ENTRY OF B-CELL ANTIGEN RECEPTOR AND ANTIGEN INTO CLASS-II PEPTIDE-LOADING COMPARTMENT IS INDEPENDENT OF RECEPTOR CROSS-LINKING

The processing and presentation of Ag by B lymphocytes are initiated b y Ag binding to the B cell Ag receptor (BCR). Using subcellular fracti onation, we recently identified a compartment in B cells in which func tional, processed Ag-class II complexes are formed following BCR-media ted Ag internalization, referred to as the peptide-loading compartment . These studies, however, did not address the transport of Ag or BCR f rom the cell surface to the peptide-loading compartment. In this work, we describe the intracellular trafficking of Ag and surface Ig (sIg) in B cells and evaluate the effect of cross-linking sIg on this intrac ellular movement. We show that sIg constitutively transports Ag from t he plasma membrane, through endosomes, to the MHC class II peptide-loa ding compartment, The cross-linking of the BCR increases the rate of i nternalization of sIg and bound Ag, but does not alter the trafficking pathway. Thus, the delivery of Ag to the class II peptide-loading com partment by the sIg is independent of BCR cross-linking, but can be in fluenced by BCR cross-linking.