Résumé
B lymphocytes express Ag receptors (BCR) that are composed of ligand binding subunits, the membrane Igs, associated with Ig α/Ig β heterodimers. One main BCR function is to bind and to internalize Ags. Peptides generated from these internalized Ags may be presented to T lymphocytes. Here, we have analyzed the involvement of BCR Ig α/Ig β components in BCR constitutive endocytosis. The role of Ig α subunit in BCR constitutive endocytosis was first determined in the context of an IgM-based BCR. In contrast with BCR that contain wild-type Ig α, surface BCR lacking Ig α cytoplasmic domain were not constitutively internalized. The respective roles of Ig α and Ig β subunits were then analyzed by expressing chimeric molecules containing the cytoplasmic domains of either subunits in a B cell line. Only the Ig α cytoplasmic domain contained an internalization signal that allowed constitutive endocytosis of Ig α chimeras via coated pits and accumulation in sorting-recycling endosomes. This internalization signal is contained in its immunoreceptor tyrosine-based activation motif. These results indicate that Ig α, through its immunoreceptor tyrosine-based activation motif, may account for the ability of IgM/IgD BCR to constitutively internalize monovalent Ags.