Imaging B-cell receptor signaling by single-molecule techniques.

B-cell activation initiates antibody responses against pathogens. Recent imaging of B cells in vivo shows that B cells move rapidly through lymphoid tissues to search for antigens captured on the surfaces of antigen-presenting cells. Recognition of antigens by the B-cell antigen receptor (BCR) leads to microclustering of the BCR and the initiation of intracellular signaling that prompts the B cells to stop and form immunological synapses with the antigen-presenting cells. Although the biochemical signaling pathways downstream of the BCR that mediate these events are becoming better characterized, the initial molecular steps in BCR microclustering and activation remain elusive. In part, this is because the dynamics of the cell-cell contact makes the observation of the antigen-induced changes in the BCR technically challenging. Here we review single-molecule imaging techniques that help to provide new information on the molecular behavior of small populations of the BCR as they initiate intracellular signaling in a dynamically moving B cell. The techniques are generally applicable to the study of a broad range of membrane receptors involved in cell-cell contacts.