Definition of germinal-center B cell migration in vivo reveals predominant intrazonal circulation patterns

Proliferation, mutation, and selection in the germinal center (GC) are thought to occur in distinct microanatomical compartments-the dark zone (DZ) and the light zone (LZ). Thus, affinity maturation has been posited to require frequent trafficking between zones. Here we report the use of multiphoton in vivo microscopy to determine migration patterns of GC B cells. Analysis of time-resolved images revealed unexpected patterns of movement as well as GC B cell morphology. Though frequent movement between the DZ and LZ was anticipated, few cells were observed to cross the interface between the two compartments. Moreover, cell-track trajectories indicated that cell movement in this region is predominantly parallel to the interface, suggesting that B cells circulate within individual LZ and DZ compartments. The results suggest a revision to our views of B cell circulation within GCs and the functional relationship of its two major compartments.