Activated Germinal-Center B Cells Undergo Directed Migration

Affinity maturation, the fundamental basis for adaptive immunity, is accomplished through somatic hypermutation of B-cell receptors followed by the expansion of rare mutants with higher affinity for the immunizing antigen. This process occurs over a period of weeks in unique microanatomic sites known as germinal centers. Two-photon microscopy has recently made it possible to track individual B cells moving within germinal centers in living animals. Characterizing the migration patterns of B cells within germinal centers is critical for understanding the mechanisms underlying affinity maturation. Here we present the results of two statistical approaches designed to test the hypothesis that the motion of B cells within germinal centers is random. Analysis of four different experiments shows that activated B cells move in a directed manner that sharply contrasts with the behavior of naive B cells.