STRUCTURE OF A MONOCLONAL ANTI-ICAM-1 ANTIBODY R6.5 FAB FRAGMENT AT 2.8 ANGSTROM RESOLUTION

The specific binding of the monoclonal murine anti-intercellular adhes ion molecule-1 (anti-ICAM-1) antibody, R6.5, inhibits the attachment o f neutrophils to endothelium and prevents the attachment of major grou p human rhinovirus (HRV) to ICAM-1. This binding interferes with the h ost immune system and, as a result, the R6.5 antibody has been develop ed as a therapeutic antiinflammatory and perhaps anti-HRV agent. The v ariable-region amino-acid sequence of R6.5 was determined from the ant i-ICAM-1 cDNA. The crystallization conditions of the Fab fragment of R 6.5 were established and the three-dimensional structure was determine d by X-ray crystallography. The crystal space group is orthorhombic P2 (1)2(1)2(1), a = 40.36, b = 137.76, c = 91.32 Angstrom, and the highes t resolution of recorded reflections is 2.7 Angstrom. The molecular-re placement method using known Fab structures was employed to solve the R6.5 Fab structure. The final R factor is 18.8% for a total of 3320 no n-H protein atoms, 39 water molecules and 10 606 unique reflections. T he protein exhibits the typical immunoglobulin fold. The surface conto ur of the antigen-combining site of the R6.5 antibody has a wide groov e which resembles more the structure of an anti-polypeptide antibody t han the structure of an anti-protein antibody.