Exo70-silenced B cells display deficient lysosome stabilization and fusion at the IS. (A) Control or Exo70-deficient B cells were stimulated with BCR ligand⁺ Dynabeads for indicated times, and synaptic membranes were isolated. Left: Detection of LAMP-1 by Western blot. Right: Quantification of LAMP-1 normalized to input protein levels in total lysates from four independent experiments. Error bars are mean ± SEM. (B) Representative images of B cells stained for the centrosome (CEP55, green) and lysosomes (LAMP-1, red) after indicated activation times with BCR ligand⁺ beads. The centrosome plane of the CEP55 staining is shown along the Z-projection of LAMP-1. Scale bar: 3 µm. (C–E) Quantification of centrosome and lysosome polarity indexes (C and D) and lysosome accumulation (E) at the IS. **, P = 0.0026; ****, P < 0.0001; ANOVA with Sidak’s multiple comparison test. n ≥ 70 cells from two independent experiments. ns, not significant.