B cells within germinal centers migrate preferentially from dark to light zone.

One of the main questions in the field of imaging immune cell migration in living tissues is whether cells fulfill their functionality via random or nonrandom migration processes. For some applications, this issue has remained controversial even after publication of various imaging studies. A prime example is B-cell migration in germinal centers (GCs) where somatic hypermutation and clonal selection of B cells are thought to occur within morphologically distinct regions termed dark zone (DZ) and light zone (LZ). Here, we reanalyze a previously published dataset on GC B-cell migration, applying a sensitive analysis technique to detect directed migration and using a procedure to correct for a number of artifacts that frequently occur in time-lapse imaging experiments. Although B cells roughly perform a persistent random walk, we present evidence that they have a small preference (of on average about 0.2-0.3 μm min(-1)) to migrate from DZ to LZ, which is consistent with classical views of the GC reaction. This preference is most pronounced among a large subset of almost half of the B-cell population migrating along relatively straight tracks. Using a computational model to generate long-lasting B-cell tracks based on the experimental motility data (including the small directional preference), we predict a time course to travel from DZ to LZ of a few hours. This is consistent with experimental observations, and we show that at the observed cellular motility such a time course cannot be explained without the small preferential migration from DZ to LZ.