Complexes of CD28 with mitogenic and non-mitogenic antibodies
(A) Ribbon representation of the complex between the CD28 homodimer and two Fab fragments from the mitogenic antibody 5.11A1 as observed in the crystal structure. The approximate position of the cell membrane, if CD28 were attached to its transmembrane segment, is indicated and the blue spheres mark the position in CD28 shown (by mutagenesis) to be bound by non-mitogenic antibodies. (B) Cryo-electron microscopy (EM) images of complexes formed between the CD28-Fc chimera and whole mitogenic or non-mitogenic antibodies. The left panel of each pair shows a class-averaged cryo-EM image of a complex in a single orientation and the right panel is a cartoon indicating the probable complex topology, with the box marking the region observed (constant regions are not seen in the averages due to their flexible linkage to the core region of the complex). To see a model for how these topological differences could account for the signalling properties of the antibodies see our 'antibody signalling model'. (C) A 3D reconstruction of the core of the CD28-Fc:5.11A1 complex produced by aligning cryo-EM images with the crystal structure (as in A) is shown fitted to the CD28-5.11A1 Fab structure or to a model of a similar complex based on a CTLA-4 like dimer. The solution structure is clearly a better fit with the crystallographic dimer.