Crystal structure of a soluble CD28-Fab complex

Evans EJ, Esnouf RM, Manso-Sancho R, Gilbert RJ, James JR, Yu C, Fennelly JA, Vowles C, Hanke T, Walse B, Hünig T, Sørensen P, Stuart DI, Davis SJ. (2005), Nat Immunol. 6, 271-9

Abstract

Naive T cell activation requires signaling by the T cell receptor and by nonclonotypic cell surface receptors. The most important costimulatory protein is the monovalent homodimer CD28, which interacts with CD80 and CD86 expressed on antigen-presenting cells. Here we present the crystal structure of a soluble form of CD28 in complex with the Fab fragment of a mitogenic antibody. Structural comparisons redefine the evolutionary relationships of CD28-related proteins, antigen receptors and adhesion molecules and account for the distinct ligand-binding and stoichiometric properties of CD28 and the related, inhibitory homodimer CTLA-4. Cryo-electron microscopy-based comparisons of complexes of CD28 with mitogenic and nonmitogenic antibodies place new constraints on models of antibody-induced receptor triggering. This work completes the initial structural characterization of the CD28-CTLA-4-CD80-CD86 signaling system.

Key figure: Proposed ligand- and 5.11A1 Fab complexes formed by putative CD28 homodimers

(a) Orthogonal views of the extracellular region of modeled CD80−CD28 homodimer complexes. With SHP, each of the CTLA-4 monomers from the CTLA-4−CD80 (PDB accession number 1i8l; dark blue) and CD86−CTLA-4 (PDB accession number 1i85; pale blue) complexes was superimposed in turn on each of the monomers in the crystallographic CD28 homodimer (red). CD80 was then docked with CD28 according to the mode of ligand binding seen for the CTLA-4 monomer. (b) Superposition of CD28 and CTLA-4 homodimers as in Figure 4b, after the docking of CD80 to both molecules, as in a, based on the binding seen in PDB file 1i8l. (c) Two orthogonal views of the complex formed by the crystallographic sCD28 homodimer (red) and bivalently bound 5.11A1 Fab (green). The position of the mutation disrupting nonmitogenic antibody binding in rat CD28 is marked with light blue spheres. Top, horizontal line indicates approximate position of the membrane; the line of view is parallel to the membrane. Bottom, the line of view is toward the T cell.