8536d_ch03_057-075 8/6/02 10:28 AM Page 67 mac79 Mac 79: 45_Bw Glasby et al. Immunology 5e Antigens CHAPTER 3 similar to hEL to be recognized by antibody to native HEL. dividual members of a given species Within an animal, cer Even though the open loop had the same sequence of amino tain epitopes of an antigen are recognized as immunogenic, acids as the closed loop, it lacked the epitopes recognized by but others are not. Furthermore, some epitopes, called im- the antibody and therefore was unable to block binding of munodominant, induce a more pronounced immune re HEL sponse than other epitopes in a particular animal. It is highly B-cell epitopes tend to be located in flexib likely that the intrinsic topographical properties of the epi unogen and display site mobility. John A. Tainer and his col- tope as well as the animals regulatory mechanisms influence leagues analyzed the epitopes on a number of protein the immunodominance of epitopes antigens(myohemerythrin, insulin, cytochrome G, myoglo bin, and hemoglobin) by comparing the positions of the Antigen-Derived Peptides Are the Key known B-cell epitopes with the mobility of the same Elements of T-Cell Epitopes residues. Their analysis revealed that the major antigenic de- terminants in these proteins generally were located in the Studies by P. G. H Gell and Baruj Benacerraf in 1959 sug most mobile regions. These investigators proposed that site gested that there was a qualitative difference between the t- mobility of epitopes maximizes complementarity with the cell and the B-cell response to protein antigens. Gell and antibody's binding site, permitting an antibody to bind with Benacerraf compared the humoral(B-cell) and cell-medi an epitope that it might bind ineffectively if it were rigid. ated (T-cell)responses to a series of native and denatured However, because of the loss of entropy due to binding to a protein antigens(Table 3-5). They found that when primary flexible site, the binding of antibody to a flexible epitope is immunization was with a native protein, only native protein, generally of lower affinity than the binding of antibody to a not denatured protein, could elicit a secondary antibody (hu- moral)response. In contrast, both native and denatured pro Complex proteins contain multiple overlapping B-cell epi- tein could elicit a secondary cell-mediated response. The topes, some of which are immunodominant. For many years, it finding that a secondary response mediated by t cells was in- was dogma in immunology that each globular protein had a duced by denatured protein, even when the primary immu- small number of epitopes, each confined to a highly accessi- nization had been with native protein, initially puzzled ble region and determined by the overall conformation of the immunologists. In the 1980s, however, it became clear that T otein. However, it has been shown more recently that most cells do not recognize soluble native antigen but rather rec- of the surface of a globular protein is potentially antigenic. ognize antigen that has been processed into antigenic pep- This has been demonstrated by comparing the antigen-bind- tides, which are presented in combination with MHC ing profiles of different monoclonal antibodies to various molecules. For this reason, destruction of the conformation globular proteins. For example, when 64 different mono- of a protein by denaturation does not affect its T-cell epi clonal antibodies to BSA were compared for their ability to topes bind to a panel of 10 different mammalian albumins, 25 dif- Because the T-cell receptor does not bind free peptides, ferent overlapping antigen-binding profiles emerged, sug- experimental systems for studying T-cell epitopes must in gesting that these 64 different antibodies recognized a clude antigen-presenting cells or target cells that can display minimum of 25 different epitopes on BSA. Similar findings the peptides bound to an MHC molecule have emerged for other globular proteins, such as myoglobin Antigenic peptides recognized by t cells form trimolecular and HEL mplexes with a T-cell receptor and an MHC molecule(Figure The surface of a protein, then, presents a large number of 3-8). The structures of TCR-peptide- MHC trimolecular potential antigenic sites. The subset of antigenic sites on a complexes have been determined by x-ray crystallography given protein that is recognized by the immune system of an and are described in Chapter 9. These structural studies of animal is much smaller than the potential antigenic reper- class I or class II MHC molecules crystallized with known t tore, and it varies from species to species and even among in- cell antigenic peptides has shown that the peptide binds to a TABLE 3.5 Antigen recognition by T and B lymphocytes reveals qualitative differences SECONDARY IMMUNE RESPONSE Primary immunization Secondary immunization Cell-mediated ToTH response Native protein Denatured protei TDTH is a subset of CD4 TH cells that mediate a cell-mediated response called delayed-type hypersensitivity (see Chapter 14)similar to HEL to be recognized by antibody to native HEL. Even though the open loop had the same sequence of amino acids as the closed loop, it lacked the epitopes recognized by the antibody and therefore was unable to block binding of HEL. B-cell epitopes tend to be located in flexible regions of an im￾munogen and display site mobility. John A. Tainer and his col￾leagues analyzed the epitopes on a number of protein antigens (myohemerytherin, insulin, cytochrome c, myoglo￾bin, and hemoglobin) by comparing the positions of the known B-cell epitopes with the mobility of the same residues. Their analysis revealed that the major antigenic de￾terminants in these proteins generally were located in the most mobile regions. These investigators proposed that site mobility of epitopes maximizes complementarity with the antibody’s binding site, permitting an antibody to bind with an epitope that it might bind ineffectively if it were rigid. However, because of the loss of entropy due to binding to a flexible site, the binding of antibody to a flexible epitope is generally of lower affinity than the binding of antibody to a rigid epitope. Complex proteins contain multiple overlapping B-cell epi￾topes, some of which are immunodominant. For many years, it was dogma in immunology that each globular protein had a small number of epitopes, each confined to a highly accessi￾ble region and determined by the overall conformation of the protein. However, it has been shown more recently that most of the surface of a globular protein is potentially antigenic. This has been demonstrated by comparing the antigen-bind￾ing profiles of different monoclonal antibodies to various globular proteins. For example, when 64 different mono￾clonal antibodies to BSA were compared for their ability to bind to a panel of 10 different mammalian albumins, 25 dif￾ferent overlapping antigen-binding profiles emerged, sug￾gesting that these 64 different antibodies recognized a minimum of 25 different epitopes on BSA. Similar findings have emerged for other globular proteins, such as myoglobin and HEL. The surface of a protein, then, presents a large number of potential antigenic sites. The subset of antigenic sites on a given protein that is recognized by the immune system of an animal is much smaller than the potential antigenic reper￾toire, and it varies from species to species and even among in￾dividual members of a given species. Within an animal, cer￾tain epitopes of an antigen are recognized as immunogenic, but others are not. Furthermore, some epitopes, called im￾munodominant, induce a more pronounced immune re￾sponse than other epitopes in a particular animal. It is highly likely that the intrinsic topographical properties of the epi￾tope as well as the animal’s regulatory mechanisms influence the immunodominance of epitopes. Antigen-Derived Peptides Are the Key Elements of T-Cell Epitopes Studies by P. G. H. Gell and Baruj Benacerraf in 1959 sug￾gested that there was a qualitative difference between the T￾cell and the B-cell response to protein antigens. Gell and Benacerraf compared the humoral (B-cell) and cell-medi￾ated (T-cell) responses to a series of native and denatured protein antigens (Table 3-5). They found that when primary immunization was with a native protein, only native protein, not denatured protein, could elicit a secondary antibody (hu￾moral) response. In contrast, both native and denatured pro￾tein could elicit a secondary cell-mediated response. The finding that a secondary response mediated by T cells was in￾duced by denatured protein, even when the primary immu￾nization had been with native protein, initially puzzled immunologists. In the 1980s, however, it became clear that T cells do not recognize soluble native antigen but rather rec￾ognize antigen that has been processed into antigenic pep￾tides, which are presented in combination with MHC molecules. For this reason, destruction of the conformation of a protein by denaturation does not affect its T-cell epi￾topes. Because the T-cell receptor does not bind free peptides, experimental systems for studying T-cell epitopes must in￾clude antigen-presenting cells or target cells that can display the peptides bound to an MHC molecule. Antigenic peptides recognized by T cells form trimolecular complexes with a T-cell receptor and an MHC molecule(Figure 3-8). The structures of TCR-peptide-MHC trimolecular complexes have been determined by x-ray crystallography and are described in Chapter 9. These structural studies of class I or class II MHC molecules crystallized with known T￾cell antigenic peptides has shown that the peptide binds to a Antigens CHAPTER 3 67 TABLE 3-5 Antigen recognition by T and B lymphocytes reveals qualitative differences SECONDARY IMMUNE RESPONSE Primary immunization Secondary immunization Antibody production Cell-mediated TDTH response* Native protein Native protein Native protein Denatured protein  * TDTH is a subset of CD4 TH cells that mediate a cell-mediated response called delayed-type hypersensitivity (see Chapter 14). 8536d_ch03_057-075 8/6/02 10:28 AM Page 67 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: