Cyclosporin A as a model antigen: immunochemical and structural studies.

The immunosuppressant drug cyclosporin (Cs) A is a cyclic undecapeptide which has been used as a model antigen because structural information and a large number of analogs, modified at each of its 11 positions, were available. This review summarizes immunochemical and crystallographic studies of the interaction between the Fab of monoclonal antibody R45-45-11 and Cs. Three points are discussed: (1) the different conformations of CsA and the question of its biologically active form; (2) the Fab-CsA recognition mechanism; and (3) the relationship between structure and binding properties of CsA analogs.