The cytoskeleton coordinates the early events of B-cell activation

Abstract

B cells contribute to protective adaptive immune responses through generation of antibodies and long-lived memory cells, following engagement of the B-cell receptor (BCR) with specific antigen. Recent imaging investigations have offered novel insights into the ensuing molecular and cellular events underlying B-cell activation. Following engagement with antigen, BCR microclusters form and act as sites of active signaling through the recruitment of intracellular signaling molecules and adaptors. Signaling through these "microsignalosomes" is propagated and enhanced through B-cell spreading in a CD19-dependent manner. Subsequently, the mature immunological synapse is formed, and functions as a platform for antigen internalization, enabling the antigen presentation to helper T cells required for maximal B-cell activation. In this review, we discuss the emerging and critical role for the cytoskeleton in the coordination and regulation of these molecular events during B-cell activation.

Figure 1.

Dual-View TIRFM for simultaneous tracking of BCR alongside the actin cytoskeleton. (A) Schematic representation of the setup of the Dual-View TIRFM. The total internal reflection of incoming laser beam (purple) generates evanescent waves capable of exciting flurophores in close proximity to the sample interface. The two colored emission beams (green and red) are passed through beam splitters, detected simultaneously by a CCD camera and overlaid in an analysis computer. (B) Snapshot of a movie collected tracking single molecules of BCR (red, anti-BCR Fab) and actin (green, LifeAct a marker of filamentous actin). BCRs located in actin-rich areas are more restricted in their diffusion (yellow tracks in schematic zoom), whereas BCRs located in actin-poor areas tend to be less restricted in their diffusion (blue tracks in schematic zoom).

Figure 2.

Two models of T-cell triggering. (A) The conformational change model states that extracellular pMHC binding to the TCR triggers intracellular conformational changes in the associated CD3 complex, opening up binding sites for the intracellular adaptor Nck. Subsequently, the “opened” TCR recruits activatory kinases (such as Lck) to phosphorylate the TCR, leading to assembly of the signalosome. (B) The kinetic segregation model postulates that, in the resting state, TCR is equally likely to be phosphorylated by activatory kinases or dephosphorylated inhibitory phosphatases. On proximity of an antigen-presenting cell, small adhesion molecules such as CD2 and CD58 establish a “close contact zone” that excludes bulky surface molecules such as CD45, but permits diffusion of TCR and Lck. TCR in the “close contact zone” is more likely to be found in a phosphorylated state, and the half-life of phosphorylated TCR in this region is increased following recognition of specific pMHC, triggering the assembly of the signalosome and T-cell activation.

Figure 3.

B-cell activation by antigen on the surface of a presenting cell. Schematic representation of the processes occurring at the B-cell surface following recognition of antigen on a presenting surface and what is currently known about the molecular requirements of these processes. On initial contact with antigen, BCR microclusters form, driving B-cell spreading. During spreading, signaling active BCR microclusters are moved (possibly via reterograde actin flow) toward the central cluster. Following achievement of maximum spread, the more prolonged contraction phase occurs, resulting in the formation of the mature immunological synapse. The central cluster of the immunological synapse acts as a platform for internalization of antigen. It appears that at every stage, cytoskeleton reorganizations play an important role in orchestrating and coordinating the molecular processes.