MC–DC synapse formation requires integrin engagement. (A) Representative images of functional blocking with small molecule inhibitors for VLA-4 (LDV) and/or LFA-1 (Zyn) disrupts trafficking and polarization of actMC endosomes. Tubulin, white; IgE–FcεRI, red; actin, green. Insets are of actin staining (for the MC located below and to the right of each inset) to demonstrate when actin clearance occurred. Values below images indicate the percentages of BMMC demonstrating polarization or the percentages of imDCs demonstrating material transfer (error is the 95% confidence interval). All images are brightness and contrast enhanced. Bars, 10 µm. (B) The force in nanonewtons needed to detach actMCs from imDCs under different integrin blocking conditions (Unactivated: n = 21 DC–MC contacts in five experiments; activated: n = 43 contacts in six experiments; anti-integrin antibodies [Abs]: n = 30 contacts in two experiments; Zyn+LDV: n = 24 contacts in three experiments; PP2: n = 19 contacts in two experiments). ***, P < 0.001 (by Mann–Whitney test). Error bars are SEM.