Antigen-B Cell Receptor Complexes Associate with Intracellular MHC Class II Molecules [Cell Biology]

September 23rd, 2015 by Barroso, M., Tucker, H., Drake, L., Nichol, K., Drake, J. R.

Antigen processing and MHC class II-restricted antigen presentation by antigen presenting cells (APC) such as dendritic cells and B cells allows for activation of naive CD4+ T cells and cognate interactions between B cells and effector CD4+ T cells, respectively. B cells are unique among class II-restricted APC in that they have a clonally restricted antigen-specific receptor, the B cell receptor (BCR), which allows the cell to recognize and respond to trace amounts of foreign antigen present in a sea of self-antigens. Moreover, engagement of peptide-class II complexes formed via BCR-mediated processing of cognate antigen has been shown to result in a unique pattern of B cell activation. Using a combined biochemical and imaging / FRET approach, we establish that internalized antigen-BCR complexes associate with intracellular class II molecules. We demonstrate that the M1-paired MHC class II conformer, previously shown to be critical for CD4 T cell activation, is selectively incorporated into these complexes and selectively loaded with peptide derived from BCR-internalized cognate antigen. These results demonstrate that in B cells internalized Ag-BCR complexes associate with intracellular MHC class II molecules, potentially defining a site of class II peptide acquisition, and reveal a selective role for the M1-paired class II conformer in the presentation of cognate antigen. These findings provide key insights into the molecular mechanisms that B cells use to control the source of peptides charged onto class II molecules, allowing the immune system to mount an antibody response focused on BCR-reactive cognate antigen.