The interaction of B cell receptors with antigens gives birth to the immunological synapse, spurring B cell activation and antigen presentation to T cells if the antigen is sufficiently well suited to the B cell receptor. Batista's group examined the cellular changes that accompany the first moments of synapse formation.
The authors exposed naive B cells to lysozyme antigens with varying affinities for a B cell receptor and watched as the cells first spread, and then contracted inward, drawing in their captured antigens. The degree of B cell spreading depended on two factors: the amount of antigen detected by the B cell and its affinity toward the antigen.
The spread–contract mechanism may help the immune system select for B cells that produce the highest affinity antibodies to a specific antigen. “If you have a high-affinity interaction, you will see higher spreading,” says Batista. More antigen will thus be aggregated, which in turn will increase B cell maturation and activation.
The researchers supported their experimental results with a mathematical model in which disabling B cell spreading led to similar amounts of antigen accumulation for both low- and high-affinity antigens. With spreading, however, abundant high-affinity antigens were gathered in higher quantities.
Batista notes that, by spreading and contracting, a B cell improves its chances of eliciting T cell help. More broadly, he says, “this could be a general mechanism by which cells recognize different ligands.” T cells, for example, were recently shown to spread in a similar manner.