Antigen endocytosis and presentation mediated by human membrane IgG1 in the absence of the IgA/Igβ dimer

Membrane immunoglobulin (mIg) M and D heavy chains possess minimal (KVK) cytoplasmic tails and associate with the Igα/Igβ (CD79) dimer to achieve surface expression and antigen presentation function. In contrast, the cytoplasmic tail of mIgG is extended by 25 residues (γct). We have tested the possibility that mIgG can perform antigen capture and presentation functions independently of the Igα/β dimer. We show that CD4/γct chimeras are efficiently endocytosed partially dependent on a tyrosine residue in γct. In addition, human mIgG was expressed on the surface of Igα/Igβ-negative non-lymphoid cells and mediated antigen capture and endocytosis. Antigen-specific human mIgG targeted antigen to MIIC-type vesicles in the Igα/β negative melanoma Mel JuSo and augmented antigen presentation 1000-fold, identical to the augmentation seen in Igα/β-positive B-cells expressing the same transfected mIgG. Thus, unlike mIgM, mIgG has autonomous antigen capture and presentation capacity, which may have evolved to reduce or eliminate the BCR's dependence on additional accessory molecules.