Processing fate of protein antigen attached to IgD or MHC molecules on normal B lymphocytes using heterocrosslinked bispecific antibodies

We have studied the internalization, processing and presentation of hen egg lysozyme (HEL) attached to surface IgD (sIgD) or MHC molecules on normal murine B cells, using heterocrosslinked bispecific antibodies (HBA). Nearly all HEL attached to sIgD was internalized within one hour, with at least a portion rapidly entering a chloroquine-sensitive, acidic environment. Degradation and presentation of HEL to hybridoma T cells began several hours after internalization. Degraded HEL was found in the medium after about 6 hr incubation, but at no time were significant amounts of HEL peptides found within the cells. When HEL was attached to class I or class II MHC molecules, its rate of internalization was low. The fraction of antigen bound to MHC molecules that was inside the cell was always low, even at later stages of culture, but the internalized antigen was located in an acidic environment. Degradation and presentation of HEL internalized via MHC molecules followed internalization. No difference was observed in the processing fate of HEL attached to class I or class II MHC molecules. These results suggest that the rate limiting step in antigen processing and presentation is antigen degradation, when the antigen is bound to sIgD, and internalization when bound to MHC molecules. The slow and steady processing of bound or internalized antigen could provide a sustained presence of antigen on the B cell surface and enhance the potential for its presentation to T cells.