B cells recognize foreign antigens by virtue of cell surface immunoglobulin receptors and are most effectively activated by membrane-bound ligands. Here, we show that in the early stages of this process, B cells exhibit a two-phase response in which they first spread over the antigen-bearing membrane and then contract, thereby collecting bound antigen into a central aggregate. The extent of this response, which is both signaling- and actin-dependent, determines the quantity of antigen accumulated and hence the degree of B cell activation. Brownian dynamic simulations reproduce essential features of the antigen collection process and suggest a possible basis for affinity discrimination. We propose that dynamic spreading is an important step of the immune response.

The integrin LFA-1 and its ligand ICAM-1 mediate B cell adhesion, but their role in membrane-bound antigen recognition is still unknown. Here, using planar lipid bilayers and cells expressing ICAM-1 fused to green fluorescence protein, we found that the engagement of B cell receptor (BCR) promotes B cell adhesion by an LFA-1-mediated mechanism. LFA-1 is recruited to form a mature B cell synapse segregating into a ring around the BCR. This distribution is maintained over a wide range of BCR/antigen affinities (10(6) M(-1) to 10(11) M(-1)). Furthermore, the LFA-1 binding to ICAM-1 reduces the level of antigen required to form the synapse and trigger a B cell. Thus, LFA-1/ICAM-1 interaction lowers the threshold for B cell activation by promoting B cell adhesion and synapse formation.

Here we describe the spatiotemporal architecture, at high molecular resolution, of receptors and signaling molecules during the early events of mouse B cell activation. In response to membrane-bound ligand stimulation, antigen aggregation occurs in B cell antigen receptor (BCR) microclusters containing immunoglobulin (Ig) M and IgD that recruit the kinase Syk and transiently associate with the coreceptor CD19. Unexpectedly, CD19-deficient B cells were significantly defective in initiation of BCR-dependent signaling, accumulation of downstream effectors and cell spreading, defects that culminated in reduced microcluster formation. Hence, we have defined the dynamics of assembly of the main constituents of the BCR 'signalosome' and revealed an essential role for CD19, independent of the costimulatory molecule CD21, in amplifying early B cell activation events in response to membrane-bound ligand stimulation.