A three-dimensional model of the Fas/APO-1 molecule: cross-reactivity of anti-Fas antibodies explained by structural mimicry of antigenic sites

Fas/APO-1 is a member of the tumor necrosis factor (TNF)/nerve growthfactor receptor family. This cell surface protein, when associated with theFas/APO-1 ligand or specific mAb, elicits an apoptotic response insusceptible cells via an oligomerization of its intracellular domains,termed the'death domains'. We have previously mapped the epitopes of apanel of Fas/APO-1-reactive mAb to a series of linear portions of theFas/APO-1 molecule. In order to gain a greater understanding of the mode ofinteraction of these antibodies with the Fas/APO-1 antigen, we constructeda homology-based model of the extracellular portion of the molecule, basedon the crystallographic coordinates of the TNF type I receptor. The modelclearly demonstrates that the antibodies do not identically mimic theendogenous ligand to achieve their effect, but probably act in an analogousmanner by recruiting Fas/APO-1 molecules into clusters which may lead tooligomerization of 'death domains'. Moreover, the apparent cross-reactivityobserved for the monoclonal anti-Fas antibodies between different linearregions of the Fas/APO-1 molecule was found to be due, most likely, to thestructural mimicry of these epitopes. Reduction of the Fas/APO-1-derivedcross-reactive peptides by dithiothreitol completely abrogated theirantigenic reactivity with the anti-Fas mAb CH-11, thus indicating that theestablishment of intrapeptide disulfide bonds is critical for antigenicreactivity.