Formation of BCR Oligomers Provides a Mechanism for B cell Affinity Discrimination

Abstract

B cells encounter antigen over a wide affinity range. The strength of B cell signaling in response to antigen increases with affinity, a process known as "affinity discrimination". In this work, we use a computational simulation of B cell surface dynamics and signaling to show that affinity discrimination can arise from the formation of BCR oligomers. It is known that BCRs form oligomers upon encountering antigen, and that the size and rate of formation of these oligomers increase with affinity. In our simulation, we have introduced a requirement that only BCR-antigen complexes that are part of an oligomer can engage cytoplasmic signaling molecules such as Src-family kinases. Our simulation shows that as affinity increases, not only does the number of collected antigen increases, but so does signaling activity. Our results are also consistent with the existence of an experimentally-observed threshold affinity of activation (no signaling activity below this affinity value) and affinity discrimination ceiling (no affinity discrimination above this affinity value). Comparison with experiments shows that the time scale of dimer formation predicted by our model (less than 10 s) is well within the time scale of experimentally observed association of BCR with Src-family kinases (10-20 s).