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 by Ag binding to the B cell Ag receptor (BCR). Using subcellular fractionation, we recently identified a compartment in B cells in which functional, processed Ag-class II complexes are formed following BCR-mediated Ag internalization, referred to as the peptide-loading compartment. These studies, however, did not address the transport of Ag or BCR from 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 intracellular movement. We show that sIg constitutively transports Ag from the plasma membrane, through endosomes, to the MHC class II peptide-loading compartment. The cross-linking of the BCR increases the rate of internalization of sIg and bound Ag, but does not alter the trafficking pathway. Thus, the delivery of Ag to the class II peptide-loading compartment by the sIg is independent of BCR cross-linking, but can be influenced by BCR cross-linking.