J Biol Chem, Vol. 274, Issue 16, 11439-11446, April 16, 1999

Trafficking of the Ig/Ig Heterodimer with Membrane Ig and Bound Antigen to the Major Histocompatibility Complex Class II Peptide-loading Compartment

Bruce K. Brown, Chang Li, Paul C. Cheng, and Wenxia Song

Department of Cell Biology and Molecular Genetics, University of Maryland at College Park, Maryland 20742 and the Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, Illinois 60208

The binding of antigen to the B cell antigen receptor (BCR) initiates two major cellular events. First, upon cross-linking by antigen, the BCR induces signal transduction cascades leading to the transcription of a number of genes associated with B cell activation. Second, the BCR internalizes and delivers antigens to processing compartments, where processed antigenic peptides are loaded onto major histocompatibility complex (MHC) class II molecules for presentation to T helper cells. The BCR consists of membrane Ig (mIg) and Ig/Ig heterodimer (Ig/Ig). The Ig/Ig, the signal transducing component of the BCR, has been indicated to play a role in antigen processing. In order to understand the function of the Ig/Ig in antigen transport, we studied the intracellular trafficking pathway of the Ig/Ig. We show that in the absence of antigen binding, the Ig/Ig constitutively traffics with mIg from the plasma membrane, through the early endosomes, to the MHC class II peptide-loading compartment. Cross-linking the BCR does not alter the trafficking pathway; however, it accelerates the transport of the Ig/Ig to the MHC class II peptide-loading compartment. This suggests that the Ig/Ig heterodimer is involved in BCR-mediated antigen transport through the entire antigen transport pathway.

[back to seminar]