The Role Of The Ig alpha /Ig beta Heterodimer In The Internalization And Intracellular Transport Of The B Cell Antigen Receptor

Abstract

The B cell antigen receptor (BCR) consists of membrane-bound immunoglobulin (mIg), which binds extracellular antigen, and the Ig alpha / Ig beta heterodimer (Ig alpha / Ig beta), the signaling component. Crosslinking of BCR by multivalent antigen initiates signaling, the first step in activating B cells, and leads to rapid internalization and transport of antigen to the major histocompatibility complex (MHC) class II containing compartments (MIICs). Here antigen is degraded to peptides which are loaded onto MHC class II molecules which are transported to the surface for presentation to T helper cells. Studies indicate that BCR-triggered signaling plays a role in accelerated internalization and transport of antigen, but the mechanism is not clear. This work examines the role of the signaling component Ig alpha / Ig beta in the efficient internalization and intracellular transport of the BCR. The effect of a lack of association between Ig alpha / Ig beta and mIgM on internalization and intracellular transport of mIgM was studied using biochemical techniques and immunofluorescence microscopy. We demonstrated that a reduction of association with Ig alpha / Ig beta leads to a reduction in signaling, a defect in internalization and transport to MIICs, and a decrease in antigen presentation. Thus, physical association of mIgM with Ig alpha / Ig beta is necessary for efficient internalization and intracellular transport of mIgM. The role of the tyrosines in the immunoreceptor tyrosine-based activation motif (ITAM) of Ig alpha / Ig beta was examined. Mutation of the N-terminal tyrosine had no significant effect on BCR signaling and antigen transport. Mutation of the C-terminal tyrosine resulted in decreased signaling and internalization and a defect in transport to MIICs. Mutating both tyrosines caused a greater decrease in BCR signaling and a greater defect in internalization and transport of antigen to MIICs than the C-terminal tyrosine mutation. The results of this study indicate that Ig alpha /Ig beta, in particular the ITAM of Ig alpha, is essential for accelerated internalization and intracellular transport of BCR to the MIICs, probably by regulating these processes through signaling.