The nature of the subset of MHC class II molecules carrying the CDw78 epitopes

A CDw78 mAb FN1 was shown to recognize DP and/or DR molecules under the con ditions of Western blotting, DP molecules were specifically retarded on a c olumn of the FN1 immunosorbent; binding of FITC-labeled FN1 to B cell lines was completely blocked by excess of mAb to DR/DP beta chains, partially by several mAb to DP and weakly by some mAb to DR, The binding of two other C Dw78 mAb, FN4 and MR11, to the B cell surface was most strongly inhibited b y excess of different mAb to DR. Kinetics of stable binding of the CDw78 mA b indicated that their monovalent binding is of low affinity and that the s table binding to the surface is due to bivalent binding to two spatially cl ose MHC class II molecules, FN1-based immunosorbent effectively immunoisola ted complexes of MHC class II proteins with several tetraspanin molecules f rom a mild detergent lysate of a B cell line. It is concluded that FN1 and most likely also the other two CDw78 mAb recognize with low affinity determ inants on MHC class II molecules (DP or DR) and preferentially bind in a st able fashion to dimerized or aggregated MHC class II molecules. Such dimers or aggregates may either exist as preformed on the cell surface or may be gradually formed and stabilized by bivalent interaction with mAb, These str uctures may be related to the previously described 'superdimers' of MHC cla ss II and/or 'MHC-tetraspanin complexes', CDw78 mAb may be valuable tools t argeting such aggregated fraction of MHC class II molecules which can exhib it important signaling and antigen-presenting properties.