INTRACELLULAR TARGETING OF ANTIGENS INTERNALIZED BY MEMBRANE IMMUNOGLOBULIN IN B-LYMPHOCYTES

An important function of membrane immunoglobulin (mIg), the B cell ant igen receptor, is to endocytose limiting quantities of antigen for eff icient presentation to class II-restricted T cells. We have used a pan el of mIg mutants to analyze the mechanisms of mIg-mediated antigen pr esentation, and specifically to explore the ability of mIg to target i nternalized antigen to intracellular processing compartments. Transfec ted mIgs carrying substitutions for the transmembrane Tyr(587) residue fail to efficiently present specifically bound antigen. However, thes e mutants internalize antigen normally, and their defect cannot be att ributed to a lack of mIg-associated Ig alpha/Ig beta molecules. A nove l functional assay for detecting antigenic peptides in subcellular fra ctions shows that wild-type mIg transfectants generate class II-peptid e complexes intracellularly, whereas only free antigenic peptides are detectable in the mutant mIg transfectants. Furthermore, an antigen co mpetition assay reveals that antigen internalized by the mutant mIgs f ails to enter the intracellular processing compartment accessed by wil d-type mIg. Therefore, mIg specifically targets bound and endocytosed antigen to the intracellular compartment where processed peptides asso ciate with class II molecules, and the transmembrane Tyr(587) residue plays an obligatory role in this process. Targeting of internalized an tigen may be mediated by receptor-associated chaperones, and may be a general mechanism for optimizing the presentation of specifically boun d and endocytosed antigens in B lymphocytes and other antigen-presenti ng cells.