The tipping points in the initiation of B cell signalling: how small changes make big differences

Abstract

B cells are selected by the binding of antigen to clonally distributed B cell receptors (BCRs), triggering signalling cascades that result in B cell activation. With the recent application of high-resolution live-cell imaging, we are gaining an understanding of the events that initiate BCR signalling within seconds of its engagement with antigen. These observations are providing a molecular explanation for fundamental aspects of B cell responses, including antigen affinity discrimination and the value of class switching, as well as insights into the underlying causes of B cell tumorigenesis. Advances in our understanding of the earliest molecular events that follow antigen binding to the BCR may provide a general framework for the initiation of signalling in the adaptive immune system.

Figure 1. The structural organization of the B cell receptor

The four-chain structure of the IgM B cell receptor (BCR), composed of the membrane form of IgM associated with a covalently linked heterodimer of Igα and Igβ, is depicted. Cell surface BCRs are shown in a compartmentalized plasma membrane such as that proposed by Kusumi and colleagues (BOX 2). This schematic is based on observations from single particle tracking experiments from live-cell imaging showing that the BCRs exist mainly as highly mobile monomers on resting B cells, with only ~20% comprising less-mobile dimers or more highly ordered oligomers. The membrane-proximal Cμ4 portion of the membrane-bound IgM (or the Cγ3 portion of membrane-bound IgG, not shown) was determined to be required and sufficient to induce BCR oligomerization and immobilization upon membrane-bound monomeric antigen binding. The immunoreceptor tyrosine-based activation motif (ITAM)-containing cytoplasmic tails of the BCR extend through the cytoskeleton into the cytoplasm.

Figure 2. The ‘conformation-induced oligomerization model’ for B cell receptor microcluster formation

Depicted is an IgM B cell receptor (BCR) in the absence of antigen and in a conformation that is not receptive to oligomerization. The binding of monovalent antigen in a fluid lipid bilayer that cannot physically crosslink the BCR exerts a force that brings the BCR into an oligomerization-receptive form. The subsequent random bumping of antigen-bound BCRs results in their oligomerization through the membrane-proximal Cμ4 domain and in the initiation of signalling. ITAM, immunoreceptor tyrosine-based activation motif.

Figure 3. B cell microclusters grow with time after antigen binding

a | B cell receptors (BCRs) in the absence of antigen (left) and in the presence of antigen (right). b | Confocal images showing the spatial distribution of BCRs on the surface of a B cell placed on a fluid lipid bilayer in the absence of antigen (left), showing the BCRs covering the entire B cell surface, and (right) on an antigen-containing fluid lipid bilayer, showing the BCRs accumulating at the interface of the B cell with the bilayer. c | Total internal reflection fluorescence microscopy images demonstrating the growth, over time, of the microcluster within the area indicated by the white box. SYK, spleen tyrosine kinase.

Figure 4. Models for how B cell receptor oligomers trigger signalling

a | B cell receptor (BCR) oligomers may simply bring the BCR-associated tyrosine kinase LYN into close proximity with the Igα–Igβ heterodimer for trans-phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs). b | Alternatively, they may perturb the local lipid environment, causing the coalescence of lipid rafts and bringing the lipid-raft-associated LYN into close proximity with ITAMs of the Igα–Igβ heterodimer for phosphorylation. c | Yet another possibility is that BCR oligomers alter the association of membrane-bound immunoglobulin and Igα–Igβ, inducing changes in the cytoplasmic tails of Igα–Igβ and allowing LYN to access the ITAMs for phosphorylation. APC, antigen-presenting cell.