Immunology Today, vol. 6, No. 1, 1985
10
Anti-idiotypic antibodies: what do they recognize? B. F. Erlanger On the basis of our present comprehension of the structure of ligand-binding proteins, B. F. Erlanger here discusses how anti-idiotypic antibodies react with their target epitopes, in particular, when anti-idiotypic antibodies cross-react with receptors or act as internal images of ligands. Antibodies can be raised to the combining sites of other antibodies, as part ofa heterologous response (' individual antigenic specificity') ~or within a species or the same individual. In the latter case the anti-idiotypic antibody response is part of an immunoregulatory network 2'3. If the antibody toward which the anti-idiotypic response is directed is specific for a well-defined ligand, then in many instances it is possible to inhibit the idiotype-anti-idiotypic interaction competitively with the ligand. If this can be done, it is evidence that the anti-idiotypic antibody is directed toward an epitope located in the site of the antibody which combines with the ligand. O n the other hand, there are idiotype-anti-idiotypicinteractions that are not competitively inhibited by specific ligands. These have been explained as being 'noncombining site specific' 4, i.e. interactions with epitopes not directly engaged in the binding interaction. These might involve amino acids in t h e 'framework' region rather than in the complementarity-determining region. Observations we have made during attempts to prepare anti-receptor antibodies by the anti-idiotypic route 5, as well as observations of others 6, have made it difficult to accept this explanation. In our approach, monoclonal anti-idiotypic antibodies were raised to antibodies that were specific for ligands of the acetylcholine and adenosine receptors. Subsets of the anti-idiotypic antibodies could be shown to bind receptor. In some cases, the idiotype anti-idiotypic antibody reaction, as well as the receptor-anti-idiotypic antibody interaction, could be inhibited competitively by a ligand. In other cases, however, competitive inhibition was not seen in either system. If it is true that lack of competitive inhibition implies that the anti-idiotypic antibody is directed at epitopes other than those engaged in the ligand-binding interaction, then it would appear that there is structural homology in the receptor and in the idiotype-bearing antibody in regions that are not part of their respective combining sites. Although this is possible, it seems unlikely since Nature does not usually solve the problem of specificity by constructing identical ligand-binding and accessory sites. For example, chymotrypsin, subtilisin and carboxypeptidase all bind aromatic/tmino acids but the amino acids in or surrounding their respective binding sites differ strikingly 7-9. The conceptual difficulties arise as a result of the simplistic way in which antibody-combining sites are customarily represented, i.e. as a cleft or groove in a solid form. The use of this archaic representation should be disDepartment of Microbiology, Cancer Center/Institute of Cancer Research, Columbia University, New York, NY 10032, USA. © 1985, Elsevlt'l S~lente P u b h s h e t s B V . A m s w r d a n l
(1167
491q/85/$02 011
continued because it is misleading and not in accord with our present understanding of the structure ofligand-binding proteins, of which the immunoglobulin molecule is only one example. In reality, the amino acids that directly participate in the binding interactions also provide idiotopes that can yield reactions with anti-idiotypic antibodies that need not be inhibited by specific ligands. As seen in Fig. 1, which represents the Fab fragment of a mouse myeloma protein specific for phosphorylcholine 1°, most of the complementarity-determining amino acid residues which function in binding are, on their 'reverse' aspects (i.e. the surfaces not directly involved in binding), exposed to solution and available as antigenic determinants. Moreover, the reverse aspects, which are as idiotypic and, in their own way, as reflective of specificity as the obverse, could very well share determinants with reverse surfaces of a receptor specific for the same ligand. Anti-idiotypic antibodies directed at these determinants of the antibody, therefore, could cross-react with receptors specific for the same ligand. Their reactions might or might not be inhibited by specific ligands, depending upon the effect of the binding interaction on the integrity of the combining site. There are anti-idiotypic antibodies that have been designated as 'internal images' or homobodies H'IL They have been described as having topographical features in common with those of the specific ligand and hence, can mimic the specific ligand functionally. Because of the aforementioned 'cleft or groove' representation, reservations have been raised about the internal image concept because of the implied requirement than an antibody must insert itself into the 'cleft' of another antibody or receptor ~3. In fact, an 'internal image' antibody could just as well bind by reacting partially with determinants on reverse aspects of the binding site of the receptor or the anti-ligand antibody. Among internal images that have been described are anti-idiotypic antibodies that mimic insulin 14 and alprenolo115.16. It might seem surprising that a protein can mimic a low tool. wt, non-peptide organic compound like alprenolol. O n the other hand, as I have emphasized above, antibody specificity is governed by the same physieochemical principles that influence the specificity of other ligand-binding proteins such as enzymes and receptors. With respect to the latter, the endorphins, which are polypeptides, bind to the same receptor as does morphine 17, an alkaloid, i.e. the polypeptide is an image of the alkaloid. Moreover, since proteins can assume complementary conformations that allow binding of an almost infinite variety of molecules and macromolecules, it is not illogical that they can also assume regional conformations which, like the endorphins, act as images of these molecules. Therefore, it should not be surprising if internal
Immunology Today, vol. 6, No. 1, 1985
G 52C
I1
N 28
G 52C
N 28
Fig. 1. Stereoscopic presentation of a-carbon backbone of McPC 603. Complementarity-determining residues are shown as filled circles ( R ~ 10; reprinted with permission).
image antibodies were to be found that can mimic components of polysaccharides and nucleic acids as well and thereby be recognized by their respective specific binding proteins. As such, they should be extremely useful biological reagents. Moreover, if they are normally present in vivo, there must be consequences with respect to immune regulation and autoimmunity. ~=[] Research in the author's laboratory is supported by grants from the National Institutes of Health and the Muscular Dystrophy Association.
References 1 Kunkel, H. B., Mannick, M. and Williams, R. C. (1963) Science 14-0, 1218-1219 2 Oudin, Y. and Michael, M. (1963) C. R. Acad. Sd. 257, 805-808 3 Jerne, N. K. (1974)Ann. Immunol. (Inst. Pasteur) 125C, 373-389 4 Eichmann, K. (1978)Adv. ImmunoL 26, 195-254
5 Cleveland, W. L., Wassermann, N. H., Sarangarajan, R., Penn A. S. and Erlanger, B. F. (1983) Nature (London) 305, 56-57 6 Strosberg, A. D. (1983) Springers Semin. ImraunopathoL 6, 67-78 7 Hartsuck, J. A. and Lipscomb, W. N. (1977) in The Enzymes Vol. 3, 3rd edn (Boyer, P. D., ed.), 1-56 Academic Press, New York, London 8 Blow, D. M. (1977) in TheEnzymes Vol. 3, 3rd edn (Boyer, P. D.ed.), 185-212 Academic Press, New York, London 9 Kraut, J. (1977) in The Enzymes Vol. 3, 3rd edn (Boyer, P. D.ed.), 165-183 Academic Press, New York, London 10 Davies, D. R. and Metzger, H. (1983)Annu. Rev. lmrnunol. 1, 87-117 11 Lindenmann, J. (1979) Ann. lrnmunoL (Inst. Pasteur) 130C, 311-319 12 Nisonoff, A. and Lamoyi, E. (1981) Clin. ImmunoL lmmunopathol. 21, 397-406 13 Rodkey, L. S. (1980) MicrobioL Rev. 44, 631-659 14 Sege, K. and Peterson, P. A. (1978) Proc. Natl. Acad. Sci. USA 75, 2443-2447 15 Schreiber, A. B., Couraud, P. O., Andre, C., Vray, B. and Strosberg, A. D. (1980) Proc. Natl. Avad. Sei. USA 77, 7385-7389 16 Homcy, C.J., Rockson, S. G. and Haber, E. (1982)J. Clin. Invest. 69, 1147-1153 17 Guillemin, R. (1977) N. Eng. J. Med. 296, 226-228
10
Anti-idiotypic antibodies: what do they recognize? B. F. Erlanger On the basis of our present comprehension of the structure of ligand-binding proteins, B. F. Erlanger here discusses how anti-idiotypic antibodies react with their target epitopes, in particular, when anti-idiotypic antibodies cross-react with receptors or act as internal images of ligands. Antibodies can be raised to the combining sites of other antibodies, as part ofa heterologous response (' individual antigenic specificity') ~or within a species or the same individual. In the latter case the anti-idiotypic antibody response is part of an immunoregulatory network 2'3. If the antibody toward which the anti-idiotypic response is directed is specific for a well-defined ligand, then in many instances it is possible to inhibit the idiotype-anti-idiotypic interaction competitively with the ligand. If this can be done, it is evidence that the anti-idiotypic antibody is directed toward an epitope located in the site of the antibody which combines with the ligand. O n the other hand, there are idiotype-anti-idiotypicinteractions that are not competitively inhibited by specific ligands. These have been explained as being 'noncombining site specific' 4, i.e. interactions with epitopes not directly engaged in the binding interaction. These might involve amino acids in t h e 'framework' region rather than in the complementarity-determining region. Observations we have made during attempts to prepare anti-receptor antibodies by the anti-idiotypic route 5, as well as observations of others 6, have made it difficult to accept this explanation. In our approach, monoclonal anti-idiotypic antibodies were raised to antibodies that were specific for ligands of the acetylcholine and adenosine receptors. Subsets of the anti-idiotypic antibodies could be shown to bind receptor. In some cases, the idiotype anti-idiotypic antibody reaction, as well as the receptor-anti-idiotypic antibody interaction, could be inhibited competitively by a ligand. In other cases, however, competitive inhibition was not seen in either system. If it is true that lack of competitive inhibition implies that the anti-idiotypic antibody is directed at epitopes other than those engaged in the ligand-binding interaction, then it would appear that there is structural homology in the receptor and in the idiotype-bearing antibody in regions that are not part of their respective combining sites. Although this is possible, it seems unlikely since Nature does not usually solve the problem of specificity by constructing identical ligand-binding and accessory sites. For example, chymotrypsin, subtilisin and carboxypeptidase all bind aromatic/tmino acids but the amino acids in or surrounding their respective binding sites differ strikingly 7-9. The conceptual difficulties arise as a result of the simplistic way in which antibody-combining sites are customarily represented, i.e. as a cleft or groove in a solid form. The use of this archaic representation should be disDepartment of Microbiology, Cancer Center/Institute of Cancer Research, Columbia University, New York, NY 10032, USA. © 1985, Elsevlt'l S~lente P u b h s h e t s B V . A m s w r d a n l
(1167
491q/85/$02 011
continued because it is misleading and not in accord with our present understanding of the structure ofligand-binding proteins, of which the immunoglobulin molecule is only one example. In reality, the amino acids that directly participate in the binding interactions also provide idiotopes that can yield reactions with anti-idiotypic antibodies that need not be inhibited by specific ligands. As seen in Fig. 1, which represents the Fab fragment of a mouse myeloma protein specific for phosphorylcholine 1°, most of the complementarity-determining amino acid residues which function in binding are, on their 'reverse' aspects (i.e. the surfaces not directly involved in binding), exposed to solution and available as antigenic determinants. Moreover, the reverse aspects, which are as idiotypic and, in their own way, as reflective of specificity as the obverse, could very well share determinants with reverse surfaces of a receptor specific for the same ligand. Anti-idiotypic antibodies directed at these determinants of the antibody, therefore, could cross-react with receptors specific for the same ligand. Their reactions might or might not be inhibited by specific ligands, depending upon the effect of the binding interaction on the integrity of the combining site. There are anti-idiotypic antibodies that have been designated as 'internal images' or homobodies H'IL They have been described as having topographical features in common with those of the specific ligand and hence, can mimic the specific ligand functionally. Because of the aforementioned 'cleft or groove' representation, reservations have been raised about the internal image concept because of the implied requirement than an antibody must insert itself into the 'cleft' of another antibody or receptor ~3. In fact, an 'internal image' antibody could just as well bind by reacting partially with determinants on reverse aspects of the binding site of the receptor or the anti-ligand antibody. Among internal images that have been described are anti-idiotypic antibodies that mimic insulin 14 and alprenolo115.16. It might seem surprising that a protein can mimic a low tool. wt, non-peptide organic compound like alprenolol. O n the other hand, as I have emphasized above, antibody specificity is governed by the same physieochemical principles that influence the specificity of other ligand-binding proteins such as enzymes and receptors. With respect to the latter, the endorphins, which are polypeptides, bind to the same receptor as does morphine 17, an alkaloid, i.e. the polypeptide is an image of the alkaloid. Moreover, since proteins can assume complementary conformations that allow binding of an almost infinite variety of molecules and macromolecules, it is not illogical that they can also assume regional conformations which, like the endorphins, act as images of these molecules. Therefore, it should not be surprising if internal
Immunology Today, vol. 6, No. 1, 1985
G 52C
I1
N 28
G 52C
N 28
Fig. 1. Stereoscopic presentation of a-carbon backbone of McPC 603. Complementarity-determining residues are shown as filled circles ( R ~ 10; reprinted with permission).
image antibodies were to be found that can mimic components of polysaccharides and nucleic acids as well and thereby be recognized by their respective specific binding proteins. As such, they should be extremely useful biological reagents. Moreover, if they are normally present in vivo, there must be consequences with respect to immune regulation and autoimmunity. ~=[] Research in the author's laboratory is supported by grants from the National Institutes of Health and the Muscular Dystrophy Association.
References 1 Kunkel, H. B., Mannick, M. and Williams, R. C. (1963) Science 14-0, 1218-1219 2 Oudin, Y. and Michael, M. (1963) C. R. Acad. Sd. 257, 805-808 3 Jerne, N. K. (1974)Ann. Immunol. (Inst. Pasteur) 125C, 373-389 4 Eichmann, K. (1978)Adv. ImmunoL 26, 195-254
5 Cleveland, W. L., Wassermann, N. H., Sarangarajan, R., Penn A. S. and Erlanger, B. F. (1983) Nature (London) 305, 56-57 6 Strosberg, A. D. (1983) Springers Semin. ImraunopathoL 6, 67-78 7 Hartsuck, J. A. and Lipscomb, W. N. (1977) in The Enzymes Vol. 3, 3rd edn (Boyer, P. D., ed.), 1-56 Academic Press, New York, London 8 Blow, D. M. (1977) in TheEnzymes Vol. 3, 3rd edn (Boyer, P. D.ed.), 185-212 Academic Press, New York, London 9 Kraut, J. (1977) in The Enzymes Vol. 3, 3rd edn (Boyer, P. D.ed.), 165-183 Academic Press, New York, London 10 Davies, D. R. and Metzger, H. (1983)Annu. Rev. lmrnunol. 1, 87-117 11 Lindenmann, J. (1979) Ann. lrnmunoL (Inst. Pasteur) 130C, 311-319 12 Nisonoff, A. and Lamoyi, E. (1981) Clin. ImmunoL lmmunopathol. 21, 397-406 13 Rodkey, L. S. (1980) MicrobioL Rev. 44, 631-659 14 Sege, K. and Peterson, P. A. (1978) Proc. Natl. Acad. Sci. USA 75, 2443-2447 15 Schreiber, A. B., Couraud, P. O., Andre, C., Vray, B. and Strosberg, A. D. (1980) Proc. Natl. Avad. Sei. USA 77, 7385-7389 16 Homcy, C.J., Rockson, S. G. and Haber, E. (1982)J. Clin. Invest. 69, 1147-1153 17 Guillemin, R. (1977) N. Eng. J. Med. 296, 226-228
