Monoclonal antibodies to cyclosporin are representative of the major antibody populations present in antisera of immunized mice

What is a B-cell epitope?

The antigenicity of proteins resides in different types of antigenic determinants known as continuous and discontinuous epitopes, cryptotopes, neotopes, and mimotopes. All epitopes have fuzzy boundaries and can be identified only by their ability to bind to certain antibodies. Antigenic cross-reactivity is a common phenomenon because antibodies are always able to recognize a considerable number of related epitopes. This places severe limits to the specificity of antibodies. Antigenicity, which is the ability of an epitope to react with an antibody, must be distinguished from its immunogenicity or ability to induce antibodies in a competent vertebrate host. Failure to make this distinction partly explains why no successful peptide-based vaccines have yet been developed. Methods for predicting the epitopes of proteins are discussed and the reasons for the low success rate of epitope prediction are analyzed.

Functional quantification of cyclosporine A and FK506 in human whole blood by flow cytometry, using the green fluorescent protein as an interleukin-2 reporter gene

The concentration of the immunosuppressive drugs cyclosporine A (CSA) and FK506 in biological fluids is routinely determined by antibody-based assays, which for several reasons do not give accurate information on the actual level of immunosuppression in the patient. To alleviate this problem, we developed a functional reporter gene assay which uses the enhancer fragment of the interleukin-2 promoter region driving the expression of the green fluorescent protein (GFP). This construct was stably transfected in the Jurkat human T lymphoblastoid cell line. Upon stimulation of the cell recipient, the GFP was produced and evaluated by flow cytometry. Immunosuppressants acting via inhibition of interleukin-2 synthesis, such as CSA or FK506, inhibited the production of GFP in a dose-dependent manner. This assay can be performed within a working day with a good reproducibility and was more sensitive than the antibody-based assays, since its detection limit was as low as 10 ng/ml for CSA and 0.5 ng/ml for FK506. We used it for the follow up of drug level present in the blood of transplanted patients, and compared the results with those obtained with the antibody-based assay routinely carried out in our hospital. The conclusions suggest that this assay is a valuable alternative to the presently available assays for the measurement of the immunosuppressive activity found in body fluids.

Glycoprotein B of Aujeszky's disease virus: topographical epitope mapping and epitope-specific antibody response

A panel of 26 monoclonal antibodies (mAbs) against glycoprotein B (gB) of Aujeszky's disease (pseudorabies) virus (ADV), a glycoprotein complex consisting of three glycoproteins, gBa, gBb, and gBc, was produced by two research groups and was used for the topographical epitope mapping of gB. An epitope map was constructed in which the identified epitopes of gB were situated in 14 topologically distinct antigenic domains; ten antigenic domains represented by 22 mAbs were localized on gBc, while four antigenic domains represented by four mAbs resided on gBb of the gB complex. All the epitopes located on gBc appeared to be conformation-dependent, whereas all the epitopes on gBb were conformation-independent. The identified epitopes of gB were conserved among laboratory, vaccine and field ADV strains. Conformation-dependent epitopes were shown to contribute largely to the overall antibody response to gB in naturally infected swine and immunized mice. Moreover, it was found that most of the infected animals responded relatively weakly to the identified conformation-independent epitopes of gB, while a group of immunodominant epitopes that induced a strong antibody response was represented exclusively by conformation-dependent epitopes from different antigenic domains. The results clearly demonstrated that conformation-dependent epitopes of gBc play a crucial role in inducing the humoral immune response to gB of ADV during the natural infection of swine and immunization of mice. The application of mAbs of our panel as research and diagnostic tools is discussed.

Immunological characterisation of glycoprotein E of Aujeszky's disease virus

A panel of 14 monoclonal antibodies (MAbs) against glycoprotein E (gE) of Aujeszky's disease (pseudorabies) virus (ADV), which constitutes a representative sample of naturally occurring gE-specific antibodies in sera from infected animals, was produced and characterised. Eleven topologically distinct antigenic domains represented by one or more MAbs were identified on gE by using these MAbs and three additional gE-specific MAbs. Three of the MAbs available recognised conformation-independent epitopes on gE, while the other 14 MAbs bound to conformation-dependent epitopes. By using the recombinant protein encompassing the N-terminal part of gE, which was expressed in Escherichia coli, all the conformation-independent epitopes of gE were mapped within the first 125 amino-terminal amino acids of gE. The epitopes of gE were demonstrated to be conserved among gE-positive laboratory, field and vaccine ADV strains. Conformation-dependent epitopes were shown to contribute largely to the overall antibody response to gE in naturally infected swine and immunised mice. Most of the infected animals responded weakly to the identified conformation-independent epitopes of gE, while the group of immunodominant epitopes of gE was represented exclusively by conformation-dependent antigenic determinants from different antigenic domains. The results clearly demonstrated that conformation-dependent epitopes play a crucial role in inducing the humoral immune response to gE of ADV during the natural infection of swine and immunisation of mice. The application of MAbs of our panel as research and diagnostic tools is discussed.

Detection and quantification of cyclosporine in body fluids using an interleukin-2 reporter-gene assay

Different assays are employed to monitor the concentration of immunosuppressive drugs in biological fluids. None of these methods gives direct and precise information on the actual level of immunosuppression in the patient. Here we describe the use of an interleukin-2 (IL-2) reporter-gene assay (IL-2 RGA) to monitor the concentrations of immunosuppressants in body fluids. This assay is based on a chimeric gene construct in which the human IL-2 promoter drives the expression of a reporter gene. Upon mitogenic stimulation the reporter gene is expressed and can be easily quantified. The assay is very sensitive and selective for immunosuppressive compounds inhibiting IL-2 gene expression such as cyclosporine (CsA) and FK506, their active metabolites and derivatives, but not for others such as rapamycin. High reproducibility, fast performance time, and high capacity are additional characteristics of the assay. The assay was developed to monitor immunosuppressive drug levels in human volunteers or in animals receiving CsA analogues as the only immunosuppressive drugs. This assay is sensitive to CsA or ascomycin/FK506 analogues and metabolites, for which there are presently no specific monoclonal antibodies available. The IL-2 reporter-gene assay may be more suitable than other in vitro systems such as MLR or mitogen stimulated PBMC which were previously used to study the immunosuppressive activity of drugs in body fluids.

Anti-cyclosporine monoclonal antibodies and their anti-idiotopic counterpart: structure and biological activity

In order to study the structural and functional mimicry of an antigen by anti-idiotypic antibodies, we generated anti-idiotopic monoclonal antibodies (anti-Id mAbs) against a mAb (R45-45-11) with specificity for the immunosuppressive cyclic undecapeptide cyclosporine (Cs; Sandimmune). Three out of five anti-Id mAbs inhibited the binding of Cs to the anti-Cs mAb R45-45-11. All anti-Id mAbs cross-reacted only with one (anti-Cs mAb V45-271-10) out of 19 anti-Cs mAbs. The anti-Cs mAb V45-271-10 recognizes an epitope on the Cs molecule which is very similar to that recognized by R45-45-11. R45-45-11 and V45-271-10 differ only by one amino acid in the variable region. The anti-Id mAbs which recognize combining site-associated idiotopes (Ids) reverse the blocking effect of the anti-Cs mAb R45-45-11 on Cs immunosuppression in vitro. The sequences of the variable regions of heavy and light chain of one anti-Id mAb were determined. X-ray analysis of the corresponding Fab fragment, either alone or complexed with the Fab fragment of the Id, is currently in progress.

Novel monoclonal antibodies to cyclosporine A: Characterization and epitope mapping with cyclosporine analogs and cyclophilin

Monoclonal antibodies to cyclosporine A (Cs), a potent immunosuppressant, were generated in BALB/c mice using a novel antigen prepared by linking Cs to a protein carrier via a photoactive cross-linking reagent, 4-benzoylbenzoic acid (BBa). Twenty-two monoclonal anti-Cs antibodies were generated, using Cs-BBa-bovine serum albumin (Cs-BBa-BSA) as the immunogen. They were characterized with respect to affinity by Scatchard analysis of a radioimmunoassay (RIA), and with respect to specificity by an ELISA in which a series of singly substituted Cs derivatives were examined as inhibitors. McAb affinities ranged from 5 x 10(-8) M to 2 x 10(-10) M. Based on ELISA inhibition data with Cs analogs, and on the binding to two Cs-BSA conjugates in which opposite sides of the Cs molecule are exposed, the antibodies fell into five epitope recognition groups. Binding to Cs was also studied by ELISA in competition with cyclophilin (CyP), a Cs-binding protein whose epitope specificity has been well characterized. Competition by CyP was found to correlate with antibody specificity, not with affinity, i.e. CyP competed best with antibodies having specificities most similar to that of CyP. Epitope mapping can, therefore, be accomplished in a system in which two different species of binding proteins compete for the same antigen. This type of characterization may be useful in identifying antibodies whose combining sites mimic those of a receptor.

Monoclonal antibody technology for cyclosporine monitoring

Monitoring blood levels of Cyclosporine (CsA) has been the basis for adjusting individual dosage regimens in the clinic. Radioimmunoassays using polyclonal antisera reacted with CsA and some CsA metabolites, leading to overestimation when compared with high-performance liquid chromatographic measurements of CsA. Monoclonal antibodies (mAbs) have the potential to discriminate between closely related molecules. MAbs with high affinity for CsA have been prepared and their fine-specificity characterized by cross-reactivity studies using a large series of CsA-derivatives. According to the known sites of metabolism on the CsA molecule and to its three-dimensional structure, it was possible to predict which mAb would be suitable for recognizing native Cs specifically.

Analysis of the structural diversity of monoclonal antibodies to cyclosporine

The immunosuppressive cyclic undecapeptide cyclosporine (Cs) represents a useful model for studying the molecular basis of antibody-antigen interactions. The three-dimensional structure of the Cs molecule is known and a large panel of monoclonal antibodies (mAbs) to Cs has been well characterized by cross-reactivity studies with numerous Cs analogs. In the present study, the sequences of the variable regions of seven mAbs to Cs were determined and a striking relationship was found between the expressed variable region genes and the Cs recognition pattern. An analysis of the length and hydrophobic content of the hypervariable regions and sequence similarities suggested that the heavy chain plays a major role in Cs recognition. Different fine specificities were observed for mAbs exhibiting identical light chains, while two antibodies differed by only a single amino acid located in the heavy chain. The presence of a duplication of 12 nucleotides within the heavy chain third hypervariable region of two antibodies suggests the existence of an additional mechanism for creating antibody diversity.