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.
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The authors thank S. Ikemizu, E.Y. Jones, P.A. van der Merwe, K.M. Dennehy, S.H. Abidi and L. Hene for comments on the manuscript; and E. Mancini, J. Grimes and the staff of ID13 at ESRF for assistance with data collection. Supported by the Wellcome Trust, the Royal Society and the UK Medical Research Council through funding of the Oxford Protein Production Facility, and by Active Biotech Research AB.
Equilibrium analysis of monomeric sCD28 binding to sB7-1. (PDF 114 kb)
Final electron density maps for the CD28/511A1 Fab structure. (PDF 117 kb)
sB7-2d1 versus full-length sB7-2 binding to the CD28 homodimer. (PDF 128 kb)
Blocking the ligand binding site of CD28 with the 7.3B6 antibody. (PDF 119 kb)
Properties of major lattice contacts seen in crystals of costimulatory proteins. (PDF 32 kb)
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