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How BCRs mingle
The B cell antigen receptor (BCR) consists of a plasma membrane–bound antibody [immunoglobulin (Ig)] that is associated with a pair of signaling proteins. Antigen binding to the BCR stimulates B cells to differentiate into antibody-secreting cells. Maity et al. used high-resolution microscopy, electron microscopy, and proximity ligation assays to visualize the organization of IgM-BCRs and IgD-BCRs on mature B cells. Under resting conditions, the different BCRs were separated in relatively large clusters called protein islands. Antigen triggered the protein islands to become smaller and more disperse, reducing the distance between the different BCRs.
The B cell antigen receptors (BCRs) play an important role in the clonal selection of B cells and their differentiation into antibody-secreting plasma cells. Mature B cells have both immunoglobulin M (IgM) and IgD types of BCRs, which have identical antigen-binding sites and are both associated with the signaling subunits Igα and Igβ, but differ in their membrane-bound heavy chain isoforms. By two-color direct stochastic optical reconstruction microscopy (dSTORM), we showed that IgM-BCRs and IgD-BCRs reside in the plasma membrane in different protein islands with average sizes of 150 and 240 nm, respectively. Upon B cell activation, the BCR protein islands became smaller and more dispersed such that the IgM-BCRs and IgD-BCRs were found in close proximity to each other. Moreover, specific stimulation of one class of BCR had minimal effects on the organization of the other. These conclusions were supported by the findings from two-marker transmission electron microscopy and proximity ligation assays. Together, these data provide evidence for a preformed multimeric organization of BCRs on the plasma membrane that is remodeled after B cell activation.