PMID: 21444918

Authors:

Niederfellner G, Lammens A, Mundigl O, Georges GJ, Schaefer W, Schwaiger M, Franke A, Wiechmann K, Jenewein S, Slootstra JW, Timmerman P, Brannstrom A, Lindstrom F, Mossner E, Umana P, Hopfner KP, Klein C

Title:

Epitope characterization and crystal structure of GA101 provide insights into the molecular basis for type I/II distinction of CD20 antibodies.

Journal:

Blood. 2011 Mar 28.

Abstract:

CD20 is a cell surface marker of normal and malignant B cells. Rituximab, a monoclonal antibody targeting CD20, has improved treatment of malignant lymphomas. Therapeutic CD20 antibodies are classified as either type I or II based on different mechanisms of killing malignant B-cells. To reveal the molecular basis of this distinction, we fine mapped the epitopes recognized by both types. We also determined the first X-ray structure of a type II antibody by crystallizing the obinutuzumab (GA101) Fab fragment alone and in complex with a CD20 cyclopeptide. Despite recognizing an overlapping epitope, GA101 binds CD20 in a completely different orientation than type I antibodies. Moreover, the elbow angle of GA101 is almost 30 degrees wider than in type I antibodies potentially resulting in different spatial arrangements of two CD20 molecules bound to a single GA101 or rituximab molecule. By protein tomography different CD20 complexes were found associated with the two antibodies and confocal microscopy showed different membrane compartmentalization of these subpopulations of the cellular CD20 pool. Taken together, our findings offer a possible molecular explanation for the different cellular responses elicited by type I and II antibodies.