A 3-DIMENSIONAL MODEL OF THE FAS APO-1 MOLECULE - CROSS-REACTIVITY OFANTI-FAS ANTIBODIES EXPLAINED BY STRUCTURAL MIMICRY OF ANTIGENIC SITES/

Fas/APO-1 is a member of the tumor necrosis factor (TNF)/nerve growth factor receptor family, This cell surface protein, when associated wit h the Fas/APO-1 ligand or specific mAb, elicits an apoptotic response in susceptible cells via an oligomerization of its intracellular domai ns, termed the 'death domains', We have previously mapped the epitopes of a panel of Fas/APO-1-reactive mAb to a series of linear portions o f the Fas/APO-1 molecule, In order to gain a greater understanding of the mode of interaction of these antibodies with the Fas/APO-1 antigen , we constructed a homology-based model of the extracellular portion o f the molecule, based on the crystallographic coordinates of the TNF t ype I receptor, The model clearly demonstrates that the antibodies do not identically mimic the endogenous ligand to achieve their effect, b ut probably act in an analogous manner by recruiting Fas/APO-1 molecul es into clusters which may lead to oligomerization of 'death domains', Moreover, the apparent cross-reactivity observed for the monoclonal a nti-fas antibodies between different linear regions of the Fas/APO-1 m olecule was found to be due, most likely, to the structural mimicry of these epitopes, Reduction of the Fas/APO-1-derived cross-reactive pep tides by dithiothreitol completely abrogated their antigenic reactivit y with the anti-fas mAb CH-11, thus indicating that the establishment of intrapeptide disulfide bonds is critical for antigenic reactivity.