CXCL13/CXCR5 signaling establishes an LFA-1–supported kinapse to facilitate BCR signal integration on motile B cells. (A) Time-lapse DIC and fluorescence images (Ca²⁺ influx, color-coded scale) of representative MD4 B cells on membranes with tethered HEL (1 molecule/μm²) alone or with CXCL13. White and black arrows identify B cells monitored in each condition. Scale bar indicates 5 μm. (B) Proportion of migratory MD4 B cells showing Ca²⁺ influx on membranes bearing HEL at the specified density and with CXCL13; data represent the means ± SEM of 4 experiments. (C) Ca²⁺ influx profiles of single migratory MD4 B cells on CXCL13-coated membranes with no antigen or tethered HEL (1 molecule/μm²). Profiles of 2 representative cells are shown in each case; dashed black line indicates maximum Ca²⁺ signal for only chemokine stimuli. (D) DIC and fluorescent images for F-actin (red) and pMLC (green) of 2 representative fixed MD4 B cells on CXCL13-coated membranes in the absence of antigen. Scale bar indicates 2 μm. (E) Frequency of B cells that show target membrane contact (IRM⁺) and migration (estimated by DIC) on CXCL13-coated membranes in the absence of antigen after treatment with 0.1% DMSO (carrier), 0.5μM latrunculin A (LAT), 50μM blebbistatin (Blebb), or no treatment (none); data of one representative experiment are shown. Gray bar indicates not detected. All experiments were performed in the presence of ICAM-1 (150 molecules/μm²).