limited proteolysis of purified p50 demonstrated that the three MAbs reacted with different epitopes of the protein. Unlike BA10 and FE6, MAb BG2 induced a decrease in arginine metabolism and a reduction of CFU in metabolic inhibition tests. F(ab)2 fragments of MAb BG2 showed the same inhibitory effect as the intact MAb molecule, while Fab and Fc fragments had no influence on vital functions. Preincubation of the mycoplasmas with MAb BG2 followed by trypsin treatment yielded the same amount of CFU as the control without antibodies. In conclusion, the cell aggregates were resolved by the trypsin treatment. These experiments and tests with the antibody fragments led to the conclusion that only the intact MAb structure or the F(ab)2 structure had metabolic inhibition potential and that the observed metabolism inhibition as well as the apparent decrease in viability were a result of agglutination by MAb BG2.     

Expression of the high molecular weight melanoma-associated antigen by pericytes during angiogenesis in tumors and in healing wounds.

In the course of immunohistochemical characterization of murine monoclonal antibodies recognizing the human high molecular weight-melanoma associated antigen (HMW-MAA), a striking reactivity with blood vessels in the tumor stroma was noted. Immunocytochemical analysis by light and electronmicroscopy of a panel of tissues and cell lines showed that the staining of microvessels by anti-HMW-MAA monoclonal antibodies was restricted to pericytes. Correspondingly, anti-HMW-MAA monoclonal antibodies were found to react with cultured pericytes from human brain, but not with endothelial cells in serologic assays, and to immunoprecipitate from biosynthetically labeled pericytes an antigen with the characteristic structural profile of HMW-MAA. At the subcellular level, the expression of HMW-MAA in cultured pericytes was mainly restricted to microspikes that are localized in clusters on the cellular membrane. Staining by anti-HMW-MAA monoclonal antibodies of pericytes was not only found in the tumor stroma, but also in other lesions associated with angiogenesis, such as granulation tissue of wound healing and synovitis. In these lesions, microvascular staining for another marker of pericytes, ie, alpha-smooth muscle actin, also was observed. These results suggest that, in conditions associated with vascular proliferation, 1) pericytes acquire HMW-MAA and 2) the number of pericytes may be increased as compared with normal tissues.     

Isolation and characteristics of anti-adenovirus monoclonal antibodies in immunoenzyme and immunofluorescent reactions

New monoclonal antibodies (MAbs) to adenovirus hexon, highly active in ELISA and immunofluorescent analysis, were prepared. According to competitive ELISA, new MAbs differed in their blocking activity and were directed to 2 different hexon epitopes. MAb 3H8 did not modify antigen binding of the rest MAbs labeled with peroxidase (PAb x Pox), and none of unlabeled MAbs suppressed the reaction of MAb x Pox 3H8. MAbs 1E8 7F1, 1E11, and 3B1 reacted with each other but differed by the spectrum and level of competitive inhibition, which indicated that they were directed to different epitopes of adenovirus hexon. Comparison of the specific activity of MAbs 7F1 and 1E8 in direct immunofluorescent detection of adenovirus antigens in infected cell cultures and clinical materials from patients showed a good coincidence (90-97%) of the results with the IMAGEN Adenovirus test (Dako) and with polyclonal FITC conjugates to adenovirus hexon.     

Anti-idiotypic antibodies to bovine herpesvirus-1 inhibit virus infection in cell cultures

Summary A panel of murine monoclonal antibodies (MAbs) to bovine herpesvirus-1 (BHV-1) was prepared. Three of them were neutralizing MAbs and reacted against 130/75/50 kDa, 77 kDa, or 97 kDa glycoproteins (gp). A fourth non-neutralizing MAb recognized the 97 kDa gp. Competition radioimmunoassay demonstrated that each of the four MAbs reacted against a different virus epitope. Anti-idiotypic antibodies (anti-id) to the four MAbs were produced in rabbits and purified by sequential immunoaffinity chromatography. Each anti-id inhibited the binding of its respective MAb to BHV-1 in competitive ELISA and blocked BHV-1 neutralizing activity of the MAb. This inhibition suggested that the anti-ids were specific for the antigen binding site of the MAbs. Treatment of MDBK cells with anti-ids inhibited BHV-1 infection, which suggested that the anti-ids block a cellular component essential for virus infection. Absence of significant cross-reactivity among the anti-ids for heterologous MAbs indicated that they recognized unique determinants on the antigen binding site of the homologous MAb.