Structural studies on the leukocyte co-stimulatory molecule, B7-1

B7-1 and B7-2 are glycoproteins expressed on antigen presenting cells. The binding of these molecules to the T-cell homodimers, CD28 and CTLA-4, generate 'costimulatory' and inhibitory signals in T cells, respectively. The crystal structure of the extracellular region of B7-1 (sB7-1), solved to 3 Angstrom resolution, consists of a novel combination of two Ig-like domains, one characteristic of adhesion molecules and the other previously seen only in antigen receptors. In the crystal lattice, sB7-1 unexpectedly forms parallel, 2-fold rotationally symmetric homodimers. The structural data suggest a mechanism whereby the avidity-enhanced binding of B7-1 and CTLA-4 homodimers, along with the relatively high affinity of these interactions, favours the formation of very stable inhibitory signaling complexes