CDW78 - A DETERMINANT ON A MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II SUBPOPULATION THAT CAN BE INDUCED TO ASSOCIATE WITH THE CYTOSKELETON

In the present study we demonstrate that CDw78 monoclonal antibody (mA b) recognizes a distinct subpopulation of major histocompatibility com plex (MHC) class II molecules. We show that the CDw78 epitope is prese nt on less than 10 % of the total number of MHC class II molecules exp ressed on different cells, is not linked to a single isotype, and exhi bits a characteristic expression pattern in tonsils. While mAb against MHC class II (DR, DP and DQ) stained the majority of cells both in th e mantle zone and in germinal centers, the CDw78 staining was more het erogeneous with the strongest reactivity and the highest number of pos itive cells in the mantle zone and in the light centrocyte-rich part o f the germinal centers. Antibodies to this MHC class II subpopulation (e.g. FN1) induced association with the cytoskeleton and a subsequent capping in more than 90 % of peripheral blood B cells. In contrast, mA b against MHC class II (DR, DP and DQ) did not induce association with the cytoskeleton and only 10-20% of B cells were induced to cap, sugg esting that CDw78 defines a population of MHC class II molecules funct ionally different from the majority of these antigens. Scatchard plot analysis indicates that FN1 mAb is of relatively low affinity (K-a = 1 .5 x 10(8) M-1) and monovalent Fab fragments fail to bind to the cell surface with measurable affinity. Our data seen in the context of the ability of FN1 to co-stimulate B cells with a suboptimal dose of anti- mu suggest that CDw78 mAb might recognize a functional important subpo pulation of MHC class II molecules so far not described. It seems like ly that this subpopulation represents dimerized or aggregated MHC clas s II molecules that can selectively bind this low-affinity mAb.