New insights into antigen recognition

I MMUNOLOGISTS STRUGGLE WITH THE PROBLEM OF HOW THE immune system manages to recognize and respond to an almost infinite number of foreign organisms and yet fails to turn on components of self. Since the finding of the structure of antibody molecules in the 1960's much progress has been made: for example, the discovery of the rearranging immunoglobulin genes and the identification of the T cell receptor. Many of these advances were discussed at a Whitehead Symposium (October 1986), "Immune Recognition," organized by David Baltimore, Herman Eisen, and David Raulet. One major topic concerned the specificities for antigens of B and T cells. These two types of cells recognize antigen in different ways. B cells and antibodies often distinguish very effectively between different forms of the same protein antigen. Antibodies to native proteins do not always bind denatured versions of the same molecule, and vice versa. There are exceptions to this rule as manifest by the frequent use of antibodies to native proteins to pick up the same protein, after SDS treatment and boiling, in Westem blot procedures. By contrast, T cells do not usually distinguish between native, denatured, and even partially degraded forms of the same protein. Solutions to the problems of antibody specificity can now be provided from x-ray crystallographic studies. Roberto Poljak (Institut Pasteur) summarized the work from his laboratory on the structure of the complex of a monoclonal antibody bound to a protein target, hen egg lysozyme. In the example most closely studied, the two proteins interact at two almost completely flat surfaces with small indentations at which amino acids from one protein penetrate slightly the structure of the other. This interaction is unlike those previously demonstrated for the binding of antibodies to small hapten molecules, in which antigens bind to marked clefts in antibody molecules. Antibody binding to lysozyme is achieved with very little distortion of the native structures of either the antibody or its ligand. The two proteins contact each other with many amino acids, which are by no means continuous on either protein; for example, lysozyme amino acids between 18 and 27 and 116 and 129 are involved in the binding. These are brought together on the same surface by folding of the native molecules. These studies illustrate the importance of protein folding to antibody binding and indicate that framework residues on the surface of the immunoglobulin molecule, as well as those in the socalled complementarity determining or hypervariable regions, may participate in antigen binding. T cells bind antigen in ways that differ from those used by B cells. Cytotoxic and helper T cells cannot bind free antigen, and only recognize antigen when it is associated with a cell surface glycoprotein coded by the major histocompatibility complex (MHC) of the host. Two major types of protein are produced by the MHC for this purpose, class I molecules, usually recognized in association with antigen by cytotoxic T cells, and class II proteins, recognized with antigen by helper T cells. Data accumulated over the last 7 years have suggested that the antigen recognized by helper T cells is a fragment of the native protein; recent studies with virus products have indicated that this may be true for cytotoxic T cells too. These fragments are thought to bind in a trimolecular complex with the T cell receptor and MHC product concemed. The nature of this temary complex has long been a mystery to immunologists and a source of confusion to nonimmunologists since it is difficult to understand how the few class II molecules (for example) produced by any given individual can "bind" the myriad of protein antigens to which the host could be exposed and thus lead to T cell receptor binding and a productive T cell response. Nonetheless, strong evidence in support of this model has recently come from B. P. Babbitt, in Emil Unanue's laboratory (Washington University, St. Louis), who showed by equilibrium dialysis that an isolated peptide from hen egg lysozyme binds to anMHC molecule. In this case, it was known that T cells could recognize the peptide in question in association with a particular class II MHC molecule, I-Ak, but not in association with a different allele of I-A, I-Ad. The work of Babbitt et al. provided an explanation for this difference, since their equilibrium dialysis experiments showed that the lysozyme peptide bound I-Ak but not I_Ad. These findings have been extended by Howard Grey, Soren Buus, and their colleagues (National Jewish Center, Denver), and were discussed at the Whitehead Symposium and are described in this issue of Science (page 1353). They demonstrated that a number of peptides bind various class II molecules in patterns consistent with their ability to stimulate T cells. In most cases, the binding constants are perhaps surprisingly high, averaging about 10-6M. Inhibition experiments indicated that all the peptides tested bind to the same site on a given class II molecule. MHC binding to antigen fragments does have some properties that are strikingly different to those of antibody-antigen interaction. The latter binding has rapid on and off rates; in the former case, antigen fragment binding is slow, but once the peptide is engaged it releases very slowly, with a half-life of perhaps 30 hours. This may indicate some unexpected process, for example, either the antigen fragment or the MHC molecule may have to adopt a rare configuration for binding to occur. A mechanism for such binding has been suggested recently in a paper by Guillet ct al. (Science, 20 February, page 865) who showed that antigenic peptides for T cells have amino acid homology with the class II molecules presenting them. They therefore suggested that the peptide binds to MHC molecules by displacing the homologous region of the protein, a process that might well have slow kinetics. These findings do leave immunologists with several problems. First, they confirm that the paradox of MHC-antigen binding does exist, but they do not explain how MHC molecules have evolved to be so promiscuous in their binding properties. Second, the results do not explain how the system avoids being permanently saturated. Results from Unanue's group indicate that peptides derived from self proteins can also bind to selfMHC molecules. If the binding constants for these interactions are similar to those found for foreign peptides, and there is every reason to suppose that this is true, how do mouse MHC molecules avoid being permanently saturated with peptides from mouse proteins, those of mouse serum albumin, for example? Perhaps the answer lies in continuous MHC protein recycling, and clearance of bound peptides in some intracellular, acidic, lysosome-related compartment. Although many mysteries remain, these studies do serve as useful