CXCR5 is differentially expressed by different B-cell populations and is required for migration of B cells into Peyer patches and the peritoneal cavity. (A) CXCR5 expression was determined for peritoneal B1 cells (CD19⁺B220^lowCD23⁻), peritoneal B2 cells (CD19⁺B220^highCD23⁺), splenic MZ-B cells (CD19⁺CD23^lowCD21^high), and splenic B2 cells (CD19⁺CD23⁺CD21^int). Numbers indicate the ratio of the mean fluorescence intensity and the corresponding isotype control staining. Light gray–shaded areas represent isotype controls for the cell populations indicated. (B) The homing efficiency of splenocytes isolated from wild-type and CXCR5-deficient mice was determined by competitive adoptive transfer experiments. One day after adoptive transfer, the ratio of wild-type to CXCR5-deficient cells (circles) was determined in different compartments. Results depicted are representative of 1 of 3 experiments performed with 4 to 6 recipient mice each. Error bars represent the SEM. Statistical differences are indicated for the mean values and variances separately as follows: *P < .05; **P <.01; ***P < .001.